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Boyle's Law (Mariotte's Law)

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Boyle's law (Mariotte's Law) states that in a perfect gas where mass and temperature are kept constant - the volume of the gas varies inversely with the absolute pressure.

Boyle's law can be expressed as

PV = constant                                   (1)


P1 V1 = P2V2(1b)


P1 / P2= V2/ V1 (1c)


P = absolute pressure (Pa, psi...)

V = volume (m3, ft3 ...)

Example - Final Pressure when Compressing an Air Volume

A gas with volume 1 m3 and initial pressure 2 MPa is compressed to final volume 0.5 m3 . With constant temperature the required final pressure can be calculated as

P2= P1 V1 / V2

= (2 MPa) (1 m3 ) / (0.5 m3 )

= 4 MPa

Example - Final Volume when Compressed Air is Expanded to Standard Conditions

Air is compressed to 114.7 psia in a 4 inches schedule 40 steel pipe . Inside diameter of the pipe is 4.03 inches and the compressed air volume in one foot of the pipe can be calculated as:

V1 = π (((4.03 in) / (12 in/ft)) / 2)2(1 ft)

= 0.09 ft3

Expansion of the compressed air to a normal pressure of 14.7 psia can be calculated by rearranging Boyle's law:

V2= P1 V1 / P2

= (114.7 psia) (0.09 ft3 ) /  (14.7 psia)

= 0.7 ft3

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Related Topics

Gases and Compressed Air

Properties of air, LNG, LPG and other common gases. Pipeline capacities and sizing of relief valves.

Related Documents

Charles' Law

Volume of an ideal gas vs. temperature.

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