This article is part of a 6-part series on automotive air conditioning and heating theory. Part 2 explains how pressure controls the boiling point of a refrigerant and describes the chemical properties of R-12 and R-134a. Knowing why a refrigerant boils at different temperatures under different pressures helps you understand how the system moves heat without any moving parts in the evaporator or condenser.
Pressure Controls the Boiling Point of a Refrigerant
The boiling point of a liquid is directly proportional to the atmospheric or internal pressure exerted upon it. Increasing pressure elevates the boiling point. In a refrigeration system, raising pressure allows a vapor to condense back into a liquid at higher ambient temperatures. Decreasing pressure lowers the boiling point. At higher altitudes, for example at 20,000 feet, the reduced atmospheric pressure causes water to boil at 160 degrees F (71 degrees C) compared to 212 degrees F (100 degrees C) at sea level. The system operates by manipulating the pressure of a refrigerant to ensure its boiling point is always significantly lower than the air temperature at the evaporator, and its condensing point is significantly higher than the ambient air at the condenser. Low pressure allows the refrigerant to boil at temperatures well below the passenger compartment air, facilitating rapid heat absorption. High pressure elevates the refrigerant temperature up to 400 psi or 2400 kPa above the outside air temperature, facilitating heat rejection.
Refrigerant Chemistry and System Compatibility
A refrigerant is a compound of chlorine, hydrogen, fluorine, and carbon that facilitates the cooling effect through rapid evaporation and condensation. Legacy refrigerants, known as R-12 or CFC-12, are dichlorodifluoromethane (CCl2F2). They are known to contribute to ozone layer depletion when released into the atmosphere, allowing increased ultraviolet radiation. Modern refrigerants, known as R-134a or HFC-134a, are tetrafluoroethane (CF3CH2F). They were developed as a non-ozone-depleting alternative. R-12 and R-134a systems are not compatible.
Local Shop Note:
This reminds me of something I heard from a tech up on GUY Park Ave in Amsterdam, N.Y. We were at an ASA A/C seminar, and he was telling me about a European sedan that came in with a complaint that the AC would blow cold for about five minutes, then gradually warm up to ambient temperature. He’d checked the system with his manifold gauges — low side looked normal, but the high side was pegged at 450 psi, way above the spec for that ambient temperature.
He recovered the refrigerant and noticed the sight glass on his recovery machine looked milky. That was the first red flag. He pulled a vacuum on the system, but it wouldn’t hold. After nitrogen testing, he found the compressor shaft seal was weeping. But that didn’t explain the sky-high pressures.
He cut open the accumulator and found a brown, sludgy residue. Turns out the previous shop had topped off a leaky R-12 system with R-134a without evacuating or flushing — just mixed them. The two refrigerant chemistries don’t play nice. The incompatible mineral oil from the R-12 system and the PAG oil from the R-134a reacted, creating a thick sludge that clogged the expansion valve and restricted flow. That restriction caused the compressor to dead-head against a blockage, driving head pressure through the roof.
He replaced the compressor, flushed the entire system with a dedicated flush solvent, installed a new expansion valve, accumulator, and condenser, and recharged it with the correct refrigerant and oil by weight. That system blew ice-cold and stayed cold.
Here’s what I took from that: the pressure-temperature chart is your roadmap, but it only works if the refrigerant is pure. Contamination changes everything. You cannot mix refrigerants, you cannot mix oils, and you cannot just top off a system without knowing its history. If the pressures don’t match the P-T chart for the measured ambient temperature, stop and find out why. Otherwise, you’re just throwing parts at a chemistry problem.
Each refrigerant type requires specific pressure ranges to achieve the necessary phase changes; components and lubricants must be matched to the specific chemical properties of the refrigerant used.
Pressure-Temperature Data for R-12
The internal pressure of R-12 is strictly dictated by its temperature when in a saturated state.
Temperature (F) Pressure (PSIG) Pressure (kPa)
-21.7 0 (Atmospheric) 0
0 9.2 63.4
32 30.1 207.5
70 70.1 482.7
80 84.1 579.9
100 116.9 806.0
125 167.5 1154.9
Pressure-Temperature Data for R-134a
R-134a exhibits a distinct P-T curve compared to R-12, requiring specific calibration for pressure-cycling switches and expansion valves.
Temperature (F) Pressure (PSIG) Temperature (F) Pressure (PSIG)
-14.7 0 70 71.1
0 6.5 80 86.7
20 18.4 100 124.2
32 27.8 150 260.8
50 45.5 200 400.0
System Logic and Contamination Effects
The pressure within a refrigerant container or static system will always correspond to the ambient temperature of the liquid. For instance, an R-12 drum at 80 degrees F (26.6 degrees C) will consistently register 84.1 PSIG. The system relies on the temperature differential created by pressure changes. If pressure is not increased sufficiently by the compressor, the refrigerant cannot reach a temperature higher than the ambient air, preventing heat rejection at the condenser. The presence of air or moisture disrupts the P-T relationship, leading to higher-than-normal head pressures and reduced cooling capacity due to non-condensable gases. Refrigerant is designed to be used repeatedly in a closed-loop system. It remains chemically stable and efficient unless contaminated by external elements such as dirt, moisture, water, or air. In its gaseous state, refrigerant is heavier than air.
The key takeaway from Part 2 is that the pressure-temperature relationship is the foundation of the entire refrigeration cycle and that different refrigerants have different pressure charts and cannot be mixed. Proceed to Part 3.