Air- Conditioning System Components

 Air- Conditioning System Components

AIR-CONDITIONING SYSTEM COMPONENTS:-

A typical automobile air-conditioning system. It includes compressor, magnetic clutch, condenser, receiver drier, strainer, expansion valve, evaporator, blower and the air distributor system. The evaporator and the expansion valves are located inside the passenger compartment, the other components usually put under the car hood. The components and the behavior of the refrigerant in them are briefly described here:

1.Compressor:-  The compressor used in the automobile air conditioning system is belt-driven from the engine crank shaft pulley. A magnetic clutch engages the compressor shaft. When the voltage is applied to the compressor clutch coil, the magnetic force so generated locks the clutch plate and the pulley together as one unit; and then the compressor shaft is turned with the pulley. Springs in clutch plate and hub assembly automatically moves the plate away from the pulley when the voltage to the clutch coil is interrupted. This causes the compressor to stop. Most compressors used are piston type compressors with one or two cylinders. The compressor draws in low pressure, high temperature vaporized refrigerant and compresses it to high pressures. The refrigerant never reaches its liquid state in the compressor. The compressor compresses the refrigerant to maximum of about 200 N/cm² and 100° C; and pushes it along the condenser.

The refrigerant used is a chlorine-fluorine mixture known by the trade name of 'Freon-12'. It has a boiling point of -22° at atmospheric pressure and exerts a vapor pressure of 125 N/cm2 at 133° F.

2. Magnetic clutch:- The electrically controlled magnetic clutch is housed in pulley assembly and the system selector switch controlling the clutch operation is mounted on the control panel inside the passenger compartment. In the Off or Vent position, the compressor and its clutch are off. In other four positions of the selector switch, the clutch is engaged or disengaged depending upon the temperature of the air in the ambient temperature. Below 40° F, the ambient switch located infront of the condenser remain open and the compressor cannot be operated. Above 55° F, this switch closes and completes the circuit to the compressor clutch. 

3. Condenser:- Condenser is basically a fin-and-tube radiator, usually placed infront of car's radiator. The condenser receives heated and compressed refrigerant vapor from the compressor and is cooled by the air passing across the condenser. The refrigerant is now in the form of a high pressure liquid which is fed to the receiver-drier unit.

4. Receiver-drier (or Dehydrator):- The refrigerant is stored under pressure in the receiver-drier or the dehydrator. The pressure in the receiver lines is between 50 N/cm² to 200 N/cm depending upon the compressor speed and the surrounding air temperature. The drier removes any traces of moisture present in the system to avoid freezing of moisture at low temperature and, thus, clogging the lines. Even a small amount of moisture can cause running troubles. Drier is usually a silica gel filter that absorbs any water that may be in the refrigerant. A liquid-sight glass is mounted on high pressure line between the receiver and the expansion valve for ascertaining the quantity of the refrigerant in the system.

5. Expansion valve:- The refrigerant goes from the receiver drier to the expansion valve where a sudden expansion to a much lower pressure occurs. The refrigerant changes back to vapor state and this causes a big chilling effect that drops the surface temperature of the evaporator coils to near freezing. It is here that the cooling effect is obtained.

The expansion valve is operated by opposing pressures on either side of the diaphragm. If the evaporator outlet temperature is higher the refrigerant expands and exerts more pressure against the diaphragm, thereby, providing a greater opening to the expansion valve and hence, a greater flow of refrigerant into the evaporator for increased cooling. When the outlet temperature of the evaporator is low, the expansion valve starts closing.

6. Evaporator:- The evaporator unit, where the cooling effect is obtained, is usually located inside the passenger compartment below the dash board. A high capacity blower circulates the air in the car interior across the evaporator coils; and this drops the temperature of the air inside the passenger compartment. The heat picked up by the refrigerant goes back to the compressor in the vapor from where the refrigerant is again compressed to a high pressure and the cycle starts again.

7. Suction throttling valve:- Suction throttling valve ensures that the refrigerant in the evaporator stays at such a pressure that the evaporator core surface temperature does not fall below the freezing point of water (0°C), thus preventing ice formation in the evaporator. A pressure equalizing tube is usually connected between the suction side of the suction throttling valve and the expansion valve diaphragm for effective expansion valve functioning.

EFFECT OF AIR-CONDITIONING ON FUEL ECONOMY

Because the cooling capacity of the automobile air-conditioning system is quite high, the engine driven compressor may absorb 0.75 to 7.5 kW and the fuel economy of the car is affected. If the car reserve power is small, it can affect its acceleration. A loss in economy level of order of 1 to 1.5 km/litre can occur due to use of air-conditioning. The maximum compressor power is required when the car is running at maximum speed under high ambient temperature conditions.

TYPES OF CONTROL SYSTEM:-

In order to control the function and operation of a unit, there are four basic cycles and mechanical systems used. These systems vary in size and installation procedures and are as follows:

1. Low side pressure control.

2. Pressure operated by-pass. 3. Solenoid operated by-pass.

4. Electromagnetic clutch.

5. Combination of above controls.

Low Side Pressure Control:-

In this system, an evaporator pressure controlled regulator valve is installed in the suction line. A constant pressure is kept in the evaporator by this valve. If the evaporator tends to go below a certain setting while holding the evaporator at constant pressure and temperature, the valve would close.

This pressure control prevents the compressor from producing a high vacuum at high speeds causing the compressor to lose its oil. A system with an automatic expansion valve having small bleeder hole in the orifice is used to overcome this problem. To prevent a very high vacuum from forming, a small amount of refrigerant enters the suction line. In this system, the compressor is run continuously.

Pressure Operated Hot Gas By-pass Valve:-

In this system, the pressure operated bypass valve is connected between the compressor discharge (high side) and compressor low side or suction line.  When the pressure difference reaches the valve setting, the valve is set to open and by-pass hot vapor from the high side to the low side. The valve will close or open if the suction line pressure decreases or increases respectively. To maintain a certain pressure in the evaporator, hot gas (vapor) is fed into the low pressure side. In this system, a continuously operated compressor is used.

Solenoid Operated Hot Gas By-pass:-

In order to open a solenoid valve and by-pass the hot gas from the high pressure side to the low pressure side when the temperature of the evaporator falls to 0° C, a thermostat mounted on the evaporator is used. It is located in the return air flow with a sensing bulb. When the return air temperature is lowered, so that the thermostat is at a temperature of 0°C, the solenoid valve opens to allow hot gas from the condenser to by-pass back into the suction line. This valve does not provide the throttling effect of a pressure operated valve since it is either closed or wide opened. In this system, either a continuous operating compressor or a rotary compressor is used. When the circuit is closed or when the current is on, the solenoid is in a closed position. When the circuit is open, the valve also opens. Hence, the thermostat opens on temperature drop. When the pressure in the system balances to reduce the starting load and the power is off, the solenoid valve also opens up.

Electromagnetic Clutch:-

In certain automobiles air-conditioning systems, a mechanism permitting the engine to run without the compressor running is used. To engage the compressor belt pulley to the compressor crank shaft or the disengage it, a clutch is used. This clutch operated by electromagnetism by forcing the clutch disk against the pulley is known as magnetic clutch. Two types of magnetic clutches used are - revolving magnetic coil and stationary magnetic coil.

***Air- Conditioning System Components