Basics of
Steam System Design
by W. M. (Bill) Huitt
Steam – The Basics
Steam is the gas phase of water. It is created when heat energy is added to the water until, at some corresponding point of pressure and temperature, the water can no longer remain as a liquid. This is called the "Saturation" point and any additional heat energy added to the water at this point, will cause some of the water to boil off as steam. This action is referred to as evaporation. The energy in the evaporated state is referred to as the "Latent Heat of Evaporation".
The amount of energy required for evaporation, at lower pressures, is significantly higher than the amount of energy needed to bring that same water to its saturation point. The condensing process of steam takes place when the steam is allowed to give up its latent heat content and condense back to a liquid state.
The latent heat content of the steam is the energy that is given up so readily for heat transfer. In fact the steam is so willing to give up this heat energy that we have to take various measures, sometimes costly, just to contain it long enough to get it to where we need to use it. Some of these measures include insulation to contain the heat, steam traps and separators to keep condensate from accumulating in the distribution piping and proper line sizing to reduce friction loss. These measures will be covered in more detail later.
Steam can exist in either a saturated condition or a superheated condition. The temperature of saturated steam is in constant correlation with its pressure. If you know what the temperature of saturated steam is you then know what the pressure is. Inversely, if you know what the pressure is you then know what the temperature is. To illustrate; if we were to take a container with a volume of 26.8 cubic feet and pour in 1 pound of water at the temperature of melting ice, 32° F, in all practicality the heat content, or enthalpy, would be zero. We would then begin adding heat to the water, which is at atmospheric pressure, until it reached 212° F. The heat content of that water will then be 180.2 BTU's. By adding additional heat, or enthalpy, to the water it will begin to change state and evaporate into steam. The heat content of this latent heat of evaporation is 970.6 BTU's. At atmospheric conditions these numbers will always be constant. Water at it's saturation point will contain 180.2 BTU/lb and any steam that is formed will contain 970.6 BTU/lb.
By sealing our container, and continuing to heat the water until it has been completely evaporated, that 1 pound of water, with a volume of .016 cubic feet, has been transformed into 1 pound of steam with a volume of 26.8 cubic feet. That is an increase in volume of 1675 times. This is indeed one of the considerations in designing for steam distribution, mass volume.
At atmospheric pressure we now know that the constants are: 212° F at saturation point, heat content of the liquid is 180.2 BTU/lb, heat content of the steam is 970.6 BTU/lb and the specific volume of the steam is 26.8 cubic feet.
Now we will remove the seal on our container and install a vertical piston, one that doesn’t leak around the edges. The only thing holding down the piston initially is atmospheric pressure at 14.7 P.S.I.A. or zero P.S.I.G. Consequently the container still has a volume of 26.8 cubic feet and the steam is still at atmospheric pressure. By adding a 10 P.S.I. weight on top of the piston, two things happen. The steam condenses back to water and the volume in the container is reduced. However the heat of the liquid still remains at 180.2 BTU's. With 10 P.S.I. of compression on the hot water in the container it will take more energy to release the molecules into steam. At 10 PSIG the saturation or boiling point of water is 239.4° F at which point the heat of the liquid will be 207.9 BTU/lb. By adding an additional 27.7 BTU to the water we can bring it to it's saturation point of 239.4° F. If we continue to add heat the water will begin to boil off to steam. If we boil off the entire pound of water under the 10 P.S.I. pressure the latent heat of the steam will then be 952.9 BTU's and occupy 16.5 cubic ft.
Under 10 PSI pressure the constants are: 239.4° F saturation point, heat content of the liquid is 207.9 BTU/lb, heat content of the steam is 952.9 BTU/lb and specific volume of the steam is 16.5 cubic feet.
Superheated steam, on the other hand, does not have a pressure/temperature correlation. Saturated steam and water exist at the same pressure and temperature. If heat transfer continues into the steam after saturation and evaporation has been accomplished, the steam temperature will continue to raise. Once it has exceeded the temperature of saturated steam at a corresponding pressure it becomes superheated. This superheat condition can be manufactured to achieve a certain amount of superheat for use in mechanical work such as steam turbines or for transmission over long distances (this will be discussed later). It can also be the unneeded result at pressure reducing stations and other cases where there is a turndown of steam pressure to effect temperature control or pressure control, the amount of superheat depends on the turndown ratio.