Basic Control Switches

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Pressure switching devices

Low pressure switch

Reacts to the pressure in the low side (suction side) of the system, generally the accumulator and disengages the compressor clutch if the pressure drops below approximately 1.5 bar. On an FOV system the low pressure switch and cycling switch are often the same device.

Dual pressure – high pressure switch and condenser fan switch

Two pressure-sensitive switches are contained in the high pressure switch. One of these switches acts as a safety switch to prevent excessive system pressure. The second switch

switches the condenser auxiliary fan on to its second setting at approximately 20.7 bar and off again at 17.2 bar. This switching process improves the performance of the system in cases of excessive heat.

High pressure switch

A single normally closed pressure switch de-energises the compressor if excessive high pressure exists within the A/C system, approximately 30–35 bar. The switch is normally positioned in the high side of the system.

Trinary switch

Three pressure sensitive switches are integrated into a trinary switch.A low pressure switch which creates an open circuit thus removing the current flowing to the compressor if the system pressure drops below approximately 1.4 bar.This could be caused by a refrigerant leak or natural discharge over a number of years. A high pressure switch operates at a pressure of approximately 30 bar which again removes the A/C compressor’s current in the event of a system blockage anywhere in the system. The third switch is used for high speed operation of the condenser fan aiding the removal of heat.This operates at approximately 18 bar.The switch is positioned on the high pressure side on TXV controlled systems. If FOV is used then a cycling switch is incorporated into the low side with a dual pressure switch on the high side. Modern A/C systems are replacing switches with a single pressure sensor.

Pressure sensors

The pressure sensor contains two metal plated ceramic discs mounted in close proximity. The disc located closest to the pressure connection is thinner and bends when subjected to pressure. By this means, capacitance between the metal plating of the discs is changed based on the pressure. A circuit integrated in the sensor converts the capacitance to an analogue voltage (see Table 2.3). For more information, see Section 3.2

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Anti Frosting Devices

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The evaporator pressure regulator (Figs 2.43 and 2.44) is mounted between the outlet of the evaporator and the compressor inlet (suction side).The valve regulates the pressure inside the evaporator to prevent icing. If the pressure drops below a certain threshold (196 kPa (28.4 psi)) then the valve closes to restrict the flow of refrigerant and increase the pressure inside the evaporator. This is to stop the evaporator temperature from reaching 0°C due to the relationship between temperature and pressure.

When the cooling load is high the vapour pressure of the refrigerant in the evaporator is high. The valve fully opens and the refrigerant flows unobstructed to the compressor.The valve operation is based on a spring bellows which expands and contracts with changes in refrigerant pressure.This device virtually eliminates the need for the compressor to cycle on and off to regulate the temperature of the evaporator.

De-ice switch (TXV system)

A de-ice switch (Fig. 2.45) is a temperature sensor and relay built as one unit. The temperature sensor is fitted to the evaporator’s fins and measures the temperature of the evaporator surface using an NTC type temperature sensitive resistor.This sensor sends the information to the relay in the form of a voltage drop and when approaching the freezing point of water (0°C) the current to the compressor clutch is interrupted to increase the pressure in the evaporator and avoid the surface water freezing. With a system threshold of 1°C at the surface, the relay will turn the compressor off. Once the surface increases to 2.5°C, the compressor, via the relay, will be switched

on again.

Evaporator sensor

An evaporator sensor is used on a system which generally uses electronic control as opposed to electrical control. These systems use the temperature sensor to feed a voltage reading to a control module which is programmed to use the data and compare it to stored data in its memory. The sensor is generally an NTC type sensor which means it has a negative temperature coefficient.This means that with an increase in temperature the resistance of the sensor will reduce. This will affect the current flowing through the sensor and the voltage across the sensor. The module can apply this data and when the corresponding voltage is sensed, associated with 1°C, the module will disengage the compressor clutch via a relay to stop the evaporator from freezing.The compressor clutch will be re-engaged when the temperature of the evaporator reaches 2.5°C. The sensor is generally fitted to the evaporator fins for direct measurement. Advanced vehicle electronic systems may use a multiplex wiring system to transfer the information digitally (see Chapter 3, section 3.2).

Compressor cycling switch (FOV system)

If the temperature in the evaporator approaches 0°C then icing will occur on the surface of the evaporator due to the water droplets which form. This will reduce the volume of air flowing through the evaporator and reduce the efficiency of the system. To prevent this a compressor cycling switch (Fig. 2.46) is fitted to the accumulator to deactivate the compressor clutch when a specific pressure is reached. For example, when 1.5 bar is reached on the low pressure side of

the system the switch contacts will open and directly or indirectly interrupt the current flowing to the compressor clutch. Once the pressure rises to 2.9 bar then the switch contacts will close again and the compressor will re-engage. Often the cycling switch is also used as a low pressure switch in case there is a system leak and the refrigerant escapes. In this case the switch will open at 1.5 bar and disengage the compressor clutch ensuring the compressor is not damaged due to no refrigerant flow (refrigerant carries the lubricant to lubricate the compressor).

