Steam Basics - The Properties of Steam

 

The steam is a medium used to carry energy in the form of heat and pressure.

In industry, steam is used because it possesses many outstanding qualities, for example:

  1. It has a very high heat content
  2. It gives up its heat at a constant temperature.
  3. It is produced from water which is cheap and plentiful.
  4. It is clean, odorless and tasteless.
  5. Its heat can often be used over and over again.
  6. It can generate power and then be used for heating.
  7. It can be readily distributed and easily controlled.

Everything, or almost everything, that needs to be known about steam is in the steam tables.

For each pressure there is a corresponding boiling or saturated steam temperature, and these are tabulated for quick reference in the steam tables, or in a form of graph of the saturated steam curve.

As far as steam for heating and process is concerned, there are just two fundamental things that govern everything:

  1. The boiling point of water decreases with reduced pressure.
  2. The latent heat (the “heating” heat) of steam increases with reduced pressure.

As far as steam for Power is concerned, there are also two basic rules:

  1. Use the highest practical initial pressure and temperature.
  2. Use the lowest predictable exhaust or back pressure.

As far as steam for any purpose is concerned there is another rule that is of universal application:

NEVER PERMIT STEAM TO EXPAND FROM ONE PRESSURE TO A LOWER PRESSURE WITHOUT GETTING SOME USEFUL RESULT FROM THE EXPANSION.

The usual unit of heat is the BTU (British Thermal Unit).

A BTU is the quantity of heat required to raise the temperature of one pound of water through 1 deg F.  So to raise the temperature of one pound of water from 32 deg F to 212 deg F (the boiling point at atmospheric pressure at sea level) requires 180 BTU.  This heat is known as sensible heat.  To produce steam under these conditions it takes another 970 BTU’s per pound.  This is nearly 5 ½ times the heat needed to raise the water from 32 deg F to 212 deg F.  This heat is known as latent heat.

The heat which produces a rise temperature is sensible heat.  It is the practice to reckon that the sensible heat of water is nil at 32 deg F and the heat which produces a change of state without  a change in temperature is the latent heat.  The heat provided by dry saturated steam is latent heat.

The total heat of dry “saturated” steam at any pressure is the sum of sensible and latent heats.  At 212 deg F and atmospheric pressure the total heat is equal to 1150 BTU’s per pound.

The steam table shows the number of BTU’s present in each pound of steam, as total heat, as latent heat, as sensible heat at various steam pressures and as sensible heat at atmospheric pressure.  It shows the variation of the heat content as the steam pressure and steam temperature vary.

Note that if the pressure increases, the sensible heat increases, but the latent heat decreases.

Saturated Steam is steam generated in contact with water.

When steam is at a temperature corresponding to the liquid boiling point appropriate to its pressure, it is said to be “Saturated” – when no liquid is present at that temperature it is called “Dry Saturated Steam”.

So far we have considered only the heat content of dry saturated steam.  But steam is seldom dry and not always saturated.

When steam is generated in a boiler, the water surface is turbulent and droplets of water are thrown up into the steam.  Particularly when steam is being extracted from the boiler at a high rate, the movement of the steam towards the outlet will carry these droplets away and into the steam system.

Steam which contains these particles of water in a finely divided state is called wet steam.  If one pound of wet steam is made up of, say, 95% dry steam and  5% water particles, it is said to have a dryness fraction of .95.  The total heat of one pound of wet steam is less than the total heat of dry steam, because the water particles have escaped without receiving any latent heat.

The important effect of a small percentage of wetness in the steam will be realized when we consider at 200 PSIG if it contains 6% of water particles, has less total heat than dry steam at atmospheric pressure.