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AC Pressure Chart of Car


From rolled down windows to complex multi-zone HVAC Systems, the Air Conditioning in automobiles has come a long way. A typical air conditioning system in a modern car consists of several components such as Compressor, Condenser, Evaporator, Refrigerant etc. With such complex system, the maintenance also becomes critical and one important factor to keep track off is the pressure of the refrigerant.

In this article, we will dig a little bit deeper into Car AC Refrigerant, a typical Car AC Pressure Chart for a commonly used refrigerant i.e., the R134a and also some troubleshooting tips with respect to the AC system pressure.

How Does Car AC Work?

Let us first see a very basic overview of how the air conditioning system in your car works. The process of removing heat from an area or location is called as Refrigeration. The unwanted heat is mechanically transferred to an external area where the heat is unobjectionable. Refrigerators and Air Conditioners are the perfect examples of this system.

A special fluid called Refrigerant is used in refrigerators and air conditioners that helps in transfer of heat by continuously cycling between high high-pressure liquid state and a low-pressure gas state. During this process, it absorbs heat and humidity from the car’s cabin and provides with cool and dry air.

All the components i.e., the compressor, condenser, accumulator (or receiver drier) and evaporator work in harmony to give you with cool air.

It is clear that the main component that helps in cooling your cabin is the refrigerant. At a low temperature and pressure, the refrigerant absorbs heat. During this phase, the refrigerant is converted from liquid to gas. To expel all the heat absorbed by the refrigerant, it is condensed from gas to liquid at high temperature and pressure.

It is important for the refrigerant to have certain properties such as:

  • It should not be explosive or flammable
  • It should have a low boiling point
  • It should be non-corrosive to metals and must mix well with oils
  • It shouldn’t be hazardous and must be safe and non-toxic
  • It should have a large latent heat value
  • It should be easy to liquify or vaporize and also moderate its pressure and temperature

There are many other properties that a refrigerant must have (there are some requirements even at the molecular level). Based on these and several other properties, some of the popular and industry approved refrigerants are:

  • R-12
  • R-134a
  • Freon (Freon-11, Freon-12, Freon-13, Freon-21, Freon-22, Freon-113, etc.)
  • Freeze-12
  • GHG-HP
  • Hot Shot
  • Ikon-12
  • R-406A

R134a Refrigerant

Earlier, the R-12 (also sold under the brand name Freon-12) was used as the main refrigerant in automobiles and household refrigeration. But due to its damaging effect on ozone, it has been banned in phases starting from 1994.

Since then, the R134a refrigerant became the industry standard in automotive ACs and other refrigeration and freezing applications. The thermodynamic properties of R134a are very similar to that of R-12 but it has significantly less impact on the depletion of ozone layer.

At atmospheric pressure, the R134a has a boiling point of -26.8oC and at 10psi pressure, its boiling point rises to -10.6oC. If R134a is released into a room at atmospheric pressure and room temperature, it would instantly turn into gas.

The temperature and pressure of the refrigerant have a direct relationship. If the pressure of the refrigerant (R134a) is 35psi or 241kPa, then its temperature will be 4.4oC. So, it is clear that temperature and pressure play an important role in the condensation and expansion of R134a.

NOTE: Since 2012, the usage of R134a as refrigerant is also being phased out and a new refrigerant called HFO-1234yf became the preferred choice in automobiles. But as majority of cars still use R134a as the refrigerant, this article and the AC Pressure Chart in it will focus on R134a only.

Low Side Pressure and High Side Pressure

Before looking at the AC Pressure Chart of R134a Refrigerant, we will try to understand why measuring and monitoring pressure is important in a car’s air conditioning system.

In an automotive AC System, a pressurized refrigerant continuously changes from liquid to gas and gas to liquid in a closed loop. The pressure of the refrigerant directly correlates with the temperature.

Based on the pressure of the refrigerant at any particular point in the loop, the whole system can be divided into High-Pressure Side and Low-Pressure Side.

The compressor receives the low-pressure gas from the evaporator. Compressor compresses this low-pressure refrigerant gas to high pressure gas (the temperature also increases). As this is the starting point of the high-pressure refrigerant gas, the High-Pressure measuring point is located after the compressor.

This high-pressure gas is then transferred to the condenser. The radiator fan near the condenser blows cool air onto the condenser and as a result, the high-pressure gas is converted into high pressure liquid, when it leaves the condenser. Also, the refrigerant releases heat here which it absorbed at the evaporator.

The high-pressure liquid then flows through the receiver / dryer, which filters out the moisture in the refrigerant. Filtered high-pressure liquid then passes through a thermal expansion valve. The high-pressure liquid will expand and convert to a low-pressure liquid at the expansion valve. The low-pressure liquid then flows to the evaporator and the refrigerant starts to boil and converts to low pressure gas.

This low-pressure gas absorbs the heat from the cabin and is then sent to the compressor and the cycle starts again. As this is the exit point of the low-pressure refrigerant gas, the Low-Pressure measuring point is located after the evaporator.

NOTE: Some systems have the low side pressure service point just before the compressor and the high side pressure service point just after the receiver / dryer.

AC Pressure Chart

To check the low side pressure and high side pressure of the refrigerant, a special testing device which contains two pressure gauges (one for low side and one for high side) connected to a manifold assembly. The high side pressure gauge and its connecting pipe are usually colored Red while the low side pressure gauge and its connecting pipe are colored Blue.

The following table shows the AC Pressure Chart for R134a Refrigerant. This AC Pressure Chart contains all the typical high side and low side pressure values of R134a refrigerant at a given temperature.

Temperature Low Side Pressure High Side Pressure
in oF in oC in psi in kPa in psi in kPa
65 18 25 – 35 172 – 241 135 – 155 931 – 1069
70 21 35 – 40 241 – 276 145 – 160 1000 – 1103
75 24 35 – 40 241 – 310 150 – 170 1034 – 1172
80 27 40 – 50 276 – 345 175 – 210 1207 – 1448
85 29 45 – 55 310 – 379 225 – 250 1551 – 1724
90 32 45 – 55 310 – 379 250 – 270 1724 – 1862
95 35 50 – 55 345 – 379 275 – 300 1896 – 2068
100 38 50 – 55 345 – 379 315 – 325 2172 – 2241
105 41 50 – 55 345 – 379 330 – 335 2275 – 2310
110 43 50 – 55 345 – 379 340 – 345 2344 – 2379

How to Use the AC Pressure Chart?

If you feel that your car’s AC is not cooling properly, then it is high time for your take your car to an AC technician. The first thing he / she will recommend is to check for the pressure of the refrigerant and see if the pressure readings of both high side and low side are within the optimum range as per the above-mentioned Car AC Pressure Chart.

Based on the readings from the pressure gauges, you can further diagnose the problem with the help of the following table.

Low Side Pressure High Side Pressure Diagnosis
Within range Within range No problem. AC is working fine.
Low Low Refrigerant is Low. Add some refrigerant.
Low High Possible Blockage in Expansion Valve or Orifice Tube. Service Needed.
High Low Problem with Compressor. Service or Replacement.
High High Refrigerant Overload. Vent extra R134a with proper care.

NOTE: Do not vent R134a into the air. It is illegal in many countries including the U.S.

Conclusion

A simple overview of Car AC System, the R134a Refrigerant and automobile AC Pressure Chart for R134a. With proper care and maintenance of your car’s AC System, you can expect a long working life from it without any major repairs or installations. We advise you to frequently check for the refrigerant levels by testing its high side and low side pressure at a trusted AC technician or workshop.



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