Friedrich Twintemp, Service Manual

Page: 28 / 68

Suction
Line
Evaporator
Coil
Metering
Device
Refrigerant
Strainer
Discharge
Line
Condenser
Coil
Compressor
Refrigerant Drier Liquid
Line
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.
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.
rEfrIGErATION SEQUENCE Of OPErATION
26