Development of Efficient Designs of Cooking Systems. I. Experimental

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Development of Efficient Designs of Cooking Systems. I. Experimental Jyeshtharaj B. Joshi,*,†,|| Aniruddha B. Pandit,*,† Shirish B. Patel,*,†,§ Rekha S. Singhal,‡ Govind. K. Bhide,§ Kishore V. Mariwala,§ Bhagwat A. Devidayal,§ Sanjay P. Danao,†,^ Ajitkumar S. Gudekar,† and Yogesh H. Shinde† †

Department of Chemical Engineering, Institute of Chemical Technology, Matunga, Mumbai 400019, India Department of Food Engineering and Technology, Institute of Chemical Technology, Matunga, Mumbai 400019, India § Land Research Institute, Second Floor, United India Bldg., P.M. Road, Mumbai 400001, India Homi Bhabha National Institute, Anushaktinagar, Mumbai 400094, India

)

‡

ABSTRACT: In the conventional cooking practice, where a pot or a pan is directly placed on a flame, the thermal energy efficiency is in the range of 1025%. It was thought desirable to increase this efficiency up to 60% or more. The cooking systems can be of various sizes. In the developing world (85% of the world’s population), open pan cooking is largely still practiced at the family level (410 people) or at the community level (502000 people or more). The latter requirement is encountered in schools, homes for senior citizens, jails, social and/or religious centers (temples, mosques, churches), social and/or educational functions (conferences, marriages, celebrations, etc.), remand homes, etc. For these different types of final application, in the present work, cooking systems have been developed. A systematic work has has been reported regarding the effect of several parameters on thermal efficiency. The parameters include the cooker size, number of pots, size and aspect ratio of the pots, heat flux, flame size, fluxtime relationship, insulating alternatives, etc. Local and global optima of the parameters have been obtained, resulting in thermal efficiency of about 70%.

1. INTRODUCTION Energy conservation has become an important need of our times. Population levels are rising. Conventional energy sources are steadily depleting. The developments in the renewable energy alternatives have been slow and are not yet economically viable. Because of these factors, it becomes important to look for the ways and means to reduce the consumption of energy from conventional sources. About 40% of the total energy consumed in the developing world (population more than 4 billion) is used for cooking. The large population size explains this astonishing figure. Out of the total energy consumed for cooking, 31% comes from commercial fuels such as gas, kerosene, or coal; the remaining 69% is from noncommercial fuels such as firewood, dung cakes, and agricultural waste1,2. Because of its increased availability, economy, and convenience, the use of liquefied petroleum gas (LPG) is increasing at a faster rate in urban as well as rural areas. The problem of energy conservation has been seriously addressed in the industrial sector, and various techniques and methods of operation that conserve energy have been developed and practiced. However, practically no information is available in the published literature on energy conservation in cooking. It may be noted that cooking is also a physicochemical process, like any other process in the Chemical Process Industry (CPI), and hence, there is a possibility of applying conservation strategies in CPI to cooking practices. This paper considers the ways and means of minimizing the consumption of LPG in cooking. It also considers the optimum use of solid fuel in stoves and solar energy for cooking. The cooking device, or cooker developed and discussed here, is restricted to foods that can be cooked by boiling or steaming. Basic chemical engineering principles have been combined to develop an energy efficient cooker for such foods. Datta and co-workers36 have discussed such methodology for baking and r 2011 American Chemical Society

microwave cooking. Datta and co-workers79 and Joshi and coworkers1022 have discussed the principles of heat and mass transfer. These works will be used in the present work. This work is presented in three parts. Part I starts with the optimum dimensions of vessel and heat flux recommended by the statutory agencies such as the Bureau of Indian Standards. Experiments have been performed for the optimization of cooker geometry and the sizes inside cooking pots. An attempt has also been made to optimize the flame dimensions and heat flux. Further experiments have been performed for the optimum use of multiple pots. For improving thermal efficiency, it is important to reduce heat losses to the surroundings. We have used an air gap as an insulator, and Part I describes an experimental methodology for the optimum selection of air gap. It may be noted that in Part I only water has been used in cooking vessels and pots. Various cooker sizes have been optimized such as 4.5, 6, 24, 72, 120, 160, and 700 L capacities. In Part II, we have used actual food materials such as rice, lentils, and vegetables for the purpose of optimization of cookers in the size range of 4.5700 L. We have employed computational fluid dynamics for understanding fluid mechanics and heat transfer phenomenon in the cooking process. Part III is concerned with the investigation of the kinetics of cooking and the estimation of the quality of cooked food (such as nutrients, antinutrients, taste, and flavor) by using various sources of heat such as gas burners, solid fuel stoves, microwave oven, etc. Special Issue: Nigam Issue Received: April 21, 2011 Accepted: December 19, 2011 Revised: November 9, 2011 Published: December 19, 2011 1878

dx.doi.org/10.1021/ie200866v | Ind. Eng. Chem. Res. 2012, 51, 1878–1896

Industrial & Engineering Chemistry Research

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Figure 1. Experimental setup to determine the thermal efficiency of the cooking device: (1) LPG cylinder, (2) regulator, (3) control valve, (4) U-tube barometer, (5) soap film meter, (6) gas stove, (7) pot, (8) thermocouple probe, (9) temperature indicator.

2. EXPERIMENTAL SECTION Figure 1 shows a schematic diagram of the experimental setup. It consists of a domestic LPG cylinder with a regulator and a flow control valve. A U-tube barometer was used to measure the pressure drop across the burner. A soap film meter was employed to measure the flow rate of LPG through the burner. The gas stove was provided with four burners: (i) smallest burner with the maximum gas flow rate of 5.6 mL/s, (ii) small burner with the maximum gas flow rate of 10.5 mL/s, (iii) medium burner with the maximum gas flow rate of 16.7 mL/s, and (iv) large burner with the maximum gas flow rate of 21.4 mL/s. Only one burner was used at a time. Pots of different capacities with different aspect ratios (height to diameter ratios), with and without compliance to Bureau of Indian Standards (BIS) (IS 4246:1992) were used for the experiments. The experiments have been performed in two stages: (i) heating of a predetermined quantity of water from temperature T1 to T2 (Sections 36) of Part I and (ii) cooking of common food items such as rice and lentil (dal) discussed in Section 3 of Part II. Temperatures were measured by means of point thermocouples located at the center of the water pool and center of the food charge in the vessel. Experiments were also performed to investigate the cooking by pressure cooker and stacking the vessels. Different methods of heat insulation have also been examined. The thermal efficiency was determined using the following equation Thermal efficiency = (enthalpy absorbed by vessel + enthalpy absorbed by the vessel contents + enthalpy absorbed by water in base + enthalpy lost in water evaporation if any)/(enthalpy supplied) ηtherm ¼

½ðMV CPV þ MG CPC þ MB CPW ÞðT2 T1 Þ þ mλ ðMG CVÞ

ð1Þ where M and CP denote mass and specific heat capacity. T1 and T2 are initial and final temperatures, respectively. CV is the calorific value of LPG and is considered as 10,900 kCal/kg. The subscripts V, C, G, and B denote the vessel, vessel contents, LPG, and base, respectively. The significance of the term MBCPW will

be explained in Section 4.4. Until then, the value of MBCPW is zero. Also, m is the quantity of water evaporated, which was estimated by noting the water weight before and after heating. Temperature measurements were within a precision of 0.1 C. LPG flow rate was measured using a soap film meter. For this, an average of five observations for each flow rate was considered, where the reproducibility was within 5%. In a few cases, where the difference in efficiency was small, the experiment was repeated three times, and the error was reduced to within 2%.

3. RESULTS AND DISCUSSION 3.1. Optimization of Vessel Geometry with Water Heating. 3.1.1. BIS Standard Vessels. Bureau of Indian Standards

(IS 4246:1992) has recommended vessel dimensions [diameter (D), total height (H)] and the respective quantities of water for getting maximum thermal efficiency. The standards also recommend burner size and gas flow rate for the optimum thermal efficiency. In view of these recommendations, it was thought desirable to perform the starting experiments with BIS standard burners and vessels. Accordingly, three vessel sizes with an aspect ratio (H/D) of about 0.55 were employed: (i) D = 180 mm, H = 100 mm; (ii) D = 220 mm, H = 120 mm; and (iii) D = 260 mm, H = 140 mm. In these three vessels, the BIS recommended water charge is 2, 3.7, and 6.1 L, respectively, and these same values were selected in the initial set of experiments. The burner sizes and gas flow rates were also according to BIS guidelines. The ratio of vessel diameter to the respective flame size was in the range of 33.5. For the estimation of thermal efficiency, two temperature ranges were selected: (i) 2690 C and (ii) 2698 C. The results in terms of gas consumption and the thermal efficiency are listed in Table 1. For the temperature range of 26 to 90 C, the experimentally observed values of thermal efficiency (ηtherm) agreed with those reported by the BIS. For the second temperature range (not recommended by BIS), the values of efficiencies are shown to be lower because of the enhanced heat losses at operating temperatures exceeding 90 C. Lentil (dal) forms an important part of a staple diet in the developing world. The cooking of lentils starts at and above 94 C and takes about 20 min for the desired extent of cooking. This means that when a temperature of 94 C is attained the 1879

dx.doi.org/10.1021/ie200866v |Ind. Eng. Chem. Res. 2012, 51, 1878–1896


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Table 1. Gas Consumption and Thermal Efficiency for BIS Standard Vesselsa burner/flow rate (mL/s)

charge (L)

small

2.0

vessel dimensions (mm)

aspect ratio (H/D)

D = 180

0.55

H = 100

10.5 medium

D = 220

3.7

large

D = 260

6.1

0.54

H = 140

21.4 a

0.55

H = 120

16.7

gas consumed (g)

efficiency (%)

18.0

66.9

(20.6)

(65.8)

32.8

67.9

(38.6)

(64.9)

53.8

67.8

(65.2)

(62.9)

The unbracketed values are for the first temperature range of 2690 C. The bracketed values are for the second temperature range of 2698 C.