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The Evaporator

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The evaporator is very similar in construction to a condenser. An evaporator will have a serpentine, tube and fin or parallel type construction. The function of an evaporator is to provide a large surface area to allow the warm often humid air to flow through it releasing its heat energy to the refrigerant inside.

The refrigerant by this time will have just had a large pressure and temperature (Fig. 2.41) drop coming through the expansion/fixed orifice valve causing it to want to boil and just requiring the heat energy to do so. The evaporator absorbs the heat energy from the air flowing over its surface. The energy is transferred and the refrigerant reaches saturation point. At this point the refrigerant can still absorb a small amount of heat energy.The refrigerant will do so and become superheated. The superheated refrigerant will then flow to the compressor (TXV system) or accumulator (FOV system).

The evaporator is extremely cold at this stage and any moisture in the air flowing through the evaporator will adhere to the evaporator’s surface. The water droplets on the surface help

clean the incoming air by trapping dirt and foreign particles. The humidity content is also reduced so cleaner drier air is delivered to the interior of the vehicle. This improves the comfort level especially in high humid conditions and allows perspiration to evaporate more quickly.The moisture drips off the evaporator’s surface into a drain which directs it to the outside of the vehicle via a duct.

Dehumidified air is very effective for window defogging due to a large number of passengers in a vehicle and/or humid conditions.

Design

The design of an evaporator is based on the size, shape, number of tubes and fins and the number of rows. This is to maximise the flow rate and surface area. The evaporator is tested for the maximum amount of heat and moisture which can be removed by the evaporator within a given period.

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The expansion Valve or Fixed Orifice Valve

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To control the amount of refrigerant volume flowing through the evaporator a metering device must be used. The function of the metering device is as follows:

● To separate the high pressure and low pressure side of the system.

● To meter the volume of refrigerant and hence the cooling capacity of the evaporator.

● To ensure that there is superheated refrigerant exiting the evaporator.

Currently there are two main categories of metering device used, a Thermostatic Expansion Valve (TXV or TEV) and a Fixed Orifice Valve (FOV). The pressure drop across the evaporator is used to determine which type of valve is the most appropriate. Simple air-conditioning systems will generally use only one of these metering devices. Dual air-conditioning systems may use both a TXV and an FOV within the system. 

Superheat

Superheat within an expansion system is very important. It ensures that the entire liquid refrigerant inside the evaporator has been vaporised from a liquid. It is generally measured by the difference in the boiling point of the refrigerant between the inlet and the outlet of the evaporator which can be as high as a 10°C difference. Evaporators, depending on their design characteristics, operate within different amounts of superheat and expansion valves are matched against this by adjusting the superheat valve spring tension.

Thermostatic expansion valve

There are also some variations in the design of a TXV. 

● Internally equalised thermostatic expansion valve.

● Externally equalised expansion valve.

● Box or H-valve type.

Operation of externally equalised expansion valve

The externally equalised expansion valve (Fig. 2.36) has the benefit of having refrigerant pressure from the outlet of the evaporator acting directly on the underside of the diaphragm of the TXV. This arrangement overcomes the problem of sensing the pressure drop across the evaporator. The TXV has a pintle valve which is controlled by a diaphragm.

There are three pressures acting on the diaphragm of a TXV valve:

1. Refrigerant within or exiting the evaporator applies pressure under the diaphragm (Pe).
2. Spring pressure (called superheat spring) applying pressure against a pintle valve (ball and seat). This pressure is applied under the diaphragm (Ps).
3. Pressure from the expansion of the liquid within the heat sensing bulb via the capillary tube (Pf) which is above the diaphragm.

The amount of refrigerant that is allowed to flow into the evaporator is determined by the vertical movement of the diaphragm and valve. This is controlled by the difference in pressure above the diaphragm Pf and below the diaphragm which is the sum of Pe and Ps.

When the load on the system is high and additional cooling is required the temperature exiting the evaporator will be high.

Figure 2.36

This high temperature will be transferred to the heat sensing tube (remote bulb) which contains an inert liquid (different to the refrigerant) which expands within the bulb increasing in pressure due to its fixed volume. The increase in pressure is transferred via the capillary tube to the top of the diaphragm enabling the diaphragm to overcome the combined pressure of the superheat spring (Ps) and the refrigerant exiting (Pe). The valve opens further and allows an increase in volume of refrigerant to flow through the evaporator to cope with the increase in load. This will lower the temperature of the refrigerant exiting the evaporator. This reduction in temperature will be transferred to the heat sensing tube and cause the liquid inside to contract. This will reduce the pressure inside the capillary tube and diaphragm. This reduction in force Pf above the diaphragm will allow the superheat sensing spring pressure Ps and refrigerant exiting the evaporator pressure Pe to force the diaphragm upwards reducing the orifice size and thus reducing the volume of refrigerant.

