Machine Design

Hotter Heat Exchangers

Air-based heat pumps typically deliver 1.5 to 3 times more heat than the electric energy they consume.

This is possible because heat pumps move heat rather than generate it from fuel. National Institute of Standards and Technology (NIST) researchers are working to make heat pumps even better by giving engineers computer-based tools to design heat exchangers.

In a typical air-based heat pump, air flows over two refrigerant-filled heat exchangers (coils). One is indoors and the other outdoors. Both have metal fins to aid heat transfer. In heating mode, liquid refrigerant in the outside coil pulls heat from the air and evaporates the refrigerant into a gas. The indoor coil releases heat from the refrigerant as it condenses back into a liquid. A valve near the compressor changes the direction of refrigerant flow for cooling.

Uneven air-flow distribution reduces the performance of air-to-refrigerant heat exchangers. But design changes that increase refrigerant flow in areas that receive more air significantly lessen degradation.

NIST researchers devised a means of tracking air-flow distribution in finned-tube heat exchangers using a high-resolution camera and laser-illuminated dust particles. The data are compared with CFD simulations of air flow. Engineers could use accurate CFD models as the basis for design changes to coils and refrigerant circuitries that improve air distribution.

The program could increase finned-tube heat-exchanger heating and cooling capacity by 5%. Such improvements would let manufacturers reduce heat-exchanger size, thus reducing material costs and the amount of refrigerant needed. NIST will issue study results on home air-based heat pumps next year. The results should also apply to large commercial heat exchangers and refrigeration systems.

A sheet of laser light

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