Ih-D USTRIAL AA'D ENGINEERIKG CHEMISTRY
February, 1930
127
labor savings, upkeep, and production of chemical very high in sulfide have exceeded all expectations. The plant, designed for 120 tons daily capacity, has consistently exceeded this by from 10 to 20 per cent.
taken to the same source. The result is an odorless pulp mill, which has not been accomplished before in this country. The steani produced has approximated very closely the theoretical shown by a heat balance, while the recovery,
Performance of Economizers in the Drying of Pulp and Paper' Frederick W. Adams and Charles M . Cooper SCH001.
OF
CHEXXCAL ENGISEERISGPRACTICE, hIASSACHUSETTS INSTITUTE
OF
TECHNOLOGY, CAMBRIDGE, MASS
A series of tests has been made to determine the of itself furnish much Inore Ry1n-G with air is One of t h e i m p o r t a n t efficiency of a Briner-type economizer in recovering the than enough heat to preheat chemical engineering heat used in the drying of P U ~ Pand paper. From the the incoming air. I n t h i s results of these experiments it is concluded that a case, unless the economizer processes involved in the production of pulp and paper. recovery 50 to 60 per cent is good practice with this is used to heat air for other economizer when the heat is used only for the drier purposes as well, the heat The heat required to accomr e c o v e r a b l e will be only a system; but this efficiency may be increased greatly plish this drying is a major item in determining manuif there is a demand outside this system for large small fraction of that availfacturing Costs, particularly amounts of tempered air for ventilating purposes. able. The economic savings Possible by the use of this This paper presents the rewhere the product has a low equipment are shown by the calculations of heat results of a series of tests perselling price per unit weight. covery and costs in a hypothetical case. formed by students of the Thus it is necessary to evapoS c h o ol of Chemical Engirate between 1 and 3 pounds neering Practice on a typiral economizer installation. of water per pound of pulp or paper produced. I n drying, relatively large quantities of fresh air displace Procedure for Tests hot. humid air, which is discharged, carrvina away as sensible Hot, humid air is removed by a hood from a 60-ton pulp heat a considerable proportion-of ' the h e 2 supjilied to the process. To recover this heat by preheating the fresh air, drier and passed through a Briner economizer, where heat is cross-current economizers have been commercially employed transferred to fresh, outdoor air, which is blown back onto for nearly twenty years (1, 2 ) . Their application to dry- the drying pulp sheet. When desired the incoming air may ing in the pulp and paper industry has recently become more be further heated by steam coils before reaching the pulp general with the development of the Briner-type economizer. machine. A by-pass is provided which allows some of the The heat-recovery efficiency of an economizer is intimately incoming air to be drawn off for ventilation purposes a t other connected with the process in which it is a part. I n some points in the building. A diagrammatic layout of the econocases a large proportion of heat may be recovered. This mizer installation is shown in Figure 1. The tests were conducted to cover a range of outdoor is true wherever hot air is used directly as a drying agent without other sources of heat. The outgoing air may be temperatures varying between 13" and 74" F. Measureused to preheat entering air and, if in the process it is cooled ments of incoming and outgoing air flows were made nith
D
T a b l e I-Results of T e s t s on T y p i c a l E c o n o m i z e r I n s t a l l a t i o n (Area of heating surface = 10,500 sq. ft.: free area for air flow = 55 sq ft RUN 1 R U N2 R U N3 RUN4 Temperature before, E;. Temperature after, ' F. Humidity, Ibs. water per Ib. air Pounds water-free air per hour Pressure drop, inches of water H e a t picked up, million:; B. t . u . per hour OUTGOING AIR: Temperature before. E . O Temperature after, F. Humidity before, Ibs. wzter per Ib. air" Humidity after. Ibs. water per Ib. air Pounds water-free air Der hour Pounds condensate pet hour Pressure drop, inches of water Sensible heat delivered I J ~humid air. million B. t. u. per hourc H e a t delivered b y condensation. million B. t . u . per hour a Calculated. b Calculated by difference
13
71 0.001
207,000
....
2.8; QI. Q
20 75 0.001 186,000
31 76 0.002 200,000
2.44
2.15
...
....
39 83 0.003 179,000
.... 1.8s
0.0261 236,000 2296
0.0371 0.0303 222,000 1505
112 94 0.0337 0.0290 226,000 1055
0.0396 0.0335 209.000
0.40 2,47
0.83 1.61
1.03 1.12
0.66 1.22
92 0.0358
....
to the entering-air temperatures and a t the same time most of its water burden is condensed, an almost perfect recovery of heat is theoretically possible. Such conditions do not exist, however, in the ordinary drying of pulp and paper, where steam condensing in drier rolls furnishes the principal source of heat. The hot outgoing air carries with it all the mater vaporized in the process, which, if condensed, would 'Received December 11, 1929. Presented a t t h e meeting of t h e American Institute of Chemical Engineers, Asheville, N. C., December 2 t o 4, 1929.
111 96
...
....
111 44
R U N5
_.
OD
ti
74 101 0.014
82 0.009 166,000 0.24
127,000
1.08
0.84
104 91 0.0300 0.0280 203,000 415 0 .li 0.64 0.44 ~~~
....
liu\
....
