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10. Its pre.~sure-teiiii,erature rclat,ioiis are such that air-cooled units can Le constructed and head prcssnres (always below 150 ponnds) maint,aincd in temperate climates. 11. Itslow boiling point (approximiLtely -11" P.)renders it possible to o b t a i n all ranges of evaporator temperatures necessary in the ordinary household or commerrisl work, always with positive siictioii pressure. 12. Its nonirritant properties and its slight but pleasant odor render it pssible at all Linics to service eqnipnicnt witlioiil miisiiig annoyance to oconpants of the prmniscs where s u d i equipment, is installed. 13. It does not injurc textiles, furs, flowers, or food products if such become exposed to its va,poys by reason of n c d e n t a l lraliage.
hlaeiiiuoiurers d iiietiiyi ohloride and other ciiemical products
Suwmiiu ox' Anv,wrmes OF M w a r i . ( ' i m n i i i > t : 1. It is noncorrosive to all ordiiiriry tri:itc,riids riscd in tlie construi:iion of refrigcriLtion 2. It, docs not decornpose or liydrdyee into wrrosivc substances wlien coming in contact with w i e r or iiioistnre accidently inbroduced into the systeni. 3. It doos not fonn explosive miat,iires witli air inidcr ordinary conditions of operation. 4. I t is stable and incapable oi' d~:conipositioii at any temperatme existing in the systern. 5. It is only nioderately inflammable. 6. Its toxicity is relatively low when compared with othcr refrigerants such as sulfur dioxide and ammonin. 7. It has no diemica1 action on oils used fur lubricdon. 5. It decomposes wlmi coming in contact with ail upcn flame but yields no highly toxic substances. 9. The displacement per ton of refrigeration is such as to allow reasonable constrisetion of the compressor and refrigerating eqniprnent as to size aiid with due consideration for the niinimiaing of frictioii loss. It is well adapted as a refrigerant in equipment having rapcities up to 10 tons.
274 (1930). Hoessler & liasslaoher Chemical (.'., uirimbiished dats. (4) .ievcrs, Yaut, Thomas, and Beracr, si. S. PuBlio Health Scrvice, Bull. 185 (1D29). (5j Umiervrriters Laboratories,R s p l . 1418 ll9X3j (6) Y m t . Shoal. and Chornyak, Public I f d t h Kept. 45, 50. 19.
(:{I
(1030).
BIBLIOGRAPHY SOC. Refrinrratine Enere., - . Circ. 2 (1926) . .
Am. Itid., ~ i m s. (i926). Churohill, J. B., C h m . Markets. 25, 687-92 (1929); 30, 140-5 (1932): Chem. Mel. En".. 38, 47 (1931); Refrigmaling Eny., 21, 269-73 (19Jlj. Edwards. H. D., Am. 806. R r / & g m d i n g I h g i n . J . , 8, 488 (1922). Herter, C. H.,Ref~iysrotin~ World, 54, No. 5 , 11 (1919); 58, KO. 4, 13 (1923); 58, KO.7, 13 (1923). Hodadon, G. C., Ice & Cold Sloraoe ( L a d o n ) .22, 215 (1919). K ~ ~M~~ sU . ~ Iurlci I ~rope, , J . A ~Mea. . .4ssoc., 93,353-.1:(IOZI). Kmmcr, G. A , , Am. Soc. Refriyeraling Enyrs. J.. 7. 29 Il920). Meaiotire, Marvel. and Ford. Re/"yeiu(ing Enpa.. 14, 116 (1927). Matthews. F. E.. Am. Soc. Nof7iyerotingEnpe. J., 6 , 4 8 7 4 5 (1920). Shorthose, D. N., Dept. Sei. Ind. Reseer& (London), Sveeial Repl. 19 (1924). St,nrr. J. E., Am. SOC.R d r w m f i n g E w ~ a J. . , 5, 157 ( I Y I X ) ~
R F C E ~ V EMnroh D 5. 1932.
