Steam Today and Steam Tomorrow
By Brian Johnston, Spirax-Sarco
Why and where steam is used
What do these things all have in common? A headache tablet, a gallon of petrol, a pair of stretchy leggings, a can of baked beans, a tin of paint, a ream of paper, a car dashboard and a freshly sterilised theatre instrument?
No, itís not a party game involving a tray and a tea towel, so letís remove the mystery and say that they are all produced with the assistance of steam.
Steam is used for a variety of purposes in a huge number of processes across industries as diverse as pharmaceuticals, food and beverages, textiles, pulp and paper, oil and petrochemicals, laundries and public buildings. We literally couldnít function in the modern world without it.
Power generation - to turn the turbines
Process & space heating - to cure rubber tyres, or
heat large buildings
Humidification - for clean rooms or
Sterilisation - for hospital instruments
Cooking - in vats, ovens and
Steam has been used to produce electrical power in thermal power stations for many years. Even in modern generation facilities today (such as nuclear power stations) steam is still the fluid used to turn the turbines. Power produced from gas turbines can also involve steam in Combined Cycle Gas Turbine (CCGT) systems. Here steam is produced from the gas turbine exhaust and used in a steam turbine to improve the overall generation efficiency.
So what makes steam so suitable for all these other different applications?
Well it takes a lot of energy to turn water into steam, energy that is Ďmade availableí again when the steam condenses back to water. This makes steam a very effective carrier of heat. A lot of energy available in a small volume means smaller pipes.
As steam condenses, its pressure drops and higher-pressure steam flows into the lower pressure region. Therefore no pumps are needed to make the steam flow, a considerable saving in installation and running costs. No pumps also mean no system balancing is required.
Another of steamís unique properties is that there is a distinct relationship between pressure and temperature. This is illustrated in steam tables, an extract from which is shown below.
So to control the temperature of the steam, and hence itís heat transfer capability, we need only control the pressure. That means the use of a simple 2-port valve rather than the mixture of 2 and 3 ports normally associated with liquid heating systems.
Steam is inherently sterile. The rapid transfer of heat it gives when condensing is the reason why it is such a common means of sterilising not only surgical instruments in a sterilizer, but also pipelines in essential industries such as food or pharmaceuticals where steam is a key component of SIP (Steam In Place) or CIP (Clean In Place) systems.
Being a sterile gas makes steam an ideal choice to humidify air in ventilation systems. This is why it is commonly used in the healthcare, pharmaceutical and electronic industries where clean, sterile and humidified air is required. Of course the steam can be used to heat the air as well (strangely, it can also cool the air, as will be shown in a later article.)
A lack of basic awareness training on steam is causing younger engineers to be nervous about its application. It is inherently a safe medium (a leak wonít poison you!) but deserves to be treated with respect, no different from how youíd treat electricity.
It is also a very forgiving medium. A steam system will seldom simply stop working but with a little ongoing maintenance, operating costs can be kept low.
In the next article, we will look at what a modern steam system could and should look like to maximise efficiency and reduce operating costs.
Spirax-Sarco, Tel: 01242 521361
Published in Valve User Magazine Issue 11
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