Automobile Air Conditioning
Chapter 1
INTRODUCTION
Twenty five years ago air conditioning in cars was rare and driving in summer was nothing short of sweaty experience. There was only a handful technicians who could install AC units on Indian car and also maintain imported car A.C. One opening the bonnet of car one could recognise who has installed that particular unit as good as recognising a friends handwriting.
The use of refrigeration and air conditioning for transporting purpose proves to be very advantageous. Air conditioning is very much used in cars i.e. Automobiles, railways, aeroplanes, and ships. The use of air conditioning in automobiles is a luxury in India but it is commonly used in western countries to provide better human comfort.
Today automobile air conditioning has acquired a growing market. The AC in automobiles is a need of persons who are suffering from the hot climate in India which may be carry about 8 to 10 months per year. The new cars are so designed as to accommodate A.C. in its cabin. Maruti 800, Maruti 1000, Premier 118NE Contessa Classic, Tata Instat., Tata Siera , Opel Astra, Ford and Mercedes Bens are some of the models which are having A.C. system.
Automobile air conditioning system works on the principle of vapour compression refrigeration cycle and employees R12 as refrigerant to run the system
The following factors are controlled by A.C. which leads to human comfort.
1) Heating of cabin,
2) Cooling,
3) Circulation of air,
4) Cleaning and filtering,
5) Humidity control.
As per the standards the temperature at 250 C and humility of 50% R.H. is maintained to provide better comfort. This can be achieved very easily in a room or office but it is very difficult to maintain such temperature and R.H. factor because of different sources of heat addition to the automobile system. This heat sources are stated later.
#MULTI FLOW SYSTEM :-
All modern cars are equipped with multiflow A.C. systems whereby we can channel the airflow to your face, feet or the wind screen. It is recommended that the face position only be used. This channels the air trough the front vent in the facia to occupants seating in the rare seat most effectively. vehicle is owner driven or when there are only two occupants in the car;, both seating in the front. Since the feet are the most sensitive to the temperature, when the airflow is set in this position it provides very effective cooling to the occupant., The wind screen position should be used only when the A.C. is off and the heater is on This position enables, the hot air from the heater to strike the windscreen on the outside.
The feet only position should be used when the heater is on. When hot air is let out, in this position ,it rises and heats the car compartment evenly. Another control lever is a multiflow system is the adjustment for fresh air intake and the recycling of air. The fresh air-intake and the recycling of air. The fresh air position allows outside air to enter the passenger compartment, including exhaust fumes.
The persons seating in a car can adjust the position of knob, which controls the cooling effect to be provided. If more cooling is desired. He can adjust the knob and he will get the more cool., If less cooling is needed he can minimises the cooling effect. If he is unintrested to run AC he can stop the A.C. unit.
The major components like evaporator compressor and expansion valve are the same as are utilise in common room AC system such as power source, types of controls, component design capacity, etc.
Chapter 2
CYCLE
Vapour (fairly dry vapour) leaves the evaporator and enters the compressor at point 1. The vapour is compressed is entropically to point 2. During compression, the pressure and temperature increases. The temperature at point 2 should be greater than the temperature of the Condenser cooling medium. The vapour leaves the compressor in dry saturated state and enters the condenser at 2. The vapour is condensed and latent heat of condensation is removed in condenser. The high pressure saturated liquid leaves the condenser and enters the throttle valve at 3. Thus the flow through valve causes decrease in pressure and temperature of refrigerant and causes it to evaporate partly. This refrigerant liquid at every low temperature enters the evaporator where it absorbs heat from the space to be cooled thus producing refrigerating effect. This increases its pressure and temperature and the refrigerant is now dry vapour , which is supplied to compressor. This completes the cycle.
Chapter 3
WORKING
Cool refrigerant gas is drawn into the compressor from the evaporator and pumped from the compressor to the condenser under high pressure and temperature due to compression, As this gas passes through the condenser, high pressure, high temperature gas rejects etc. Heat to the outside air as the air passes over the surface of condenser. The coding of the gas causes it to condense into a liquid refrigerant. The liquid refrigerant still in high pressure passes to receiver drier (dehydrator), The receiver acts as a reservoir for refrigerant The liquid refrigerant flows from the receiver dehydrator to the thermostat expansion valve refrigerant will looses its pressure and temperature. This low pressure low temperature liquid enters the evaporator. The evaporator coil is mounted below front dash board. As the temperature of refrigerant passing through evaporator is low„ it absorbs heat and continues to boil, drawing heat from the surface of the evaporator core warmed by the rush of air passing over the surface of the evaporator core.
