UNDERSTANDING A/C

BASIC OVERVIEW

The air conditioning system in your car is compromised of a compressor, condenser, expansion valve, receiver/drier and evaporator. Refrigerant (also known as freon) is compressed in the compressor and turns into a gas. In the condenser, this gas is cooled to a liquid state and travels to the expansion valve. As the liquid refrigerant goes through the expansion valve, its rapidly cools in the evaporator. A fan blows over the evaporator and cools the air that blows out of your vents. The receiver/drier separates gas and liquid.

EVAPORATOR:

In order to remove the heat from the air in the vehicle, the evaporator allows the refrigerant to absorb the heat form the air passing over it. The blower fan moves cool air out into the car’s interior.

COMPRESSOR:

The compressor pumps and circulates the refrigerant through the system.

CONDENSER:

The condenser is a heat exchanger mounted at the front of the vehicle. Heat drawn out of the interior of the car is expelled here.

RECEIVER/DRIER:

The drier not only dries refrigerant, it also filters the refrigerant and stores it under certain operating conditions.

HI/LO PRESSURE SWITCH:

A pressure switch is used to shut down the system if high or low pressure is detected. It basically acts as a safety switch.

THEORY

THE MAJOR FUNCTIONS

To be effective, the automotive air conditioner must control these conditions within the vehicle interior:

  •      • it must cool the air
  •      • it must circulate the air
  •      • it must dehumidify the air

These functions are essential if passenger comfort is to be maintained when the ambient temperature and humidity are high. By performing these functions, the air conditioner maintains the body comfort of the passengers.

UNDERSTANDING HEAT

To understand how an air conditioning system works, we must first understand the nature of heat. For a simple definition, we may say heat is energy. The meshing of gears, the turning of wheels, cause of friction which results in heat. Heat in either extreme will be uncomfortable. The control of temperature means the control of comfort. Air conditioning is a method of controlling heat.

All substances contain heat. Something feels hot when it is warmer then our body temperature. When something contains less heat that our bodies, it feels cold. Cold is merely the removal of some heat. Science tells us that a measurement called “Absolute Zero” is the point at which all heat is removed from an object (approximately -273 C). Any substance above absolute zero temperature contains some heat.

The average person requires a comfort zone of approximately 70-79 F, with a relative humidity of 45-50%. All objects within this range is comfortable to touch.

HEAT MEASUREMENT: A temperature reading gives us the heat intensity of a substance and not the actual quantity of heat. Heat quantity is measured by kilocalories (KCAL). One KCAL is the amount of heat required to raise the temperature of one kilogram of water 1 C (at sea level.) This quantity measurement is used in air conditioning to describe heat transfer during changes of state.

CASES OF HEAT MOVEMENT: Heat always moves from the hotter objects to the colder one. Whenever there is a transfer difference between 2 objects, the heat energy will be transferred from the warmer object to the cooler until both are stabilized at the same temperature. This is known as the law of heat transfer and is the basis go air conditioning operation.

HOW DOES HEAT GET INSIDE A VEHICLE: When a car is driven or parked in the sun, heat enters the vehicle through many sources. These sources include:

CHANGES OF STATE

EVAPORATION

The term used when enough heat is added a liquid substance to change it into a vapor.

CONDENSATION

The term used when the opposite of evaporation occurs. Condensation is when you take a vapor and remove enough heat from it and the vapor becomes a liquid.

FREEZING

The term used when heat is removed from a liquid substance until it becomes a solid. Remember that anything above -273 C (-459.4 F) still contains some heat. In an air conditioning system, freezing must be avoided otherwise component damage will occur.

R134A PROPERTIES

Commonly referred to as R134a, Tertra Flouroethane is a non-ozone depleting refrigerant HFC 134a chemical. R134a was selected as a replacement refrigerant for R12 because R12 contains chlorine and has a major effect to ozone layer depletion. R134a and water have the same abilities to change the state but R134a can do this more rapidly and at a much lower temperature than water. At any time above -15.34 F, R134a changes state and becomes a vapor absorbing large quantities of heat from inside the vehicle. This is what creates the cooling effect you feel inside the vehicle. R134a is stored in containers under high pressure. If it is released into the atmosphere, it will boil at -15.34 F.

