A typical pressure drop for large silencers... ?

[mammut74]

Hi all,

Thanks for reading this, I am a newbie in Silencers, and I hope for some pointers.

Is there a 'typical' or rule of thumb value for pressure drop across a large silencer? It will be placed after an ID Fan, before the stack, with a maximum flue gas flow of ~100,000 Nm3/h. It is for a power plant.

I Googled and came up with the following formula:

http://www.eiwilliams.com/steel/index.php?p=FanSilencers

Pressure drop = C x ( V/ 4005)^2 x (530/ {T+460}) * ({ P+14.7} / 14.7)

where C = 0.85 (pressure drop coeff),
T= temp, deg. F (= 390 deg. F)
P= pressure, psig

with this I got a low pressure drop of ~0.4" WG.

I am not asking for anyone to check my work, but I would like to know if the 0.4" WG is realistic for such a large flow?

Is there any considerations that I may have missed out? Thanks

Kai

 

[Hatch]

The basics of silencer selection are as follows:

Higher Insertion Loss = higher pressure drop.
Longer Silencer = higher pressure drop.
Higher low frequency performance = Longer Silencer
Higher blockage = Higher velocities in silencer (passage velocity)
Higher Passage Velocity = higher pressure drop.

The formula you used is valid for a particular silencer design which has a corrosponding attenuation curve which I don't see, (mentioned but not shown). Each attenuation curve (silencer type/design) will have a unique Silencer pressure drop coefficient. Your calculated pressure drop (which I did not check) may be correct but the silencer insertion loss, designed by the vendor may not meet your requirements. You have to determine your insertion loss requirements first (and hence the corrosponding silencer pressure drop coefficient) before the pressure drop is calculated.

Putting it another way, a straight duct section would provide an almost zero pressure drop but also zero attenuation. The other extreme is to block the duct completely. You then have zero noise propagating down the duct but a back pressure equivalent to the maximum pressure which can be generated by the fan. You want something in-between.

 

[MikeHalloran]

I'm used to buying exhaust silencers for Diesels between 6 and 64 liters displacement, where 10"wc would be a low- restriction, noisy silencer, and 20"wc would be a quieter or smaller silencer, and about all I could stand to allocate from the backpressure budget.

Larger, locomotive- size Diesels have lower backpressure budgets, say 5"wc total, and effective silencers for them tend toward the size of the locomotive.

In my limited world, 0.4"wc seems like extraordinarily low backpressure for a silencer.

Powerplants are surely different.



Mike Halloran
Pembroke Pines, FL, USA

 

[SylvestreW]

well, 100K m3/h is about 58K CFM. That's a very high flow.

You didn't state what the dims of the stack exhaust are but I'd assume (hope) it's quite tall.

We did a biofilter project once with 6 fans ducting into 1 stack, each fan ~40K cfm (~40K cfm total). Stack was ~100 ft tall. each fan (on the suction side) had about 8-11" H20. The silencers we used weren't the "plug" type (i.e. no centre core), they just had a wrap on the inner dia. of the stack. The attenuation was minimal (but all we really needed, plus there wasn't budget for more pressure loss). Those silencers were about 0.5"

In general, anything off the shelf usually has a much higher pressure drop (>1 to 2").
I've seen custom made systems that were designed for ultra low drop but it was $$$ (and done by a specialist).

Gut thought is that 0.4" is too low, but you'll need to describe the silencer.

 

[SylvestreW]

edit:
meant ~250K cfm total.

 

[pat0]

0.4" isn't impossible provided the face velocity is low enough. The pressure drop of a silencer is directly related to the free area of the baffles and the exit configuration. (ie Blunt, rounded, or flared)

A silencer for that application isn't an "off the shelf" unit. It typically is custom fabricated along a standardized design.
For a given application the final design is governed by the required attenuation and the allowed pressure drop.

This form

http://www.vawsystems.com/media/files/ssrform.html

shows most of the information required by a silencer vendor to provide an accurate quote.

Regards,

Pat0

 

[mammut74]

Thank you gentlemen for the information and explaination.

Kai

 

[fredt]

A typical ID fan silencer system would have a pressure drop of the order 1.5-2" WG. The silencer face area will generally be much larger than the duct cross-section. This means that losses in the transitions can be almost as high as the silencer.

The above is a broad generalisation. As mentioned previously it all depends on the amount of attenuation and other factors such as silencer type.

Beware of absorptive type silencers on flue gas. They plug up and become much less effective in a relatively short time. Consider reactive-absorptive type.