Auto Air-conditioning System

 Auto Air-conditioning System

INTRODUCTION:-

Due to varying conditions of heating, ventilating, cooling and dehumidification in the atmosphere at various places, the air conditioning of automobiles is very essential. To maintain human comfort and improve internal atmosphere in an enclosed space, proper control of freshness, temperature, humidity and cleanliness of the air is required.

automobile, the air-conditioning system involves three main processes of heating, cooling and dehumidification. In the heating system, the heat needed to warm the automobile is derived from the engine coolant or circulating warm water after passing through a heating coil. The system either consists of the recirculatory heaters or fresh air intake systems with provisions of recirculation. For producing a cooling effect, an evaporator coil operated by a refrigerating system is placed inside the chamber of automobile. The air to be circulated inside the passenger compartment is cooled by the evaporator coil. Depending upon the speed of vehicle, frequency of stops, atmospheric and traffic conditions, the air conditioning loads vary from instant to instant and place to place. Due to moving nature of the vehicle, the equipment used should be light, compact, quickly serviceable and unaffected by vibrations.

The dust particles are entrapped by the wet surfaces of the evaporator core and are drained of with the condensed moisture. This provides clean and pure air for breathing.

In comparison to domestic air-conditioning, very large amount of air-conditioning capacity is required for a car. This is due to the metal construction of the car, the flow of air around a moving car and relatively large glass areas in the passenger compartment. Typically, a car air-conditioning system may be between 1 to 4 tons.

A refrigerating machine driven by the car engine is used in the automobile air conditioner to maintain the desired comfortable temperature level. For heating purposes, the warm water from the engine cooling system is used. The heat required to warm the automobile is generally provided by circulating warm water through a heating coil.

The air-conditioning system of the most of the car consists of 1.5 kW compressor located under the engine hood and belt driven from the crank shaft. The evaporator and the blowerunit were located in the trunk compartment while the air cooled condenser was placed infront of the radiator.

A typical air-conditioning system used in automobiles, as shown in primarily consists of compressor, condenser, receiver, expansion, evaporator, dehydrator, filter, sight glass, blower and the air distribution system as well as certain type of capacity control.

Operating conditioning:-. In an automobile, the air-conditioner g should provide comfort and control conditions in the car during mild or damp weather as well as hot weather. The operation should include heating, defogging, as well as removing dust, smoke and odor.

The compressor is belt driven from the engine. Since the engine speed changes with the change in car speeds, therefore, the compressor speed will also vary. Only the hottest day in the sun as well as underside wind conditions the compressor as well as the cooling is at idling speed. Considerable excess capacity, particularly under cool weather conditions, is provided of by the above capacity condition. The problems of controlling the temperature as well as the refrigerant flow (both liquid and vapor) within the system is provided by this condition. The high side pressure is liable to build up while the low sidepressure will drop very low when the compressor is operating very rapidly; and little or no refrigeration is needed.

The evaporator temperature will be lowered by dropping of the low side pressure. Since the evaporator surface is liable to circulation through it, it is, therefore, necessary that the evaporator temperature should not be allowed to drop below 0°C. The compressor valves are liable to damage as well as result in its burning out if the system is operated under a low side pressure.

To overcome these problems, different types of cycle and mechanical systems are used. To obtain maximum cooling during high heat loads in cooling season as well as in washing or during heavy rainstorm, it is necessary that the fresh air ducts should be closed. About 10 minutes are taken by a standard automobile air-conditioner to cool an automobile from 43.3°C to 29.44°C standing in the sun. Generally, the inside temperature of a car parked in sun with windows closed is 65.5°C. The sun load and the heat conducted through the car windows is the highest heat load or the heat gains. Usually 100% fresh air or no fresh air (all circulated) to 25% fresh air are used by different units.

Performance rating:-The compressor capacity will decrease when the automobile slows down, while it will increase when it speeds up. Except during the interval of parking or moving in a slow moving traffic, the variation in compressor capacity is directly proportional to the changing heat loads. This changed capacity is below the required capacity. To obtain the required capacity, the engine may be idled at higher speed and for higher engine speeds, travel in traffic in some intermediate gear.

At high speeds, the refrigerating system consumes as much as 6 kW with a unit capacity of 3 tons or 9072 kcal/h approximately. For each ton of refrigeration effect, about 0.4 kW to 2 kW are required which is comparably more than a motor driven constant speed compressor comparably built and with the evaporator coil and condenser more ideally located requiring 0.75 kW for each ton of refrigeration.

A hot water coil using engine heat is installed in the air duct for heating purposes. During both the cooling and heating purposes, the same fans may be used.

Typical installation. A factory installed air-conditioner to be fitted as a compact unit or cooling unit which can be fitted in any standard make of automobile are manufactured by most of the automobile manufacturers. A small mixing chamber is by a V-belt from a special pulley on the engine crank shaft. In certain systems, the compressor driven pulley contains an electrically operated clutch to declutch it from the pulley. The condenser is mounted at the front of the car while the receiver is mounted at the side of the engine. The expansion valve, which is the controlling valve in the system, is placed near the evaporator. The evaporator is fitted in the tank compartment.

The cooled air enters the passenger compartment at its rear before distribution. Below the rear seat, the warm air is returned through a grill to the cooling unit after passing through a filter.

Units of generally 1 to 1.5 tons capacity, with compressor ranging from 1.5 to 3 kW (maximum) are used in most of the cars. To control the car temperature fitted under the dash board receives air from both the evaporator coil as well as the heater coil. The passenger compartment receives the conditioned air travelling from the plenum or mixing chamber.

For heating the motor vehicle, the controls are set. The hot water from the engine block water jacket circulating through the heater coils in the passenger compartment is used for heating purposes. To provide air-conditioning cooling, the controls are again set. The air blown through the cold evaporator coils is circulated through the passenger compartment of the motor vehicle for this purpose. Certain units are also operated by automatic systems.

A typical installation of air-conditioning system used in automobiles is shown in

The compressor is generally mounted on the side of the engine. It is motor driven and compensate for the variable compressor speeds a thermostat is also used. A recent development in automobile air-conditioning is a complete factory assembled unit which can be bolted near the engine.

**Describe about.....................Auto Air-conditioning System