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Introduction of Steam Generator & Steam Boiler

Water and steam are typically 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
– for instance a pressure of 10 barg (11 bar abs) equals an evaporation temperature of 184°C.

During the evaporation process, pressure and temperature are constant, and
during this 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 called dry-saturated.

In this
100% dry condition the steam
thus contains a huge amount of latent heat, corresponding the heat that was
provided during the
evaporation process. If you heat the steam, which is now a gaseous fluid,
further then it becomes superheated steam.

Despite temperature and pressure is the same
for the liquid and the vapour, the amount of heat is thus very much higher in vapour compare to the
liquid. This latent heat in the dry-saturated steam can efficiently be
utilised in
different applications mainly within process heating. Very superheated steam
is used for e.g. steam turbines, whereas slight superheated steam is
sometimes used in process heating in order to compensate for heat loss in
steam piping.

Only boilers for saturated steam is discussed in the following. Boilers for
superheated steam are never steam generators, but special often very big
water-tube and fire-tube boilers

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) it can then be returned to the feed water tank, from where it
again is pumped to the steam boiler / steam generator. However sometimes the
steam is 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 maybe humidifying of air).

In the closed system, the steam condensate is
instead returned to
the condensate and to the feed water tank. A pressure reduction is normally
established tanks are normally atmospheric or low pressurised. This causes
generation of flash steam – typically just after the steam trap(s) providing
the pressure drop from steam pressure to tank pressure.

This gives the well-known large condensate heat loss in the steam system,
which is actually mostly high-energy flash steam being generated and led to
the ambient.

This loss of flash steam also represents physical loss of feed water volume, which then requires
fresh and pre-treated make-up feed water to the circuit. The higher steam
pressure, the higher heat loss and the higher demand for expensive new
treated boiler feed water.

We are not speaking moderate losses, but losses between 10 and 30% – in both
heat loss and feed water loss ! This is the huge disadvantage using steam
with low pressure / atmospheric feed water tank.

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 a complete different heat carrier – for instance
Thermal Oil, where you can operate atmospheric at temperature over
300°C. This is however a complete different system and you cannot exposed
you existing steam system to another heat carrier like thermal oil. You can
get more information on this subject following this link :
Thermal Oil / Thermal Fluid versus Steam.

Steam Generator

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.

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.

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 authorization
Rapid start-up and establishing full
steam pressure
Compact and easy to adapt in the
existing machinery arrangement
Price attractive – especially at low
steam rates.

Generator Design

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 heated, including
EX-design if required
Stainless steel – all parts in
contact with steam made in stainless steel.
High Pressure design for special
applications up to 190 bar / 350°C
Complete skid-mounted with tanks and
pre-treatment equipment.
Build in a container or on a trailer
for mobile operations.

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).

A heat exchanger utilisation the waste heat in flue gas of
the steam boiler or steam generator itself for increasing the boiler
effeciency,  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.

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