Pressure And Boiling Point

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The boiling point given in tables for a liquid is always referenced to an atmospheric pressure of 1 bar (14.5 psi). If the pressure acting on a fluid changes, its boiling point also changes.

NOTE:

Pressure is measured in different units: 1 MPa (mega Pascal) corresponds to 10 bar (145.04 psi) positive pressure or 145 psi. 1 bar (14.5 psi) absolute pressure corresponds to 0 bar (0 psi) positive pressure and thus to the ambient pressure (atmospheric pressure).

It is known that, for example, water boils at a lower temperature when the pressure is lower.

The vapor pressure curves for water and refrigerant R134a show that, at constant pressure, reducing the temperature changes vapor to liquid (in the condenser) or that reducing the pressure causes the refrigerant to change from liquid to vapor (inside the evaporator).

Vapor pressure curve of water

Fig 1: Identifying Vapor Pressure Curve Of Water Graph

Courtesy of AUDI

A - Liquid

B - Gaseous

C - Vapor pressure curve of water

1 - Pressure acting on liquid in bar (absolute)

2 - Temperature in °C

Vapor pressure curve of refrigerant R134a

Fig 2: Identifying Vapor Pressure Curve Of Refrigerant R134a Graph

Courtesy of AUDI

A - Liquid

B - Gaseous

D - Vapor pressure curve of refrigerant R134a

1 - Pressure acting on liquid in bar (absolute)

2 - Temperature in °C