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

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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

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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