Table 2. Effect of Aspect Ratio (H/D) on Thermal Efficiencya

burner/flow rate (mL/s) smallest 5.6

vessel V2

vessel V3

vessel V4

D = 140 mm H = 160 mm

D = 230 mm H = 125 mm

D = 180 mm H = 220 mm

H/D = 0.55

H/D = 1.14

H/D = 0.55

H/D = 1.22

parameters

charge = 2 L

charge = 2 L

charge = 5 L

charge = 5 L 4560

time required (s)

1520

1580

4510

gas consumed (g)

19.6

20.3

58.1

58.8

η (%)

68.6

66.3

59.3

58.6

small 10.5

time required (s) gas consumed (g)

840 20.6

900 21.9

2250 54.3

2450 59.1

η (%)

65.8

60.4

63.8

57.8

medium 16.7

time required (s)

640

670

1360

1660

large 21.4

a

vessel V1 D = 180 mm H = 100 mm

gas consumed (g)

24.7

25.7

52.3

63.9

η (%)

56.7

51.6

66.3

52.6 1370

time required (s)

540

580

1090

gas consumed (g)

26.7

28.4

53.7

67.6

η (%)

52.4

47.2

64.6

50.3

Temperature range is 2698 C.

remaining cooking can be achieved by just maintaining the temperature of the vessel mass at 94 C. Therefore, it was thought desirable to explore the possibility of reducing (or stopping) the gas supply after a certain temperature is reached. By taking into consideration a temperature drop of about 34 C in a 20 min period over which the cooking temperature is to be maintained, heating of the contents up to 98 C is desired. Hence, the temperature range of 2698 C is considered hereafter for all the experiments in order to calculate the values of thermal efficiency. It has been observed that the heat losses during the heating process occur via three modes: (i) with the flue gases, (ii) from the hot surface of vessel to the surroundings and (iii) the evaporation losses from the exposed part of water charge in the vessel. In order to understand the relative contribution of these three modes of heat losses, it was thought desirable to undertake a systematic investigation of the effects of (i) vessel geometry, (ii) gas flow rate, (iii) ratio of vessel diameter to flame diameter and (iv) various methods of insulation. 3.1.2. Effect of Vessel Geometry. In order to investigate the effect of vessel geometry, vessels of 2 and 5 L capacity with different aspect ratios were employed. In the case of 2 L capacity, the standard vessel with a H/D ratio of 0.55 having a diameter of 180 mm and a height of 100 mm (V1) and the tall vessel with a H/D ratio of 1.14 having a diameter of 140 mm and a height of 160 mm (V2) were employed. For the 5 L capacity, the standard vessel had a ratio of 0.55 with a diameter of 230 mm and a height

of 125 mm (V3), whereas the tall vessel had a H/D ratio of 1.22 with a diameter of 180 mm and a height of 220 mm (V4). Thus, two BIS standard (H/D ∼ 0.55) and two nonstandard vessels have been investigated. For all four vessels, all four burners were employed and were operated at their recommended gas flow rates. The results are listed in Table 2. It is observed that, for the same capacity, the standard (flatter) vessels give higher efficiency than the tall vessels. This is because, in standard vessels, base diameter is large compared to that of a tall vessel of the same volume. Therefore, the flue gases are in contact with the vessel bottom for a longer time and hence transfer more quantity of heat to the vessel before escaping to the surroundings. Further, among the standard vessels, it appears that there is an optimum burner configuration. In fact, there are several factors that govern performance: (i) as mentioned earlier, a larger bottom area is useful for heat absorption, (ii) for a given area, an increase in gas flow rate (or heat input) reduces the proportion of heat absorption, (iii) for a given bottom area, an increase in flame size reduces the contact time between the flue gases and the vessel bottom, and (iv) for a given aspect ratio (H/D), an increase in the bottom area (concomitant increase in the water volume) prolongs the heating time and hence the overall heat losses. The combined effect of these four factors determines an optimum performance. Thus, for a 180 mm diameter standard vessel, the smallest burner was found to be optimum, whereas for 230 mm diameter standard vessel, a medium burner was found to be optimum. These results are in line with the BIS guidelines. 1880



Figure 2. Effect of LPG flow rate on the thermal efficiency: D = 180, H = 220, charge = 5 L.

The tall vessel gives low efficiency because of longer heating time during which more heat losses occur from the sidewall. For instance, in a 2 L capacity vessel, the time to heat requirements were 1520 and 1580 s for the standard and tall vessel, respectively, when the smallest burner was used. Also, the percentage difference of heating time was reducing as the gas flow rate decreased. Though the tall vessel gives low efficiency, it was thought desirable to investigate the various features of this vessel in view of the possibility of using it for multilayer cooking. The features include the effect of gas flow rate and various methods of reducing heat losses. The results have been described in the next two subsections. 3.2. Effect of LPG Flow Rate. Vessel V3 was used to study the effect of LPG flow rate. All burners were used with varying flow rates. A water charge of 5 L was selected, as the cooked food quantity equivalent to this is sufficient for a family of 56 members. The results are shown in Figure 2. There appears to be an optimum gas flow rate for every burner and vessel configuration. The optima can be observed at the flow rates of 5.6, 9, 9.9, and 10.6 mL/s for the smallest, small, medium, and large burner, respectively. The corresponding values of heat flux works out to be 19355, 31106, 34217, and 36636 kCal/h m2, respectively. Maximum efficiency was obtained for the smallest, small, and medium burner. As the burner size was increased to large, a reduction in the maximum efficiency value was observed. Every vessel and its content have a particular overall heat transfer coefficient (this subject will be discussed in detail in Section 2 of Part II). Optimum heat flux for the vessel depends upon the combination of the vessel itself, its dimensions, surface area, and contents, as well as the area occupied by burner and flame. In these experiments, the vessel and the quantity of its contents was essentially kept the same. In addition to this, the temperature gradient that was to be achieved, 2698 C, was also kept same for all the burners and their respective gas flow rates. When the LPG flow rate to the burner is increased, more quantity of heat energy is made available to the vessel. Heat absorption capacity and hence the thermal efficiency of the vessel increases until the heat flux across the vessel bottom matches with that of the burner. If the heat supplied is less than this capacity, it would result in increased surrounding losses from the

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vessel and its contents. If the heat supplied is more than this capacity, it would have no effect on the absorption, resulting in wastage. Thus, efficiency is maximum when both heat fluxes (absorption and loss) are matched. Heat flux, either less or more than this, would result in a decrease in thermal efficiency. Because of this, maximum efficiency is obtained for the burner at a particular gas flow rate, as observed in Figure 2. As burner size increases, flame size also increases, which results in an increase in temperature of flue gases leaving the vessel without supplying its enthalpy content to the vessel. Hence, the optimum gas flow rate increases with an increase in burner size. The subject of the “optimization of heat flux” will be discussed in detail in Part II. 3.3. Effect of Insulation. It was stated earlier that the tall vessels show lower efficiency compared to the standard vessels. In order to make an improvement in the thermal efficiency, various ways were attempted for reducing the heat losses. 3.3.1. Heat Loss from Flue Gas. In order gain information on the heat losses from flue gases, the flue gas temperatures were measured along the vessel side wall. A medium size burner was used with the flow rate of 14.7 mL/s for all four vessels described previously (Table 2). As shown in Figure 1, temperature was measured at the flame center (at point F) as well as at point P, 20 mm below the vessel side wall or periphery, (hereafter referred as the peripheral point). Some typical measurements of temperatures for vessels V1, V2, V3, and V4 are listed in Table 3. The flame temperature was observed to be 1150 C in all the cases. Further, considerable reduction in temperature is observed to occur just a short radial distance away from the flame center. Various vessel wall temperatures along the vertical distance when the water temperature reaches 98 C are listed in Table 3. Again, a substantial reduction in temperature is observed to occur along the side wall. These reductions in temperature from the peripheral point give an indication of heat loss through flue gas. The temperature of the flue gas at the peripheral point increases as the charge temperature increases. This may be due to the reduction in the heat absorption capacity of water with a rise in temperature. This also results in more heat loss with the flue gas. The peripheral and wall temperatures for the standard vessel were observed to be lower compared to those for the tall vessel. This is because, in the case of the standard vessel, the flue gas is in contact with the vessel bottom for a longer time transferring its enthalpy before escaping. Moreover, for the vessel with the diameter of 180 mm, the temperatures of vessel wall and the peripheral point are higher in the case of the BIS standard vessel compared to those of the nonstandard, tall vessel. This is due to the difference in water quantity in the two vessels. As a result of the above-mentioned factors, the thermal efficiencies for the 2 L capacity vessel are 56.9% and 50.1% for standard and nonstandard vessels, respectively. The corresponding values for the 5 L capacity vessels can be observed to be 64.5% and 57.4%, respectively. The significant difference in thermal efficiencies of the standard and tall vessels reinforces the need for reducing energy loss through the flue gas, especially for tall vessels when employed for heating and/or cooling. For this purpose, two methods of insulation were employed. 3.3.2. Glass and Asbestos Insulation. In this case, two types of insulation methods were examined. In the first method, the vessel was covered by a glass cylinder, and in the second method, a spiral winding of asbestos rope insulation (20 mm diameter) outside the vessel was provided, which was then covered by a glass cylinder. Practically no improvement was observed in the performance. This is because of the specific heat requirement of 1881



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Table 3. Temperatures of Water, Flame, and Flue Gas along the Vessel Wall Medium Burner at LPG flow rate of 14.7 mL/s vessel V1 with D = 180 mm, H = 100 mm, charge = 2.0 L, water depth = 79mm flue gas temperature on the side wall (C) distance from vessel bottom (mm) water (C)

flame center (C)

30

1150

40

1150

50

1150

60

1150

70 80

peripheral point (C)

0

30

60

90

time (s)

