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TXV vs. Orifice-Tube Car AC Systems: Operation and Diagnostics

Michael Thomas, automotive and diesel technician, is the author of the McGraw-Hill textbook "Truck and Trailer Systems."

Angled swashplate AC compressor

Angled swashplate AC compressor

How the Two Types of Car AC Systems Work

This article first describes how AC systems remove heat from a car's interior and then describes the two common types of car AC systems and the refrigerants they use.

How an AC System Works

An AC system works by removing heat from air circulated in the passenger compartment of the vehicle and transferring this heat to the front of the vehicle to release it to the atmosphere. The air coming out of the vents will generally be about 20°F cooler than the air entering the AC system. The other function of air conditioning, besides cooling the air, is dehumidifying or drying the air. Drier air feels cooler.

Three Ways Heat Can Be Transferred in an AC System

It's the function of an AC system to move heat: to move it from the place you want to cool out into the atmosphere. Here are three ways heat can move from place to place.

  1. Conduction is the movement of heat through a conductor. Copper, aluminum, and other metals make great conductors. In an AC system, heat travels through the aluminum AC evaporator to be absorbed by the refrigerant. The heat from the hot coolant travels through the aluminum or copper in the radiator to the atmosphere. The heat travels through the engine block to reach the coolant. In all three of these examples, the energy is transmitted through a metal. The fins on the radiator add more metal surface area to make it more efficient.
  2. Convection is the movement of hot air or other fluid from one place to another. A convection oven has a fan that moves the air around the oven for more even baking temperatures. Convection also happens in our homes when we circulate the hot or cool air from the furnace around the house through ducts. The engine cooling system takes the heat from the block and circulates it to the radiator.
  3. Radiation is the release of heat waves to the atmosphere or to a cooler environment. A radiator has that name because it "radiates" the heat from the engine to the air. The heater core under the dash uses the same principle; it transmits the heat from the engine to the inside of the car.

Heat is Measured in BTUs

One BTU or British Thermal Unit is approximately the heat given off by one wooden match. Technically, a BTU is the amount of heat energy it takes to raise the temperature of one pound of water (about 1 pint) by 1°F at sea level, or to cool a pound of water 1°F.

Two Types of Refrigerant Systems Used in Car and Truck ACs

Cars and trucks basically use two types of AC systems. Both systems work by using a pressurized refrigerant, which boils after a pressure drop, absorbing heat from the vehicle's cabin.

1. Thermal Expansion Valve (TXV) system

The first type of system, the Thermal Expansion Valve (TXV) system, uses a thermal expansion valve (TXV) to provide the pressure drop in the refrigerant. The TXV is a variable control valve located at the entrance of the evaporator. This type of system also has a receiver-drier on the liquid line. An H-Block system is a variation of the TXV system.

1. TXV and H-Block AC system diagram

1. TXV and H-Block AC system diagram

Notice the receiver-drier on the liquid line on the TXV system above.

2. Fixed-Orifice Tube System

The second system is called a Fixed-Orifice Tube system (this system used to be called CCOT, or Compressor Cycling Orifice Tube). Instead of a TXV, it has an orifice tube with an opening of fixed size, usually located like the TXV in the entrance to the evaporator, to create the pressure drop. The flow of refrigerant flow is controlled by cycling the compressor on and off, or with a variable-output compressor. There is an accumulator in the line from the evaporator to the compressor.

2. System diagram of an AC loop with a fixed-orifice tube

2. System diagram of an AC loop with a fixed-orifice tube

Note the accumulator on the low side of the fixed-orifice system above.

The Thermo Expansion Valve (TXV) and the H-Block are both variable refrigerant control valves that maintain the evaporator temperature. The fixed-orifice tube comes in a variety of orifice sizes and uses a cycling or variable compressor.

The Thermo Expansion Valve (TXV) and the H-Block are both variable refrigerant control valves that maintain the evaporator temperature. The fixed-orifice tube comes in a variety of orifice sizes and uses a cycling or variable compressor.

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Check HOT and COLD Lines in Both Systems

Both types of AC systems operate similarly. In both, the AC system forms a loop that starts at the compressor, which is the pump in the system. In both, either the TXV or the orifice tube controls the flow of refrigerant into the evaporator. Both systems should be cold and hot in the same places.

