Volkswagen Golf / Golf GTI / Jetta. Manual - part 994

  

 

When water is heated in a vessel (heat absorption), water 
vapor can be seen to rise. If the vapor is further heated 
through heat absorption, the visible vapor turns into 
invisible gas. The process is reversible. If heat is extracted 
from water in gaseous form, it changes first to vapor, then 
to water and finally to ice.

A - Heat absorption 

B - Heat emission 

Heat always flows from warmer to colder 
substance

 

 

Every substance consists of a mass of moving molecules. 
The fast moving molecules of a warmer substance give off 
some of their energy to the cooler and thus slower 
molecules. As a result, the molecular motion of the warmer 
substance slows down and that of the colder substance is 
accelerated. This process continues until the molecules of 
both substances are moving at the same speed. They are 
then at the same temperature and no further heat 
exchange takes place.

Pressure and boiling point

 

 

The boiling point given in tables for a liquid is always 
referenced to an atmospheric pressure of 1 bar. If the 
pressure acting on a liquid changes, its boiling point also 
changes.

 

For example, water boils at a lower temperature the lower 
the pressure.

The vapor pressure curves for water and refrigerant R134a 
show for example that, at constant pressure, reducing the 
temperature changes vapor to liquid (in condenser) or that, 
for instance, reducing pressure causes the refrigerant to 
change from liquid to vapor state (evaporator). 

 

Vapor pressure curve of water

A - liquid 

B - gaseous 

C - Vapor pressure curve of water 

1 - Pressure acting on liquid in bar (absolute) 

2 - Temperature in C  

General information for A/C system

13/2/2005

  

 

 

Vapor pressure curve of refrigerant R134a

A - liquid 

B - gaseous 

D - Vapor pressure curve of refrigerant R134a 

1 - Pressure acting on liquid in bar (absolute) 

2 - Temperature in C  

Vapor pressure table for refrigerant R134a

 

 

The vapor pressure table for every refrigerant is published 
in literature for refrigeration system engineers. This table 
makes it possible to determine the vapor pressure acting 
on the column of liquid in a vessel if the temperature of the 
vessel is known.

 

As there is a characteristic vapor pressure table for every 
refrigerant, refrigerant can be identified by measuring 
pressure and temperature.

Note: 

- At absolute pressure, 0 bar corresponds to absolute 
vacuum. Normal ambient pressure (positive pressure) 
equals 1 bar absolute pressure. 0 pressure corresponds to 
an absolute pressure of one bar on most pressure gauges 
(indicated by -1 bar below 0). 

Temperature in   C 

Pressure in bar (positive pressure) of R134a

-45

-0.61

-40

-0.49

-35

-0.34

-30

-0.16

-25

0.06

-20

0.32

-15

0.63

-10

1.00

-5

1.43

0

1.92

5

2.49

10

3.13

15

3.90

20

4.70

25

5.63

30

6.70

35

7.83

General information for A/C system

13/2/2005

 

40

9.10

45

10.54

50

12.11

55

13.83

60

15.72

65

17.79

70

20.05

75

22.52

80

25.21

85

28.14

90

31.34

Refrigerant R134a

 

 

Vehicle air conditioning systems make use of the 
vaporization and condensation process. Use is made of a 
substance with a low boiling point, which is called 
refrigerant.

The refrigerant employed is tetrafluoroethane R134a, which 
boils at - 26.5   C at a vapor pressure of 1 bar. 

Physical data of refrigerant R134a

 

Chemical formula

CH2FCF3 or CF3CH2F

Chemical designation

Tetrafluoroethane

Boiling point at 1 bar

- 26.5   C 

Solidification point

-101.6   C 

Critical temperature

100.6   C 

Critical pressure

40.56 bar (absolute)

Critical point

 

 

The critical point (critical temperature and critical pressure) 
is that above which there is no longer a boundary between 
liquid and gas.

 

A substance above its critical point is always in the 
gaseous state.

 

At temperatures below the critical point, all types of 
refrigerant in pressure vessels exhibit both a liquid and a 
gas phase, i.e. there is a layer of gas above the liquid.

As long as both liquid and gas are present in the vessel, 
the pressure is governed by ambient temperature 

 

00-1, 

Vapor pressure table for refrigerant R134a

 . 

Note: 

- Different types of refrigerant are never to be mixed. Only 
the refrigerant designated for the corresponding A/C 

General information for A/C system

13/2/2005

system may be used. 

Environmental aspects of refrigerant R134a

 

- R134a is a fluorocarbon and contains no chlorine.  

- R134a has a shorter atmospheric lifespan than refrigerant 
R12. 

- R134a does not deplete the ozone layer.  

- The global warming effect of R134a is ten times less than 
that of refrigerant R12. 

Characteristics of refrigerant R134a

 

Trade names and designations

 

 

The refrigerant R134a is currently available under the 
following trade names:

- H-FKW 134a 

- SUVA 134a 

- KLEA 134a 

Note: 

- Different trade names may be used in other countries.  

- Of the wide range of refrigerants available, this is the only 
one which may be used for vehicles. The designations 
Frigen and Freon are trade names. They also apply to 
refrigerants which may not be used in automotive vehicles.  

Color

 

 

Like water, refrigerants are colorless in both vapor and 
liquid form. Gas is invisible. Only the boundary layer 
between gas and liquid is visible. (Liquid level in tube of 
charging cylinder or bubbles in sight glass). Refrigerant 
R134a liquid may appear colored (milky) in a sight glass. 
This cloudiness is caused by partially dissolved refrigerant 
oil and does not indicate a malfunction.

Vapor pressure

 

In a partially filled, closed vessel, the quantity of refrigerant 
evaporating from the surface equals the quantity returning 
to the liquid state as vapor particles condense. This state of 
equilibrium occurs under the influence of pressure and is 
often called vapor pressure. Vapor pressure is a function of 

General information for A/C system

13/2/2005