Ind. Eng. Chem. Process Des. Dev. 1986, 25, 843-849
843
Heat Shield for High-Temperature Kiln Nam K. Kim' Department of Chemism and Chemical Engineering, Michlgan Technological Unlversiv, Houghton, Michigan 4993 1
Jay E. Lyon FMC Corporetion, Green River, Wyoming 82935
Naradpur V. Suryanarayana Department of Mechanical Engineering -Engineering
Mechanics, Michigan Technological Universiv, Houghton, Michigan 4993 1
It is difficutt to esminate heat losses from a hightemperature (500-750 O F ) rotary kiln since it is often not desirable to insulate the surface due to shell overheating problems. Often, the kiln bare shell is exposed to the ambient air to keep it under the critical temperature, and the large energy loss is evident. Installation of a simple concentric shield around the kiln not only cools the shell but also recovers 90% of the heat that otherwise would be lost. Furthermore, it acts as an effective air seal at the firing end of the kiln, reducing unwanted excess air into the system. The shield has been proved to save energy as well as to provide a safer and cooler environment. Fuel savings of $34940 per year have been realized for use of such a shield over a 20-ft-long 8-ftdiameter kiln at 400 OF. This paper shows how to evaluate these heat savings and to design a shield for such a kiln.
The direct fired rotary kiln is widely used for calcination of various types of ores in the chemical processing industries. The normal kiln is fired with gas, oil, coal, or a combination of them. The slightly inclined kiln consists of a long cylinder rotating to convey the solid materials to the lower end in direct contact with the combustion gas. The kiln is rotated by a motor through a speed-reducer and ring gear-pinion arrangement. A typical kiln layout is shown in Figure 1. There are no lifters in the combustion zone of the kiln for full flame development and in the heat soaking zone to subdue dust prior to disengagement of gas from solid product (Vailant, 1975). This energy intensive unit is often lined with refractory which protects the shell from being overheated. One frequent maintenance problem is detached bricks which can cause product contamination and often damage subsequent processing equipment. The need for refractory lining can be successfully eliminated by proper design of the burner and combustion zone in some kilns having a shell temperature not exceeding the critical value (Le., approximately 750 O F for a mild steel: Clarke, 1962). However, if it exceeds the critical value, the other means of insulation must be sought to protect the shell from being overheated. This paper will discuss the case where the kiln shell of mild steel operates under the critical temperature in either the absence or the presence of refractory lining. The kiln shell without refractory lining operates well above the ambient temperature and requires some type of external insulation to reduce heat losses. However, the excessive thermal expansion and shrinkage of the shell, particularly during the startup and shutdown, cause the circumferential variations that severely stress insulating material, if it were in contact with the shell (Pippitt, 1976). When the shell temperature of the combustion zone remains near 600 OF, the external insulating material often produces an adverse effect on the shell temperature. The resulting higher temperature reduces the tensile strength 0196-4305/86/1125-0843$01.50/0
of the shell substance. It may often exceed 750 O F . When such a high shell temperature persists for a long period of time, the shell may be permanently deformed or warped from thermal fatigue. The magnitude of the totalheat loss from the bare shell of the kiln and subsequent replacement cost of the warped shell can be enormous. Factors Affecting Shell Temperature The shell temperature is affected by a number of variables; burner design, rate of heat release, combustion chamber design, excess air, fresh ore handling, and many others. The two variables that can be easily identified and characterized by material and energy balances are flame temperature and rate of heat dissipation from it. Measurement of the flue gas temperature and composition is necessary to verify the flame temperature obtained by direct measurement or by computation based on fuel rate and excess air. The compositions obtained by Orsat analysis represent the combustion products as well as reaction products. They are good enough for reasonably accurate mass and energy balance computations. However, since these compositions are given on a moisture-free basis, the moisture content of the kiln off-gas must also be measured. For an existing burner and kiln the quantity of excess air and the method of introducing it into the burner system may affect the total length and configuration of the flame. It is desirable to operate the flame envelope within the combustion zone of the kiln so that complete combustion may take place prior to entering the lifter zone. These conditions also influence the shell temperature, and the excess air may be altered to some extent to avoid the critical temperature of the shell substance (Perry, 1967). The shell temperature can be substantially reduced by effectively handling the fresh cool ore under the flame in cocurrent operation. Proper design in the combustion zone may help the ore to absorb the excessive heat from the 0 1986 American Chemical Society
844
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 4, 1986 E x i t f l u e sa5
4
7b Wind Velocity Ore 'i.ed
,\
C o ~ b u s t i o rz o n e
I
- 1 f t e r 2@ne
i i e a t s o a k i n g zone \
\
Wind across t h e kiln
"L
1 1
2 0 mph
15 mph
f r e e convection I
0
I
I
1
200
100
300
400
1
50C
GO0
700
hood
Shell Temperature, F ( b ) Meacbrement Of Air I n f l i t r a t l o r R a t e
# , ( I ,!eat cl,ield
Figure 2. Convective heat-transfer coefficient at various temperatures and wind velocities.