30

30

30

30

30

0

250

147

99

67

70

120

267

173

116

78

89

180

275

182

124

91

90

300

1150 1150

282 288

189 194

134 142

97 103

91 93

380 480

90

1150

296

201

155

116

95

570

98

1150

307

208

160

120

97

650

vessel V2 with D = 140 mm, H = 160 mm, charge = 2.0 L, water depth = 130 mm flue gas temperature on the side wall (C) distance from vessel bottom (mm) water (C)

flame center (C)


0

40

80

130

time (s)

30 40

1150 1150

30 250

30 161

30 118

30 40

1150 1150

30 250

50

1150

267

182

137

50

1150

267

60

1150

275

196

151

60

1150

275

70

1150

282

211

163

70

1150

282

80

1150

288

234

170

80

1150

288

90

1150

296

253

177

90

1150

296

98

1150

307

278

182

98

1150

307

time (s)


flame center (C)

30 40


0

40

80

120

1150

30

30

30

30

30

0

1150

175

85

57

46

44

240

50

1150

195

93

68

59

56

420

60

1150

213

105

79

68

65

660

70

1150

218

119

85

77

77

840

80 90

1150 1150

220 224

127 132

95 104

83 87

81 86

1080 1260

98

1150

226

144

114

101

98

1440


flame center (C)

30

1150

30

30

30

30

30

30

0

40 50

1150 1150

286 292

120 133

77 91

55 73

47 61

43 53

240 480

60

1150

303

144

101

75

71

61

720

70

1150

309

153

114

88

82

71

960


0

40

1882

80

120

180

time (s)



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Table 3. Continued vessel V4 with D = 180 mm, H = 220 mm, charge = 5.0 L, water depth = 196 mm flue gas temperature on the side wall (C) distance from vessel bottom (mm) water (C)

flame center (C)


0

40

80

1150

311

164

90

1150

316

175

98

1150

320

178

80

120

180

time (s)

123

97

89

78

1200

130

103

92

87

1440

138

108

99

93

1620

Figure 4. Effect of metal covers on thermal efficiency (A) flat vessel: charge = 5.0 L, D = 230 mm, H = 125 mm; (B) tall vessel: charge = 5.0 L, D = 180 mm, H = 220 mm.

Figure 3. Schematic of the assembly of the vessel with metal covers: (A) covers without holes on the top surfaces (front view); (B) covers with 8 holes of 10 mm diameter on the top surfaces (top view); (1) rubber spacer, (2) lid, (3) water level, (4) cover-1, (5) cover-2, (6) vessel.

the glass cover and the insulating rope. In addition, in actual cooking practice, it is difficult to use a glass cover or asbestos as a choice for insulation. Therefore, further work was directed toward selection of material with low heat capacity.

3.3.3. Effect of Metal Covers. In this case, the vessel was enclosed by two concentric metal vessels, called covers, of appropriate sizes with a 5 mm air gap between the two acting as insulation. Rubber spacers were used to ensure that the two covers and the vessel do not touch each other as shown in panel (A) of Figure 3. Different burners at their maximum flow rates were employed. The following cases were considered: (i) vessel without covers, (ii) vessel with covers, and (iii) vessel with covers, each cover having on the top surfaces 8 holes of 10 mm diameter (Figure 3B) to allow the escape of hot flue gas. In this investigation, both, the standard vessel (V3) and the tall vessel (V4) were used. The results are shown in panels (A) and (B) of Figure 4, respectively. It is shown that the metal covers without the escape holes show relatively higher efficiency. 1883



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Figure 5. Schematic of cooker assembly: (1) vessel lid; (2) stand for holding pots; (3) outer cover; (4) cover; T = thermocouples; V1, V2, V3, V4 = cooking vessels.

Thermal efficiency is a function of the following parameters: geometry of the vessel, heat transfer area, heat holding capacity of the vessel and its contents, and burner size and its flow rate. It should be noted that the metal cover serves a dual purpose: first, it reduces the heat losses from the vessel and its contents to the surroundings during heating, and second, it entraps the flue gas that helps in recovering more heat energy associated with it. These factors determine the balancing of heat fluxes for the vessel assembly and the burner with its respective gas flow rate. In this case, vessel charge and the required temperature gradient was the same for both kinds of vessels. Because of this, the heat holding capacity of both vessels can be assumed as equal. Because of the different geometry of the vessels, the tall vessel has more heat transfer area (side and bottom) of 0.15 m2 as compared to 0.13 m2 for the standard vessel, although the bottom area of the standard vessel was more than that of the tall vessel. For a particular burner, with its respective gas flow rate, flue gas holding capacity of the tall vessel was more than that of the standard vessel. This results in transfer of more quantity of heat to the tall vessel than to the standard vessel. Hence, the rise in thermal efficiency due to the metal covers is more in the case of the tall vessel. Because of the equal heat holding capacity and entrapment of flue gas in the cover, balancing of heat fluxes for both vessels occurs for the same burner, small burner in this case, at the similar gas flow rate in the range of 910 mL/s. This shows the same maxima in the efficiency values in both cases. Further, when holes were drilled in the top surfaces of the covers to allow the escape of flue gas, efficiency was found to decrease for both cases. This is because the flue gas entrapped in the covers vented through these holes. This resulted in a decrease in flue gas retention time. Because of this, relatively more heat energy was lost along with the flue gas without getting absorbed by the vessel and its contents.

4. DEVELOPMENT OF COOKER ASSEMBLY: PHASE I 4.1. Multiple Stacking of Pots. Cooking practices employ simultaneous cooking of 2 to 4 items such as rice, lentils, vegetables, etc. Therefore, it was thought desirable to try stacking

Figure 6. Temperature profiles: (A) heating profiles, (B) cooling profiles (LPG flow rate = 7.3 mL/s, charge = 4.5 kg).

of multiple vessels in the cooker. Toward this objective, additional aspects need to be considered. A schematic of stack design is shown in Figure 5. A stack of four cooking vessesl V1V4 (supported by a stand) was placed in a vessel having a 180 mm diameter and a height of 360 mm, henceforth called an inner container vessel or just container vessel. Each pot had a capacity of 1.5 L with a diameter of 160 mm and a height of 80 mm. The uppermost vessel (V4) was covered with a flat lid. The container vessel was also covered with a lid as shown. Another vessel having a diameter of 190 mm and a height of 400 mm was kept inverted over this whole assembly acting as an outer cover. Thus, the container vessel and the outer cover together form a double cover as shown in panel (A) of Figure 3. The bottom of the inner container vessel was in contact with the flame. A known quantity of water was placed in the container vessel to maintain a thermal contact between vessel V1 and the container vessel. This assembly was investigated in a stepwise manner: (1) estimation and analysis of thermal efficiency, (2) mechanism of heating and cooling (during retention period), (3) selection of material of construction, (4) effect of LPG flow rate, and (5) effect of the quantity of water in the container vessel. The results have been reported in the following subsections. 4.2. Time for Heating and Cooling. Experiments were performed with 1 kg of water in each of the inner four pots and 500 g water in the container vessel. As the best thermal efficiency for the 180 mm diameter vessel was observed in the gas flow range of 79 mL/s (Table 2), heating was carried out at 7.3 mL/s of LPG flow rate. About 5560 min are required for the four pots to reach a temperature of 98 C. At this point, the gas supply was stopped, and then cooling (during retention period) was allowed for 240 min. The heating profile is shown in panel (A) of Figure 6 , and a cooling profile is shown in panel (B) of Figure 6. 1884



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Figure 7. Gas consumption for different charges.

From heating profiles, it is observed that the temperature of water in the container vessel rises fast as it is in direct contact with the flame. As the bottom vessel (V1) is in contact with water in the container vessel, its temperature follows the temperature of water in the container vessel. When the temperature of water in the container vessel reaches ∼80 C, some water quantity starts evaporating, and steam moves up, comes in contact with outer surfaces of the upper vessels (V2, V3, and V4), and condenses. The condensate descends back to the container vessel. The cover temperature is less compared to that of others as it is exposed to the atmosphere. From the cooling profiles (Figure 6B), it is shown that the cooling rate for the outer cover and the water in the container vessel is faster than the inner vessels, as they are exposed to the surroundings. Among the vessels, the bottom vessel (V1) cools relatively fast as it is in contact with the water in the outer vessel. However, other vessels, V2, V3, and V4, cool relatively slowly and almost simultaneously as they are not in contact with either the container vessel or the outer vessel or the surroundings. A temperature of more than 94 C is maintained for almost 45 min in these vessels, except for vessel V1, which cools to 88 C in that time. It is observed that a temperature of more than 94 C for up to 45 min is possible even without further supply of heat. 4.3. Material of Construction. In order to select the material of construction of cooking pots, experiments were performed using aluminum and stainless steel (SS-304) pots under otherwise identical conditions. The temperature of vessel V3 was noted because its response was relatively slow during both heating and retention. All results are shown to be practically comparable. In this case, aluminum is preferred as the material of construction for the container vessel and the outer cover. 4.4. Effect of Water Quantity in Pots. Experiments were performed at two gas flow rates of 8.3 and 18.4 mL/s for water quantities in pots in the range from 400 to 6000 g. An equal amount of water was placed in all four pots. Figure 7 shows the gas consumption for different water quantities. It was observed that although the heating rate was lower at the low LPG flow rate the quantity of gas consumed to heat the charge to 98 C was also lower than that for the higher flow rate. In the case of the higher gas flow rate, the rate of flue gas formation is also higher compared to that at a lower flow rate. As the heat transfer area is the same in both cases, the lower LPG flow rate results in more

Figure 8. Effect of LPG flow rate on the thermal efficiency for the cooker assembly with a 4.0 kg charge and 0.5 kg water in the outer vessel.

