CHAPTER III REFRIGERANTS (Part 1)

CHAPTER III REFRIGERANTS General Requisites. — The most desirable refrigerant should possess the following" properties : 1. A high latent heat as well as a high ratio of the latent heat to the specific heat of the liquid, in order to produce a large refrig- erating effect per cycle of operation. 2. A boiling point at ordinary atmospheric pressure low enough to obtain the temperature desired. 3. A condensing temperature at a relatively low pressure. 4. A low specific volume of vapor. 5. A high critical temperature. 6. A low ratio of compression. 7. A non-corrosive action on metals. 8. A chemical composition which is stable under working condi- tions and inert on lubricants and gaskets. 9. A non-inflammable and non-explosive nature even when mixed with air. 10. An inoffensive odor, non-injurious to health. 11. A behavior whereby its presence in small quantities may be visibly detected by a simple test. 12. A low cost of production for a product of necessary chem- ical purity for commercial use. 13. No affinity for constituents of the atmosphere whereby leaks might form gases or acids effecting the normal operation of the system. 14. A non-corrosive action on desirable bearing materials. Refrigerants for Household Systems. — There are approxi- mately 500,000 household refrigerating machines in operation 31 32 HOUSEHOLD REFRIGERATION in the United States. Sulphur dioxide is the refrigerant used in more than 75 per cent of these systems. Some of the other mediums employed are : methyl chloride, ethyl chloride, butane, isobutane, ammonia, propane, carbon dioxide, ether, air and water vapor. Amonia is used in more than 90 per cent of the larger or commercial refrigerating plants. Carbon dioxide is now used extensively for refrigerating systems in boats where formerl}- ethyl chloride and air ma- chines were favored. Carbon dioxide and air machines are considered safer than machines with other refrigerants, in case of accident or fire. Carbon dioxide is used rather ex- tensively in Europe for small household machines and its use in cooling theatres and public buildings is increasing in the United States. Ether has some use in small hand operated machines which are manufactured in Europe and sold in the tropics. Nitrous oxide has a limited use in the chemical industries when very low temperatures are desired. Pressure of Condensation, — The condensing pressure should be comparative!} low. Assuming 86° F, as the con- densing temperature, the following pressures are obtained with the refrigerants in common use: Ether 2.4 Lbs. Gauge Ethyl Chloride 12.40 Lbs. Gauge Sulphur Dioxide 51.75 Lbs. Gauge Methyl Chloride 80.83 Lbs. Gauge Propane 143.0 Lbs. Gauge Ammonia 154.5 Lbs. Gauge Ethane 666.0 Lbs. Gauge Nitrous Oxide 915.3 Lbs. Gauge Carbon Dioxide 1024.3 Lbs. Gauge The high condensing pressure reached with carbon dioxide and even ammonia, necessitates very strong and w^ell made apparatus. The carbon dioxide machines in use today are water cooled. The ammonia machines are also water cooled. Air cooled ammonia machines have been built but have not been used commercially. Sulphur dioxide machines have been placed on the market, both as w^ater cooled and air cooled. The air cooled operate at a condensing pressure of 10 to 20 pounds higher than the water cooled type. Air cooling lowers REFRIGERANTS 33 the efficiency, but increases the simplicity of the refrigerating system. A study of the development of household machines indicates that it is very desirable to use air cooled condensers to obtain simplicity, lower initial cost, and lower installation costs. Air cooled condensers are now used almost universally in household machines of the compression type. It has not proven practical to use air cooling for refriger- ants operating at a condensing pressure of more than 150 lbs. gauge. It is usually necessary .to centralize the piping with refrigerants having a condensing pressure of over 150 lbs. o-auo-e and distribute the refrigeration by means of a brine system. Pressure of Vaporization. — The following evaporating pressures are obtained with the refrigerants in common use at 5° F., evaporating temperature. Ether — 13.19 Lbs. Gauge Ethyl Chloride —10.05 Lbs. Gauge Sulphur Dioxide —2.88 Lbs. Gauge Methyl Chloride 6.19 Lbs. Gauge Ammonia 1957 Lbs. Gauge Propane 30.5 Lbs. Gauge Ethane 221.0 Lbs Gauge Nitrous 'Oxide 318.3 Lbs. Gauge Carbon Dioxide 319.7 Lbs. Gauge The evaporating pressure has an important influence on the stuffing box. The packing is usually made to take up wear automatically. It is advantageous to have nearly the same pressure on both sides of the packing. Sulphur dioxide