3.3 Heat Transfer and Pressure-Temperature Relationships

3.3 Heat Transfer and Pressure-Temperature Relationships

Understanding heat transfer and the relationship between pressure and temperature is fundamental to diagnosing system problems and passing the EPA 608 exam.

Three Methods of Heat Transfer

The Pressure-Temperature Relationship

For any pure substance at its saturation point (where liquid and vapor coexist), there is a direct relationship between pressure and temperature:

• Increase pressure → Temperature increases (boiling point rises)
• Decrease pressure → Temperature decreases (boiling point drops)

This is the entire basis of the refrigeration cycle:

• In the evaporator (low pressure), refrigerant boils at a LOW temperature — cold enough to absorb heat from the conditioned space
• In the condenser (high pressure), refrigerant condenses at a HIGH temperature — warm enough to reject heat to the outdoor air or cooling water

Saturation Temperature

The saturation temperature is the temperature at which a refrigerant will boil (or condense) at a given pressure. At saturation:

• Liquid and vapor coexist
• Temperature remains constant as the substance changes phase
• This is the temperature you read on a pressure-temperature (PT) chart

For example, R-22 at 68.5 psig has a saturation temperature of 40°F. This means at 68.5 psig, R-22 will boil at exactly 40°F.

Temperature-Pressure Chart for Common Refrigerants

Using PT Charts

Every HVAC technician should be proficient with pressure-temperature charts. Here is how they are used in practice:

1. Read the suction pressure on your gauges
2. Look up the saturation temperature for that pressure and refrigerant type
3. Compare with actual temperature measured with a thermometer
4. The difference tells you superheat (at evaporator) or subcooling (at condenser)

Example: You measure 121 psig suction pressure on an R-22 system. The PT chart shows the saturation temperature at 121 psig is approximately 70°F. If the actual suction line temperature is 80°F, the superheat is 80°F - 70°F = 10°F.

Gauge Pressure vs. Absolute Pressure

• Gauge pressure (psig): Reads pressure relative to atmospheric pressure. A reading of 0 psig means atmospheric pressure (14.7 psia). Negative psig readings indicate vacuum.
• Absolute pressure (psia): Includes atmospheric pressure. To convert: psia = psig + 14.7
• Vacuum: Measured in inches of mercury (in. Hg). Full vacuum = 29.92 in. Hg.

For the Exam: Know how to read a PT chart and convert between psig and psia. Understand that low-pressure systems (Type III) operate below atmospheric pressure (in vacuum), so their pressures are measured in inches of mercury vacuum rather than psig.