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Freeze Failure of BPHEs in the Air to Water Heat Pump

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Failure Effects of BPHE Freezing

BPHE is made by several SUS plates(0.3~0.4mm thickness) with copper brazing. Although its design pressure (normally is 30 or 45 bar) is quite high as well as the burst pressure (more than 5 times the design pressure).  Because the water volume will be expansive when water freeze to ice, the deformation of plate or of the exterior of BPHE will be happened even the break. 

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The internal leakage will usually happen since the plate is broken or the external leakage is also possible in some rare case. But the internal leakage is not easy be seen in the early status and finally it will make the heat pump shout down and even permanent break the compressor.

Reasons of BPHEs Freezing in ATW Heat Pump

Scenario A

The BPHE is applied as the condenser of the air to water(ATW) heat pump. There are two common design for air to water heat pumps.
(1) Mono Block Type BPHE is in the outdoor unit. Its connection pipe is delivered the hot water to the indoor equipment. When the ambient is below zero degree in the cold climate and the heat pump is stop, there is possible freezing to ice inside the pipe and the BPHE in once the insufficient thermal insulation or lack the auto drain device for the BPHE or the water pipe.
(2) Split Type BPHE is in the indoor unit. The pipe connect to outdoor unit is just for the refrigerant cycle. In majority, there is no risk of BPHE freezing problem.

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Scenario B

The ATW heat pumps all have the defrost mode and some of them will take the reversed refrigeration cycle during this mode . The low temperature refrigerant will flow into the BPHE under the defrost mode. If the heat pump control is abnormal(ex. Defrost time is too long) or the water flow is not sufficient, the temperature of water inside the BPHE will become lower and also bring the freezing risk when temperature is below the ice point.

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Scenario C

The reversible heat pump is designed for both heating and cooling applications. The same BPHE is applied as the condenser(heating mode) and the evaporator(cooling mode). Freezing failure is usually happened in the cooling mode. The possible reasons could be:
(1)The low pressure of refrigerant is under poor control or low pressure switch is fail.
(2)The temperature controller for water is not work.
(3)The compressor don’t stop when the water stop flowing through the BPHE.

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