Heat Pump Operation

A heat pump uses refrigerant to remove heat from a given area and transfer it to another area. For example, a heat pump can remove heat from an office during the summer and transfer it outside. Conversely, a heat pump can remove heat from outside during winter (what little heat there is) and transfer it into an office. The heat pump consists of four main parts: the compressor, the condenser, the expansion valve, and the evaporator.

The compressor raises the pressure and temperature of the refrigerant gas. Heat is taken out of the high-pressure refrigerant gas in the condenser, converting it to a liquid at high pressure, thus reducing the temperature of the refrigerant. The expansion valve allows the pressure and temperature of the high-pressure refrigerant liquid to decrease, converting it to a mixture of liquid and gas. Heat is added to this mixture in the evaporator converting it back to a gas, thus raising the temperature of the refrigerant. The cycle begins again as the gas enters the compressor.

Figures 3 and 4 (below) are schematics of a heat pump in the heating and cooling modes, respectively. In the heating mode, cool air (inside the office) is blown over the condenser, removing heat from the refrigerant as described above, so the condenser is a refrigerant/air heat exchanger. Relatively warm water (60 degrees F) is circulated through the evaporator, adding heat to the refrigerant as described above, so the evaporator is a refrigerant/water heat exchanger. The refrigerant flow is reversed for operation in the cooling mode.

Examination

The two heat pumps reportedly leaked large amounts of water. In the heating mode, water can come from two places in this heat pump system. Water vapor that is usually present in the air we breathe can condense on the cool refrigerant line entering the evaporator.

It is unlikely that enough water could condense from the atmosphere to cause the reported leaking.

Water was also found in the refrigerant/water heat exchanger described above. In these installations, water is normally pumped into the heat exchanger at 45 psig, and exits at 15 psig.

Prior to any leak testing, the copper lines in both heat pumps were examined. There was no evidence of any punctures or weak solder joints, and the lines showed no signs of overpressurization. However, the top covers on both heat pumps were slightly corroded in a small circular area just above the temperature-controlled refrigerant pressure relief valve on the refrigerant line entering the compressor.

Leak Testing

Both of the removed heat pumps were tested in order to determine the location and severity of the leaks. The leak testing was accomplished by circulating water through the refrigerant/water heat exchanger. A test valve installed at the water exit was gradually closed, thereby increasing the internal water pressure in the heat exchanger.

Water leaked out of the temperature-controlled refrigerant pressure relief valve in both of the heat pumps.

Both refrigerant/water heat exchangers were removed from their respective heat pumps and cut circumferentially into three pieces, revealing a small hole in the internal bellows-type water tube near the water inlet port in each of the heat exchangers. Furthermore, the water tube near each hole appeared as if it had expanded or bulged significantly. (See Figure 2.)

This type of heat exchanger is known as a coaxial heat exchanger. That is, the water flows through a tube nested inside of the tube in which the refrigerant flows. The small hole allowed water to flow from the internal water tube into the refrigerant tube and to exit the heat pump through the open temperature-controlled pressure relief valve on the refrigerant line.

Additional Testing

The thermocouples attached to the water outlet lines on each of the refrigerant/water heat exchangers were removed and tested. The thermocouples were placed in an ice water bath and the ice was allowed to slowly melt. As the ice melted, the resistance across the poles 1 and 2, poles 2 and 3, and poles 1 and 3 was monitored.

Near freezing, poles 2 and 3 and poles 1 and 3 registered open circuits; poles 1 and 2 registered a closed circuit. When the temperature of the bath reached approximately 41 degrees to 42 degrees, both thermocouples switched. The switching was distinguished by an audible click. Now poles 1 and 2 and poles 1 and 3 registered open circuits, while poles 2 and 3 registered a closed circuit.

In addition to the above testing, a filter (which removes large particulates from the refrigerant/water heat exchanger water line) was removed from a third heat pump installation at the same facility, and replaced with a new filter. The removed filter was examined for any signs of clogging.

Particulates, including leaves, small rocks, and a foam rubber-like compound, were found in the removed heat exchanger water filter. Each of these particulates existed in the filter in relatively small quantities. Visual examination of the filter revealed that the above particulates did not completely block the passage of water.

Conclusions

High temperatures (in excess of the refrigerant pressure/temperature relief valve setting) at the refrigerant inlet line to the compressor tripped the temperature-controlled refrigerant pressure relief valve, creating a refrigerant leak in the heat pump system. These high temperatures were likely caused by the swelling of the bellows-type water tube within the refrigerant tube in the refrigerant/water heat exchanger.

This bellows-type water tube swelled and ruptured near the water inlet port, allowing heat exchanger water to enter into the refrigerant lines and eventually exit the heat pump system at the temperature-controlled pressure relief valve. The formation of ice in the heat exchanger water likely caused this swelling.

Ice may form at the water inlet port to the refrigerant/water heat exchanger if the water flow through the heat exchanger is restricted. The thermocouple that prevents ice formation is located at the water outlet port; there is no such thermocouple located at the inlet port.

Thermocouples from both of the subject units were found to be operable; no evidence of freezing was found near the water outlet port (thermocouple attachment point) of either refrigerant/water heat exchanger.

Particulates, such as those found in the third heat exchanger water supply line, may have existed in sufficiently large quantities in the water supply lines of the two failed heat pumps to significantly inhibit the flow of water through the refrigerant/water heat exchanger. A significant reduction in the water flow rate may lead to ice formation in the refrigerant/water heat exchanger.

Duffner and Hopkins are with Exponent Failure Analysis Associates Inc., 149 Commonwealth Drive, Menlo Park, Calif. 94025; 650-688-7282; 650-326-8072 (fax).

Publication date: 05/19/2003