This continual adjustment and balancing of forces controls the volume of refrigerant to ensure that a superheated refrigerant exists within and exiting the evaporator.The manufacturers adjust expansion valves to ensure they operate under superheat conditions. Factory settings must not be tampered with and the correct TXV must always be used as a replacement for a faulty one.

Some expansion valves have a small V cut into the valve seat to ensure that if the valve is closed a small quantity of refrigerant may still flow around the system if a fault occurs or they are seized closed.

Operation of internally equalised expansion valve The operation of the internally equalised expansion valve is almost the same as the externally equalised expansion valve. The difference is the loss of the benefit of sensing the pressure of the refrigerant as it leaves the outlet of the evaporator. The pressure is sensed on the inlet of the evaporator or just before, using an internal drilling inside the housing of the TXV where refrigerant entering can apply pressure on the underside of the diaphragm.This means that the pressure drop across the evaporator is unknown with this type of valve.

Box or H-valve type

The box type expansion valve (Fig. 2.37) includes the pressure sensing and temperature sensing functions of the externally equalised TXV but has no external tubes (capillary or pressure

Figure 2.37

sensing).This is achieved by using two passages – one entering the evaporator and one exiting

the evaporator.

Liquid refrigerant enters the block valve housing (1). The orifice is very small and there is a large pressure drop on the other side of the ball valve (8).The liquid and small amount of vapour refrigerant (flash gas) enters the evaporator. The liquid/vapour will boil due to the drop in pressure and the vapour will become saturated. The saturated vapour will continue to flow through the evaporator becoming superheated.The superheated refrigerant will enter the box type expansion valve at point (7). The valve position is controlled by the temperature and pressure of the superheated vapour entering it from the evaporator at point (7). If the temperature of the refrigerant is high due to a high cooling demand then this additional heat will be transferred to the sensing element and diaphragm head. The liquid will expand and apply pressure downwards on the ball valve and superheat spring enlarging the orifice and allowing an increased volume of refrigerant to flow through the evaporator. The increased volume of refrigerant will provide additional cooling capacity and the refrigerant temperature entering the valve at point (7) should reduce. When this occurs the sensing element will transfer the reduced temperature to the diaphragm head which will then contract and reduce the pressure applied to the ball valve and superheat spring. The pressure of the superheated vapour is sensed directly under the diaphragm head via internal drillings. If the pressure is high then the diaphragm head flexes upward reducing the pressure applied to the ball valve and superheat spring. This causes the orifice to reduce in size thus reducing the volume and pressure of the refrigerant flowing into the evaporator. If the pressure applied to the diaphragm is low then the diaphragm will flex downward and apply additional pressure to the ball valve and superheat spring. This will increase the size of the orifice and allow a larger volume of refrigerant at a higher pressure to flow through the evaporator.

2.38

Fixed orifice valve (FOV)

The fixed orifice valve (Fig. 2.38) is positioned inside the high pressure line between the condenser and the evaporator. This can often be seen when looking at a system with an FOV fitted due to the increased diameter of the aluminium tube where the valve is situated. This is

also where you can sense the change in temperature from hot to cold.

The volume of refrigerant flowing through the orifice is determined by the pressure of the refrigerant and the size of the orifice.The orifice is fixed so the only way to control the volume is to vary the pressure of the refrigerant (1.5–2.9 bar low pressure side). This type of system tends to cycle (switch on/off) the compressor on a regular basis to vary the volume/pressure of refrigerant matched against the load on the system. The compressor is cycled by a cycling switch located in the low side of the system. A way to improve this system is to fit a variable orifice valve or smart valve or a variable displacement compressor.

As previously stated the function of the FOV is to divide the system creating a high pressure and low pressure side and to meter refrigerant into the evaporator.

Liquid refrigerant flows from the condenser at high pressure to the inlet of the FOV. The refrigerant travels through filter screens to remove any foreign particles and then to the calibrated fixed diameter tube inside the plastic valve body. Dependent on the pressure of the refrigerant a small volume flows through the orifice into the evaporator inlet. This changes the refrigerant into a low pressure liquid ready to boil off inside the evaporator. The orifice size of the FOV is matched to deliver the correct volume of refrigerant under maximum cooling loads. This causes problems under light loads when the evaporator is in danger of becoming flooded due to not enough heat available to vaporise the entire refrigerant inside the evaporator.This is the reason why an accumulator must be fitted between the outlet of the evaporator and the compressor ensuring no liquid refrigerant flows to the evaporator, except through the oil bleed inside the accumulator which is used to deliver oil to the compressor (5%). 

