5 replies to this topic

[lookingtolearn]

I have seen the following equation, but I question the source:

L = 0.00038*Q/D

L - length
Q - capacity, (Btu/hr)
D - diameter, (inches)

If you have a graph that shows how the efficiency changes based on the firetube length and heat input, that will be helpful too.

There is no tube bundle or heat exchanger involved.

Thank you!

[Root]

Hi,

To find out the Tube length and diameter you need to calculate:

Q=UA (T2-T1)

From this equation, once you calculate the A then you can calculate the tube length and dia.

Hope you will get it.
Cheers

Toor

[lookingtolearn]

Thanks Toor!

Question:

Q = capacity
delta T = temp change
A = area
U = ???

[Qalander (Chem)]

Dear

U = Heat Transfer coefficient i.e. relates to the property of material/system involved.

[djack77494]

LTL,
Your original equation,
L = 0.00038*Q/D
looks to be a design equation that is based on limiting the heat flux through the surface area of the tube. The other equations suggested to you are entirely different; they are the basic heat transfer performance equations.
Doug

[Propacket]

lookingtolearn,

djack77494 is right. Your equation is dedicated solely for sizing of the fire tubes based on 10,000 Btu/hr.ft2 heat flux. You can find it in the following book:
Surface Production Operations, Volume 2, 2nd edition
By: Ken Arnold & Maurice Stewart
However, this book does not explain how the equation has been derived.

If you have calculated the fire tube diameter, you can also calculate the length of fire tube using this simple formula:
Surface Area (ft2) =Heat Duty (Btu/hr)/Heat Flux (Btu/hr.ft2)
Surface Area (ft2) = pi x diameter (ft)*length (ft)
Length (ft) =Surface Area (ft2)/ (pi x diameter (ft))
You will find that for 10,000 Btu/hr.ft2 heat flux, length calculated by the above equation will be fairly in agreement with your design equation.
Thanks