Lesson HVAC fundamentals · Gases and energy
Gas laws
Gases in HVAC/R follow predictable rules when you use absolute pressure and temperature. Boyle: volume and pressure move opposite (constant T). Charles: volume and temperature move together (constant P). Dalton: total pressure of a gas mixture = sum of partial pressures. Compressors and ducts rely on this physics daily.
Before any formula: absolute scales only
Gas-law math fails if you mix gauge pressure or Fahrenheit without converting to absolute:
- Pressure: use psia, not psig
- Temperature: use Rankine (°R = °F + 460) or Kelvin for SI work
Absolute zero ≈ −460°F → 0°R. Room 75°F = 535°R.
If answers look “almost right but wrong,” check absolute conversion first.
Boyle’s law (Robert Boyle, 1600s)
At constant temperature, volume and absolute pressure vary inversely.
Squeeze a gas into smaller volume → pressure rises. Expand volume → pressure falls.
Formula form:
P1 × V1 = P2 × V2 (T constant, use psia)
Example: 40 psia, 30 in³ → what volume at 50 psia?
V2 = (40 × 30) / 50 = 24 in³
Field tie-in: refrigerant gas in a compressor cylinder during compression—volume down, pressure up (temperature also changes in real compressors, but Boyle is the first half of the story).
Charles’s law (Jacques Charles, 1800s)
At constant pressure, volume and absolute temperature vary directly.
Heat a gas → it expands if pressure stays fixed. Cool it → it contracts.
Formula form:
V1 / T1 = V2 / T2 (P constant, absolute T)
Also: at constant volume, pressure and absolute temperature vary directly.
Example: 2000 ft³ air at 75°F (535°R) heated to 130°F (590°R) in a furnace at constant pressure:
V2 = V1 × (T2/T1) = 2000 × (590/535) ≈ 2206 ft³
That is why heated air needs more duct volume—or higher velocity—and why cfm changes with temperature.
Combined gas law (preview)
Real processes often change both P and T. A combined form merges Boyle and Charles. Compressor discharge is neither constant T nor constant P—but understanding the two laws separately explains gauge behavior and line sizing intuition.
Dalton’s law (John Dalton)
In a mixture of gases, total pressure = sum of each gas’s partial pressure.
Each gas acts as if it occupied the whole volume alone.
Examples:
- Atmosphere: nitrogen, oxygen, water vapor, CO₂ each contribute partial pressure; total = barometric pressure
- Combustion / flue gas: analyzing CO or oxygen requires partial-pressure thinking
- Moist air psychrometrics (later courses): water vapor partial pressure drives condensation
Field habit: when you add nitrogen for pressure test, or when air enters a system, you are changing mixture behavior—not just “one gas.”
Practical HVAC/R applications
| Situation | Law idea |
|---|---|
| Compressor cylinder | Volume ↓ → pressure ↑ (Boyle); temperature rises (Charles at constant V) |
| Duct air heated | Volume ↑ at constant P (Charles) |
| Evaporator air cooled | Volume ↓; same mass flow ideas need density correction |
| Leak test with nitrogen | Dalton—know what pressure you added vs refrigerant remnant |
| Altitude | Lower atmospheric partial pressure changes air density |
Why refrigerant cylinders change pressure in the truck
A sealed cylinder of mixed liquid and vapor sits at equilibrium: temperature and saturation pressure match ambient.
Warm the truck → pressure rises. Cool the walk-in → pressure falls. That is not a mystery leak—it is Charles + saturation (detailed in lessons 08 and 10).
Work habits for calculations
- Convert psig → psia
- Convert °F → °R
- Write given values with units
- Solve; sanity-check direction (did pressure go up when volume went down?)
Field case
Situation. Duct design table assumes 400 cfm at 70°F. Installer measures at register on a hot attic run—air at 110°F, lower density. Customer complains of weak cooling upstairs.
How to think with this lesson.
- Same mass flow matters for capacity; volume flow (cfm) changes with temperature at constant pressure (Charles).
- Hotter, expanded air → fewer lb/min for the same cfm → less capacity delivered.
- Fix path: reduce heat gain on duct, improve insulation, or adjust airflow accounting for density.
Learning takeaway: cfm without temperature is an incomplete story.
In the field
Symptom
Math answers wrong by ~15 psi or ~460°; “impossible” pressure/temperature predictions
Where to look
Calculator setup; gauge type; which T/P chart
Likely causes
- psig used as psia
- °F used instead of °R
- wrong law for process (constant V vs constant P)
What to measure
- psig then convert
- dry-bulb for air density estimates
- document assumptions
What not to do
- Plug gauge numbers into Boyle without +14.7
Checklist
- I state Boyle’s law (inverse P and V at constant T)
- I state Charles’s law (direct V and T at constant P)
- I state Dalton’s law (total P = sum of partials)
- I convert psig → psia and °F → °R before gas-law math
- I connect heated air to increased volume at constant pressure