Opel 1900 1973 Service Manual page 535

1973 OPEL SERVICE MANUAL
greater than the opposing pressure in the power ele-
ment. Therefore, the valve remains closed. When the
compressor is started, it will reduce the pressure and
temperature of the refrigerant in the cooling coil to
a point where the vapor pressure in the power ele-
ment becomes the stronger. The seat then moves off
the orifice and liquid starts to flow through the valve
orifice into the cooling coil.
The purpose of the power element is to help deter-
mine the quantity of liquid that is being metered into
the cooling coil. As the temperature of the low pres-
sure line changes at the bulb, the pressure of
vapor in the power element changes, resulting in a
change of the position of the seat. For example, if the
cooling coil gets more liquid than is required, the
temperature of the low pressure line is reduced and
the resultant lowering of the bulb temperature
reduces the pressure of the vapor in the power ele-
ment, allowing the seat to move closer to the orifice.
This immediately reduces the amount of liquid leav-
ing the valve. Under normal operation, the power
element provides accurate control of the quantity of
refrigerant to the cooling coil.
To employ our tire pump analogy once more for
clarity, it is the same situation that would exist if you
were inflating a tire with a very slow leak. Providing
you pumped the air into the tire as fast as it leaked
out, you would be able to maintain pressure even
though the air would merely be circulating through
the tire and leaking out through the puncture.
To Sum Up
So far, we've discussed only what each unit in an air
conditioning system does. We've learned that the
evaporator is the unit in which liquid refrigerant
soaks up heat from the air, the compressor is a pump
for squeezing this heat out of the vapor, the con-
denser is a radiator for getting rid of the heat, and the
thermostatic expansion valve is a device for regulat-
ing the pressure on the refrigerant. Now, let's find
out how the temperature of the cooled air is con-
trolled.
METHOD OF TEMPERATURE CONTROL
To achieve temperature control, the compressor is
run intermittently, automatically turning on and off
as necessary to maintain proper temperature.
Thermostatic Switch
The compressor can be started and stopped au-
tomatically through the use of an electro-magnetic
clutch and a thermostat affected by variations of
temperature.
The job is usually done by a gas bulb thermostat (Fig.
the
Figure 9B-21 Thermostatic Switch Schematic
With the gas bulb type of thermostat, a highly expan-
sive gas is sealed into a metallic bulb which is located
in the air stream as it leaves the evaporator. A small
tube leads from the bulb to a bellows operated
switch. As air temperature rises, the gas inside the
bulb expands, travels through the tube to the bellows
and closes the electrical switch that engages the com-
pressor clutch.
Of course, as soon as the compressor starts running,
the temperature begins to go down. As the air being
cooled gets colder, the gas in the thermostat bulb
begins to reduce the pressure on the switch bellows.
This Ilips "off' the switch and disengages the com-
pressor clutch.
REFRIGERANTS
No matter how scientifically refrigerating machinery
is built or how efftciently it runs, it alone cannot
remove heat. The only thing that carries heat out of
a refrigerator cabinet or an automobile is the sub-
stance we call the refrigerant.
There are many refrigerants known to man. In fact,
any liquid that can boil at temperatures somewhere
near the freezing point of water can be used.
But a boiling point below the temperature at which
ice forms is not the only thing that makes a good
refrigerant. A refrigerant should also be non- poiso-
want a refrigerant that is non-corrosive and one that
will mix with oil.
Since Nature did not provide an ideal refrigerant,
chemists went to work to see if they could do any
better. They did! But it wasn't as simple as that.
At first, they tried to improve existing natural refrig-
erants. But after exploring innumerable trails along