LV35 cooling systems (3)

LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3) LV35 cooling systems (3)

Student Workbook

LV35 Cooling Systems (3)

kap all phase 2 & 3 6/11/03 11:35 am Page 7

Student Workbook for Technical Certificates in

Light Vehicle Maintenance and Repair

MODULE LV35 COOLING SYSTEMS (3)

Contents

Page ……… Page

Climate Controlled Systems 3 Air Conditioning Control: 36

Air Conditioning: 3 Evaporator temperature control 37

Temperature Control – Heating: 5 Compressor belt protection system 40

Electrical System Overview: 41

Health and safety information 45

Temperature Control – Cooling: 9 Legal requirements 46

Latent heat of evaporation 10

Basic operation of the refrigeration Layout of an air conditioning system 48

Page ……… Page

Connecting the manifold gauge 58

Installing the manifold gauge 60 In-car temperature sensor 81

Installing the refrigerant container tap Evaporator sensor 83

Fitting a new refrigerant container 65 Blower motor (interior fan) speed 88

Fault Diagnosis using a Manifold

Climate Control Systems

Drivers demand a heating system that maintains a comfortable temperature and a steady stream of fresh air Due to the development of motorways and the resulting traffic congestion caused by a more mobile society, drivers are spending more time in their vehicles It is therefore essential in the interests

of health and safety that the occupants of vehicles are provided with a

continuous supply of fresh temperature controlled air This has lead to very sophisticated heating and ventilation systems being fitted in motor vehicles

Air Conditioning

Four functions of A/C

The purpose of air conditioning is to maintain a comfortable environment inside the vehicle, which includes the following:

• temperature control

• air circulation

• humidity control

• air purification

Functional requirements

The equipment necessary to provide air conditioning in the motor vehicle consists of a cooler or refrigeration system, heater, moisture controller and a ventilator

• the temperature must be adjustable

• heat must be available as soon as possible

• heat needs to be directed to different areas of the inside of the vehicle

• ventilation must be provided with minimum noise

• the system must be able to demist all windows

• ease of operation is essential for safety reasons

Temperature Control – Heating

Varies the amount of air flowing

over the heater matrix

Varies the amount of coolant flowing through the heater matrix

Varies the amount of air flowing

over the heater matrix

Varies the amount of coolant flowing through the heater matrix

In vehicle heater systems the coolant is pumped through a hot radiator matrix,

a blower is used to blow cool air through the matrix causing the air to heat up The engine must be hot for this to happen

Two methods are used, one method is air mix and the other is water flow control

Air mix type

In this type of heater the proportion of cool air is passed over the heater matrix and a proportion bypasses it

Water flow control type

In this type of system, the flow of water through the heater matrix is controlled

by a valve, which causes the temperature of the matrix to be varied

Some manufacturers combine the two methods using two heater matrix each with their own coolant control valves, one to supply controlled heat to the driver’s side (right hand valve) and one to supply controlled heat to the

passenger side of the vehicle (left hand valve)

An electric circulation pump may be integral with the ‘pump valve unit’ as it is known

Air Circulation Control

Damper operation (flaps)

There are two types of operating methods a manual or lever and a push button type

In the lever type, no electric motors are used The driver operates the

controls manually, the cables move the flaps as the levers are moved

Push button type

Air flow selector switches

Air inlet selector switches Air flow selector

Humidity control

Humidity is the term used to describe the degree of dryness or wetness in the atmosphere, dry air contains no moisture If ice is added to a glass of water, drops of water appear on the outside of the glass as the water is condensed from the surrounding air This demonstrates that humidity is the amount of water vapour contained in the air

The cooling process brought about by air conditioning reduces the humidity of the air through this condensing effect

Relative humidity

Humidity levels are quantified using the term ‘relative humidity’ The relative humidity level of air is a reflection of the amount of moisture that it actually contains compared to how much moisture it is physically capable of

containing If we describe the humidity level as being 50%, this means that the air is at half moisture capacity Think of it as being half full!

