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Heat exchanger for heat pump

• Condenser for heat pump
• Overview of condensers by heat load in kW
• The design pressures of the exchangers (in bars)
• Combo connections (i.e. both soldering and external threading)
• Heat pump freezing, SWEP B26FH
• Evaporator for heat pump
• Separation exchanger

Condenser for heat pump

The condenser is the heat pump exchanger on its secondary circuit. The hot refrigerant in the condenser transfers heat to the heating water. The hot refrigerant enters the exchanger in the vapor phase and is cooled, condensed and subcooled after condensation. The heat transferred to the heating water comes mainly from the change of state (i.e. condensation of the refrigerant). One of the heat exchanger calculation parameters for a heat pump is the condensing temperature of the refrigerant.

The list shows plate heat exchangers often used as condensers:

• SWEP B8LASH (for capacities of 3–10 kW, B8LASH is asymmetric; connections are combo ¾"); the exchanger is suitable as a condenser for the Sinclair Split ASGE air conditioning unit when being rebuilt to a heat pump,
• SWEP B26H, B26FH (asymmetric exchanger 5–20 kW designed for heat pumps; it has soldering connections towards the primary, external thread ISO G 1" towards the secondary circuit),
• SWEP B25TH, B85H, B86H (for capacities 10–50 kW, air conditioning; connections combo 1" or combo 1 ¼"),
• SWEP B18H, B185H, B16DW (for natural gas, CO2 up to 140 bar; connections according to customer's request),

The asymmetric exchanger has narrower channels in the internal circuit (designed for refrigerant). There is usually about 10 times more flow on the water side than on the refrigerant side. So asymmetric heat exchanger is optimized for air conditioning and heat pumps.

The SWEP B25TH version is popular among technicians because it has pure solder connections on the refrigerant side. SWEP B85H and B86H have a higher efficiency compared to B25TH, their connections are combo: the connections are externally threaded and also provide inner pipe for soldering (see picture, click to open the connection's drawing). B86H achieves the highest efficiency, but it also has the highest pressure losses. Pressure losses can be reduced by increasing the number of plates.

All SWEP heat exchangers have stainless steel connections and a silver solder containing at least 45% silver has to be used.

The design pressure of the exchanger can be obtained from the graph provided in its product sheet. The design pressures of common exchangers are approximately as follows:

• 46 bar(g) at 75 °C for B8TH, B8LASH;
• 48 bar(g) at 75 °C for B26H, B26FH;
• 46 bar(g) at 75 °C for B25TH;
• 50 bar(g) at 75 °C for B85H, B86H.

Pressure-temperature charts of individual refrigerants are commonly available on the Internet. For clarity, the pressures bar(g) for refrigerants R410A, R407C, R134a and R32 are summarized in the table (source A-GAS):

The pressure bar(a) is absolute, the pressure bar(g) is relative to the atmospheric pressure (excess pressure to the surrounding air). Some refrigerants (e.g. R407C) are a mixture of several refrigerants, each having their own condensing temperatures. As a result, two temperatures are indicated for these refrigerants: a) Boiling Temperature refers to the point at which the liquid refrigerant begins to boil and transitions into a vapour state. b) Condensing Temperature represents the point at which the vapour refrigerant begins to condense back into a liquid state.

Heat pump freezing, heat exchanger failure

The exchanger rupture most often happens in these two cases:

• The operating pressure of the refrigerant is higher than the design pressure of the exchanger. The system must include a high pressure switch. This switches off the compressor when the working pressure is exceeded (e.g. in the event of a fault).
• The heat exchanger must not get frozen. There is a risk of freezing the media inside condenser when the heat pump is running in reverse. Reverse operation is started for a few minutes to defrost the evaporator. Also, when starting cold, the evaporator temperature is very low, the evaporator can freeze.

The refrigerant can have a temperature of -20 °C. Therefore, under unfavorable circumstances, there is a risk of water freezing in the condenser. Even if the water at the exchanger outlet is 3 °C, inside the heat exchanger might be a space with a temperature below freezing point. Measures against freezing are, for example:

• Temperature sensor at the outlet of the water from the heat exchanger: when it drops below a certain temperature, the compressor turns off.
• Antifreeze, electric heating of the exchanger during reverse.
• Flow switch: to prevent the exchanger from freezing, it is necessary to maintain full flow on the water side: use the constant speed on the circulation pump. The valves on the radiators must be open.
• A strainer is required to prevent particles from entering the exchanger and disturbing the flow locally inside the heat exchanger. The strainer should stop particles larger than 1 mm.
• Delayed water pump stop when stopping the compressor. The pump can be allowed to run for some minutes after the compressor is stopped and vice versa: start the water pump before starting the compressor.
• Stopping the fan during the defrost cycle raises the evaporator temperature.
• The compressor is started at as low a capacity as possible. This will minimize the fall in evaporation temperature during the start-up.

Freezing water in the heat exchanger means damaging the heat exchanger and usually also the overall damage to the heat pump (water might get into the refrigerant circuit). That's why SWEP also supplies a special version of the most commonly used SWEP B26H heat exchanger for R410A refrigerant: the modified B26FH version has no channels in the corner at the refrigerant inlet, where the exchanger is most susceptible to freezing. This reduces the overall risk of the "heat pump freezing".

Evaporator for heat pump

The evaporator is the heat pump exchanger on its primary circuit. In this exchanger, the cold liquid refrigerant evaporates. The system is usually set so that the expansion valve in front of the evaporator reduces the pressure. This reduces the boiling temperature. The evaporator refrigerant temperature is set to a temperature of about 0 °C, but it may be less. Heat must be supplied to the refrigerant in order for the refrigerant to evaporate. This is taken, for example, from the ambient air or from the ground (and later transferred to the heating water in the condenser). Most of the energy that is thus transferred from the environment to the refrigerant is stored in the change of state.

For small applications, the classic SWEP plate heat exchanger can be used. The refrigerant inlet connection should never be larger than the refrigerant outlet connection. For proper operation, the recommended refrigerant speed of 10 to 25 m/s at the inlet and 5 to 10 m/s at the outlet (2.5 to 5 m/s if the connection is horizontal) should be ensured; this also prevents oil accumulation in the heat exchanger.

High performance pumps require more plates in the exchanger. If more than 30 plates are needed for the evaporator, it is usually necessary to select a specialized type of plate heat exchanger (V-type, P-type, F-type or Q-type). V-series heat exchangers are classic heat exchangers equipped with a system for even distribution of refrigerant (e.g. V25, V80). Without this measure, with a larger number of plates, the refrigerant would only flow through the plates closest to the inlet. The exchanger would not have the expected efficiency and could get frozen. The distribution system is not an obstacle if such exchanger is used also as a condenser.

Specialized types (i.e. most of V-series heat exchangers and especially P-type and other evaporators) are not in stock and must be manufactured.

Separation exchanger for heat pump

The separation exchanger is used, for example, to separate the antifreeze circuit from the heating water circuit. Then a mixture with glycol can be used outside and there is only heating water in the heating circuit inside the building. The separation exchanger can also be used to separate the heat pump from dirty or aggressive media.

To maintain the efficiency of the heat pump, it is necessary to bring the temperatures of both circuits as close as possible. The pressure losses increase with the square of the flow rate.