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,
- It has a very high heat content
- It gives up its heat at a constant temperature.
- It is produced from water which is cheap and plentiful.
- It is clean, odorless and tasteless.
- Its heat can often be used over and over again.
- It can generate power and then be used for heating.
- 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:
The boiling point of water decreases with reduced pressure.
heat (the “heating” heat) of steam increases with
As far as steam for Power is concerned, there are also two basic rules:
Use the highest practical initial pressure and temperature.
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
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
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.