Figure I. General layout of kiln and heat shield.
shell and to act as a dynamic interior insulation. The alignment of the burner angle is another factor directly influencing shell temperature. When a local overheating problem in the combustion zone is detected, the possibility of flame impingement upon the shell should be investigated. Use of an optical pyrometer is recommended for a round-the-clock surveillance. If the hot spots persist, the heat shield should be designed in such a way that the temperature sensor would not be hindered. When pulverized coal is used as the main fuel, the average size of coal powder and the amount of primary excess air may dominate the flame length. Overheating problems with the calciner shell are not a simple matter but a complex one, for a specific problem in this case requires a joint effort of the burner and kiln designers as well as process engineers who operate the unit. The more indepth discussion on the factors that affect the flame can be found elsewhere (Wendt, 1978).
Identifying Sources of Heat Losses The sensible heat of the hot product can be recovered to heat the combustion.air by means of an additional cooler as practiced in the clinker process (Peray, 1972). When the subsequent step is dissolving of the hot kiln spill (as commonly seen in ore refining processes), heat recovery from the kiln product is not necessary. In this case the major portion of energy loss comes from the bare shell to the ambient air and energy required to heat the infiitrated air to the exit flue temperature. These are the prime targets for the heat recovery, and the proposal will be limited to these two key areas. 1. Convection and Radiation Heat Losses. The kiln dimension varies over a wide range (from 5 f t to nearly 20 f t in diameter), and shell temperatures of 300-700 O F are commonly found. Several empirical equations widely used for the estimation of the convective heat-transfer coefficient (h,) are described as a function of wind velocity, orientation of the kiln to the wind, and rotational speed of the kiln (Kreith, 1973; Suryanarayana, 1983). To provide a design basii for the heat shield, an 8-ftAameter kiln is selected. The hottest part of the &ft-diameter kiln, the first 20-ft section from the firing end, is selected to provide
a basis for computations of the convective heat-transfer coefficients (h,) and radiation heat-transfer coefficient (hJ. There are four major working empirical equations for estimation of the convective heat-transfer coefficients. (1) h ,for Free Convection. This is a situation where the kiln stands still with no wind. The heat-transfer rate is dictated by the Grashof number as shown in eq 1. This equation is also used to estimate the heat loss from the heat shield in the absence of wind.
(y )
= 0.53(GrPr)0.25
(2) h , for Kiln Rotation. When the kiln rotates in the absence of wind, the convective heat-transfer rate increases. The equation used for this calculation is eq 2. The resulting convective heat-transfer coefficient slightly increased from 1.6 to 1.8, as the kiln rotation speed increased from 1 to 4 rpm.
(y )
+ Gr)Pr]0.35
= 0.11[(0.5Re2
(2)
(3) h , for Wind along the Kiln. The empirical equation is derived in the absence of kiln rotation (eq 3).
(7)
= 0.036(Pr)0.333(Re)0.8
(3)
(4) h , for Wind across the Kiln. The conditions are the same as in case 3 except for the direction of wind (eq 4).
(7)
= 0.0239(Re)0.805
(4)
The last two convective heat-transfer coefficients are plotted in Figure 2 as a function of shell temperature with wind velocity as the parameter. The h, obtained for a situation where the wind blows along the kiln showed a slightly higher value than h, across the kiln. The h, values increased at a higher wind velocity but decreased at a higher shell temperature.
