Steam Properties

When heating water and steam at constant pressure, as in a boiler, one of five conditions exist:

1) Water only

2) Water and steam

3) Saturated steam only

4) Superheated steam

5) Supercritical steam

When heat is added to water in a boiler at constant pressure, the temperature of the water increases. The continued addition of heat will cause the temperature to increase until the pressure of the vapor generated is equal to the boiler pressure (the pressure boost of the boiler feed pumps). At this point, the liquid and vapor are said to be saturated.

Boiling point is the temperature at which water begins to boil. It is a function of pressure: as pressure increases the boiling point increases. At atmospheric pressure at sea level, for example, the boiling point is 212°F (100°C). As more heat is added at constant pressure after the boiling point is reached, the saturated liquid starts to vaporize. This vaporization of liquid is known as a phase change from liquid to vapor. The boiling liquid is called a saturated liquid and the vapor that is generated is called a saturated steam. The enthalpy of saturated liquid refers to the sensible heat required to raise the temperature of 1 lbm (or kg) of water from 32°F (0°C) to the saturation temperature, in this case, the boiling temperature. The heat required to change 1 lbm (or kg) of water from liquid to vapor is called the latent heat or heat of vaporization.

Saturation temperature is the temperature at which both the water and steam are in equilibrium for any given pressure. As long as liquid is present, vaporization of a two-phase mixture of liquid and vapor will continue at constant saturation temperature as heat is added. Whereas at atmospheric pressure, the saturation temperature is 212°F (100°C), at 300 psia (20.7 bar), for example, the saturation temperature is about 417°F (214°C).

A mixture of vapor and liquid is at a state that is somewhere between the saturated liquid and the saturated vapor states. This is referred to as the wet region. As long as liquid is present, the mixture is said to be wet. After all of the liquid is vaporized and only steam is present, it is said to be dry saturated steam. The enthalpy of the saturated steam is the enthalpy of the saturated liquid, plus the heat of vaporization. This is the total amount of heat that must be added to convert water initially at 32°F (0°C) to 100% steam. At 250 psia (17.2 bar), for example, the sensible and latent heat content of dry saturated steam are about 376 and 826 Btu/lbm (875 and 1,921 kJ/kg), respectively. The total enthalpy is the sum of these two, which is about 1,202 Btu/lbm (2,796 kJ/kg).

The thermodynamic state of steam properties in this saturated region requires specification of two independent variables, and pressure and temperature are not independent in this region. Typically, one of these (temperature or pressure) along with vapor (steam) quality is used to specify conditions. Steam quality is a measure of the amount of vapor in the two-phase liquid-vapor mixture. It is defined as the ratio of the mass of vapor present to the total mass of the mixture and may be represented as a percent ranging from 0 to 100%:

mass vapor

massliquid + massvapor

The further addition of heat to saturated dry steam at constant pressure causes the temperature of the vapor to increase. This state is called the superheat state and the vapor is said to be superheated. In the superheat region, steam quality remains at 100%.

Superheated steam is thus steam heated to a state where it has a higher enthalpy than is associated with its saturation temperature. Unlike saturated steam, which has only a single associated temperature for any given pressure, superheated steam may exist at any temperature above the saturation temperature. Since pressure and temperature are independent in this superheat region, their specification uniquely defines the thermodynamic state of the vapor. Superheated steam properties must, therefore, be tabulated as a function of both temperature and pressure.

The critical point is the point at which water turns to steam without boiling and their states are indistinguishable. This occurs at a pressure of 3208.2 psia (22.09 MPa or 221.25 bar) and a temperature of 705.5°F (374.2°C or 647.3°K). As the pressure and temperature of water approach the critical point, the value of the heat of vaporization decreases and becomes zero at the critical point.

Figure 6-2 is a temperature-enthalpy diagram for steam at pressures ranging from 0 to 5,500 psia (0 to 382.8 bar) and temperatures ranging from 300 to 1,200°F (422 to 922°K). Curves are provided showing steam by weight (SBW), or quality, in percent. Figures 6-3 (English units) and 6-4 (SI units) show the values of enthalpy and specific volume for steam and water over a wide range of pressure and temperatures.

Temperature Enthalpy Diagram
Fig. 6-2 Temperature-Enthalpy Diagram. Source: Babcock & Wilcox
Fig. 6-3 Pressure-Enthalpy Chart for Steam, English Units. Source: Babcock & Wilcox
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