Sulfur Dioxide as a Refrigerant CHAS.W. JOHNSTON, \ irginia Smelting Company, West Norfolk, Va.
I
K THIS article tlie specifications for refrigeration-grade sulfur dioxide, the effects of sulfur dioxide on metals and oil, and sonie of tlie reasons why sulfnr dioxide is nsed so extensively in small units are discusscd n t h g t h . The tticrinodynamic properties ( f ) , the rriilnui'actiire of sulfur dioxide (S), facts usually covered in folders (9),the testing tion-Fade sulfur dioxide (7), :tiid the quest.ion of toxicity ;ire discussed briefly, fur tlicst: point,s ha7.e been quite fully covered in other ppers. The t,herniodynnmic properties of snlfur dioxide linre loiig bccir linomi and appreciated. As those facts regarding it %\-hiellhave been discovered and demonstrated through experience in its actual use as a refrigerant becomo more widely knorvri to engineers, it may well be that sulfur dioxide will be used in machines of increasingly larger capacity, and in more and more of the refrigerating equipment that is needed in industry. I t is
hoped that bhe article may be interesting and helpful to niaiiy who liave littlr kiiowledge of refrigeration engineeriug, but are interested io refrigerants, either because they are errgaged in some way in the refrigeration business, or have piirchased or will purchase some refrigernt;on device.
EAIILY HmEsiexms TO S ~ L I * UI ~RI O X I D E Sulfur dioxide is not a new product. I t has been kiiuwi and nsed for many years, and was even nientioned by Homer. Priestley prepared it over mercury in 1775. Michael Farnday liqurficd and solidified it. As a gas largely diluted with air, it is known to many, for it occurs in the air ii: volcanic regions nnd around snielt,ers, acid plants, and chemical plants; in fact, it is present in the air wherever coal is burned, for practically all coal contains sulfur. Sulfur dioxide is t.he gas formed by the burning of sulfur, and it. Was commonly
I N D US T R I A L AN D E N G I N E E R I N G C H E M I ST R Y
June; 1932
TABLEI. PROPERTIES
O F SATURATED
VOLUME TE3fP. O
F.
- 40
-20 0 20 40 60 80 100 120 140
PRES6CRE Lb./sq. in. 3.136 5.883 10.35 17.18 27.10 40.93 59.68 84.52 120.93 168.61
Liquid Cu.Jt./lb. 0.01044 0.01063 0.01082 0.01103 0.01126 0 . 0 1150 0.01176 0.01204 0.01236 0.01272
HEATOF LIQCID Vapor ( A B O V E -40' F.) Cu. ft./lb. B . f. u . 22.42 0.00 12.42 5.98 1 2.44 7.280 19.20 4.487 26.12 2.887 33.10 1.926 40.05 1.321 46.90 0.926 53.58 0.660 60.04 0.476
used a few years ago for fumigation purposes, being obtained by the burning of sulfur candles or brimstone. Raoul Pictet used it as a refrigerant in France about 1875. The earliest German patent known to the author for making liquid sulfur dioxide was taken out in 1883 For many years sulfur dioxide was used in France and England as a refrigerant in machines for the making of ice; these machines had ice-making capacities of from 10 to 20 tons per day and perhaps larger. I t is only during the past few years, with the large increase in production of the small unit, that sulfur dioxide has been used in this country to an important extent as a refrigerant. Probably 90 per cent of all these small units use sulfur dioxide. One reason why these machines were not developed sooner mag be that the sulfur dioxide available until the last few years was not of sufficient purity to make its use as a refrigerant in the small unit successful.