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 4, 1986 845 I
I
I
I
I
I
I
I
I
She1 1 temperature
600 F
-
N ‘c c, Y
LL
v
L L
..
Y
c, 3
m 42
E . c, T
w-
.r U
.r Y
‘c ‘c
a-
u O o E .r
Lc, a m
‘c
.r
V I5 E 0
m a +-a c,m 5 I W O E .r
0
WE10 W U .E r > W
5
n c
15
10
20
K i l n Diameter, f t
E O
V o U v
Figure 4. Heat loss from the kiln bare shell a t various kiln diameters and shell temperatures in the absence of wind.
I
i
I
0
5
10
I
I
20
1
I
30
I
I
40
Wind Velocity, mph Figure 3. Combined heat-transfer coefficient for the surface of a 8-ft-diameter by 20-ft-long kiln.
For computational convenience, the radiation heattransfer coefficient, h,, for the same portion of the kiln is obtained from eq 5 .
The total heat-transfer coefficient, hT (combined convective and radiative heat-transfer coefficients), is plotted against the wind velocity at different shell temperatures (Figure 3). The hT value increased at higher wind velocities, but it also showed an increase at higher shell temperatures. This is contributed by the radiation coefficient which sharply increased at elevated shell temperatures, reversing the relative magnitude of hT against the wind velocity. The heat loss (QT)from the surface area, A, of a 20-ft-long cylinder with various kiln diameters was obtained from eq 6. The combined hourly heat loss from QT =
(hc + hr)Adth - tc)
(6)
this portion of the kiln surface area is plotted against the kiln diameters at various shell temperatures in Figure 4. Figure 5 shows the hourly heat loss from a 400 OF kiln shell at various wind velocities. For each curve in Figure 4 a group of curves as in Figure 5 can be constructed.
0
5
10
15
20
K i l n Diameter, f t
Figure 5. Heat loss from the kiln bare shell at various kiln diameters and wind velocities at a shell temperature of 400 O F .
2. Heat Loss due to Air Infiltration. Excess air to the kiln burner in terms of primary and secondary air is normally well monitored and controlled. However, the air infiltration through the circumferential gap between the stationary firing hood and rotating kiln is easily neglected. The primary and secondary air can be individually measured’to evaluate the combustion excess air. The air infiltration around the burner hood can be directly measured by simple installation of a water-filled Pitot tube as shown in Figure Ib. When the average differential pressure and
848
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 4, 1986 12,000
I
1
I
Circumferential
L
I
0
1
2
3
4
5
6
7
8
9
10
Air Infiltration Rate, 1000 CFM
e
0.5
1.0
1.5
2.0
P , inches of Water
Figure 6. Air infiltration rate into 8-ft-diameter kiln with 5-ft-dim e t e r firing hood at various differential pressures and circumferential gaps.
circumferentialgap are known, the air infiltration rate may be estimated by using eq 7 (Jorgensen, 1970; Myer and Seider, 1976). With typical values of da = 0.075 73 lbm/ft3, dP =
(;)( 2)
(7)
d, = 62.363 58 lb,/ft3, g = 32.174 ft/s2, and field air temperature = 60 OF, a simple but useful working equation is obtained (eq 8). V = 3987(dP)0.5 (8) The air infiltration rate through the various radial gaps was calculated and plotted against differential pressure in Figure 6. With 8000 operating h per stream year and $5 per million Btu (these two parameters were used throughout the text for energy computation), the annual fuel cost is shown with the exit gas temperature as a parameter in Figure 7. The total annualheat losses obtained by summing up the heat losses due to the convection and radiation and due to the air infiltration are depicted by the two top curves in Figure 8. Also shown are the measured heat losses obtained from the plant operating data. The overall material and energy balances over the kiln produced the heat losses attributed to this portion of bare kiln shell. The balance requires the hourly fuel consumption rate and compositions and rates of flue gas and kiln products. The predicted values reasonably agreed with the measured data. Figure 8 also shows the heat losses following installation of a heat shield. Heat Shield Design The heat shield is a large concentric tube encircling the fiiing end of the kiln (Figure IC).Twelve-gauge corrugated galvanized steel has been chosen for this purpose, since this segment of the kiln shell operated under 450 OF. If
Figure 7. Cost of energy at various air infiltration rates and exit gas temperatures.
