Filters And Driers; Expansion Valve - Bosch DOMESTIC HOT WATER HEAT PUMP Technical Manual

4.2 Filters and driers

Moisture or water vapour may cause problems in any type of refrigerant system, because moisture may freeze in
the orifice of the expansion valve, causing corrosion of metal parts and wet the motor of the compressor, damaging
it (motor burnout and oil sludge). On the other hand, foreign matter may contaminate the compressor oil and
become lodged in valve parts, making system inoperative.
The filter drier, positioned in the liquid line protects the expansion valve, absorbing potential humidity in the
refrigerant (R134a). The combination of humidity and high discharge gas temperatures will accelerate
decomposition of the oil and increase risk of compressor failure.
Attention:
In case of need to access refrigerant circuit and in case of remove and charge of new gas when components are
replaced, it is mandatory the replacement of the filter the replacement is recommended because if a filter in the
liquid line becomes blocked, there will be a pressure drop refrigerant to boil and create flash gas, which will disrupt
the operation of the expansion valve.

4.3 Expansion valve

The expansion valve has two main functions:
- Controlling the amount of refrigerant entering the evaporator:
- Maintaining the pressure difference between the condenser (high pressure) and the evaporator (low
pressure):
The pressure difference created by the work of the compressor is kept by the expansion device.
As much of the evaporator surface as possible should be covered with liquid refrigerant without liquid being carried
over to the compressor. If the capacity of the evaporator increases, the expansion valve should allow a larger flow
of refrigerant (bulb detects high superheating) and vice versa.
A smaller refrigerant mass flow results in a higher level of superheating, because less surface area is required for
evaporation
The expansion valve or orifice tube represents the point of separation between the high pressure and low pressure
sections in the refrigerant circuit and is installed before the evaporator. The liquid refrigerant is injected into the
evaporator through the expansion valve or orifice tube. The refrigerant expands, evaporates and thus releases
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Pict.36 – Filter Drier (cut model for internal view)
6720649497 SM HP270-1 2012/09 en