June, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
air or noncondensable gas reduces the capacity of the condenser, with a corresponding increase in condenser pressure and power consumption. Calculations show that the introduction of 5 per cent of air in an ammonia compression system
TABLE111.
CALCULATED PERFORMANCES FOR AIR AYD AMYOSI.4 ( 4 )
COMPRESSING
WORK
AIR
RE-
COEFFI-
FRIGERATIOX PERLB.
CIENT OF
AM-
PER-
IN
PERLB.
TURK
NONIA
JlONIh
FORMAKCE
5%
B . t . u.
0 1 3 5
119.6 120.2 121.6 122.8
B . t . u. 482 473 444 9 420.2
g:3:. 4 2
MIX-
.4x-
4.40
MIXTURESOF
INCREASE OF
RE-
613
reduces the coefficient of performance by the surprising figure of 22.3 per cent (Table 111). There is no excuse for the presence of excessive amounts of noncondensable gas in an ammonia system, as there are on the market several very satisfactory “air separators” which a t a very nominal cost eliminate these objectionable gases.
LITHRATURE CITED
DCCTION IN
(1) Am. SOC. Refrigerating Engrs , Circ. 2. COM- CREASE COEFFI(2) Bur. of Standards, Czrc. 142 (1933). OF CIENTOF PRESSREFRIQ-PERFORM- (3) Macintire, H. J., “Principles of Mechanical Refrigeration,” ING \TORK ERATION ANCE 2nd ed., p. 191, hlcGraw-Hill, 1928. B . 1. u . B . 1. u. % (4) Mawson, H., Proc. Inst. Mech. Eng. (London), 1932; Refriger... .. ating Eng., 173 (March, 1933). 3.2
DE-
9 37 62
12 17 16.8 22.3
RECEIVED April 16, 1932.
Liquid Carbon Dioxide as a Refrigerant J. H. PRATT, The Liquid Carbonic Corporation, Chicago, Ill.
C
ARBON dioxide is a colorless, odorless, tasteless gas, approximately one and one-half times heavier than air. It has the following characteristics:
Critical temperature, O F. Critical pressure, Ib. per sq in Boiling point a t a t m pressure, F Latent heat of vaporization a t 0’ F , B t. u
’
SS 6
1078
-109 3 117 5
This gas exists to a slight extent in the atmosphere (0.3 per cent in country air, and often as high as 0.6 per cent in city air). It is nonpoisonous but does not support respiration or fire, owing to the exclusion of oxygen. Ordinary flame is extinguished by from 10 to 15 per cent carbon dioxide, “but even 20 per cent is not dangerous in an hour to animals, and probably not to man” (3’). The pathological effect of a small percentage of carbon dioxide is to increase the respiration This property has recently been used in establishing a new method of artificial respiration in gas-poisoning cases. COMMERCI.4L PRODUCTIOK O F CARBON DIOXIDE At least 90 per cent of the liquid carbon dioxide made today is manufactured by the so-called coke process (I). This consists in burning high-grade coke under specially constructed boilers with carefully regulated draft, producing flue gases with 17 to 19 per cent carbon dioxide. These gases are scrubbed free of mechanical dirt and sulfur compounds, and are then passed into large coke-filled absorbing towers where they meet a countercurrent flow of lye solution, made up of sodium or potassium carbonates dissolved in water. The lye solution dissolves the carbon dioxide from the flue gases, is then circulated through a series of heaters, and is finally pumped into an expeller or still, where it is boiled out by the condensation of the exhaust steam used by the engine which operates the plant. The gas released a t the still or expeller contains a large quantity of steam which is removed in an atmospheric cooler. It then passes through a series of traps and separators and is delivered direct to the low-pressure suction of the liquefying compressor. Carbon dioxide is sold and shipped in two standard sizes of cylinders. The small one, containing 20 pounds, is 5 X 51 inches, and the larger one, containing 50 pounds, is 8.5 x 51 inches. These cylinders, which are all built under Interstate Commerce Commission regulations, must be tested a t least once in five years by a 3000-pound hydraulic test. USES.Approximately 100 million pounds of liquid carbon dioxide are produced annually, and about 85 per cent of this is
used in the soft-drink trade, either by the bottlers of carbonated beverages or direct by the soda fountain. Large quantities of carbon dioxide are used a t present in the mining industry as a safety explosive replacing black powder and dynamite. The refrigeration industry today uses from 1 to 2 million pounds annually, or approximately 2 per cent of the entire production. Carbon dioxide is generally used as a fire extinguisher for ships, electrical fires, and telephone switchboards, and has been employed in large quantities in mine fires. It is used in the chemical industry as a base for the manufacture of salicylic acid and aspirin, and as a nonoxidizing agent in the rubber industry and for the aging of cement products. Physicians and beauty doctors use it as a local anesthetic, and one large city utilizes it as a substitute for illuminating gas in the humane killing of animals. PURITY.Ordinary coke-process carbon dioxide will analyze 99.5 to 99.8 per cent pure, the impurities consisting of less than 0.1 per cent water, and the balance air. I n ordering carbon dioxide for refrigeration purposes, the manufacturer should be advised of its destined use, as cylinders returning from bottling shops contain varying quantities of water which are not always