Opel 1900 1973, Service Manual

Page: 531 / 625

1 9 7 3 O P E L S E R V I C E M A N U A L
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
that the heat removed
just exactly equals the amount of heat that was neces-
sary to make the substance vaporize in the place.
At last the lost is found! The latent heat of vaporiza-
tion the heat that apparently disappeared when
a liquid boiled into a vapor again reappears on
the scene 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 tempera-
ture 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.
But pressure moves the fence! Water will boil at 212
d e g r e e s under normal conditions. Naturally, we ex-
pect 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 de-
grees. 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 at 21.7 degrees below 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. Also, the condensing point temperature
must be above that of room air or else the vapors
won’t
condense.
This is where pressure comes to the rescue. With
pressure, we can compress the vapor, thereby con-
centrating the heat it contains. When we concentrate
heat in a vapor that way, we increase the intensity of
the heat or, in other words, we increase the
temperature is merely a measurement
of heat intensity. And the most amazing part of it all
is that we’ve made the vapor hotter without actually
adding any additional quantity of heat (Fig.
Use of Pressure in Refrigeration
Because we must live by press&s and gauges in air
conditioning work, 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).
Figure Compressing a Vapor Concentrates its
Heat
Atmospheric Pressure is pressure exerted in every
direction by the weight of the atmosphere. At higher
altitudes air is
and has less weight. At sea
level atmospheric pressure is 14.7 psi.
Any pressure less than atmospheric 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
lbs. gauge pressure is
equivalent to 14.7 lbs. atmospheric pressure. Pres-
sure greater than atmospheric is measured in pounds
(psi) and pressure below atmospheric is measured in
inches of vacuum. The “0” on the gauge will always
correspond to the surrounding atmospheric pressure,
regardless of the elevation where the gauge is being
used.
Basic Refrigerator Operation
We’ve now covered all the ground-rules that apply to
refrigeration. Most likely they still are a little hazy,
but it is easy enough 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 knowledge, here is how we can build a
refrigerator.
We can place a flask of refrigerant in an ice-box. We
know it will boil at a very cold temperature and will
draw heat away from everything inside the cabinet
(Fig.
We can pipe the rising vapors outside the cabinet and
thus provide a way for carrying the heat out. Once