the shell temperature approaches the critical value, a mild steel or stainless steel of heavier gauges may be considered. The one end of the heat shield is completely closed against the firing hood so that the infiltration air must be induced from the other end of the shield. This induced air flow is reduced due to extended resistance created by the narrow concentric gap along the air passage (Kim, 1969). In addition the air cools the shell as it sweeps around the hot shell surface. To make this concept clear, a 8-ft-diameter mild steel kiln (trona ore calciner) having a shell temperature of 400 O F is chosen. The 20-ft-long shield covers the kiln with 2-in. radical clearance from the feed end. The diameter of the firing hood protruding the feed end of the kiln is 5 ft. 1. Heat Losses from the Kiln with a Shield. To calculate the heat loss from the kiln with a heat shield, the shield temperature must be known under the various ambient temperatures and wind velocities. A BASIC computer program was written to determine the shield temperature. A heat shield temperature was first assumed, and the energy balance across the shield was made. The computation was iterated until the relationship in eq 9 and 10 was satisfied. (9) Qr(k-a) = Qe(s-b) + Qrcs-a) + Qc(s-a)
A heat flow diagram and program flow chart for this computation are shown in Figures 9 and 10, respectively. The geometric and emissivity factor Fk*) was calculated from the equation and data
kiln shell shield
A, ft2 503 524
t
0.95 0.4
material mild steel galvanized steel
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 4, 1986 847 3.0
Predicted heat loss
2.0
Measured heat loss from bare shell
Predicted heat loss w i t h shield
Predicted h e a t loss w i t h shield
1 .o 1.0-
Measured heat loss w i t h shield
10
20
30
Q>
Measured h e a t loss w i t h shield
0
I-
0
Q
8
w
40
A
0
10
20
30
40
Wind Velocity, mph
Wind Velocity, mph
( a ) Heat L o s s from 400 F Shell Figure 8. Heat loss prior to and following installation of heat shield.
(b)
Heat Loss from 600 F S h e l l
Ambient Air ( T a )
-
Y
Figure 9. Heat flow diagram.
The shield temperature (T,) varies as a function of kiln shell temperature (Tk) and wind velocity as shown below. example wind velocity = 10 mph shield temp, assumed = 130 O F Tf(film temp) = (130 + 60)/2 = 95 (OF)
Re = 10(1.467)20/(1.9 X lo4) = 1.544 X lo6 hc(s-a)= 2.25; hr(s-a)= 0.52; hc(k-b)= 2.92 Q(c+r)(s-a)= (2.25 + 0.52)524(130 - 60) = 101 600 = 2.9205(524)(130 - 100) = 45910
Qr(k-) = 0.171(0.4)503(8.64- 5.g4) = 146 509
Table I. Measured Plant Data vs. Predicted Values kiln temp (initial), O F 400 600 kiln temp with shield measd (325-342) (505-523) shield temp, O F calcd 170 0 mph 290 measd (130-150) (215-229) 0 mph calcd 130 10 mph 213 measd (100-118) (156-177) 10 mph air inlet temp, O F calcd 145 195 0 mph measd (125-142) (162-180) 0 mph calcd 118 178 10 mph measd (98-115) 10 mph (142-163)
2. Infiltrated Air Rate with Heat Shield. When a pressure drop of 1 in. of water is applied across the 20ft-long shield and the feed side of the kiln, the air flow in
848
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 4, 1986
Table 11. Summary of Energy Savings4 rate of enerev loss with bare kiln, shield, annual Btulh Btulh savings, $/vr A. Kiln Shell Temperature = 400 OF 1. no wind 670 400 41 000 25 170 2. wind 10 mph 740520 101 450 25 560 3. wind 20 mph 1003000 121 000 35 280 4. wind 40 mph 1460 500 142 000 52 740 air leaksb 839 000 9 770 1 083 230
START
Operatlng Conditione: Tk, Dk, Lk. Ten Vw DE, Le
I
B. Kiln Shell Temperature = 600 O 1. no wind 1 468 900 110 110 I 2. wind 10 mph 1634 140 228 370 3. wind 20 mph 1861 200 276 000 Symbols 2 520090 4. wind 40 mph 311 000 T: temperature D: diameter L: length V : velocity d: denaity p : viacoaity Cp: apecific heat k: thermal cond. Gr: Grashof No. R e : Reynolds No. Pr: Prandtl No. 0: rate o f heat tranafer
ASSUME Tb EVALUATE Tf (k--b)
Subscript k: kiln a: entry w : wind 8 : ahield a: alr f: film b: bulk air (infiltrated)
F
54 350 50 160 63 400 88 360
"Kiln length 20 ft from firing hood. Kiln diameter: 8 ft. Wind direction: along the kiln. Exit flue temperature: 400 O F . Ambient air temperature: 60 "F. bBased on 1-in. water pressure drop across the circumferential gap of 1 in. between the firing hood and rotating kiln.