As air is cooled, it becomes denser and therefore less capable of holding moisture

A human being feels at their most comfortable when the atmosphere has a temperature of around 22°C and a relative humidity of about 25%

Temperature Control – Cooling

After swimming water vaporises from the body causing it to feel cool

Alcohol or any liquid that will readily vaporise such as petrol, if spilled on the hand, will cause the hand to feel cool This is because heat is extracted during the vaporising process

A more scientific way of examining what happens is described If a container

is placed in an insulated box (fitted with a tap) and a liquid that will vaporise readily is placed in the container, when the tap is opened the liquid in the container will vaporise as heat is removed from the air inside the box,

therefore the temperature of air inside the box falls If the gas is condensed and returned to the container no loss of liquid will result

This use of heat energy in order to evaporate is called ‘latent heat of

evaporation’

Latent heat of evaporation

When heat energy is supplied to a liquid, its temperature increases The relationship between the amount of heat energy supplied and the temperature increase experienced is a fairly linear one

This linear relationship is maintained until the liquid starts to boil (evaporate) and then something rather strange happens Heat energy is absorbed without any subsequent increase in temperature The heat energy is ‘hiding’ Latent means hidden So where is the heat energy going? It is being used to bring about a change of state in the substance – liquid to gas Substances will not change state (solid to liquid, liquid to gas) unless they have a supply of energy

in order to do so

This is the underlying principle of refrigeration A liquid is allowed to

evaporate under controlled conditions in order to absorb heat energy and cool the surroundings

Cooling

Air is cooled by:

• storing the refrigerant in a container known as a receiver

• the liquid refrigerant is released through a small hole into the evaporator via an expansion valve, the temperature of the refrigerant is lowered and some of it changes to vapour

• the lower temperature refrigerant flows into the evaporator, the liquid refrigerant evaporates and removes heat from the surrounding air in the process

Condensing

1 and 2 converted

into a liquid by raising the pressure without

changing the temperature

3 and 4 converted into a

gas by lowering

the pressure without changing

the temperature

or by raising the

temperature without changing

the pressure

1 and 2 converted

into a liquid by raising the pressure without

changing the temperature

3 and 4 converted into a

gas by lowering

the pressure without changing

the temperature

or by raising the

temperature without changing

the pressure

To ensure a constant supply of refrigerant to the receiver is it is necessary to change the gaseous refrigerant back into a liquid

When a gas is compressed its temperature and pressure increase

The graph above shows the characteristic curves of R134a, which expresses

the relationship between the temperature and pressure The upper portion of the graph shows R134a in a gaseous state and the lower portion below the curve in a liquid state If the pressure is raised and the temperature remains constant the gas will change into a liquid

The refrigerant can be changed into a gas by decreasing the pressure without changing the temperature or by raising the temperature without changing the pressure

The refrigerant in the form of a gas leaves the evaporator and is compressed

by the compressor The compressed gas releases heat to the surrounding air and it condenses back into a liquid The refrigerant in liquid form returns to the receiver

It is critical that only gas enters the compressor otherwise hydraulic locking will occur, therefore stalling the compressor The gas drawn in is compressed

to over 14.1 kg/cm² (201 p.s.i 1,383 kPa), which causes it to get very hot The heat that is now in the refrigerant will work to prevent it from achieving a liquid state The refrigerant is now sent to the condenser where it is cooled The condenser is in effect a radiator Its ability to cool the refrigerant is aided through the use of a condenser fan drawing air across the condenser fins

Liquefying

The now liquefied refrigerant is passed through the bulkhead of the car to the expansion valve and evaporator (behind the dashboard) It is able to expand and absorb heat from the air flowing across the evaporator (flowing through the use of the interior blower motor) It changes state into a gas (absorbing heat from the interior of the car in the process)

Basic operation of the refrigeration cycle

Evaporator

Receiver/Dryer Condenser

H E A T

H E A T

as it flows through the condenser and is then stored in the receiver/dryer until needed The expansion valve allows very small amounts of the hot refrigerant

to spray through its tiny opening into the evaporator

When the refrigerant evaporates it absorbs heat from the air blowing across

Progress check 1

Answer the following questions:

1 State the four functions of the air conditioner:

2 Explain the differences between the water flow control type heaters and the air mix type:

3 When the tap is opened in the device shown below, does the thermometer

in the box show a lower temperature than the thermometer in ambient air?

