Temperature and Heat - More Information

When water is heated, the molecular structure reacts in two ways. The velocity of the molecules increases, raising the temperature, and the distance between the water molecules expands. This promotes evaporation, which is the escaping of vapor from the surface of the water into the atmosphere, as indicated by the cloud or mist that forms above an open pan of water as it is heated on a stove.

Most liquids can react to only so much heat in this manner (called the heat of the saturated liquid). For water, the amount is about 180 Btu per pound (418.8 kJ/kg) at sea level, atmospheric pressure. Heat added above that amount is absorbed by the liquid. The speed of the molecules and the distance between the molecules of the liquid are constrained by the ambient pressure in which it exists.

When enough additional heat (called latent heat) has been absorbed by the water so that the pressure forcing the molecules apart (vapor pressure) exceeds the ambient pressure, the water changes state. This is characterized by vaporization within the body of water as opposed to evaporation, which happens only at the surface. For water at atmospheric pressure, the heat of vaporization (latent heat that must be added above the saturation temperature of the water) is about 970 Btu per pound (2256.2 kJ/kg).

The vapor that is formed when water boils is called steam. Steam is the pure gaseous state of water. The most telling dissimilarity between a liquid and a gas is usually the difference in density or distance between the molecules. At atmospheric pressure, the molecules of steam are slightly less than 1600 times farther apart than the molecules of water. It is important to realize that the density varies with pressure. As the steam is pressurized, the specific volume, like the latent heat, decreases.

This combination of facts leads engineers to design distribution systems with higher pressures so that smaller piping can be used (the steam takes up less space). When the steam arrives at the heat exchanger, the pressure is usually let down, or reduced, providing more latent heat for work.

Condensation is the opposite of boiling. It is the conversion of matter from the gaseous state to a liquid. In order for the steam to condense, heat must be drawn away from it. At sea level atmospheric pressure, once the 970 Btu per pound (2256.2 kJ/kg) heat of vaporization has been removed from the steam, it will condense to water at 212°F/100°C (with 180.07 Btu of heat/418.8 kJ/kg).