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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