The EPA listing of reclaimers is updated when additional refrigerant reclaimers are approved. Reclaimers appearing on this list are approved to reprocess used refrigerant to at least the purity specified in appendix A to 40 CFR part 82, subpart F (based on ARI Standard 700, “Specifications for Fluorocarbon and Other Refrigerants”). Reclamation of used refrigerant by an EPA-certified reclaimer is required in order to sell used refrigerant not originating from and intended for use with motor-vehicle air conditioners.
The EPA encourages reclaimers to participate in a voluntary third-party reclaimer-certification program operated by the Air-Conditioning and Refrigeration Institute (ARI). The volunteer program offered by the ARI involves quarterly testing of random samples of reclaimed refrigerant. Third-party certification can enhance the attractiveness of a reclaimer program by providing an objective assessment of its purity.
Since the world has become aware of the damage the Freon® refrigerants can do to the ozone layer, there has been a mad scramble to obtain new refrigerants that can replace all those now in use. There are some problems with adjusting the new and especially existing equipment to the properties of new refrigerant blends.
It is difficult to directly replace R-12, for instance. It has been the mainstay in refrigeration equipment for years. However, the automobil air-conditioning industry has been able to reformulate R-12 to produce an acceptable substitute, R-134a. There are others now available to substitute in the more sophisticated equip- ment with large amounts of refrigerants. Some of these will be covered here.
FREON® REFRIGERANTS
The Freon® family of refrigerants has been one of the major factors responsible for the impressive growth of not only the home-refrigeration and air-conditioning industry but also the commercial-refrigeration industry. The safe properties of these products have permitted their use under conditions where flammable or more toxic refrigerants would be hazardous. Following are descriptions of commonly used Freon® refrig- erants.
Freon-11 has a boiling point of 74.8 degrees F and has wide usage as a refrigerant in indirect industrial and commercial air-conditioning systems employing single or multistage centrifugal compressors with capacities of 100 tons and above. Freon-11 is also employed as brine for low-temperature applications. It pro- vides relatively low operating pressures with moderate displacement requirements.
The boiling point of Freon-12 is -21.7 degrees F. It is the most widely known and used of the Freon refrigerants. It is used principally in household and commercial refrigerators, frozen-food cabinets, ice-cream cabinets, food-locker plants, water coolers, room and window air-conditioning units, and similar equipment. It is generally used in reciprocating compressors, ranging in size from fractional to 800 horsepower. Rotary compressors are useful in small units. The use of centrifugal compressors with Freon-12 for large air-condi- tioning and process-cooling applications is increasing.
The boiling point of Freon-13 is -144.6 degrees F. It is used in low-temperature specialty applications em- ploying reciprocating compressors and generally in cascade with Freon-12 or Freon-22.
Freon-21 has a boiling point of 48 degrees F. It is used in fractional-horsepower household refrigerating systems and drinking-water coolers employing rotary vane-type compressors. Freon-21 is also used in com- fort-cooling air-conditioning systems of the absorption type where dimethyl ether or tetraethylene glycol is used as the absorbent.
The boiling point of Freon-22 is -41.4 degrees F. It is used in all types of household and commercial re- frigeration and in air-conditioning applications with reciprocating compressors. The outstanding thermo- dynamic properties of Freon-22 permit the use of smaller equipment than is possible with similar refriger- ants, making it especially suitable where size is a problem.
The boiling point of Freon-113 is 117.6 degrees F. It is used in commercial and industrial air-condition- ing and process-water and brine cooling with centrifugal compression. It is especially useful in small-tonnage applications.
The boiling point of Freon-114 is 38.4 degrees F. It is used as a refrigerant in fractional-horsepower household refrigerating systems and drinking-water coolers employing rotary vane-type compressors. It is also used in indirect industrial and commercial air-conditioning systems and in industrial process-water and brine cooling to -70 degrees F, employing multistage centrifugal-type compressors in cascades of 100 ton’ refrigerating capacity and larger.
The boiling point of Freon-115 is -37.7 degrees F. It is especially stable, offering a particularly low dis- charge temperature in reciprocating compressors. Its capacity exceeds that of Freon-12 by as much as 50 percent in low-temperature systems. Its potential applications include household refrigerators and automo- bile air conditioning.
Freon-502 is an azeotropic mixture composed of 48.8 percent Freon-22 and 51.2 percent Freon-115 by weight. It boils at -50.1 degrees F. Because it achieves the capacity of Freon-22 with discharge temperatures comparable to Freon-12, it is finding new reciprocating compressor applications in low-temperature display cabinets and in storing and freezing of food.
Properties of Freons®
The Freon® refrigerants are colorless and almost odorless, and their boiling points vary over a wide range of temperatures. Those Freon® refrigerants that are produced are nontoxic, non-corrosive, nonirritating, and nonflammable under all conditions of usage. They are generally prepared by replacing chlorine or hy- drogen with fluorine. Chemically, Freon® refrigerants are inert and thermally stable up to temperatures far beyond conditions found in actual operation. However, Freon® is harmful when allowed to escape into the atmosphere. It can deplete the ozone layer and cause more harmful ultraviolet rays to reach the surface of the Earth.
