Modelling of Ammonia Heat Pump Desuperheaters

This paper presents a study of modelling desuperheating in ammonia heat pumps. Focus is on the temperature profile of the superheated refrigerant. Typically, the surface area of a heat exchanger is estimated using the Log Mean Temperature Difference (LMTD) method. The assumption of this method is that the specific heat is constant throughout the temperature glide of the refrigerant in the heat exchanger. However, considering ammonia as refrigerant, the LMTD method does not give accurate results due to significant variations of the specific heat. By comparing the actual temperature profiles from a one-dimensional discretized model with the LMTD, it is found that the LMTD method provides a higher temperature difference than the discretised model, and would therefore lead to an underestimation of the needed condenser area. The lower temperature difference in the discretized model can be compensated for in two ways. The area of the heat exchanger can be increased or the condensation temperature can be raised to achieve the same temperature difference for the discretized model as for the LMTD. This would affect the compressor work, hence the COP of the system. Furthermore, for higher condenser pressure, and thus higher pressure in the desuperheater, a larger deviation between the two temperature difference models is observed. Using the discretization model the number of discretizations to get accurate estimates is found to be 20.