entirely removed for the ordinary trade. Special cylinders are used for the refrigeration trade to insure against this moisture. USE AS A REFRIGERAXT Carbon dioxide has been generally employed as a common refrigerant in European countries for many years, and its use is now becoming more common in this country. Refrigerating units are now built by several American manufacturers in sizes ranging from 0.5 to 250 tons capacity. Because of the fact that carbon dioxide, in fairly large quantities, is not dangerous to plant or animal life, these systems are generally used in connection with ships, hospitals, department stores, hotels, and theaters, where slight traces of other refrigerants would be dangerous or disagreeable. The United States Kavy now uses carbon dioxide exclusively on its capital vessels. The entire charge of carbon dioxide from the ordinary refrigerating plant can be safely emptied into the engine room, as the amount can run up as high as 10 to 15 per cent without danger, while 0.5 per cent of other refrigerants in the air may be fatal. The fact that it is heavier than air keeps it in the engine room, where it is not obnoxious, whereas other lighterthan-air refrigerants will escape through the upper part of the ship. Special safety devices, including outside vent pipes, are not required by city ordinance, as is true of other re-
614
INDUSTRIAL AND ENGINEERIKG CHEMISTRY
frigerants. Besides its general use for purposes already mentioned, it is now becoming popular in connection with fur storage vaults, markets, delicatessen stores, and florist shops, where the escape of other refrigerants might injure merchandise. STABILITY.Carbon dioxide gas is absolutely inert and stable under all conditions. It never forms noncondensable gases which have to be vented from the system; nor can it form explosive mixtures, either inside the refrigerating system or around it. This inert quality allows the use of babbitt, bronze, and, in fact, any metal for bearings or structural purposes throughout the plant. The use of copper coils in shell type condensers is an advantage, as the amount of cooling surface is reduced as well as the space needed for the apparatus. Any good grade of low cold-test oil may be used in connection with carbon dioxide refrigerating plants, as it has no chemical or emulsifying effects on oils. The life of the oil is therefore practically indefinite except as it picks up dirt and foreign matter in the course of operation. DISTRIBUTIOK.Carbon dioxide a t the present time is sold for refrigerating purposes a t costs varying from 8 to 12 cents in different parts of the country. This is considerably less than other refrigerants. It can be purchased in reasonable quantities in practically every fair-sized city in the United States and Canada. Low TEMPERATURE SYSTEMS. Many new industries, such as the manufacture of helium, oxygen, and nitrogen from liquid air, require extremely low temperatures and must turn to carbon dioxide as the refrigerant. One of the more recent developments is the combination of a carbon dioxide and ammonia system which uses ammonia for carbon dioxide condensers and is able to produce commercial temperatures as low as -50" F. for commercial operations such as the quickfreezing of fruit and the hardening of ice cream. Several companies have developed this system to the point of operating both compressors from a single motor. It is a well-known fact that the rapidly developing frozenfood industry is depending upon extremely cold temperatures, and it has recently been shown that the quality of ice cream is materially improved by hardening it as low as -35" F. as against ordinary temperatures of approximately 0" F. For extremely low temperatures, carbon dioxide is the only well-known refrigerant in general use which may be operated under positive pressure a t all points in the system. It is possible to produce temperatures down to -69" F. before the triple point is reached. By combining ammonia and carbon dioxide, the extremely high pressures commonly met in carbon dioxide plants are eliminated. The main argument used in this OPERATING PRESSURES. country against carbon dioxide has been the comparatively high operating pressures. These ordinarily range from 250 pounds suction to 900 pounds condenser pressures. While these are high as compared t o ordinary steam engineering work, they are not excessive as compared t o Diesel engine and liquid air practice. With present day construction methods, there is no real difficulty in handling these pressures The elimination of fittings by the use of welded joints and bent pipe overcomes most of the trouble, although satisfactory flange joints are easily made by experienced engineers. In connection with the high pressures, it is not often understood that the pressure ratio-i. e., condenser pressure divided by suction pressure-is much less for carbon dioxide than for ordinary air compressors or other refrigerating systems. For carbon dioxide the range is from two to three, as compared to six to ten for ammonia, This low ratio gives maximum compressor economy, owing to the small amount of clearance, reexpansion, smaller superheating losses, and much less piston leakage and valve wire drawing.
