In order to decrease the use of refrigerants in heat pumps with high global warming potential, new ways are sought. There are basically two ways to go: reduce the amount of refrigerant used, or switch to refrigerants with lower GWP. To this end, a new type of evaporator has been tested, where a lower refrigerant charge is possible. It is called falling film. Various units have been tested with two different types of refrigerant.

The traditional equipment is a flooded evaporation chiller, and this is compared to three set-ups of falling film evaporation chillers, where the type of refrigerant, refrigerant charge, and the tube pass arrangement (bottom-up or top-down) were altered. They were tested under the same conditions, with e.g. flow rates and temperatures kept the same.

From the testing, some conclusions could be drawn. It was clear that with increasing refrigerant charge, certain critical characteristics were improved. For both types of tube pass arrangements, the cooling capacity, the coefficient of performance (COP) and the evaporation temperature increased. All three characteristics increased more in the beginning and then grew more slowly, and eventually not at all.

Another conclusion is that, for all characteristics, the performance is higher for the bottom-to-top tube pass arrangement. For the cooling capacity, the difference is relatively similar independent of refrigerant charge, while for the COP the difference decreases with increased charge. For the evaporation temperature, the pattern is not as stable. It seems possible to reduce the refrigerant charge with as much as one third (34%) if a falling film evaporation chiller with a bottom-to-top tube pass arrangement is used, instead of the traditional flooded evaporation chiller. This set-up is thus preferable form the global warming perspective.

Also, different refrigerants were tested with the falling film evaporation chillers: R1233zd(E) and R134a. The characteristics proved to depend also on this choice. For the refrigerant charge, the former refrigerant showed a lower cooling capacity but also a higher COP and evaporation temperature.

Thus, the use of falling film evaporation chiller seems to be a way to go in order to decrease the amount of refrigerant used, compared to the flooded evaporation chiller. And for them, between the tube pass arrangements, the bottom-to-top alternative showed better performance.

Hua Liu, China
Dongbing Hu, China
Sheng Wang, China
Ying Zhang, China


The text is shortened by the HPC team

Read the full article here
Read the full HPT Magazine here.