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CHAPTER 3

AC REFRIGERATION SYSTEMS

To understand the physics of how the temperature and humidity is controlled in the cabin of cars and trucks, it is important to expand on the discussion about heat transfer from chapter 2. The heat transfer process of convection is used to move heat into and out of the cabin depending on how the control head is set. The AC system will be explained in this chapter and the heat system will be covered in more detail in chapter 8.

Heat Transfer

In the AC system, refrigerant is the substance that is used to move the heat out of the cabin. In a similar way, coolant is the substance that is used to move heat away from the engine. A main difference between these two systems is that the refrigerant changes from a liquid to a gas during the process of moving the heat, but the engine coolant maintains its liquid state when the system is operating normally.

To understand how the heat is moved and how these state changes take place, more explanation is needed in the area of sensible heat and latent heat.

Sensible Heat

Sensible heat is the measure of heat that can be felt and measured on a thermometer. When a thermometer gives a reading of 75°F, sensible heat is being measured.

Latent Heat

Latent heat is different from sensible heat because it is not able to be measured on a thermometer. Latent heat is a hidden heat that causes a substance to change states. For example, when water changes from a liquid to a gas, the latent heat of evaporation takes place. When water vapor or steam is cooled down and changes from a gas back to a liquid, the latent heat of condensation takes place.

These two processes are continually happening with the refrigerant in a functioning AC system. The last type of latent heat is when water turns into a solid and this is called the latent heat of fusion. This is not supposed to happen in a normally functioning AC system. However, if the AC system malfunctions, then ice may be seen forming on some of the low side components while in operation.

Heat quantity is a unit of measure for heat and there are two terms that are commonly used to measure this: the British thermal unit (BTU) and the calorie. The BTU is the amount of heat it takes to change 1 pound of water 1 degree Fahrenheit. The calorie is the amount of heat it takes to change 1 gram of water 1 degree Celsius.

As stated above, the refrigerant is continually moving between the liquid and vaporous states when the AC system is functioning. A key point to remember is that large quantities of heat are transferred during the changes of state of the refrigerant. It takes 180 BTU of heat to change 1 pound of water from 32°F to 212°F. It then takes 970 BTU to change the 212°F water into a vapor. On the contrary, 970 BTU of heat must be released from the water vapor to change it back to a liquid. The processes of great heat absorption during evaporation and great heat released during condensation is why the AC system is so efficient at moving heat out of the cabin.

Orifice Tube Systems

One design for automotive AC systems uses an orifice tube as the metering device. This system has been used for a long time by many manufacturers and is still being used on modern cars and trucks. As stated previously, the main purpose of the AC system is to remove heat from the cabin, which leaves cool and dry air that cools the cabin and passengers.

It takes 180 BTU to raise a pound of water from 32 to 212°F, but it takes 970 BTU to change 212°F water into a gas because of the large amount of heat absorbed during the latent heat of evaporation. This concept is why the warm air that is pushed through the evaporator core loses its heat and the result is very cool and dry air that is then directed to the passenger compartment through the mode doors.

Orifice tubes are devices that are used to meter a fixed amount of refrigerant into the evaporator core. They can be located in the liquid line at some point after the condenser and before the evaporator core. These devices are available in several different sizes, which is why they are made to be different colors. The color of the orifice tube assists the individual who is servicing the system in choosing the correct size for replacing the original device. It is not advisable to change the color/size of the original orifice tube when replacing it during service procedures.

Orifice tubes are available in a variety of sizes that are designed by engineers to meter the correct amount of refrigerant into the evaporator core. The different colors represent the different sizes of the internal passage that the refrigerant must pass through during the operation of the AC system. Orifice tubes also have a screen that prevents contaminants from passing into the evaporator core. If the orifice tube screen is covered by a large number of foreign particles, then the system needs to be closely checked to determine where it came from.

The following chart gives the size of the opening for each color of orifice tube.

Orifice tubes can be located at any point between the exit side of the condenser and the entry point of the evaporator. Ease of assembly is the main factor when manufacturers choose the location to mount the orifice tube. Most orifice tubes are in a location where the line can be opened up to allow the orifice tube to be serviced. To locate the orifice tube, look for a spot on the liquid line that is a little larger in diameter than the rest of the line.

Variable orifice valves can be used in place of standard fixed orifice tubes because they are the same size and will fit in the same space. These devices vary the flow rate of refrigerant as the heat load changes in a similar way that a thermal expansion valve operates.

The orifice tube is often located near the entry point of the evaporator. A pair of needle-nose pliers can usually be used to remove the orifice tube from the mounting location. Remember that the orifice tube is made of plastic and can be easily broken if it is stuck in place. A good penetrant or lubricant can be sprayed into the area to free up a stuck orifice tube.

