have heard about Atmospheric Pressure. In case you are not quite sure
what it means, let's use a simple (but painful) illustration. If, in
imagination, you will allow yourself to be flattened by a 100 tonne
steam hammer you will at once recognise it as an unpleasant experience.
we put you into a sealed chamber which is absolutely devoid of air ("atmosphere")
the effect would be the opposite of the steam hammer pressure. While
the result would be less messy it would still be decidedly unpleasant.
The air ("atmosphere") in your body would exert a pressure
outwardly and in all directions and you would tend to burst whether
you liked it or not.
atmosphere exerts a pressure on you and on everything else. The pressure
is exerted in all directions and is approximately 101.3 kPa at sea level.
steam coming out of the kitchen kettle (and out of the imaginary spout
on our boiler) is exposed to atmosphere and is therefore at Atmospheric
Pressure of 101.3 kPa. The temperature of steam at this atmospheric
pressure is the same as the temperature of boiling water at atmosperic
pressure, which is 100 deg.C. So long as the imaginary spout remains
on our boiler, everything is at atmospheric pressure and its specified
us explain what happens at atmospheric pressure, the imaginary spout
has served its purpose and can be removed. We are still generating steam
in the boiler and - being minus the spout - there is no outlet for the
steam. What happens?
is virtually a closed vessel. So as more steam is generated inside this
vessel, the steam must compress to find room for itself. And because
it is now compressed it tries to push out in all directions and exerts
a pressure on everything surrounding it. As yet more steam is generated
it becomes even more compressed and its pressure continues to increase
accordingly. This pressure of the steam is exerted in all directions.
So besides pressing on the inside surfaces of the boiler, the steam
also creates a pressure on the surface of the water.
things - the increasing pressure exerted by the steam and the effect
of that increasing pressure on the surface of the water - cause other
things to happen. These other things have a very important bearing on
the practical use of steam for process heat and space heating purposes.
Properly understood they will help you to use your steam to the best
As the pressure
on the surface of the water increases it has the effect of increasing
the temperature at which the water boils. While at atmospheric pressure
(101.3 kPa) water boils when it reaches the temperature of 100 deg.C.
we find that at a pressure of, for example, 690 kPa the boiling point
has gone up to 169.95 deg.C.
readily see that to keep the water boiling (and giving off steam) at
this higher temperature it has to be supplied with a greater proportion
of Sensible Heat units. On the other hand, at the higher pressure and
temperature, the quantity of Latent Heat units needed to convert the
boiling water into steam is reduced.
the steam pressure increases more Total Heat is available,
but not much (and the increase becomes less and less as the pressure
that this discussion has been about steam as a conveyor and giver-up
of heat. Steam as a means of supplying Power (through engines etc.)
has to be approached from a slightly different angle.
are expressed as kPa (kiloPascals)
may be given as either "Gauge" pressure or "Absolute"
pressure, usually gauge pressure. "Gauge" pressure is the
pressure above atmospheric pressure. "Absolute" pressure begins
at absolute zero, which is a point 101.3 kPa below atmospheric pressure.
below zero gauge (that is below atmospheric pressure) are generally
expressed as "inches of mercury vacuum" (abbreviated to "inches
mercury" or simplified by the letters "Hg" the chemical
symbol for mercury).
is the pressure shown on an ordinary pressure gauge which is fitted
on every boiler. As gauge pressure is the pressure above atmospheric,
the "0" on the dial of the gauge is really 101.3 kPa absolute.
But for all ordinary purposes "0" is zero gauge.