ISDUSTRIAL, AXD ENGINEERING CHEMISTRY
AUGUST, 1935
(2) E g e r t o n a n d Pidgeon, Proc. Roy. SOC. (London), -1142, 26 (1933). (3) Rassweiler a n d WithroTv, =IutoirrobileEngT., 2 4 , 385 (Aug.. 1934). (4) Rassweiler a n d WithroR-, ISD. ESG.CHEM.,2 4 , 528 (1932). ( 5 ) Ibid., 2 5 , 1359 (1933). (6) Shawhan a n d M o r g a n , P h y s . Ra., 4 7 , 1 9 9 (1935). (7) TTithrow a n d B o y d , ISD. ESG.Cirmf., 2 3 , 5 3 9 (1931).
a79
(8) Kithrow, Lorell, a n d B o y d , I b i d . , 22, 945 (1930) (9) Withrow a n d Rassweiler, Ibid., 2 3 , 769 ( 1 9 3 1 ) . (10) Ibid., 2 5 , 923 ( 1 9 3 3 ) . (11) I b i d . , 2 6 , 1256 (1934).
RECEIVED April 4, 1935.
Concentrated Solutions in Air-conditioning F. R. BICHOWSKI
AND
GILBERT A . KELLEY
Surface Combustion Corporation, Toledo, Ohio
HE usual method of conditioning for summer \Teather is to cool the air by passing it in contact with a refrigerating liquid or with a refrigerated surface. Experience has shown that it is undesirable to pass air at temperatures below 65" F. into an occupied enclosure, since undesirable drafts are produced. If air v-ith a typical summer condition of 84" F. and 51 per cent relative humidity is cooled t o 65' F., no moisture will be precipitated. Even if sufficient air is so cooled as to maintain the enclosure a t a comfortable temperature, the moisture content will soon go so high, because of infiltration and moisture given off by the body, that the enclosure will become uncomfortable because of the humidity. I n order t o remove moisture from air by refrigeration, it is necessary to cool the air considerably below the dew point. In fact, it is desirable, in order to maintain proper humidity conditions, t o cool the air as low as possible without precipitation of frost on the cooling coil or without freezing of the circulated water. In practice, the air is cooled down to the neighborhood of 50" F. Air a t 50" is too cool to introduce directly into the room. In order to make it useful, such air has to be reheated, either by the extravagant process of adding heat directly t o the air or by mixing a portion of the 50'
T
Lithium chloride solutions of high concentrations have been successfully used in the drying and conditioning of air. Humidities as low as ll per cent may be obtained. The automatic cycle employed is described, and data are given on the coefficients for the transfer of moisture and heat to lithium chloride solutions in packed tow-ers employing Raschig rings. When dehumidification is employed in connection with cooling in air-conditioning, large savings are possible both in operating cost and first cost. air with a portion of recirculated air which has not been cooled, and introducing the mixture into the room. This patented process is the dominant process in present air-conditioning practice. Figure 1 is a psychrometric chart on which is shown the graphical solution of a typical air-conditioing problem in accordance with this process. If the heat balance is worked out, it nil1 be found that approximately 78 per cent of the refrigeration has been utilized in cooling the air and 22 per cent in removing water vapor from the air. Refrigeration is expensive, and it is natural, therefore, to investigate the possibility of saving on refrigeration by finding methods of reducing the moisture content of the air without the necessity of excess cooling. Two methods suggest themselves: absorption of the moisture by a solid absorbent such as silica gel or solid calcium chloride, and absorption of the moisture from the air by means of liquid absorbents. Both processes are now represented by commercial apparatus. The process utilizing silica gel is well known.
Liquid Drying Agents
I
50
60
DRYBur 5 EMPERRNR~ IN O L
70
80
I
I
90
100
FIGURE1. PSYCHROMETRIC CHARTFOR SOLVING A TYPICAL AIR-CONDITIONING PROBLEM
This paper describes the process employing liquid drying agents. All chemists are familiar with the use of such desiccants as sulfuric acid and phosphoric acid for drying air However, these acids have certain obvious disadvantages for drying. They are highly corrosive, and they cannot readily be re-used since reconcentration of the acid ib a process which is subject t o technical difficulty. This is true even in a chemical plant where the employees are accustomed to handling corrosive material. It is almost unthinkable that sulfuric acid could be successfully used in a domestic air-conditioning unit. Phosphoric acid is somewhat less obnoxious than sulfuric in that it does not fume on reconcentration, but it is more expensive and is equally corrosive, and very little practical
INDUSTRIAL AND ENGINEERING CHEMISTRY
880
TABLEI. NO.