PROPERTIES OF SULFURDIOXIDE Sulfur dioxide is a permanent gas. It is colorless, both in the liquid and gaseous state. It is nonexplosive, noninflammable, tends to put out a fire, and in case of contact with a fire is not decomposed into any harmful substances. Some of its properties are as follom: Melting point ( 6 ) , F. Boiling point ( 6 ) ' F. Critical temp. (8j 0 F. Critical Dressure (8).Ib./sq. in. abs. Sp. heat-of gas at constantpressure (8) Ratio of sp. heats ( C p / S v at 61-93' F. (8) Sp. heat of liquid at 50 Latent heat of vaporizatioh at 1 atm , B. t . u /lb. Density of liquid (water = 1) ( 6 ) : At -40' F. At 14' F. At 60' F. Density of gas (at atm. pressure and 32' F.) (8): Grams per liter Referred to air as 1 Lb. per cu. ft.
4
-103.4 14.00 311.7 1159.8 0.1544 1.256 0.35 167 1,5331 1.4601 1.4096 2.9267 2.2636 0.1827
The properties of saturated sulfur dioxide' are shown in Table I. The volume of saturated vapor of sulfur dioxide to condense for one ton refrigeration (12,000 B. t. u. per hour) a t different evaporator and condenser temperatures is as follows: 1 1 1 . . . 1
EVAPORATOR TEMPER.ATURE
F. 30 25 20 16 10 5 0
- 5 -10
Condenser at 70' F. Cu. ft./min. 4.825 5.390 6.066 6.829 7.715 8.741 9.939 11.340 12.972
Condenser at 86' F. Cu. f t t . / m i n . 5.012
5.599 6.302 7.102 8.017 9.084 10.329 11.786 13.485
Condenser at 100O F. Cu J t . / m i n 5.185 5.793 6.526 7.344 8.297 9.402 10.594 12.202 13.961
To get the required compressor displacement, divide the volume given above by the volumetric efficiency of the compressor, and correct for superheat, if any. The volumetric efficiency will vary from 44 per cent in small refrigerator compressors up to 90 per cent in large machines. 1 A more complete tabulation of the thermodynamic properties of sulfur dioxide and also standard ton data are given in Circ. 9 of the .imerican Society of Refrigeration Engineers.
SULFUR DIOXIDE
LATENTHEAT Internal Total B. 1. u . B. t . u. 165.57 178.61 161.58 175.09 156.70 170.63 151.08 165.32 159,26 144.82 137.98 152.49 130.65 145.12 122.89 137.20 114.70 128.78 106.30 119.90
TOTALHEAT OF VAPOR B . f. u . 178.61 181.07 183. 07 184.52 185.37 185.59 185.17 184.10 18%.36 179.94
627
---
Liquid
0.000 0.01366 0.02795 0.05668 0.04241
0.07060 0.08399 0.10829 0.09657 0.11893
ENTROPY---Evaporation Vapor 0,4256 0.3983 0.3712 0.3447 0.3187 0,2935 0.2690 0,2452 0,2222 0,1999
0.4256 0.4119 0.3992 0.3871 0,3754 0.3641 0.3529 0.3417 0.3305 0.3189
Example. The volume for one ton refrigeration, 20" F. in evaporator and 86" F. in condenser, is 6.302 cubic feet per minute. Assume 80 per cent volumetric efficiency compressor and no superheat displacement required: 2*6 = 7.878 cu. f t . per minute 0.80
~IASUFACTURE OF LIQUIDSULFURDIOXIDE(3) Liquid sulfur dioxide is manufactured in this country from brimstone, although it has been made in the past from smelter gases and is probably made from roaster gases in Europe. The usual method of manufacture is to absorb the sulfur dioxide from the sulfur burner or roaster in water, and then boil it out of the water by heating. The water vapor is removed from this sulfur dioxide gas and the gas is compressed and cooled to the point where it becomes a liquid. The resulting product is purified for refrigeration in two different ways. I n some plants the final drying and purification is accomplished by passing the sulfur dioxide through sulfuric acid towers. In the plant of the Virginia Smelting Company, final drying and purification are accomplished by careful distillation. Either method yields a pure product. CHARACTERISTICS INFLUEKCING SELECTION AS REFRIGER.4NT
A refrigerating machine is a device to absorb heat units a t a comparatively low temperature, and radiate or discharge these same heat units a t a higher temperature. For the work for which the small units are built, the temperatures a t which heat units are to be absorbed and discharged are such that sulfur dioxide can be and is used most advantageously because of its thermodynamic properties. A temperature in the evaporator of about 14" F. (- 10" C.) can be obtained without having a vacuum on the suction side of the machine. To avoid the possibility of leaks into a system that is not entirely sealed, it is desirable to keep the suction pressure above atmosphere. Compressors can be built of iron and of comparatively light cdnstruction, for sulfur dioxide condenses a t low pressures a