Ask the Expert: What is an enthalpy chart and how will it help diagnose an automotive A/C system?

May 28, 2020
An enthalpy chart shows the A/C cycle and at what temperature and pressure the refrigerant changes from a gas to a liquid and back to a gas.

Question: What is an enthalpy chart and how will it help diagnose an automotive A/C system?

Answer: 

Enthalpy is the measure of energy in a thermodynamic system. In short, it shows the A/C cycle and at what temperature and pressure the refrigerant changes from a gas to a liquid and back to a gas. This has been used in the HVAC industry for years and is being taught as a diagnostic tool by CTI, Carquest Training Institute. See the reference chart below for an explanation of what is occurring in an A/C system.

In the chart:

  1. The compressor engages.
  2. The refrigerant gas enters the condenser.
  3. The sub-cooled gas is changing from a vapor to a liquid as it passes through the condenser.
  4. The refrigerant is all liquid.
  5. The liquid refrigerant enters the expansion valve.
  6. The liquid refrigerant enters the evaporator and becomes a mixture of liquid and gas becoming increasingly more gas.
  7. The super-heated refrigerant is all gas as it enters the compressor. 

Why is this important?

Everything to the right of the “shark fin” is a gas. Everything to the left of the “shark fin” is a liquid. 

Everything inside the “shark fin” is a mixture of gas and liquid as it is changing state or reaching saturation.

We need to make sure we have the right side of the trapezoid extending a little past the right side of the “shark fin” to ensure that the compressor is seeing superheated vapor. We also need to make sure the left side of the trapezoid is extended slightly outside the “shark fin” on the left or we don’t have subcooled liquid entering the expansion valve, suggesting an undercharged system. This is a simplification that just touches on the use of enthalpy charts. The chart to the right shows an actual enthalpy chart from the Mantooth app.

Information provided by Ritchie Engineering Co.

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