Understanding Steam

Steam is so familiar to all of us that we easily forget what a marvelous thing it is.  Steam will carry twenty times the BTU's per pound that Freon 12 will, nearly fifteen times the BTU's per pound of F22, and over twice that of ammonia.  Even when we use nuclear energy for power, it is only to heat water to make steam.  For all of this, we tend to take steam for granted without much thought to how it really is produced or how it works best for us.  So let's take a very basic, but serious look at steam.

Boilers make steam by boiling water.  That's about the most oversimplified statement we could make.  Some very interesting things happen when water is boiled.  At atmospheric pressure water expands 1,600 times its original volume when it turns to steam.  Which explains why a teakettle may put off a plume of vapor half the morning before running dry.

Water at atmospheric pressure boils at 212 degrees F.  The boiling water in an open vessel measures 212 degrees F, the steam coming off the surface of the water also measures 212 degrees F.  All the while, heat is being applied to the vessel but nothing is increasing in temperature.  The applied energy is going into changing the water into steam.  The steam will give up this energy before returning to liquid.  This energy is called Latent Heat and for most purposes is considered the usable energy of steam. 

Now, below the boiling point, the heat applied to the vessel goes into the water and raises its temperature.  We are able to sense this temperature change with a thermometer and we call this heat Sensible Heat.

For most purposes for which steam is used, it is the Latent Heat of steam that we utilize, for after the Latent Heat is given up, the steam recondenses into liquid.  Wherever possible we return this condensed steam, or condensate, back to the boiler for reuse.  This conserves a large part of the Sensible Heat we had to apply to raise the temperature of the water up to 212 degrees F.  Even if the condensate has come into contact with contaminating materials that make it unsuitable for return to the boiler, it can often still be used to help heat new incoming water through a heat exchanger and thus reduce fuel requirements.

So far we have only talked about an open vessel for boiling.  Close off the vessel to the atmosphere and, as we continue to add heat above the boiling point, the expanding steam causes the pressure to raise.  This increase in pressure, however, causes the temperature at which water boils to raise.  This calls for a further supply of Sensible Heat to get the water to its new, higher, boiling point.  At the same time, though, the Latent Heat required to convert the higher temperature water into steam is reduced.  The net result is only a slight increase in the Total Heat required for each pound of steam.  But since that same pound (by weight) of steam will occupy 26.8 cubic feet of space at atmospheric pressure and only 2.15 cubic feet at 200 lbs. per sq. inch pressure, it is easy to see why higher pressure is used.  While occupying only 1/8 the volume, 200 P.S.I. steam has only required 4 1/2% more heat per pound.  Smaller piping will serve the same system and will reduce piping heat loss as well.

Steam can be in one of three forms, wet, dry, or superheated.  Wet steam contains small water droplets entrained within it.  These droplets contain no Latent Heat and therefore have nothing to contribute to the process before becoming returned condensate.  Just 6% of water particles at 200 P.S.I.G. reduces the Total Heat in pound of steam to less than the Total Heat in a pound of dry steam at atmospheric pressure.

Dry steam requires the elimination of water particles from the steam line.  This can be done mechanically with baffles in the steam flow design so that water particles are deposited and left behind, or by removing the wet steam from the water surface and applying additional heat to drive all the water particles into steam.  Flue gas waste heat is increasingly used for this additional heat.

Superheated steam occurs when enough additional heat is applied to raise the steam temperature to any level above that of saturated dry steam.  Some superheat is sometimes required to make sure that dry steam is available at the end of a long steam line.  It would seem that superheat would be the course to take in all cases as a means to transfer more heat.  In reality, the amount of additional heat transferred is not great.  One pound of steam at 200 P.S.I. superheated 100 degrees F more has gone from 1200 B.T.U.'s Total Heat to 1260 B.T.U.'s,  a gain of only 4.8%.  While that small change was taking place, the saturated steam started acting like a perfect gas and expanded in volume from our original 2.14 cubic feet, a volume change of 14.5%.  The heat content per cubic foot of steam has actually decreased by superheating.

Reflecting then, we have eight times the volume of steam in the same space by raising the pressure from atmospheric to 200 P.S.I.G. but we have a 14.5% decrease through superheating another 100 degree F at the same 200 P.S.I.G.  Superheat is therefore an advantage only when the heat requirement can be met by the small amount of energy in the superheat and where the higher temperature gradient from the hotter steam is required by the process job.

For steam engines or turbines, it is often part of the power unit's design to use superheated steam.  This is a specialized application of steam utilizing the entropy of steam (which needs to be treated separately.)  Suffice it to say that after expansion through the engine, this steam has nearly all of the heat units that were put in it, and can often be put to additional use in process applications.

We have not attempted to discuss all areas of steam use.  For example, we have said nothing about steam trapping, a very necessary consideration in most steam systems.  But we have zeroed in on steam's basic simplicity, its great heat transfer capabilities and the versatility of its application.  When these are taken together, it is obvious that steam will always continue to be a very important tool to industry.  It will not become obsolete for there is nothing else that even comes close to having these properties.