Vol. 24, No. 6
HIGHCOSDESSERTEMPERATURES. Since the critical temperature of carbon dioxide is 88.4' F., it is a common fallacy that there is practically no refrigerating effect with condenser temperatures above this. The fact is that carbon dioxide is not a perfect gas around the critical point, and there is no sudden change in properties in passing from 88" to 89" F. Authorities do not agree as to whether the refrigerating effect is due to the liquefying of part of the gas through the expansion of the balance, or entirely to the effect of the high specific heat of the gas a t this point. The efficiency and capacity of the system are considerably higher than for a dense-air machine under similar conditions, Practically all British, United Fruit Company, and United States Kavy vessels operate such systems satisfactorily in equatorial waters, although the capacity of their plants is reduced as much as 50 per cent as compared to the usual 70" F. condenser temperatures. SIMPLICITYOF OPERATIOS. Because of the fact that cylinders, stuffing boxes, and other compressor details must be well designed and built, and that the high-pressure piping must be heavier than for other refrigerants, the first cost of the ordinary carbon dioxide refrigerating plant is somewhat higher than others. There is one exception to this in connection with the aircooling of theaters where, owing to the safety feature of carbon dioxide, it is possible to cool the air with directexpansion coils instead of using brine piping, as required by other refrigerants, where a slight leakage would render the entire ventilating system obnoxious. This saving in brine piping somewhat more than offsets the additional cost of the rest of the system. GENERAL EFFICIENCY.Unfortunately, there are no good tests on record for larger carbon dioxide refrigerating plants, although it is understood that some are now being made, and that preliminary results show the general efficiency of carbon dioxide systems to be as good as that of others. Macintire (9) states that "contrary to the expectations of most engineers, the power required by the different refrigerating machines per unit of refrigeration is about the same. In fact, ammonia leads slightly in this respect, but the net result is so close that the matter of horsepower per ton of refrigeration for usual operating conditions need not be considered when deciding on one or the other refrigerant.'' PHYSICAL DATA Until recently, accurate physical data on carbon dioxide were very incomplete. Research work recently done in Germany by Plank and Kuprianoff has been translated into English units by The Liquid Carbonic Corporation engineers and is now available to the refrigeration industry in the form of a convenient temperature entropy diagram, which will be furnished gratis on receipt of request a t the Chicago office of the company. SUhlMARY
Briefly summing up the advantages and disadrantages of carbon dioxide as compared to other refrigerants, it is generally believed that its use will become more and more popular as the great advantage of safety, coupled with the fact that the operating efficiency is practically as good as other mediums, more than overcomes the imaginary difficulties with the higher pressures, particularly when operated in northern climates with ordinary cooling-water temperatures. LITERATURE CITED (1) Howe, H. E., ISD ESG.CHEW,20, 1091 (1928) (2) Macintire, "Principles of Mechanical Refrigeration," 2nd ed., p. 1.55, McGraw-Hill, 1928. (3) Sellman, Manual of Pharmacology, p. 605, W. B. Saunders Co., 1918.
RECEIVED .4pril 18, 1932.