4 Which refrigerant is used in a modern motor vehicle?

5 What does retrofitting mean?

6 Name the parts from 1 to 5:

4 5

3

a

b

d

7 Describe the condition of each component (a) to (d) when the cooler is turned on:

(a)

(b)

(c)

(d)

Main Components

Compressor

This is commonly referred to as the heart of the system It is a belt driven pump and is responsible for compressing and transferring the refrigerant gas The air conditioning system is split into two areas, high pressure and low pressure and these can be broadly defined as discharge and suction The intake or suction side of the compressor draws in refrigerant gas from the outlet of the evaporator In some cases this is done via an accumulator

Piston type

The rotation of the crankshaft is changed to reciprocating motion of the piston Two valves are fitted on the valve plate The suction valve is installed on the lower surface of the valve plate and the discharge valve is fitted to the top surface of the valve plate

Operation

Suction

When the piston is on the down stroke the discharge reed valve is held in the closed position because the pressure in the discharge hose is greater than that in the cylinder At the same time, the suction reed valve is opened by the action of the piston moving down the cylinder, thus allowing refrigerant to enter the compressor

Discharge

When the piston is on the up stroke, the refrigerant is forced out of the

cylinder through the discharge reed valve and is sent to the condenser as a high temperature and pressure vapour At the same time, the suction reed valve is kept closed by the high pressure

It is lubricated using a sump in which the connecting rod splashes oil around the inside of the compressor Some compressors use an oil pump to circulate the oil, in a similar way to an engine lubrication system

Swash plate type

A number of paired pistons are set on the swash plate at intervals of 72

degrees for a 10-cylinder compressor or 120 degrees for a 6-cylinder

compressor When one side of the piston is on the compression stroke, the other is on the suction stroke

Lubrication is by oil splash The swash plate splashes the oil around the internal parts of the compressor Some oil is mixed with the refrigerant

The diagram above shows the swash plate and piston with suction valves and discharge valves at each end of the cylinder

Through vane type

Vanes

The through vane type has a higher pumping volume and is lighter and more compact than the piston or swash plate type There is also less frictional loss and lower noise levels

It is important that the vane type compressor is not dismantled due to the very close clearances and high precision of the front bearings, rotor housing, rotor, rear housing, and the vanes

The tolerances are very small and the components will not fit together if

assembled in the workshop Attempts to overhaul this type of compressor will

Operation

Each vane forms an integral component with its opposite number There are usually two pairs of vanes, which are mounted at right angles to each other in slots in the rotor As the rotor rotates the ends of the vanes slide across the inside of the cylinder, the vanes shift in a radial direction as they slide

Beginning of suction

This view is from the rear of the compressor

Note: For ease of explanation and understanding the word suction is used throughout The definition of this word is that a depression is formed and that

a higher pressure, forces its way into the lower pressure volume

Suction (intake)

Suction complete

As the rotor turns it increases the volume in the suction port, which draws refrigerant into the low pressure chamber of the compressor

Suction is completed when the vane passes the suction port

Compression

Beginning of compression

At the beginning of compression the volume bounded by vanes decreases, thereby compressing the refrigerant

Compression complete

Further movement of the vanes increases the pressure on the refrigerant Compression is now complete

Exhaust

Beginning of discharge

The vane passes over the exhaust port, and the pressure generated by the compressed refrigerant opens the discharge valve port

Discharge complete

The refrigerant flows into the high pressure chamber (oil separator case) If the pressure is higher than the exhaust pressure the exhaust port will not open This will serve to prevent the refrigerant reverse flowing

To improve cooling, the oil separator case separates the oil required for

lubrication from the mixture of oil and refrigerant gas The interior oil

separator is always at a high pressure whenever the compressor is in

operation

Scroll type

In a scroll type compressor, two offset spiral discs compress the refrigerant The upper disc is stationary while the lower disc is allowed to move in an orbital fashion The orbiting lower disc inside the stationary disc creates sealed spaces of a continuous varying volume

Operation

A quantity of refrigerant is trapped in one of the sealed spaces As the disk rotates, orbiting the enclosed space containing the refrigerant, it transfers the refrigerant towards the centre of the disk, its volume decreases and the

refrigerant is compressed

The compressed refrigerant is discharged through a discharge (exhaust) port

in the centre of the upper disk Scroll compressors have become more

common in recent years as they are quiet, smooth-operating units with the highest efficiency of all compressor types (very low friction)