The FOV comes in a range of orifice sizes (1.19 mm–1.70 mm) which are colour coded to

enable them to be matched to the manufactured vehicle (see Table 2.2). Special tools are required

to remove the valve (Chapter 5).

Variable orifice valve (Smart VOV)

The fixed orifice valve (Fig. 2.39) is limited due to its design which often leads to poor performance and excessive cycling of the compressor (if fixed displacement).To improve the performance a VOV can be fitted which can respond to the change in pressure of the refrigerant

and thus vary its orifice size to compensate. It is important for the correct operation of the

Table 2.2

Figure 2.39

A/C system to create the correct pressure distribution throughout the system. This must be matched to the load on the system. This will promote the correct condenser and evaporator performance.

The valve assembly contains two ports, a fixed and a variable port. The fixed port is matched against the required refrigerant flow for high vehicle speed.The variable port reacts to the temperature applied to it from the refrigerant exiting the condenser.The temperature is sensed by a bimetallic spring which expands and contracts with changes in refrigerant temperature. At idle and low compressor speeds the orifice size can be reduced. This creates a larger pressure differentiation across the valve and reduces the volume of refrigerant and evaporator flooding.When either the output of the compressor due to vehicle speed or the external load changes the orifice size can respond to this change. The main benefits which are published by aftermarket companies that sell VOV state that the valve is suited to drivers who spend a great deal of time idling in traffic or moving slowly. Performance is improved through reduced compressor loading and greater cooling. There are also published data on improved fuel economy due to reduced compressor loading. Currently VOV are not in widespread use by Original Equipment Manufacturers (OEM). Whenever working on an A/C system the OEM must always be contacted for advice when deviating/changing the original set-up of a system (replacing an FOV with a VOV).

The pressure in the high pressure line pushes the metering pin against a spring with a force which depends on the type of driving (idle, town or motorway/highway). The aperture of the valve is increased or decreased by the different diameters of the metering pin and thus matched to the driving situation.

Figure 2.40

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The Receiver-Drier or Accumulator

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The receiver-drier

A receiver-drier (Fig. 2.32) is used when a thermostatic expansion valve metering device is

used and is positioned between the condenser and the thermostatic expansion valve.

The function of the receiver drier is as follows:

1. To ensure the system is free from dirt preventing any excessive wear or premature failure of components.
2. To remove moisture from the refrigerant ensuring no ice can form on any components within
3. the system which may cause a blockage and to ensure no internal corrosion can form.
4. To act as a temporary reservoir to supply the system under varying load conditions.
5. To allow only liquid refrigerant to flow to the expansion valve.
6. To act as a point for diagnostics (sight glass sometimes fitted).

Operation

Refrigerant entering the receiver-drier in an ideal system will be in a liquid state. If the system is under heavy load the condenser may have not been efficient enough to completely condense the refrigerant. This means a small amount of vapour may be present. Liquid and vapour can enter the receiver through the inlet where it will separate. Liquid will fall to the bottom of the receiver while vapour will rise to the top. The outlet is connected to a receiver tube internally which has a pickup point at the bottom of the receiver where the filter is positioned.

The refrigerant flows through the desiccant and filter to get to the outlet pickup tube. This ensures that only liquid refrigerant flows to the expansion valve.

Sight glass

A sight glass is not used on R134a systems because the refrigerant has a cloudy appearance in its normal condition. (For more information see section 4.5.)

Fusible plug (pressure relief valve)

If adequate cooling of the condenser is not provided or the cooling load becomes excessive a fusible plug will release the excessive pressure to reduce the possibility of a burst pipe. The fusible plug contains metal which has a melting point of approximately 100–110°C. Fusible plugs or ‘melt bolts’ are used generally on older models or as fail safe measures on new ones.

The main safety device for high pressure control is the high pressure switch (for more information, see section 2.7 for control switches).

Pressure relief valve

The problem with a fusible plug is that once the metal melts you lose the full contents of the refrigerant. This is harmful to the environment and can cause damage to vehicle components.

To reduce the pressure in the system the pressure relief valve ejects only enough refrigerant to reduce the pressure in the system. Pressure relief valves are fitted to compressors and sometimes to receiver-driers.

Accumulator

An accumulator is used when an FOV metering device is used. The accumulator is fitted between the evaporator and the compressor.

The function of the accumulator is as follows:

1. To ensure that the refrigerant leaves the accumulator as a vapour and not a liquid state for the compressor to induce.

