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View Subscription Details: Letter ( 08/28/2012 ) - HTML Format

Ammonia heat pumps


In previous newsletter I showed that wasted heat of the typical ammonia refrigeration plant can be reclaimed to heat media (air, water, and glycol) up to 60 degF. However, many food and beverage processing facilities require water with temperature higher that 60 degF to provide production and sanitation needs. These companies are paying for the electrical energy to remove heat from their refrigerated spaces via ammonia refrigeration systems and rejecting this heat to the atmosphere. Additionally, they are paying for the natural gas to add heat to the hot water.

Due to recent high pressure compressors development, rejected heat could be captured and used to provide water heating and substantial amount of energy would be saved. The highest pressure and temperatures in the refrigeration system (compressor discharge gas) provided an optimal source for heat to be transferred to the water. Discharged gas of the refrigeration system, at relatively high pressures of 100 - 180 psig, can be fed directly into the suction of heat pump compressor and compressed to even higher pressures of 450 - 800 psig. Condensing this higher pressure ammonia with cool water in a heat exchanger would capture much larger quantities of heat energy than heat reclaim and enable the full temperature elevation of the cool water from 60 degF to 145 degF. Typically, employment of such an ammonia heat pump system would require the use of a secondary loop, there reducing the efficiency of heat transfer.

The coefficient of performance (COP) is the ratio created by dividing beneficial energy supplied to the system by the energy required to develop the benefit. In fossil fuel fired heater or electrical heater the COP can never be more that one and is less that one due to heat loss to the surroundings. An ammonia heat pump operated at mentioned conditions has COP of 4 - 6. It means that pump uses 1 unit of energy and it releases 4 - 6 units of energy to the heating water. This is a major benefit of the ammonia heat pump.

When added to the existing refrigeration system, the ammonia refrigeration pump system acts as a condenser. Compressor discharge gas is diverted before reaching the evaporative condensers, is routed to the heat pump system, and returned as condensed liquid. Additional condensing capacity of the heat pump would reduce the condensing pressure of the existing refrigeration system, or if operated at minimum allowable condensing pressure (winter operation), would offset the operation of the equivalent capacity of the existing condensers. Energy use of the existing refrigeration system would be reduced. This is the second benefit of ammonia heat pump.

I think that many companies should consider investments in ammonia heat pumps and payback of these investments can be attractive.   



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