Once the refrigerant is drawn into the suction side of the compressor it is compressed and sent to the condenser where it then transfers the heat that is absorbed from the inside of the vehicle

Condenser

Heat is dissipated by radiation from the condenser very much like in the case

of a cooling radiator It is located in front of the vehicle radiator, although in some cases due to vehicle aerodynamics its location may be different

Condensers must be in an adequate airflow and the engine cooling fan is often incorporated as a function of cooling the condenser a separate cooling fan may be used

Evaporator

The evaporator’s main function is to absorb heat from the inside of the vehicle but it has several other functions As warmer air passes over the aluminium fins of the cooler evaporator coil, the moisture contained in the air condenses

on its surface Dust and pollen passing through stick to its wet surface and it then drains off to the outside of the vehicle The water can often be seen to drip from the bottom of the vehicle on humid days

The ideal temperature of the evaporator is about 0°C As the refrigerant enters the evaporator under low pressure the warm air passing over the fins causes the refrigerant to boil and it absorbs a very large amount of heat This heat is carried off with the refrigerant to the outside of the vehicle

The evaporator works with other components to regulate the temperature and pressure Whilst there are many variations of devices used their main

functions are the same, they keep the pressure in the evaporator low and keep the evaporator from freezing A temperature sensor is used to prevent the compressor from working in the event of the evaporator freezing An alternative method of protecting the evaporator from freezing is to regulate the amount of refrigerant flowing to the compressor from the evaporator

Thermal expansion valve

Valve

Inlet liquid

Outlet vapour

Adjusting screw

Equalizing port

Heat sensing tube

Equalizing circuit Diaphragm

Diaphragm chamber

Capillary tube

Pressure spring

The thermal expansion valve (TXV), is used to sense both temperature and pressure, it is very efficient at regulating the flow of refrigerant to the

evaporator The valve is a simple spring loaded ball which has a diaphragm attached to a spring Carbon dioxide, which is temperature sensitive, acts on the diaphragm and is enclosed in the capillary tube and heat sensing tube

Diaphragm

Equalizer line

Heat sensing tube Evaporator

Pressure spring Valve

Capillary tube

The sensing tube, which senses the temperature, is secured to the

evaporator If the evaporator temperature rises, the gas in the sensing tube expands and acts on the diaphragm, opening the ball valve allowing a greater flow of refrigerant If the evaporator becomes too cold the gas in the sensing tube will contract and the ball valve will close The flow of the refrigerant is controlled, and therefore the temperature of the evaporator is held near

constant under varying airflow conditions

Box type expansion valve

refrigerant passing through the orifice The thermostatic expansion valve has three main functions, a metering action, modulating and a controlling action

Restrictor (orifice tube)

Strainer for atomising refrigerant Calibrated

bore

Dirt strainer

Arrow points to the evaporator

The restrictor, sometimes known as the ‘orifice tube’ serves the same basic function as the thermal expansion valve It is a straight tube of sintered metal

or plastic Some of these tubes have a filter strainer to remove contaminates and a calibrated meter tube to meter the refrigerant flow The orifice opening

is fixed and it does not affect compressor operation, as does the thermal expansion valve that cycles the compressor clutch

There is a constant flow of refrigerant through the fixed orifice, which provides

a constant pressure drop The refrigerant pressure drops rapidly when it goes through the restrictor orifice The refrigerant is cold at low pressure The restrictor is an interface between the high pressure and low-pressure side of the refrigerant circuit A seal ensures the refrigerant only passes the restrictor

at the narrowing of the orifice

The pressure in the refrigerant circuit limits the amount of refrigerant flow through the calibrated orifice The restrictor keeps pressure on the high

pressure side of the refrigerant, maintaining it in a liquid state when the

compressor is running

When the pressure in the restrictor drops, the refrigerant cools down before it enters the evaporator through partial evaporation The refrigerant is atomised and there is a dirt strainer in front of the narrowing orifice and a strainer for atomising the refrigerant before it reaches the evaporator, which is located downstream of the narrowing orifice

To compressor

The tank is installed in a warm area of the engine compartment It serves as

an equalizing vessel and reservoir for the refrigerant and oil and it also

protects the compressor The refrigerant, as a gas, enters the tank and all traces of moisture are removed and are retained in the integrated dryer The refrigerant collects in the upper part of the plastic cap and is in a gaseous state before being drawn into the compressor through the U-tube, ensuring that the compressor is protected from damage (no liquid droplets) As the name implies the refrigerant collects in the base of the collecting tank

