Vapour pressure of water | Water Vapour Pressure temperature chart

Water Vapour Pressure Table at Different temperatures

Vapour pressure definition: It is the pressure exerted by the saturated vapour in contact with the surface of the liquid at that temperature. Vapour pressure is also called the vapour tension

Vapour pressure of water Concept:

All liquids have a tendency to evaporate when exposed to atmosphere. The rate at which this evaporation to atmospheric. The rate at which this evaporation occurs depends on the molecular energy of the liquid.

Consider a liquid contained in a sealed breaker and assume a constant temperature is maintained. In Fig – 1 some of the liquid molecules have sufficient energy to break away from the liquid and enter the air in the vapour form. As some increased container temperature the air  will contain vapour molecules to the extent that some of them will be forced to re-enter the liquid by partial air pressure as shown in Fig – 2.

Finally in Fig- 3 an equilibrium condition will exists, when the rate at which molecules are leaving the liquid will be equal to the molecules re-entering the liquid. Under such conditions the air above the liquid is saturated with liquid molecules and the pressure on the liquid surface is called its vapour pressure at prevailing temperature.

Water vapour pressure at 100 ⁰C is 1 atm or 1 kg/cm2 abs. at sea level.

Vapour Pressure of water versus temperature Table

Vapour Pressure table for water from 1 ^oC to 100 ^oC

Vapour Pressure table for water from 102 ^oC to 370 ^oC

Vapour pressure of liquids at different temperatures for pump NPSH calculation

The term vapour pressure is frequently used in connection with calculation of  Net Positive Suction Head available (NPSHa) in pumps.

In NPSH available calculation vapour pressure head of the liquid is very essential. It is calculated by the following formula.

Example – 1 :   Calculate vapour pressure head of the gasohol at 50 ⁰C and its specific gravity 0.8.

According to above water vapour pressure vs temperature table

Water vapour pressure at 50 ⁰C = 0.12335 bar = 1.2335 mwc (mwc –  metres of water column)

Now according to above formula

Vapour tension / vapour pressure of the liquid = 1.2335 / 0.8 = 1.54 metres

Example – 2 : Calculate vapour pressure head of the sulphited sugar syrup at 65 ⁰C and its specific gravity 1.25

According to above water vapour pressure vs temperature table

Vapour pressure of water at 65 ⁰C =0.2501 bar = 2.501 metres.

Now according to above formula

Vapour pressure head of the sulphited syrup = 2.501 / 1.25 =  2.0 metres.

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