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.

Eventually it leads to significant advantages by making the right choice.


 
   
 Dansk Version

by Arvid Blom,
Senior Engineer & Partner
at AB&CO

        

          FAST LINKS TO DETAILS :

 

        Go directly to :

        CLASSIC
       
FIRE-TUBE
       
 STEAM BOILERS

Go directly to :

STEAM
GENERATOR

BOILERS

        Go directly to :
  
          
ELECTRIC STEAM BOILERS
 


        Go directly to :
   
          
EXHAUST GAS BOILERS

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 eventually condensates also) 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 pdf-file).


 

     AB&CO Steam Table
 



During the evaporation (and condensation) process the pressure and temperature are maintained constant (isobaric). During this thermodynamic phase 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 a kind of a released "flying" vapour phase. At this process the steam is "wet" in different degrees until all water 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 (beyond evaporation temperature) as common known gaseous matter are at normal known temperatures (like for instance air and natural gas).

At this point - the dry saturated condition - the steam 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 - as previous mentioned - so-called superheated steam, and actually it becomes an ordinary gas like air and gas, that can have any temperatures independent of the pressure - and where heating just makes the molecules moves faster and energy level increases, at same 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 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 does not become wet but stay as high quality dry saturated steam at the location where you need to use it.

Only boilers for saturated steam and steam heating systems are discussed in the following. Boilers and systems for superheated steam for other different thermo-dynamic applications is quite a different subject that is covered by other literature.



 

The Steam Supply

In steam heating system, the steam boiler (including the steam generator boiler) is connected to the consumers by 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 when 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 in steam heating applications are 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) - i.e. before the condensate is eventually returned to the feed water tank (which are normally atmospheric or low pressurised) and the steam boiler.

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 (whether it might be a heat exchanger or a vessel of any kind).

This gives the well-known large "condensate heat loss" in the steam system, i.e. high-energy flash steam being generated and quite noisy led into the condensate line and back to the atmospheric tank where is steamed up into the ambient.

This mass loss of flash steam also represents corresponding physical and expensive loss of the feed water content, which then requires constant amount of fresh and pre-treated make-up feed water added 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 flash steam heat recovery solutions when designing and adapting the steam system into the relevant application processes.

The heat and feed water losses can not only be reduced, sometime its fully eliminated by investing in "smart" 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.

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 established.

A steam boilers does not work like a machine. It does not impose steam to the system, it only compensate 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 by others from safety reasons.

 

The Alternative to Steam

An alternative to steam for heating purposes, to use a  heat carrier without evaporation and condensation. Most known is of course a conventional hot water system. However fr industrial processes another more interesting alternative is HEAT TRANSFER FLUID (HTF) - a special thermal 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 using this link :
THERMAL FLUID OIL VERSUS STEAM.


 

Steam Generator Boiler
versus
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 so-called 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 very small total water and steam volume i.e. a small pressure vessel. 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 often 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. But another solution 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). This will give some of the advantages of the fire-tube steam boiler. The ultimate alternative solution is of course to install a real fire-tube steam boiler instead, - less sensitive to steam pressure fluctuation (fluctuation is steam consumption).
 

The Advantages using a steam generator compared to fire-tube 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.
 
More safe due to small pressure vessel of small dimensions tubes. 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 fire-tube steam boilers are:

   Not able to have even small and short peaks in steam consumptions beyond the maximum capacity. Separate buffer arrangement to be added.
 
Not capacities above 3.000 kg/h
 
Not steam pressure less than 3 -4  bar meaning that steam velocity gets to high. If so, the use of pressure reduction station is required.
 
Loads below 50% easily lead to severe pressure variations. Especially when gas operated (requiring time for venting before start-up). Separate buffer arrangement to be added.
 
Not heavy fluctuating loads below 30 - 40% - leads to risk of operation failure.
 
Cannot be used for to produce steam for turbine operation (requiring high-grade and typically very superheated steam).
 
Will always have a little water moisture (water content) in the steam. If application is sensitive to this, a steam separator is to be added,

 Steam Generator Delivery & Options

Steam generator boilers can be delivered in horizontal execution (with low height), or in vertical execution (occupying limited floor space). Like the fire-tube 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 2014/68/EU code and EN-standards for steam boilers.

 


Electrical Heating - Option of the Future ?

Today there is an 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. They are today considered environmental as electricity in many regions increasingly comes from non-fossil energy sources. But often the electricity is both very expensive in consumption (kWh) since price for distribution of electricity and taxes are still very high worldwide. But also if a complete new larger size electrical supply must be establish to obtain the requested steam capacity. Rule of thumb is that each kg/h steam requires 1 Amp (@ 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 EX-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/gas-fired burners and by electrically heating. The steam boilers can also be design as so-called recuperators utilising the substantial amount of waste heat in for instance 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.

 

 


 

 

 

Important Legal Announcement

This article including all illustrations are made by AB&CO and must be considered legally as property of AB&CO. It can freely be referred to, but must not be copied in parts or in whole without written permission by AB&CO Group.


Latest revision :
Copenhagen, 8th February 2021
by Arvid Blom,
Senior Engineer & Partner


 




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