t the temperatures that can be readily secured even in air-cooled condensers of modest size built of copper. Water-cooled condensers are not necessary with small units. If water-cooled condensers are used, they lower the condensing pressure, and this means also a smaller ratio of compression. The ratio of the pressure of a refrigerant in the evaporator to the pressure in the condenser is important for, in case this ratio becomes excessive, the efficiency of the compressor drops off, and there may be excessive heat generated in the compresbor. This point tends to place a limit on the use of sulfur dioxide a t very low temperatures, but the difficulty might be overcome by the use of two-stage compression. The low suction pressure of sulfur dioxide has the further advantage that the chance of leaks through the stuffing box and the consequent loss of the charge is less than it would be if the pressure were higher. The latent heat of sulfur dioxide, affecting as it does the quantity of sulfur dioxide to be circulated for a given amount of refrigeration, has certain advantages in the small unit.
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The amount of refrigerant required in any system depends on that system. I n some of the small, compactly built units, 1.5 pounds (0.71 kg.) are all that is needed or used. With ammonia, sulfur dioxide gives a dense white, so-called smoke. Testing for leaks with ammonia is easy, simple, and very positive, and even extremely small leaks are disclosed by the test. The fact that sulfur dioxide is nonexplosive and noninflammable, and is not decomposed by fire into harmful substances is a real advantage to it as a refrigerant. Lubrication is comparatively simple, and good inexpensive oils are available. Sulfur dioxide costs less than any other refrigerant. In the absence of water it has no corrosive action on metals. The toxic effect of sulfur dioxide has been an outstanding argument against it, but experience has developed a very large amount of information on this subject, some of which is cited later in this article. SPECIFICATIOKS FOR REFRIGERATION-GRBDE SVLFL-R
DIOXIDE Various specifications have been written to cover sulfur dioxide that is to be used as a refrigerant. These usually state that the sulfur dioxide shall be water-white in color and shall leave no residue on the evaporation of 100 cc. -4clean dry glass vessel in which a sample is evaporated should be just as clean and dry after the liquid sample has evaporated as it was before the sample was placed in it. All the specifications limit the water content of the sulfur dioxide. I n some, the limit is set a t 100 p. p. m. and in some as low as 20 p. p. m. Another point usually covered is the percentage of inert gases (largely air) which is usually limited to 0.01 per cent by volume. This means that, if a given quantity of the liquid is evaporated and the gas formed analyzed, it will show less than 1 volume of inert gas for 10,000 volumes of sulfur dioxide. Very often the sulfur trioxide content of the sulfur dioxide is limited. An investigation of the sulfur trioxide content of sulfur dioxide (which is difficult to determine accurately) was made a t the Bureau of Standards ( 2 ) . An evaporation test of sulfur dioxide can be easily made because the rate of evaporation in an open glass is reasonably slow. The outside of the glass gradually becomes coated with ice. If the glass is placed in quiet air, an ordinary drinking glass of sulfur dioxide will usually require 3 hours or more to evaporate completely even Tvith an air temperature of 80" F. The liquid will usually appear quiescent n-ith only slight signs of boiling after the first quick evaporation which takes place while drawing the sample. It should always be remembered in drawing a sample that sulfur dioxide takes up moisture from the air, and precautions must be used to guard against this if an accurate moisture test is t o be made. The problem of sampling, as well as the method of accurate analysis, has been so fully covered recently in an article by Scribner ( 7 ) that it is necessary only to mention that methods which give reliable and accurate results are now in use by manufacturers and purchasers of sulfur dioxide. These give check results, and liquid sulfur dioxide, which by these tests passes requirement of refrigerating grades, can be used safely and satisfactorily for refrigeration purposes. Refrigerationgrade sulfur dioxide as shipped is ready t o be used and is transferred to machines by charging directly from the containers in which it is shipped. 1
