Part 1

Air conditioning system





Cooling process (overview)

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).

Components





Hoses, pipes
All connections have O-rings as seals. The O-rings are manufactured in a specially developed material (HNBR = hydrated nitrile rubber). The O-rings are in different colors (black - blue - yellow - green) depending on the type of refrigerant they are suited to.
There are three different types of connection depending on the connection and the car and model year.
- A: Connection with cap nut
- B: Block connection
- C: Quick-release connector (a special tool is required to separate the connection)
The plastic ring hanging on one of the pipes shows that the connection is correctly assembled. At fist the ring is located in one section of the connection but that jumps free when the connection is pressed together with the correct force.

Evaporator (air heated heat exchanger)
Available in different versions depending on the car and model year. However the version that is used has nothing to do with the type of system the car has.
In principle it consists of pipe loops in which the refrigerant flows. The loops have flanges to increase the heat absorbing surface.

Condenser (air cooled heat exchanger)
Available in different versions depending on the car and model year. However the version that is used has nothing to do with the type of system the car has.
In principle it consists of pipe loops in which the refrigerant flows. The loops have flanges to increase the heat radiating surface.
There are two main types - serial and parallel. Serial = one, two or three pipes that run forwards and backwards through the condenser (the loops are in series).
Parallel = several pipes ("flat pipes") which run parallel through the condenser. Each pipe has a large number of channels and the pipes are connected together at the face of the condenser.

Receiver drier
In principle it is a reservoir containing a drying agent.
The internal design with piping depends on the system it is intended for. On systems with a TEV valve there is a filter in the receiver drier. The filter collects any impurities in the system.
On systems with an expansion pipe, the corresponding filter is integrated with the expansion pipe.

Compressor
There are a number of different makes and versions depending on the engine variant and the car and model year.
The different versions are:
- wing compressor.
- piston compressor with 2 - 4 - 5 - 6 or 7 cylinders.
- piston compressor with fixed or variable cylinder displacement.
This is mechanical and is belt driven by the engine.
Has an electro-magnetic clutch which makes it possible to engage/disengage the compressor.

Electro-magnetic clutch
The clutch consists of:
- a solenoid coil mounted directly on the compressor housing.
- a flexible plate mounted on the compressor shaft.
- a pulley positioned on the front face of the compressor.
When the compressor is disconnected the pulley rotates freely without effecting the compressor.
When the solenoid coil is supplied with current the flexible plate is drawn into contact with the pulley.
Power is transferred from the pulley via the plate to the compressor shaft.
The way in which the engagement/disengagement (the current to the solenoid coil) is governed varies depending on type of system, engine variant and the car and model year.

Compressor with variable cylinder displacement




The pistons are driven by a cam pulley, the angle of which can vary.
When the compressor is disengaged, the angle is determined by the springs.
When the compressor is engaged, the angle is determined by the pressure affecting the top side of the pistons (= intake pressure) and lower side (= pressure in the crank case) during the intake phase.
The pressure on the lower side of the pistons (= in the crank case) is governed by a valve which keeps the intake pressure constant.

High intake pressure = large cylinder displacement
The valve opens and lowers the pressure in the crank case. The back pressure on the reverse of the pistons is then reduced and the angle of the cam pulley increases.
An increased angle gives an increased cylinder stroke which results in a greater volume of refrigerant being sucked in and the outlet pressure dropping.

Low intake pressure = small cylinder displacement
The valve closes and the pressure in the crank case increases. The pressure is built up by the refrigerant which is led via a calibrated channel from the outlet side to the crankcase.
The back pressure on the reverse of the pistons is then increased and the angle of the cam pulley reduces.
A reduced angle gives an reduced cylinder stroke which results in a smaller volume of refrigerant being sucked in and the outlet pressure increasing.

The advantages of this type of compressor are:
The flow of refrigerant is continually adapted to requirements.
Few engagements/disengagements providing good driving comfort.

System with expansion pipe





Expansion pipe ("orifice")
Passive flow regulator (= fixed choke valve).
Consists of a thin metal pipe which is positioned in a plastic housing with a filter on both the intake and outlet sides. The filter collects any impurities in the system. Passive flow regulator (= fixed choke valve).
Governs the volume of refrigerant which is released from the evaporator.
The volume is affected by the dimensions of the pipe (diameter, length) and the pressure and temperature on both sides of the pipe.
The pipe is adjusted for its system and for the type of refrigerant that is used.
The pipe for the R12 has a greater diameter than the pipe for the R134a. This is because the R12 and oil mixture has a higher viscosity than the oil and R134a mixture.

Governs the system's capacity and functions as an anti-freeze
Consists of a switch which is affected by the pressure of the refrigerant downstream of the evaporator, in other words on the lower pressure side (the pressure switch (Pressostat) must be positioned as close to the evaporator outlet as possible.
If the pressure is too low (= too low temperature the pressure switch (Pressostat) interrupts a current circuit and the compressor is switched off. This is so that the water that condenses on the evaporator does not freeze to ice and obstruct the airflow through the evaporator. Heavy build up of ice can also cause damage to the evaporator.
When the pressure (= temperature) increases again, the pressure switch (Pressostat) closes the current circuit and the compressor is engaged.
The pressure switch (Pressostat) opens at approximately 1.6 bar and closes at approximately 3.2 bar.

Receiver drier
On systems with an expansion pipe, the pipe may in some cases allow through a little too much refrigerant. The result is that a little of the refrigerant will be released from the evaporator in liquid form. To prevent damage to the compressor, the receiver drier must be positioned on the low pressure side and be able to separate liquid from gas.
The receiver drier outlet is located so that only refrigerant in gas form can leave the receiver drier. Refrigerant in liquid form is collected in the bottom of the receiver drier.
The refrigerant in the bottom of the receiver drier contains a relatively high amount of oil. This mixture (refrigerant and oil) is led through a calibrated hole out into the outlet pipe and is transferred with the gas back to the compressor.
The hole plays a very important role in the return of oil to the compressor.