GMC 4000 Series Service Manual page 82

from the evaporator through the suction throttling
valve, then back through the suction line to com­
pressor thus completing the cycle.
AIR CIRCULATION
Air circulation is common to both the heating
and air conditioning system s. For detailed des­
cription of air circulation in vehicle, refer to
"Operating Instructions" explained previously.
OPERATION OF INDIVIDUAL UNITS
Figures 1 and 4 illustrate the location of the
Air Conditioning (and Heating) System units. Each
of the units in the air conditioning system is d es­
cribed following:
AIR OUTLETS
Refrigerated air enters the interior of the cab
through four outlets below the instrument panel
(fig. 1).
Air outlets can be individually controlled to
provide a comfortable air flow in any direction de­
sired by the occupants.
EXPANSION VALVE
The expansion valve (fig. 5) consists of a cap­
illary bulb and tube which is connected to an oper­
ating diaphragm (which is sealed within the valve
itself) and an equalizer line which connects the
valve and the low pressure return line.
The purpose of the expansion valve is to reg­
ulate the flow of liquid refrigerant into the evap­
orator automatically in accordance to the require­
ments 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 super heat before more refriger­
ant is allowed to enter the evaporator. Superheat
is an increase in temperature of the gaseous re­
frigerant above the temperature at which the re­
frigerant vaporizes.
A capillary tube filled with carbon dioxide and
the equalizer line provide the temperature regu­
lation of the expansion valve. This capillary tube
is fastened to the low pressure refrigerant pipe
coming out of the evaporator so that it communi­
cates 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 automatically reduce the amount of refrigerant
entering the evaporator, thus reducing the amount
of cooling. If the superheat increases, the expan­
sion valve will automatically allow more refriger-
G M C S E R V I C E M A N U A L
ant to enter the evaporator, thus increasing the
cooling.
valve with the area behind the operating diaphragm
acts with the capillary to measure superheat.
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 passing over the evaporator is cool, the heat
transfer is small and a lesser quantity of refrig­
erant is required to cool the air and to achieve the
proper superheat on the refrigerant gas leaving
the evaporator.
tionate to the amount of heat (superheat) picked up
by the refrigerant gas in passing through the evap­
orator, it can be seen that adjusting spring tension
which works against capillary pressure and equal­
izer line pressure controls the volume of refrig­
erant entering the evaporator as signaled by the
temperature and pressure in the evaporator outlet
pipe.
operating, all pressures within the expansion valve
assembly will have equalized at the ambient (sur­
rounding air) temperature, thus the pressure above
and below the operating diaphragm and at the inlet
and outlet side of the valve will be equal (fig. 5).
AIR CONDITIONING
F ig u r e S — Expansion Valve
The equalizer line joining the suction throttling
It is the temperature of the air passing over
Since the evaporator outlet pressure is propor­
When the air conditioning system has not been
Sec. 1
Page 75