1 2 3 4
5 6 7 8 9 10 11 12
13 14 15 16 17
18 19 20 21 22 23 24 26 26 27 28 29 30
Experimental Data Air rate Tower cross section Tower length Packing surface Lithium chloride: In out Sp. gr. Rate -4ir in: Dry bulb Wet bulb Air out: Dry bulb Wet bulb Lithium chloride: In out Water: In out Rate Partial pressure of water: In out Moisture content: In
out
Total heat: In out Sp. vol. Amount Total heat removed Total water removed Latent heat of water Latent heat removed Sensible heat removed
Engineering Units 655 cu. ft./min. 2 . 4 sq. it. 1 , 3 3 ft. 27 aq. ft./cu. ft.
VOL. 27, NO. 8
SAMPLE CALCULATIOX
Metric Units Dehumidifying Tower 0 . 3 0 9 cu. m./sec. 0 . 2 2 3 sq. m. 0 . 4 0 7 m. 8 9 . 5 Bq. m./cu. m.
Source Experimental Experimental Experimental Experimental
c.
70' F. 7 8 . 2 ' F. 1.20 2 . 7 5 gal./min.
21.10 2 5 . 8 ' C.
8 2 . So F. 6 6 . 0 ' F.
27.20 c. 18.90
c.
Experimental Experimental
7 9 . 0 ' F. 6 1 . 2 " F.
28.1O C. 16.2'C. Cooling Tower
Experimental Experimental
7 8 . 2 ' F. 7 0 . 0 ' F.
25.8' C. 21.10
Experimental Experimental
6 5 . 0 " F. 7 3 . 0 ' F. 2 0 . 3 lb./min
Experimental Experimental Experimental Experimental
0 . 2 0 4 kg./seo.
c.
1 8 . 3 ' C. 2 2 . 8 ' C. 0 . 1 6 3 kg./sec. Derived Data for Air
Experimental Experimental Experimental
1 1 . 4 mm. Hg 8 . 6 mm. Hg 6 6 . 8 grains/lb. dry air 5 0 . 0 grains/lb. dry air
Psychrometric tables Same
9 . 5 5 grams/kg. 7 . 1 5 grams/kg.
Same Same
1 6 . 9 Cal./kg. 1 5 . 0 Cal./kg. 0.856 cu. m./kg. 0 . 3 6 2 kg./sec. 0 . 6 8 2 Cal./sec. 0 . 8 7 k ./see. 0 . 5 7 2 Eal./kg. 0 . 4 9 6 Cal./sec. 0 . 1 8 6 Cal./sec.
~~~
Same Same Same No. 1/No. 24 No. 25 ( N o . 22 - No. 23) No. 25 ( N o . 20 - No. 21) International Critical Tablee No. 27 X No. 28 No. 26 - No. 29
Derived Data for Brine 31 32 33 34 35
Vapor pressure: In out Air in equilibrium: In -~~ out Mean vapor pressure difference Mean temp. difference
3 1 . 4 grains/lb. dry air 4 3 . 1 grains/lb. dry air
5 . 4 mm. Hg 7 . 4 mm. Hg
International Critical Tablea Same
4 . 4 8 gramdkg. 6 . 1 6 grams/kg. 3 . 6 mm. Hg
Psychrometric tables Same (NO. 18 - NO.31) ( N O . 19 32)/2 (NO.9 NO. 13) ( N O . 10
lA\ /0
6 . 8 ' F.
3 . 8 ' C.
37
0.0224 lb./hr., sq. f t . , 4 mm. Hg
0 . 3 0 gram/sec.. sq. m., 4 mm. Hg
38
4 . 6 B. t. u./hr., sq. ft. A
6 . 2 Cal./sec., sq. m.,
36
a
F.
C.
- NO.
-
KO.
No:i??(No. 2 X No. 3 X No. 4 X No. 35) No. 30/(No. 2 X No. 3 X No. 4 X No. 36)
I t is poseible to obtain checks on K and h by measurements on note of change of density of brine and other measurements given here.
use of its drying power has ever been made. Another proposed solution is a concentrated liquid solution of calcium chloride. At temperatures above 86" F. calcium chloride is a reasonably satisfactory drying agent, the saturated solution being in equilibrium with air a t a relative humidity of 20 per cent. Because of the fact that calcium chloride hexahydrate has a true melting point a t 86", the use of calcium chloride in high concentration is limited to applications in which there is no danger that the temperature of any part will ever come below 85'. If it should fall below 85", the entire solution would solidify in the pipes. Calcium chloride is thus not suitable for comfort air-conditioning, except a t relatively low concentrations. I n these concentrations the solution is in equilibrium with air of greater than 35 per cent relative humidity. Combination apparatus employing solid calcium chloride in contact with the saturated solution are available and undoubtedly will have certain applications. Various other liquid drying agents are known which do not have this disadvantage. The most completely studied of these are the aqueous solutions of lithium chloride. Lithium chloride ha3 the present disadvantage of high first cost, but in almost every other respect it proves t o be a highly satisfactory drying solution for its range of application, which is down to approximately 11 per cent relative humidity a t normal room-temperature condition. Lithium chloride solution has proved in practice to be entirely stable, hydrolysis
being entirely suppressed a t the high concentration. It has relatively low viscosity, is relatively noncorrosive toward most structural materials, is unreactive toward carbon dioxide in the air, is nonvolatile, and is entirely suitable for reconcentration by boiling in a commercially practical type of apparatus. Furthermore, it has the advantage that its vapor pressure and relationship are known over a wide range of temperature and concentration, as are its specific heat and heat of vaporization. Figure 2 shows one of the various types of cycles which can be employed in the automatic control of the humidity of air by means of concentrating aqueous solutions. Various other cycles may be used and are described in the patent literature.
