The Theory of Producing
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.
By higher pressure, water evaporates at higher
temperature - for instance a pressure of 10 bar gauge (11 bar
absolute) equals an evaporation temperature of 184°C. You can see all these
comparison figures in a steam table.
During the evaporation (and condensation) process, pressure and temperature are constant, and
during this phase a substantial
amount of heat are use for bringing the water from liquid phase to vapour phase.
The steam is wet until all liquid is evaporated - and the steam is then
defined as dry-saturated.
100% dry condition the steam
thus contains a huge amount of so-called latent heat, that corresponding the heat that was
provided during the
evaporation process. If you heat the steam further from the dry saturated
condition (that has now just become a 100% gaseous fluid) - then it becomes
superheated steam, and actually a ordinary gas like air that can have any
temperatures independent of the pressure.
Despite temperature and pressure is constant in start and in the end of the
evaporation (and condensing) i.e. for the liquid and the vapour, - then the amount of heat is thus very much higher in vapour
phase compare to the
liquid phase. This is called latent heat, and in the dry-saturated steam
(steam at boiling point) this heat energy can efficiently be
different applications mainly within process heating. Superheated steam
- on the other hand - is used for thermo-dynamic processes e.g. 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
The Steam Supply
The steam boiler (including the steam generator
boiler) is connected to
the consumers through the steam and condensate piping. When the steam is provided to the
consumers, it condensates and thereby releases a high amount of latent heat
described above. The condensate (hot water) can then be returned to the feed water tank, from where it
again is pumped and provided to the steam boiler / steam generator. However sometimes the
steam is taken-out of the system - 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).
In the closed system, the steam condensate is
instead returned to
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 in the beginning
of the condensate line, i.e. before the condensate is returned to these tanks
(which are normally atmospheric or low pressurised). This pressure drop causes
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, unless you invest in some special heat recovery
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.
There are solutions where
you can minimise this loss, 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.
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.
||Explore a full
a the steam production process in a TT BOILERS
fire-tube steam boiler (do expect some load-up time, and you should use Windows Media Player).
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 solution to prevent
this, is to install and connect a separate buffer tank next to the steam
generator - or to choose a classic fire-tube steam boiler. The demand for
extra steam buffer occur in about 10 - 15% of all installations.
advantages using a steam generator compared to conventional steam boilers
Easy to operate - normally no
requirement for boiler authorisation
Rapid start-up and establishing full
Compact and easy to adapt in the
existing machinery arrangement
Price attractive - especially at low
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 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
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.
Latest revision :
26th January 2016
by Arvid Blom, Senior Engineer
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