effective heat transfer from the flue gases to the vessel before their escape to the surroundings and hence requires less gas consumption. Moreover, the metal assembly to be heated is the same in all cases. This requires the same quantity of sensible heat to heat to 98 C. A further supply of heat is utilized in heating the contents. Thus, the sensible heat of metal assembly per unit water quantity decreases with an increase in the total water quantity. The time required for cooling of water in vessel V3 was noted. It was observed that the rate of cooling reduces with an increase in the quantity of charge. This is because the heat transfer area (for heat losses) per unit volume decreases with an increase in the volume of water. 4.5. Effect of LPG Flow Rate. The effect of gas flow rate on thermal efficiency was reported in Section 3.2. For a 5 L capacity, a BIS standard vessel was employed with an aspect ratio of 0.55 (D = 230 and H = 125). One more vessel was also investigated having an aspect ratio of 1.22 (D = 180, H = 220 mm). The quantity of water was 5 L in both the cases. The optimum flow rates were found to be 15 and 5 mL/s, respectively, with respective efficiencies of 63.8 and 58.6%. From the above discussion, it is clear that the efficiency is lower for the high aspect ratio vessel. In order to improve this number, a provision was made as described in Section 4.1 for stacking of multiple pots as well as a double cover insulation system (Figure 5). For such an assembly, it was thought desirable to investigate the effect of LPG flow rate where the container vessel had a 180 mm diameter. Experiments were performed for the fixed charge of 4 kg of water (with 1 kg in each vessel). A burner that has a flame diameter one-third of the container vessel (optimum flame diameter as has been observed in Section 3.2) was employed, and the LPG flow rate was varied in the range of 4.721.4 mL/s. Water quantity of 0.5 kg was added in the container vessel. The results are shown in Figure 8. It is observed that the efficiency initially increases up to a particular LPG flow rate and then decreases. The obtained maximum efficiency was found to be 64% in the flow rate range of 79 mL/s. It may be emphasized that the efficiency value at optimum gas flow rate is comparable to the efficiency value (63.8%) for the BIS standard vessel with similar capacity. However, it should be noted that the BIS standard vessel cannot accommodate multiple pots. 1885



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Table 4. Effect of Water Quantity in Outer Vessel on Thermal Efficiency distribution water

of charge

charge (kg)

in vessels

2.0

0.5 kg in each vessel

4.0

6.0



water in outer LPG flow

thermal

vessel (kg) rate (mL/s) efficiency (%) 0.150

9.5

43.0

0.180

8.3

45.1

0.200

10.1

52.3

0.250

9.8

53.5

0.300

10.3

50.8

0.300

9.4

47.1

0.400

8.9

59.6

0.500 0.600

9.7 9.8

60.2 54.2

0.750

9.7

51.1

0.300

8.8

58.8

0.400

9.1

62.2

0.500

9.0

64.0

0.600

9.2

65.1

0.750

8.9

59.8

4.6. Effect of Water Quantity in Container Vessel. Water quantity in the container vessel serves many functions. It provides good thermal contact between the vessel bottom and the bottom pot. As mentioned earlier, water after evaporation provides heat to the upper vessels. However, this water quantity needs to be optimized because too high a quantity of water may result in wasted fuel, and too low a quantity of water may result in complete evaporation, which would be disastrous causing melting of the bottom of the container vessel (as was indeed experienced). Further, while loss of contact between water in the container vessel and the bottom pot may be desirable once the water starts boiling, loss of contact is not desirable during the preheating period. In order to optimize the water quantity, experiments were performed with a 6 L cooker having total water charge in the range of 26 kg. For these charges, water quantity in the container vessel was varied in the range of 0.150 0.750 kg. The LPG flow rate was maintained at 9 ( 1 mL/s. The results are summarized in Table 4. From these results, it is observed that the optimum amount of water in the container vessel is approximately onetenth of the total charge taken in the inner pots of the cooker. 4.7. Analysis of Phase I Development. Though the cooker assembly (Figure 5) has shown good promise, some of the limitations were found to be as follows: • Escape of steam from the lid of the container vessel and its condensation and accumulation on the top of lid of the container vessel is undesirable. • Condensation of steam and its accumulation on the top of the lid over the uppermost pot is undesirable. • The two types of condensate accumulation result in a reduction in the quantity of water in the container vessel. This may eventually result in breaking of the contact between the bottom pot and the container vessel, thus affecting further heating of the bottom pot if the loss of contact occurs too early in the preheating process. To overcome this loss, more quantity of water has to be added to the container vessel, which consumes additional energy as sensible heat during the heating period.

Figure 9. Schematic diagram of improved design of the cooker assembly: (1) lid, (2) inner metal cover, (3) outer metal cover, (4) handle, (5) base, (6) vessels.

• Entrapped flue gas transfers its heat to the container vessel as well as the metal cover, thus increasing the temperature of both. In addition to this, convective currents are also present in the gap between the container vessel and the metal cover. However, with the temperature gradient being higher on the metal cover side, more heat is transferred to it. As the metal cover is in contact with the surroundings, the overall result this is enhanced heat loss to the surroundings. • Once heating is discontinued, the contact of the bottom vessel (V1) with the remaining water in the container vessel results in faster cooling of the bottom pot. • It is an inconvenience to cooks to add a measured optimized quantity of water in the container vessel. • A relatively tall assembly is likely to be unstable.

5. DEVELOPMENT OF COOKER ASSEMBLY: PHASE II 5.1. Description of Hardware. In view of the abovementioned drawbacks, it was thought desirable to incorporate the modifications in the design shown in Figure 5. A schematic of the improved assembly is shown in Figure 9. Some of the salient features of this assembly are as follows: • easier removal of the metal cover • replacement of container vessel with the specially designed container vessel, hereafter termed as base • replacement of the lid with an inverted vessel that fits exactly in the base, thus reducing steam losses (now acts as a cover) • entrapment of flue gases in lower side of the base to utilize heat contents to heat water in the base • flat lid on top pot replaced with upwardly convex-shaped lid to trickle condensate down to the base • arrangement made in the base to accommodate exactly the measure of water quantity that is to be added • height of each vessel reduced from 80 to 70 mm • stand eliminated • handles provided • mechanism provided such that lower part of pot exactly fits the base, which also supports the other pots • notches provided at the basevessel contact level to allow free movement of steam and condensate, which reduces 1886



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Figure 10. Temperature profiles for inner cover, outer cover, base, and three vessels (V1, V2, and V3) during retention time.

spillage of water outside the base due to steam pressure during heating 5.2. Heat Losses during Retention Time. In order to quantify the heat losses taking place in the improved design of the cooker assembly shown in Figure 9, the temperatures were measured at several locations of the assembly, three pots, base, and metal covers. Figure 10 shows the plots of temperature for the three vessels, base, inner cover, and outer cover. The temperature of the base, inner cover, and outer cover during the retention time decreases rapidly, whereas it remains practically constant for the content of the vessels. As the temperature of the content of the vessels are maintained to cooking temperature during the retention time, this ensures the complete cooking of vessel contents. It is interesting to note the order of magnitude contribution of radiation to overall heat loss. The overall heat loss is estimated by the following equation QL ¼ mCP

dT dt

ð2Þ

The radiative heat loss is given by the following equation QR ¼ εσAðT 4 T0 4 Þ

ð3Þ

The value of emissivity for aluminum is about 0.09. From the cooling curve (Figure 10), the temperature of the outer cover decreases from 84 to 61 C. The values of overall and radiative heat loss at 84 C is calculated to be about 24,000 and 100 W, respectively. At 61 C, these values are 1500 and 50 W, respectively. Thus, the radiative heat loss was found to be always less than 5% of the overall heat loss. 5.3. Insulation Alternatives. In order to insulate the metal cover, three alternatives of insulation were considered: (i) natural wool (sweater), (ii) glass wool, and (iii) coaxially fitting a metal cover such that there is an air gap of 5 mm on all the sides and the top between the two covers. It was observed that the outer insulating cover (henceforth called the outer cover) reduces the cooling rate of the pot, coaxially fitting the metal cover (henceforth called the inner cover), giving the best result. 5.4. Water Spillage Problem. 5.4.1. Some Observations. When two metal covers were used, it was observed during the operation of the assembly that water spillage takes place from the base through the gap between the base and the metal covers, thereby reducing the water quantity in the base. Careful observation showed that the leakage occurs in two stages: first, during the initial phase of heating, and second,

during the last phase of heating. The total height of the cover was 200 mm, which includes the supporting height of 16 mm that fits inside the base. Water in the base may or may not be in contact with the lower edge depending upon the quantity of water. When heating starts, water in the base is heated first. Its level rises when boiling starts and because of the bubbling of steam, the liquid comes in contact with the upper edge of the base, if not earlier. Simultaneously, air inside the cover also becomes heated and its pressure increases. This creates a pressure differential that results in escape of water, which is possible only through the joint between the base and the cover. In fact, the inner higher pressure pushes the water in the base to the outside, thereby reducing the water level. This continues until the inside and outside pressures become practically equal. In the last phase of heating, the contents are near boiling point, and their heat absorption capacity reduces. This results in accumulation of steam inside the cover thereby increasing its pressure. This also expels some water quantity outside the base due to the reasons explained earlier. This problem may be solved in two ways: either by reducing the overlap of the inner cover that almost brings it in contact with the water, thereby breaking the contact, or providing some mechanism to facilitate the escape of air in the initial phase and steam in last phase, although the later approach may not be advisible because of heat loss. Both of these approaches were investigated and are described below. 5.4.2. Reduction in Overlap of Inner Cover. The original overlap of the inner cover that rests in the base was 16 mm. It was reduced by 5 mm in two stages: first to 11 and thento 6 mm. The effect on the water spillage was investigated. It was observed that as the overlap reduces and the bottom of the inner cover is raised, spillage also reduces. Reduction of the overlap of the cover does not affect thermal efficiency to any significant extent. Thermal efficiency is maximum for a 375 mL water quantity in the base. However, very low overlap is not acceptable, as the inner cover does not fit properly and results in an unstable system. In view of these observations, a second option allowing heated air to escape was investigated in which openings were provided to the inner cover in the form of holes having suitable cross-sectional area. 5.4.3. Holes in the Inner Cover. During the heating operation, air inside the cover needs to escape and be replaced by steam. This can be done by providing an opening of suitable area. The area of the opening should be such that it reduces spillage but at the same time does not have an adverse effect on thermal efficiency. Spillage was investigated for different sizes of holes starting from 1 mm diameter and upward. It was observed that two holes of 2 mm diameter (total opening area of 6.3 mm2) were just sufficient to stop the spillage. However, a single hole of 3 mm diameter (opening area of 7.1 mm2) was also adequate to stop the spillage. In order to determine its location on the cover and its effect on the heating and cooling mechanism and thermal efficiency, experiments were performed with three locations of holes on the side wall of the inner cover at 20, 90, and 160 mm from the top. It was observed that the location has no significant effect on thermal efficiency which was about 65.8% (for a 4.5 L charge with 375 mm water in the base at the LPG flow rate of 8.3 mL/s). The heating trend was similar to that observed earlier. From the cooling profiles, it is clear that during the first 30 min of retention, temperatures of all cooker components are maintained above 95 C. This indicates that the presence of a 3 mm diameter hole (at any location) has no adverse impact on the heating and retention trend. However, for the sake of convenience, a lower position for the hole was adopted. 1887



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Figure 11. Torch type burner head.