Orifice Tube Sizes
Color	English (inches)	Metric (mm)
Green	0.047	1.19
Brown	0.053	1.35
Oranqe	0.057	1.45
Red	0.062	1.57
Black/Blue	0.067	1.70

While most of the vehicles that use orifice tubes are like the ones described above, there are a couple of variations that need to be discussed. The variable orifice valve and the electronic orifice tube have been used in certain applications. Both of these types allow for some variable operation.

The variable orifice valve can be used in place of a standard orifice tube and is recommended for vehicles that idle for long periods of time, such as police cars and taxis. Electric orifice tubes are used on a few late-model vehicles, and they have the capacity to increase the flow rate under certain conditions when commanded by the vehicle’s electronic control module.

Normal Operating Characteristics

Orifice tube AC systems are very effective designs that have been used for many years. The one drawback of this system design is that the orifice tube does not change the flow rate of refrigerant into the evaporator core, so the compressor needs to be cycled periodically to keep the evaporator from getting too cold and becoming covered with ice. There are two ways to control the output of the compressor: turn it off for short periods of time or change the internal displacement of the compressor.

The first of these strategies is typically called the cycling clutch orifice tube (CCOT) design. CCOT systems use pressure switches or temperature switches to monitor the conditions of the AC system, which then causes the AC compressor to be turned off to prevent evaporator freeze up. The cycle rate of these systems will vary depending on the ambient temperature and the humidity level. It is common to notice the compressor turning on and off during operation. A clicking sound will be heard in the engine area as the compressor is cycled, which is often accompanied by a slight change in engine RPM.

In recent years, vehicles that use orifice tubes have used compressors that are a variable-displacement style. Many variable-displacement compressors use mechanical operation to change the internal capacity of the compressor. This action was accomplished by using a valve that sensed the temperature of the suction side of the compressor and changing the angle of the wobble plate, which caused the internal pistons to change the length of the stroke.

Flow of Refrigerant Through the AC System

It is helpful to understand the refrigerant flow through an operable AC system. The refrigerant is continuously moving and changing pressures and states while the system is running. Here is a step-by-step description of what is happening to the refrigerant as it moves through the components of the AC system.

1. The compressor pulls in low-pressure refrigerant gas from the suction line and then compresses it to raise the pressure and temperature. The refrigerant exits the compressor as a high-pressure gas.

2. The refrigerant is routed to the condenser through the dis charge line. The refrigerant enters the condenser as a heated high-pressure gas. Airflow through the condenser fins causes the refrigerant to cool down, which causes the gas to condense into a liquid. The refrigerant exits the condenser as a hot, high-pressure liquid. The temperature of the liquid is typically 20 to 50 degrees cooler at the exit than it was on entry of the condenser. The temperatures at the condenser inlet will vary greatly because of the wide variance of pressures that can exist at the condenser. On very hot days, the pressures can be as high as 300 psi, which would create inlet temperatures in the 275 to 300°F range. On a 70°F day, the pressure would be approximately 150 to 175 psi, which would create inlet temperatures in the 150 to 175°F range.

It is helpful to understand the flow of refrigerant as it passes through each section of the AC system. The low side of the AC system runs from the exit of the orifice tube up to the entry point of the compressor. The high side of the AC system runs from the exit point of the compressor to the entry point of the orifice tube.

3. The refrigerant is routed to the orifice tube through the liquid line. The orifice tube acts as a restriction device since the passage pipe is much smaller than the liquid line. High-pressure liquid enters the orifice tube and low-pressure atomized liquid exits the orifice tube and then is routed into the evaporator core. Atomized liquid will have small droplets of bubbles due to the pressure drop as it exits the orifice tube.

4. The refrigerant enters the evaporator core as a cool low-pressure atomized liquid. Duct box air is routed through the fins of the evaporator core, which causes the liquid refrigerant to begin to vaporize into a gas. This process of latent heat of evaporation absorbs the heat from the duct box air, which causes the air to be cool and dry as it exits the evaporator fins. The refrigerant exits the evaporator core as a low-pressure gas and is then routed to the accumulator dryer.

5. The refrigerant enters the accumulator dryer as a cold low-pressure gas. The accumulator dryer stores and dries the refrigerant and prevents any liquid refrigerant from being routed to the compressor. The exit point is near the top of the accumulator to prevent any liquid from being sent to the compressor through the suction line.

On late-model vehicles, the mechanical valve that adjusted the output of the compressor has been changed to a solenoid, which works much better. The engine computer monitors the conditions of the pressure and the temperature and adjusts the command to the solenoid, which adjusts the output of the compressor. Both of these variable-style compressors did not need to be cycled on and off to control the temperature in the evaporator core.

Normal Operating Characteristics

The key characteristics to look for under the hood while an orifice tube system is running are: • The suction line will be cold and likely have water droplets on the line. • The accumulator dryer will be cold to the touch and likely have water droplets on the device. • The discharge line, condenser, and liquid line will be hot and should be checked with a laser thermometer/pyrometer. • There will be water draining from the duct box drain tube after the system has run for several minutes.

The compressor is driven by the engine drive belt and is energized when power is sent to a stationary coil that creates a magnetic field to pull the front clutch plate into the drive pulley. This action causes the internal parts of the compressor to begin compressing the refrigerant.

Variable-displacement compressors mechanically adjust their output by monitoring the temperature of the suction gasses that are drawn in from the suction line. When low temperatures are sensed by the internal valve, the angle of the swash plate is changed, which reduces the stroke of the pistons inside the compressor. Many manufacturers used this type of compressor as a method to keep the evaporator temperature from getting too low and causing ice to form on the surface of the evaporator core.

It is common to hear a clicking sound when the compressor cycles on and off during the normal operation of a cycling AC system. The clutch coil creates a magnetic field that pulls the front plate into the rotating pulley. This causes the internal compressor components to begin operating.

The suction line will be cold and have water droplets on the surface when the AC system is operating as designed. This line contains cold refrigerant gas that is being routed into the compressor. The cold surface of this line collects moisture from the surrounding air.

The accumulator dryer will be cold and wet on an AC system that is operating in an efficient manner. One point to keep in mind is that there should not be any ice or frost on any of the lines or components during operation. Visible ice or frost is a sign that the deicing systems are not working correctly.

It is common for the suction line to be cool or cold when the AC system is operating as designed. It is a good practice to check the temperature of the suction line to see if it is cold as a diagnostic step.

The accumulator dryer will be cold and wet when the AC system is working well. The accumulator is located between the exit of the evaporator core and the entry point of the AC compressor in orifice tube systems. If the accumulator is not cold while the system is operating, then the system is not functioning properly.

The high side components of the AC system will be warm or hot when the system is operating correctly. The compressor raises the pressure and temperature of the refrigerant gas, which is then routed to the discharge line that connects to the condenser. The liquid line connects the exit of the condenser to the inlet of the evaporator core. All of the components in the high side of the system will be at a high temperature while the system is operating.

The evaporator drain tube will have water droplets dripping when the AC is used on hot days. The water originates from the hot, humid air that is pushed through the evaporator core. As the heat is absorbed into the boiling refrigerant, the water that was in the hot air sticks to the surface of the evaporator and then runs down the bottom of the duct box where it drains out of the drain tube. Refrigerant boils at very low pressures/temperatures. The refrigerant is turning from a liquid to a gas in the evaporator while taking on the latent heat of evaporation, which is why the duct air turns cool and dry after it passes through the evaporator core.

Operation of Each Major AC Component

The following list explains what each component of the AC system is and what it does.

Compressor: The AC compressor serves as the pumping device that causes the refrigerant to move through the system when the AC is commanded to run. The different compressor types include the piston, scroll, and vane pump styles. The compressor is needed to pull in cold, low-pressure gaseous refrigerant into hot, high-pressure gaseous refrigerant. The temperature of the gaseous refrigerant needs to be increased in order to use the ambient air flowing past the condenser to create a cooling effect that causes the refrigerant to condense into a liquid.

Condenser: The condenser is a large heat exchanger located in front of the radiator. It is used to cool the hot gaseous refrigerant, which causes it to condense back into a liquid. Airflow across the condenser is very important and is accomplished by the ram air effect when the vehicle is traveling at highway speeds and by the fan assembly when the vehicle is traveling slowly or stopped.

It is common to see a puddle of water under the area below the evaporator drain tube when a vehicle is parked after running the AC on a hot and humid day. This water is a sign that the system is working to remove the heat and humidity from the cabin air, which results in cool and dry air being directed into the passenger compartment through the mode doors.

The compressor has a pulley that spins at all times when the engine is operating. The front plate only turns when the AC is commanded to be turned on. Power is sent to the AC clutch coil when the AC compressor needs to run.

The compressor should be carefully tightened with equal torque at all of the mounting bolts to minimize the chance of the body getting in a bind, which could cause a leak. Some compressors use a pigtail connector that connects to the vehicle harness connector.

Many electric and hybrid vehicles use compressors that are driven by a high-voltage battery instead of the engine drive belt. This design increases fuel economy by reducing the need to have the internal combustion engine run. Caution must be followed when performing service and repair on vehicles with electric compressors because these compressors operate on three-phase AC high voltage.

The condenser is the heat exchanger located in front of the engine radiator and allows for generous airflow to pass through the fins. The condenser is connected to the discharge line on the inlet and the liquid line on the exit side.

Orifice Tube: The orifice tube is used as a metering device that causes the pressure to be reduced in the AC system. The refrigerant pressure needs to be lowered in order for it to be able to absorb heat from the duct box air and begin to boil and take on the latent heat of evaporation. Orifice tubes also have a screen that serves as a filter to prevent contaminants from passing into the evaporator core.

Orifice tubes are made in different colors that align with the various sizes, and care should be taken to use the one that is designed for the vehicle being repaired. It is important to remember to use the correct orifice tube during a service procedure.

Evaporator Core: The evaporator core is a heat exchanger located inside the duct box that is used to provide a place for the low-pressure atomized liquid refrigerant to begin to boil. This change of state of the refrigerant causes it to take on the latent heat of evaporation, which absorbs large amounts of heat from the duct box air. The result is that the duct box air is much cooler and drier after moving past the evaporator core. The cool air is distributed to the passenger compartment, and the moisture drops to the bottom of the duct box and drains outside the vehicle.

Accumulator Dryer: The accumulator dryer container is located between the evaporator core and the compressor and is used to store and dry the refrigerant. The dryer contains a desiccant that is used to absorb any moisture that could be present inside the AC system. The location of the accumulator is important on orifice tube systems because it prevents any liquid refrigerant from being supplied to the compressor. The compressor would be damaged if liquid refrigerant was pulled in. The exit point of the accumulator dryer is near the top, which only allows gaseous refrigerant to be sent to the compressor. There typically is a small feed hole at the bottom of the dryer that feeds refrigerant oil to the compressor.

Many condensers used on vehicles with a thermal expansion valve (TXV) have a condenser that combines with the receiver dryer. This design is very effective and it reduces the potential leak points by welding the dryer to the condenser.

The evaporator core is the heat exchanger on the low side of the AC system and is located in the duct box. It is important to transfer the insulation from the old evaporator core to the new unit when replacing it. A block-type TXV would be mounted on the flat surface of this evaporator core.

Evaporator cores often collect dirt and debris in the fins. This problem is usually reduced on vehicles that use a cabin air filter that gets replaced on a regular basis. This evaporator core has one threaded connecting point as well as a quick-connect coupling.

Accumulator dryers can be connected to the lines and hoses with bolted connections that use O-rings to seal the joint from potential leaks. Low side pressure switches are often mounted to the accumulator dryer as well.

Many accumulator dryers are connected to the lines and hoses with quick-connect couplings. The low side service port is often mounted on the accumulator dryer as well as a fitting to mount a low side pressure switch.

Basic Cycle for TXV Systems

A second style of metering device that is used in many AC systems is the thermal expansion valve (TXV). The TXV metering device has been used for many years and is increasing in use in recent years because it varies the flow rate based on operating conditions. There are two styles of TXVs in use: the standard type and the block type. The standard type TXV is almost always located inside the duct box, while the block style is almost always located on the firewall.

As stated, the TXV metering device varies the flow rate through the valve based on operating conditions. This variable operation occurs by the valve sensing the temperature of the exit line of the evaporator and changing the internal opening as the temperature changes. When the sensing element is exposed to warm temperatures, the valve opens, and when the sensing element is exposed to cold temperatures, the valve closes. The continual opening and closing helps control the temperature in the evaporator core, which helps prevent the temperature from reaching freezing levels, which would cause the core to form ice on the surface.

Thermal expansion valves (TXVs) are a popular choice to use as a metering device to feed low-pressure atomized liquid into the evaporator core. The two types of TXVs include the block type (left) and standard type (right). Both styles operate by varying the opening inside the valve to feed refrigerant into the evaporator core.

Block type TXVs are located near the firewall and are visible from under the hood. O-rings and metal gaskets are used to seal the lines to the body of the valve.

Standard-type TXVs are usually located inside the duct box and sealed with O-rings. It is important to make sure that the sensing element is securely attached to the exit line of the evaporator core so that it can accurately sense the temperature of this line. The valve opens up when the sensing element is warm and the valve closes when the sensing element is cold.

Flow of Refrigerant Through the TXV System

The flow of refrigerant through the TXV system is very similar to the flow through the orifice tube system. The refrigerant is the substance that flows through the components, which is used to absorb heat from the passenger compartment and carry it outside the cab and release it at the condenser to the outside air. Here is a step-by-step description of the refrigerant flow through a TXV system:

1. The compressor pulls in low-pressure refrigerant gas from the suction line and compresses it to raise the pressure and temperature. The refrigerant exits the compressor as a high-pressure gas.