Chevrolet 70 Series 1970, Service Manual

Page: 135 / 841

CAB AND BODY MOUNTINGS 1-104
REFRIGERANT-12
PRESSURE-TEMPERATURE RELATIONSHIP
The table below in d icates the p re s s u re of
R efrigerant-12 a t v arious tem p e ra tu res. F or
instance, a d ru m of R efrig eran t a t a te m p e r­
a tu re of 80°F. w ill have a p re s s u re of 84.1
p si. If it is heated to 125 F . the p re s s u re will
in cre as e to 167.5 p sl. It a lso can be used con­
v e rsely to d eterm ine the te m p e ra tu re a t which
R efrigerant-12 boils u nder v arious p re s s u re s .
F o r exam ple, at a p re s s u re of 30.1 psl, Re­
frig e ra n t boils at 32 F.
TEMP. I*F.) PRESSURE (PSIG)
TEMP. (°F.) PRESSURE (PSIG)
-21.7
0 (atm ospheric
55
52.0
p re ssu re ) 60 57.7
-20 2.4 65 63.7
-10 4.5 70 70.1
- 5 6.8 75 76.9
0 9.2 80 84.1
5 11.8 85 91.7
10 14.7 90 99.6
15 17.7 95
108.1
20 21.1 100 116.9
25 24.6 105 126.2
30 28.5 110 136.0
32
30.1
115 146.5
35 32.6 120 157.1
40
37.0
125 167.5
45 41.7
130
179.0
50 46.7 140
204,5 T-4884
Figure 7—Pressure Temperature Chart
1. T em perature is a m easurem ent of the in­
ten sity of heat.
2. Heat is a form of energy. When heat is
added to a su b stan ce, it u sually i s noticed by an
in c r e a se in tem perature. For exam ple, in order
to r a is e the tem perature of w ater from 35 °F . to
1 0 0 °F ., it is n e c e ssa r y to add a certain amount of
heat.
3. When an object c o o ls , it does not absorb
cold , but rather it lo s e s heat to a colder object or
substance nearby. When a bottle containing w arm
liquid is placed on a cake of ic e , the ic e w ill m elt
and the b ottle and it s contents w ill b ecom e co o l.
Heat from the bottle and it s contents i s lo s t to the
ic e . *
4. When a liquid b o ils, turning to vapor, it
ab sorb s a great amount of heat. For in stan ce,
w ater boiling on a sto v e is absorbing a great
amount of heat fro m the burner a s it i s changing
to the vapor com m only ca lled stea m . B oilin g i s a
rapid form of evaporation.
When a liquid b o ils, it ab sorb s heat without
changing tem p eratu re. F or exam ple, when heat is
added to w ater at s e a le v e l, as when heating on a
sto v e , the tem perature of the w ater w ill r is e until
it rea ch es 212°F . If the w ater rem ain s on the hot
sto v e , it w ill b o il, but the tem perature w ill rem ain
at 2 1 2 °F . The heat being absorbed by the w ater is
changing it to stea m rather than ra isin g the tem p­
erature.
R e fr ig e r a n t-12 u sed in a ir conditioning sy ste m
b o ils at 2 1 .7 ° F ., below z e r o . Thus, if it w ere ex ­
p o sed to the a ir at norm al room tem p eratu re, it
would absorb heat from surrounding a ir and boil,
im m ed iately changing to a vapor.
5. When heat i s rem oved from w ater vapor,
it w ill condense back into a liquid. F or exam ple,
the stea m cau sed by boiling w ater on a sto v e w ill
condense into w ater on the u n derside of the co v er.
T h is i s due to the fa c t that the co v er is not a s hot
a s the stea m . The co v er, th erefo r e, tak es heat
fro m the steam , condensing it back to w ater.
6
. The tem perature at which su b stan ces w ill
b o il o r condense is affected by p r e s s u r e . R efer to
" P r e ssu r e T em perature Chart" (fig. 7). If the
p r e s s u r e is in c r e a se d , the liquid w ill not boil until
a higher tem perature is reached. Thus, w e can
p reven t refrigeran t fro m boiling if it i s kept under
high p r e s s u r e . If th is high p r e s s u r e i s suddenly
r e le a se d , refrigeran t w ill im m ed iately b oil. T his
h as been dem onstrated on m odern v e h ic le s with
p r e s s u r e cooling s y s te m s .
When the p r e s s u r e of a vapor i s in crea sed ,
the tem p eratu re at w hich it w ill condense i s a lso
r a ise d . Steam condenses below 2 1 2 °F ., if heat is
rem oved from it, but it can be made to condense
at higher tem perature by in c rea sin g the p r e ssu r e .
7. C om p ressin g a vapor in c r e a s e s it s tem p­
era tu re. For exam ple, when pumping a ir into a
tir e with hand pump, the pump w ill b ecom e w arm
due to the heating of the a ir a s it i s co m p ressed .
8
. When a liquid is heated until it i s converted
to a g a s, then th is g a s i s heated additionally w ith­
out changing p r e s s u r e , the gas i s sa id to be su p er­
heated. For in stan ce, in the evaporator, r e fr ig e r ­
ant ab sorb s heat and b o ils at a constant tem p er­
ature and p r e s s u r e until it has been com pletely
vap orized , and it continues to absorb heat fro m the
w arm a ir p a ssin g ov er the evaporator without any
in c r e a se in p r e s s u r e . Since th is heat is no longer
b eing u se d to convert the refrigeran t from a liquid
to a g a s, it w ill now cau se the tem perature of the
refrig era n t to r is e . The refrigeran t i s then su p er­
heated.
REFRIGERANT
The refrig era n ts u sed a r e com m only known
by th eir trade nam e of " F reon-12," "U con-12," or
"G enetron-12." R eg a rd less of brand, refr ig e ra n t-
12 m ust be u sed . The ch em ical nam e o f r e fr ig e r ­
ant
- 1 2
is dichlorodifluorom ethane (CCL
2
F
2
).
REFRIGERANT CHARACTERISTICS
R efrigerant e x is ts a s a gas at atm ospheric
p r e s s u r e and m ust be held under p r e s s u r e to r e ­
m ain liquid. At ordinary tem p era tu res, it w ill ex ist
a s a liquid under a p r e s s u r e of about 75 pounds
p e r -sq u a r e -in c h .
CHEVROLET SERIES 70-90 HEAVY DUTY TRUCK SHOP MANUAL