Heating / Air-Conditioning / Ventilation Systems

Heating and air-conditioning systems do little for the operation of a vehicle; they merely provide comfort for the passengers of the vehicle. Both systems are dependent on the proper operation of the engine.

The heating system basically adds heat to the vehicle's interior, whereas air conditioning removes heat.

To do this, the systems rely on many parts to put basic theories to work.

Heating Systems

To meet federal safety standards, all vehicles must be equipped with passenger compartment heating and windshield defrosting systems. The main components of an automotive heating system are the heater core, the heater control valve, the blower motor and the fan, and the heater and defroster ducts. The heating system works with the engine's cooling system and converts the heat from the coolant circulating inside the engine to hot air, which is blown into the passenger compartment. A heater hose transfers hot coolant from the engine to the heater control valve and then to the heater core inlet. As the coolant circulates through the core, heat is transferred from the coolant to the tubes and fins of the core. Air blown through the core by the blower motor and fan then picks up the heat from the surfaces of the core and transfers it into the passenger compartment. After giving up its heat, the coolant is then pumped out through the heater core outlet, where it’s returned to the engine's cooling system to be heated again.

Transferring heated air from the heater core to the passenger compartment is the job of the heater and defroster ducts. The ducts are typically part of a large plastic shell that connects to the necessary inside and outside vents. Contained inside the duct are also the doors required to direct air to the floor, dash, and/or windshield.

Air-Conditioning Systems

An air-conditioning (A/C) system is designed to pump heat from one point to another. In an automotive A/C system, heat is removed from the passenger compartment and moved to the outside of the vehicle.

The substance used to remove heat from the inside of a vehicle is called the refrigerant. To understand how a refrigerant is used to cool the interior of a vehicle, the effects of pressure and temperature on it must be first understood. If the pressure of the refrigerant is high, so is its temperature. Likewise if the pressure is low, so is its temperature. Therefore, changing its pressure can change the refrigerant's temperature.

To absorb heat, the temperature and pressure of the refrigerant are kept low. To get rid of the heat, the temperature and pressure are high. As the refrigerant absorbs heat, it evaporates or changes from a liquid to a vapor. As it dissipates heat, it condenses and changes from a vapor to a liquid. These two changes of state occur continuously as the refrigerant circulates through the system.

|||| 9 A simple look at an air conditioning system. The blue signifies low pressure and the red is high pressure. Condenser Compressor Evaporator Dryer Flow control device

An A/C system is a closed, pressurized system. It consists of a compressor, condenser, receiver/dryer or accumulator, expansion valve or orifice tube, and an evaporator. The best way to understand the purpose of the components is to divide the system into two sides: the high side and the low side. High side refers to the side of the system that is under high pres sure and high temperature. Low side refers to the low-pressure, low-temperature side.

Compressor --The compressor separates the high and low sides of the system. Its primary purpose is to draw the low-pressure and low-temperature vapor from the evaporator and compress this vapor into high-temperature, high-pressure vapor. The secondary purpose of the compressor is to circulate or pump the refrigerant through the system. The compressor is located on the engine and is driven by the engine's crankshaft via a drive belt.

Compressors are equipped with an electromagnetic clutch as part of the compressor pulley assembly. The clutch is designed to engage the pulley to the compressor shaft when the clutch coil is energized. When the clutch is not engaged, the compressor shaft does not rotate, and the pulley freewheels.

The clutch provides a way for turning the compressor on or off.

Condenser The condenser consists of coiled tubing mounted in a series of thin cooling fins to provide maximum heat transfer in a minimum amount of space. The condenser is normally mounted just in front of the vehicle's radiator. It receives the full flow of ram air from the movement of the vehicle or air flow from the radiator fan when the vehicle is standing still.

The condenser condenses or liquefies the high pressure, high-temperature vapor coming from the compressor. To do so, it must give up its heat. Very hot, high-pressure refrigerant vapor enters the inlet at the top of the condenser, and as the hot vapor passes down through the condenser coils, heat moves from the refrigerant into the cooler air that flows across the coils and fins. This loss of heat causes the refrigerant to change from a high-pressure hot vapor to a high pressure warm liquid. The high-pressure warm liquid flows from the bottom of the condenser to the receiver/dryer or to the refrigerant metering device if an accumulator is used.

Receiver/Dryer--The receiver/dryer is a storage tank for the liquid refrigerant from the condenser. The refrigerant flows into the receiver tank, which contains a bag of desiccant (moisture-absorbing material). The desiccant absorbs unwanted water and moisture in the refrigerant.

Accumulator--Most late-model systems have an accumulator rather than a receiver/dryer. The accumulator is connected into the low side at the outlet of the evaporator. The accumulator contains a desiccant and is designed to store excess refrigerant. If liquid refrigerant flows out of the evaporator, it will be collected by and stored in the accumulator. The main purpose of an accumulator is to prevent liquid from entering the compressor.

Thermostatic Expansion Valve/Orifice Tube--The refrigerant flow to the evaporator must be controlled to obtain maximum cooling while ensuring complete evaporation of the liquid refrigerant within the evaporator. This is the job of a thermostatic expansion valve (TEV or TXV) or a fixed orifice tube. The TEV is mounted at the inlet to the evaporator and separates the high-pressure side of the system from the low-pressure side. The TEV regulates the refrigerant flow to the evaporator by balancing the inlet flow to the outlet temperature.

Like the TEV, the orifice tube is the dividing point between the high- and low-pressure sides of the sys tem. However, its metering or flow rate control does not depend on comparing evaporator pressure and temperature. It’s a fixed orifice. The flow rate is determined by pressure difference across the orifice and by the additional cooling of the refrigerant in the bottom of the condenser after it has changed from vapor to liquid.

Evaporator --The evaporator, like the condenser, consists of a refrigerant coil mounted in a series of thin cooling fins. The evaporator is usually located beneath the dashboard or instrument panel.

The low-pressure, low-temperature liquid refrigerant from the TEV or orifice tube enters the evaporator as a spray. The heat at the evaporator causes the refrigerant to boil and change into a vapor. The transfer of heat from the evaporator to the refrigerant causes the evaporator to get cold. Because hot air always moves toward cold air, the hot air from inside the vehicle moves across the evaporator. As the process of heat loss from the air to the evaporator core surface is taking place, any moisture in the air condenses on the outside of the evaporator core and is drained off as water. This dehumidification of air adds to passenger comfort.

Refrigerant Lines--There are three major refrigerant lines. Suction lines are located between the outlet side of the evaporator and the inlet side or suction side of the compressor. They carry the low-pressure, low temperature vapor to the compressor. The discharge or high-pressure line connects the compressor to the condenser. The liquid lines connect the condenser to the receiver/dryer and the receiver/dryer to the inlet side of the expansion valve. Through these lines, the refrigerant travels in its path from a gas state (compressor outlet) to a liquid state (condenser outlet) and then to the inlet side of the expansion valve, where it vaporizes at the evaporator.

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