Cooling Process (Overview)

WARNING: This page is about a different car, the 2002 Volvo C70, 2001 Volvo C70, 2000 Volvo C70, 1999 Volvo C70, and 1998 Volvo C70. However, it is still accessible from the selected car via links, so may be relevant.

Contained system

The main components in the system are: evaporator - receiver drier - compressor - condenser - choke valve. The components are connected to each other by pipes and/or hoses.

The system is filled with a suitable quantity of refrigerant and oil. The role of the oil is to lubricate and seal (piston-cylinder) the compressor.

Because it is the refrigerant that draws the oil around the system, a lack of refrigerant can result in noise from the compressor or cause the compressor to break down due to a lack of oil.

It is important that the system is used regularly. This is so that:

the compressor is kept lubricated.
the refrigerant circulates so that moisture (water) that diffuses into the system (= forces in through the hoses etc.) can be taken up by the drying agent in the receiver dryer.

Evaporator

The evaporator is located in the climate control system's passenger compartment unit and the blower fan blows air through the evaporator.

In the evaporator there is a low pressure due to the choke valve and the suction effect of the compressor.

When the refrigerant enters the evaporator via the choke valve, it expands and both its pressure and temperature lower. The refrigerant then begins to boil and evaporate. The heat (energy) used for evaporation is taken from the air that is cooled.

When the warm air encounters the cold evaporator, the humidity condenses on the evaporator. The heat (energy) that is released during condensation is transferred to the refrigerant which evaporates. The temperature difference between the air and the refrigerant reduces. High humidity results in an increased need for cooling.

To obtain sufficient cooling capacity, the refrigerant's evaporation temperature must be considerably lower than the temperature desired in the passenger compartment. But to prevent the moisture that has condensed on the evaporator from freezing to ice, the air is not cooled below approximately +30°C (86°F) (at which stage the temperature of the refrigerant in the evaporator is approximately -30°C (-22°F)).

Receiver drier

Receiver drier:

contains a drying agent that collects and binds moisture (water) in the system.
stores refrigerant in the system which is temporarily not needed.
separates refrigerant into gas form and liquid form.

The receiver drier is located on either the low pressure or the high pressure side, depending on the type of system (= type of choke valve).

Compressor

Compressor:

sucks refrigerant in gas form from the evaporator.
compresses the gas thereby increasing its pressure and temperature.
expels the gas with high pressure and temperature to the condenser.

The compressor is mechanical and is driven by the car engine. When the compressor is operating it "steals" 0.5-8 kW (0.7-11 bhp) from the engine. This may be noticed as slight jerks when driving as the compressor is engaged/disengaged.

Condenser

The condenser is located in front of the engine radiator and charge air cooler (CAC) if installed. The breaking wind and engine cooling fan (FC) blow air through the condenser.

When the refrigerant enters the condenser it is in gas form and has both high pressure and temperature.

In the condenser the hot refrigerant loses some of its heat to the cooler air. The refrigerant then condenses and changes into liquid form.

The amount of heat given off = the amount of heat taken up in the evaporator + the amount of heat supplied by the compressor.

In order to supply sufficient cooling capacity, the condensation temperature of the refrigerant must be greater than the temperature of the outside air.

Choke valve

Upstream of the choke valve is the refrigerant in liquid form at high pressure.

The choke valve ensures that the correct amount of refrigerant is released to the evaporator. The quantity of refrigerant must be sufficient that the final remains evaporate precisely before the evaporator outlet. Too little refrigerant the refrigerant evaporates quickly and the steam is super heated = reduced cooling capacity. Too much refrigerant some of the refrigerant does not evaporate but is merely heated up = reduced cooling capacity. In addition there is the risk that some of the refrigerant in liquid from reaches the compressor. This may damage the compressor because liquid cannot be compressed.

The choke valve consists of an expansion pipe or a TEV valve (thermostatic Expansion Valve) depending on the type of system.

Pressure and temperature in the air conditioning system

The values may vary depending on the type of system, operational conditions etc.
The values listed below are only intended to give an indication of size.
Downstream of the evaporator gas form, 1.6-3.5 bar; -2 to -12°C (28 to 10°F).
Downstream of the compressor, upstream of the condenser gas form, 10-28 bar; +65 to +120°C (+149 to +248°F).
Downstream of the condenser, upstream of the choke valve liquid form, 9-24 bar; +40 to +80°C (+104 to +176°F).
Directly downstream of the choke valve partly in liquid form, 1.5-3.2 bar; -4 to -10°C (25 to 14°F).