Alternative Refrigerants

Both low-ozone depleting and non-ozone depleting refrigerants are used as alternatives to CFCs. Low-ozone depleting refrigerants include HCFC refrigerants such as HCFC-22 and HCFC-123, which are referred to as bridge refrigerants because of the longer timetable governing their own phase-out. Other alternatives for vapor compression-based refrigeration systems include HFCs, notably HFC-134a, which has no chlorine atoms and no ozone depleting characteristics, and ammonia. Other alternatives include use of the two absorption-cycle refrigerants: water and ammonia-water solution.

Refrigerants such as HCFC-22, HCFC-123, and HFC-134a and other newly developed refrigerant blends can replace CFCs in original or slightly modified equipment. HCFC-22 is a refrigerant commonly used in systems with reciprocating and screw compressors. Production of large capacity systems that use HCFC-22 has also increased rapidly over the past several years. Some centrifugal compressor-based systems are designed to use HCFC-22, HCFC-123, or HFC-134a. Lower pressure centrifugal compressors are now replacing R-11 (CFC-11) with HCFC-123. Mid-sized systems previously operating on R-12 (CFC-12) now generally use HFC-134a.

There are no perfect drop-in replacements refrigerants and retrofit applications are generally more problematic than new equipment applications. Many systems, particularly hermetic designs, will be costly to retrofit, requiring new gaskets, seals, and possibly motors. Others simply cannot be retrofitted. However, with newly developed optimizing equipment and some hardware modifications, many systems can be retrofitted with a relatively straightforward, but lengthy (2 to 4 week), procedure. This is often done during scheduled overhauls.

The result of using replacement refrigerants is generally a loss of efficiency and cooling capacity output. This may be somewhat counteracted by efficiency and capacity increases achieved during traditional overhaul procedures. To compensate for losses, new equipment utilizing improved heat transfer surfaces and more efficient compressors have been developed. Computer models of engineering conversion techniques are also used to design optimal conversion systems which minimize losses.

When HFC-134a is used as a substitute for CFC-12, significant capacity reductions may result unless a gear set (or, in some cases, a motor change) is included as part of the retrofit. When replacing CFC-500, no hardware changes are necessary, but capacity can be reduced by 8 to 10%. HCFC-123 can be used to replace CFC-11, but may experience efficiency losses of up to 5% and capacity losses of up to 15%. Losses can be minimized by varying the impeller size or by over-sizing the compressor.

Retrofitting can be quite expensive and must be weighed against the cost of equipment replacement. In a chiller retrofit, typical conversion costs range from 30 to 80% of the cost of replacement. Replacement is more likely to be selected when existing equipment is fairly old, has already experienced significant efficiency losses, or where

Table 18-2 Ozone Depletion Potential, Global Warming Potential, and Atmospheric Lifetime of Various CFC, HCFC and HFC Refrigerants

Refrigerant

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