While in years past there were some differences, today, most automotive A/C systems contain 5 major components. These are the evaporator, compressor and condenser, and two other items, either a receiver/drier and expansion valve (also called a thermal, or thermostatic expansion valve, or TXV for short), or an accumulator and orifice tube; which two of these four last items depends on the type of A/C system.
Automotive A/C systems also contain “minor” components, such as rubber hoses and metal piping (sometimes referred to as the “lines”), the air routing ductwork and controls, and also electrical devices such as relays, switches, electronic control units, etc. These additional components will vary by system and vehicle manufacturer.
Vehicles equipped with Automatic Temperature Control (ATC) also have a computer that handles the ATC system functions. And speaking of computers, on most vehicles equipped with computerized engine control systems (most since the early ’80s), the engine control computer usually has some authority concerning A/C system operation. There may even be other computers having a role in A/C system operation; it all depends on the vehicle. In any case, even though the engine control computer, or others, may not “directly be” air conditioning system components, they must be taken into consideration during any discussion of A/C system operation, as their presence can have a great affect on A/C system service and diagnostic issues.
The compressor is a pump that moves the refrigerant through the system. The refrigerant is carried by hoses and pipes from one component to another. Compressor designs vary, but they all essentially work the same way.
The compressor is belt-driven by the engine through an electromagnetic clutch (although some hybrid vehicles use an electric motor to operate the compressor). The clutch allows the compressor to disengage when the A/C is switched off, or at times during A/C system operation when compressor function is not called for. The clutch usually receives its electrical signal from a component called a relay, which in turn, receives its activation signal, in most cases, from the fuel injection or engine control computer.
Besides pumping the refrigerant, the compressor has another job; at a certain point in the system, it raises the pressure of the refrigerant from low to high, and as the refrigerant’s pressure goes up, so does its temperature. Raising the refrigerant’s pressure and temperature enables it to release the passenger compartment heat it absorbed while inside the evaporator. The heat release process takes place in the next component to be discussed, the condenser.
The compressor can somewhat be compared to the water pump in an engine cooling system:
- The water pump circulates the engine coolant throughout the system. The coolant absorbs heat from the engine, and the water pump moves it to the radiator where it releases the heat to the atmosphere. It also circulates hot coolant through the heater core to warm the interior of the vehicle.
- The compressor circulates the refrigerant through the system. The refrigerant absorbs the heat inside the vehicle while passing through the evaporator. The refrigerant is then passed on to the condenser, where it gives up the heat to the atmosphere.
The evaporator, also sometimes referred to as the evaporator core, is one of the two (maybe three) heat exchangers in a mobile A/C system. In a typical passenger car or pickup truck, the evaporator is usually located inside the passenger compartment, quite often deeply buried in or under the instrument panel. Some vehicles, usually vans or SUVs, have two evaporators; one under the instrument panel, or elsewhere at the front of the vehicle, and another one located in or toward the rear of the vehicle. The rear evaporator is often located behind a side panel or in the ceiling above the rear passengers.
Evaporators are usually made of aluminum. They look like, and in fact are, similar to radiators, only thicker and smaller in overall size. Like radiators, evaporators consist of a series of internal tubes or “flow paths” with fins attached to them. Air can pass freely through the fins, just like a radiator. But unlike a radiator, where the internal tubes carry moving engine coolant, the passages in the evaporator carry moving refrigerant. When many people talk about refrigerant, they refer to it by its most popular brand name from years back, “Freon,™” or R-12. In the United States, Freon™/R-I2 was the type of refrigerant used in mobile A/C systems until about 1994, but it was replaced with a different refrigerant in all vehicles after the 1995 model year. The new refrigerant is HFC-134a (or R-134a).
But getting back to the term heat exchanger, what does that mean?
In a mobile A/C system, cold, low-pressure liquid refrigerant enters the evaporator. Warm air from the interior of the vehicle passes through the evaporator by action of the blower fan. Since it’s a fact of nature that heat always travels from a warmer area to a cooler area, the cooler refrigerant flowing inside the evaporator’s absorbs heat from the warm air. At the same time, humidity in the air condenses on the cool evaporator’s surface, then eventually drips out of a drain tube to outside the vehicle (think of how moisture forms on a cold bottle of soda pop on a humid day and forms a puddle on your kitchen counter). This is why you see water dripping underneath a car while the air conditioner is on. After the (now slightly warmer) refrigerant has completed its path through the evaporator, it moves on to the compressor.
So, as you can see, air conditioning does not actually cool the interior of the vehicle. What it reallv does is remove heat and humidity from it.
The evaporator can be somewhat compared to a heater core working in reverse:
- Both are located inside the passenger compartment, often in very close proximity, or even inside the same housing under the dashboard.
- The heater core has hot engine coolant flowing through it, bringing heat from the engine into the interior of the vehicle, where it is distributed by the blower fan.
- The evaporator has cool refrigerant flowing through it, which absorbs passenger compartment heat as the blower fan moves the warm air across it.
The condenser is the other heat exchanger in a mobile A/C system. Nowadays, condensers are usually made of aluminum, but in the past, some were made of copper/brass. Condensers look very much like radiators, just a little thinner, and since they also depend on air flowing through them, are usually located in front of the radiator.
Like radiators and evaporators, condensers are also constructed as a series of tubes with fins around them. But unlike an evaporator, whose job is to absorb heat, the condensers job is to release heat. More specifically, to release the heat the refrigerant absorbed while it was flowing through the evaporator, very much the same way the radiator releases the heat from engine coolant that the coolant absorbed while it was flowing through the engine. The refrigerant enters the condenser as a high-pressure vapor, but as it flows through the condenser and cools, it turns back into a cooler high-pressure liquid.
