GMC 1976 ZEO 6083 Maintenance Manual page 51

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AIR CONDITIONING SYSTEM
the reason why pressure cookers cook food faster,
since the pressure on the water permits it to boil out
at a higher temperature.
We know that R-12 boils at 21 .7°F . (-5 .7°C .) be-
low zero . A thermometer will show us that the rising
vapors, even though they have soaked up lots of heat,
are only slightly warmer . But the vapors must be
made warmer than the room air if we expect heat to
flow out of them . The condensing point temperature
must be above that of room air or else the vapors
won't condense .
This is where pressure helps, with pressure, we
can compress the vapor, thereby concentrating the
heat it contains . When we concentrate heat in a va-
por that way, we increase the intensity of the heat or,
we increase the temperature, because temperature is
merely a measurement of heat intensity (figure 9) .
Pressure in Refrigeration
Because we must use pressures and gauges in air
conditioning service, the following points are men-
tioned so that we will all be talking about the same
thing when we speak of pressures.
All pressure, regardless of how it is produced, is
measured in pounds per square inch (psi) .
Atmospheric Pressure is pressure exerted in ev-
ery direction by the weight of the atmosphere . At sea
level atmospheric pressure is 14 .7 psi . At higher alti-
tudes air has less weight (lower psi) .
Any pressure less than atmospheric (14.7) is
known as a partial vacuum or commonly called a
A COMPRESSOR'
INSTALLED HERE
WOULD COMPLETE
THIS BASIC
SYSTEM
THESE THREE COMPONENTS
" EVAPORATOR
" CONDENSER
" COMPRESSOR
ARE USED IN EVERY
REFRIGERATION CIRCUIT
Figure 9-Basic Refrigerant Circuit
vacuum . A perfect vacuum or region of no pressure
has never been mechanically produced .
Gauge pressure is used in refrigeration work .
Gauges are calibrated in pounds (psi) of pressure and
inches of Mercury for vacuum . At sea level, "O" lbs.
gauge pressure is equivalent to 14 .7 lbs. atmospheric
pressure . Pressure greater than atmospheric is mea-
sured in pounds
(psi)
and pressure below atmos-
pheric is measured in inches of vacuum . The "O" on
the gauge will always correspond to the surrounding
atmospheric pressure, regardless of the elevation
where the gauge is being used .
Pressure-Temperature Relationships of R-12
A definite pressure and temperature relationship
exists in the case of liquid refrigerants and their satu-
rated vapors . Increasing the temperature of a sub
stance causes it to expand . When the substance is
confined in a closed container, the increase in tem-
perature will be accompanied by an increase in pres-
sure, even though no mechanical device was used .
For every temperature, there will be a corresponsing
pressure within the container of refrigerant . A table
of the temperature-pressure relationship of R-12 is
presented below. Pressures are indicated in gauge
pressure, either positive pressure (above atmos-
pheric) in pounds or negative pressure (below atmos-
pheric) in inches of vacuum.
Thus if a gauge is attached to a container of R-12
and the room temperature is 70°F. (21 .1°C .), the
gauge will register approximately 70 psi pressure; in
a 100°F. (37.8°C .) room, the pressure would be 117
psi.
°F. °C .
Pressure
(psi)
-40 -40 11 .0*
-35 -37.2 8 .3*
-30 -33.3 5 .5*
-25 -31 .7 2.3*
-20 -29 0.6
-15 -26.1 2.4
-10 -23.3
4.5
-5
-20.6 6.8
0 -17.8 9.2
+5 -15 11 .8
+10
-12 .2 14 .7
+15
-9 .4 17 .7
+20 -6 .7 21 .1
+25 -3 .9 24 .6
+30 -1 .1 28 .5
+32 0 30 .1
+35 +1 .7 32 .6
+40 +4 .4 37 .0
+45 +7 .2 41 .7