The Theory of Producing Steam
Steam and vapour are actually the same. The term 'Steam' is used more along with the process application, whereas the term 'Vapour' or 'Vapor' is the theoretically used general term for gaseous matter generated from liquid.
Water and steam are often used as heat carriers in heating systems. It is well known that water boils and evaporates at 100°C under atmospheric pressure. And also when exposed to higher pressure, water evaporates (and condensates) at higher temperature. It means that the water molecules are suppressed and retained in liquid form (by higher pressure) even when the molecules increase their internal velocities and thus level of energy (by higher temperature). For instance a pressure of 10 bar gauge (11 bar absolute) equals an evaporation temperature of 184°C. These temperature / pressure relations and other thermal properties appears from a so-called steam table (see below).
During the evaporation (and condensation) process, pressure and temperature are constant.
During this phase a substantial
amount of heat are use for bringing the water molecules from liquid phase -
and to be released into vapour phase.
At this point the steam is "wet" until all the liquid is evaporated - and
the steam is then defined as dry-saturated. It is just being 100% evaporated
but not superheated as a gaseous matter.
The Steam Supply
In steam heating system, the steam boiler (including the steam generator boiler) is connected to the consumers through the steam and condensate piping. When the steam is applied to the consumers, it condensates and thereby releases a high amount of latent heat described above. The condensate (which is hot water) can then be returned to the feed water tank, -from where it again is pumped and provided as feed water to the steam boiler / steam generator. However sometimes the steam is taken out of the system and consumed in an open system - for instance if the steam is injected into a product or in other way discharged or sprayed out (e.g. steam cleaning or humidifying of air).
So in the closed system, the steam condensate is
the condensate tank and to the feed water tank respectively. Since steam
pressure is normally quite high (beyond atmospheric pressure) a pressure reduction
in the form of a steam trap or orifice must be established at the condensate
outlet of the consumer(s) - before the condensate is returned to these tanks
(which are normally atmospheric or low pressurised). Due to the above
discussed thermo-dynamic relations, this pressure drop causes a generation of flash steam - typically just after the steam trap(s) after the
consumer /heat exchanger.
The Steam Boiler Operation Principle
Any steam boiler works in the principle the
A steam boiler delivery exactly what is being consumed in the system The steam boiler is always set for a specific steam pressure, and the operation of the steam boiler is solely controlled by means of this steam pressure set point.
The consumer in the system calls for steam by the decreasing steam pressure since too much steam is condensed at the consumers compared to what the steam boiler actually delivers. The reduction of steam pressure in the system is consequently detected by the control and the pressure sensors in the steam boiler, which initialise heat (more heat) in the boiler for evaporating more steam.
When sufficient steam flow seems to be established, you will have a balance with the consumption of steam (consumers of the system) and the steam pressure will return into a stabile condition.
Then when the consumers eventually stop demanding steam, the steam pressure starts increasing - and this detected by the steam boiler control too, and the heat for evaporating steam is then being turned down to a lower level where the new balance will be.
A steam boilers does not work like a machine. It does not impose steam to
the system, it only covers the lack of steam that is being consumed by the
The alternative to steam
alternative is, instead of steam, to use a complete different heat carrier - for instance
Thermal Oil, where you can operate atmospheric (unpressurised) at
300°C. This is however a complete different system, and you cannot just use
or for that matter exposed your existing steam system to another heat carrier like thermal oil.
In this long tube of tube coil assembly the feed water is heated up to the evaporation temperature in the first part of the tube coil and then evaporated in the second part. The intensity of the heat, the feed water flow and the size/length of the tube are adapted, so that the water is just about being fully evaporated at the exit of the tube. This ensures a total very small water and steam volume i.e. content of the pressure vessel. Thus there are no extra volume of water at boiling point forming an evaporation buffer in a steam generator, and is the steam generator temporary overloaded beyond its nominal steam capacity, it will gives a operation failure due and alarm for high steam temperature (superheated steam). The solutions to prevent this are normally just to place a pressure sustaining valve in the steam line. This valve will protect the steam generator against critically low steam pressure due to uncontrolled high steam consumption beyond its max. capacity. Another solution often used is to install and connect a separate buffer tank next to the steam generator that absorb a majority of steam pressure fluctuations (the demand for extra steam buffer occur in about 10 - 15% of all installations). The ultimate alternative to these two solutions is of course to install instead a classic fire-tube steam boiler, which is less sensitive to steam pressure fluctuation (fluctuation is steam consumption).
The advantages using a steam generator compared to conventional steam boilers are:
There is a still increasing demand for
electric steam boilers. Particular the small sizes (up to 250 -
300 kg/h steam) are very popular - they are typically very price
competitive and very easy to install. No chimney and no fuel arrangement
- and very clean. On top of this they are today considered environmental
as electricity in many regions comes from non-fossil energy sources. But
often the electricity is both very expensive in consumption (kWh) and
also if a complete new larger size electrical supply must be establish.
Rule of thumb is that each kg/h steam requires 1 Amp (at 3 x 400V).
Boiler / Generator Design
A heat exchanger utilisation the waste heat in flue gas of
the steam boiler or steam generator itself for increasing the boiler
efficiency, is called an
ECONOMISER. It can be used for preheating the feed water, but also for
external purposes including preheating of make-up water, domestic water or
central heating water.