Generation of thermal energy for heating and cooling often has a negative environmental impact due to the fuels used. The need for such fuels decreases with installation of heat pumps. But the heat pump in itself is a potential source of greenhouse gases, as the refrigerants used often have a high global warming potential, GWP. This is regulated in the Kigali amendment to the Montreal protocol, and the heat pumping industry is searching for alternatives. Two major challenges regarding this is that many of the alternatives are either expensive or toxic. Therefore, the development of heat pumps must be in the direction of high efficiency, and low need for refrigerant.
The heat exchangers in industrial heat pumps are divided into two groups, based on the technique that they are using: “shell and tube” or “plate”. These groups then include various types of heat exchangers, each with their own pros and cons regarding characteristics such as size, geometry, and area of use. This article has predominantly looked at the difference regarding refrigeration charge per kilowatt, i.e., how much refrigerant that is needed for a certain thermal output.
Four types of heat pumps were compared under the same conditions: 1000 kW evaporator capacity, ammonia with saturated temperature of 2 °C as refrigerant, and the same inlet and outlet temperatures. The comparison showed that there is a large difference in charge per kW between the heat pumps. The one that required the lowest amount of refrigerant, 46 kg, was the DX-Shell type, which is the latest innovation. Some positive characteristics with this type are a simple control system and the possibility of cleaning of the tubes.
The ratio between the DX-Shell type and the one with the highest need of refrigerant per kW is more than a factor four: the PHE type needed as much as 209 kg to reach the same level. Additionally, their costs were compared. The DX-Shell type is 12% more expensive than the PHE type. Which, on the other hand, is 3% more expensive than the cheapest one, the DX-tube type, but then the refrigerant charge is much lower, at 65 kg. Thus, apart from the various types being relevant for various implementations, in the choice there is also a trade-off between purchase cost and refrigerant charge.
Zahid Ayub, Ph.D., P.E., USA (Isotherm)
Adnan Ayub, D.Eng, P.E., USA (Isotherm)
The text is shortened by the HPC team