Steam Generator  VS.  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.

by Arvid Blom, B.Sc. Senior Engineer & Partner at AB&CO.

 

 DANSK VERSION
 


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The Theory of Producing Steam

Steam and water 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 or solid phase.

Water and steam are often used as heat carriers in heating systems. It is known to everyone that water boils and evaporates at 100C at atmospheric pressure. And it is also common knowledge for most, that when exposed to
higher pressure, water evaporates (and condensates again) at corresponding higher temperature.

This 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 184C. 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 maintained constant. Here the thermodynamic phase contains a substantial amount of heat are use for bringing the water molecules in higher speed, and thus from liquid phase to be released  - a take-off into "flying" vapour phase. At this particular point the steam is "wet" until all the liquid particles are vaporised - and the steam is then defined as in dry-saturated condition. It is just being 100% evaporated, but not in a superheated condition as a common known gaseous matter at normal known temperatures (like air and gasses).

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 and gas mentioned above, that can have any temperatures independent of the pressure - and where heating makes the molecules moves faster and energy level increases, at unchanged pressure (isobaric).

In other words and in short, - 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 and 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 for instance 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.
 

 

The Steam Boiler Operation Principle
"
Demand & Delivery"

Any steam boiler works in the principle the same way.

A typical misunderstanding is that you control the production rate on a steam boiler. This is not correct.

A steam boiler delivery exactly what is being consumed in the system The steam boiler is always set for a specific steam pressure, and the operation of the steam boiler is solely controlled by means of this steam pressure set point.

The consumer in the system calls for steam by the decreasing steam pressure since too much steam is condensed at the consumers compared to what the steam boiler actually delivers.

The reduction of steam pressure in the system is consequently detected by the control and the pressure sensors in the steam boiler, which initialise heat (more heat) in the boiler for evaporating more steam.

When sufficient steam flow seems to be established, you will have a balance with the consumption of steam (consumers of the system) and the steam pressure will return into a stabile condition. Then when the consumers eventually stop demanding steam, the steam pressure starts increasing - and this detected by the steam boiler control too, and the heat for evaporating steam is then being turned down to a lower level where the new balance will be.

A steam boilers does not work like a machine. It does not impose steam to the system, it only covers the lack of steam that is being consumed by the system.

A steam boiler is an autonomic device. It is purely self-controlled unit and must thus never be manually controlled from safety reasons.

 

The Alternative to Steam

An alternative is, instead of steam, to use a complete different heat carrier - for instance THERMAL FLUID, a special oil where you can operate atmospheric (unpressurised) at temperature above 300C. 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 FLUID OIL VERSUS STEAM.


 

Steam Generator Boiler
versus the "classic" fire-tube Steam Boiler

THE PRINCIPLE IN THE FIRE-TUBE STEAM BOILER, 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.

 


 
   Steam Boiler Operation
AB&CO animations
for PC and Smartphone





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 solutions to prevent this are normally just to place a pressure sustaining valve in the steam line. This valve will protect the steam generator against critically low steam pressure due to uncontrolled high steam consumption beyond its max. capacity. Another solution often used is to install and connect a separate buffer tank next to the steam generator that absorb a majority of steam pressure fluctuations (the demand for extra steam buffer occur in about 10 - 15% of all installations). The ultimate alternative to these two solutions is of course to install instead a classic fire-tube steam boiler, which is less sensitive to steam pressure fluctuation (fluctuation is steam consumption).


 

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

   Easy to operate - simple and safe, normally no requirement for boiler authorisation
 
Rapid start-up - establishing full steam pressure within 10 - 20 minutes depending on size.
 
Compact - easy and fast to adapt in the existing machinery arrangement
 
Price attractive - especially at low steam rates below 1.500 - 2.000 kg/h
 
Safe - small pressure vessel of small dimensions tubes means no risk of steam explosions and thus normally easier to get approved by authorities and insurance companies.





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

   Not able to have even small and short peaks in steam consumptions beyond the maximum capacity. Separate buffer arrangement to be considered.
Not attractive for capacities above 2.500 - 3.000 kg/h where the units approaches the same dimensions as other steam boilers.
 
Not good for steam pressure less than 3 -4  bar since steam velocity gets to high and cause water in the steam.
 
Problems with loads below 50% easily lead to severe pressure variations. Separate buffer arrangement to be considered.
 
Not heavy fluctuating loads below 30 - 40% - leads to risk of operation failure.
 
Cannot be used for turbine operation (requiring production of high-grade superheated steam for instance for production of electricity).

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.

 


Electrical Heating - Option of the Future ?

There is a still increasing demand for ELECTRIC STEAM BOILERS. Particular the small sizes (up to 250 - 300 kg/h steam) are very popular - they are typically very price competitive and very easy to install. No chimney and no fuel arrangement - and very clean. On top of this they are today considered environmental as electricity in many regions comes from non-fossil energy sources. But often the electricity is both very expensive in consumption (kWh) and also if a complete new larger size electrical supply must be establish. Rule of thumb is that each kg/h steam requires 1 Amp (at 3 x 400V).

Electrical Steam Boiler - Industrial Design

 


Electrical Steam Boiler - Commercial Design

 

Optional Steam Boiler / Generator  Design

Beside the standard execution the steam generator boilers can be delivered in for instance following variations:

   ELECTRICAL HEATED in special design
STAINLESS STEEL - all parts in contact with steam made in stainless steel.
HIGH PRESSURE design for special applications up to 190 bar / 350C
COMPLETE SKID-MOUNTED with tanks and pre-treatment equipment.
BUILD IN 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
EXCHAUST GAS STEAM BOILERS (EGSB) or exhaust gas steam generator (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.
 

 




recommends these links for further useful
 "FACTS ABOUT" information :

>> Steam Generators vs. Boilers

>> Thermal Fluid vs. Steam

>> Classic Steam Boilers

>> Steam Generator Boilers

>> Electric Steam Boilers

>> About ProcessHeating.biz

>> Technical Tools for Engineers

 

 

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 Group.


Latest revision :
Copenhagen, 22nd January 2020
by Arvid Blom, Senior Engineer & Partner