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A good understanding of the basic operation of the refrigeration
system is essential for the service technician. Without this
understanding, accurate troubleshooting of refrigeration
system problems will be more difficult and time consuming, if
not (in some cases) entirely impossible. The refrigeration
system uses four basic principles (laws) in its operation they
are as follows:
1. "Heat always flows from a warmer body to a cooler body."
2. "Heat must be added to or removed from a substance
before a change in state can occur"
3. "Flow is always from a higher pressure area to a lower
pressure area."
4. "The temperature at which a liquid or gas changes state
is dependent upon the pressure."
The refrigeration cycle begins at the compressor. Starting
the compressor creates a low pressure in the suction line which
draws refrigerant gas (vapor) into the compressor. The
compressor then "compresses" this refrigerant, raising its
pressure and its (heat intensity) Temperature.
The refrigerant leaves the compressor through the discharge
line as a hot high pressure gas (vapor). The refrigerant enters
the condenser coil where it gives up some of its heat. The
condenser fan moving air across the coil's finned surface
facilitates the transfer of heat from the refrigerant to the
relatively cooler outdoor air.
When a sufficient quantity of heat has been removed from
the refrigerant gas (vapor), the refrigerant will "condense" (i.e.
change to a liquid). Once the refrigerant has been condensed
(changed) to a liquid it is cooled even further by the air that
continues to flow across the condenser coil.
The RAC design determines at exactly what point (in the
condenser) the change of state (i.e. gas to a liquid) takes place.
In all cases, however, the refrigerant must be totally condensed
(changed) to a liquid before leaving the condenser coil.
The refrigerant leaves the condenser coil through the liquid line
as a warm high pressure liquid. It next will pass through the
refrigerant drier (if so equipped). It is the function of the drier to
trap any moisture present in the system, contaminants, and large
particulate matter.
The liquid refrigerant next enters the metering device. The metering
device is a capillary tube. The purpose of the metering device is to
"meter" (i.e. control or measure) the quantity of refrigerant entering
the evaporator coil.
Refrigeration SystemRefrigeration System
Refrigeration SystemRefrigeration System
Refrigeration System
Sequence of OperationSequence of Operation
Sequence of OperationSequence of Operation
Sequence of Operation
Liquid
Line
Suction
Line
Evaporator
Coil
Metering
Device
Refrigerant
Dryer
Discharge
Line
Refrigerant
Drier
Condenser
Coil
Compressor
In the case of the capillary tube this is accomplished (by design)
through size (and length) of device, and the pressure difference
present across the device.
Since the evaporator coil is under a lower pressure (due to the
suction created by the compressor) than the liquid line, the liquid
refrigerant leaves the metering device entering the evaporator coil.
As it enters the evaporator coil, the larger area and lower pressure
allows the refrigerant to expand and lower its temperature (heat
intensity). This expansion is often referred to as "boiling". Since
the unit's blower is moving Indoor air across the finned surface of
the evaporator coil, the expanding refrigerant absorbs some of
that heat. This results in a lowering of the indoor air temperature,
hence the "cooling" effect.
The expansion and absorbing of heat cause the liquid refrigerant
to evaporate (i.e. change to a gas). Once the refrigerant has
been evaporated (changed to a gas), it is heated even further by
the air that continues to flow across the evaporator coil.
The particular system design determines at exactly what point (in
the evaporator) the change of state (i.e. liquid to a gas) takes
place. In all cases, however, the refrigerant must be totally
evaporated (changed) to a gas before leaving the evaporator coil.
The low pressure (suction) created by the compressor causes the
refrigerant to leave the evaporator through the suction line as a
cool low pressure vapor. The refrigerant then returns to the
compressor, where the cycle is repeated.