EFFECTS OF SULFURDIOXIDE ox METALS Refrigeration-grade sulfur dioxide does not attack metals used in refrigeration machines. Sulfur dioxide containing water above some small amount does attack metals. Just what amount of water there must be in the liquid sulfur dioxide before it begins to attack metals depends on several
Vol. 24, No. 6
factors. Some few years ago the sulfur dioxide shipped in steel containers ran from 0.2 to 0.3 per cent water (2000 to 3000 p. p. m.), and little or no action on the cylinders occurred. Yet if a piece of bright steel is placed in liquid sulfur dioxide containing more than 60 p. p. m. of water (0.006 per cent) and allowed to remain in it until the liquid has entirely evaporated, the steel will show that it has been acted upon. It is important, however, to note that this action occurs just as the last drops of the liquid sulfur dioxide evaporate. The steel remains bright and unattacked up to this time. The action is due t o the fact that as the liquid sulfur dioxide containing water evaporates, the water does not evaporate a t the same rate as the sulfur dioxide, but much more slowly. The unel-aporated sulfur dioxide is constantly increasing in water content, and the last drops contain a high percentage of water; they finally even become water containing sulfur dioxide, which is sulfurous acid. Sulfurous acid is very corrosive. It is not only essential for successful operation to use sulfur dioxide with low moisture content, but care must be taken that the moisture content is not increased by addition of water from any source. To avoid this introduction of water, machines are dried before charging, and dry oil is used. Khere the water content of sulfur dioxide in the machine after charging is kept below 50 p. p. m. (0.005 per cent), not only is corrosion avoided, but also action on oil is reduced to a minimum.
EFFECTS o s OIL AND OTHER SUBSTASCES Liquid sulfur dioxide dissolves certain oils and also is soluble in oils to varying degrees. The so-called carbonization of oils in the presence of sulfur dioxide is greatly increased by the presence of water. Local carbmization may result from sulfur dioxide containing over 0.006 per cent water at the point where the liquid sulfur dioxide entirely evaporates because of a concentration of water a t that point. Two other causes for carbonization of oils in the presence of sulfur dioxide are air and temperature. Machines are designed to run a t reasonable temperatures, and, before charging, the air in them is carefully removed. The questions of oil for use with sulfur dioxide and the action of sulfur dioxide on oils were recently covered in an article by the author ( 4 ) . Sulfur dioxide dissolves certain resins and other substances, and acts on rubber and other materials, so that it is necessary to be sure that any material used in a system that might come in contact with sulfur dioxide will not be affected by it. Sulfur dioxide is absorbed by some metals; hence, the texture of iron, for example, used in compressors should be such that the sulfur dioxide mill not filter through it, Leaks of this nature mag not be noticed for many weeks, or even months, but once even the most minute leak of this kind does develop, it will become larger with great rapidity. TOXIC EFFECTS OF SULFURDIOXIDE If sulfur dioxide escapes from a refrigeration system because of a leak in that system, its irritating effect makes its presence immediately knom-n so that anyone breathing air containing it will immediately and naturally get out into the fresh air, away from the leak. If for any reason sulfur dioxide is breathed, it is the experience of the author that only temporary discomfort is experienced and that exposure to very strong concentrations of sulfur dioxide rarely result in any permanent injury or serious after-effects. The extent of irritation resulting from breathing air containing sulfur dioxide varies greatly with different individuals; those n-ho are accustomed to breathing such air are able to stand concentrations which cause severe discomfort to those not accustomed to it. Those connected with the Virginia Smelting Company have repeatedly been exposed to and have breathed
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