Chevrolet Light Duty Truck 1973 Service Manual page 50

1A-24 HEATER A ND AIR C O N D IT IO N IN G
Fig. 3 1- R eu sin g R e frig e ra n t
That is where we find the biggest difference between the
old icebox and the modern refrigerator. We used to put
in new ice to replace that lost by melting. Now we use
the same refrigerant over and over again.
We can change a vapor back into a liquid by chilling it,
or do the same thing with pressure. When we condense a
vapor we will find that the heat removed just exactly
equals the amount of heat that was necessary to make
the substance vaporize in the first place.
This is called the latent heat of vaporization - the heat
that apparently disappeared when a liquid boiled into a
vapor—again reappears - when that same vapor reverts
back into a liquid. It is just like putting air into a balloon
to expand it and then letting the same amount of air out
again to return the balloon to its original condition.
We know that any substance will condense at the same
temperature at which it boiled. This temperature point is
a clear-cut division like a fence. On one side, a substance
is a liquid. Immediately on the other side it is a vapor.
Whichever way a substance would go, from hot to cold
or cold to hot, it will change its character the moment it
crosses over the fence.
W ater will boil at 212° under normal conditions.
Naturally, we expect steam to condense at the same
temperature. But whenever we put pressure on steam, it
doesn't. It will condense at some temperature higher
than 212°. The greater the pressure, the higher the
boiling point and the temperature at which a vapor will
condense. This is 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
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.
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at 21.7° below zero. A
This is where pressure helps, with pressure, we can
compress the vapor, thereby concentrating the heat it
contains. When we concentrate heat in a vapor that way,
we increase the intensity of the heat or, we increase the
temperature, because temperature is merely a measure­
ment of heat intensity (fig. 32).
Pressure in Refrigeration
Because we must use pressures and gauges in air
conditioning service, the following points are mentioned
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 every
direction by the weight of the atmosphere. At sea level
atmospheric pressure is 14.7 psi. At higher altitudes air
has less weight (lower psi).
Any pressure less than atmospheric (14.7) is known as a
partial vacuum or commonly called a 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 measured in pounds
(psi) and pressure below atmospheric 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 saturated vapors.
Increasing the temperature of a substance causes it to
expand. When the substance is confined in a closed
container, the increase in tem perature will be accompa­
nied by an increase in pressure, 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 atmospheric) in pounds or negative
pressure (below atmospheric) in inches of vacuum.
Thus if a gauge is attached to a container of R-12 and
the room temperature is 70°, the gauge will register
approximately 70 psi pressure; in a 100° room, the
pressure would be 117 psi.
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
LIGHT DUTY TRUCK SERVICE MANUAL