12s
113 0.0417 0.0413 203.000 79 '0'76
o os
pitot tubes and an anemometer. Air temperatuies and humidities were obtained before and after the economizer from wet- and dry-bulb readings or by determining den points. Stratification rendered impractical direct measurement of the temperature and humidity of the outgoing air before the economizer, necessitating the calculation of these values. The water removed by the drip line was measured to determine the amount of condensate: In run 5 pressure drops resistance Of the economizer to air to flow \yere measured on differential manometers.
INDL‘STRIAL A X D ENGINEERIXG CHEMISTRY
128
Results of Tests
These data are presented in Table I. It will be noticed that, while the rate of flow of outgoing air is substantially constant, that of incoming air decreases appreciably under summer conditions, where less air is required for heating and ventilating the room. The quantities of heat transferred are also tabulated.
ECONO-, MlZER
INCOMING AIR
ICONDEWTE
I
T O VENTILATE DRYER ROOM
FAN
I
1. OUTGOING AIR
Figure 1-Diagrammatic Layout of Economizer System
Discussion of Heat Recovery
The chief value of an economizer is its ability to recover heat. The heat absorbed by the incoming air represents the heat recovered, and this has been plotted in Figure 2 against the incoming air temperature. The recovery in the winter is two or three times that in the summer. It should be emphasized that, although the ends of this curve do not represent extreme conditions, its interpretation offers the opportunity for reasonable extrapolation. The large quantity of heat recovered in the winter is due to the condensation of a considerable proportion of the moisture carried by the hot, humid air. This is the distinctive feature of this type of heat recovery, Figure 3 brings out the fact that, in the range of incoming air temperatures studied, an increase of almost thirty fold in the amount of condensation is obtained. Thus a small drop in air temperature in the winter causes a large increase in heat recovery. In the summer condensation is of minor importance and a small increase in temperature causes a very slight decrease in heat recovery.
VOl. 22, No. 2
recovered in the winter is in this case due principally to the larger quantity of cold air drawn through the economizer, amounting to about 200,000 pounds per hour at that time in contrast to 127,000 pounds per hour when no ventilating air was used. But for this fact the percentage recovered would not change greatly between summer and winter, although, as already pointed out, the amount of heat saved may change several fold. For the specific case, such as this one, where the economizer supplied heat only for the drier system, the performance of the economizer should be based on the maximum heat recoverable in preheating air for the drier. This performance is shown on a percentage basis in Figure 5, where it varies from about 30 to 60 per cent largely on account of the changes in rate of air flow. A comparison of the amounts of incoming and outgoing air in run 1 shows that in the winter they are approximately balanced, nearly all of the air leaving the drier having been replaced by air heated in the economizer. Under ideal conditions the machine room should operate at balance, the air flows being equal, thus eliminating the infiltration of cold air through doors or windows. Therefore, we may conclude that a performance of 50 to 60 per cent is good practice with the present equipment.
TUIPCRANRE OTNOOUING AIR-%
Figure 3-Rate
d
O
I 20
40
M
80
100
TCUPERATURC cf l N C O ” G AIR-T.
Figure 2-Heat
Recovery in Economizer
The maximum heat recoverable for drying in this systeni is the sensible heat which can be put into the drying air. This is plotted in Figure 4 (curve A ) as a percentage of the heat available in the hot, humid, outgoing air. As pointed out previously, this is a relatively small fraction, being always less than one-third of the available heat. Curve B shows the measured heat recovery. The higher percentage of heat
of Condensation
The maximum performance which an economizer may deliver under balanced conditions is 100 per cent. This is, however, only possible with infinite area of heating surface, since the temperatures of hot outgoing air and heated incoming air would have to be identical. It is therefore apparent that the factors entering into the design of the economizer, such as area of heating surface, arrangement of surface, air velocity, direction of air flow, and condensate removal, are very important in determining performance. Figure 4 shows that the maximum heat recoverable in preheating air for a drier is only about one-third of the total heat available in the outgoing air. A big increase in recovery is possible where there is a demand outside the drier system for large amounts of tempered air for ventilating purposes. Under these conditions a “double-effect” use of steam i. accomplished in fact and really high recovery is possible. Sufficient air is withdrawn from the economizer. t o satisfy the demands of the drier and the remainder, which may be as high as 80 per cent of the total, depending on local conditions, is utilized a t other points in the mill for heating and ventilating. Economic Considerations
A discussion of heat recovery is incomplete and of little significance without a consideration of the economics involved.