The pressures required in liquefying the refrigerant vapor affect the design of the system. The refrigerat- ing effect and specific volume of the refrigerant vapor determine the compressor displacement. The heat of vaporization and specific volume of the liquid refrigerant affect the quantity of refrigerant to be circulated through the pressure-regulating valve or other system device.
Flammability
Freon® is nonflammable and noncombustible under conditions where appreciable quantities contact flame or hot metal surfaces. It requires an open flame at 1,382 degrees F to decompose the vapor. Even at this temperature, only the vapor decomposes to form hydrogen chloride and hydrogen fluoride, which are irritating but are readily dissolved in water. Air mixtures are not capable of burning and contain no elements that will support combustion. For this reason, Freon® is considered nonflammable.
Circulation
It should be noted that the Freon® refrigerants have relatively low heat values, but this must not be con- sidered a disadvantage. It simply means that a greater volume of liquid must be circulated per unit of time to produce the desired amount of refrigeration. It does not concern the amount of refrigerant in the system. Actually, it is a decided advantage (especially in the smaller or low-tonnage systems) to have a refrigerant with low heat values. This is because the larger quantity of liquid refrigerant to be metered through the liq- uid-regulating device will permit the use of more accurate and more positive operating and regulating mech- anisms of less sensitive and less critical adjustments. Table 12.1 lists the quantities of liquid refrigerant me- tered or circulated per minute under standard ton conditions.
For reason of compactness, cost of equipment, reduction of friction, and compressor speed, the volume of gas that must be compressed per unit of time for a given refrigerating effect generally should be as low as possible. Freon-12 has a relatively low volume displacement, which makes it suitable for use in reciprocat- ing compressors ranging from the smallest size up to 800-ton capacity, including compressors for household and commercial refrigeration. Freon-12 also permits the construction of compact rotary compressors in the commercial sizes. Generally, low-volume displacement (high-pressure) refrigerants are used in reciprocat- ing compressors; high-volume displacement (low-pressure) refrigerants are used in large-tonnage centrifu- gal compressors; intermediate-volume (intermediate-pressure) refrigerants are used in rotary compressors. There is no standard rule governing this usage.
Condensing Pressure
Condensing (high-side) pressure should be low to allow construction of lightweight equipment, which af- fects power consumption, compactness, and installation. High pressure increases the tendency toward leak- age on the low side as well as the high side when pressure is built up during idle periods. In addition, pres- sure is very important from the standpoint of toxicity and fire hazard.
In general, a low-volume displacement accompanies a high condensing pressure, and a compromise must usually be drawn between the two in selecting a refrigerant. Freon-12 presents a balance between volume displacement and condensing pressure. Extra-heavy construction is not required for this type of refrigerant, and so there is little or nothing to be gained from the standpoint of weight of equipment in using a lower- pressure refrigerant.
Evaporating Pressure
Evaporating (low-side) pressures above atmospheric are desirable to avoid leakage of moisture-laden air into the refrigerating systems and permit easier detection of leaks. This is especially important with open- type units. Air in the system will increase the head pressures, resulting in inefficient operations, and may ad- versely affect the lubricant. Moisture in the system will cause corrosion and may also freeze out and stop op- eration of the equipment.
In general, the higher the evaporating pressure, the higher the condensing pressure under a given set of temperatures. Therefore, to keep head pressures at a minimum and still have positive low-side pressures, the refrigerant selected should have a boiling point at atmospheric pressure as close as possible to the lowest tem- perature to be produced under ordinary operating conditions. Freon-12, with a boiling point of -21.7 de- grees F, is close to ideal in this respect for most refrigeration applications. A still lower boiling point is of some advantage only when lower operating temperatures are required.
Freezing Point
The freezing point of a refrigerant should be below any temperature that might be encountered in the sys- tem. The freezing point of all refrigerants, except water (32 degrees F) and carbon dioxide (-69.9 degrees F,
triple point), are far below the temperatures that might be encountered in their use. Freon-12 has a freezing point of -247 degrees F. (See Appendix A for more details on newer refrigerants.)
Critical Temperature
The critical temperature of a refrigerant is the highest temperature at which it can be condensed to a liq- uid, regardless of a higher pressure. It should be above the highest condensing temperature that might be encountered. With air-cooled condensers, in general, this would be above 130 degrees F. Loss of efficiency caused by superheating of the refrigerant vapor on compression and by throttling expansion of the liquid is greater when the critical temperature is low.
All common refrigerants have satisfactorily high critical temperatures except carbon dioxide (87.8 degrees F) and ethane (89.8 degrees F). These two refrigerants require condensers cooled to temperatures below their respective critical temperatures, and thus generally need water.
There are some hydrofluorocarbon refrigerants (such as R-134a) that are made to eliminate the problems with refrigerants in the atmosphere caused by leaks in systems. R-134a is a non-ozone-depleting refrigerant used in vehicle air-conditioning systems. DuPont’s brand name is Suva, and the product is produced in a plant located in Corpus Christi, Texas, as well in Chiba, Japan. According to DuPont’s Web site, R-134a was globally adopted by all vehicle manufacturers in the early 1990s as a replacement for CFC-12. The tran- sition to R-134a was completed by the mid-1990s for most major automobile manufacturers. Today, there are more than 300 million cars with air conditioners using the newer refrigerant.
