Those who design, install, or have charge of steam heating plants certainly should have some knowledge of steam and its formation and behavior under various conditions.

Steam is a colorless, expansive, and invisible gas resulting from the vaporization of water. The white cloud 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 partway with a white cloud; in traversing the main, the little particles combine, forming drops of condensation too heavy to remain in suspension, which accordingly drop to the bottom of the main and drain off as condensation. This condensation flows into




Figure 2-10 Three types of steam.

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 be applied only to 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

Three of these classes of steam (wet, saturated, and super­heated) are shown in the illustration of a safety valve blowing in Figure 2-10. 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 is steam containing no



Collapse of bubble (in cold zone)

Disengaging a bursting of bub

Change of sta


Heating surface

Figure 2-11 The phenomenon of vaporization.

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 that take place in the making of steam are known as vaporization and are shown in Figure 2-11. For the sake of illustration, only one bubble is shown in each receptacle. In actu­ality there is a continuous procession upward of a great multiplicity of bubbles.

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. As mentioned elsewhere in this chapter, sensi­ble heat is the part of the heat that 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


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Steam Steam Steam Steam

Figure 2.12 Variation of the boiling point when pressure changes.

Temperature at which a liquid begins to boil (Figure 2-12), and it depends upon both the pressure and the nature of the liquid. For instance, water boils at 212°F, ether at 9°F, under atmospheric pres­sure of 14.7 psi.

The relationship between boiling point and pressure is such that there is a definite temperature or 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 re­established.

One’s knowledge of the fundamentals of steam heating should also include an understanding of the role that condensation plays. 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, V2o, or 5 percent, of air by volume (at atmo­spheric pressure). This air is liberated during vaporization and does not recombine with the condensation. As a result, trouble is experi­enced in heating systems when one attempts to get the air out and keep it out. Suitable air valves are necessary to correct the problem.

Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers