Duct sizing

[horseysi]

Hi folks,

I am having a bit of a brain ache here. I know that this may be a simple question, but I seriously do not know the answer.
I have a fan with an inside diameter of 700mm. The duct-fan motor width is 200mm, so to work out the necessary duct size do I just have to subtract the 200 from the 700 to give me a duct size of 500mm?
I cannot run the fans to work out the correct airflow, but the fan does have 295M3/1' on the label.
I'm not that up to scratch on duct sizing, so any help at all would be of great help!

[horseysi]

By the way, I want to reduce the size as much as possible without causing any issues to airflow or noise.

[frank]

Duct sizing does not have anything to do with the physical size of the fan but with the amount of air the fan produces.

You also need to decide which type of duct system you want - low, medium or high velocity, as this determines the air speed you design to.

Too small a duct for a large volume of air and you create large static pressures which then requires a large fan to overcome them, and if you oversize the duct which results in low velocities, you can then suffer from poor distribution at the terminals.

Like most things, there is a science to good duct design which takes training, effort and experience.

Not an easy thing to explain on here in a short simple reply I'm afraid.

[eggs]

.....................

[B G Scott]

I sent this link the other day to some one else looking for duct design information.
Its a link to the whole book so don't be overwelmed by the early chapters, scroll through to the simple duct design options I am sure it will help.
The rest of the book will stand you in good stead for future reference both to ductwork design and also the understanding of fan engineering in general.
I also note that you don't mention what type of fan you have, prop fans are not designed for operation with large external resistance for example, axial and centrifugal both forward and backward curved are suited to this type of application.
If you can provide more details about the fan I am sure this forum will provide the help you need.
www.flaktwoods.com/34bd869d-e1da-4f17-9209-8bb33b2ffa8a

[TRASH101]

fan does have 295M3/1' on the label.

That information, if correct, I would take as 295m3/1" as 295m3/h @ 1" water gauge (250pa).
This alone doesn't tell you more than that. Ideally you would need the complete fan curve chart.

[horseysi]

I should say, this isn't for a AC system. It's the exhaust system already in place on a wooden Dow. The problem is that the Engine room ventilation exits directly onto the main deck where guests are collected. This creates noise and excessive heat. As a way to overcome this problem I'm simply going to fit a small amount of ducting, incorperating a silencer and carbon filters and 'push' the exhaust over the side of the ship. We are talking about 2.5m of ducting.
This was a problem that was overlooked in the build and needs to be addressed.
There seems to be no information about the room space, or climate condition therein. Unfortunately I have nothing to go on here. I could simply extend the ducting in the same diameter as the fan, however, in the interest of taste, I want to reduce it if possible. So I figured that if I remove the motor diameter from the overall fan size, it shouldn't effect the condition too much. Does that make sense?
Thanks for helping, and please keep the thoughts coming.
Si

[B G Scott]

I should say, this isn't for a AC system. It's the exhaust system already in place on a wooden Dow. The problem is that the Engine room ventilation exits directly onto the main deck where guests are collected. This creates noise and excessive heat. As a way to overcome this problem I'm simply going to fit a small amount of ducting, incorperating a silencer and carbon filters and 'push' the exhaust over the side of the ship. We are talking about 2.5m of ducting.
This was a problem that was overlooked in the build and needs to be addressed.
There seems to be no information about the room space, or climate condition therein. Unfortunately I have nothing to go on here. I could simply extend the ducting in the same diameter as the fan, however, in the interest of taste, I want to reduce it if possible. So I figured that if I remove the motor diameter from the overall fan size, it shouldn't effect the condition too much. Does that make sense?
Thanks for helping, and please keep the thoughts coming.
Si

OK but tell us what type of fan you have, we can I am sure help is it an axial or a prop or centrifugal.
You say you intend to fit a silencer that makes me think this is an axial 2 pole, which are screamers.
Let us have some more information to help you.

[Brian_UK]

Also, check that the extract fan won't affect the engine operation.

[refcon32]

If you reduce the size of duct the speed will increase and also the noise will increase with this. Ideally you need to open the duct size out bigger than fan and also take into mind area loss of silencer, if the silencer baffling is in air flow. You would also need to open out for filters area loss.