In addition to the warm air passing over- the evaporator rejecting its heat to the cooler surfaces of the evaporator core, any moisture in the air condenses on the cool surface of the core resulting in cool dehydrated air passing into the compartment of the car. By the time the gas leaves the evaporator, it gets completely vapourised and is slightly superheated. The pressure in evaporator is controlled by suction throttle valve. R12 vapour passing through the evaporator flows through the suction throttle valve and is returned to compressor where refrigeration cycle is repeated.
Chapter 4
SOURCES OF HEAT TO THE SYSTEM
The cooling load is affected by many factors. Some of them are listed below
1) Faster the car moves, the greater amount of infiltration into the car and better rate of heat transfer .
2) The sun baking down on the blank road will raise the temp. up to 500 to 600 C and thus increases the amount of heat transferred into the car through the floor.
3) Because of the relatively large glass areas, metal construction and the flow of air around the moving vehicle (automobile) is very large, so the air conditioning capacity is also large in comparison with A. C. installed at home.
4) Quantity of fresh air in.
5) Number of occupants.
6) Quantity of heat directly rejected by sun on car.
For all the above sources, it is necessary that capacity of automobile A.C. should be large, be capable to take ovealoads and operate for relatively long
periods.
The cooling capacity of automobile A.C. system ranges from 1 to 4 tones, which is the amount of refrigeration needed to cool a small house
Chapter 5
COMPONENTS
1)COMPRESSOR :-
Compressor is a driver of the system. The construction is much rigid and the unit is semisealed. i.e. the power to drive the compressor is directly taken from the crank shaft by means of v-belt pulley and electromagnetic clutch. The heavy-duty gaskets are provided at joints to prevent vibration, noise and leakage. A typical value arrangement is provided to suit the requirements. The high and high pressure refrigerant enters in compressor which further gets compressed causing hot vapour exit from the compressor unit, The compressor can start or stop by means of thermostat arrangements which engages or disengagtes the electromagnetic clutch so as to run compressor as per requirements. Lubrication oil is placed inside the chamber. The noise of compressor is very least as compared to that of engine. The vibration of compressor creats problem in Diesel air conditioning system.
2] ELECTROMAGNETIC CLUTCH –
The pulley assembly contains an electrically controlled magnetic: clutch, permitting the compressor to operate only when air conditioning is actually desired. All automobile A.C. systems employs the clutch to drive the compressor on demand from the thermostat inside the car (i.e. the knob).
When the compressor clutch is not engaged, the compressor shaft does not rotate, although the pulley is being rotated by belt from the engine. The clutch armature plate, which is movable member of the drive plate assembly is attached to the thrive hub through drive springs and is riveted to both driver and armature plate. The hub of this assembly is pressed over the compressor shaft is aligned with a square drive key located in the key way on the compressor shaft. The pulley assembly. Consist of pulley rim , pulley hub and power element ring.
3] CONDENSER -
A condenser is similar to an ordinary automobile radiator but are designed to withstand much high pressure, It contains a fan to provide forced circulation of air. This whole assembly is fitted infront of the car radiator. So that it receives high volume of air. The high temperature and high pressure refrigerant vapour looses its heat to forced air flowing through it causing change of this phase into high pressure liquid. The fan and electromagnetic clutch are electrically coupled. Rapid condensation of refrigerant can be done by fan. This high pressure liquid refrigerant then passed to receiver drier.
4) Receiver Drier :-
The purpose of receiver drier is to ensure a solid column of liquid refrigerant to the thermostatic expansion valve. Automobile A.C. units are more susceptible to leaks than units because of vibration. Over a period of time, small leaks will occur, which may requires addition of refrigerant,, Also the evaporator requirements vary because of the changing heat load. A small receiver is used in the system to compensate all the above variables, Refrigerant is stored in the unit untitled it is needed by the evaporator. A liquid indicator or slight glass is provided at outlook pipe of receiver unit. The appearance of bubbles or foam in the slight glass indicates the shortage of refrigerant in the system. Drier part of this unit consists of sillica gel to absorb moisture if any in the system, also it traps foreign material which may have entered the system during assembly. It is temporary storage and purifying unit.
5) EXPANSION VALVE:
The expansion valve fulfils the following two functions.
1) The temperature and pressure of refrigerant is reduced to such a low-level due to sudden expansion by throttling process. This is helpful to create low temperature than the evaporator .
2) According to cooling load, the quantity of refrigerant supplied to evaporator can be controlled.
It automatically regulates the flow of liquid refrigerant. The valve is located at the inlet to evaporator core. It consists of a (capillary bulb and tube, which are connected to an operating diaphragm (sealed within the valve).
When the cooling load increases, the refrigerant evaporates at a faster rate in evaporator than the compressor can suck. As a result., the degree of superheat and pressure in evaporator increases which cause the valve to open more allowing more refrigerant to enter into the evaporator. At, the same time the increases in suction pressure also enables the compressor to deliver increased refrigerating capacity.