Receiver/dryer

The receiver/dryer has a filter and desiccant sealed in a container, called the receiver dryer Its purpose is to store temporarily, refrigerant that has been liquefied by the condenser and to remove dirt and moisture that could cause damage to the refrigeration system A sight glass is fitted in the top, (normally only for systems with R12 refrigerant) which allows vision of the refrigerant flowing out of it and to the expansion valve

Since the characteristics of R134a are like that of water it can be absorbed by the desiccant silica gel, together with the moisture If there is a large

decrease in moisture absorption capacity, it can cause poor cooling due to condensation of moisture in narrow areas of the system which could freeze in the expansion valve orifices Moisture also causes corrosion within the air conditioning system To achieve adequate removal of the moisture in R134a systems a greater amount of desiccant was needed However, Zeolite, which does not absorb R134a, is now used in air conditioning systems

reaches 30kg/cm² and 95°C - 100°C the fusible plug melts and allows the refrigerant to discharge into the atmosphere, which although adding to ozone layer problems, prevents damage to the equipment

Sight glass and fusible plug

Note: The fusible plug and the sight glass are not normally used for R134a refrigerant systems (see pressure release valve)

Evaporator pressure regulator (EPR)

The EPR is installed between the evaporator and the compressor When the cooling load becomes small the evaporation pressure will decrease and the temperature of the fins falls

If the temperature of the evaporator fins should fall below 0°C the water

vapour passing through the fins will turn to frost and adhere to the fins To prevent frosting a thermostat may be used to turn the magnetic clutch of the compressor on and off On some vehicles an EPR is used instead of a

thermostat

Low cooling load

When the cooling load is low, the pressure of the refrigerant vapour is low The valve begins to close to prevent vapour pressure from falling below

2kg/cm²

High cooling load

When the cooling load is high the pressure of the refrigerant in the evaporator

is high and is vaporized

The valve opens up fully and the refrigerant that evaporated in the evaporator

is drawn directly into the compressor without being regulated

Air Conditioning Control

Magnetic clutch

Principle of operation

Attractive force

Attractive force Battery

Switch Iron

Magnetic flux Windings

The purpose of the magnetic clutch is to provide a means of engaging and disengaging the compressor to affect control over the running of the air

conditioning It works on an electro-magnetic principle very similar to that of a solenoid

Current is fed to the coil and the magnetic force generated in the iron core acts as a very powerful magnet, which attracts the iron bar

The magnetic clutch itself consists of a rotor, stator pulley, and pressure plate

as shown in the above diagram

The stator is fixed to the compressor housing and the pressure plate is

attached to the compressor shaft

When the engine is running the pulley rotates driven by the crankshaft via a belt The compressor does not operate until the clutch is engaged, this only happens when the air conditioning system is switched on When current is applied to the magnetic coil it attracts the pressure plate, pulling it against the friction surfaces on the pulley, this causes the clutch assembly to rotate as a unit, therefore driving the compressor

Evaporator temperature control

Thermistor type anti-frosting device

EVAPORATOR SENSOR

This is used to switch the magnetic clutch of the compressor on and off to prevent frosting on the evaporator fins The sensor is an NTC type thermistor and will therefore experience an increase in electrical resistance as its

temperature reduces (and vice versa)

This change in resistance provides a detectable temperature condition

through the use of a very simple ECU When the sensor indicates that the evaporator is at risk of frosting, the ECU will disengage the magnetic clutch to prevent this (ice is an excellent heat insulator and the efficiency of the air conditioning will reduce drastically if the evaporator ices)

Dual pressure switch

The switch is fitted between the expansion valve and the receiver It detects pressure on the high side of the system and switches off the magnetic clutch when the pressure is abnormal, thus preventing damage to the system

Pressure abnormally low

If the refrigerant quantity should fall short of capacity or leaks out completely,

a lack of lubrication of the compressor would result and it could seize

Therefore a lack of refrigerant will cause a pressure drop and the switch will then switch off the compressor preventing damage

Gas ejection method

Pressure of the refrigerant forces the valve to open preventing damage to the

if a pressure release valve is fitted to the system R134a systems use

pressure release valves