Using your hands to feel the lines can be a quick way to see if there is a problem with the AC: are the lines HOT where they are supposed to be (on the high side) and COLD where they are supposed to be (on the low side)?

1. How a Refrigeration System With a Thermal Expansion Valve (TXV) Should Work

With the system running correctly, the following conditions should be found.

  1. The compressor compresses the refrigerant in a gas state (liquids can’t be compressed.) The compressor also increases the pressure and temperature of the refrigerant. The pressure is typically about 225 to 250 PSI, but it can vary depending on the ambient temperature. The refrigerant leaves the compressor as a gas through the discharge port. Then it is carried by the discharge line which is typically the smaller line coming from the compressor and should be HOT to the touch.
  2. The condenser is connected to the compressor by the discharge or high-pressure-side line. The discharge line should be HOT to the touch. The condenser is like a radiator with the refrigerant flowing through it and giving off heat to condense the refrigerant from a gas to a liquid. Removing latent heat changes the state of the refrigerant from a gas to a liquid while still maintaining high temperature. An engine-driven or electric fan is used to keep air flowing across the condenser. This fan may be temperature-controlled or electrically controlled.
  3. The liquid line connects the condenser to the TXV at the evaporator. The outlet of the condenser should have liquid coming out. The small liquid line should be HOT to the touch. With the TXV system, a receiver-drier is installed in the liquid line. Sometimes (not usually) the receiver-drier has a sight glass (a window) to allow observation of the refrigerant flow.
  4. The thermal expansion valve controls the flow of the refrigerant into the evaporator. At the TXV there is both a temperature and pressure drop. The liquid line side should be HOT and the evaporator side should be COLD. This temperature change should be dramatic. If not, there could be a problem with the TXV.
  5. The evaporator is set up like a small radiator with the refrigerant circulating through it. The blower pushes air across the evaporator to remove heat from the air. The cold surface of the evaporator also condenses humidity from the air. An evaporator housing drain allows the water to drain away, under the vehicle. The evaporator inlet and outlet should both feel COLD. At the outlet of the evaporator, the TXV has a temperature-sensing bulb clamped to the line, usually under insulating tape. Some models have the TXV inside the evaporator housing.
  6. The suction line connects the evaporator outlet to the inlet of the compressor, to restart the loop at the compressor. The suction line is usually a larger-size line. The suction line should feel COLD.

2. How a Refrigeration System With a Fixed Orifice Tube Should Work

With the system running correctly, the following conditions should be found.

  1. The compressor compresses the refrigerant in a gas state (liquids can’t be compressed). The compressor also increases the pressure and temperature of the refrigerant. The pressure is typically about 200 to 250 PSI. This pressure can vary depending on the ambient temperature. The refrigerant leaves the compressor as a gas through the discharge port, and is then carried by the discharge line which is typically the smaller line coming from the compressor. The discharge line should be HOT to the touch.
  2. The condenser is connected to the compressor by the discharge or high-pressure-side (or "high-side") line. The discharge line should be HOT to the touch. The condenser is like a radiator, with the refrigerant flowing through it and giving off heat so that it condenses from a gas to a liquid. Removing the latent heat changes the state of the refrigerant from a gas to a liquid while still maintaining high temperature. An engine-driven or electric fan is used to keep airflow across the condenser. The fan may be temperature or electrically controlled.
  3. The liquid line connects the condenser to the orifice tube at the evaporator. The outlet of the condenser should have liquid coming out. The small liquid line should be HOT to the touch.
  4. The orifice tube controls the flow of the refrigerant into the evaporator. At the orifice tube there is a temperature and pressure drop. The liquid line side should be HOT and the evaporator side should be COLD. This temperature change should be dramatic; if not, there could be a problem with the orifice tube. The flow of refrigerant is controlled by the size of the orifice in the orifice tube. A pressure switch or sensor is used to determine the pressure in the evaporator.
  5. The evaporator is also set up like a small radiator with the refrigerant circulating through it. The blower pushes air across the evaporator to remove heat from the air. The cold surface of the evaporator also condenses humidity from the air. An evaporator housing drain allows the water to drain away, under the vehicle. The evaporator inlet and outlet should both feel COLD. At the outlet of the evaporator, an accumulator is used to store any liquid refrigerant that may have made it through the evaporator. The accumulator should also feel COLD.
  6. The suction line connects the outlet of the accumulator to the inlet of the compressor, to complete the loop that starts at the compressor. The suction line is usually a larger size line and should feel COLD.
Since 1994, R-134a has been the typical refrigerant used in cars, trucks, and mobile equipment, with with PAG oil being the typical refrigerant oil.

