The decision to be made in the next several decades by designers and owners of industrial refrigeration plants in the choice of refrigerant is primarily between ammonia and HCFC-22. The decision may be a quick one if ammonia is not permitted at the location or its use is inadvisable. Codes in certain cities or counties may restrict the type of facility in which ammonia can be used. In general, its use is permissible in locations physically separated from spaces to which the general public has access. Even if no codes legislate against it, placing a large ammonia system close to a school, hospital, or similarly occupied building would not be prudent. Progress is being made in the development of packaged ammonia liquid chillers with low refrigerant charges that open opportunities for applications not previously considered, such as water chillers for air conditioning.
The first point of comparison will be cost. The prices of refrigerants fluctuate and are a function of the quantity bought, but a rough comparison on a mass basis of ammonia, HCFC-22, and HFC-134a is shown in Figure 1.9. The costs are relative to that of HCFC-22 and show ammonia with the lowest cost and HFC-134a the highest. The cost comparison becomes a factor in plants containing tens of thousands of kilograms or pounds. The cost comparison is even more striking when it is realized that to fill a plant, a certain amount of volume (not mass) of liquid must be supplied. Because the density of liquid ammonia is about half that of the halocarbons, only half the mass needs to be purchased.
Table 12.5 showed comparisons of mass and volume flow rates for HFC-134a, HCFC-22, R-507, and ammonia. The volume flow rates that must be handled by the compressor in systems of the latter three refrigerants are comparable, so there is no advantage for one of the refrigerants. The mass flow rate for a given refrigerating capacity of ammonia is 1/7 that of HCFC-22, and this comparison exerts itself both in required pipe sizes and in requirements for liquid recirculating systems. Only 1/7 of the liquid need be pumped for a given refrigerating capacity, which means that the liquid pump can be much smaller and pumping power will be much smaller in an ammonia system.
Table 12.5 also showed that the volume flow rate in the suction lines is comparable for HCFC-22, R-507, and ammonia, which may suggest that the pipe sizes for low-pressure vapor should be of the same order of magnitude. Another factor is concealed, however. The principal criterion in selecting vapor
line sizes is the drop in saturation temperature. Table 12.12 compares the refrigeration capacities for several different pipe sizes and drops in saturation temperature for HCFC-22 and ammonia. This advantage for ammonia shows itself particularly in industrial facilities spread over a large area where the suction lines are long. Either the pipe size for ammonia could be smaller than that for HCFC-22 or for a given pipe size the pressure drop penalty will be less.
Ammonia is more tolerant of water that might inadvertently get into the system. In most halocarbon systems, water remains separate from the refrigerant and could freeze, especially immediately after an expansion valve. With ammonia, however, water stays in solution and contaminations less than 100 ppm, for example, cause no penalty to system operation.
Ammonia enjoys higher heat-transfer coefficients than HCFC-22, primarily because most of the thermodynamic and transport properties that affect heat transfer are favorable to ammonia. The ratios of these properties for ammonia relative to HCFC-22 are the following.
• Specific heat of liquid and vapor, 4 to 1
• Latent heat of vaporization, 6 to 1
• Liquid thermal conductivity, 5.5 to 1
• Viscosities, 0.8 to 1
• Liquid density, 0.5 to 1
A survey conducted among designers of heat exchangers who work with both ammonia and HCFC-22, as shown in Table 12.13, indicates that heat-transfer coefficients used in industrial practice are two to three times higher for ammonia.
It is fair to point out by way of additional perspective that the copper tubes used for halocarbon refrigerants lend themselves conveniently to heat-transfer enhancement, so that the values that apply to bare tubes shown in Table 12.13 for HCFC-22 can be improved. On the other hand, research results are showing that heat-transfer enhancement is possible with the steel or aluminum tubes used in ammonia heat exchangers as well. So far, the possibilities have not generally been commercialized.