Using the Heat of Flash Steam

The heat absorbed by the water in raising its temperature to the boiling point is sensible heat because it can be sensed or measured by a thermometer. The heat required to convert water at the boiling point to steam at the same temperature is called latent heat. The unit of heat in common use is the Btu (or kilojoule), which is the amount of heat required to raise the temperature on one pound of water 1°F (.55°C), at atmospheric pressure. In SI units, the kilojoule is equal to a kilowatt-second.

		Bar	psig
		1.7
		3.4
		6.8
		13.7

If water is heated under pressure, however, the boiling point is higher than 212°F (100°C), so the sensible heat required is greater. The higher the pressure, the higher the boiling temperature and the higher the heat content. If pressure is reduced, a certain amount of sensible heat is released. This excess heat will be absorbed in the form of latent heat, causing part of the water to "flash" into steam.