Duct Sizing for Selected Fan

[titanum456]

Hi guys,
I am lost in calculating the duct sizing for a selected fan.

The fan details based on the fan catalog are :
Requirements :
Volume : 300 L/s
Static Pressure : 50 Pa
Selection Pressure : 50 Pa at std condition
Temperature : 20 deg c

Fan Data (at STP)
Type : Centrifugal Fan
Diameter : 310 mm
Speed : 23 r/s
Weight : 25 kg

Above are the fan details.

I need to build a series of ductwork based on the fan specification. But I dont know what thickness of the duct I should be installing and the duct flanges sizes?

I hope someone can clear this for me.

[B G Scott]

I think you may be starting this from the wrong end.
First what are you trying do with the air?
Is this a supply duct or an exhaust duct?
Normal practice is to design a system to satisfy the design criteria then work out the external Resistance and select a fan to match both the volume flow against your calculated system resistance.
Give us some more information on your intended use, duct layout, numbers of take offs from the duct etc and I am sure we can help you.

[titanum456]

I think you may be starting this from the wrong end.
First what are you trying do with the air?
Is this a supply duct or an exhaust duct?
Normal practice is to design a system to satisfy the design criteria then work out the external Resistance and select a fan to match both the volume flow against your calculated system resistance.
Give us some more information on your intended use, duct layout, numbers of take offs from the duct etc and I am sure we can help you.

Thx for the quick reply.

Anyways, I am basicaaly designing a test rig for an experiment of inlet installation conditions on the performance of the fan.
So, I am currently supplied with the fan.
I have designed an assembly test rig which Ive attached.

In short, this is more of a small scale testing of the fan performance. The noise level and pressure level will be measured in the shown ducts. As you can see from the file, there is an elbow and straight line of duct to the inlet of the fan. I need to determine the design material thickness of the ducts.

[B G Scott]

OK, as I see it your TEST will be influenced by a couple of factors.

1. Do you plan to have a cone on the inlet to the fan, you have not indicated direction of air flow so at this stage I have assumed that the air enters the system in the duct attached to the fan.

2 have you designed the radius of the bend as this will have significant effect on the way the air flow behaves in the duct, let me know what the bend design is, or I will recommend a design.

Get back asap it's Sunday evening at 18:57 here, time for a beer or three.

[titanum456]

OK, as I see it your TEST will be influenced by a couple of factors.

1. Do you plan to have a cone on the inlet to the fan, you have not indicated direction of air flow so at this stage I have assumed that the air enters the system in the duct attached to the fan.
No, there is no cone inserted into the inlet of the fan. It is just connected to the fan by the flanges. And the air enters the system through the throttle and heads all the way to the fan.

2 have you designed the radius of the bend as this will have significant effect on the way the air flow behaves in the duct, let me know what the bend design is, or I will recommend a design.
The bend elbow is already designed.

The bend radius is the same dimension as the circular duct's diameter. So, its basically the same dimension as all the other duct which are connected by the flanges.
Get back asap it's Sunday evening at 18:57 here, time for a beer or three.[/quote]

[B G Scott]

Second time of trying to reply, typed it out and the system gobbled it up.

1, You can get a sheet metal shop to fabricate the duct sections, the lengths are not mentioned on the drawing so am assuming they are short.
They would also fabricate the flanges for assembly with sealant and held together with nuts and bolts.
I would think for your application 20 gauge galvanised steel would be OK.

2, They other option would be to use "Off the shelf" galvanised steel spiral duct which is available in 300mm diameter 3M lengths so you can just cut the sections you need.
The spiral duct manufactures also stock flanges bends and seals.
I don't know the names of any suppliers in your part of the world but I am sure if you do a quick search locally you will find a stockist.
3, You say the bend has been designed, all I would add is that standard design would have the inner radius not less that half the duct diameter.

[B G Scott]

One more point in your post you say that the fan is a centrifugal but your drawing states that it is an axial, which is correct?

[titanum456]

I am designing both the axial fan and centrifugal fan. But the main assembly as shown in the diagram is the same.

May I know how you come up with the 20 gauge thickness value? Did you did some calculation or it just popped in your head?

[B G Scott]

I am designing both the axial fan and centrifugal fan. But the main assembly as shown in the diagram is the same.

Ok just interested in what you are going to do with the test results.

May I know how you come up with the 20 gauge thickness value? Did you did some calculation or it just popped in your head?

20 Gauge would be the standard construction material for this size of ducting, you could go down to 22 gauge but it would not be so rigid.

PM me with the results of your test when they are finished, is this a new fan design or are you testing it for a new type of application?

[titanum456]

No, its not a new fan design.

I am just testing the effects of the distance of the elbow to the inlet of the fan. So, I would be adding more circular duct in between the elbow and fan to add the distance of them.

I would measure the noise and pressure readings of the air when they are connected together.

But at the moment, I am still stuck at the designing phase of the duct.

Anyway, now that the thickness of the duct is solved, do you have any recommendation for the material of the flange of the duct. Do you think they could probably be stamped to each other?
You also have to realize that the flange should be able to support the weight of the duct .

[B G Scott]

Get the sheet metal shop to fabricate the flanges from 20 gauge also, the flanges should have and ID so that they can be slipped over the duct and either pop riveted into position or PK screwed.
The actual flange dimension would be large enough to allow the fixing bolts to to be assembled.

The one issue you will have to address is to support the fan, but I guess you have already sorted this.

[titanum456]

thanks heaps man.

certainly helped me on the sizing.

i will post more once I would get some feedback from the supplier.

[B G Scott]

Any time, best of luck.

[Brian_UK]

Info on basic duct flanges types...
http://www.denhaco.co.uk/Pages/products.htm

[titanum456]

Those links were mainly for rectangular ducts.
I am looking for round duct flanges.