GMC 4000 Series Service Manual page 83

Sec. 1
Page 76
AIR CONDITIONING
(Pressure under the diaphragm is evaporator p res­
sure. It reaches this area by means of clearance
around the operating pins in the valve body which
connects the area under the diaphragm with the
evaporator 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 com pressor will immediately begin
to draw refrigerant from the evaporator, lowering
the pressure in the evaporator and in the area
under the operating diaphragm. As the pressure in
this area decreases, the pressure above the dia­
phragm exerted by the carbon dioxide in the capil­
lary tube will overcome spring pressure and push
the diaphragm against the operating pins, which in
turn will force the needle valve off its seat.
Refrigerant will then pass through the expan­
sion 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, and, also it will begin to cool
the evaporator outlet pipe.
As the evaporator outlet pipe cools, the p res­
sure of the carbon dioxide in the capillary tube
(contacting this outlet pipe) decreases, exerting
le ss force on the operating diaphragm.
The valve adjusting spring is calibrated so that
the pressure of the refrigerant in the evaporator,
plus the spring force, will equal the force above
the operating diaphragm when the temperature of
the refrigerant in the evaporator outlet is 10.6°F.,
above the temperature of the refrigerant entering
the evaporator. In other words, the refrigerant
should remain in the evaporator long enough to
LEVER HELPER
SPRINGS
ADJUSTIN G
PIN NUT ADJUSTING
SCREW
ADJUSTING
PIN
CONTRO L / INNER
LEVER / SPRING
ASSEM BLY OUTER
SPRING
Figure 6 — Suction Throttling V a lve
G M C S E R V I C E M A N U A L
FROM
EVAPORATOR
PISTON
DIAPHRAGM
TO COMPRESSOR
T-3310
completely vaporize and then warm (superheat)
10.6°F.
If the temperature differential begins to go
below 10.6 F. (outlet pipe becomes too cold), car­
bon dioxide pressure in the capillary tube and area
above the diaphragm decreases, allowing the valve
adjusting spring to move the needle valve toward
its seat, closing off the flow of refrigerant past the
needle valve.
If the temperature differential begins to go
above 10.6°F. (outlet pipe too warm), the pressure
in the capillary tube and area above the operating
diaphragm will increase, pushing this diaphragm
against the operating pins to open the needle valve
further, admitting more refrigerant to the evap­
orator.
EVAPORATOR
The evaporator is actually the device which
cools and dehumidifies the air before it enters the
cab. High pressure liquid refrigerant flows through
the valve orifice in the expansion valve into the
low pressure area of the evaporator. This regu­
lated flow of refrigerant boils immediately. Heat
from the core surface is lost to the boiling and
vaporizing refrigerant, which is cooler than the
core, thereby cooling the core. The heat in the air
passing over the evaporator loses its heat to the
cooler surface of the core, thereby cooling the air.
As the process of heat loss from the air to the
evaporator core surface is taking place, any m ois­
ture (humidity) in the air condenses on the outside
surface of the evaporator core and is drained off as
water.
Since Refrigerant-12 will boil at 21.7°F., be­
low zero at atmospheric pressure and water freezes
at 32°F., it becomes obvious that the temperature
in the evaporator must be controlled so that the
water collecting on the core surface will not freeze
in the fins of the core and block off the air p ass­
ages. In order to control the temperature, it is
necessary to control pressure inside the evapor­
ator and this is done by the suction throttling valve.
To obtain maximum cooling the refrigerant
must remain in the core long enough to completely
vaporize and then superheat a minimum of 10.6°F.
If too much or too little refrigerant is present in
the core, then maximum cooling efficiency is lost.
An expansion valve in conjunction with the suction
throttling valve is used to provide this necessary
refrigerant and pressure control.
SUCTION THROTTLING VALVE
The main function of the Suction Throttling
Valve (fig. 6) is to maintain the evaporator p res­
sure at a pressure sufficiently high to avoid freez­
ing of moisture on the evaporator core and at the
same time provide maximum cooling efficiency.