Modelling of boiler combustion air system 1

[Norvin]

I have an assignment to model an existing boiler combustion air system, in order to advise on fan upgrades that would be required to meet revised air flows that the end user requires

The system is quite complex with several fans and fresh and recircualted air being mixed at certain points

I am looking for suggestions as to a suitbale software to model the existing system and also trial changes in fan / fan impeller size to achieve the revised air flows

My search so far has found software designed for HVAC which I don't think really fits this application

The more powerful software such as PlantFLOW and Hysys are expensive and I think overkill for this exercise.

I am expereinced with using Pipe-FLO software for centrifugal pumps and ideally I am looking for a similar sofware which allows me to enter fan curves and then asses fan duty / performance at different operating scenarios

I could do the exercise via spreadsheet but if I can find suitbale software it would make the whole exercise more efficient

Any suggestions as to suitbale software that is not too expensive? (I have also posted this on the pipework forum!)

 

[chicopee]

So we are not looking at induced draft fans, are we?

 

[Norvin]

Correct , forced draft fans

 

[chicopee]

"I have an assignment to model an existing boiler combustion air system" Since you are already have an existing system, what's the reason to model it when you should have all the paper work (spec sheet, performance test, etc.) from the manufacturer and installer when the system was installed.

 

[Norvin]

For various reasons the majority of the paperwork in not available form the manufacturer/ installer. Also as per orignal post the aim is to revied the air flows (potentially quantity and where the airflow is directed) so once a model is established various modifications can be trialled in the model.

 

[chicopee]

This is the procedure that I carried out with a battery of old Bigelow fire tube boilers. First I had to determine each boiler heating surface (H.S.) and this was stamped on their front tube sheets. If you have skid mounted boilers you may have nameplates fastened to their casings with that information. If you have National Board numbers on the boilers, you can order manufacturer's data sheets (on which the H.S.'s are noted) from the National Board for a nominal fee; also,ASME power code I and heating code IV also has details on estimating the heating surfaces of boilers.

From the H.S. (in Sq.Ft.) values, you can determine their steaming capacities (S.C.'s) in Lbs/Hr for power steam boilers or heating rate (Btu/Hr)for hot water boilers which are used to rate the relieving capacities of their safety or relief valves. The terms safety valves is used for steam boiler and relief valves is for hot water boilers. The ASME code I and IV has the factors needed to multiply the H.S.(in Sq.Ft.)values to find the steaming rate in Lbs/Hr. Note for a steaming capacities in LBS/Hrs, you must multiply that value by 970 Btu/Lb to convert to Btu/Hr.

From S.C. in Btu/hr, you can determine the heat output of each burner as long as you can approximate the heat transfer efficiency from the firebox to the boiler heating drum and that value is somewhere between .75-.80 with boilers manufactured within recent times (i.e. in the last 40 years or so)) Knowing the burner heat output, you can estimate the burner heat input(in Btu/Hr) as long as you know the burner efficiency and that value is estimated at 80%.

From burner heat input (BHI), you can determine the amount fuel in GPH or CFH to the burner, depending if you burn oil or gaseous fuel. From BHI, you can then determine the amount of combustion air which must include excess air in CFM. Excess air value depends on the fuel burned. If you are lucky enough, burners will have fastened tags or permanent labels showing the minimum and maximum fuel consumption which you can use to determine the combustion air moved by the forced draft fans.

There is an abundance of literature on these subjects. I know that libraries of communities that have or had industrial sectors have books by such author as Shield which you should research for a better understanding of what you are trying to accomplish.

 

[chicopee]

One last not mentioned above is the back pressure that needs to be overcome by the forced draft fans. Without going into details about how these back pressures are determined, you should realize that back pressure will range from about 2 inches of water column (in.w.c.) to somewhere 7 in.w.c.. Again do your research about your boiler. If there are no ME's in your company, get a boiler representative for help.

 

[Norvin]

Thanks , however what you have described has already been carried out ...ie determining the required increased airflow and where it should in theory be directed. From my several years of experience as a professional engineer in boilers and industrial ventilation and air pollution control, the fundamentals such as including back pressure are not an issue. My question related to advice as to appropriate software for the modelling of the FD fan ductwork to determine the new duty points for the FD fans. Any advice from users out there?

 

[chicopee]

You may have to write your own spread sheet or your own program since it appears that you already have all the information.