When cooling load decreases;, the refrigerant evaporates
at a slower rate than the compressor can suck, As a result the evaporator pressure drops and the degree of superheat will decrease., The valve tenets to close and the compressor delivers less refrigerant capacity .
Thus this valve is capable of meeting the varying load requirements,, This valve keeps the evaporator full of refrigerant, thus ensures safety to compressor
6)Evaporator:-
Evaporator is a place where the refrigerant evaporates and absorbs heat from the air passed over it. Air is forced to flow over the evaporator with the help of blower, which is installed in the evaporator itself and cooled before distributing in seating compartment.
The design of evaporator is more critical as the space limitations are very severe and worse than compact room conditioners.
The evaporator is placed under dashboard of car, We can provide more ducts if the car seating capacity is more. The purpose of evaporator is to cool and dehumidity the air passing over it into passenger's cabin.
The refrigerant in cooled liquid state boils immediately in evaporator when air looses its heat and moisture to it. Heat from the core surface is lost to boiling and vapourising refrigerant, which is cooler than the core, thereby cooling the core. The moisture collected is then drained of as it may reduce the cooling effect.
Dirt or other foreign matter on the core surface or in evaporator housing will restrict the airflow. A cracked or broken housing can result in insufficient air-or warm air supply to passenger’s compartment. The dirt can be removed by forcing dry air on it under pressure.
7) Suction throttle valve:-
The device which is used to regulate the refrigerant flow from evaporator to compressor is termed as suction throttle valve. It is located in the suction throttle valve. It is located in the suction line between the outlet of evaporator and inlet to compressor.
It controls the evaporator pressure and evaporator out temperature. It also prevents the foreign materials to enter the compressor.
8) FAST IDLING CONTROL DEVICE :-
This is a speed raising device, Specially when in city operation when the vehicle is moving at very low speed or is stationary, the idle rpm of moving at very low speed or is stationary , the idle rpm of the engine is raised to the rated value in order to get sufficient coiling and air draft on condenser and radiator. When A.C. is on, the vaccum solenoid valve , which is electrically operated , directs vaccum to actuator installed on the injection pump. With the help of actuator, the accelerator lever is pushed to get higher idling rpm.
9) CONTROLS:-
These units ensures safe operation of air conditioner. The thermostat is used to prevent the formation of frost on the evaporator coil. The cabin air temperature is also controlled to the desired level. Once the evaporator fins temperature approaches near freezing point, the thermister sends signals to the thermoamplifier which in turn cuts all power supply to electromagnetic clutch, thereby A.C. operation stops temporarily.
Chapter 6
INSULATION
It is absolutely essential to provide sufficient insulation to the car to reduce the refrigeration capacity required. The glass or fiberglass is used for the car insulation, Insulation is provided over the back of the rear seat of the car to avoid the air-conditioning of the trunk space. The tinted glass are preferred instead of plain glass for windows as large areas of windows allows a considerable heat leakage and allows high sun load. The insulation also reduces the noise level.
Another insulation should must provided at the floor to reduce heat admitted through floor. Also the tubing surrounding the engine containing refrigerant should also be properly insulated.
Chapter 7
ADVANTAGES
The main advantage of this system is to travel with comfort for a long distance. In any type of atmospheric conditions without tired. During summer the temp. inside the car can he maintained low and this is very necessary for comfort conditions. There may be more advantages rather than this.
Chapter 8
DISADVANTAGES
1) Engine speed, average and power will reduce due to power supplied
to run A.C. system.
2)Maintenance and initial cost of unit is high.
3)Proper attention should be given to all the components.
4)Very complicated assembly for repair work.
5)Leakage due to vibration is the major problem, which can be reduced .
6)Separate electrical fuse is required for compressor.
7)Cleaning and servicing should be on time.
8)It can be used mostly for luxurious purpose.
9)“Sunstroke”
This is the major problem. But it can be minimised by allowing the A.C . to stop for certain time till the temperature inside the cabinet reaches the atmospheric temperature , which will not provide to be danger.
Though there are lot of disadvantages, the automobiles having A.C. built in has captured the market.
Chapter 9
TIPS FOR MAINTAINANCE
1) Keep The interior of the car clean in order to prevent drawing dust from the mats and clogging the A.C.
2)If the vehicle is parked outside during the day, the interior temperature will be on the high side. Keep the blower on maximum, lower the windows and drive some distance to ventilate the hot air tapped inside.
3)Have the A.C. system examined before the onset of summer as all A..C. installers are over loaded with work during this season. Have the has levels examined every eight months to year and also have the A.C. serviced or charged once every 30,000 km. Avoid using the fresh air intake over since it will clog your evaporator with dust or leaves that have accumulated under the bonnet grille.