Question

Water is forced through a contraction causing low pressure. The water is observed to boil at a pressure -80kPa(gage). If atmosphere pressure is 100kPa, what is the temperature of the water?

Answer 1

Water boils at 100 ⁰C at atmospheric pressure of 100 KPa. But it will boil at lower temperatures if the pressure on it reduced. This happens because the resistance to the water molecules on the water surface trying to go up in the air is reduced. Thus as the pressure is lowered, the temperature at which the water will boil also reduces.

When water is boiling, only one of the properties among pressure and temperature needs to be specified. The other property is automatically fixed. Problems involving boiling of water or condensing of steam are solved with the help of standard steam tables which contain all the experimental data of water at various pressures and temperatures.

The gage pressure is given to be -80 KPa.

Now,

Gauge pressure = Absolute pressure - Atmospheric pressure.

We need absolute pressure for taking reference of steam tables.

From the steam tables for saturated water at absolute pressure of 0.2 Bar,

Temperature can be read as

Thus the temperature of water is 60.09 degrees celcius.