How to calculate industrial heat-pump temperature lift

Use refrigerant-side temperatures

Process lift is sink supply temperature minus source temperature. The compressor experiences a larger lift because the refrigerant must evaporate below the source stream and condense above the sink stream. Heat-exchanger approach temperatures therefore belong in the calculation.

Worked screening example

Suppose wastewater cools from 45°C to 35°C and a hot-water loop returns at 60°C and leaves at 80°C. With a 5 K cold-side approach, a first evaporating estimate is 30°C. With a 5 K hot-side approach, a first condensing estimate is 85°C. The refrigerant temperature lift is therefore about 55 K, not the 35 K suggested by comparing 45°C with 80°C.

What the number does and does not say

A smaller lift usually supports a higher COP, but lift alone does not select a machine. Refrigerant properties, compressor envelope, glide, pressure ratio, source cooling and sink heating profile all matter. Request a manufacturer map for the full duty.

Approaches add on both sides

If a source leaves an intermediate loop at 35°C and the refrigerant needs a 5 K approach, a first evaporating estimate is 30°C. If the process needs 80°C water and the condenser needs a 5 K approach, a first condensing estimate is 85°C. The cycle lift is about 55 K.

Adding another intermediate exchanger can widen the lift again. This may be necessary for hygiene, corrosion or refrigerant-charge separation, but the performance effect belongs in the model.

Use profiles rather than one difference

A source may cool through the evaporator while a sink heats through the condenser. The temperature difference changes along both exchangers. A single inlet-to-outlet subtraction can miss the closest approach and the actual refrigerant conditions.

Gliding refrigerants and multistage cycles add further detail. Early screening can use conservative fixed approaches, while concept design should use the supplier's exchanger and cycle calculation.

Reduce lift before buying efficiency

Lower process return temperature, larger exchanger area, source segregation and direct preheating can reduce compressor lift. These changes may increase usable capacity as well as COP.

Compare process modifications with a larger machine on a whole-system basis. A modest change to a water loop can be more valuable than selecting equipment around an avoidably high supply temperature.