4.3 Heat Transfer and Pressure-Temperature Relationships

Section 3.3 ties together the physics behind every gauge reading you take. Understanding heat transfer, the pressure-temperature relationship, and superheat/subcooling lets you diagnose a system and answer the calculation-style questions that appear on the EPA 608 exam — especially in the Type II and Type III sections.

Three Methods of Heat Transfer

Convection is the workhorse of the cycle — air and refrigerant are both fluids in motion carrying heat.

Sensible Heat vs. Latent Heat

This distinction underpins everything that follows.

• Sensible heat changes the temperature of a substance with no change of state — you can sense it on a thermometer. Heating 60°F vapor to 70°F vapor is sensible heat. Superheat and subcooling are both sensible heat.
• Latent heat changes the state of a substance — boiling or condensing — at constant temperature. While a refrigerant boils in the evaporator, it absorbs huge amounts of latent heat without its temperature rising. This is why phase change carries so much heat, and why the evaporator and condenser are so effective.

The Pressure-Temperature Relationship

For any pure refrigerant at saturation (where liquid and vapor coexist), pressure and temperature move together — a direct relationship:

• Raise the pressure → the boiling/condensing temperature rises.
• Lower the pressure → the boiling/condensing temperature falls.

This is the whole engine of refrigeration:

• In the evaporator (low pressure), refrigerant boils at a low temperature — cold enough to pull heat out of the conditioned space.
• In the condenser (high pressure), refrigerant condenses at a high temperature — hot enough to dump heat to the outdoor air or cooling water.

The metering device and compressor set these pressures, so the technician effectively dials the boiling and condensing temperatures by controlling pressure.

Saturation Temperature and the P-T Chart

The saturation temperature is the temperature at which a refrigerant boils (or condenses) at a given pressure. At saturation, liquid and vapor coexist and the temperature stays constant during the phase change. A pressure-temperature (P-T) chart lists the saturation temperature for each pressure and refrigerant. For example, R-22 at 68.5 psig has a saturation temperature of 40°F — at that pressure, R-22 boils at exactly 40°F.

Notice how R-410A always shows a much higher pressure than R-22 at the same temperature — that is why R-410A is a high-pressure refrigerant and uses heavier gauges and recovery equipment.

Superheat: What It Indicates and How to Measure It

Superheat is the number of degrees a vapor has been heated above its saturation temperature at the same pressure. It is sensible heat added after the last drop of liquid has boiled.

• Where measured: at the evaporator outlet (TXV bulb location) for evaporator superheat, or at the compressor suction line for total superheat.
• What it indicates: that all the liquid has boiled, so only vapor reaches the compressor. Adequate superheat protects against liquid slugging. Too much superheat can mean an undercharge or starved evaporator; too little can mean an overcharge or flooding.
• How to measure: (1) read the suction pressure on your gauge; (2) look up the saturation temperature on the P-T chart; (3) measure the actual suction-line temperature; (4) superheat = actual temperature − saturation temperature.

Subcooling: What It Indicates and How to Measure It

Subcooling is the number of degrees a liquid has been cooled below its saturation temperature at the same (high-side) pressure. It is sensible heat removed after the last vapor has condensed.

• Where measured: at the condenser outlet / liquid line.
• What it indicates: that a solid column of liquid (not vapor) is reaching the metering device, which improves efficiency. Low subcooling can mean an undercharge; high subcooling can mean an overcharge or restriction.
• How to measure: (1) read the discharge (high-side) pressure; (2) find the saturation temperature on the P-T chart; (3) measure the actual liquid-line temperature; (4) subcooling = saturation temperature − actual temperature.

Worked Example: On an R-22 system you read 121 psig on the suction gauge. From the P-T chart, 121 psig corresponds to a saturation temperature of about 70°F. You measure the actual suction-line temperature at 80°F. Superheat = 80°F − 70°F = 10°F — a healthy value confirming only vapor returns to the compressor. Now check the high side: discharge pressure reads 278 psig, whose saturation temperature is roughly 125°F, and the liquid line measures 115°F, so subcooling = 125°F − 115°F = 10°F, confirming solid liquid feeds the metering device.

Gauge Pressure vs. Absolute Pressure and Vacuum

• Gauge pressure (psig): measured relative to atmosphere; 0 psig = atmospheric. Negative readings indicate vacuum.
• Absolute pressure (psia): includes atmosphere. psia = psig + 14.7.
• Vacuum: measured in inches of mercury (in. Hg); full vacuum = 29.92 in. Hg.

Low-pressure (Type III) systems operate below atmospheric pressure — in a vacuum — so their pressures are read in inches of mercury rather than psig. This is why Type III recovery is measured to an evacuation level in inches of mercury and why air can leak into these systems.

For the Exam: Superheat = actual temp − saturation temp (evaporator/suction). Subcooling = saturation temp − actual temp (condenser/liquid line). Both are sensible heat. Higher pressure = higher boiling point. psia = psig + 14.7. Low-pressure Type III systems run in a vacuum measured in in. Hg.