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DIY Auto Service: Air Conditioning (AC) System Operation with TXV or Orifice Tube

Updated on March 21, 2016

TXV and H-Block AC System Diagram

The AC system loop with an H-Block or TXV. Notice the Receiver Drier on the Liquid Line.
The AC system loop with an H-Block or TXV. Notice the Receiver Drier on the Liquid Line.

AC System Operation

The Basics of Air Conditioning

The AC system works by removing heat from air circulated in the passenger compartment of the vehicle and circulating this heat to the front of the vehicle to then release it to the atmosphere. The air coming out the vents will generally be about 20°F cooler than the air entering the AC system. The other facet of air conditioning is the dehumidifying or drying of the air. Drier air feels cooler. Each person has a different comfort temperature level.

BTU’s

Heat is measured in BTU’s or British Thermal Units. One BTU is approximately the heat given off by one wooden match. Technically a BTU is the amount of heat energy it takes to raise one pound of water (about 1 pint) 1°F at sea level. Cooling would be removing this amount of heat.

There are three types of heat transfer:

  1. Conduction is heat that travels thru a conductor like; copper, aluminum and other metals. In the AC system, heat travels thru the aluminum AC Evaporator to be absorbed by the refrigerant. The heat from the hot coolant travels thru the aluminum or copper in the radiator to the atmosphere. The heat travels thru the engine block to reach the coolant. All three of these examples the heat transmitted the energy thru a metal. The fins on the radiator add more metal surface area to make it more efficient.

  2. Convection is the movement of heat from one place to another. A convection oven has a fan that moves the air around the oven for more even baking temperatures. In our homes, we heat or cool the air at the furnace and circulate the air around the house. The engine cooling system takes the heat from the block and circulates it to the radiator.

  3. Radiation is the release of heat to the atmosphere. The radiator is named for giving the heat from the engine to the air or radiating the heat to the air. The heater core under the dash uses the same principle it uses the heat from the engine and gives it off under the dash in the heater box.

Three Part Series

This series is divided into three parts. The AC system operation, AC System Service and the Diagnosis of a malfunctioning system. To be able to service and repair an AC system, read through all three Hubs.

DIY Auto Service: Air Conditioning (AC) System Operation with TXV or Orifice Tube

DIY Auto Service; AC System Service and Component Replacement

DIY Auto Service; AC System Diagnosis by Symptom

Refrigerants

Two types of refrigerant have been used in cars and trucks; R-12 Freon and R-134a. R-12 was the subject of the Montreal Protocol held in 1987. The US and 22 other countries agreed to limit the production of Ozone depleting refrigerants. The Clean Air Act of 1990, agreed to cease production of R-12 refrigerant at the end of 1995. New vehicles would be required to use another refrigerant, R134a. New vehicles 1994 and newer came with R-134a refrigerant.

The differences in the refrigerants are:

  • R-12 is dichlorodifluoromethane which is a CFC or chlorofluorocarbon and attacks the ozone layer. The ozone layer limits harmful radiation or ultra-violet rays from reaching the earth. It was found that CFC refrigerants were a major cause of this damage. R-12 was a major refrigerant identified in this cause. Most R-12 refrigerant was lost during service to AC systems. The name Freon is a DuPont trademark name. The 30lb container for R-12 is White in color.
  • R-134a is tetrafluorothane which is a HFC or hydrofluorocarbon. HFC refrigerants are much less harmful to the ozone layer, but still has some affect. R-134a is also a smaller molecule than R-12. New barrier hoses had to be fitted to these vehicles with R134a. Most manufacturers started installing barrier hoses in the 1980’s, since they knew R-12 was being replaced. The 30lb container for R-134a is Light Blue in color.


Service Tip: R134a is the only OEM recommended refrigerant retrofit for R-12 systems. The OEM’s also 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 oil carried by the refrigerant may be the only oil the compressor has to lubricate with. 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 the oil specified is what is being used. Viscosity is the thickness of the oil. Use the viscosity oil specified as well. (SP-20 vs SP-45) PAG oil can also absorb moisture, so keep it in tightly sealed metal containers.

