Evaporator - Pontiac FIREBIRD 1972 Service Manual

1A-26 1972 PONTIAC SERVICE MANUAL
As the evaporator outlet pipe cools, the pressure of the
carbon dioxide in the capillary tube (contacting this outlet
pipe) decreases, exerting less force on the operating dia-
phragm.
The valve adjusting spring is calibrated so that the pres-
sure 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 com-
pletely vaporize and then warm (superheat) 10.6"F.
If the temperature differential begins to go below 10.6"F.
(outlet pipe becomes too cold), carbon dioxide pressure in
the capillary tube and the area above the diaphragm de-
creases, allowing the valve adjusting spring to move the
needle 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
evaporator.
EVAPORATOR
DESIGN
The evaporator core consists of a series of plates which
when joined together form the refrigerant tubes and the
top and bottom tanks. Between the tubes corrugated strips
of aluminum serve as air fins. This type of construction is
called a channel plate-type core. The nature of this design
is such that the refrigerant travels a relatively short dis-
tance with little or no pressure drop resulting between the
inlet and the outlet. Therefore, the inlet pressures and
outlet pressures are about equal and exactly controlled to
maintain the refrigerant boiling point at a temperature
which cools the air passing over the evaporator to a tem-
perature at or just above the freezing point of water.
The evaporator core with this design permits a very effi-
cient distribution of refrigerant at the moment refrigerant
enters the core.
The evaporator housing is constructed of a reinforced
plastic material for strength. A self-opening rubber nozzle
serves as a water drain and is located at the bottom of the
housing. A typical unit is shown in Fig. 1A-20.
FUNCTION
The evaporator is actually the device which cools and
dehumidifies the air before it enters the car. High pressure
.ARY
IVER
'OR
I Z E R
I E
E V A P O R A T O R
DRAIN
Fig. 1 A - 2 0 T y p i c a l Evaporator Core a n d Case Assembly
liquid refrigerant flows through the valve orifice in the
expansion valve into the low pressure area of the evapora-
tor. This regulated flow of refrigerant boils immediately.
Heat from the core surface is lost to the boiling and vapo-
rizing refrigerant, which is cooler than the core, thereby
cooling the core. 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 moisture (hu-
midity) in the air condenses on the outside surface of the
evaporator core and is drained off as water.
Since the refrigerant will boil at 21.7"F. below 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 passages. In order to control the temperature, it is
necessary to control pressure inside the evaporator and
this is done by the P.O.A. 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 P.O.A. valve is used to provide this necessary refriger-
ant and pressure control.