operates at an evaporating pressure very close to atmospheric pressure, thus favoring this condition better than any of the other refrigerants in common use. With a refrigerant such as ethyl chloride, which normally operates with a partial vacuum on the evaporator, it is very difficult to locate a leak as air could enter the system un- noticed, and would greatly reduce the efficiency of the appa- ratus. Some household machines have all moving parts entirely enclosed, thus eliminating this packing gland difficulty. The compressors so far designed with a method of eliminating the packing gland include the design features which have not as yet proven practical in large quantity production. Other 34 HOUSEHOLD REFRIGERATION machines have an oil reservoir on both sides of the stuffing box, so that any small leak would be of oil either into or out of the compressor crank case. This would depend upon the pressure inside the crank case being above or below atmos- pheric pressure. Latent Heat of Vaporization. — The latent heat of vaporiza- tion should be carefully considered in selecting a refrigerant for a household machine. One of the most difficult problems is the expansion valve, float valve, or liquid restriction device, which controls the rate of flow of liquid from the condensing to the evaporating side of the system. With a high latent heat of vaporization, this problem is more difficult, as it is then necessary to control through a more sensitive valve (the amount of liquid circulating per minute being less). In mak- ing this comparison it is also necessary to consider the con- densing and evaporating pressures. These determine the pressure differential tr\ing to force the liquid through the expansion valve. This problem is more difficult with ammonia than with sulphur dioxide, as it is necessary to circulate three to four times more refrigerant in the sulphur dioxide system, because of its lower latent heat of vaporization, while the pressure differential between the condensing and evaporating sides are less than in an ammonia system. On larger refrigerating sys- tems, the liquid control problem is less difficult ; therefore, a refrigerant with a high latent heat of vaporization is preferred. Carbon dioxide has a very low latent heat of vaporization, about half that of sulphur dioxide. However, the pressure differential is so great as to more than offset the advantage of having a loAver latent heat. The latent heat of vaporization of the household refriger- ants in common use at 5° F. is: Carbon Dioxide 115.30 B.t.u. per Lb. Nitrous Oxide 121.4 B.t.u. per Lb. Sulphur Dioxide 169.38 B.t.u. per Lb. Propane 169.5 B.t.u. per Lb. Ethane 176.0 B.t.u. per Lb. Ethyl Chloride 177.0 B.t.u. per Lb. Methyl Chloride 178.5 B.t.u. per Lb. Ammonia 565.0 B.t.u. per Lb. REFRIGERANTS 35 Corrosion of Metals. — An important factor in choosing a refrigerant is the corrosive action on metals. Sulphur dioxide has no corrosive action on iron or steel, unless there is water present. Water and sulphur dioxide combine chemically as follov^'s : H,0 plus SO2 = H.SO.-; Water plus sulphur dioxidei^Sulphurous acid Suli)liur(nis acid is f(jrnicd, which will attack won This condition sometimes occurs, resulting in a so-called "frozen" compressor. The pistons will "freeze" to the cylin- ders so tightly that it is necessary to take the compressor apart and remove such material before operating again. Sulphur dioxide has no chemical or corrosive action on copper or copper alloys, thus permitting the use of copi)er tubes for the condensing and cooling elements. This is an advantage, as the thermal conductivity of copper is seven or eight times greater than that of steel or iron. Copper or cop- per alloys cannot be used with ammonia when there is water present. Copper can be used with anhydrous ammonia. Cop- per lines are used on some absorption machines using a solid absorbent and charged with anhydrous ammonia. Methyl chloride, ethyl chloride, butane, and carbon diox- ide have no chemical or corrosive action on copper, copper alloys, iron or steel ; therefore, these refrigerants may be used with any of these metals. Testing for Gas Leaks. — Sulphur dioxide is one of the two refrigerants, ammonia being the other, with which it is pos- sible to find leaks by means of a visible method called the "smoke" test. The smoke test consists of placing aqua ammonia near the sulphur dioxide leak. A chemical reaction occurs and dense white smoke apparently issues from the opening. SO. + H.O = H.SO3 2NH4OH H.SO:. = (NH4).S03 + 2H.O (NH4)2SOy is a white solid ammonium sulphite. A burn- ing sulphur stick is used in testing for an ammonia leak. A small alcohol