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Heat recovery

 

Heat recovery is utilization of thermal energy that is normally rejected from the system.

In industrial refrigeration, thermal energy of hot gas can be used for water heating, glycol under floor heating, space (docks, coolers) heating, hot gas defrosting and etc. If this wasted heat can be put to use, it is possible to improve the over all efficiency of the facility because less heat will be required from primary energy sources (natural gas or electricity) used to generate heat.

To evaluate the viability of heat recovery in industrial refrigeration plants, quantity and quality of the heat need to be considered.

Heat quantity is how much heat is available.

Heat quality is a temperature of available heat.

A common misunderstanding with heat recovery in industrial ammonia refrigeration is the belief that, because discharge temperature is high, a lot of heat is available. However, reality is that, although discharge temperature can be high, the quantity of high quality heat is relatively low.

Assume that temperature of discharge gas is 180 degF. Condensing temperature is 80 degF (138 psig). First step of heat recovery is desuperheating. It means reduction of gas temperature from 180 degF to 80 degF. Unfortunately, this high quality (temperature) heat is about 10% of total heat available. Second step of heat recovery is ammonia condensing. This heat has relatively low temperature of 80 degF, but quantity of condensing heat is 90% of total heat. It means that we have a lot of low temperature heat (condensing) and limited quantity of high temperature heat (desuperheating) is available. Typically, heat recovery project can be viable when condensation can be used. To use condensation, low temperature (60 degF or lower) medium (air, water, glycol) should be heated.

However, the decision of how and even whether to use heat recovery can be a tricky one that is based on relative fuel cost, how much of a capital investment is required, how much operating cost will be saved, and whether the timing of the availability and requirement of the heat is coincident.

 

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