×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Generic Boiler Output Formula?

Generic Boiler Output Formula?

Generic Boiler Output Formula?

(OP)
Hello,
I am looking for a little help.  I was asked to find the exhaust flow of a boiler, but I am a little overwhelmed with where to start.  I was wondering if there is a general formula to calculate what the exhaust flow would be?  If there is not, what would I need to be able to calculate this?

Thanks for any help!
Chrissy

RE: Generic Boiler Output Formula?

Approximation:
Takes 1000 BTU to produce 1 pound of steam.
Natural gas contains 1000 BTU/cu. ft,
boiler about 80% efficent, so requires 1.25 cu. ft NG/lb steam.
NG is mostly methane, CH4, so it burns as

  CH4 + 2 O2 = CO2 + 2 H2O

Air is about 21 vol.% O2, and 3% excess air is often used.  Thus, combustion air required is

  2x (1/0.21) x 1.03 = 9.81 x NG volume.

Since combustion equation has 3 moles of gas on both LHS & RHS, for exhaust gas can use (NG + combustion air) volume.  Only need to correct for temperature per gas law.  Best to use the actual exhaust T, but if not available, use 400oF.  The boiler mfr. recommends the stack ID, so from volumetric flow, you get the exhaust velocity.
 

RE: Generic Boiler Output Formula?

You did not mention the size or type of fuel for your boiler. If you want to calculate the flue gas velocity for a fossil-fueled boiler, the EPA provides and describes 3 test methods. Not very enjoyable reading, but if you wade thru the legal BS, there is some good technical information;

www.epa.gov/AIRMARKET/monitoring/flow/rule.pdf

RE: Generic Boiler Output Formula?

(OP)
The only information that I was given is 70 gph of #6 fuel oil is fired into a 250 HP boiler (I am assuming that is BHP).

RE: Generic Boiler Output Formula?

From Perry's Chemical Engineers' Handbook, 7th Edn., pages 27-9 & 27-10, Low sulfur No. 6 F.O. contains 87.26 wt% C and 10.49 wt% H, and weighs about 7.144 pounds/gallon. (I've neglected other components).
Calculate the O2 and thus the air to combust 70 GPH of this to the stoichiometric amounts of CO2 and H2.
Then, subtract the O2 and add the CO2 and H2 to the initial air in calculating the volumetric exhaust flow.   

RE: Generic Boiler Output Formula?

Typo above: The F.O. (presuming low sulfur) weighs about 8.187 pounds/gallon & thus contains 7.144 pounds carbon/gallon.

RE: Generic Boiler Output Formula?

(OP)
Do I need to know the molecular weight?

RE: Generic Boiler Output Formula?

Not if you only need to find the combustion air/flue gas when firing a fuel of known C & H content at a constant 70 GPH.  

But, the composition above is a typical analysis for No. 6 Fuel Oil, of 12.6o API gravity [SG = 0.982]. Always preferable to use hard data if available.  The heat content will vary with fuel density per Fig. 27-3 in Perry's, 7th edn.  This automatically factors in MW and C/H ratio.

RE: Generic Boiler Output Formula?

(OP)
Thank you for all your help!  I found a copy of Perry's Chemical Handbook in the office so everything is much clearer now!

RE: Generic Boiler Output Formula?

(OP)
Does 1,600 cu ft/min sound realistic??  I am not familiar with what type of results to expect, just wondering if I was in the ballpark.

My Solution:
I took the percentages given in Table 9-11 and inserted them into eq 9-13 to get 35.54.

I then did the following:  
70 gph x 150000 BTU/gal x (1/18230 BTU/lb) x 35.54 cu ft/ lb fuel x 1hr/60 min

I am not sure if this is right, but the units worked out...

By multipling and then dividing the heating value, did I end up leave something out?  This is the only glaring flag that I can see.  My main problem was trying to get the units in cu ft/min, I didn't think I could do a straight conversion since gallons are typically a liquid volume.

RE: Generic Boiler Output Formula?

Looks as if you took the fuel's heat content & divided by the heat of formation of CO2 [(Eq. 9-13) in Perry's 6th Edn.] to get gas volume?

I came out with a higher flow by the old-fashioned method:

For the combustion of 1 gallon of fuel oil of composition above:

From C:  7.144 lbs/12  =  0.5933 lb·mol C requires 0.5933 lb·mol O2 creating 0.5933 lb·mol CO2
From H: 0.8588lbs/2 = 0.4294 lb·mol H2 requires  0.2147 lb·mol O2 creating  0.4294 lb·mol H2O

Total O2 required is 0.8080 lb·mol, so N2 of air is 0.8080x (79/21) = 3.0396 lb·mol N2.  Also,  N2 of combustion air =  N2 of exhaust.

Molar exhaust flow (CO2 + H2O + N2) = 4.0623 lb·mol per gallon fuel.
Assume ideal gases & vapor.
At 32oF (273.15 K) and 1 atm pressure, 1 lb·mol occupies 359.0 ft3
At final T = 400oF (477.59 K) and 1 atm, 1 lb·mol occupies 627.7 ft3

For 70 GPH, then 70 gal/hr x (4.0623 lb·mol/gal) x (627.7 ft3/lb·mol)  = 178,493 ft3/hr
Dividing by 60 gives the exhaust flow as 2975 ft3/min at 400oF.

A small excess of air brings the actual exhaust volumetric flow to 3000 ft3/min.  
An exhaust stack cross-sectional area of 2 ft2 gives a linear flow of 1500 ft/min = 25 ft/sec.

RE: Generic Boiler Output Formula?

(OP)
Hmm, I only have the 5th edition Of Perry's Handbook.  I will have to try and find a later addition since this one only goes to chapter 25.

Thank you for your help, I will try and work through the solution you provided and see if that will work for what I need.  

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources