Apply the combined gas law to find how pressure, volume or temperature changes when a fixed amount of gas is compressed, heated or cooled.
Boyle’s lawCharles’s lawAbsolute valuesReceivers and cylinders
°C
°C
P29.0916 bar(a)
P28.0783 bar(g) at sea level
How this result was calculated
P2 = P1 V1 T2 ÷ (T1 V2) = 9.0916 bar(a)
Advertisement
Combined Gas Law
P1V1 ÷ T1 = P2V2 ÷ T2
Pressures must be absolute and temperatures in kelvin. The law assumes ideal gas behaviour and a fixed mass of gas.
Worked example
A closed 1 m³ receiver at 8 bar(a) and 20 °C heated to 60 °C rises to 8 × 333.15 ÷ 293.15 = 9.09 bar(a).
Key insight: a sealed, liquid full line behaves very differently: liquids are almost incompressible, so thermal expansion can raise pressure by several bar per °C. Fit thermal relief valves on blocked in liquid lines.
Advertisement
Frequently Asked Questions
Why must I use absolute pressure?
Gas laws describe the physical pressure from zero. Gauge pressure is relative to atmosphere and gives wrong answers.
What is Boyle’s law?
At constant temperature, pressure times volume is constant: halving the volume doubles the absolute pressure.
How much free air does a receiver hold?
Use Find V2 with P2 = atmospheric pressure: the result is the volume the stored air would occupy at atmospheric pressure.
You May Also Like
Learn more on the blog
Pressure transmitter installation tips
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.