You may 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.

Now, if 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.

The 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.

Now the 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 temperature.

Having helped 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?

The boiler 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.

These two 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 possible advantage.

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.

You will 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.

As the steam pressure increases more Total Heat is available, but not much (and the increase becomes less and less as the pressure rises).

Please remember 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.

Steam Pressures are expressed as kPa (kiloPascals)

Steam Pressures 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.

Pressures 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).

Gauge pressure 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.