GMC 4000 Series Service Manual page 79

Sec. 1
Page 72
AIR CONDITIONING
Subject
General Operation
Fundamental Principles of Refrigeration
R e fr ig e r a n t............................................................................................
Refrigerant Circulation
Air C ir c u la tio n .....................................................................................
Operation of Individual U n i t s ............................................................
GENERAL OPERATION
Some controls and units used with the air con­
ditioning system are common to the heating sy s­
tem. These controls and units are: Control switches,
heater core, air intake and distribution ducts.
The heating and cooling system s operate in­
dependently of each other, except under certain
conditions of cooling system operation when there
is an overlapping operation of both system s as
explained previously under "Operating Instructions."
NOTE
Refer to applicable Wiring Diagram
Manual for schem atics of air conditioning
electrical circuits.
REFRIGERANT-12
PRESSURE-TEMPERATURE R ELATION SH IP
The tab le b elow in d ic a te s the p r e ss u r e of
R e fr ig e r a n t-12 at v a rio u s te m p er a tu re s. For
in sta n ce, a drum of R efrig era n t at a te m p e r ­
atu re of 8 0 ° F . w ill have a p r e ss u r e of 84.1
p s i. If it is heated to 1 2 5 °F . the p r e ss u r e w ill
in c r e a se to 167.5 p s i. It a ls o can be u sed co n ­
v e r s e ly to d e te r m in e the tem p era tu re at which
R e fr ig er a n t-1 2 b o ils under va rio u s p r e s s u r e s .
F or exam ple, at a p r e ss u r e of 30.1 p s i, R e­
fr ig e r a n t b o ils at 32 F .
TEMP. (°F.) PRESSURE (PSIG)
0 (a tm o sp h eric
-2 1 .7
p r e ssu r e )
2.4
-2 0
4.5
-1 0
6.8
- 5
9.2
0
11.8
5
14.7
10
17.7
15
20 21.1
25 24.6
28.5
30
32 30.1
32.6
35
40 37.0
41.7
45
46.7
50
Figure 3 Pressure-Tem perature Chart
—
G M C S E R V I C E M A N U A L
SYSTEM OPERATION
Contents of This Sub-Section are as Follows:
..............................................................................
...................................................................
PRESSURE (PSIG)
TEMP. (°F.)
52.0
55
57.7
60
63.7
65
70.1
70
75 76.9
84.1
80
91.7
85
90 99.6
108.1
95
116.9
100
126.2
105
110 136.0
115 146.5
157.1
120
167.5
125
179.0
130
140 204.5
Page No.
....................................
FUNDAMENTAL PRINCIPLES OF
REFRIGERATION
The principle of operation of the refrigeration
system is based on a few simple laws of physics
which are stated informally as follows:
1. Temperature is a measurement of the in­
tensity of heat.
2. Heat is a form of energy. When heat is
added to a substance, it usually is noticed by an
increase in temperature. For example, in order
to raise the temperature of water from 35°F. to
100°F., it is necessary to add a certain amount of
heat.
3. When an object cools, it does not absorb
cold, but rather it loses heat to a colder object or
substance nearby. When a bottle containing warm
liquid is placed on a cake of ice, the ice will melt
and the bottle and its contents will become cool.
Heat from the bottle and its contents is lost to the
ice.
4. When a liquid boils, turning to vapor, it
absorbs a great amount of heat. For instance,
water boiling on a stove is absorbing a great
amount of heat from the burner as it is changing
to the vapor commonly called steam. Boiling is a
rapid form of evaporation.
When a liquid boils, it absorbs heat without
changing temperature. For example, when heat is
added to water at sea level, as when heating on a
stove, the temperature of the water will rise until
it reaches 212°F. If the water remains on the hot
stove, it will boil, but the temperature will remain
at 212 F. The heat being absorbed by the water is
changing it to steam rather than raising the temp­
erature.
Refrigerant-12 used in air conditioning s y s ­
tem boils at 21.7 F., below zero. Thus, if it were
exposed to the air at normal room temperature, it
would absorb heat from surrounding air and boil,
immediately changing to a vapor.
5. When heat is removed from water vapor,
it will condense back into a liquid. For example,
the steam caused by boiling water on a stove will
condense into water on the underside of the cover.
72
72
73
74
75
75