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How Your Truck’s Air Conditioner (AC) Works

The Science Behind Staying Cool

Alex Steele
Nov 2, 2015
In principle, air conditioners and refrigeration units don't produce cold air. These systems transfer heat from one place to another and therefore reduce temperature in a confined area (passenger compartment, inside the refrigerator, your home, etc.).
The concept is believed to go as far back as ancient Egypt with the use of wet reeds hung in a window. As a breeze flowed through the reeds, the moisture would evaporate. The evaporation process removed heat and cooled the incoming air.
This same theory of evaporation to reduce temperature is used in modern automotive A/C systems, only with the use of a highly volatile liquid (refrigerant) sealed inside a network of components.
The volatility or low boiling point of a liquid refrigerant is essential to an A/C system's functionality in the real world. Unlike water, which boils or vaporizes at 212 degrees, the current most-used automotive refrigerant, R134a (tetrafluoroethane), becomes a vapor at 15.1 degrees below zero. Now, keep in mind that another relative aspect of volatility is that a liquid's boiling point increases with higher pressure applied.
The refrigeration cycle begins at the A/C compressor, a pump taking in low-pressure vaporized refrigerant. The compressor expels the vapor under high pressure to the condenser. The condenser, appropriately named, is where the refrigerant condenses back to a liquid form. Remember, higher pressure increases the temperature evaporation occurs, hence the point of condensation. The reverse of evaporation absorbing heat, condensation disburses that same heat. In this case, heat travels out of the refrigerant, through the condenser, and into the atmosphere.
An A/C condenser resembles, and is located in front of, the engine's radiator. Just like a radiator, it consists of multiple rows of passages and metal fins, which supply a wide area of air contact. The heat transfer is finalized with sufficient airflow through the vehicle's grill, aided by mechanical and/or electric motor-driven cooling fans.
Exiting the condenser and have disbursed the heat, we now have a liquid form of refrigerant remaining under high pressure. Next step in the cycle is the expansion valve, as refrigerant crosses the firewall and enters the passenger compartment. The expansion valve is a variable (or sometimes fixed) orifice, which is significantly smaller than the lines and hoses the refrigerant travels through. This reduction in size reduces the pressure of the liquid refrigerant passing through. This reduction in pressure lowers the boiling point and the refrigerant is forced to evaporate.
The evaporation process takes place in the next component in line, the "evaporator," which is located within the passenger compartment. Just like airflow through the condenser disbursed heat into the atmosphere from the condensing high-pressure refrigerant, the vehicle's blower motor and fan routes air through the evaporator. Here the vaporization of low-pressure refrigerant absorbs the heat from the passenger compartment. The evaporator is designed very similar to the condenser and the engine's radiator, allowing a wide area of air contact for heat absorption.
Photo 2/2   |   Air Conditioning Components
The full cycle ends up where we started. The low-pressure refrigerant vapor carrying the heat from the passenger compartment travels back to the intake side of the A/C compressor.
Additional components, depending on the type of A/C system, are referred to as the receiver-drier and accumulator. A system using a thermal expansion valve, which varies the size of the orifice with system temperature, requires a receiver-drier. It's located between the condenser and expansion valve. Its purpose is to store liquid refrigerant so there's an adequate amount at the expansion valve under all conditions. It also contains a filter and desiccant to remove unwanted moisture and debris.
A system using a fixed orifice tube includes an accumulator, which similar to the receiver-drier but is located between the evaporator and compressor on the low-pressure side of the system. It prevents any liquid refrigerant from reaching and damaging the compressor, while also filtering moisture and particles.
We keep hearing about the high and low pressures within the system. That's where the terms high-side and low-side came about. The high-side of the system includes the output side of the compressor, the condenser, hoses, and lines up to the expansion valve. The low-side begins after the pressure drop at the expansion valve and includes the evaporator and lines/hoses, and leads up to the intake side of the compressor. Pressures vary depending on temperature and types of systems, but ballpark numbers while the system is running are about 20 psi low-side and 150 psi high-side. With the system off, both sides will equalize to around 80 psi.
If you ever noticed the two refrigerant service fittings under the hood, one is in a low-side line and the other is in a high-side line. These fittings are used for recovery, evacuation, and recharging of the system's refrigerant, but they're also critical in diagnosing system problems.
The electrical engagement part of each A/C system is important in both maintaining performance and protection from damage. It's all based on the engagement and disengagement of the AC compressor clutch. The clutch consists of a steel pulley mounted to the compressor housing and a matching disk, which turns the compressor, input shaft. When 12 V is applied to the electromagnetic coil behind the pulley, the magnetic force locks the disk to the pulley and the compressor is engaged (rotating with the engine by means of a drive-belt).
