Getting the most from a Heat Pump Heat Exchanger

A common mistake made when specifying and selecting heat exchangers to be used as circuit breakers between a heat pump and a building is as follows.

Example:

16 kw heat pump

Heat pump providing a hot fluid at 55 deg flow and 35 deg C return.

Needs to heat cold water in the heating loop from 10 up to 53 deg C where 53 C is the design temperature that the building radiators have been sized upon.

To size a heat exchanger correctly for use with a heat pump it is important to base the selection upon the final “steady state” running conditions and not the start up conditions as given in the example above.

If the heat exchanger is selected on the start-up temperatures, the Log Mean Temperature Difference (LMTD for short) is large resulting in a smaller heat exchanger. A reduced size heat exchanger may be desirable in terms of price and space needed, but when the cold water is gradually heated as it is recirculated around the heating loop, the once large LMTD reduces in value resulting in the need for a larger heat exchanger to transfer all of the energy from the heat pump ( a lowering of the LMTD value requires more heat transfer surface area to pass across the same amount of heat)

Selecting the heat exchanger on the figures given in the above example will result in a heat exchanger that transfers 16 kw only when the water is at 10 deg C. and not when the water is at any higher temperature.

Therefore, it is important to determine the likely steady state running condition and then use temperatures in the selection of the exchanger to counter-act this loss of heat exchanger performance as LMTD reduces.

The temperature figures given below shall ensure the heat exchanger has enough surface area to transfer 16 kw at the time when needed most (this being at the steady state running conditions that the building heating appliances have been selected upon to tie in with the building heating demands)

16 kw heat pump

Heat pump providing a hot fluid at 55 deg flow and 35 deg C return.

Heating warm water from 33 up to 53 deg C

Using the correct temperatures in the heat exchanger selection will help avoid reports of a disappointing performance from a well installed heat pump.