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Specifying Steam Generators,Waste Heat Boilers  V.Ganapathy

Steam generators and waste heat boilers are critical equipment in any plant and have a life of 20 to 30 years. Unfortunately specifications developed by consultants/engineers often lack clarity and basic process/operating cost information  to design the optimum system. Often operating costs are ignored and equipment are purchased based solely on initial costs,which may be appealing at first but in the long run,it is the plant which loses. Operating costs for Waste heat boilers arise due to gas pressure drop and fuel consumption if fired.A good design optimizes the initial and the operating costs so that the life cycle cost of owning and operating the equipment is minimized. For steam generators, fan power consumption,fuel consumption and maintenance costs (refractory/superheater failures etc) have to be evaluated in addition to initial costs. The completely water cooled furnace design in packaged boilers for example results in lower maintenance costs (compared to refractory filled boilers with associated maintenance and startup costs),though the initial cost may be slightly higher.However over a period of time additional investment in these features pays off. Secondary heat recovery in HRSG systems is not often looked into. See the article on Methods to Improve HRSG Efficiency .
Several specifications have hardly a page or so on process information,while they are filled with hundreds of pages on aspects such as painting,legal mumbo jumbo,reservoir water table etc etc! Here are a few pointers from process viewpoint which I feel should be included as a minimum by engineers involved in developing specifcations for packaged steam generators and waste heat boilers.
 
Waste Heat boiler specifications
The following discussions pertain to waste heat boilers in incineration plants,chemical plants,gas turbine cogeneration systems,refineries and boilers recovering energy from various flue gas sources.
1.Describe the application. Where is the gas coming from? How is it generated? Process or flow diagrams help.
2.Gas flow in mass units such as lb/h and NOT in volumetric units such as acfm,scfm etc. Also provide the flue gas analysis such as % volume CO2,H2O,N2,O2,SO2,Hcl etc. This helps to determine if low or high temperature corrosion problems are likely. Gas specific heat and hence boiler duty,steam generation and temperature profiles are also impacted by gas analysis. Gas pressure is also important. High gas pressure (say above 2 to 3 psig) may call for special casing designs to contain the gas.
3.Nature of the gas,whether clean or dirty or slagging. The boiler design is very much impacted by this. For clean gases as in gas turbine HRSGs or fume incineration heat recovery boilers,extended surfaces can be used to make the boiler compact. If the gas is dirty bare tubes may have to be used,making the boiler expensive and heavy. If slagging,a radiant furnace may be required to cool the gases to below slagging levels before it enters the convection bank. Also soot blowers or other cleaning systems may be required.
4.Suggested gas pressure drop.. This helps one to compare various designs. In small HRSGs 3 to 6 in wc is typical,while in large HRSGS it could range from 6 to 12 in wc. 
5.Suggested exit gas temperature or duty or steam output. This may not always be possible initially. In gas turbine HRSGs and some incineration applications, a simulation using the software "HRSGS" may be performed to arrive at gas/steam temperature profiles. In incineration plants, a scrubber may be used behind the HRSG and hence the exit gas temperature may not exceed say 450 -500 F. In some applications such as sulfuric acid plants or hydrogen plants,the exit gas temperature from the Waste Heat Boiler is dependent on the process downstream of the boiler. 
6.Emission levels of NOx,CO desired. This may require catalysts for NOx,CO reduction located within the HRSG surfaces at the optimum gas temperatures. 
7.Fuel analysis in case auxilliary firing is required in the HRSG.
8.Steam purity requirements in case steam is used for injection in gas turbines or steam turbines. 
9.Avoid specifying surface areas. Surface areas can be misleading. Since energy transferred Q=USDT(U=overall heat transfer coefficient,S=surface area and DT=log-mean temperature difference),unless you know how to evaluate U for each surface,it is not a good idea to look at S alone. By using different velocities,optimum tube diameters or fin configuration,it is possible to transfer more energy with lesser surface area..Redistribution of energy among radant and convective surfaces can also distort the picture.By using a lower transverse pitch for the tubes and with the same surface area,one can transfer more energy at higher gas pressure drop..and vice versa.So don't select a boiler simply because it has more surface area..With finned tubes,by using higher fin densities you can have a higher surface area (and a lower U) transferring the same duty.The tube wall temperatures and fin tip temperatures can run hotter with higher fin densities.(higher ratio of external to internal tube surface). Heat flux inside tubes will also be much higher if a large ratio of external to internal surface area is used can lead to DNB problems or tube failures due to overheating...
10.While using interstage attemperation for superheated steam temperature control,be sure to mention if demineralized water is available. If not then other systems have to be considered such as use of condensate spray obtained by condensing saturated steam..
More detailed discussions may be found in my "Waste Heat Boiler Deskbook". 
Specifying Steam Generators
1.Provide basic steam parameters,fuel analysis,space limitations if any and whether the boiler is for continous or standby operation. This may impact the need for an efficient or inefficient boiler. 
2.While specifying steam generators,be sure to mention if deaeration steam is to be included in the steam output of the boiler..this can be a substantial amount if make up is quite large.. 
3.It is not a good idea to select a large boiler capacity and operate it at very low loads..this is moreso if you have a radiant superheater..At low loads,the flow distribution through the tubes and outside the tubes are unpredictable..(if the steam pressure drop is 50 psi at 100 % load,it is hardly 3 psi at 25 % load)..hence flow maldistribution can cause overheating and possible tube failures..With convective superheaters,the prblem may not be as severe but it is better to avoid low loads..also the fan operating point has to be checked at low loads,whether it is capable of stable operation....
4.State emission levels of CO and NOx upfront..You cannot buy the boiler first and then check for emissions later..If flue gas recirculation is to be used to meet the new NOX levels,the existing fan system may not be adequete..also the superheater performance and boiler efficiency will be affected. 
5.Surface areas,again,should NOT be specified. There are several ways of arriving at the same overall performance (by transferring different amounts of energy in the furnace,convection section and economizer). One should look for overall efficiency,fan power consumption and maintenance costs.
6.If steam temperature is controlled using spray attemperator,then demineralised water should be provided or condensate may have to be used.
7.Specifying volumetric heat release rates is not meaningful in gas or oil fired steam generators,though many consultants do this. This is more applicable to fuels which are difficult to burn such as solid fuels or fuels which require some residence time for combustion. Area heat release rate is more meaningful as it relates to heat flux and DNB concerns.
8.Include  cost of fuel,electricity in the specifications and hours of operation per year,so that the boiler designer can optimize the design considering both the initial and the operating costs.
This  applies to both  steam generators and waste heat boilers.

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