Subcooler

A subcooler is a heat exchanger used in the circuits of heat pumps and refrigeration systems. It cools the already condensed working fluid (refrigerant) below its condensation temperature before it expands at the expansion valve and subsequently evaporates again. In industrial high-temperature heat pumps, the subcooler increases the amount of usable heat delivered by the system, for example for preheating purposes, thereby improving the coefficient of performance (COP).

Principle of operation in the cycle

After condensation in the condenser, the refrigerant is present as a high-pressure liquid at a temperature that still allows further heat recovery. In the subcooler, this liquid stream is cooled further so that, for example, feedwater for steam generation can be preheated. An additional benefit arises in the evaporator: because the refrigerant enters the evaporator at a lower temperature, it can absorb more heat from the source, thus increasing the evaporator capacity. This measure enhances the COP of refrigeration and heat pump systems.

Many systems use a subcooler for so-called economizer operation: a small partial stream of the refrigerant is taken from the condenser, expanded to an intermediate pressure level, evaporated in the subcooler, and injected into the compressor for intermediate cooling. This reduces the compressor’s thermal load and improves not only its efficiency but also the overall COP of the system. The use of an economizer is particularly beneficial at large temperature lifts.

Heat integration in practice

In process heat applications, the heat released during subcooling is typically used to preheat feedwater or the return flow of the heat sink (i.e. the fluid entering the heat pump). This improves the COP and thus the overall economic efficiency of the system. For high-temperature heat pumps supplying process heat up to approximately 150 – 200 °C, this additional heat utilization is particularly effective at low return temperatures, i.e. when there is a large temperature difference between return and supply.

Effects on efficiency and operation

• Higher coefficient of performance (COP): Subcooling increases both the usable evaporator capacity and the effective heating capacity of the heat pump.
• Operational reliability: A stable liquid feed to the expansion valve protects it from vapor bubbles and ensures consistent control behaviour.

Design and limitations

The benefit of a subcooler depends on the boundary conditions of the heat source and sink, the refrigerant used, and the design of the heat exchanger. In cases of small temperature differences or unfavourable hydraulic conditions, the additional effort may outweigh the gains. In high-temperature processes, material selection, pressure levels, and thermomechanical stresses are key factors for successful integration.

Overall, a subcooler can be an effective key component in industrial high-temperature heat pumps. It enhances cycle efficiency, enables extended heat recovery, and thus supports a resource-efficient and economically viable supply of process heat.