A/C System Basic Principles
In order to understand how an A/C system works, we must first understand a few basic principles. The first, and most important principle is that heat always moves toward a state of less heat. This is how an engine’s cooling system works. Heat created by the combustion process in the engine’s cylinders is transferred to the coolant in the water jackets next to the cylinders. The coolant is then pumped to the radiator where the heat is transferred to the cooler air traveling through it. The second principle is that it requires a large amount of heat to change a liquid into a gas. Heat quantity is measured in British Thermal Units (BTUs) and a BTU is the amount of heat that is required to raise the temperature of one pound of water by 1°F. Consider that it takes only 180 BTUs of heat to raise the temperature of one pound of water from 32°F (0°C) to 212°F (100°C),but it requires another 970 BTUs to change that same pound of water into steam at the same temperature. The amount of heat needed for a liquid to change state to a vapor, without the temperature changing, is called the latent heat of evaporation. Conversely, for a vapor to change back to a liquid it must release a large amount of heat. For steam to change back to liquid it must release 970 BTU’s of heat. The amount of heat given off as a vapor changes state from a gas to a liquid, without the temperature changing, is called the latent heat of condensation. The last principle is that when the pressure on a liquid increases, its boiling point also increases. This is why the cooling system on a modern vehicle is pressurized. To remove the heat from today’s powerful, accessory laden engines, the coolant must be able to stay in liquid form and not boil over. Armed with the knowledge of these principles, let’s look at the A/C system. Since hot always moves to cold, to remove heat from inside a vehicle we must put something cold inside the vehicle that can carry the heat away, this is the refrigerant in the evaporator of the A/C system. Refrigerant has a boiling point below 32°F, which enables it to boil and absorb heat at low temperatures. The A/C compressor pumps refrigerant through the system. Refrigerant is metered into the evaporator by a Thermostatic Expansion Valve (TXV) or an orifice tube, depending on the type of system. If the system is equipped with a TXV, liquid refrigerant will pass through a receiver/drier after it leaves the condenser and before it enters the TXV. If the system uses an orifice tube, the refrigerant flows through an accumulator before it enters the compressor. Both the TXV and the orifice tube create a restriction in the system against which the compressor forces the refrigerant. Before the refrigerant passes through the TXV or orifice tube, it is under high pressure. Beyond that point the refrigerant is under low pressure. In fact, it is common to refer to the high side and low side of the A/C system when discussing function and problems. Everything between the compressor and the TXV (or orifice tube) is on the high side; everything from the TXV (or orifice tube) back to the compressor is on the low side. Liquid refrigerant is metered into the evaporator, where warm air blown through the evaporator by the blower motor causes the liquid refrigerant to boil and change into a vapor, absorbing the latent heat of evaporation.The refrigerant leaves the evaporator in the form of a gas and is drawn into the NC compressor. The compressor raises the pressure of the refrigerant gas until its temperature is above ambient. The warmer refrigerant is pumped to the condenser, where relatively cooler ambient air removes the latent heat, changing the state of the refrigerant back to a liquid. The liquid refrigerant then leaves the condenser and returns to the refrigerant metering device, where it completes the refrigerant cycle.The specifications for the high pressures and low pressures vary based on system design, and the type of refrigerant being used. These pressures must be kept in balance in order for the A/C system to function properly. Or, to put it more accurately, we must prevent the evaporator or condenser from becoming too cold or too warm. To insure proper service, always look up the system pressure specifications and testing requirements in the appropriate service information source. The TXV and orifice tube act as system control devices, but these components by themselves do not control the refrigerant flow accurately enough to maintain precise evaporator temperature.
Over the years various methods have been employed to achieve this, but today there are two that are most common:
- Applying or removing electricity to the compressor electromagnetic clutch, having it engage or disengage.
- By changing the actual operating displacement of the compressor.
