About the authorHeat Recovery Methods
I often see people interested in heat recovery asking questions about desuperheating. This mode of heat recovery is based on circulating a fluid such as water through a heat exchanger to aid in transferring heat from one location to another. One of the major issues with utilizing the superheat of the discharge gas from a refrigeration systems is; there is not a lot of available heat! The available superheat in the discharge gas from a compressor is only a small percentage of the the total available heat energy.
It is helpful to think of superheat as temperature (what I call quality of heat). Having a high temperature available such as the discharge gas stream seems to imply a lot of heating potential. While a high temperature may exist when ammonia is utilized as the refrigerant the fraction of high temperature heat is low when compared to the total quantity of heat energy available if condensing is considered.
Desuperheating is a sensible heat transfer process. You are only transferring sensible (heat you can feel) energy to the fluid being circulated. Once the superheat is reduced to a limit dictated by the heat exchanger used no addition heat is transferred. Therefore, having a small percentage of available heat at high temperature will limit how much heat you can recover and re-use.
Alternative methods may use the latent heat of condensation, which is a phase change. Being able to recover the larger fraction of low-grade heat may provide the ability to heat a larger volume of fluid in circulation, however this would be performed at a much lower temperature. In many cases this lower temperature will approximate the condensing temperature of the refrigerant at the expected operating conditions. It would certainly seem reasonable at this point to take fluid warmed up and then use the superheat to actually increase the fluids temperature above the condensing temperature of the refrigeration system.
In some instances, the use of industrial heat pumps can amplify the temperature to provide a more useful heating source. In other circumstances it might be prudent to utilize refrigerant as the heat transfer fluid.
Remember, using a sensible heat transfer process requires a larger heat exchanger than a heat exchanger sized for a phase change heat transfer process. The differences in film coefficients between sensible and phase change heat transfer can be substantial.
When you approach a heat transfer process for heat recovery, be sure to investigate the actual process requirements, not just the equipment that seems to be popular.
Tags: condensing temperature, discharge pressure, energy conservation, energy reduction, System design, vapor condensation
