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R-22 Hard Act to Follow: Ammonia Low-Pressure Receiver Systems

 

©2013 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 55, no. 2, February 2013.

This article is based on a presentation at the 2012 IIAR Industrial Refrigeration Conference. The paper received the Andy Ammonia Award for best paper at the conference.

By S. Forbes Pearson, Ph.D.

About the Author
S. Forbes Pearson, Ph.D., is the president of Star Refrigeration in Glasgow, U.K., and is past president of the Institute of Refrigeration in Carshalton, Surrey, U.K.

 
R-22 is a hard act to follow. Significant disadvantages are associated with all possible substitutes. Some provide less capacity, some have lower critical temperatures (and will tend to be less efficient in use), some have higher global warming potentials (GWPs), some are flammable, some are toxic, and some are inherently expensive to produce.

Ammonia can be applied to any type and size of refrigeration system from domestic refrigeration to commercial refrigeration and air-conditioning to industrial refrigeration. However, it is desirable to minimize the charge to prevent toxic effects and, sometimes to use secondary refrigerants, such as volatile carbon dioxide, to keep ammonia away from the general public.

This article describes a method by which the toxic refrigerant ammonia can be used in low-charge automatic systems with maximum efficiency and minimum risk.
 

Possible Alternatives to R-22

Phaseout of R-22 is taking place under the Montreal Protocol because R-22 contains chlorine, which can damage the ozone layer. However, the ozone depletion potential (ODP) of R-22 is only 0.055, which probably would not have had significant long-term effects on the ozone layer if R-22 had been effectively confined within the systems in which it is used. The main damage to the atmosphere associated with R-22 has resulted from a by-product of the production of R-22, namely R-23, which has a very high GWP of 12,000. Recently, responsible producers of R-22 have been destroying their unwanted R-23, but in the past much of it was vented to atmosphere.

Some hydrocarbons are good substitutes for R-22, but for code and safety reasons they should be limited to small, fully sealed systems.

A recently promoted family of refrigerants, the hydrofluoroolefins (HFOs), has zero ODP and very low GWP. However, these refrigerants are not yet in full commercial production, and they will be very expensive to produce. It is unlikely that HFOs will develop into universal replacements for the once ubiquitous R-22.

Carbon dioxide has been used in European supermarkets to a significant extent, but it is inherently inefficient in use compared to R-22 except when used in cascade with another refrigerant. Subcritical carbon dioxide systems are less efficient than ammonia systems, and transcritical systems are even less efficient. It will be difficult to justify the use of carbon dioxide as a general substitute for R-22 except in cool and temperate regions.

Table 1 compares some physical properties of R-22 with properties of other refrigerants that might be considered as a substitute. All the refrigerants in the table are non-flammable except for ammonia, which has a minimal fire risk when used in small systems. R-23 is included for comparison although it would not be a credible substitute.

The table shows that ammonia (R-717) is the only substitute for R-22, having zero GWP. Ammonia also has a high critical temperature that tends to make systems using ammonia more efficient than systems using other refrigerants. Despite their very different properties, the volumes of vapor to be pumped to produce similar amounts of refrigeration are similar for ammonia and for R-22. This is not the case with the widely used R-134a that requires about 30% more swept volume and has significant global warming potential.
 

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