What size ducting for cooker hood?

[pyxis]

I am installing a chimney cooker hood with an extraction rate of 480m/hour. Ducting runs 50cm vertically above the fan itself then horizontally 150cm to vent through an exterior wall.
The manufacturer recommends using 125mm pipe ducting (fan outlet connector is 120mm) but I want to use flat ducting.
Obviously I would prefer it as small as possible whilst maintaining extraction efficiency so is 110 x 54mm sufficient or would 150 x 70mm or even 204 x 60mm be best?

 

[seasickstevie]

I suspect you know the answer.

 

[pyxis]

I don't know the answer which is why I'm asking.
I'm presuming it's not as simple as calculating the volume capacity of round duct & using the same volume rectangular duct as perhaps there are other considerations when changing profile eg. length of ducting and number of bends. However as I don't have any experience of this I'd appreciate help from anyone else who does.
Thanks.

 

[RedHerring2]

If you compare the csa (cross sectional area) you'll draw your own conclusions.
125mm diameter (radius = .0625m)
Area = .01227m²

Rectangular section 204mm X 60mm (.204 X.06m)
Area = .01224m²

need I say more?

 

[pyxis]

I had compared csa of 125mm round & 204 x & 60mm & came to the same conclusion.

I then saw this site;
www.i-sells.co.uk/ducting-system-125?page=2

The rectangular profile duct on this page is described as 125mm ducting system & I assume that means ok to use in place of 125mm round. It also says it is suitable for extraction rates up to 650m/hour.
It measures 150 x 70mm & will obviously fit in the cramped space available better than 204 x 60mm.

But this gives a csa of only .0105m...approx. half what I thought I needed!

I am confused!!

 

[mointainwalker]

You should not take what you read as gospel, especially when someone is trying to sell you something.

With this kind of product they can say what they like as it is difficult/impossible to disprove. This pipe will undoubtedly pass this volume of air but at what sound level ?

If the manufacturer recommends 120 mm and this is flexible accordeon tubing ( usually is), then I would expect that the equivalent of 100 mm round, smooth bore would be more than adequate as a replacement.

Also, do not forget that the figs quoted are max extraction and most hoods have 3 speeds, so it will likely be a small proportion of total time you use top speed.

I'm presuming it's not as simple as calculating the volume capacity of round duct & using the same volume rectangular duct as perhaps there are other considerations when changing profile eg. length of ducting and number of bends.

I'm sure you;re right, but i very much doubt if anyone has thought it worthwhile to put any time into studying it.

 

[pyxis]

Thanks, that has clarified things for me.

I have also just found an airflow conversion tool on the i-sells website which, if you enter the dimensions of the rectangular duct, calculates the diameter of compatible rigid round duct for you;
150 x 70mm equates to 115.62mm diameter round
204 x 60mm equates to 124.83mm diameter round
These answers vary from those you'd expect from simple cross section area ratio comparisons so must presumably take other factors into account.

Bearing in mind points made by Mountainwalker, fan spec, space constraints & the above calculation I will go with the 150 x 70mm system.