Figure 2.33

Figure 2.34

2. To ensure it is free from dirt to stop any excessive wear or premature failure of components.

3. To remove moisture ensuring no ice can form on any components within the system which may cause a blockage and to ensure no internal corrosion can form.

4. To act as a temporary reservoir to supply the system under varying load conditions.

5. To add lubricating oil for system components like the compressor.

6. Often to house the low pressure switch/sensor.

Operation

The refrigerant enters the accumulator from the evaporator in liquid/vapour form. It enters through the inlet (3) creating a vortex and flowing around the cap (4). The refrigerant passes through the desiccant where it is cleaned and moisture is removed. The vapour collects under the cap (4) where it is extracted through the outlet. During extraction it passes through a U tube in which it is mixed with oil from the small bleed hole (6).This bleed hole allows very small quantities of liquid refrigerant mixed with lubricating oil (3%) to flow with the refrigerant vapour to the compressor. Because the liquid refrigerant vapour is in such small quantities there is no danger of compressor damage.

Note – a desiccant is situated inside the receiver to absorb moisture and filter particles. Zeolite is used for R134a and silica gel is used for R12.

The receiver-drier or accumulator must be replaced under any of the following circumstances:

● If the A/C system is opened to moisture for more than 3 hours then the receiver/accumulator must be replaced.

● Under service conditions – every two years.

● Compressor seizure or any possibility of foreign matter in the system.

● Excessive moisture in system causing icing.

Note – often the receiver/accumulator also contains a contrast medium (dye) which dissolves in the compressor lubricant. If there is a leak in the refrigerant circuit, the compressor lubricant, together  ith the contrast medium, will escape and can be detected with a special UV lamp.

Figure 2.35

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The Condenser

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The function of the condenser is to act as a heat exchanger to dispel the heat energy contained in the refrigerant. Superheated vapour enters the condenser at the top and subcooled liquid leaves the condenser at the bottom. The condenser must be highly efficient but as compact as possible.

The pressure and temperature has been raised by the compressor. There is a need to lower the temperature of the heat laden refrigerant to change it back into a liquid enabling it to act as a cooler again later in the system. To do this the refrigerant flows into the condenser as a vapour and gives off heat to the surrounding area and most of the refrigerant (depending on system load) condenses back into a liquid which then flows into the receiver/drier.

The condenser is located at the front of the vehicle where strong air flow through its core can be achieved when the vehicle is in motion. To aid the removal of heat when the vehicle is stationary or at a low speed the condenser is fitted with a single or double fan system. Shrouds are often used to direct the air flow over the surface of the condenser.

Condenser design

The ideal condenser should have no pressure drop between the inlet and the outlet. Condensers are generally made from aluminium to prevent any chemical reaction between the metal and refrigerant/oil mixture. They are generally constructed with tubes and fins. Tubes to carry the

 refrigerant and fins to increase the surface area in contact with the outside air. Their shapes vary and include:

Serpentine fin type

Tube and plate type

Tube and plate and fin types have been used for a number of years and can be seen on R12 systems but are not generally used on R134a systems. This type of condenser can also be backflushed to remove any foreign particles within the system.

Parallel flow type (a flat tube condenser)

Parallel flow condensers are very efficient, the condenser breaks up the flow into tiny streams enabling it to transfer heat more rapidly. This type of condenser cannot be flushed and if it becomes blocked can only be replaced.

Condenser refrigerant flow

The flow of refrigerant is either serpentine (Fig. 2.29) or parallel flow (Fig. 2.30). Serpentine flows through the tube(s) evenly eventually condensing while following the same path.

Parallel flow allows the path of the refrigerant to go vertically as well as horizontally across the condenser. Parallel flow is considered to be the more efficient layout.The key to the design is the header tanks/manifolds fitted to the sides of the core allowing the flow to break up into small streams.

Dual condenser

This layout includes a condenser with an integrated multi-flow condenser (Fig. 2.31) and a gas/liquid separator (modulator) – a subcool cycle. In simple terms these are two condensers stacked on top of each other with a receiver drier called a modulator between the two. They are generally used in vehicles with large internal space and cooling requirements. Refrigerant flows through the first parallel flow condenser and then into the modulator as a liquid. Any gaseous refrigerant which the first condenser was unable to condense will travel with the liquid refrigerant to the subcooling portion of the condenser to ensure only liquid refrigerant flows to the FOV (Fixed Orifice Valve) or receiver-drier.

The condenser is the point within the A/C system which is used to remove the unwanted heat. This makes it very important in the overall efficiency of the system. As the latent heat of condensation is transferred to the air stream, the refrigerant vapour makes the necessary change into liquid.

