Steam Generator OR Steam Boiler ?


A very important question to ask yourself. Are you in doubt about the exact differences in these two kind of industrial steam boilers, then it might be a good idea to take a few moments reading the below guidance.

Because there is quite a difference in where the two type of boilers should preferable be used, and it does can gives substantial advantages when making the right choice.

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The Theory of Producing Steam

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.

  steam table

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.

An 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 Boiler
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 animation 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.


The advantages using a steam generator compared to conventional steam boilers are:

   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.
Steam 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 be designed and equipped according to European regulations including EU's pressure equipment directive PED 97(23 CE code and EN-standards for steam boilers.

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.


 

This article is made by AB&CO and is the property of AB&CO. It must not be copied in part or in whole without written permission by AB&CO.



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