One interesting observation was noted at this stage. When the charge achieved the temperature of about 97 C, some water droplets started coming out of the gap between the base and the cover, followed by some steam puffs. This is a good indicator to determine that the charge has achieved the desired temperature at which point the gas supply can be stopped. 5.5. Burner Design. 5.5.1. Torch Type Burner Head. In order to study the temperature profile patterns of cooker components and the charge in different vessels during the heating and retention periods, it was thought desirable to measure temperatures of the charge in all cooking vessels, water in the base, and cooker cover temperatures. As the cooking of common food items resembles water heating because of similar specific heat capacities, we can analyze the cooking pattern for cooker components from the temperature profiles for plain water. A commercial LPG burner with the torch type or “T” type head was used, as it is very commonly used in practice. It has a cylindrical wind shield of 400 mm diameter and 400 mm height and a three point support stand for the vessel. A schematic of this type of head is shown in Figure 11. The burner head has holes on top from which the LPG comes out. The flame of burnt LPG rises straight upward, strikes the vessel bottom, and then spreads out radially. Then, flue gases leave the base at its periphery and escape to the atmosphere. During the travel of flue gases, which occurs fairly close to the bottom of the vessel, heat transfer takes place from flue gases to the vessel and then to the contents in the vessel. The cooker was charged to 72 L capacity with 6 L water in each vessel. Eight liters of water was added to the base, which is about 10% of the total charge (this proportion was observed to be optimum, as discussed in Sections 4.4 and 4.6). After assembling the cooker components, the burner was run at the maximum LPG flow rate of about 160 mL/s. Heating was continued in order to attain water boiling point temperatures in all components of the cooker. Water in the base achieved boiling temperature within 5 min, as the base was in direct contact with the flame. After 40 min of heating, the part of the base equivalent to the area of the flame striking the base melted. Heating was immediately stopped. At this moment, temperatures of the vessel charges were 99, 92, 91, and 96 C for the vessels V1, V2, V3, and V4, respectively.

Figure 12. Sunbeam type burner head.

The reasons behind the melting of the base of the cooker were thought to be as follows in decreasing order of severity: (1) The burner has a torch type head. The flame emanating from the burner head strikes the aluminum base in a very limited area (equivalent to the area of the burner head) before spreading out. This resulted in the creation of a hot spot at each flame striking point. (2) With the LPG flow rate being comparatively high, it contributed to increasing the intensity at the hot spots. (3) Water in the base may have been evaporated totally. (4) The base is made of aluminum that has a low melting point. However, aluminum was the material of choice for the construction. Hence, it was thought desirable to concentrate on the first three parameters for further research. Subsequently, the torch type head was replaced by a sunbeam type head. The LPG flow rates used were also reduced. 5.5.2. Sunbeam Type Burner Head. A schematic of the “S” or sunbeam type burner head is shown in Figure 12. It has two sets of holes. The first set is at the top of the head and gives a flame that is directed inward. Another set at the sides of the head is inclined outward. These form the main flame. This configuration results in a spreading of the flame before it strikes the bottom of base. Hence, the formation of hot spots at the cooker base did not occur. This burner head was used for future experimentation. 5.5.2.1. Heating and Retention Cooling Characteristics. In order to study the heating and cooling patterns of the charge in the cooker for the sunbeam type burner head, it was thought desirable to measure temperatures of the charge in all the cooking vessels as well as of the water in the base. Heating and retention were carried out with only water as a charge. Each vessel was charged with 6 L of water, thus making a total charge of 72 L. About 10 L of water quantity was added in the base. The cooker was assembled. Heating was carried out at about a 85.1 mL/s LPG flow rate. The charge was heated until all the vessel contents reached boiling point, after which the gas supply was shut off. Retention was carried out for 4 h. Temperatures were noted at regular time intervals during the heating and retention periods. When the cooker was opened, the water quantities in all the 1888


Industrial & Engineering Chemistry Research vessels and in the base were measured. It was observed that the quantity of water in the base was reduced due to evaporation, thus reducing its level. Typical heating and cooling profiles are shown in panels (A) and (B) of Figure 6, respectively. Cooling profiles indicate that all the vessels are cooled at a very low rate, and a temperature above 97 C is maintained for well above 60 min, which is sufficient for cooking all food items. The water in the base cools faster as it is in direct contact with the surroundings through the metal base. However, as there is no contact between vessel V1 and the water in the base (due to evaporation of some base water, water level was reduced and contact was broken), this vessel does not cool along with the base. All vessels are thermally well insulated from the surroundings, and hence, their cooling rates are low. From these temperature profiles, it can be concluded that the food items that require longer cooking time such as lentils or whole grams should be kept in the bottom vessels (V1), rice in the top vessel (V4) as well as in any other middle level vessels (V2 or V3), and potatoes or vegetables in any of the middle level vessels (V3 or V2). 5.5.2.2. Limitations of Sunbeam Burner. Although the present assembly of cooker and burner gives about 72% thermal efficiency and saves about 70% gas consumption over traditional open pan cooking practices, it has the following drawbacks. (1) Occasional lift off of the flame from the burner head was observed. This results in leakage of unburnt LPG to the surroundings that reduces the thermal efficiency of the cooker. (2) Heating time of the contents is relatively high, at about 85 min. Adding 30 min of retention time, the total cooking time may become unacceptable. (3) The burner support was not adequate to hold the base of the cooker in a stable position. Hence, there was a chance of tumbling of the whole assembly during setting or opening, as has actually been experienced. (4) In order to remove these drawbacks, a moonbeam type burner was employed, and a separate stand was provided for stability of assembly. 5.5.3. Moonbeam Type Burner Head. A schematic of the moonbeam type burner head is shown in Figure 13. It has no holes from the top of the head. Instead, holes are provided from the sides of the head. There is a provision for the pilot flame that burns continuously. This helps in complete combustion of the LPG, including leakage if any, either by lift off or for any other reason. The M-22 type of burner has a gas consumption rate of about 1000 g per hour at 130.6 kPa of LPG delivery pressure. A distance of 6 mm was maintained between the burner head and the bottom of the base. The cooker was charged to 72 L capacity with 6 L water in each vessel. Fifteen liters of water was added to the base. Heating was carried at a gas flow rate of about 122 mL/s. It required about 60 min for the charge in all vessels to attain 98 C. At this instant, heating was stopped, the drain tap opened, and retention was allowed for 4 h. The cooker was opened, and water quantities in all the vessels and what had been drained from the base were measured. Heating and retention cooling profiles were found to be similar to those shown in panels (A) and (B) of Figure 6, respectively. 5.6. Effect of Gap between Burner Top and Container Vessel. To determine the effect of the gap between burner top and container vessel on thermal efficiency, experiments were carried out for different cooker sizes. At the domestic scale,

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Figure 13. Moonbeam type burner head.

Figure 14. Base of 6 L cooker with bubble plate: (1) base, (2) bubble plate.

experiments were performed using a 6 L capacity cooker and a domestic LPG stove supplied by HPCL (Hindustan Petroleum Corporation Limited, India) and LPG cylinders supplied by a local dealer. The LPG stove has two burners, small and large, and the small burner was used for the experiments. For medium scale cooking, a 24 L cooker was used. This cooker is sufficient for cooking lentils and rice for about 60 people. The gap between the burner tip and the cooker bottom was varied from 3 to 15 mm. A minimum gap of 3 mm could be obtained only by removing the normal supporting stand from the stove. The cooker assembly consisted of a base, inner and outer covers, cooking vessels, and a bubble plate to ease the boiling of water in the base as shown in Figure 14. The temperatures were monitored using RTD thermocouples, which were mounted at the center of each vessel, in the base and on the two covers. Water quantities were monitored before and after the experiment. LPG flow rate was measured using a soap 1889


Industrial & Engineering Chemistry Research film meter, with an average of three readings over the entire heating period. The typical procedure for the experiment involved the following steps: (1) Remove the normal stand from the stove and adjust the gap between cooker vessel and the burner top to the desired value. (2) Charge the cooker with a measured quantity of water and mount the thermocouples at the center of each of the four vessels. (3) Put a measured amount of water in the base of the cooker with a thermocouple mounted in the same. (4) Arrange the vessels in the form of a stack and cover that with the inner cover and outer cover. (5) Switch on the LPG supply and immediately place the cooker assembly on the stove. (6) Monitor the LPG burning rate, temperatures, etc. The thermal efficiency of the cooker was calculated by eq 1. For the domestic cooker (6 L), the thermal efficiency variation with the gap between burner and cooking vessel is shown in Figure 15. It is observed that the efficiency decreases as the gap increases. This can be explained on the basis of the heat transfer rate. The rate of heat transfer is directly proportional to the overall heat transfer coefficient, heat transfer area, and temperature difference. Now, in the case of any flame, different colors are observed depending on the flame temperature. Blue and yellow are the two extremes for high and low temperatures, respectively.