PRINCIPLES OF AIR CONDITIONING

HIGH PRESSURE SIDE: Low pressure R134a vapor entering the compressor is compressed to become high pressure/temperature R134a vapor. This is then circulated along with lubricant oil to the condenser. As the high pressure/temperature vapor travels through the condenser, heat is released to the cooler ambient air passing over the condenser tubes condensing the vapor into a liquid. This high pressure/temperature liquid then travels through the filter drier onto the expansion valve where a small variable orifice provides a restriction against which compressor pushes.

LOW PRESSURE SIDE: Suction from the compressor pulls the high pressure/temperature liquid R134a through small variable orifice of the TX valve and into the low-pressure side of the A/C system. The R134a is now under low pressure/temperature vapor where heat from the cabin being blown over the evaporator coil surface is absorbed into the colder low pressure refrigerant The R134a is then pulled through the evaporator and into the compressor. The A/C cycle begins again as the R134a vapor is compressed and discharged under pressure.

HEAT TRANSFER: R134a in the LOW-PRESSURE side is COLD and can absorb large quantities of heat from the air moving over the evaporator. R134a in HIGH-PRESSURE side is HOT and the cooler ambient air moving over the condenser can absorb the heat from it.

SYSTEM TYPES

PARALLEL FLOW CONDENSER SYSTEM

The function of a condenser is to act as a heat exchanger and allow heat tot flow from the hot refrigerant to the cooler outside air. R134a entering the condenser will be high pressure, high temperature vapor. As the R134a vapor travels through the tubes of the condenser, heat is given off to the cooler ambient air; the refrigerant vapor condenses and changes to a liquid state. At this point, a large amount of heat is given off by the R134a. The refrigerant will now be a hot, high pressure liquid.

PARALLEL FLOW DESIGN

(recommended for R134a) This design is very similar to a cross flow radiator. Instead of refrigerant traveling through one passage, it can now travel across numerous passages. This will give larger surface area for the cooler ambient air to contact.

R13A - R12 COMPARISON

As R134a operates on higher pressures, less internal flow, restrictive and improved heat rejection condensers are required. Most manufacturers select the parallel flow design for this version. They are approximately 25% more efficient than the serpentine condensers.

CONDENSER ELECTRIC FAN

Most vehicles with air conditioning require an electric fan to assist air flow, either pushing or pulling the air through the condenser, depending on which side of the condenser the fan is placed. The majority of vehicles using R134a require this additional condenser cooling due to the higher operating pressures of R134a. Also, some vehicles have smaller grills or bumper bar openings. This causes poor air flow conditions by the amount of air flow over the condenser.The condenser fan is operator with a/c engaged in various ways:

  •      • medium pressure switch
  •      • indirect connection to the compressor clutch
  •      • via the electronic control module (ECM)
  •      • signal from the a/c switch activation

COMPRESSOR

There are various makes and types of compressors used in automotive air conditioning systems operating on R134a. The internal design could be Piston, Scroll, Wobble plate, Variable stroke or Vane. Regardless, all operate as the pump in the A/C system to keep the R134a and lubricating oil circulating, and to increase the refrigerant pressure and thus temperature.

SANDEN - WOBBLE PLATE

A reciprocating piston, fixed displacement compressor. The pistons are operated by a wobble plate, which moves them backwards and forwards in the cylinders. As the front shaft turns the wobble plate angle changes, causing the pistons to move in and out, pulling refrigerant vapor in through the suction side, compressing it and discharging this high pressure vapor into the condenser.

VARIABLE STROKE - HARRISON V5

The Delphi (Harrison) V5 compressor is a non-cycling variable displacement compressor. The compressor varies displacement to control capacity to meet A/C system demand at all operating conditions. The compressor features a variable angle wobble plate in five (V5) cylinder axial piston design. Displacement is controlled by a bellows actuated control valve located in the rear cylinder head. This control valve senses and responds to the system suction pressure or A/C system demand. Through regulation of compressor crankcase pressure, the wobble plate angle, and therefore compressor displacement is variable. In general, the compressor discharge pressure is much greater than the compressor crankcase. Which is greater than or equal to the compressor suction pressure. At maximum displacement , compressor crankcase pressure is equal to the compressor suction pressure. At reduced or minimum displacement, the compressor crankcase pressure is greater than the suction pressure.

