Buick LeSabre 1965 Service Manual page 774

ACCESSORIES OPTIONAL HEATER-AIR CONDITIONER SYSTEM 1 1 - 4 1
NOTE: The superheat setting on
the 49000 expansion valve is 10°F.
For explanation purposes, the 6
U
F
setting of the 45000, 46000 and
48000 will be used.
A capillary tube filled with carbon
dioxide and the equalizer line
provide the temperature regula-
tion of the expansion valve. This
capillary tube is fastened to the
low pressure refrigerant pipe
coming out of the evaporator so
that it communicates the tem-
perature of the refrigerant at this
point to the expansion valve. If
the temperature differential be-
tween the inlet and outlet de-
creases below 6°F., the expansion
valve will automatically reduce
the amount of refrigerant enter-
ing the evaporator.
If the temperature differential in-
creases, the expansion valve will
automatically allow more refrig-
erant to enter the evaporator.
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 e v a p o r a t o r .
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 refrig-
erant gas leaving the evaporator.
When the air passing over the
evaporator is c o o l , 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 refrig-
erant gas leaving the evaporator.
A mechanical adjusting nut loca-
ted within the valve is provided
to regulate the amount of refrig-
erant flow through the valve; when
turned, move the spring seat to
increase or decrease the tension
on the needle v a l v e carriage
spring. By varying the tension
on this spring, it is possible to
regulate the point at which the
needle valve begins to open or
close, thereby regulating refrig-
erant flow into the evaporator.
As this adjustment feature is in-
side the valve, no external ad-
justment is possible. All expan-
sion valves are preset at the
time of manufacture.
When the air conditioning system
has not been operating, all pres-
sure within the expansion valve
assembly will have equalized at
the a m b i e n t (surrounding air)
temperature, thus the pressure
above and below the operating
diaphragm and at the inlet and
outlet side of the valve will be
equal. (Pressure under the dia-
phragm is evaporator presssure.
It reaches this area by means of
clearance around the operating
pins which c o n n e c t s the area
under the diaphragm w i t h the
evaporator pressure area.) While
pressures in the expansion valve
are almost equal, the addition of
the valve adjusting spring pres-
sure behind the needle will hold
the needle valve over to close
the needle valve orifice.
When the air conditioning system
first begins to operate, the com-
pressor will immediately begin
to draw refrigerant f r o m the
evaporator 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 a g a i n s t the operating
pins, which in turn will force the
needle off its seat.
Refrigerant will then pass through
the expansion valve into the evap-
orator where it will boil at a
temperature corresponding to the
pressure in the evaporator. This
will begin cooling the air passing
over the evaporator, and, also it
will begin to cool the evaporator
outlet pipe.
The valve adjusting spring is cal-
ibrated so that the pressure of
the refrigerant in the evaporator
outlet pipe and equalizer line to
the valve, plus the spring force,
will equal the force above the
operating diaphragm when the
temperature of the refrigerant in
the evaporator o u t l e t is 6°F.
above the temperature of the r e -
frigerant entering the evaporator.
In other words, the refrigerant
should remain in the evaporator
long enough to completely vapor-
ize and then warm (superheat)
6°F.
If the temperature differential be-
gins to go below 6° F. (outlet pipe
becoming too cold) carbon dioxide
pressure in the capillary tube and
area above the diaphragm de-
creases, allowing the valve ad-
justing spring to move the needle
valve towards its seat closing off
the flow of refrigerant past the
needle valve.
If the temperature differential be-
gins to go above 6° F. (outlet pipe
too warm), the pressure in the
capillary tube and area above the
operation diaphragm w i l l in-
crease, pushing this diaphragm
against the operating pins to open
the needle valve further, admit-
the needle valve further, ad-
mitting more refrigerant to the
evaporator.
The equalizer line permits the
STV outlet pressure to be im-
posed on the expansion valve dia-
phragm, thus, over-riding i t s
normal control of liquid refriger-
ant. As the compressor capacity
becomes greater than the evapo-
rator load, the drop in compres-
sor suction line pressure forces
the expansion valve to flood liquid
through the evaporator and STV,
thus preventing the suction pres-
sure from dropping below a pre-
determined pressure.
The equalizer line is used prim-
arily to prevent prolonged and
constant operation of the com-
pressor in vacuum conditions.
This operation is considered un-
desirable both from a noise angle
and from possibili;y of subjecting