Figure 15. Effect of gap between burner top and bottom of cooking vessel on thermal efficiency.

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In the case of LPG, one can observe the central core of a blue color flame representing maximum temperature. When the gap between the burner tip and bottom of the vessel is decreased, the blue part of the flame comes in direct contact with the base. Also, nearly all of the flame touches the base and spreads uniformly over the base. Because the cooker is operated at low gas flow rates, the flame height is relatively small, and any increase in the gap causes a decrease in the contact (even no contact for the highest gap) between the high temperature flame and the base. Thus, the temperature driving force decreases thereby decreasing the rate of heat transfer and reducing thermal efficiency. Though the efficiency is highest at the minimum gap of 3 mm, it was observed that sometimes the flame is not stable due to the very small area available for free flow of flue gases, and hence, it was decided to use a gap of 6 mm as safe and desirable for a sustained flame. In the case of the 24 L cooker (using a different burner), it is interesting to note an observation about the gap minimization. For this, we need to understand the geometry of the burner (Figure 16). The burner has three circular rings of holes on the outer side and two rings on the inner side for the flowing out of LPG. These rings of holes for gas exit surround a central hole with about a 3 cm diameter for secondary air. For initial experiments, the gap between burner and cooker base was kept as 10 mm, but it was found that the flame from the inner ring holes turns downward through the central hole probably due to insufficient upward draft. Hence, the gap was increased gradually until downward penetration of the flame into the central hole was stopped. This gap was found to be 20 mm, and hence, it was selected as the minimum gap for further experiments with the 24 L cooker and its corresponding burner assembly. Similar to the results obtained for the domestic burner, efficiency decreases with an increase in gap. From the two models of the burner (with and without stand, Figure 16), the efficiency for the burner with a stand and without a stand for a 28 mm gap was 50.1% and 58.6%, respectively. As shown, the burner without a stand gives a much higher efficiency compared to the burner with a stand. As mentioned previously, heat transfer from the flame to the cooking vessel is maximum if the flame travels along the base and gives out heat efficiently, but in the case of a burner with a stand (which is provided with three legs), these legs are not far away from the burner itself. This results in a direct contact between flame and stand and is also a hindrance to the smooth passage of flue gases along the base. Thus, a substantial amount of heat from the flame is absorbed by the stand and is lost. A further

Figure 16. Designs of Sunbeam burners and arrangement for supporting vessel (A) with stand and (B) without stand. 1890



Figure 17. Effect of distance between burner tip and bottom of cooking vessel on thermal efficiency for M-22 burner. Distance between burner tip and bottom of cooking vessel: ) = 1 mm, 0 = 3 mm, and Δ = 6 mm.

reduction in the gap from 28 to 20 mm increased the efficiency from 58.6% to 59.7%. For the 72 L cooker, (with M22 burner, Figure 13) experiments were performed for three distances, 1, 3, and 6 mm. The range of LPG flow rate for the study was narrowed between 110150 mL/s. Figure 17 shows the thermal efficiencies at different LPG flow rates for three distances between the burner head and the cooker base. Similar to Figure 15, it is observed that as the distance decreases thermal efficiency increases. 5.7. Effect of Wind Shield. Earlier cooker assemblies rested on a four-legged stand without any protection from the wind. Hence, it was thought desirable to study the effect on the thermal efficiency of a wind shield surrounding the stand. An aluminum sheet was placed around the stand keeping a 25 mm gap from the ground and 10 mm from the cooker base bottom. It was observed that the wind shield enhances the thermal efficiency by about 23%. It arrests the flow of air passing under the burner and reduces the heat losses that take place from the flue gases to the surrounding air. In these conditions, the average gas consumption to cook a full charge was about 930 ( 10 g

6. DEVELOPMENT OF COOKER ASSEMBLY: PHASE III 6.1. Characteristic Features and Performance Evaluation. The assembly of the cooker is shown in Figure 9. Cooking pots and the lid were fabricated of stainless steel. The base and two covers were fabricated with aluminum. This cooker combines the four principles of energy conservation in the form of LPG savings and can be summarized from the subsequent discussion: (i) rate of heat supply, i.e., optimum gas flame in combination with the specially designed base to utilize maximum energy associated with the flue gas, (ii) proper insulation in the form of a 5 mm air gap between the two metal covers to reduce heat loss to the surroundings, (iii) multiple effect evaporation, employing stacking of the cooking pots, and (iv) early gas shut off before cooking is complete. Each principle helps save about 30% gas consumption as has been discussed in the previous sections. In addition to this, a fifth parameter of a timetemperature relationship has an effect on the thermal performance of the cooker.

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The upper part of the base has an arrangement to support the cooking vessels stacked one over another. The base is provided with a mark that helps to place the optimized water quantity in it. It may be noted that once the charge is placed in the cooker any kind of stirring/agitation/mixing is not possible of the content in the cooker. In order to examine the performance of the cooker, heating and retention experiments were performed using plain water as the charge. The typical food composition of the average Indian family consists of rice, lentils, and vegetables, especially potatoes. Taking into consideration the requirement of a small family consisting of 45 members, a 4.5 L charge was selected. The charge could be distributed into the four cooking pots, one having a 1.5 L capacity that can be used for rice cooking. Of the remaining three pots having a 1 L capacity each, one can be used to cook lentils, one to cook another lentil (or more rice), and the remaining pot to cook vegetables. Typical dimensions for this capacity are shown in Figure 9 and Table 5. The efficiency of the cooker was estimated using eq 1 From eq 1, it is observed that the numerator has four terms. Out of these, the heat absorbed by the charge is useful for cooking the material. Therefore we define another “useful efficiency” as ηuse ¼

MC CPC ðT2 T1 Þ ðMG CVÞ

ð4Þ

It may be noted, however, that the term ηuse represents the sensible heat for heating the charge from ambient temperature to the desired cooking temperature. So, heating the charge is necessary for the onset of cooking. The actual energy required for the cooking reaction is a small fraction of the numerator, and the cooked mass is available at a much higher temperature. Most of the heat with the cooked mass is available for recovery. This particular feature has been employed in continuous cooking process and is discussed in Section 5 of Part II. 6.2. Performance of 4.5 L Cooker. In order to study the heating and cooling profiles of the 4.5 L capacity cooker (as described previously), the cooker was charged with water to the full capacity of 4.5 L by placing 1.5 L in vessel V1 and 1 L each in the other vessels, V2, V3, and V4. A quantity of 0.4 L was placed in the base. The base was placed on a gas burner. Vessels were stacked in the base in the sequence of V1, V2, V3, and V4 from bottom to top. A lid was placed on the top vessel (V4). The inner metal cover was placed and set on the base. Similarly, the outer metal cover was also placed and set on the base. While assembling the cooker components, thermocouple probes were placed at the center of all four vessels, in the water in the base, and on top of the outer cover. After assembling the cooker, gas was supplied at a flow rate of 6.6 mL/s. All temperatures were noted during the heating and retention periods. When the charge in all the vessels (usually the slowest heating vessel) reached a temperature of 98 C, gas was shut off. Retention was then allowed for a period of 240 min. When retention was over, the cooker was opened and the water quantities in all the vessels and the base were measured. The heating profile shown had characteristic features similar to panel (A) of Figure 6 and to the cooling profile (during the retention period) of panel (B) of Figure 6. The charge took about 60 min during heating to reach 98 C. It is observed from the heating profile that the temperature of water in the base rises fast, followed by vessel V1. It was followed by vessel V2 and then by vessel V4. The third vessel from the bottom (V3) heated the slowest. These results are similar to 1891



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Table 5. Dimensions for Different Capacities of Cooker base (mm) capacity (liter) 4.5

Di 180

D0 240

inner cover (mm) H 50

D 210

H a

180 + 20

outer cover (mm)

vessel (mm)

D

H

D

220

210

180

mass (kg)

H

no. of vessels

S.S.

Al

V1 = 70

4

0.87

1.38

4

1.04

1.32

V2,3,4 = 50 6.0

180

240

50

210

240 + 20a

220

270

180

V1 = 90 V2,3 = 70 V4 = 50

a

24.0

380

395

65

320

600

355

650

255

140

4

4.8

7.4

72.0

720

760

110

720

560

760

640

255

140

12

14.4

26.1

Effective height (exposed outside) + additional height required for support (inserted).

those described in Section 4.1. An additional observation was that when the temperature of the vessel charge was approaching 98 C, steam puffs started coming out from the gap between the outer cover and the base. This can be considered as an indication of the charge being near its boiling point, and soon after such a stage gas supply can be stopped. From the cooling curve, it is observed that the temperatures of all four vessels are sustained well above 96 C for 30 min and above 90 C for 60 min. Common food items are capable of being cooked in this time period at this temperature level. It was previously observed that rice takes about 11 ( 1 min (660 ( 60 s) to cook and lentils take about 18 ( 2 min (1080 ( 120 s) to cook after reaching the temperature of 98 C. The temperature at a high level is sustained due to the insulation provided by the 5 mm air gap between the covers. During heating, steam formed replaces the air between the covers. When heating is stopped, the formation of steam stops, and steam that is already formed is gradually condensed. Because of this, air enters the gap that acts as insulation. Low heat capacity of air makes it a poor conductor of heat, and the narrow gap (less than 6 mm) ensures that convection currents are not developed in the gap. This results in a reduction in heat losses. Moreover, during heating, some quantity of steam escapes through the gap between the covers and the base, thereby reducing the water level in the base. This breaks the contact between water in the base and vessel V1. This avoids the faster cooling of vessel V1 in the retention period (as was observed in Section 4.1). Thus, the stack of four vessels is nearly isolated in terms of heat transfer from any other cooker component (except the minor area of contact between the bottom vessel and the base through the bubble plate), thereby minimizing conductive heat losses. As the bottom vessel (V1) remains at a higher temperature for a longer period, it can be employed to cook a food item that requires a longer time to cook (for instance, lentils). The top vessel (V4) and vessel V2 can be employed to cook rice. Vessel V3 can be employed to cook a food item that requires less time compared to cooking rice, e.g., potatoes. Accordingly, the experiments were performed to cook rice, lentils, and vegetables by stacking them in the manner described previously. It was observed that heating the food charge to 98 C and retaining it for 30 min without opening the cooker resulted in the complete cooking of all food to the required texture (no overcooking or mushing). In view of the successful performance of the 4.5 L capacity cooker, it was decided to scale it to higher capacities and examine its performance. Consequently, cooker models were designed and tested for a 6 L capacity for a family consisting of 78 members and