flame is sometimes used in testing for an appreciable leak of methyl chloride. The flame is passed near the connections to be tested. A leak of methyl chloride will 36 HOUSEHOLD REFRIGERATION impart a green color to the nearly colorless alcohol flame. There is no danger of igniting an explosive mixture of methyl chloride and air in making- this test. It is necessary to have at least 10 per cent and not more than 15 per cent of methyl chloride present by volume to form an explosive mixture with air. It is impossible to remain in a room for more than a minute or tw"o with this concentration present because of the physiological effect upon breathing. Another method used to find leaks of methyl chloride is to use a small electrically heated wire. The wire is heated to a dull red temperature. While the wire is being applied, the fumes of ammonia arc brought near. If methyl chloride is present a fume will result, due to the decomposition of the methyl chloride to hydrochloric acid and carbon and the recon- struction of the hydrochloric acid set free with the ammonia. Comparisons of Refrigerants for Household Machines. — From foregoing considerations it will be observed *that the operating pressures, latent heat of evaporization, facility for testing for gas leakage, inflammability, corrosive action on TABLE XI. REFRIGERANTS FOR HOUSEHOLD MACHINES Relative Advantage for Use in Household Machines. Listed in order of preference under each heading. Operating Pressures Latent Heat of Vapori- zation Testing for Gas Leaks Inflamma- bility Corosive Action on Metals Danger of Breathing Small Con- centration of Gas in Air Lubrica- tion Sulphur dioxide Carbon dioxide Ammonia Carbon dioxide Methyl chloride Carbon dioxide Sulphur dioxide Methyl chloride Sulphur dioxide Sulphur dioxide Sulphur dioxide Ethyl chloride Ethyl chloride ,\mmonia Ammonia Ethyl chloride Methyl chloride Ammonia Ether Methyl chloride Methyl chloride Ethyl chloride Methyl chloride Ether Methyl chloride Carbon dioxide Ether Ether Ether Ether Ethyl chloride Ethyl chloride Sulphur dioxide Ammonia Carbon dioxide Carbon dioxide Ammonia Carbon dioxide Ether Ammonia Sulphur dioxide Ethyl chloride metals, danger of breathing, and lubrication, are the principle factors to be considered in the selection of a suitable refriger- ant for household refrigerating machines. With these factors REFRIGERANTS 2,7 in mind, the author has prepared Table XI, to show the rela- tive advantages of various refrigerants in household machines. These are listed in order of preference, under each of the head- ings for sulphur dioxide, ethyl chloride, ammonia, methyl chloride, ether, and carbon dioxide. Characteristics Influencing Selections. — The following are some of the general characteristics influencing the selection of refrigerants : 1. The condensing pressure should be reasonably low at tap water or atmospheric air temperatures, depending upon the cooling medium used. The evaporating pressure necessary to freeze ice in a reasonable length of time should be close to atmospheric pres- sure, preferably above, to prevent gas leaks when a stuffing box is used. The ratio of compression between the condensing pressure and pres- sure of vaporization should be small in order to facilitate the function- ing of the expansion valve. 2. A low latent heat of vaporization is preferred so that a larger amount of liquid refrigerant circulates to do the same amount of cooling. This makes the expansion valve or liquid control restriction less sensitive and permits the valve to leak more without affecting normal operation. 3. A refrigerant having a visible or "smoke" test for leaks is preferable as it is then not necessary to test every joint with oil or soap water. It is extremely difficult to find leaks if a refrigerant oper- ates at a pressure less than atmospheric as air can leak into the appara- tus affecting normal operation before the leak is detected. 4. A non-inflammable refrigerant is preferred in order to prevent danger in case of a gas leak in the refrigerating system in a home and also to prevent danger in case of fire. 5. A refrigerant is favored which does not have a corrosive or chemical action on metals. It is advantageous to be able to use cop- per and copper alloys for heat interchange apparatus on account of the higher rate of heat conductivity. Some refrigerants have a corrosive efi'ect on metals when water or gases from the atmosphere are allowed to enter the refrigerating system. 6. Preference is given to the different refrigerants in accordance with the percentage of gas, which, when mixed with air, will not give discomfort when breathed for a considerable length of time. 7. It is preferable to use oil as a lubricant. It is desirable to eliminate the oil trap. The lubricant problem is more difficult when larger volumes of gas must be compressed, often .necessitating a ro- tary compressor. 38 HOUSEHOLD REFRIGERATION Amount of Refrigerant to Be Evaporated. — The relative amount of the liquid refrigerant to be evaporated to produce refrigeration at a given rate depends upon the relative latent heat of vaporization and sensible heat of the respective re- frigerant. Generally, those refrigerants which have high latent heat of evaporization require a small amount of liquid to be evaporated to produce a given refrigerating effect. This is illustrated by ammonia, which has a fairly large latent heat of evaporization. On the other hand, certain refrigerants have Carbon Dioxide Ethyl Chloride Methyl Chloride Sulphur Dioxide Ammonia FIG. 2— AMOUNT OF LIQUID REFRIGERANT TO BE EVAPORATED low latent heats of evaporization, in which case, the sensible heat of the liquid corresponds to a large proportion of the available latent heat of evaporization. By sensible heat of liquid is meant the heat required to cool the liquid refrigerant from the temperature at the exit from the condenser, or at a point just before the expansion valve to the temperature exist- ing in the evaporator. Carbon dioxide is one of the representa- tive refrigerants which has a fairly small latent heat of evapo- rization. Fig. 2 shows graphically the amount of refrigerant which must be evaporated per minute to produce one pound of ice melting effect per 24 hours for carbon dioxide, ethyl chloride, methyl chloride, sulphur dioxide, and ammonia. REFRIGERANTS 39 Use of Refrigerants in the United States. — The various types of refrigerating plants using different refrigerants in the United States may be classified into large commercial plants, small commercial plants, marine installations, and household refrigerating machines. In a large commercial plant, it will be found that ammonia is used extensively; in small commercial plants ammonia is used extensively also; in marine installations, carbon dioxide is used extensively, and in the household machines, sulphur dioxide is used exten- sively. Table XII shows the use of the different refrigerants in the United States at present. TABLE XII. — USE OF REFRIGERANTS IN UNITED STATES Table Showing Present Usage in U. S. for Various Types of Refrigerating Plants. Large Commercial Plants Small Commercial Plants Marine Installations Household Machines Ammonia (Compression) Extensive Extensive Limited Very Limited Sulphur Dioxide None None Very Limited Extensive Methyl Chloride None None Very Limited Limited Ethyl Chloride . None None Very Limited Limited Carbon Dioxide . Limited Limited Extensive Very Limited Air None None Very Limited Very Limited Ammonia (Absorption) . Limited Limited None Limited Isobutane None None None Limited Comparative Cylinder Displacements. — On account of the fact that the different refrigerants have different latent heats of evaporation and sensible heats of liquid, as well as specific volumes of vapors, it is evident that the cylinder displace- ments will be individual with each kind of refrigerant. Those refrigerants which have high refrigerating effects with corre- sponding low specific volumes of vapor, will require the mini- mum cylinder displacements, while those which have low re- frigerating effects, and correspondingly large specific volumes of vapor, will require the maximum cylinder displacements. The converse of this may be stated by giving the refrigerat- ing effect per cubic foot of cylinder displacement. Table XIII has been prepared to show the relative refrigeration per cubic foot of cylinder displacement for an evaporating temperature of 5' F., and a condensing temperature of 86° F. for some of the common refrigerants. From this table, it will be noted 40 HOUSEHOLD REFKIGERATION that ethyl chloride has a very small refrigerating effect per cubic foot of cylinder displacement, that carbon dioxide has a high refrigerating effect per cubic foot, and that sulphur diox- ide, methyl chloride, and ammonia, have a medium refrigerat- ing eff'ect per cubic foot of cylinder displacement. TABLE XIII COMPARATIVE REFRIGERATION PER CU. FT. OF CYLINDER DISPLACEMENT For 5° F. Suction Temperature and 86° F. Condensing Temperature S :lphur Mcthvl Carbon Ethyl Dioxide Ammonia Chloride Dioxide Chloride Chemical S"mbol SO2 mh CH3CL CO2 C2H5CL Latent Heat at 5° F 1G9.3S oGo.O 17S.5G 115.3 177.0 Heat to Cool Liquid 2<S.01 90.55 ■ 38.15 58.61 34.7 Refrigerating Effect per lb . UL37 474.45 140.41 56.69 142.3 Specific Volume Vapor at 5° F. (cu. ft. per lb.)...