Latent Heat of Evaporation
A refrigerant should have a high latent heat of evaporation per unit of weight so that the amount of re- frigerant circulated to produce a given refrigeration effect may be small. Latent heat is important when con- sidering its relationship to the volume of liquid required to be circulated. The net result is the refrigerating effect. Since other factors enter into this determination, they are discussed separately.
The refrigerant effect per pound of refrigerant under standard ton conditions determines the amount of refrigerant to be evaporated per minute. The refrigerating effect per pound is the difference in Btu con- tent of the saturated vapor leaving the evaporator (5 degrees F) and the liquid refrigerant just before pass- ing through the regulating valve (86 degrees F). While the Btu refrigerating effect per pound directly de- termines the number of pounds of refrigerant to be evaporated in a given length of time to produce the required results, it is much more important to consider the volume of the refrigerant vapor required rather than the weight of the liquid refrigerant. By considering the volume of refrigerant necessary to pro- duce standard ton conditions, it is possible to make a comparison between Freon-12 and other refriger- ants so as to provide for the reproportioning of the liquid orifice sizes in the regulating valves, sizes of liq- uid refrigerant lines, and so on.
A refrigerant must not be judged only by its refrigerating effect per pound; the volume per pound of the liquid refrigerant must also be taken into account to arrive at the volume of refrigerant to be vaporized. Al- though Freon-12 has relatively low refrigerating effect, this is not a disadvantage, because it merely indicates that more liquid refrigerant must be circulated to produce the desired amount of refrigeration. Actually, it is a decided advantage to circulate large quantities of liquid refrigerant, because the greater volumes re- quired will permit the use of less sensitive operating and regulating mechanisms with less critical adjustment.
Refrigerants with high Btu refrigerating effects are not always desirable, especially for household and small commercial installations because of the small amount of liquid refrigerant in the system and the diffi- culty encountered in accurately controlling its flow through the regulating valve. For household and small commercial systems, the adjustment of the regulating-valve orifice is most critical for refrigerants with high Btu values.
Specific Heat
A low specific heat of the liquid is desirable in a refrigerant. If the ratio of the latent heat to the specific heat of a liquid is low, a relatively high proportion of the latent heat may be used in lowering the temperature of the liquid from the condenser temperature to the evaporator temperature. This results in a small net refrigerating effect per pound of refrigerant circulated and, assuming other factors remain the same, reduces the capacity and lowers the efficiency. When the ratio is low, it is advantageous to precool the liquid before evaporation by heat interchange with the cool gases leaving the evaporator.
In the common type of refrigerating systems, expansion of the high-pressure liquid to a lower-pressure, lower-temperature vapor and liquid takes place through a throttling device such as an expansion valve. In this process, energy available from the expansion is not recovered as useful work. Since it performs no ex- ternal work, it reduces the net refrigerating effect.
Power Consumption
In a perfect system operating between 5- and -86-degree F conditions, 5.74 Btu is the maximum refrigeration obtainable per Btu of energy used to operate the refrigerating system. This is the theoretical maximum coefficient of performance on cycles of maximum efficiency (for example, the Carnet cycle). The minimum horsepower would be 0.821 horsepower/ton of refrigeration. The theoretical coefficient of performance would be the same for all refrigerants if they could be used on cycles of maximum efficiency.
However, because of engineering limitations, refrigerants are used on cycles with a theoretical maximum coefficient of performance of less than 5.74. The cycle most commonly used differs in its basic form from the Carnet cycle, as already explained, in employing expansion without loss or gain of heat from an outside source, and in compressing adiabatic ally (compression without gaining or losing heat to an outside source) until the gas is superheated above the condensing medium temperature. These two factors, both of which increase the power requirement, vary in importance with different refrigerants. But it so happens that when expansion loss is high, compression loss is generally low, and vice versa. All common refrigerants (except carbon dioxide and water) show about the same overall theoretical power requirement on a 5- to -86-degree F cycle. At least the theoretical differences are so small that other factors are more important in determin- ing the actual differences in efficiency.
The amount of work required to produce a given refrigerating effect increases as the temperature level to which the heat is pumped from the cold body is increased. Therefore, on a 5- to 86-degree F cycle, when gas is superheated above 86 degrees F on compression, efficiency is decreased and the power requirement increased unless the refrigerating effect caused by superheating is salvaged through the proper use of a heat interchanger.
Volume of Liquid Circulated
Volumes of liquid required to circulate for a given refrigerant effect should be low in order to avoid fluid- flow (pressure-drop) problems and to keep down the size of the required refrigerant change. In small-capac- ity machines, the volume of liquid circulated should not be so low as to present difficult problems in accu- rately controlling its flow through expansion valves or other types of liquid-metering devices.
With a given net refrigerating effect per pound, a high density of liquid is preferable to a low volume. However, a high density tends to increase the volume circulated by lowering the net refrigerating effect.