GMC 1976 ZEO 6083 Maintenance Manual page 52

* Inch es of Vacuum
Pressure and Flow
When we use a tire pump to inflate an automobile
tire, we are creating pressure only because we are
"pushing" against the air already entrapped inside
the tire . If a tire has a puncture in it, you could pump
all day, and still not be able to build up any pressure .
As fast as you would pump the air in, it would leak
out through the puncture . Unless you have some-
thing to push against - to block the flow of air -
you can't create more than a mere semblance of pres-
sure .
The same situation holds true in an air condition-
ing system . The compressor can pump refrigerant
vapor through the system, but unless it has some
thing 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 tem
peratures. This must be done sometime after the va-
por 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.
AIR CONDITIONING SYSTEM 1- 35
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 refrig-
erant, also drops lower and lower as the liquid goes
down .
By means of a simple system of mechanical link-
age, the downward movement of the float opens a
valve to let refrigerant in . The incoming liquid raises
the fluid level 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 con-
dition 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
We'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. With 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 cabi-
net (figure 9) .
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 out of "cold" vapor even in a
warm room . An ordinary radiator will help us get rid
of heat .
By removing the heat, and making the refrigerant
into a liquid, it becomes the same as it was before . So,
we can run another pipe back into the cabinet and
return the refrigerant to the flask to be used over
again.
°F .
`C .
I
Pressure
(psi)
+50 +10 46 .7
+55
+12.8
52 .0
+60 +15 .6
57 .7
+65 +18 .3
63 .7
+70
+21 .1
70 .1
+75
+23 .9 76 .9
+80 +26.7 84 .1
+85 +29 .4 91 .7
+90 +32.2 99 .6
+95 +35 108 .1
+100
+37.8 116.9
+105 +40.6 126.2
+110 +43 .3 136.0
+115 +46.1 146 .5
+120 +49 157.1
+125 +51 .7
167.5
+130 +54.4 179 .0
+140 60 204.5
+150 +65 .6 232.0