24 and 72 L capacity for canteens, hostels, hotels, etc. The dimensions of these models are listed in Table 5. 6.3. Performance of 6 L Cooker. Design of this cooker was similar to the cooker described in Section 6.2. The diameters of cooker components were the same as for the 4.5 L capacity. However, heights were increased such that four vessels could be accommodated with a total charge of 6 L. The capacities of the four vessels wereas follows: bottom vesse, 2 L; middle vessels, 1.5 L each; and top vessel, 1 L. In order to examine its performance, a 6 L water charge was placed in the vessels according to the capacity. Water quantity of 0.5 L was placed in the base. Thermocouple probes were positioned in the vessels and in the water in the base. Gas was supplied at 7.3 mL/s for heating. When the charge temperature reached 98 C, gas supply was stopped, and retention was allowed for 240 min. Temperature of the cooker components were noted during heating and retention. After retention was over, water quantities in the vessels and the base were measured. The nature of heating and cooling profiles was similar to that of the 4.5 L cooker (Figure 6). 6.4. Performance of 24 L Cooker. Design of this cooker was similar to the cooker described in Section 6.2. It utilizes four BIS standard vessels, each with a 6 L capacity and a diameter of 255 mm and 140 mm height. These vessels were stacked in a base supported by a ring that maintains a gap between the bottom vessel and the base. In order to examine its performance, a 6 L water charge was placed in each vessel, thus making the total charge of 24 L. A quantity of 2 L was placed in the base. The cooker was assembled on the burner with thermocouples inserted in the vessels and water in the base. A normal domestic stove burner was used to supply gas at 27.6 mL/s during heating. When the charge temperature reached 98 C, gas supply was stopped, and retention was allowed for 240 min. The temperatures of the cooker components were noted during heating and retention. After retention was over, water quantities in the vessels and the base were measured. The natureof heating and cooling profiles were similar to those shown in Figure 6. 6.5. Performance of 72 L Cooker. In order to design a 72 L capacity cooker, one has to use four vessels, each having 18 L capacity, if a strategy similar to that for smaller capacity cookers is to be employed. This may create the following related problems: (i) Effective radial heat transfer from the vessel wall to the vessel center may not take place due to the large diameter of the vessel, especially when a food item has to be cooked. This is because the food components swell with water during cooking. This limits the fluid convection currents and reduces radial heat transfer (further details are in Part II). This may result in the undercooking of food items at the center of the vessel, although these 1892



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Table 6. Gas Consumption and Thermal Efficiencies for Different Capacities of Cookers S.N.

capacity of cooker (L)

charge in vessels (L)

water in base (L)

LPG flow rate (mL/s)

time (s)

gas consumption (g)

ηtherm (%)

1

4.5

4.5

0.45

6.6

4350

66

65.9

2

6.0

6.0

0.55

7.3

5062

85

66.2

3

24.0

24.0

2.0

27.6

5360

340

67.2

4

72.0

36.0

12.0

84.3

2600

504

71.4

5

72.0

48.0

12.0

83.2

3360

643

71.5

6

72.0

60.0

12.0

84.6

4050

788

71.8

7

72.0

72.0

12.0

85.1

4850

950

72.1

may get somewhat overcooked near the vessel wall. (ii) Handling of the vessels containing larger quantities of food mass would be cumbersome. (iii) The larger height of stack may result in an unstable assembly. These issues have indeed been experienced when an assembly consisting of a stack of four 15 L vessels was tested. These problems can be solved in two ways: (i) by using annular vessels if a stack of four vessels has to be used and (ii) by dividing the stack of four vessels into a larger number of stacks with smaller capacity vessels. As the first option is operationally difficult, the second option was opted for and investigated. Initially, it was thought to employ three similar stacks of 24 L each (as has been employed in the 24 L assembly) placed in an equilateral triangle arrangement inside a single circular base with a set of two covers. In this manner, the 72 L charge was distributed in 12 vessels, each accommodating 6 L of cooked charge. Three stands provide support to the three stacks. Nine rings, three in each stack, separate the vessels from each other during stacking. Three plates cover the uppermost vessels in each stack. In order to examine the performance of this cooker, a base containing 10 L of water was placed on a commercial M22burner (LPG gas flow rate of 85.1 mL/s). The 72 L water charge was distributed equally in all 12 vessels. Thermocouples were located in all the vessels and the base. Two metal covers were placed and set on the base. When the temperature of the charge in all the vessels reached 98 C, gas supply was stopped, and retention was allowed for a further 240 min. Temperatures of all the components were noted during heating and retention. Water quantities in all the vessels and the base were measured after the experiment. Heating and cooling profiles were found have features the same as in Figure 6. From these, it can be observed that the heating as well as retention trends are similar to those for the 4.5 L capacity cooker. Temperatures of the charge in vessels at the same level (V1 in all three stacks) differed by about (3 C in the initial period of heating. However, this difference was reduced when all the vessels were near 98 C. Hence, the average temperature of the three vessels at the same level in each stack has been considered when reporting the temperature profile. As with earlier experiments, the set of bottom vessels (V1) in each stack are heated first and maintain a higher temperature for a longer time. The uppermost vessels (V4) in the stacks follow these lowermost vessels in terms of rate of heating. Middle level vessels (V2 and V3) are heated simultaneously and are the slowest. From this trend, food that requires a longer cooking time such as lentils or whole grams can be kept in the bottom vessels, rice in the top vessels as well as any other middle level vessels, and potatoes or vegetables in any of the middle level vessels. Assuming that every time the cooker may not be used to full capacity, experiments were performed to examine the effect of charge, 50, 67, and 83% of the total charge. For this 3, 4, and 5 L charges, respectively,

were placed in each vessel. Heating and cooling profiles for these capacities were similar to those for the full capacity (6 L in each vessel). Gas consumption and thermal efficiency to heat the charge up to 98 C for different capacities of cooker (full charge in 4.5, 6, and 24 L capacity cookers and 36, 48, 60, and 72 L charge in 72 L capacity cookers) are listed in Table 6. It is observed that the value of efficiency increases with an increase in the cooker size due to the reasons described in Section 4.3. It is observed that the efficiencies of the cooker are comparable and even higher (by 10 12%) than the thermal efficiencies of BIS standard vessels of respective capacities. 6.6. Performance of 120 L Cooker. The 120 L cooker contains five stacks of three vessels, V1, V2, V3, of 6.3, 7.8, and 9.5 L capacities (in each stack), respectively. Each vessel stack is put on an aluminum base plate. The cooker was placed on a specially designed MS stand, such that there was a very low gap (∼5 mm) between the burner and the cooker base. To avoid unnecessary heating of the stand, the cooker was supported only at certain points, at the four corners and at the center of each edge of the stand. Special burners, supplied by M/s United Works, Mumbai, operating at a specified discharge pressure were used. For a typical run, 21 kg of water was placed in the base of the cooker, and 5.5, 7, and 8.5 kg of water was placed in each vessel, totaling 126 kg of water charge (in 15 vessles, 3 5 stacks). LPG was supplied at the rate of 1.1 kg/h (133 mL/s). During heating, the temperature of the water in all vessels reached above 95 C, which is sufficient for cooking of food items like rice, lentils, etc. After this, the cooker assembly was allowed to cool. During the retention period, the heat is retained and the food continues to cook. Heating and cooling profiles were measured using thermocouples. For the case of the 120 L cooker, it was thought desirable to extract the possible enthalpy from the flue gases. For this purpose, 105 holes were drilled in the upper rim of the base for the entry of flue gases into the annular space between the two covers. Considering the high LPG burning rate, it was thought that providing a skirt-like structure around the base (like that discussed in Section 5.6) would provide some resistance to flue gases and will force the same to travel through the gap between the inner and outer cover. For this purpose, a thin cylindrical band of GI (galvanized iron) of 50 mm height was fixed on the outside of the base extending downward. Also, a hole of 80 mm diameter on the top of the outer cover was provided to facilitate the passage of flue gases during the heating period, and it could be shut off during the cooling period. The temperature profiles for a typical experiment for water in the base and three vessels for heating and retention were found to be similar as shown in panels (A) and (B) of Figure 6, respectively. It was observed that the vessel temperature drops only marginally (34 C) over the 1893