The condenser can be compared to a radiator in an engine cooling system:
- The radiator releases heat from the hot engine coolant passing through it, to the atmosphere.
- The condenser releases heat from the hot A/C system refrigerant passing through it, to the atmosphere.
Receiver/driers (also sometimes called “filter/driers” or “receiver/dehydrators”) look like small metal cans with an inlet and outlet. They are only used in A/C systems that use expansion valves. Receiver/driers are located in the high-pressure section of the sys- tem, usually in the plumbing between the condenser outlet and the expansion valve inlet, although some may be connected directly to the condenser.
Receiver/driers serve three very important functions:
- They act as a temporary storage containers for oil and refrigerant when neither are needed for system operation (such as during periods of low cooling demand). This is the “receiver” function of the receiver/drier.
- Most receiver/driers contain a filter that can trap debris that may be inside the A/C system.
- Receiver/driers contain a material called desiccant. The desiccant is used to absorb moisture (water) that may have gotten inside the A/C system during manufacture, assembly or service. Moisture can get into the A/C components from humidity in the air. This is the “drier” function of the receiver/drier.
Damage can occur if there’s excessive moisture inside an A/C system. It can cause corrosion, as well as possibly degrade the performance of the compressors lubricating oil. The receiver/drier should be replaced any time the system is opened for service, and most compressor warranties require it. The desiccant is only capable of absorbing a certain amount of moisture, and when the inside of the system and/or the receiver/drier are exposed to the atmosphere, the desiccant can become very quickly saturated from humidity in the air. If this occurs, the desiccant is no longer effective, and will not provide future protection. Additionally, the filter inside the receiver/drier could be restricted by debris that may have been inside the system. This could diminish refrigerant and oil flow.
An accumulator is comparable in purpose to a receiver/drier. It serves similar, but slightly different functions. An accumulator is also a metal cylinder, but differs from a receiver/drier in these three ways:
- An accumulator is considerably larger than a receiver/drier, usually around twice the volume.
- The accumulator is connected to the evaporator outlet, in the low-pres- sure section of the system.*
- The accumulator’s primary function is to store liquid refrigerant that is exiting the evaporator, to prevent it from reaching the compressor. If liquid refrigerant were to enter the compressor, it could cause damage, as the compressor is not designed to pump liquid, only vapor.
Accumulators are only used on systems that contain orifice tubes. It is a characteristic of orifice tube systems to have large amounts of liquid refrigerant leaving the evaporator. In other words, unlike in expansion valve systems, where all or most of the refrigerant turned into a vapor while passing through the evaporator, in orifice tube systems, the refrigerant leaves the evaporator still as a liquid. The accumulator is the component in which the refrigerant gets the opportunity to warm up and change from a liquid to a vapor before being drawn back into the compressor Like receiver/driers, accumulators also serve as a temporary storage containers for oil when the oil is not needed by the system. Lastly, accumulators also contain the system desiccant and a small filter, so compared to receiver/driers, the same “rules of replacement” apply.
The expansion valve’s place in the system is at the evaporator inlet. Like any other valve, its job is to control flow; in this case, the amount of refrigerant entering the evaporator. Since system operating conditions vary (sometimes high cooling demand, sometimes low cooling demand) it is necessary to be able to adjust the amount of refrigerant entering the evaporator. For any given operating condition, if we were to allow too much refrigerant to enter the evaporator, it would get too cold, and the moisture collected on it could freeze. This would not allow the hot interior air to pass through its fins, and the refrigerant flowing inside the evaporator would not be able to absorb the heat from the air. This would eventually bring cooling to a halt. If we were to allow too little refrigerant to enter the evaporator, there may not be enough to properly absorb the interior heat, which would also result in inadequate, or no cooling. This process of varying refrigerant flow based on system cooling demand is referred to as “metering” the refrigerant into the evaporator.
So how does the expansion valve know how much refrigerant to meter into the evaporator, and how does it do it? First the “how it does it”, and it’s quite simple. Expansion valves contain a movable rod which travels up and down inside the valve. As the rod moves up and down, it can open and close the passage inside the valve that serves as the flow path for the refrigerant. The valve does not have to be fully opened or fully closed at any given time. Its position can vary, or modulate, between the fully opened and fully closed positions. Because of this, it can very accurately meter the precise amount of refrigerant needed to meet any given cooling demand.
This internal passage inside the TXV is much smaller than that of the refrigerant flow pipe that delivers the refrigerant to it. Because of this, as the refrigerant flows through this passage, its pressure drops, and it becomes the low-pressure liquid we referenced earlier. So as you can see, the expansion valve also serves as a “dividing line” between the high and low pressure sections of the system.
Now the “how does it know how much” part. This is based on the evaporator s outlet temperature. The warmer the evaporator is, the more refrigerant flow needed, and vice-versa. The expansion valve has a temperature sensing device called a sensing bulb. The sensing bulb measures temperature at the evaporator’s outlet and sends a signal
to the movable rod inside the expansion valve. This signal corresponds to the amount of refrigerant needed, the rod moves to the proper position, and the correct amount of refrigerant enters the evaporator.
An expansion valve could somewhat be likened to the thermostat in an engine cooling svstem:
- The thermostat controls the flow of coolant from the engine to the radiator based on cooling system temperature.
- The expansion valve controls the flow of refrigerant entering the evaporator based on evaporator temperature, or A/C system load/cooling demand.