Unit 3 — Refrigeration System Fundamentals & Maintenance
Section 3 — Pressure and Temperature Relationship
3.1 — The Gas Laws
Before a technician can read a gauge set with confidence, they need to understand
why pressure, temperature, and volume are linked. The four classical gas
laws describe exactly that — and each one shows up in real refrigeration work.
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3.1.1 — Absolute Temperature — The Foundation
All three of the pressure–volume–temperature gas laws require temperature
to be expressed on an absolute scale — one where zero truly
means zero molecular motion. Using everyday Fahrenheit or Celsius scales will give
wrong answers because both have negative numbers that would produce impossible
(negative) pressures or volumes.
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Rankine (°R) — Imperial
Used with Fahrenheit-based calculations. °R = °F + 459.67
Absolute zero = 0°R = −459.67°F
Example: 70°F = 529.67°R
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Kelvin (K) — SI
Used with Celsius-based calculations. K = °C + 273.15
Absolute zero = 0 K = −273.15°C
Example: 21°C = 294.15 K
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Always convert before calculating
Plugging °F or °C directly into a gas law formula is one of the most
common errors on certification exams. Convert to °R or K first — every time.
3.1.2 — Boyle’s Law — Pressure & Volume
Robert Boyle (1662) discovered that for a fixed mass of gas held at
constant temperature, pressure and volume move in
opposite directions: squeeze the gas and the pressure rises; allow it to
expand and the pressure falls.
P₁ × V₁ = P₂ × V₂
P = absolute pressure | V = volume | Temperature = constant
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Compression Stroke
The compressor reduces gas volume. By Boyle’s Law, pressure must
rise. The low-pressure suction vapour exits the compressor as high-pressure
discharge gas.
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Expansion Device
The metering device (TXV or capillary tube) suddenly increases the
refrigerant’s available volume. Pressure drops sharply, which
lowers the boiling point and allows evaporation to begin.
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Worked Example — Boyle’s Law
A gas occupies 4 ft³ at 50 psia.
The volume is reduced to 2 ft³ at constant temperature.
What is the new pressure?
P₂ = (P₁ × V₁) ÷ V₂
= (50 × 4) ÷ 2 = 100 psia
Halving the volume doubles the pressure — exactly what a compressor does
on every stroke.
3.1.3 — Charles’ Law — Volume & Temperature
Jacques Charles (1787) showed that for a fixed mass of gas at
constant pressure, volume and absolute temperature are
directly proportional: heat a gas and it expands; cool it and it contracts.
V₁ / T₁ = V₂ / T₂
V = volume | T = absolute temperature (Rankine or Kelvin) | Pressure = constant
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Hot Discharge Gas
Discharge gas leaving the compressor is hot. At constant discharge
pressure, the high temperature means the gas occupies a larger volume —
which is why discharge lines must be adequately sized.
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Suction Line Sizing
Cold suction vapour is more dense (smaller volume at the same pressure).
Suction lines can be smaller in diameter than discharge lines, but velocity
still needs to be maintained to carry oil back to the compressor.
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Worked Example — Charles’ Law
A gas occupies 10 ft³ at 60°F
(519.67°R). It is heated to 120°F (579.67°R)
at constant pressure. What is the new volume?
The 60°F temperature rise causes roughly an 11.5% increase in volume
— a meaningful change in system design.
3.1.4 — Gay-Lussac’s Law — Pressure & Temperature
Joseph Gay-Lussac (1808) found that for a fixed mass of gas in a
rigid container (constant volume), pressure and absolute
temperature are directly proportional. This is also called the
Pressure–Temperature Law — the same principle behind the
P–T chart.
P₁ / T₁ = P₂ / T₂
P = absolute pressure | T = absolute temperature (Rankine or Kelvin) | Volume = constant
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Cylinder Safety
A refrigerant cylinder in direct sunlight can reach temperatures that
raise internal pressure to dangerous levels. Gay-Lussac’s Law explains
exactly why — and why cylinders must never exceed 125°F (52°C).
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Saturation Curve
For a saturated refrigerant at a fixed volume, every increase in
temperature produces a predictable increase in pressure. This is the
basis of the entire P–T chart (covered in lesson 3.2).
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Worked Example — Gay-Lussac’s Law
A sealed refrigerant cylinder contains gas at 100 psia
at 70°F (529.67°R). The cylinder is left in the sun
and warms to 120°F (579.67°R). What is the new pressure?
A 50°F rise increases cylinder pressure by nearly 10 psi — significant
on a cylinder already at working pressure.
3.1.5 — Dalton’s Law — Partial Pressures
John Dalton (1801) established that in a mixture of gases, each gas behaves
independently. The total pressure of the mixture is simply the sum of the
partial pressures — the pressure each gas would exert if it
alone occupied the container.
Pₜₒₜₐₗ = P₁ + P₂ + P₃ + …
Each Pn is the pressure that component gas would exert alone at the same temperature and volume.
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Moist Air (Psychrometrics)
Atmospheric air is a mixture of dry air and water vapour. Total pressure
(14.696 psia at sea level) = partial pressure of dry air + partial pressure
of water vapour. This split drives humidity calculations.
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Non-Condensable Gases
Air or nitrogen left in a refrigeration system adds to the measured
high-side pressure. The P–T chart reading will be lower than
the actual gauge pressure — a tell-tale sign of contamination.
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Leak Checking with Nitrogen
When pressurising a system with nitrogen for a leak check, the nitrogen
adds its own partial pressure to any residual refrigerant. Dalton’s Law
explains why the gauge reads higher than the refrigerant alone would.
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Diagnostic tip — Non-condensables
If the P–T chart says your high-side pressure should correspond to
110°F but you measure 115°F on the liquid line, the system is probably
normal. But if the pressure is higher than the P–T chart predicts for
the measured condensing temperature, suspect air or nitrogen contamination —
Dalton’s Law at work.