Sequence Of Operation - Friedrich PDE07K Service Manual

SEQUENCE OF OPERATION

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 diffi cult 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 fl ows 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 fi nned surface
facilitates the transfer of heat from the refrigerant to the
relatively cooler outdoor air.
When a suffi cient 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 fl ow 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.
6
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 fi nned 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 fl ow 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.
Suction
Line
Evaporator
Coil
Metering
Device
Refrigerant Drier
Refrigerant
Strainer
Discharge
Line
Condenser
Coil
Compressor
Liquid
Line