The Theory of Producing
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 temperature - 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.
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.
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 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.
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
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.