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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 PTAC 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.
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.
Sequence of Operation
Liquid
Line
Suction
Line
Evaporator
Coil
Metering
Device
Refrigerant
Strainer
Discharge
Line
Refrigerant Drier
Condenser
Coil
Compressor
