Efficiency and low cost are the major reasons ammonia is the preferred industrial refrigerant in the food processing industry. Ammonia is often a natural refrigerant and has no ODP/GWP. However, ammonia is a dangerous substance and there are several advantages to lowering the overall amount of ammonia used at a facility.
To avoid these risks and increase safety, many plant owners are looking at ways to reduce their ammonia charge. Two of the most common approaches include:
Limiting ammonia charge through a cascade system — Most refrigeration systems use only one refrigerant in the system. In a cascade system, two refrigerants are used with a heat exchanger in the middle. This allows the engineer to take advantage of the best characteristics of each. At lower temperatures, generally between –31°F and –60°F, a NH3/CO2 cascade system becomes more efficient than a two-stage ammonia system, As with any cascade system, the charge of each refrigerant will be reduced. There is also the option to design this process so that ammonia is confined to the machine room to increase safety throughout the rest of the plant.
Limiting ammonia charge through use of a secondary refrigerant — Typically, ammonia is piped from the machine room out to the facility to all points of use. By incorporating a secondary refrigerant, ammonia is limited to the machine room and the secondary refrigerant is piped to the points of use. This reduces the total quantity of ammonia required for a system. It also limits ammonia to the confines of the machine room where there are measures to deal with an accident. Compared to a conventional system, it greatly reduces the probability that a facility worker would be exposed to an ammonia leak inside the facility or on the roof.
A glycol/brine solution is often used as a secondary refrigerant for temperatures above 0°F. When temperatures fall below that level, special heat transfer fluids are required. While most secondary refrigerants only transfer sensible heat, CO2 can be used as a volatile brine. This allows much higher heat transfer, dramatically reducing pipe size and pumping horsepower. Because a heat exchanger is required to go from the ammonia to the secondary refrigerant, there is decreased efficiency. In addition, the pumping horsepower associated with a secondary refrigerant will be higher than directly distributing ammonia.
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