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The Compressor

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The function of the compressor is to compress and circulate superheated refrigerant vapour around a closed loop system (any liquid or dirt will damage the compressor). Compressors vary in design, size, weight, rotational speed and direction and displacement. Also compressors can be mechanically or electrically driven. Some compressors are variable displacement and some are fixed. The compressor uses 80% of the energy required to operate an air-conditioning system. This means that the type of compressor used in the system will determine the overall efficiency of the system. This is particularly important for fuel economy and pollution which is monitored through government regulation.

Operation

The compressor is driven by an engine driven pulley system (Fig. 1.68). At the front of the compressor is a magnetic clutch which when given power engages the compressor. The compressor draws in refrigerant vapour from the suction side which is the outlet of the accumulator (fixed orifice valve system) or the outlet of the evaporator (expansion valve system). Because the refrigerant that left the evaporator/accumulator is a vapour it can no longer absorb heat energy and act as a cooler.

During the compression of the refrigerant inside the compressor the pressure and temperature rapidly increase.An ideal system will increase the pressure from 200 to 2250 kPa (29 to 326 psi). The temperature increase can be as much as 0°C–110°C. When the air-conditioning system is running the suction pressure is between 120 and 300 kPa (17.5 and 43.5 psi), when the system is under high load the pressure and temperature of the refrigerant can reach as high as 2800 kPa (406 psi) and 125°C.

Figure 2.2

The compressor can only compress refrigerant vapour. Any liquid or dirt allowed to enter

the compressor will cause damage.

The boiling point of refrigerant at 326 psi is 57°C so the refrigerant will remain in a gaseous state until the refrigerant gives off enough heat to drop below 57°C. To do this the refrigerant flows from the outlet of the compressor to the condenser.

Variable capacity compressors

Variable capacity compressors can vary the volume of refrigerant depending on the demands of the system. The demands of the system are sensed by the pressure of the refrigerant coming out of the evaporator. The demand is the amount of heat transferred to the refrigerant. This increase in temperature will affect the pressure that will eventually enter the inlet of the compressor housing.The minimum displacement of a variable type compressor is about 10 cm3 and is not zero because the refrigerant carries the lubricant for the compressor which would cause damage if no refrigerant was present inside the compressor while it was operating. Variable capacity compressors greatly reduce the amount of on/off cycles that non-variable capacity compressors are subjected to. This reduces noise from the clicking of the magnetic clutch, increases fuel efficiency through variable loading of the system and reduces the wear of the magnetic clutch plate.Variable types of compressor generally differ from the non-variable type due to the addition of a control valve. The control valve is used to vary the displacement of the compressor to match the demands of the system.

Types of compressor

There are three main categories of compressor:

1. Reciprocating – crank and axial piston (swash plate).

2. Rotary – vane.

3. Oscillating – scroll type (helix).

Crank type compressor (reciprocating)

Crank type compressors (Fig 2.3) are not generally used in the automotive industry any more. They may have up to two cylinders including ‘V’ shape configuration. They are driven by the engine pulley system which rotates a crankshaft inside the pump. The crankshaft is connected to a piston via a connecting rod which travels up and down the bore.Above the piston there is a valve assembly to direct the flow of refrigerant.

Pumping operation

The crankshaft and connecting rod convert the pump’s motion from rotary (Fig. 2.4) to reciprocating. The piston travels up and down the bore inducing, compressing and discharging the refrigerant. Two valves are fitted per bore: a suction valve and a discharge valve.

On the downward stroke the refrigerant enters the compression chamber through the suction port due to a vacuum being created above the piston and the low pressure of the refrigerant.

On the upward stroke the refrigerant is compressed and an increase in pressure and temperature occurs.When the refrigerant pressure overcomes the force of the discharge valves the high temperature and high pressure (superheated vapour) refrigerant leaves the compression chamber.

Axial piston (swash plate) type variable capacity compressor

The axial piston type (Figure 2.5) is one of the most common types of compressor and can be fixed or variable capacity. The pumping cylinders are circumferentially situated around the outside of the drive shaft (4) and parallel to its axis. Each cylinder has a double ended piston with a separate pumping chamber at each end. Each pumping chamber has a set of inlet and discharge reed valves. The inlet reed valves are connected to the inlet port of the compressor via internal drillings and the discharge valves are connected to the discharge port of the compressor via internal drillings.

Pumping operation

A swash plate (3) is attached to the drive shaft. The swash plate is located at an angle. The pistons inside the pumping chamber are connected to the swash plate via swivelling ball joints. The rotation of the swash plate causes the pistons to reciprocate inside their pumping chambers. When the volume above a piston increases the refrigerant flows into the chamber via the inlet reed valves – induction. When the volume above a piston is reducing the pressurized

refrigerant exits the pumping chamber via the pumping chamber discharge reed valves. The position of the swash plate will determine the length of travel by the piston which allows the compressor to vary its output.

