Unwanted heat
- A heat may enter the vehicle by radiation, e.g.
- Heat radiated by the sun is conducted through the roof,
windows and body panels.
- Heat, which radiates upwards from the road, is conducted
through the floor panel.
- Occupants’ body heat, which radiates into the interior.
- Alternatively by conduction, e.g.
- Engine heat conducted through the bulkhead panel.
- Exhaust heat conducted through the floor panel.
Relative air humidity
Relative air humidity is very important for
driver and passenger comfort. A level of around 60% is desirable
as the air can easily absorb the moisture given off by the human
body. The role of the air conditioning system is therefore to maintain
a comfortable level of humidity at all times.
Evaporation
Evaporation is the process whereby the state
of a liquid is changed to a vapour. If for example water is boiled
it evaporates and changes its state to steam, as the evaporation
takes place the vapour produced absorbs heat: If more heat is applied
the temperature of the water will not increase, but the rate at
which it evaporates will.
Condensation
Condensation is the process whereby a vapour
when cooled, changes its state to a liquid. If for example the vapour
given off by boiling water is allowed to cool it will return to
a liquid state.
Refrigerant evaporation and condensation
Water is not a suitable refrigerant for air
conditioning as its boiling point is too high. Even at low pressures
no usable cooling effect could be achieved when it evaporates. For
that reason special refrigerants are used that evaporate at very
low temperatures (R134a boils at approximately -27°C at normal
atmospheric pressure). The temperature at which refrigerant evaporation
takes place varies according to pressure.
Refrigerant circuit
The heat from the car interior that is absorbed
during evaporation must be dispelled into the atmosphere and the
refrigerant vapour condensed back to a liquid. Therefore the refrigerant
in an automotive air conditioning system needs to be circulated
continuously |
