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Basics of
Steam System Design Introduction This section is intended for people involved in the design, operation and/or maintenance of steam systems. It covers some basic terminology, principles, steam generation in general, and the design of steam distribution and condensate collection piping. Also covered are the sizing, specifying and installation of steam traps.
Absolute Pressure:
Gauge Pressure:
Enthalpy:
Specific Enthalpy:
Specific Heat Capacity:
Heat:
Heat Transfer:
Heat of the Liquid (Enthalpy of Saturated Water):
EXAMPLE Let us assume that 50° F water is available as feedwater to a boiler at atmospheric pressure. The water will begin to boil at 212° F. 1 BTU will be required to raise each pound of water 1° F. Therefore, for each pound of water in the boiler, the increase in enthalpy required to raise the temperature from 50° F to 212° F is: (212 - 50) x 1 = 162 BTU If the boiler holds 22000 pounds mass (2638 gallons) the increase in enthalpy to bring the total mass of water to it's boiling point is therefore: 162 BTU/lb x 22000 lb or 3,564,000 BTU. It must be remembered, this figure is not the sensible heat, but merely the increase in sensible heat required to raise the temperature from 50° F to 212° F. The datum point of the steam tables is water at 32° F, which is assumed to have a heat content of zero for our purposes. (The absolute heat content clearly would be considerable, if measured from absolute zero at minus 459° F). The total sensible heat of water at 212° F is therefore: (212 - 32) x 1 = 180 BTU/lb. Latent Heat of Evaporation (Enthalpy of Evaporation):
Total Heat of Steam (Enthalpy of Saturated Steam):
Flash Steam:
This is the oldest and most widely used form of energy in industry. Yet in most plants and engineering offices it is still not understood to a large degree. It is neglected in an even larger degree. In a larger sense, plant operations quite often can’t see the forest for the trees. That is, until there’s a forest fire. What is meant by that is that steam is used in such a wide spread manner in most heavy industrial manufacturing plants that it’s taken for granted until something fails. A good example of that is the care and maintenance of strainers installed upstream of steam traps in order to prevent the trap from plugging up from various deposits of pipe scale and chemical residue. That same thinking would logically conclude that if scale and residue are anticipated then it would stand to reason that the strainers should be blown down and/or cleaned out periodically. It is surprising the number of times a systems failure or reduction in efficiency can be attributed to something as simple as the impacted build-up of scale in strainers; something that could have been averted with a little planned preventative maintenance. In conjunction with that is the periodic testing of the steam traps, all too often steam systems are installed and forgotten. Until, as mentioned earlier, a system breaks down or gradually becomes more and more inefficient. Inefficiency translates into additional operating costs. In older plants the savings potential is enormous. Not only from a fiscal standpoint, but also from an environmental standpoint. A good way to find out how current a plant is with it's steam system maintenance is to find out if there is a steam distribution list, a location plan or any kind of a record accounting for all of the steam traps throughout the plant. If any of these items do exist, when was the last time they were checked or updated? And, are they actually used as a preventative maintenance tool? In a surprising number of plants the answer is that there is no formal record of any kind for steam traps. If this is true in your plant then this is an indication that the potential for both cost savings and increased production is a likely possibility. |
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