©2012 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 54, no. 10, October 2012
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.
As refrigerant charge increases, more onerous legal requirements are being placed on owners and operators of refrigerating systems using ammonia. In the U.S., facilities containing 500 lb (227 kg) of ammonia or more must be reported to the local emergency planning committee. Facilities containing more than a threshold quantity (TQ) of ammonia, in addition, must submit a risk management plan to the U.S. Environmental Protection Agency. In California the TQ is only 500 lb (227 kg), but, generally, it is on the order of 10,000 lb (4536 kg). For all ammonia refrigerating systems, irrespective of charge, losses of more than 100 lb (45 kg) of ammonia must be reported to the National Response Center within 15 minutes of the person in command of the facility being made aware of the loss.
Ammonia is not the only refrigerant under legal pressure. There is also an increasing tendency to prosecute and fine companies found guilty of negligently releasing fluorocarbon refrigerants to the atmosphere. Recent problems have occurred in the bakery industry, which continues to use R-22, and a West Coast fish processing factory was prosecuted for continuing releases of R-22. It is obvious that the possibility of a large leak of any type of refrigerant diminishes as the charge is reduced.
This article shows some ways refrigerant charge may be reduced without significant penalty in terms of efficiency.
Secondary refrigerants have been used since the earliest days of mechanical refrigeration. Reducing the primary refrigerant charge is usually achieved at the cost of reduced efficiency because of the extra temperature differences required to transfer heat through the secondary refrigerant. Secondary refrigerants went out of favor when non-toxic, non-flammable halo-carbon refrigerants became available. However, the current threat to halo-carbon refrigerants because of their effect on the environment has led to a return to using secondary refrigerants.
Extensive use has been made of various secondary refrigerants including glycols and organic salt solutions, but the author proposed a better method using carbon dioxide in 1993. Since that time there has been a return to using carbon dioxide both as a volatile secondary refrigerant and as the low-temperature stage in cascade systems.
Ammonia is often used in the high-temperature stage, but other refrigerants have also been used.
Carbon dioxide has remarkable properties as a heat transferring fluid as pointed out by Hans Quack. Figure 1 shows the contrasting effectiveness of carbon dioxide, R-134a and water when used as secondary refrigerants. Andy Pearson, modified the Quack Factor to take account of equivalent pressure drop effects, emphasizing the advantages of carbon dioxide. Carbon dioxide is significantly better than any other fluid in this respect.
A large portion of supermarkets in Europe now use carbon dioxide as the primary and secondary refrigerant of choice. The most common arrangement consists of a transcritical system for the high-temperature duties with additional carbon dioxide compressors being used to produce refrigeration at lower temperatures. To avoid difficulties with lubricants, the two systems are often kept separate with the low-temperature carbon dioxide refrigerant being condensed by heat exchange with an evaporator of the high-pressure system.
When used as a volatile secondary refrigerant, it has been found that system efficiency is comparable with system efficiency of direct expansion halocarbon systems. There is not yet consensus as to the most suitable primary refrigerant. Systems have been installed using R-290 (propane), R-404A and R-717 (ammonia). Primary refrigerant charge can be reduced by a factor of 50 or more.
Pump power required to circulate carbon dioxide as a volatile secondary refrigerant is about 5% of the power that would be required to circulate a non-volatile secondary refrigerant such as water or propylene glycol.
Carbon dioxide has also been used to reduce refrigerant charge in industrial refrigerating systems in the U.S., Japan and in Europe where the technique was pioneered in 1993 by Nestle for the freeze drying of coffee. Carbon dioxide cascade systems would appear to have become the de facto standard for this application worldwide.
Carbon dioxide cascade systems have also been used for low-temperature cold stores and freezing systems. At some evaporating temperature between –31°F and –40°F (–35°C and –40°C), the cascade system becomes more efficient than a two-stage ammonia system. This is because of the relatively poor performance of ammonia as a refrigerant at low evaporating temperatures.
Risk of ammonia leakage into production areas is also minimized because the ammonia system is confined to the open air and to a special machinery room.
Citation: ASHRAE Journal, vol. 54, no. 3, October 2012
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