Chevrolet Light Duty Truck 1973 Service Manual page 51

Pressure (psi)
°F
11 0
-4 0
. *
8 3
-3 5
. *
5 5
-3 0
. *
2 3
-2 5
. *
0.6
-2 0
2.4
15
-
10 4.5
-
6.8
5
-
0 9.2
11.8
5
+
14.7
10
+
15 17.7
+
20 21.1
+
24.6
25
+
30 28.5
+
32 30.1
+
32.6
35
+
37.0
40
+
45 41.7
+
In c h e s o f V a c u u m
pump the air in, it would leak out through the puncture.
Unless you have something to push against—to block the
flow of air— you can't create more than a mere semblance
of pressure.
The same situation holds ture in an air conditioning
system. The compressor can pump refrigerant vapor
through the system, but unless it has something to push
against, it cannot build up pressure. All the compressor
would be doing would be to circulate the vapor without
increasing its pressure.
We can't just block the flow through the system entirely.
All we want to do is put pressure on the refrigerant
vapor so it will condense at normal temperatures. This
must be done sometime after the vapor leaves the
evaporator and before it returns again as a liquid. High
pressure in the evaporator would slow down the boiling
of the refrigerant and penalize the refrigerating effect.
Controlling Pressure and Flow
Pressure and flow can be controlled with a float valve, or
with a pressure-regulating valve.
The float valve type will give us a better idea of pressure
and flow control, let's look at it first.
It consists simply of a float that rides on the surface of
the liquid refrigerant. As the refrigerant liquid boils and
passes off as a vapor, naturally the liquid level drops
lower and lower. Correspondingly, the float, because it
rides on the surface of the refrigerant, also drops lower
and lower as the liquid goes down.
By means of a simple system of mechanical linkage, the
downward movement of the float opens a valve to let
refrigerant in. The incoming liquid raises the fluid level
°F Pressure (psi)
50 46.7
+
55 52.0
+
60 57.7
+
65 63.7
+
70 70.1
+
76.9
75
+
80 84.1
+
91.7
85
+
90 99.6
+
108.1
95
+
100 116.9
+
105 126.2
+
110 136.0
+
146.5
115
+
157.1
120
+
125 167.5
+
130 179.0
+
204.5
140
+
150 232.0
+
HEATER AND AIR C O N D IT IO N IN G
and, of course, the float rides up along with it. When the
surface level of the refrigerant liquid reaches a desired
height, the float will have risen far enough to close the
valve and stop the flow of refrigerant liquid.
We have described the float and valve action as being in
a sort of definite wide open or tight shut condition.
Actually, the liquid level falls rather slowly as the
refrigerant boils away. The float goes down gradually
and gradually opens the valve just a crack. At such a
slow rate of flow, it raises the liquid level in the
evaporator very slowly.
It is easy to see how it would be possible for a stablized
condition to exist. By that, we mean a condition wherein
the valve would be opened enough to allow just exactly
the right amount of refrigerant liquid to enter the system
to take the place of that leaving as a vapor.
Refrigerator Operation
W e've now covered all the scientific ground-rules that
apply to refrigeration. Try to remember these main
points. All liquids soak up lots of heat without getting
any warmer when they boil into a vapor, and, we can use
pressure to make the vapor condense back into a liquid
so it can be used over again. W ith just that amount of
scientific knowledge, here is how we can build a
refrigerator.
We can place a flask of refrigerant in an icebox. We
know it will boil at a very cold temperature and will
draw heat away from everything inside the cabinet (fig.
32).
We can pipe the rising vapors outside the cabinet and
thus provide a way for carrying the heat out. Once we
get the heat-laden vapor outside, we can compress it with
a pump. With enough pressure, we can squeeze the heat
Fig. 32--Basic Refrigerant Circuit
1A-25
LIGHT DUTY TRUCK SERVICE MANUAL