I X D U S T R I A L Ah'D ENGINEERIXG CHEMISTRY
February, 1930
In every case the value of steam saved, equivalent to heat recovered, must be balanced against operating and fixed charges on the equipment. Any net saving available then determines the real value of an economizer system. From the data presented in Table I and Figure 2 and a knowledge of the average air temperature throughout the year, heat recovery for any location may be determined. The results obtained will always be conservative, since the rate of increase in heat recovery for temperatures below the average is alwrrya greater than the rate of decrease in heat recovery for temperatures above the average. TOTAL HEAT IN O U T G O l K AIR $ ' O O ~ ? O Y ETEUPCPATURE OF INCOMING *R
129
240 2ooo = 571,000 pounds per hour 24 X 0.0350
The maximum possible recovery of heat for drying is that required to heat this amount of outdoor air to its temperature when leaving the drier stacks. This is: 571,000 X 0.24 (110
- 45)
=
8,900,000 B. t . u. per hour
With an economizer of the type tested, 10,500 square feet of heating surface handle 210,000 pounds of air per hour a t a performance (Figure 5 ) of above 50 per cent when balanced conditions obtain. Hence the heating surface required is:
I which will recover 8,900,000 X 0.50 = 4,450,000 B. t. u. per hour From these preliminary calculations it follows that the cost of economizer is: 28,550 X 0.60 = $17,100
and the value of heat recovered during a 350-day operating year is:
TEMPERATUUE OF I W M N G AIR--T.
F i g u r e 4-Efficiency of Available H e a t Recovery
The value of heat saving available to meet operating and fixed charges on the equipment can then be readily determined from a knowledge of local cost data. The usefulness of these data is illustrated by the calculation of the savings to be expected through heat recovery in a hypothetical case. The following average conditions which have been assumed for the purpose of these calculations are those which might exist on a newsprint machine producing 120 tons of paper a day and have been intentionally chosen to give conservative results: Production of paper = 120 tons per 24 hours Water evaporated on driers = 240 tons per 24 hours Dew point of air leaving drier stacks = 94" F. Temperature of air leaving drier stacks = 110" F. Yearly average outdoor air temperature = 45' 12. Steam cost = 50 cents per million B. t. u. Economizer cost = 60 cents per square foot of heating surface Fixed charges (interest, depreciation, insurance, taxes, etc.) = 20 per cent per annum of cost of equipment
4,450,000 X 0.50 X 24 X 350 = $18,700 1,000,000 Fixed charges = $17,100 X 0.20 = $3420
Operating and maintenance charges include the cost of power to handle the air and maintenance charges on the fans and motors required for this purpose. It is estimated that about 40 kilowatts a t 1.5 cents per kilowatt-hour are necessary Operatingcost = 40 X 24 X 350 X 0.015 = $5040
This figure will be increased to $5300 to include maintenance. Total charges on the economizer are therefore the sum of the fixed charges and operating and maintenance charges, which is $8720 per annum. To this must be added fixed charges on ducts to connect the economizer with the drier installation. I n the case assumed these charges should not exceed $2000 per annum and would probably be much lower, leaving a net saving of $7980 per annum on an initial investment of $27,100 for economizer and ducts. The results of these calculations are tabulated in Table 11. It should be remembered that the operating conditions and costs assumed for these computations have been conservatively chosen, so that the net saving indicated is probably lower than would be realized in practice. Savings in a H y p o t h e t i c a l E c o n o m i z e r 1 Installation Air t o be handled, lbs. per h o u r . . . . . . , . . . . . 571,000 Heat recovery, B. t. u. per h o u r . , . . . . . . . . . . . 4,450,000 28,550 Economizer heating surface, sq. f t . . . . . . , . . , . . 40 Power requirements, kw... . . . . . . . , . . . . . . . . Cost of economizer. , , . . . . , . . . . , , . . , . . . . , , . . . , . . . . . , 917,100 10,000 Cost of d u c t s . , , , . , , , , , , . . , , . . . . . . . . . . . . . , . . . . . . . . , T a b l e 11-Calculated
.
.
Total equipment c o s t . . . , . , . . , . . . . . . , . , , . . . . . , . . . ANNUALCHARGES Fixed charges on economizer.. . . . , . , . . . . . . . . , . . . . . . , Fixed charges on ducts.. , . . , . . . . . . . . . . . . . . . . . . . . , . . . Operating charges.. . , , , . . , , . . . . , . . . . . . . . . , . . . . . . . , Maintenance charges, . . . . , . . . . . . . . . . . . . . . . . . . . . . . . .
.
.
Total charges.. . . . , . . . . . . . . . . , . . , . . . . . . . . . . . . , . . Value of heat recovered.. . . , . . . . . , . . . . . . . . , . . . . . , Net s a v i n g . . . . . . . . . . . . . . . . . . . . . , . . . . . . . , . . . . . .
$27,100 $ 3,420 2,000 5,040 260
--
$10 720 $18'700 $ 7:980
Acknowledgment TEMPERATURE OF INCOMING AIR O F
F i g u r e 5-Economizer
Performance
These assumed operating conditions determine the amount of air to be handled and the maximum possible recovery of heat for drying. The humidity corresponding to a dew point of 94" E". is 0.0350 pound of water per pound of moisture-free air. Therefore, the amount of air to be handled is:
The authors wish to acknowledge the helpful cooperation throughout these tests and in preparing this paper of John Harlowe, of the Penobscot Chemical Fibre Company. L i t e r a t u r e Cited (1) Lewis and Hedden, Massachusetts Institute of Technology Thesis, 1911. (2) Monto, I b i d . , 1910.
