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 this guidance.

There is quite a difference in where the two types of boilers should preferable be used, and it eventually leads to significant advantages by making the right choice.

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The Theory of Producing Steam

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 liquid.

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 (see below).

  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.

In this 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 utilised in 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 water-tube boilers.


 

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 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 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 the steam 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.

 


An alternative is, instead of steam, to use a complete different heat carrier - for instance Thermal Oil, where you can operate atmospheric (unpressurised) at temperature above 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.

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
 

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 authorisation
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 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 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.


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.
 


 

 

 

 

 

Important Legal Announcement

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 :
Copenhagen, 2nd August 2016
by Arvid Blom, Senior Engineer

 


 

 

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