Gauge Pressure and Latent Heat
 
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Observe the relationship between gauge pressure and latent heat.As steam pressure increases, the latent heat of the steam decreases. The amount of decrease diminishes as the pressure rises. Since it is the latent heat available for heat transfer that provides steam's greatest utility for non-temperature dependent processes, it should be apparent that lower pressure systems are most suitable for those applications. The decrease in latent heat occurs because there is a physical limit to the amount of heat that water can absorb in any state. As the amount of heat that is used to raise the water temperature before vaporization can take place increases, the amount of latent heat used to accomplish that vaporization must decrease. At approximately 3206 psia the amount of latent heat in the steam drops to zero Btu/lb. 3206 psia is the critical pressure for water. At that pressure it absorbs the maximum amount of heat while in the water state, and requires no more heat to turn it into a gas. At this pressure water can exist as either a liquid or a gas at the same temperature.  |
