Steam

Steam is so familiar to all of us that we easily forget what a marvelous thing it really is. Steam will carry twenty times the BTUs per pound that Freon 12 will, nearly 15 times the BTUs 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 turned into steam. This explains why a tea kettle 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.  Once 212 degrees F is reached, all heat  applied to the vessel does not increase the water temperature. The applied energy is going into changing the water from a liquid into steam, a vapor. The steam will give up this energy before turning back to a liquid. This energy is called Latent Heat and for most purposes this is considered the usable energy of steam.

Below the boiling point, the heat applied to the vessel goes into the water and raises the water’s 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 re-condenses back into a liquid from a vapor. Wherever possible we return this condensed steam or water, commonly called 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 this condensate has come into contact with contaminating materials that make it unsuitable for return to the boiler, it can often be still be used to help heat new incoming water through a heat exchanger and reduce fuel requirements.

So far, we have only talked about an open vessel to the atmosphere. If we close this to 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 the water boils to rise as well. This calls for a further supply of Sensible Heat to get the water to its new, higher boiling point level. At the same time though, The Latent Heat required to convert the higher temperature water into steam is reduced. The next 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 square inch pressure, it is easy to see why higher pressure is used in industry throughout the world! While occupying only 1/8 the volume at 200 psi, steam has only required 4 ½% more heat per pound. Smaller piping will serve the same system and will reduce piping radiant heat loses 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 they are returned to condensate. Just 6% of water particles at 200 PSIG reduces the Total Heat in a pound of steam to less that 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 heat is often 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 that great. One pound of steam at 200 PSIG superheated another 100 degrees F more has gone from 1200 BTUs Total Heat to 1260 BTUs, 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 to a volume change of 14.5%. The heat content per cubic foot of steam has actually decreased by superheating. Available latent Heat or usable heat in BTUs actually increases when you use lower pressure. This is why lower pressure is used for comfort heating in buildings in colder climates.

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

For steam engines or turbines in simple cycle and combined cycle power plants (HRSG or, heat recovery steam generators), 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 used all of its heat units that were put into it and can often be put into additional use as in process applications, such as preheating supply water for makeup use.

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 and most of its basic simplicity, its great heat transfer capabilities, and the versatility of its applications. 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.

Randall J. Harwood, circa 1980.

Misc Readings & Research;

Click to view:     Pipe Fitters Handbook

Click to view:     3 Element Boiler Steam Drum Control

Click to view:     Saturated Steam Tables

Click to view:      understanding_steam_cycle_chemistry

Click to view:  Fluid Compatability Table:  FluidCompatTable

Click to View:     Steam Conditioning Valves: Case Study

American National Standards Institute
www.ansi.org

ASTM International, formerly known as the American Society for Testing and Materials (ASTM)
www.astm.org

American Supply Association
www.asa.net

International Organization for Standardization
www.iso.org

Process Industry Practices
www.pip.org

Valve Manufacturers Association
www.vma.org

American Petroleum Institute
www.api.org

American Society of Mechanical Engineers
www.asme.org

E.I. Dupont Elastomer Chemical Resistance Guide
www.dupontelastomers.com

National Association of Purchasing Management
www.napmsyr.org