tions, and sudden expansion have been computed at a given air velocity. The computation has been repeated on trial-and-error basis until a total pressure drop of a 1-in. water column is summed up (Bird et al., 1960). dP = K(VP)
(12)
Example. The pressure drop in inches of water at entry is calculated as follows: VP = 162/[2(32.174)] = 3.9784 (ft of air) dP = 0.93VP
e 3
d P = 0.93(3.9784)[0.068(27.9)]/144 = 0.0487 (in. of water)
PRINT
Figure 10. Flow sheet for energy balance.
The computed infiltrated air velocity of 16 ft/s falls within the range of average velocities of 14.8-16.6 ft/s measured by a Pitot tube. The bulk air velocity in the annular space remained fairly constant at different kiln bare shell temperatures. These infiltrated air rates also agreed well with overall material and energy balances over the kiln using the hourly operating data. The total heat loss from the heat shield is calculated in terms of million Btu/hour against wind velocity with the kiln bare shell temperature as the parameter. The results of a field test on a 20-ft-long shield over the soda ash producing calciner are shown in Table I along with the predicted values. The measured shield temperature and the temperatures of the infiltrated air remained lower than the predicted values. One of the key attributing factors is the assumption of constant skin temperature of the kiln upon which the computation of heat losses is based. The surface temperature of the kiln has actually reduced by 70 O F from the initial shell temperature of 400 O F . For example, some part of the kiln which had suffered high temperature of around 740 O F prior to the installation of a shield showed as low as 620 O F with the heat shield. The effects of reduced skin temperature of the kiln on energy losses are reflected in Figure
K i l n shell
temperature 1.0
8. 0 0
10
20
30
40
Wind V e l o c i t y , mph
Figure 11. Energy savings realized by installation of a heat shield at various wind velocities and kiln shell temperatures.
the annular space is in turbulent mode. The velocity pressures at entry, annular space, right angles, contrac-
Conclusion The energy savings contributed by the heat shield installed on a kiln of 8-ft in diameter are obtained by the difference between the original heat loss from the bare kiln and the heat loss from the kiln with a heat shield. The energy loss consists of both heat losses by convection radiation and by infiltrated air. Measured heat losses are
Ind. Eng. Chem. Process Des. Dev. 1986, 25, 849-854
shown against the calculated heat loss curves in Figure 8. The temperature curves with heat shield have been calculated based on the assumption that the kiln shell temperatures would remain unchanged. Since the rate of infiltrated air through the annular space varies depending upon the pressure drop and geometrical configuration, the skin temperature of the kiln will change to some extent accordingly. Therefore, the computed values of heat losses from the kiln (400 OF) with heat shield are used for the computation of energy savings instead of the measured values. This will ensure us the conservative side of economic analysis. Combined annual energy savings contributed by the heat shield amount to $34940 ($25 170 from the reduction in convection and radiation losses plus $9770 from the reduced infiltrated air) for a shell temperature of 400 O F in the absence of wind. Contribution of the reduced infiltrated air accounts for 28% of the total energy savings. Rates of energy losses a t various wind velocities are calculated with the wind blowing along the kiln. Fiftypercent of the initial heat loss can be recovered with this simple heat shield. The measured value of energy savings is greater than the calculated value due to the cooling effect of the infiltrated air, and the payoff is generally less than a year. The heat shield also provides a safer working environment around the hot kiln. The shell temperature dropped considerably with the shield, and practically no maintenance work has been required.
Acknowledgment This paper is written in memory of the late Reinert Kvidahl, Resident Manager of FMC Corp., Green River Plant. He was a model chemical engineer who shared the joy of success and responsibility of failure. We thank John W. Coykendall, Robert S. Simokat, James Taylor, and Marc E. Bowman for helpful information.
Nomenclature A = surface area D = kiln diameter d, = air density, lb,/ft3 d, = water density, lb,/ft3 dP = pressure drop due to heat shield, in. of water F = geometric and emissivity factor Gr = Grashof number g = gravitational acceleration, ft/s2 h = heat-transfer coefficient, Btu/(ft2 h OF)
Fluid Dynamics of Gas-Liquid-Solid
849
K = velocity pressure constant (0.93for entry; 0.9 for right angle turn; 0.56 for contraction; and 0.7 for sudden expansion) k = thermal conductivity, Btu/(ft h O F ) L = kiln length under consideration, ft Pr = Prandtl number Q = heat-transfer rate, Btu/h Re = Reynolds number T,= cold absolute temperature, O R Th = hot absolute temperature, O R t, = cold temperature, O F th = hot temperature, O F V = average air velocity, ft/s VP = velocity pressure, ft of flowing fluid Greek Symbols e = emissivity u = Stephan-Boltzmann constant, 0.171 for eq 5 and 10, Btu/(h ft2 OR4) Subscript a = ambient air
b = infiltrated bulk air in annular space c = convection e = air entering kiln k = kiln m = mass r = radiation s = shield w = water - = direction of heat flow Literature Cited Blrd, R.; Stewart, W.; Llghtfoot. E. Transport Phenomena; Wiley: New York, 1960; Chapter 6 . Clarke, L.; Davkison, R. Manual for Process Engineering Calculations, 2nd ed.;McGraw-Hill: New York, 1962; p 59. Fan Engineering, 7th ed.; Jorgensen, R., Ed.; Buffalo Forge: Englewood Cliffs, NJ. 1970; p 71. Kim, N. “Heat Shield”; Memo to G. Peverley and R . waggener; FMC Corp.: Green Rlver, WY, Nov 17, 1969. Krelth, F. Prlnclpbs of /feat Transfer; I E P New York, 1973; Chapters 7-9. Myers, A.; &Mer. W. Introductlon to Chemlcal Engineering and Computer CompufaHons; Prentice Hell: Englewood Cllffs, NJ, 1976. Peray, K.; Waddell, J. The Rotary Cement Kiln; Chemical Publishing: New York, 1972; Chapter IV. Chemical Englneers’ Hendbook. 4th ed.; Perry, J., Ed.; McGraw-Hill: New York, 1976; pp 23-66. Plppltt, R. Chem. Eng. Prog. 187& 72(2). 41. Suryanarayana, N. V.; Scofleld, T.; Kleiss, R. E. Trans. ASME 1989, 105. 519-526. Vailant, A. Kiln Operation Optimization and Pollution Abatement; Center for Professional Advancement (CPA): Englewood Cliffs, NJ, 1975. Wendt, J. Applied Combustion Technology; CPA: Englewood Cliffs, NJ, 1978.
Received for review August 27, 1984 Revised manuscript received March 8, 1985 Accepted March 4, 1986
Fluidized Beds
Enrique Costa,’ Antonlo de Lucas, and Pedro Garcia Departamento de Ingenie& Qdmica, Facuhd de Cienclas Qdmicas, UniversMad Complutense, 28040 Madrid, Spain
Correlations for prediction, holdups, porosity, pressure drop, and minimum fluidization velocity in a three-phase fluldization bed with cocurrent flow have been deduced. Two systems were considered for derivation of these correlations: a first system formed by the liquid and the gas (homogeneous flow model) and a second system formed by these two fluids and the solid (drift flux model).
Fluidized beds are widely used as a solid-fluid contact method especially in processes that involve large temperature changes or frequent regeneration of the solid (Van 0196-4305/86/1125-0849$01.50/0
Landeghem, 1980). The expression three-phase fluidization is used to describe fluidization of solid particles by two fluids (IZlstergaard, 1971). Gas and liquid are the 0 1986 American Chemical Society
