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Govind Rao
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Velocity of the Flue Gas

08/21/2012 6:15 AM

What should be the velocity of flue gas in the flue stack, if the volume of flue gas generated is known. The said chimney is for reheating furnace for rolling of steel sections
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PWSlack
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#1

Re: Velocity of the Flue gas

08/21/2012 6:16 AM

Average velocity = volumetric flowrate/duct cross-section area.

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Govind Rao
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#2
In reply to #1

Re: Velocity of the Flue gas

08/21/2012 6:23 AM

Normally, it is advised to be in the range of 15m3/sec to 30m3/sec. How do we arrive at suitable velocity for designing the flue stack?
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PWSlack
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#4
In reply to #2

Re: Velocity of the Flue gas

08/21/2012 6:32 AM

The best bet is to keep it in the range of 15m3/sec to 30m3/sec.

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#5
In reply to #2

Re: Velocity of the Flue gas

08/21/2012 6:49 AM

Just reread the first sentence of your post....

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PWSlack
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#6
In reply to #2

Re: Velocity of the Flue gas

08/21/2012 7:23 AM

Volumetric flowrate is not velocity.

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Learner42
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#9
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Re: Velocity of the Flue gas

08/22/2012 5:58 AM

Velocity should be 500ft/sec to1200ft/sec or approximately 15M/sec to30M/sec
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simonsd
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#10
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Re: Velocity of the Flue gas

08/22/2012 7:27 AM

I am sorry Learner 42 but from which parallel universe have you stepped down

Your conversions are going to cause chaos 1m = 3.281ft

so 500 - 1200 fps = 150 - 365 m/s

or 15 - 30m/s = 50 - 100 fps

Please do NOT offer such confusing information

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Learner42
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#16
In reply to #10

Re: Velocity of the Flue gas

08/23/2012 12:02 AM

Sorry for the slip!. the units for time will be minute and not second. I.e 500Ft/min to 1200Ft/min.
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simonsd
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#17
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Re: Velocity of the Flue gas

08/23/2012 3:13 AM

I am trying to decide if you are just winding me up

500 - 1200 ft/min = 2.5 - 6m/s

The best I can guess that you are trying to say is that

500 - 1200 ft^3/s = 14 - 34 m^3/s

Is this your objective?

Or perhaps the intention is to ensure Mr Govind learns the lesson of double checking anything he reads on CR4 and the interweb in general

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dhayanandhan
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#23
In reply to #2

Re: Velocity of the Flue gas

03/19/2013 10:26 AM

Dear Mr. Govind Rao,

Perhaps you are referring/meaning 15 M/Sec. to 30 M/Sec which is VELOCITY, and 15m3/sec to 30m3/sec. refers to VOLUME since m3 ( M^3) as Cubic Metre is referred by you as mentioned in your Post No. 4 for Reply to Post No.2

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Govind Rao
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#24
In reply to #23

Re: Velocity of the Flue gas

03/20/2013 3:34 AM

Yes it is M/Sec
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Govind Rao
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#3
In reply to #1

Re: Velocity of the Flue gas

08/21/2012 6:26 AM

Normally, it is advised to keep the velocity between 15m3/sec to 30m3/sec. How to arrive at the right velocity in this case?
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PWSlack
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#7
In reply to #3

Re: Velocity of the Flue gas

08/21/2012 7:24 AM

Divide by the cross-section area of the duct. See #1↑.

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#11
In reply to #3

Re: Velocity of the Flue gas

08/22/2012 9:09 AM

(I think that no one here knows what you mean by "right velocity.")

You are giving the volume flow rate of the flue gas. 15 cubic meters of gas per second, as an example. The velocity of this much gas will be very different in different size flue stacks.

Think of it, if you were to flow 15 cubic meters per second through a flue that was 100 meters in diameter the flow would be very slow. The same volume through a flue 1 meter in diameter would be very fast.

NO ONE can calculate the velocity of a flue gas if only given the volume and rate. That is why you have been repeatedly advised to divide the flow rate by the flue cross sectional area.

You can even work out the problem with just units to see how it works.

volume rate [m3/sec] divided by area [m2] equals velocity [m/sec].

I hope this helps.

-A-

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Stedou73ish
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#14
In reply to #3

Re: Velocity of the Flue gas

08/22/2012 3:07 PM

Is this homework?

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#21
In reply to #1

Re: Velocity of the Flue gas

03/19/2013 5:31 AM

Dear Mr.PWSlack,

Your Formula is CORRECT. But in order to decide or arrive at the cross sectional area for the flow, VELOCITY IS TO BE DECIDED.

Then only cross section can be arrived at. Normally 15 M/Sec. is used for Chimney or stack.

