Steam is a colorless expansive and invisible gas resulting from the vaporization of water. The white could associated with steam is a fog of minute liquid particles formed by condensation. That is to say, finely divided condensation. This white cloud is caused by the exposure of the steam to a temperature lower than that corresponding to its pressure.

If the inside of a steam heating main were visible, it would be filled part way with a white cloud and in traversing the main the little particles combine, forming drops of condensation too heavy to remain in suspension and accordingly drop to the bottom of the main and drain off as condensation. This condensation flows into a drop leg of the system and finally back into the boiler, together with additional condensation draining from the radiators.

Although the word "steam" should only be applied to the saturated gas, the five following classes of steam are recognized:
1. Saturated steam
2. Dry steam
3. Wet steam
4. Superheated steam
5. Highly superheated or gaseous steam

It should be pointed out that neither saturated steam nor superheated steam can be seen by the naked eye.

Saturated steam may be defined as steam of a temperature due to its pressure. Steam containing intermingled moisture, mist, or spray, is referred to as wet steam. Dry steam, on the other hand, is steam containing no moisture. It may be either saturated or superheated. Finally, superheated steam is steam having a temperature higher than that corresponding to its pressure. The various changes which take place in the making of steam are known as vaporization.

The amount of heat necessary to cause the generation of steam is the sum of the sensible heat, the internal latent heat, and the external latent heat. Sensible heat is that part of the heat which produces a rise in temperature as indicated by the thermometer. The internal latent heat is the amount of heat that water will absorb at the boiling point without a change in temperature---that is, before vaporization begins. External latent heat is the amount of heat required when vaporization begins to push back the atmosphere and make room for the steam.

Another important factor to consider when dealing with steam is the boiling point of liquids. By definition, the boiling point is the temperature at which a liquid begins to boil and it depends upon both the pressure and nature of the liquid. For instance, water boils at 212 degrees Fahrenheit and ether at 9 degrees Fahrenheit under atmospheric pressure of 14.7 lbs. per square inch.

The relationship between boiling point and pressure is such that there is a definite temperature of boiling point corresponding to each value of pressure. When vaporization occurs in a closed vessel and there is a temperature rise, the pressure will rise until the equilibrium between temperature and pressure is re-established. By definition, condensation is the change of a substance from the gaseous to the liquid (or condensate) form. This change is caused by a reduction in temperature of the steam below that corresponding to its pressure.

The condensation of steam can cause certain problems for steam heating systems unless they are designed to allow for it. The water from which the steam was originally formed contained, mechanically mixed with it, 1/20 or 5 percent of its volume of air (at atmospheric pressure). This air is liberated during vaporization and does not re-combine with the condensation. As a result, trouble is experienced in heating systems when attempting to get the air out and keep it out. Suitable air valves are necessary to correct the problem.