COMPRESSOR MOUNTING

MOUNT & DRIVE

Manufactured of either plate, cast iron, steel or aluminum, this bracket should exhibit excellent noise absorption qualities especially if using a piston type compressor.

COMPRESSOR MOUNT

Consists of a bracket to mount the compressor to the engine, a belt idler pulley, compressor drive belt and possibly and extra drive pulley for the crankshaft.

IDLER PULLEY

A small pulley normally used in conjunction with a belt adjusting mechanism, also used when a belt has a long distance between pulleys to absorb belt vibrations.

DRIVE PULLEY

Some vehicles do not have an extra pulley to accommodate an A/C drive belt, in these cases an extra pulley is bolted onto the existing crankshaft pulley.

COMPRESSOR CLUTCH

The clutch is designed to connect the rotor pulley to the compressor input shaft when the field coil is energized. The clutch is used to transmit the power from the engine crankshaft to the compressor by means of a drive belt. When the clutch is not engaged the compressor shaft does not rotate and refrigerant does not circulate the rotor pulley free wheels. The field coil is actually an electromagnet, once energized it draws the pressure plate towards it, locking the rotor pulley and the pressure plate together causing the compressor internals to turn, creating pressure and circulating refrigerant.

LUBRICATION

R134a is part of the air conditioners lubrication system. NEVER operate an A/C system without refrigerant as there will be no lubrication for the compressor and internal damage will occur. Refrigerant oil is circulated around the A/C system saturated in the refrigerant.

CONDENSERS

The condenser function is to act as a heat exchanger and allow heat to flow from the hot refrigerant to the cooler outside air. R134a entering the condenser will be a high-pressure high temperature vapor. As the R134a vapor travels through the tubes of the condenser heat is given off to the cooler ambient air; the refrigerant vapor condenses and changes to a liquid state. At this point a large amount of heat is given off by the R134a. The refrigerant will now be a hot, high pressure liquid.

SERPENTINE

This type of condenser consists of one long tube which is coiled over and back on itself with cooling fins in between the tubes.

PARALLEL FLOW

This design is very similar to a cross flow radiator. Instead of refrigerant traveling through one passage (like serpentine type), it can now travel across numerous passages. This will give larger surface area for the cooler ambient air to contact.

COMPONENTS

THERMAL EXPANSION VALVES

Refrigerant flow to the evaporator must be controlled to obtain maximum cooling, while ensuring that complete evaporation of the liquid refrigerant takes place. This is accomplished by the thermal expansion valve (TXV).

PRESSURES IN CONTROL

As shown in the illustration, the TXV controls the refrigerant flow by using a system of opposing pressures which will call:

F1: Temperature sensing capillary tube Sealed tube filled with refrigerant. This refrigerant is also filled above the diaphragm (7). The capillary tube sensing bulb (3) is attached to the evaporator outlet tube surface.

F2: Pressure compensation tube This is a hollow tube connected to the evaporator outlet tube and senses the pressure of the R134a refrigerant leaving the evaporator coil. (Other TX valves may not use this tube as pressure is provided internally within the valve).

F3: Pressure spring This spring (6) is located under the ball valve (5)

OPERATION

OPEN: When the evaporator outlet tube temperature increases, the refrigerant (3) in the capillary tube expands , forcing the diaphragm (7) downwards and thus pushing pin (A) also downwards causing the ball valve (5) to move away from the metering orifice (4), allowing more R134a to enter the evaporator inlet side.

CLOSED: As the evaporator outlet tube becomes cooler, the refrigerant in the capillary tube (3) contract s. Forces F2 and F3 cause the diaphragm (7) and pin (A) to move upward allowing the ball valve to move towards the metering orifice (4), restricting the R134a flow. The outlet tube gets warmer and the process starts over.

An expansion valve is utilized in all Classic Auto Air’s evaporator systems. The only function of the valve is to regulate the amount of refrigerant that is released into the evaporator coil. The valve works on the same principal as a needle and seat in a carburetor. The sensing bulb of the capillary tube is attached to the exit (return) side of the coil. Based on the temperature of the return line, the needled will move in and out of the seat. When the sensing bulb reads a high temperature at the return tube, the needle lets the refrigerant flow through the seat unobstructed. As the return tube becomes colder, the needle returns to the seat thereby restricting the flow of refrigerant.