Since 1994, R-134a has been the typical refrigerant used in cars, trucks, and mobile equipment, with with PAG oil being the typical refrigerant oil.

Refrigerants: R-12 and R-134a

Two types of refrigerant have been used in cars and trucks; R-12 (Freon) and R-134a.

  • R-12 is dichlorodifluoromethane, a chlorofluorocarbon or CFC. Freon is a DuPont trademark name for R-12. Before 2020, the 30-lb container for R-12 used to be white in color; now it is gray.
  • R-134a is tetrafluoroethane, a hydrofluorocarbon or HFC. Before 2020, the 30-lb container for R-134a was light blue in color; now it is gray.

R-12 was the subject of the Montreal Protocol treaty of 1987, in which the US and 22 other countries agreed to limit the production of ozone-depleting refrigerants. It was found that CFC refrigerants including R-12 were a major cause of damage to the ozone layer, a part of the atmosphere that limits the amount of harmful radiation or ultraviolet rays that reaches the earth. When AC systems were serviced, most of the R-12 refrigerant was lost and could go up into the ozone layer. HFC refrigerants are much less harmful to the ozone layer than CFCs like R-12, though they still have some effect.

In the Clean Air Act of 1990, the US agreed to stop production of R-12 refrigerant at the end of 1995. New vehicles (1994 and newer) came with R-134a refrigerant instead. Because R-134a is a smaller molecule than R-12, new barrier hoses had to be fitted to vehicles so they could use R-134a. Most manufacturers started installing barrier hoses in the 1980s, since they knew R-12 was going to be replaced.

Service Tip: R-134a is the only refrigerant retrofit for R-12 systems that is recommended by OEMs (Original Equipment Manufacturers). OEMs do not recommend refrigerants like Freeze 12, or "Stop Leak" additives.

Refrigerant Oils

Each refrigerant has compatible refrigerant oils that should be used. Using the wrong refrigerant oil could cause serious damage to the compressor. The compressor depends for its lubrication on the oil carried by the refrigerant.

The following are the common refrigerant oils and their uses:

  • Mineral oil is used with R-12 refrigerant only. Do not use mineral oil in an R-134a system as it will not mix with the refrigerant and will not be circulated. The compressor will run dry and seize up.
  • PAG or synthetic Polyalkyline Glycol is used with R-134a refrigerant systems. There are a variety of PAG oils. Make sure to use the oil that is specified, in the viscosity (thickness) that is specified (SP-20 vs SP-45). PAG oil can absorb moisture, so keep it in tightly sealed metal containers.
  • Ester oil is the oil some manufacturers recommend when doing a retrofit changing an old R-12 system over to R-134a. It is said that this oil will mix with the mineral oil and be carried by the R-134a refrigerant.

Service Tip: Most manufacturers recommend removing as much mineral oil from the retrofitted system as possible and then adding a full amount of PAG oil. Reduce the amount of refrigerant by about 15% when recharging the system, as the extra oil will take up space.

To know all you need to know to service and repair an AC system, read these other two articles as well.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: I have an 03 Avalanche the AC works at an idle but stops once you accelerate. My low-pressure line reads within the specs on the gauge as long as the compressor is running. When the compressor kicks off the pressure on the low side jumps up to the red about 95lbs. I know the gap in the clutch may need to be adjusted but, is the raise in low side pressure normal?

Answer: Without knowing what the high side pressure is at, I would guess that either the system is overcharged or the cooling fan hub may be bad. The compressor should cycle back on when the pressure reaches about 45 PSI and off at about 24PSI.

© 2014 Mike Thomas

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