Pontiac FIREBIRD 1972 Service Manual page 84

CUSTOM AIR CONDITIONING
mesh screen and an outlet connection (which attaches to
the evaporator). The fine mesh screen at the inlet of the
valve provides protection to the valve by preventing dirt
and other foreign material from entering the valve.
While this valve is located at the inlet of the evaporator
(at the bottom of the evaporator), the thermo bulb is
attached to the evaporator outlet pipe and is insulated
from temperature other than that of the evaporator outlet
pipe.
The equalizer line joins the expansion valve to the P.O.A.
valve so that compressor inlet pressure will register in the
expansion valve. Under high load conditions this pressure
is essentially the same as evaporator pressure, and the
expansion valve functions in a normal manner. Under
light load conditions (low ambient temperature or extreme
modulation of outlet nozzle temperatures) the pressure
transmitted to the expansion valve diaphragm is consider-
ably lower than evaporator pressure.
This low pressure, plus the thermo bulb reading on the
evaporator outlet pipe, tricks the expansion valve into
admitting more liquid refrigerant into the evaporator than
is required for the cooling demand. This high refrigerant
flow rate insures that compressor oil will flow through the
evaporator and back to the compressor, thereby keeping
the compressor adequately supplied with oil, and prevent-
ing the evaporator from becoming oil-logged.
It is the temperature of the air passing over the evaporator
core that determines the amount of refrigerant that will
enter and pass through the evaporator. When the air is
very warm, the heat transfer from the air to the refrigerant
is great and a greater quantity of refrigerant is required to
cool the air and to achieve the proper superheat on the
refrigerant gas leaving the evaporator. When the air pass-
ing over the evaporator is cool, the heat transfer is small
and a lesser quantity of refrigerant is required to cool the
air and to achieve the proper superheat on the refrigerant
gas leaving the evaporator.
A mechanical adjusting nut located within the valve is
provided to regulate the amount of refrigerant flow
through the valve and moves the spring seat to increase or
decrease the tension on the valve carriage spring. By vary-
ing the tension on this spring, it is possible to regulate the
point at which the valve begins to open or close, thereby
regulating refrigerant flow into the evaporator. As this
adjustment feature is inside the valve, no external adjust-
ment is possible. All valves are preset at the time of manu-
facture.
Since the evaporator outlet pressure is proportionate to
the amount of heat (superheat) picked up by the refriger-
ant gas passing through the evaporator, it can be seen that
adjusting spring tension, which works against capillary
pressure and equalizer line pressure, controls the volume
of refrigerant entering the evaporator as signaled by the
temperature and pressure in the evaporator outlet pipe.
FUNCTION
OPERATION
The purpose of the expansion valve is to regulate the flow
of liquid refrigerant into the evaporator automatically in
accordance to the requirements of the evaporator.
The valve is the dividing point in the system between high
pressure liquid refrigerant supplied from the receiver-
dehydrator and relatively low pressure liquid and gaseous
refrigerant in the evaporator.-It is so designed that the
temperature of the refrigerant at the evaporator outlet
must have 10.6"F. of superheat before more refrigerant is
allowed to enter the evaporator. Superheat is an increase
in temperature of the gaseous refrigerant above the tem-
perature at which the refrigerant vaporizes.
A capillary tube filled with carbon dioxide and the equal-
izer line provide the temperature regulation of the expan-
sion valve. This capillary tube is fastened to the low
pressure refrigerant pipe coming out of the evaporator so
that it communicates the temperature of the refrigerant at
this point to the expansion valve. If the superheat at the
outlet decreases below 10.6"F., the expansion valve will
reduce the amount of refrigerant entering the evaporator,
thus reducing the amount of cooling. If the superheat
increases, the expansion valve will automatically allow
more refrigerant to enter the evaporator, thus increasing
the cooling.
The equalizer line joining the P.O.A. valve with the area
behind the operating diaphragm acts with the capillary to
measure superheat.
When the air conditioning system has not been operating,
all pressures within the expansion valve assembly will
have equalized at the ambient (surrounding air) tempera-
ture. Thus, the pressure above and below the operating
diaphragm and at the inlet and outlet side of the valve will
be equal (Fig. 1A-19). Pressure under the diaphragm is
evaporator pressure. It reaches this area by means of clear-
ance around the operating pins in the valve body which
connects the area under the diaphragm with the evapora-
tor pressure area. While pressures in the expansion valve
are almost equal, the addition of the valve adjusting spring
pressure behind the valve will hold the valve over to close
the valve orifice.
When the air conditioning system first begins to operate,
the compressor will immediately begin to draw refrigerant
from the evaporator, lowering the pressure in the evapora-
tor and in the area under the operating diaphragm. As the
pressure in this area decreases, the pressure above the
diaphragm exerted by the carbon dioxide in the capillary
tube will overcome spring pressure and push the dia-
phragm against the operating pins, which in turn will
,force the needle valve off its seat.
Refrigerant will then pass through the expansion valve
into the evaporator where it will boil at a temperature
corresponding to the pressure in the evaporator. This will
begin cooling the air passing over the evaporator. It will
also begin to cool the evaporator outlet pipe.