Packed-Tower Method From the point of view of the chemical engineer, two of the most interesting portions of this cycle are the processes occurring in the drying tower and in the concentrating tower. Various forms of towers for contacting concentrated aqueous solutions with air have been developed and tested, including spray towers and packed towers of various sorts. The data presented here relate to towers packed with 2-inch Raschig rings, because this type of packing has been successful, and because the results obtained with lithium chloride can be compared for this packing with data obtained for other agents. The elementary theory states that three processes are going
AUGUST,
1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
881
on in the tower, two of w h i c h a r e indep e n d e n t . The first is the transfer of moisture from the air to t h e d r y i n g solution. T h e second is t h e transfer of heat to or from the air to or from the drying s o l u t i o n by convection. And r%C,4'//fG sl/RF.C< t h e third is t h e p f 44'!k!OC/TYFr:OV€R fm Mff. t r a n s f e r of heat to FIGLRE 3. COEFFICIENTS OF SENthe solution by conSIBLE HEATTRANSFER FROM AIR TO d e n s a t i o n of water LITHIUMCHLORIDE vapor in the solution. FIGURE2. CYCLE FOR ,kUTOM4TIC COUTROL OF HUWIDITY Brine rate = 1.8 gallons per hour per square E a c h of these procBI COWEYTRATING AQLEOLS SOLUTIONS foot of packing surface. esses is r e D r e s e n t e d by an equation of the form: cause of the great numb e r of v a r i a b l e s in7 - i = A K ( C 1 - C,) volved. Work is of the dt ordinary c o m m e r c i a 1 where C = a quantity of heat or a quantity of water vapor t y p e a n d t h e coeffiK = transfer coefficient A = area cients can probably be depended upon within 5 per cent. A> is well known, there is a relationship between K , for transThe example of exfer of latent heat, and for the transfer of moisture, so that in periments and calculaeffect we have to deal with two relations. The quantity K tions given in Table I depends on (1) the nature of the packing, (2) the temperature, shows the quantity as (3) the rate of air flow, (4) the character and concentration of m easured and the the solution, and ( 5 ) the rate of liquid flow. method of calculation Various theoretical studies have been made showing the employed. dependence of K for simple types of surfaces on all of the facFigure 3 shows values tors except concentration but, fortunately, observation indif o r t h e coefficient of cates that, in the range of concentrations generally used, K is s e n s i b l e heat transfer not a sharp function of the concentration, except when confrom air to l i t h i u m centration affects the other cpecific properties of the solution, chloride solutions f o r suRFAC< frfrRPliN. such as the viscosity and spwific heat. Unfortunately it is packed towers of this FIGURE 4. COEFFICIEKT OF usually desirable not to use simple surface of contacts between SIOISTURE TRANSFER FROM - 4 I R type and for counterthe air and the drying solution. Since K is dependent on the TO LITHIUM CHLORIDE current flow as a functype of such ;.urface particularly the nature of the turbulence Dehumidification in clay ring tower, tion of tile linear verotating di.stributor. that happens t o be in the neighborhood of the surface, it belocity of the l i t h i u m comes practically necessary to determine coefficient K for chloride. every type of tower employed. Since it is further true that, for the more complicated surfaces, theoretical relationshipgiving the dependence of K on the temperature and air flow do not hold, it becomes practically necessary to make te+ over a wide variety of te-t conditions. It would unduly enlarge thiq paper to present the experimental data in f u l l d e t a i l . Instead the varioui coefficients ~ d ~ i c h may be obtained from thebe esperim e n t a l d a t a are giren 'n the form of chart? (Figures 3 , 4 , 5 ) . T h e information on these c h a r t s is sufficiently complete to FIGURE5 . H E ~ T4 \ D XIOISTLRE show the range of TR4hSFER COEFFICIEbTS FHO\I %IR TO the data. S o eati\\. 4TER mate can be given FIGCR E 6. PSYCHROMETRIC CHART FOR WORKINGOUT A Water r a t e = 2.9 gal,ons per hour per 8 4 u a ~ 8 of the accuracy beHE.ATBALASCEPROBLEM foot of packing surface r__-----
0
Steel Foundry H y Cvnwtantinr Ennil