Clutch Cycling Systems
Cycling the clutch is a simple way to control the A/C system. Cycling clutch control works with both the TXV and orifice tube systems, but because orifice tubes don’t open and close like expansion valves, they will always be found working in conjunction with cycling clutch control. The amount of heat that the evaporator must remove affects the system pressure. Refrigerant pressure (which is directly proportionate to temperature). Often, the pressure cycling switch is mounted on the accumulator. Since system pressure is quite low during cold weather, the pressure cycling switch keeps the compressor from running when it isn’t needed. It also protects the compressor from damage should the refrigerant charge escape, since the compressor is lubricated by oil flowing with refrigerant. A typical TXV system has a bulb and/or capillary tube or some other type of temperature sensing device embedded in the evaporator fins or attached to the evaporator outlet pipe. This device works in conjunction with a switch. When evaporator temperature increases, the switch closes and the compressor clutch engages. When evaporator temperature decreases, the switch opens and the clutch disengages. For both orifice tube and expansion valve systems, the temperatures (and therefore pressures) at which the compressor clutch is cycled vary by system, vehicle and refrigerant.pressure and temperature are directly related (that is, if we increase system pressure we increase the refrigerant’s temperature). So if we control the pressure within the evaporator, we also control its temperature. The compressor cycling rate automatically changes to control the temperature. The clutch is turned off and on by a switch to prevent the evaporator from icing. During operation, the clutch may be cycled several times each minute. As the change in heat load on the evaporator affects system pressures, the compressor cycling rate automatically changes to achieve the desired temperature. Many orifice tube systems use some type of pressure cycling switch. The switch senses the low side pressure. Since low pressure is when the system is cold it keeps the compressor from cycling when it’s not needed. It also keeps the compressor from cycling should the refrigerant escape since the lubricant oil is mixed in with the refrigerant. A typical TXV system has a bulb or capillary tube attached into the evaporator fins or the evaporator outlet pipe. The device works in conjunction with the pressure cycling switch. When the evaporator temperature raises the switch closes and engages the compressor clutch and disengages the clutch when the temperature gets low enough. For both orifice tube and expansion valve systems the temperatures at which the compressor clutch cycles varies by system, vehicle and refrigerant.
Variable Displacement Compressors
A variable displacement compressor is usually an axial compressor, with the pistons arranged around and parallel to the driveshaft. One-way reed valves in the cylinder head control refrigerant flow into and out of each cylinder. Depending on the design, the pistons are driven by a wobble plate or a swash plate. In a wobble plate compressor, the pistons are connected to the plate with short push rods. An angled yoke on the driveshaft causes the plate to wobble when the shaft rotates, driving the pistons back and forth in their bores. In a swash plate compressor, the plate itself rotates with the driveshaft. A bearing in the bottom of each piston clamps around the edge and rides on either face of the swash plate. The plate is set at an angle to the shaft, so as it rotates, the pistons are forced back-and forth in their bores. The angle of the wobble plate or the swash plate determines the length of the piston stroke. In a variable displacement compressor, that angle can be changed, which changes the length of the pistons’ stroke and, therefore, the amount of refrigerant displaced on each stroke. The angle is adjusted using springs and linkage that move lengthwise along the driveshaft, and it’s controlled with refrigerant pressure in the compressor housing. When housing pressure is increased, the pressure exerted on the back side of the pistons keeps them higher in their bores and closer to the cylinder head. This shortens the stroke, reducing displacement. When housing pressure is reduced, a spring pushes the adjusting linkage away from the cylinder head, increasing plate angle and lengthening the piston stroke to increase displacement. Housing pressure is adjusted using a control valve with ports and passages that connect it to the suction (low side) and discharge (high side) chambers of the compressor head. Two different types of control valves are used. The traditional mechanical valve has a precision pressure-sensitive diaphragm that senses low-side pressure. When the temperature inside the vehicle is warm, evaporator temperature increases, which increases low-side pressure. This pushes on the diaphragm, opening a port that vents a little bit of housing pressure to the suction side. Reducing housing pressure increases piston stroke, which increases refrigerant flow volume through the system. As evaporator temperature decreases, so does low-side pressure. The diaphragm rebounds to close the low-side vent port and at the same time open a port that admits high side pressure into the housing. This reduces piston stroke and, therefore, refrigerant flow volume. Most new vehicles use a solenoid valve and temperature and pressure sensors in the refrigerant system instead of a diaphragm valve. This allows a computer to control the valve and adjust compressor displacement to control evaporator temperature, rather than using evaporator temperature to control displacement. Today almost all manufacturers offer electronically controlled variable displacement compressors, and some applications have no clutch, meaning the compressor runs whenever the engine is running. Most vehicles already operate the compressor any time the windshield defogger is turned on, even in winter. The electronic displacement control valve makes it easier to run the compressor continuously because displacement can be reduced closer to zero than with a mechanical valve. Continuous operation keeps seals lubricated, minimizes oil pooling and prevents other kinds of damage that result from long periods of inactivity. An electronically controlled variable displacement compressor puts less load on the engine, improves fuel economy and improves idle quality by eliminating clutch cycling or the idle speed surge that sometimes accompanies it.
Additional A/C System Benefits
In addition to removing heat from the passenger compartment of the vehicle, the A/C system also removes moisture (humidity) from the air inside the vehicle as well as some airborne dust and pollen. Some vehicles are even equipped with dust and pollen filters. The hot air inside the vehicle contains an elevated level of humidity. As it passes over the surface of the cooler evaporator, moisture is removed as it condenses and collects on the evaporator. This moisture collected on the surface of the evaporator then drips out of the evaporator case onto the ground. The airborne dust and pollen become trapped in the water droplets collected on the evaporator, and then drip out onto the ground with the moisture.