In many ways, chlorodifluoromethane (R22) is a near-ideal refrigerant. Relatively low temperatures are achievable with systems operating above atmospheric pressure, discharge pressures are low, condensation temperatures are high, power-to-heat load ratios are low, and personnel-exposure problems and toxicity also are low. Not surprisingly, R22 systems of varying sizes are used in a multitude of process plants and myriad other applications.
Unfortunately, however, R22 also is an ozone-depleting substance and its production and use will be phased out. So, it will be necessary to switch from R22 to another refrigerant. This is not an option, but a requirement mandated by the Montreal Protocol. Manufacture of R22 is to stop in 2010, while use of existing stocks in developed countries may continue until 2030. Some nations (Germany among others) have already banned the use of R22 or have accelerated the restrictions on production or use.
Given this mandate, R22 has not been an attractive choice for new refrigeration systems for at least five years. Nevertheless, a great many legacy systems remain in use. If the earlier phase-out of R12 is any guide, shortly after manufacture of R22 finally halts, its price will quickly escalate.
The projected rapidly rising costs, coupled with the community relations benefits of a proactive move to a more environmentally friendly refrigerant, are increasingly pushing many sites to consider an early switchover. Proper planning for the switch can save a lot of money — for instance, by enabling changes to be scheduled at the same time as other plant work rather than on a “must do” crisis basis.
No alternative is a full drop-in replacement for R22. All choices require equipment modification or system operating changes. The closest halogenated replacements include R407C and R417. Specific users may find no straightforward replacement that fits within their equipment and use constraints. In these cases, nonhalogenated refrigerants should be considered. Table 1 compares the properties of R22 to nonhalogenated replacement alternatives for industrial users in existing systems: ammonia and propane. Additionally, the table includes two often-overlooked alternatives: chemical-grade propylene and refinery-grade propylene (both of which are propane/propylene mixtures).
Common refrigerants
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R22
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R717
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R290
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R1270-R290
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R1270-R290
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Material
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Chlorodifluoromethane
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Ammonia
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Propane
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Refinery-grade propylene
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Chemical-grade propylene
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Composition
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100%
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100%
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100%
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70% propylene,
30% propane
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95% propylene,
5% propane
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Molecular weight
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86.5
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17.0
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44.1
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42.7
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42.4
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Saturated temperature
at 0 psig, ºF
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-41.4
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-28.0
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-43.7
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-51.8
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-53.5
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Saturated pressure
at -40ºF, psig
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0.6
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-4.3
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1.4
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4.8
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5.7
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Condensation pressure at 100ºF, psig
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196.0
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196.7
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175.0
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205.5
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213.0
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Condensation pressure
at 68°F, psig
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117.3
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109.3
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106.9
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127.7
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127.7
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Compared to an R22 system at fixed duty.
Operation at -40ºF evaporation and 68ºF condensation
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Power required*
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1.000
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0.982
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1.040
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1.024
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0.998
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Suction volume*
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1.000
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1.120
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1.097
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1.174
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1.071
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Mass flow*
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1.000
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0.147
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0.592
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0.583
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0.573
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Compression ratio
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8.65
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11.95
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7.55
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7.30
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6.98
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Issues
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Toxicity
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Lubricant
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Lubricant
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Lubricant
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|
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Composition
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Composition
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Composition
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|
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Ignition
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Availability
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Availability
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Ignition
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Ignition
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* Ratio to R22 system.
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While no replacement refrigerant is ideal everywhere, each option has particular situations where it generally is preferred:
Ammonia — for systems that do not require a cooling medium below -28ºF, have room for the vapor-handling capacity and sufficient compression ratio. Ammonia’s low molecular weight limits compression ratios available in turbocompressor-driven systems.
Propane — for systems with some extra power available and a small amount of surplus vapor-handling capacity. Lubricant separation from propane requires larger lubricant knockout drums than with R22 systems.
Propane/propylene mixtures — for systems that have excess vapor-handling capacity, can tolerate higher discharge pressures, and would benefit from cooler refrigerant temperatures. Propane/propylene mixtures have similar lubricant separation concerns as pure propane systems.Of course, individual circumstances and other technical and economic factors will dictate actual selection. Major factors to consider include lubricant systems, safety and availability. Propane/propylene mixtures, in particular, can be difficult to get in some areas and of varying composition depending upon the supplier.