Control of the compressor swept volume is regulated by the control valve ((5) in Fig. 2.5), located in the compressor rear end plate. The control valve attempts to keep the compressor low pressure side at a constant pressure, also known as the control point, which is determined by factory settings and cannot be adjusted in service.

The operation of the variable capacity swash plate compressor is based on the creation of three distinctive pressures which are sensed and controlled by a control valve and adjusted to create a mechanical equilibrium within the compressor (this can be seen on Figs 2.7 and 2.8):

A. High pressure (discharge pressure).

B. Low pressure (suction pressure from the evaporator (TX system or accumulator FOV).

C. Compressor internal pressure (generated by the high pressure but only a fraction of it).

The displacement is the difference between the low pressure and the internal pressure, this is called ‘delta P (P)’. The change in delta P varies the angle of the swash plate causing the capacity of the compressor to vary.

Control valve operation

The control valve consists of a spring loaded piston and ball valve and a bellows which can change its length according to pressure.

In order to achieve this constant pressure, the control valve changes the pressure in the compressor housing as follows:

Low demand When the load on the air-conditioning system is minimal the low pressure from the evaporator will be low. This is due to the expansion valve (TX valve) opening only a small amount and allowing only a small amount of refrigerant to flow to the evaporator.This low pressure will flow to the compressor into the suction side and be sensed at point (C), Figure 2.7.The

result of this low pressure, which is lower than the control pressure set by the manufacturers,

means that the compressor needs to reduce its displacement. The bellows (6) expand and

forces the ball valve up which increases the bleed of refrigerant from the high pressure (A) to

the low pressure (C). Because the high pressure (A) comes from the high pressure acting on the pistons inside the compressor the pressure acting on those pistons reduces.

High demand When the load on the air-conditioning system is high, the low pressure (suction side) will be high. This will act at point (C) on Figure 2.8, and will be higher than the control point (set at manufacture).This will cause the bellows to be compressed and the ball valve to be closed. This will stop the bleed from the high side to the low side and will cause the pressure

above the valve to increase against spring pressure moving the valve body downwards. The bypass is closed and the entire delivery volume is passed to the compressor high pressure connection. When no refrigerant is added from the high pressure side, the pressure in the crankcase will drop, the displacement increases and the compressor works at a higher capacity. This is achieved through linkages between the swash plate and shaft, which alter its angle and will continually increase as the pistons and the rocker discs move.The increased angle results in an increased displacement. The increased displacement increases the cooling effect due to increased refrigerant volume.

A weak spring, called a start spring, is fitted on the compressor shaft and is used to return the pistons and the swash plate towards the valves as soon as the compressor stops operating and the pressure difference between the high and low pressure sides evens out. In this position, the compressor delivers 5% of its maximum swept volume and the starting torque is low

the next time the compressor starts.

Low load If the low pressure decreases the low pressure force FBP decreases and therefore the low pressure refrigerant force RFBP decreases. If the internal pressure of the refrigerant force is constant, this becomes greater than the low pressure refrigerant force RFBP. At the

The compressor positions itself to maximum capacity to output as much refrigerant as possible to fulfil the system’s requirements.

As previously discussed the displacement of the swash plate compressor is determined by the angle of the swash plate. This is a function of the relationship between the high and low pressures inside the compressor which is controlled by a regulating pattern. The graph will vary from one type of compressor to another.

Vane type compressor

The vane type compressor is compact and has low frictional losses. It is quiet and has few moving parts. It is a rotary compressor using rotating vanes to increase the flow of refrigerant. There are two types of vane type compressor:

1. Through vane.

2. Eccentric vane.

Through vane type rotor

The through vane type has two vanes mounted at right angles to each other in slots in a rotor housing. As the rotor rotates the vanes slide radially to maintain contact with the surface, this is maintained through centrifugal force. The circulating refrigerant oil with the centrifugal force seals the moving parts relative to one another.

Pumping operation It is important to note that the vane type compressor has three compression spaces. When the refrigerant is discharged, a space is available on the suction side of the compressor and is filled with refrigerant. At the same time the compression space of the compressor will have refrigerant being pressurised. This means that suction, compression and discharge are continually occurring during every full rotation.

Eccentric vane

The eccentric vane (Fig. 2.16) works in a similar manner to the through vane except the vanes are organised separately and are not mounted at right angles to each other. The rotor inside the vane compressor rotates on an eccentric which is used to increase and decrease the volume within the compressor.

Scroll (helix) type compressor

The scroll compressor (Fig. 2.17) consists of two helices with one lying within the other. They are both mounted in a cylindrical housing. One helix is fixed and the other is attached to the drive shaft of the compressor. The driven helix does not rotate itself but does orbit the other helix. The two helices through movement create crescent-shaped compression chambers.