Industrial & Engineering Chemistry Research desired period of 30 min, though that of water in the base decreases by about 10 C. Although the losses from the cooker to the surroundings are reduced by using two covers with an air gap, some heat losses still occur. Also, as described previously, the vessel has the capacity to absorb heat at a particular rate. So, at too low a heat supply rate, the efficiency is less because of losses from the vessel due to longer duration of heating. Thus, in the range of low gas flow rates, an increase in the thermal efficiency occurs with an increase in the gas flow rate. However, at a particular gas flow rate, an optimum value of efficiency is reached. The optimum value of heat flux of 20,000 kCal/m2 hr represents the maximum heat flux that can be transferred across the base, and even if we increase the rate of gas supply and the heat flux, it is not effectively transferred through the base. Hence, there is no further substantial heat picked up by the vessel. Thus, the extra heat supplied by burning gas at a higher rate results in a higher flue gas temperature and decreased thermal efficiency. Though most of the heat from the flue gases is extracted, it leaves the base at a fairly high temperature (>200 C) as listed in Table 3. For the 120 L cooker, the thermal efficiency was found to increase from 65.9% to 73.6% because of proper utilization of the heat of flue gases. Similar results were obtained for the domestic cooker during the study of effect of flue gases. In the case of a domestic cooker, overall thermal efficiency was found to increase from 62.3% to 65.5%. 6.7. Performance of 160 L Cooker. On the basis of the findings in Section 6.6, two improvements were incorporated in the cooker design: (a) optimum gap between the burner and the cooker bottom and (b) recovery of enthalpy from hot flue gases by passing these through the gap between the two covers. The dimensions of this cooker are similar to those of the 120 L cooker except the height of the covers. Because this cooker has five stacks of four vessels each (120 L cooker has five stacks of three vessels each), the inner and outer covers are of 610 and 650 mm height, respectively. Thus, there are 20 pots of 8 L individual capacity. This cooker has a capacity to cook 30 kg rice and 10 kg lentils, sufficient for nearly 400 people. As another option, this cooker has a capacity for 24 kg rice, 8 kg lentils, and 810 kg vegetables for 320 people. For the water trial on this cooker, 8 kg water was charged in each vessel and 16 kg in the base (later 16 kg water in the base was reduced to 12 kg after reducing the height of the base plates). Thermal efficiency of this cooker was found to be 70%. A temperature of 98 C was achieved in 90 min of heating, with overall LPG use of 2200 ( 50 g of gas for heating the entire change. 6.8. Performance of 700 L Cooker. Though the 160 L capacity cooker is large enough to be called as a community cooker, there are still places where cooking is done on a still larger scale. Large scale cooking is also followed at organizations providing mid-day meals at school. At Naandi Foundation (Hyderabad), about 100,000 meals are prepared at a time. The Akshay Patra Organisation also provides more than 1,000,000 meals at each of several locations. Keeping in mind this application, it was thought desirable to design a large scale cooker that would be sufficient to cook food for about 2000 people at a time. 6.8.1. Cooker Dimensions. For such a large capacity, proper utilization of space is important; hence, square vessels with square bases were selected over circular vessels. Panel (A) of Figure 18 shows schematics of this cooker. It consists of a base, two covers, bubble plates for supporting vessels, and 48 pots. All the components were made from aluminum. A square stand was

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Figure 18. 700 L cooker dimensions (mm) and cooking vessel arrangement.

fabricated from MS angles to support the cooker. Small metal spacers (5 cm 5 cm 6 mm high) were welded on the top of the stand at regular intervals (∼30 cm) for smooth passage of the flue gases between the stand and the base of the cooker. The details of the bubble plate are shown in panel (B) of Figure 18. It separates the cooker base from the bottom surface of the cooking vessels, thus maintaining a water level on the cooker base at all times to absorb heat supplied by the burner. A total of 44 holes of 20 mm diameter were provided on the bubble plate for free passage of steam generated from the water in the base. The pot dimensions are shown in panel (C) of Figure 18. Vessels have a slight vertical taper for compact stacked storage when empty. Various kinds of burner (supplied by M/s United Works, Mumbai) were tested for the thermal efficiency. 6.8.2. Water Trial with Single Burner. Because a single burner is always easy to operate and maintain compared to multiple burners, initial water trials were carried out with a single burner. The burner used, M100, is shown in panel (A) of Figure 19 and has a design flow rate of 4.5 kg/h at a cylinder discharge pressure of 0.31 kPa (gauge). For all previous cookers, a single LPG cylinder was sufficient to deliver the desired gas flow rate (though there is a marginal decrease in the flow rate toward the end of the heating period because of evaporative cooling of LPG in the cylinder). For a burner with much higher flow rates (compared to ∼1 kg/h for the M22 burner), the cooling effect is predominant. Hence, a six cylinder bank of cylinders was used as recommended by the burner supplier. The burner was mounted exactly at the center of the stand, and the cooker base was properly placed above. The bubble plate was 1894



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As is obvious from the photographs, the L type burner gives a single linear flame, while the V type burner gives two linear flames. The two combinations of these burners studied are shown schematically in panels (D) and (E) of Figure 19. For the combination of three V burners (Figure 19D), time taken to reach desired temperature levels was 2.5 h (LPG flow rate of 3.8 kg/h (459 mL/s), thermal efficiency was 72.8%, and heat flux was 28,000 kCal/h/m2. For the other combination of two V burners and two L burners (Figure 19E), corresponding values were 2 h and 68.8% (LPG flow rate of 4.4 kg/h (531 mL/s) and heat flux was 32,222 kCal/h/m2. Thus, from the various types of burners and combinations studied for this cooker, it was seen that the arrangement of three V type burners gives the maximum efficiency. Because the volume of the cooker is very large, it would have required nearly 150 kg rice and 35 kg lentils for a full scale cooking trial. This was not carried out. Thus, the 700 L capacity cooker was designed and tested only in principle for cooking with a thermal efficiency of 72.8%, similar to those obtained in the case of smaller cookers (up to ∼160 L). Substantial additional work is needed for the standardization of a 700 L design and for developing procedures for further scale up . 6.9. Comparison of Various Sizes of Cooker. Gas consumption and thermal efficiency to heat the charge to 98 C for different capacities of cooker (full charge in 4.5, 6, and 24 L capacity cookers and 36, 48, 60, and 72 L charges in 72 L capacity cookers) is listed in Table 6. It is observed that the value of efficiency increases with an increase in the cooker size due to the reasons described in Section 4.3. It is observed that the efficiencies of the cooker are comparable and in some cases even higher than the thermal efficiencies of BIS standard vessels of respective capacities.

Figure 19. Photographs and schematics of burners for 700 L cooker: (A) M100 burner, (B) Linear V type, (C) Linear L type, (D) 3 V type, (E) 2 V type + 2 L type.

placed in the base and nearly 60 kg water was added to the base. The cooking pots were then filled with water (14.4 kg each) and arranged in three layers (V1, V2, and V3). Each layer was formed using 16 vessels in a 4 4 arrangement. Thermocouples were mounted at the center of vessels and in the base. The inner and outer covers were fixed on the base and the burner was switched on. The burner was kept on until the desired temperature was achieved in all the vessels from all layers. The temperature profiles for heating the full charge of around 750 L over the time of 4.25 h. Though slow in nature, these are similar to that for other cookers. Thermal efficiency for this trial was found to be 55.8% with LPG flow rate of 2.7 kg/h (326 mL/s) and heat flux in the range of 20,000 kCal/h/m2. This efficiency is substantially lower than that of other cookers, perhaps due to the use of a single burner which causes highly uneven distribution of heat and extended heating time (∼ 4.5 h) contributing significantly to the losses. To overcome this disadvantage, two different combinations of linear burners were studied and the results are discussed in the next subsection. 6.8.3. Water Trial with Linear Burners. Two types of linear burners (L and V) were used in the present study. Panels (B) and (C) of Figure 19show photographs of both of these burners.

7. CONCLUSIONS (a) In the majority cases of cooking, a heat source is kept below a vessel/pot/pan, and the heat is received by the contents. The Bureau of Indian Standards (BIS) and those of many other countries have recommended a standardized procedure for the estimation of thermal efficiency of a gas burner as the ratio of heat received by the vessel/pot/pan and content to the heat released by the burning of the gas. For this purpose, recommendations have been made regarding the vessel size (diameter and height), flame size, and heat flux. The values of efficiency have been reported in the range of 6570%. However, the efficiency of cooking observed in the practice of conventional cooking is in the range of 1525%. This paper describes a methodology of optimization of cooking systems so as to enhance the efficiency from the conventional level from 1520% to 6575%. (b) For a given cooker diameter, the optimum flame diameter was found to be one-third of the cooker diameter. The optimum distance between the burner tip and the cooker bottom was found to be in the range of 23 mm. (c) The optimum heat flux was found to depend upon the balance between the rate of heat supply and rate of heat uptake by the cooker contents. The value was found to be in the range of 20,00025,000 kcal/h m2. (d) A desired quantity of food can be cooked in a single pot or the same quantity can be distributed in multiple pots. Multiple pots can be arranged as a vertical stack, and many 1895


Industrial & Engineering Chemistry Research such stacks can be selected depending upon the ratio of vessel size to pot size. We have studied the number of stacks up to 16. The optimum number of pots in a stack was found to be in the range of 35. (e) Optimization was carried out for 4.5, 6, 24, 72, 120, 160, and 700 L cookers. In this part of the work, only water has been used for optimizing the thermal efficiency. In Part II, we have developed a theoretical basis for optimization. Further, actual food materials such as rice, lentils, vegetables, etc. have been used in the cooker.

’ AUTHOR INFORMATION Corresponding Author

*Phone: +91 22 33611111 (J.B.J). Fax: +91 22 33611020 (J.B.J). E-mails: [email protected] (J.B.J), [email protected]. in (A.B.P.), [email protected] (S.B.P.). Present Addresses ^

AISSMS College of Engineering, Pune 411001, India

’ NOTATIONS A = area of heat transfer (m2) CP = specific heat capacity (kJ kg1 K1) CV = calorific value (kJ kg1) D = diameter of vessel (m) H = height of vessel (m) M = mass (kg) m = quantity of steam evaporated (kg) T1 = temperature at the start of cooking (C) T2 = temperature at the end of cooking (C) T0 = ambient temperature (C) T = temperature of outer cover (C) t = time (sec) V1 = standard pot/vessel V2 = tall pot/vessel V3 = standard pot/vessel V4 = tall pot/vessel Greek Symbols

Δ = difference in quantity, e.g., temperature λ = latent heat of vaporization (kJ kg1) η = efficiency ε = emissivity σ = StefanBoltzmann constant (5.66 108W/m2 K4) Subscripts

B = quantity in base C = contents G = LPG gas therm = thermal use = useful V = vessel W = water

ARTICLE

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Development of Efficient Designs of Cooking Systems. I. Experimental

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