There are three main classes of heat pump heat exchanger technology used in modern domestic and commercial water heating heat pump systems.
These heat exchange technology classes discussed here are common in both integrated or split systems.
The three classes of heat exchange technology used in hot water heat pump systems are: static-heating type or dynamic-heating type. These can also be described as refrigerant-cycle type or water-cycle type.
The coils utilize a coaxial "tube-in-tube" design. Warm air flows through the inner tube while cold liquid refrigerant flows in the 'donut' between the inner and outer tubes.Refrigerant flows in a counterflow direction to the airflow resulting in the greatest temperature difference between the two streams to yield optimum heat transfer.
They are ideal where there is a significant temperature difference between the two 'liquids' ( air behaves like a liquid in a heat exchanger).
When designing this type of heat pump heat exchanger care must be taken in determining the correct tube wall thickness as well as tube diameter, to allow optimum heat exchange from the air to the refrigerant.
Tube in tube heat exchangers are used extensively in heat pumps, refrigeration and air conditioning units, where there are large temperature differences.
Flat plate heat exchangers consist of thin plates, usually stainless steel, joined together, with a small amount of space between each plate, typically maintained by a small rubber gasket.
The surface area is large, and because plate heat pump heat exchangers have such a large surface area, they are often more effective than shell and tube in tube heat exchangers.
However they are susceptible to accumulations of scale deposits especially if water hardness is a factor, and they can be difficult to clean as the distance between the plates can be as little as 2 mm.
Even so, they are a widely used and effective heat exchange technology.
This type of heat exchanger consists of a shell (a large pressure vessel) with a bundle of tubes inside it. One fluid runs through the tubes, and another fluid flows over the tubes (through the shell) to transfer heat between the two fluids.
The set of tubes is called a tube bundle, and may be composed by several types of tubes: plain, longitudinally finned, etc.
Water resistance of this type can be very little, and so the volume of the exchanger can be large. Tube-in-shell heat exchangers are not well suited to big temperature difference applications. The temperature difference should not be more than 10℃. Their large size is also a disadvantage.
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