The Theory of Producing
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
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
AB&CO Steam Table
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.
point - the dry saturated condition - the steam
thus contains a huge amount of so-called latent heat, that corresponding the heat that was
provided during the
evaporation process. This heat correspond the energy of all the released
gaseous water molecules, moving at high velocities and thus with a high
content of energy. If you heat the steam further from the dry saturated
condition (100% gaseous fluid) - then it becomes
superheated steam, and actually an ordinary gas like air that can have any
temperatures independent of the pressure - and where the molecules moves
faster and energy level increases, at unchanged pressure.
In other words, despite temperature and pressure being constant in the
start and in the end of the evaporation (or condensing) i.e. for the liquid and the vapour
respectively, the amount of heat is very much higher in vapour
phase compare to the
liquid phase. This retained potential energy is called 'latent heat', and in the dry-saturated steam
(steam at boiling point) this thermal energy can efficiently be
different applications mainly within process heating. Superheated steam
- on the other hand - is mainly used for high performance thermo-dynamic processes e.g.
to drive a steam turbines.
However slightly superheated steam is
often used in process heating in order to compensate for heat loss in steam
piping - and thus to ensure that the steam is high quality dry saturated
steam at the location where you need to use it and not too very wet steam
(containing a lot of liquid water particles).
Only boilers for saturated steam is discussed in the following. Boilers for
superheated steam (thermo-dynamic applications) are never defined as steam generators,
even though they often a type of
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.
This gives the well-known large "condensate heat loss" in the steam system,
which is actually mostly high-energy flash steam that is being generated, and
quite noisy led into the condensate line and back to the tanks where is
steam up into the ambient.
This loss of flash steam also represents physical and expensive loss of the feed water
content, which then requires
constant amount fresh and pre-treated make-up feed water to the circuit. The higher
pressure is, the higher the heat loss becomes (equals higher demand for expensive new
treated boiler feed water).
We are not speaking moderate losses, but losses between 10 and 30% - in both
heat energy loss and loss in expensive treated feed water ! This phenomenon is the huge disadvantage using steam
for heating - and today is is more or less required that you therefore
invest in heat recovery solution when designing and adapting the steam
system into the application processes.
Both the heat and feed water losses can be reduced and sometime fully
eliminated by investing in special heat recovery features, preferable
integrated in the complete heating system design. Also other solutions can
minimise these losses, for instance free- circulation steam system,
where you utilise a static height and gravity in a self-controlled
evaporation-condensation-loop, but it can only be used in small and
quite tall systems on local spots - not large steam distribution systems.
The Steam Boiler Operation Principle
"Demand & Delivery"
Any steam boiler works in the principle the
A typical misunderstanding is that you control the production rate on a
steam boiler. This is not correct.
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
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
A steam boiler is an autonomic device. It is purely self-controlled.
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.
get more information on this subject following this link :
Thermal Oil / Thermal Fluid versus Steam.
versus the "classic" fire-tube Steam Boiler
The principle in the fire-tube steam
boilers, is that from the surface of a large volume of feed water, steam
is evaporated. This boiling process is heated by the wall of the combustion
chamber (the radiant part) and by the exhaust gasses passing through a
bundle of so-called
fire-tubes or smoke-tubes forming the the convection part of the boiler.
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In the steam generator boiler the
operation is quite different. The feed water
and steam are in the principle passing through one long tube - designed as
a number of winded-up tube coils that are being serially connected.
Horizontal or Vertical Design
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).
advantages using a steam generator compared to conventional steam boilers
Steam generator boilers can be delivered in horizontal execution (with low
height), or in vertical execution (occupying limited floor
space). Like the classic steam boilers they are delivered insulated with stainless steel cover
sheets and complete with burner, armatures, instrumentation, safeties and a control panel
- and with full documentation including necessary certificates.
The steam generator boilers are made with coils made
of seamless tubes, where the feed water is preheated and evaporated
during the flow through these. The heat is transferred to the
water/steam mixture as radiant heat in the combustion chamber, where
the inner cylindrical tube coil and a flat tube coil forms the
chamber wall and the bottom respectively. Consequently refractory
concrete at the end of the combustion chamber is avoided. The
combustion gasses are hereafter cooled in the outer convection part,
as the gasses pass the space between the two tube coils.
The thermal design of the steam generator ensures a modest volume of
steam relative to the size of the heater, and allows unlimited thermal expansion due to
the high temperatures. All steam generators and steam boilers must in Europe
designed and equipped according to European regulations including EU's
pressure equipment directive PED 97(23 CE code and EN-standards for steam
Boiler / Generator Design
Beside the standard execution the
steam generator boilers can be delivered in for instance following variations:
Electrical Steam Boiler Design
Exhaust Gas Steam Boilers
Steam can be produced
not only by oil or gas-fired burners, and as electrically heated. They can
also be design as recuperators utilising the
substantial amount of waste heat in hot flue gasses or exhaust air. The
steam evaporation is done like the steam generators, and are gives
therefore a rapid acting and compact unit.
These are called
exhaust gas steam boilers (EGSB) or exhaust gas steam generators (EGSG).
Economiser using up to 5 heat sources
and extractable / replaceable inserts
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.
This article including all illustrations are
made by AB&CO and must be considered legally as property of AB&CO. It must not be copied in
part or in whole without written permission by AB&CO Group.
Latest revision :
Copenhagen, 7th November 2017
by Arvid Blom, Senior Engineer & Partner