Water, debris and air have a devastating effect on the expansion valve. A plugged condenser, plugged drier or a damaged hose can also be the culprits. Leaving a drier open to the environment could cause the desiccant bag to tear open releasing the contents of the bag into the refrigerant hose. This hose is connected to the expansion valve. If the condenser fittings are left open, small creatures tend to seek out the security of the condenser.

When installing o-rings on the hose connections, be certain the o-ring is properly sealed. Also check for sharp edges on the seat. We recommend a drop or two of mineral oil on the o-ring and the seat. If installed wrong, or the seat has a sharp burr, the o-ring can be cut. A portion of the o-ring can plug the expansion valve.

HIGH PRESSURE SIDE: Low pressure R134a vapor entering the compressor is compressed to become high pressure/temperature R134a vapor. This is then circulated along with lubricant oil to the condenser. As the high pressure/temperature vapor travels through the condenser, heat is released to the cooler ambient air passing over the condenser tubes condensing the vapor into a liquid. This high pressure/temperature liquid then travels through the filter drier onto the expansion valve where a small variable orifice provides a restriction against which compressor pushes.

LOW PRESSURE SIDE: Suction from the compressor pulls the high pressure/temperature liquid R134a through small variable orifice of the TX valve and into the low-pressure side of the A/C system. The R134a is now under low pressure/temperature vapor where heat from the cabin being blown over the evaporator coil surface is absorbed into the colder low pressure refrigerant The R134a is then pulled through the evaporator and into the compressor. The A/C cycle begins again as the R134a vapor is compressed and discharged under pressure.

HEAT TRANSFER: R134a in the LOW-PRESSURE side is COLD and can absorb large quantities of heat from the air moving over the evaporator. R134a in HIGH-PRESSURE side is HOT and the cooler ambient air moving over the condenser can absorb the heat from it.

HOSES

OWING TO THE SMALLER MOLECULAR SIZE AND HIGHER OPERATING PRESSURES OF R134a, the refrigerant hose now incorporates a nylon inner lining. This is to reduce the normal refrigerant leakage that would naturally occur through the porosity of rubber hoses.

Most R134a hoses have a smaller outside diameter and thinner hose walls to improve flexibility and reduce noise levels within the A/C system.

O-RINGS

The “O” ring rubber compound used for R134a A/C system joints, fittings and components is a hydrogenated nitrile butadiene rubber (HNBR) and identified by the color green.

“O” ring lubrication can be carried out using mineral oil. All hoses tubes and components included in an A/C kit are pre-lubricated, as are the “O” rings supplied as a spare part. Other manufacturers could use “O” rings of a different color and size. Ensure that only the approved “O” ring is used for the type of system being serviced or repaired.

FILTER DRIER RECEIVER

The filter drier acts as an article filter, refrigerant storage container and most importantly, moisture absorber.

R134a filter driers DO NOT use sight glass for charging. This is because at approximately 700 C refrigerant temperature the PAG oil will foam, giving a false impression of low gas charge.

Note: Ensure the connection indicated with the word “IN” is connected to the condenser outlet.

CLUTCH DIODE

The clutch coil is an electromagnet with a strong magnetic field when current is applied. This magnet field is constant as long as the clutch is applied. When the power is removed the magnetic field collapses and creates high voltage spikes. These spikes are harmful to the ECM and must be prevented. A diode placed across the clutch coil provides a path to ground. This diode is usually taped inside the clutch coil connector.

REFRIGERANT PRESSURE SWITCHES

HIGH PRESSURE: The power supply is interrupted when the refrigerant pressure is too high or a problem exists in the A/C refrigerant system.

LOW PRESSURE: Used to interrupt the electrical circuit to the compressor clutch. If the refrigerant pressure is too low or a problem exists in the A/C refrigerant system.

BINARY SWITCH: High/Low switch.

TRINARY SWITCH: High/Medium/Low switch.

CONDENSER FAN CONTROL

MEDIUM PRESSURE: Used to engage the condenser fan at a predetermined refrigerant pressure.

EXAMPLE: Condenser fan high speed activation at 1770kPa refrigerant pressure.

These switches can be individual or a combination of the two or even three pressure ranges.