Numerical Example for Latent Heat and Steam Formation

Calculate the amount of heat needed to convert one pound of water at 70 deg F into saturated steam at 212 deg F. The specific heat of water is given as 1 BTU per pound per deg F, and the latent heat of vaporization for water is given as 970 deg F.

This example is same as what we discussed in the last few pages. We are presenting this information to summarize the earlier discussion.

Solution

Heat is added to the water in two phases, first we need to give it sensible heat to raise its temperature to 212 deg F. The second phase begins when the water reaches 212 deg F. In the second stage, the water needs to be provided with the latent heat to allow conversion of water into steam. During the conversion period, the temperature of the steam/water mixture stays at 212 deg F.

Heat Required in Phase 1

Sensible Heat Addition = (Specific Heat) x (Mass) x (Temperature Increase)

In our case we know the following:

Mass = 1 lb

Specific of Water = 1

Temperature Increase = 142

Therefore when we plug these numbers in the formula we get:

Sensible Heat Addition = 1 x 1 x 142 = 142 BTU