A. The volume within the compressor increases with the movement of the driven helix. This allows refrigerant to enter the compression chamber.The shape of the helix and its movement mean that no physical inlet valve is required. Induction or the suction phase ends when the position of the helix is at the bottom of its eccentric movement.

B. Compression occurs by trapping the refrigerant in the centre of the helix and then reducing

the volume. This reduction will increase the pressure and temperature of the refrigerant to

the required state.

C. Discharge occurs at the centre of the helix where a valve is positioned to ensure that no

refrigerant is allowed to flow back into the compressor when not being operated.

During operation all gas spaces are in various states of compression which results in a continuous inlet and outlet flow. The compressor has few moving parts which means less wear.

Electric compressor

The electric compressor has to be used due to the absence of a mechanical drive mechanism for the compressor. Electric compressors are generally used on vehicles with hybrid or electric power units. Hybrid vehicles have both a small engine, generally diesel, and an electric motor powered by a battery unit. The engine intermittently runs in the event that the electric power unit energy level becomes low or high torque output is required. Electric powered vehicles have no engine units and generally take the form of fuel cell vehicles. A fuel cell, which is an inversion of the process of the electrolysis of water, is used to produce electrical energy. Fuel cells do not store energy; they produce it and replace the energy used by the batteries which provide the primary source to drive the electric motor on the vehicle. These technologies will over the next 10 years change the way in which we view, design, manufacture, maintain and repair our vehicles.

Pressure relief valve

If the load within the system is excessive or a blockage occurs and the pressure of the system is excessively high then a pressure relief valve fitted to the compressor and often the accumulator opens and reduces the system pressure. The valve is only used if other safety devices fail. The valve reduces the possibility of a burst pipe or fractured evaporator or condenser due to excessive pressures in the system. The valve operates at approximately 3.5–4.0 MPa.

Thermo cut-out

Some compressors have a thermo cut-out (Fig. 2.19), an electrical switch that works on the principle of a bimetallic strip. If the temperature of the refrigerant in the compressor exceeds a safe limit (approx. 150°C) then the bimetallic strip bends and breaks the electrical connection to the compressor clutch. This shuts the system down.

Compressor magnetic clutch

The compressor is driven by the engine crankshaft via a pulley system. In Figure 2.21 the pulley system provides permanent drive to the multi groove drive belt (4). This means once the engine is started the multi groove drive belt pulley is rotating. There is an air gap between the multi groove drive belt pulley and the drive plate (1). When the engine is running the compressor is stationary until the A/C button is selected and electrical power flows through the clutch field coil generating an electrical magnetic field which attracts the drive plate towards the multi groove drive belt pulley (Fig. 2.20). The drive plate which is attached to the compressor drive shaft is pulled towards and held against the multi groove drive belt pulley system.The clutch is now held together as one unit and the compressor’s rotational speed matches the engine speed. When the A/C system is either being cycled or is no longer required the current is switched off and the magnetic force created in the clutch field coil depletes (Fig. 2.21). The drive plate disengages through the help of return springs and the compressor stops.

Classification of compressor clutches

Compressor clutches are classified according to their shape (Fig. 2.22).

● F type and G type – crank type compressors.

● R type and P type – swash plate and through vane type compressors.

Clamping diodes

When an electromagnetic actuator is de-energised a voltage spike can be produced within the controlling circuit. This is due to the collapse of the magnetic field. This is often viewed on oscilloscope waveforms and referred to as sawtooth signals. Clamping diodes are used to filter out the voltage spikes and protect driver circuits. If a clamping diode becomes faulty the electrical system may become unstable due to excessive electrical interference. A clamping diode reduces the peak voltage of a signal used to switch any electromagnetic type actuator. For more information (see section 3.3).

Externally regulated compressor (ECC only)

This is a compressor with an externally regulated compressor stroke. The swash plate angle is varied by a Pulse Width Modulated (PWM) controlled valve which is regulated by the Electronic Climate Control (ECC).This enables the compressor to be precisely regulated between minimum and maximum output. For this reason, the magnetic clutch can be dispensed with. Compressor oil supply is guaranteed, even when the compressor is operating at minimum output.

Advantages:

• Reduced overall weight – weight saving thanks to omission of magnetic clutch.
• Minimum fuel consumption thanks to precise control of compressor output according to respective requirements.
• No operating jolts when delivery quantity increases.

Compressor oil

Compressor oil is necessary for the lubrication of the moving parts of the compressor.The compressor oil lubricates the compressor by dissolving in the refrigerant and circulating throughout the refrigeration circuit. For this reason the recommended oil and quantity must be used. Because the refrigerant oil for R12 is mineral oil and R134a is synthetic PAG oil they must not be mixed. Only the correct oil must be used matching the compressor type and refrigerant.

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