Steam Properties and Qualities: A Primer
When the laws and characteristics of steam are respected, fewer operating problems occur and unnecessary costs are minimized.
Qualities of Steam
- Produced from water
- Clean, odorless, and tasteless
- Easily distributed and controlled
- Heat can be used over and over
- High usable heat content
- Gives up its heat at constant temperature
- Well-known characteristics
- Pressure, temperature, volume
Properties of Steam
The physical phenomenon of converting water from a liquid to a vapor (steam) is the same, whether it occurs on a stove with a tea kettle, in a small package boiler, or in a the most sophisticated high-pressure boiler in a power plant. Steam obeys a precise, well defined, documented set of physical laws.
Two atoms of hydrogen and one atom of oxygen make up one molecule of water (H2O). The energy that the molecules of any substance possess is of two forms:
Kinetic energy - energy in motion
Potential energy – energy of position
When energy is added to a substance under conditions which do not change its state, the energy takes the form of motion (kinetic energy); and an increase in motion of molecules results in an increase in temperature. When energy is added to a substance under conditions which do change its state, the energy takes the form of change of position of the molecules (potential energy), and results in a change of state without any change in temperature.
In the case of steam, sensible heat has kinetic energy and latent heat has potential energy.
Types of Steam
Steam is an invisible gas created by adding heat energy to water. It is liquid water changed to its gaseous state.
- Saturated steam - steam in immediate contact with the water from which it is being generated. If the pressure remains constant, any loss of heat or BTUs will result in condensation.
- Superheated steam – If more heat is added to dry saturated steam at a constant pressure, increasing its temperature and specific volume, superheated steam is produced. Heat must be lost, and temperature reduced before condensation occurs.
- Flash steam - when condensate, at saturation temperature and pressure, is discharged into a region of lower pressure, it automatically adjusts to the saturated conditions at the lower pressure. In effect, some of the condensate is “re-evaporated” into steam.
Definition of Steam Terms
Heat – A form of energy
Temperature – The degree of hotness with no implication of amount of heat energy. Expressed in terms of degrees Fahrenheit: 32° is freezing, 212° is the boiling point at atmospheric pressure.
BTU - (British Thermal Unit) The amount of heat energy required to raise the temperature of one pound of water 1°. More correctly, 1/180 of the amount of heat required to raise 1 pound of water from 32° to 212° at atmospheric pressure.
Pressure - The collision of molecules of a gas with the walls of a container, represented in PSI, PSIA, or PSIG.
PSI: Pounds per square inch
PSIA: Pounds per square inch absolute - includes atmospheric pressure (0 PSIG=14.7 PSIA)
PSIG: Pounds per square inch gauge – measures pressure above atmospheric
Sensible Heat – Expressed in BTU/lb. The heat required to bring one lb. of water from the freezing point to boiling point corresponding to any pressure. Higher pressures mean higher boiling points.
Latent Heat of Evaporation – Expressed in BTU/lb. The amount of heat required to convert one pound of water to steam. Latent heat is the potential energy of steam or the useable part of steam. When these BTUs are released or given up, condensation takes place and a pound of condensate results. Note that as pressure increases, the BTU requirement to change one pound of water to steam decreases.
Specific Volume – Expressed in ft3/lb. The space occupied by one pound of steam at a particular pressure. As pressure increases, specific volume decreases.