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).
Conversion of Air Conditioners to Heat Pumps
The main reason for converting an air conditioner to a heat pump is the
lower initial cost. Air conditioners are optimized for cooling, not heating.
While they do not achieve the same efficiency as heat pumps, considering the
overall costs, they can sometimes make more sense.
During the conversion, the cooling function is usually disabled, and the
indoor wall unit is replaced with a condenser. The outdoor unit of the air
conditioner is designed for summer operation and may be more prone to frost
formation during winter operation (compared to the outdoor unit of a heat
pump). The control unit of the air conditioner must be replaced with a new one
to support the reverse cycle (defrosting the outdoor unit).
Small air conditioners use rotary compressors, and with these, the heat pump
will not achieve as high a heating factor as with scroll compressors. The air
conditioner should have an inverter to regulate the output.
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 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.
Click on the image to compare the standard connection against combo connection.
Some SWEP heat exchangers are equipped with connections
combo ¾",
combo 1"
or
combo 1 ¼"
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.
Heat exchangers for heat pumps
Overview of condensers for heat pumps, i.e. the exchanger serves as
a refrigerant condenser. The pressure losses for the water circuit (heating)
are shown within the table. The condenser calculation is for the R410A refrigerant on the
primary side, the water on the secondary side has gradient of
.
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:
Pressure-temperature charts of individual refrigerants are commonly available on the Internet. For clarity, the pressures bar(g) for refrigerants R410A, R407C, R134a, R22 and R32 are summarized in the table
(source A-GAS):
The pressure bar(g) is relative to the
atmospheric pressure (excess pressure to the surrounding air of 1 bar). 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.
Basic scheme (heating mode): without protection against freezingScheme with hot gas bypass valve (heating mode): when the evaporation temperature drops below a set level, this valve leads part of the hot gas directly into the evaporator inlet. This prevents the evaporation temperature from decreasing, and thereby protects it from freezing.
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.
Strainer at the water inlet of the heat exchanger to capture particles over 1 mm. Dirt can block flow and cause the water in the channel to freeze.
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.
Air conditioning units are optimized for summer operation. When modified into a heat pump, there may be increased difficulties with frost on the outdoor unit equipped with a fan. This is because air conditioning units have smaller gaps between the fins compared to typical heat pumps.
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.
Classic heat exchanger without distribution system used as an evaporator. For larger capacity, a specialized exchanger (evaporator) is used. Most often it is SWEP V and P series.
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.
up to 140 bar; connections according to customer's request),
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