  • Ester Oil is oil that some manufacturers recommend when doing a retrofit of changing an 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 out of the retrofitted system and then add a full amount of PAG oil. Reduce the amount of refrigerant about 15% when recharging the system as the extra oil will take up space.

R134a Refrigerant and Refrigerant Oil

Since 1994 R134a Refrigerant has been used in mobile (cars trucks and equipment) AC Systems with PAG oil being the typical oil.
Since 1994 R134a Refrigerant has been used in mobile (cars trucks and equipment) AC Systems with PAG oil being the typical oil.

The AC Refrigerant Cycle

The AC systems used in cars and trucks basically have two types of systems.

Thermal Expansion Valve (TXV) system

The first system is called a Thermal Expansion Valve (TXV) system. The TXV is a variable control valve located at the entrance of the Evaporator. This system also has a Receiver Drier on the liquid line. An H-Block is a variation of the TXV.

Fixed Orifice Tube

The second system is called a Fixed Orifice Tube. The difference in this system is the Orifice Tube usually located in the entrance to the evaporator, in about the same location as the TXV, this is a fixed orifice and not variable. Refrigerant flow control is accomplished by cycling the compressor on and off or with a variable output compressor. This system used to be called CCOT or Compressor Cycling Orifice Tube by Delco. There is an accumulator in the line from the evaporator to the compressor.

Both systems operate very similar. The main differences are the TXV and the Orifice Tube which control the flow of refrigerant into the evaporator. Both systems are 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 (High Side) where they are supposed to be and COLD (Low Side) where they are supposed to be?

AC Compressor

Angled Swashplate AC Compressor.
Angled Swashplate AC Compressor.

Refrigeration System with an Expansion Valve

The AC system forms a loop that starts at the compressor. The compressor is the pump in the system.

With the system running 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 250PSI. This pressure can vary depending on the ambient temperature. The refrigerant leaves the compressor as a gas thru the discharge port. Then 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 thru it and giving off heat to condense (gas to liquid) the refrigerant into a liquid. Removing latent heat is used to change 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. This may be temperature or electrically controlled.

  3. The Liquid Line connects the Condenser to the TXV at the Evaporator. The outlet of the condenser should have solid 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. The receiver drier may have a Sight Glass (window) to observe the refrigerant flow. Most do not.

  4. The Thermal Expansion Valve controls the flow of the refrigerant into the Evaporator. At the TXV 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 TXV.

  5. The Evaporator is also set up like a small radiator with the refrigerant circulating thru 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. This is 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. This is usually a larger size line. The suction line should feel COLD. This restarts the loop at the compressor.

Thermo Expansion Valve (TXV) H-Block and Orifice Tubes

The Thermo Expansion Valve (TXV) and the H-Block are both variable refrigerant control valves to 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 to maintain the Evaporator temperature. The Fixed Orifice Tube comes in a variety of orifice sizes and uses a cycling or variable Compressor.

AC System with a Fixed Orifice Tube

The AC loop with a Fixed Orifice Tube. Note the Accumulator on the Low Side of the system.
The AC loop with a Fixed Orifice Tube. Note the Accumulator on the Low Side of the system.

Refrigeration System with a Fixed Orifice Tube

The AC system forms a loop that starts at the compressor. The compressor is the pump in the system. With the system running 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 250PSI. This pressure can vary depending on the ambient temperature. The refrigerant leaves the compressor as a gas thru the discharge port. Then 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 thru it and giving off heat to condense (gas to liquid) the refrigerant into a liquid. Removing latent heat is used to change 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. This 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 solid 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. 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. The compressor is either cycled on and off or a variable output compressor is used. This temperature change should be dramatic. If not there could be a problem with the Orifice Tube.

  5. The Evaporator is also set up like a small radiator with the refrigerant circulating thru 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 thru the evaporator. The accumulator should also feel COLD.

  6. The Suction Line connects the outlet of the accumulator and to the inlet of the compressor. This is usually a larger size line. The suction line should feel COLD. This restarts the loop at the compressor.

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      alpesh jagani 5 months ago

      Hello...sir.,

      Thanx for share this excellent knowledge...

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      Vasco 10 days ago

      PERFECT !!!

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