Your car’s expansion valves and orifice tubes keep your A/C flowing

What Is An Orifice Tube?

The orifice tube is a tube with a given length and cross-section which expands the refrigerant but, unlike the expansion valve, it is unable to regulate flow rate and superheating. The orifice tube is always associated with an accumulator at the evaporator outlet.

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.

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


What exactly is Freon?

Sounds like an automotive A/C history class is in order. Well, welcome, pull up a chair and put on your thinking cap.

Freon, better known as R-12, was the primary chlorofluorocarbon (CFC) used in car A/C systems up until it was banned in the mid 1990s after the EPA discovered that it was depleting our planet’s ozone Layer. Talk about toxic.

The auto industry has since switched to the CFC free R-134a. Since the elimination of CFCs from the auto industry —and all industries for that matter—the ozone layer has regained its structure, and the ozone holes caused by CFCs are expected to fully close by about 2050 according to the EPA.

Did you know? The CFC ban thankfully also marked the end of the CFC heavy hairspray craze that swept the nation in the 70s and 80s. For many people, this change was a welcomed advancement in American hair culture.

Next Step

Schedule Heating AC Inspection

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What is a compressor clutch?

Before the compressor turns on, a special electro-magnetic clutch, conveniently called the “compressor clutch,” is necessary to engage and disengage the compressor cycle. The compressor clutch tells the compressor when to turn on or off so that the Freon (refrigerant) is correctly pressurized for use by the condenser which is then delivered to the evaporator where the chill begins.

Experiencing A/C problems? .

2. Frost on AC evaporator or coming from the vents

Another symptom of a potential problem with the vehicle’s AC expansion valve or orifice tube is frost coming from the vehicle’s vents. If the AC evaporator or orifice tube malfunctions, it can cause refrigerant to flow unmetered through the vehicle’s AC system. This can cause the evaporator to freeze over, or frost to form or come from the vehicle’s AC vents. Either symptom is a sign that the system is getting too much refrigerant, which will actually hinder its efficiency and performance.

Two Types of Refrigerant Systems Used in Car and Truck ACs

The AC systems used in cars and trucks basically have two types of 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, a Thermal Expansion Valve (TXV) system, uses a 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 is a variation of the TXV.

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, by Delco. Instead of a TXV, it has an orifice tube with an opening of fixed size, usually located in the entrance to the evaporator, in about the same location as the TXV, to create the pressure drop. Refrigerant flow control is accomplished 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.

Both systems operate very similarly. In both, either the TXV or the Orifice Tube controls 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 where they are supposed to be (High Side) and COLD where they are supposed to be (Low Side)?

1. The AC loop with a Fixed Orifice Tube.

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

What do I do if my AC isnt blowing cold air?

Try cleaning your air conditioner’s filter and the coils to see if improving the airflow fixes the problem. If there is ice buildup, you’ll need to run the unit with just the fan in order to melt it off. If that doesn’t get the unit blowing cold air again, it could be refrigerant levels are low (see below).


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