*Car Air-conditioning System
CAR AIR-CONDITIONING SYSTEMS Some of the important air-conditioning systems used and manufactured by Ford and general Motors are given here:
Ford Air-conditioning System:-
This system uses a factory installed unit. It has the blower, heat exchangers and housing on the engine side of the finned wall. A thermostat expansion valve with a built in sight glass and a two cylinder compressor usually mounted horizontally is used. There are two service valves mounted on the head of the compressor. A thermostat having its sensitive bulb clamped to the evaporator fins operates the compressor magnetic clutch. For providing the air circulation, duct dampers operated by a vacuum activating system are used.
General Motors Air-conditioning System:-
This system uses a rotary compressor. To control the evaporator pressure and exhaust muffler on the compressor, a pressure operated altitude valve is used. Basic refrigeration circuit of automatic air-conditioning system consists of an electrically controlled heating system, air cooling system and air duct flow using a vacuum activator system for controlling the dampers and the valves. Three temperature sensors (one outside the car, one inside the car and another in the discharge disc) connected to an amplifier and a programmer are used in this system.
Major components and refrigerant flow of the air conditioning system for Maruti MF 410 vehicle is shown
in picture
The recent research has called for a ban on 'R134a' refrigerantalso and to replace the same with CO2. In addition to the environmental benefits, CO, as a refrigerant has also the following advantages:
1. 20% less fuel consumption than today's systems, due to better average efficiency throughout the year.
2. Cooling capacity of the present air conditioning systems is limited by the compressor piston displacement, which itself is limited by the insufficient space in the engine compartment. CO, air conditioning systems only need about 13% of the volume flow of an 'R134a' system, making it possible to use smaller compressors enabling CO, system to cool the passenger compartment faster.
3. As the process is also reversible, unlike today's air conditioners, it also has a heat pump function. This can be used to heat up the vehicle interior very quickly. replacing the electric supplemental heaters that are common with modern direct-injection diesel engines.
It is expected that due to the above clear advantages, CO₂ technology will start replacing present air conditioners in 2007. 'Luk' has specialized in the development of compressors for the new technology.
Important tips for the maintenance of car AC system are:
1. Clean the condenser during normal servicing. Clogged condenser can result in frequent AC tripping and lesser cooling.
2. Maintain correct refrigerant level. Less refrigerant would result in less cooling. Excessive refrigerant would also result in lesser cooling and excessive tripping.
3. Top up the compressor with adequate oil.
4. Clean the evaporator periodically. Clogging of evaporator would cause poor airflow and hence lesser cooling.
5. Maintain proper belt tension. A loose belt will slip whereas overtight belt would cause noise and premature compressor failure.
Important precautions while using car AC systems are:
1. Do not use AC with fresh air mode open always since this mode is for occasional use just to release the air in circulation for a long time. Frequent use of fresh air mode may result in the accumulation of dust and clogging of evaporators.
2. Never operate AC with heater on.
3. Never run AC without refrigerant, otherwise compressor may cease. 4. Do not leak-test AC with more than 2 MPa pressure,
higher pressure can damage the components.
5. Do not leave AC joints open, which may cause the moisture to enter the system. 6. Do not charge the refrigerant in the AC system before flushing.
7. Do not switch on the AC at high speeds, which may result in the ceasing of compressor.
Bus Air-conditioning:-
Air-conditioning system used for a bus is of standard design. Its condensing unit is very compact and is generally installed in the bus in such a way that for servicing it is readily accessible. Due to large size of the unit, a separate gasoline engine with an automatic starting device is used to drive the compressor. To permit sliding of the condensing unit out of the bus body during servicing the condensing unit is generally mounted on rails with flexible suction and liquid lines. Standard thermostat expansion valves refrigerant controls and air cooled condensers are used. Evaporator used are finned blower. The ducts, usually one on each side of the bus. run between a false ceiling and roof of the bus. Grills controlled by passengers are provided at their seats.
TRUCK AIR-CONDITIONING:-
Due to varying conditions of heating, ventilating, cooling and dehumidification in the atmosphere at various places, long distance hauling trucks and truck tractors carrying perishable products are air-conditioned. Air-conditioned equipment used is of the "hang on" type. It is generally installed after the body has been made. Generally, two evaporators, one on the cab and other for product compartment, are used. A remote condenser mounted on the roof of the cab is generally used. To enable the radiator to operate at fuel efficiency, it helps to remove the condenser from infront of the radiator.
ROVAC AIR-CONDITIONER:-
This system of air-conditioning of the car uses air. It is yet to be commercially deployed in cars. The system has been designed by Dr. Thomas C. Edwards. It is rotary vane air cycle air-conditioning system. The system has the following distinguishing features:
1. It does not use Freon gas. Hence, there is no high pressure gas to store, seal against leaks or replace. The system is thus much simpler.
2. It has high coefficients of performance when compared with conventional type. This makes the system to consume less power of engine and hence there is fuel economy.
3. It gives instant cold. There is no detectable time lag from start up to cold air.
4. It gives instant heat. This makes the heat available from intermediate coils for defrosting long before the engine gets warmed up.
ROVAC system alternately compresses and expands the air. The conventional sealed units make use of reversed Rankine cycle with a gas-to- liquid and back-to-gas phase change. ROVAC uses no phase change like the conventional sealed units, except for the incidental moisture normally present in the air. Edwards calls this a reversed Brayton cycle.
The oval shaped housing acts as a compressor on the inlet side and an expander on the exhaust side. The air enters the compressor at atmospheric pressure and passenger compartment temperature. This air is compressed to a pressure of about 21N/cm and temperature of about 140° C. From the compressor, the air travels through heat exchanger which is provided with fins. In fact, the heat exchanger has large tubes. The inside air is cooled by the outside air flowing past the tubes. This drops the temperature of inside air to 45° C. There is no appreciable change in the pressure.
Now, this air is lead through the expander where the pressure drops to atmospheric and the temperature of the air drops to about 6° C. This air is circulated inside the passenger compartment, thus cooling the occupants. The cycle gets repeated.
It may be mentioned that the ROVAC can be designed to produce outlet temperatures well below freezing point. Modulation and moisture control is handled in the distribution ducts. It is worth mentioning that when the hot air is passing through the expander, it returns part of its heat energy to the vaned rotor, thus saving engine power.
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