On most current systems, the powertrain control module (PCM) controls compressor engagement in conjunction with cooling fan operation. However, there are several important data inputs that must be confirmed before this ever happens. It all takes place through communication between the heating, ventilation, and air conditioning (HVAC) control module and the PCM.
Needed Inputs
  • AC switch "ON"
  • System pressure within high and low parameters (above 40psi, below 300psi)
  • Blower motor "ON"
  • Evaporator temperature above freezing
  • AC compressor temperature below a dangerous level (on some systems)
System pressure sensors are sometimes used as a failsafe. If static pressure is too low due to a loss of refrigerant, the compressor clutch will not engage to prevent compressor damage. Traveling along with the system's refrigerant is refrigerant oil, which lubricates the compressor. With no refrigerant, there's no oil flow and the compressor takes a beating.
Excessively high system pressure can be caused by a lack of airflow through the condenser (increasing temperature and pressure), possibly due to a bad cooling fan or blockage of airflow. This too will cause the system to disengage the compressor as a safety measure.
Normal cycling of the A/C compressor is controlled by two basic data inputs. Cycling is the compressor clutch engaging and disengaging in a uniform manner. Some systems use a low-pressure cycling switch that results in disengagement when low-side pressure decreases below a certain point. Others will see temperature at the evaporator approaching freezing (evaporator thermal sensor) and shut down the system to prevent freeze-up. When air passes though the cool evaporator, moisture from the air condenses and drips downward. The water collects in the HVAC case, eventually draining out on the street. If the evaporator temperature reaches 32°, this condensation will freeze, and the ice buildup on the evaporator will prevent cooling and block airflow.
A/C systems are pretty much routine maintenance-free, aside from replacement of an interior cabin filter or the drive belt. But problems do happen.
Common A/C Failures
At the top of the list are refrigerant leaks. Symptoms will range from a lack of performance due to a low refrigerant charge to the system not working at all. Refrigerant can leak from hoses, O-rings, the compressor, evaporator, and so on. The condenser being punctured by a rock coming through the grill happens fairly often.
Electric cooling fan failures will often be noticed as a lack of A/C performance only at idle because wind through the grill at higher speeds is often sufficient airflow through the condenser. Fans not working can also result in an engine overheating.
There are many additional electronic components and wiring which can hinder or disable A/C when a failure occurs. A bad compressor relay, electro-magnetic coil for the compressor clutch, a blower motor, pressure switch, A/C controller, even a rat chewing through related wiring.
Improper servicing of a system can cause problems. Correct and well-maintained recovery equipment is a no-brainer. Cross-contamination of refrigerants may occur, commonly R12 with R134a. When recovering refrigerant from a system, a thorough evacuation must be performed. Evacuation applies near-vacuum to the system to remove all air and moisture. Moisture within the system can freeze-up, causing performance symptoms and damage. Recharging of the refrigerant must be done with the specified amount, along with refrigerant oil to replenish what was lost during recovery and parts replacement.
A/C compressor failure is often the result of low refrigerant charge preventing adequate flow of refrigerant oil. The system pressure may not be low enough for the pressure sensor to indicate a problem and force a system shut down. That's why it's important to have any lack-of-performance symptoms addressed as soon as possible. Damaged compressors may not produce adequate pressures, can produce abnormal noise, or can even seize.
Blockage of refrigerant flow through the system does happen and is the result of system contamination, a stuck closed expansion valve, or a damaged/kinked line. This is where analyzing abnormal high- and low-side pressures with the proper equipment helps locate the problem.
And then there is the Black Death! This is the A/C failure of all failures, where a damaged compressor distributes debris throughout the entire system. The contamination often includes a black film everywhere. In this case, the entire system must be replaced: compressor, evaporator, condenser, hoses, expansion valve—everything.
There are a few variances in modern automotive A/C systems. Hybrid compressors are powered by both the engine's drive belt and an internal high-voltage motor. Plus, some hybrid and EV compressors powered by electric motors alone.
A quick run-through of the air-conditioning cycle
1. High-pressure liquid refrigerant passes through the expansion valve, which provides a sudden drop in pressure.
2. The pressure drop lowers the boiling point of the refrigerant, causing it to evaporate.
3. The vaporization within the evaporator absorbs heat from the warm air passing through it, courtesy of the blower motor/fan.
4. The low-pressure refrigerant vapor travels through lines/hoses back to the A/C compressor.
5. The compressor compresses. Forcing refrigerant to the condenser under high pressure.
6. The refrigerant returns to a liquid form inside the condenser. Disbursing the heat from the interior into the outside air traveling through the grill.
7. High-pressure liquid travels back to the expansion valve and the cycle repeats.
- OF



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