DHAYANANDHAN.S
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phoenix911
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#8

Re: Velocity of the Flue Gas

08/21/2012 12:21 PM

remember to use ACSF and not SCSF for calculations

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welderman
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#12

Re: Velocity of the Flue Gas

08/22/2012 9:28 AM

Velocity of gas in a stack(V) = Q/kA where k is the coefficient of velocity(0.3-0.5),Q is the volume of gas to be handled in (cfs), V is the velocity (ft/sec) and A is the the cross sectional area of the chimney or stack (ft*2). If a stack is used (as opposed to a chimney) the velocity will be higher due to the smoother internal surfaces.
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davlamb
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#13

Re: Velocity of the Flue Gas

08/22/2012 10:13 AM

We design steel stacks ASME STS-1-2006. As a point of reference, quoting from section 1.3.2 Diameters, "Velocities in round stack between 2400 and 3600 ft/min are most common. Stacks venting saturated gases sometimes limit velocities between 1800 and 2400 ft/min to reduce entrained or condensed moisture from leaving the stack exit."

Having said this there are several other things that must be considered:

· Shipping limitations

· Structural stability

· Future increases in gas volume

· Local environmental regulation (plume rise)

· Draft

· etcetera

For wet stack design consult EPRI Wet Stack Design Guide TR-107099.

You can also refer to CICIND Model Code for Steel Chimneys

Stack design is not something to take lightly. Please engage an engineer competent in this design area.
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simonsd
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#18
In reply to #13

Re: Velocity of the Flue Gas

08/23/2012 3:28 AM

I have given this a GA but thought the poster might need a bit more clarification

Typical velocity 12 - 18m/s

Velocity with staurated gases 9 - 12m/s

This is low compared with general gas flows in pipes but when you think about a stack the gas plume doesn't usually rise very high so this seems ok.

Future increases in gas volume. This is critical - is there likely to be expansion of the steel processing so that the stack has to vent larger quantities of gas.

If expansion may be needed then you need to size based on low velocities so that when you get more gas you do not exceed the velocity criteria.

If you have a higher velocity then you will need a higher draft (ie the lift generated by warm gases compared to air that causes stack gases to rise) and there are usually limits on the furnace as to the maximum draft.

When calculating flows as noted you MUST use ACFH or Am3/hr that is ACTUAL volumetric flow and not SCFH (Sm3/h) STANDARD flows. Actual flows are calculated at the flowing conditions - in particular for your case the temperature has a big impact on density if you use Standard flows you may have as little as 30% of the actual flow which will completely wreck your calc.

Convert the gas flow from your funace to Am3/sec

Divide by say 14m/s

This will give you an area for the stack in m2

The local regulations on plume rise will limit the velocity / draft in the stack so that the visible plume from the stack isnt too visible. (also remember to allow for future expansion when calculating plume rise)

I am not sure where the figure you are quoting of 15 - 30m3/s comes from. This is a volumetric flow - which maybe the output of the furnace but will reflect the size of the furnace. It would not be normal to size a stack based on volumetric flows as the stack has to handle all the flow from the furnace. I would expect a velocity limit as given above.

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Govind Rao
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#19
In reply to #18

Re: Velocity of the Flue Gas

08/23/2012 3:58 AM

That's it !! thank for your advice.
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Stedou73ish
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#15

Re: Velocity of the Flue Gas

08/22/2012 3:09 PM

This should help.

http://en.wikipedia.org/wiki/Flue-gas_stack

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dhayanandhan
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#20

Re: Velocity of the Flue Gas

03/19/2013 5:27 AM

Dear Mr. Govind Rao,

15 METRES/Sec.is normally considered for flue gas in Chimney or Stack.

DHAYANANDHAN.S
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Govind Rao
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#22
In reply to #20

Re: Velocity of the Flue Gas

03/19/2013 8:16 AM

Thanks!
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kenoniuwe
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#25
In reply to #20

Re: Velocity of the Flue Gas

11/25/2013 9:23 AM

pls sir, how do i calculate for the diameter of the chimney?

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PWSlack
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#26
In reply to #25

Re: Velocity of the Flue Gas

09/17/2024 10:03 AM

There's a lot to think about:

  • The cross-sectional area is the volumetric flowrate divided by the velocity.
  • Assuming it is circular, then the diameter is the positive square root of [4 times the cross-sectional area divided by pi].
  • Then go to the next standard size diameter down.
  • It might need lagging and cladding to keep the tip of the stack at a temperature above that of the dewpoint of sulphuric acid, otherwise it won't last very long.
  • It might need spiral strakes attached to the top so as to obviate the risk of it oscillating due to vortex shedding in the passing wind.
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