Inline extractor fan placement in duct run. 2

[acrmnsm]

I wonder where is the best place to locate an inline fan in a kitchen extractor design?
Its 5m run of 150mm dia round duct.
Layout is like this:

From kitchen hood 400mm straight through wall.
90 degree bend to vertical 2.5m length vertical
90 degree bend to horizontal 2m run to outlet.

My gut feeling is that it would be best to put the inline fan about halfway along the run - to maximise suck and blow? However this is not my area of expertise. Or does it matter?

 

[Compositepro]

Ideally the fan should be at the end of the duct because you do not want positive pressure in the duct. That would result in any leakage spreading fumes and odors.

 

[acrmnsm]

That is a good point.

 

[GT-EGR]

Aside from Compositepro's good point, you also want to consider a location that is accessible for maintenance/service/replacement, has an available power supply, and if possible has a straight inlet and discharge duct on either side to maximize airflow (if you want that). Also the further away you are from the opening, the less you will hear the fan.

 

[acrmnsm]

Thanks GT-EGR - luckily all of those coincide with the end of the run as Compositepro suggested so serendipity rules here. Pretty rare..

 

[LittleInch]

However a fan can "blow" much better than it can "suck" You are unlikely to get more than a few mms of water guage below atmosphere at the fan inlet. Place the fan at the end and all you have to generate flow is those few mm.

Positive Pressure on the outlet of the fan could be 5 to 10 times the negative pressure.which will give you much more flow.

Try to get some data on your particular fan.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Compositepro]

Fans create a differential pressure, they do not know if they are sucking or blowing. The only difference is that the air leaving the fan is very slightly higher in density, and will therefore create slightly less friction or pressure drop when flowing through a duct. Ambient air pressure pressure is about 32 feet of water column (w.c.). Axial fans typically operate at 0.1 inch to 1 inch w.c., so density change is minimal. Squirrel cage blowers typically operate at less than 12 inches w.c.

 

[acrmnsm]

Fan curve is here. It is a two speed fan, so the two lines are for the high speed and low speed.
From here.

https://www.airflow.com/AIR1/AIR1-SHOP2/PDF/Commercial fans pdfs/Aventa In-line.pdf

 

[LittleInch]

Compositepro - True, I tend to think of bigger units.

It is actually quite difficult to find any data as I understand most fans are tested on the basis of an open inlet to air.

Any fan / blower / compressor though that can put out more than 10 to 20kPa differential you will see a difference between the end or the start of a duct. Domestic duty ones like this I'll accept it shouldn't make any appreciable difference to the air flow whether you put it at the start, the midpoint or the end.

This fan is approx. 0.8" water guage (200Pa)...


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[acrmnsm]

To be honest I have difficulty understanding the meaning of the fan curve test.

Based on the curve, with airflow being the independent variable on the graph, I assume they control the airflow and that affects the pressure created at some opening? So this airflow variable represents varying system resistance, which affects the pressure the fan can generate?

(edit apologies for sounding like a student but I am a materials scientist and I have forgotten large aspects of my fluids course, graduation was 23 years ago...)

 

[LittleInch]

The fan curve is the power side of the equation.

Your system ( the duct) will create differing levels of friction / resistance to flow which varies as a curve with flow. Each system (length, diameter, other losses such as filters or louvres) will be different.

Where the two curves intersect will be your flow.

If you want a certain minimum or maximum flow then you need to design your ducting accordingly to get the two parts to match.

https://mechguru.com/machine-design/understanding-fan-curves/

or similar - search "axial flow fan curve"

Most axial fans like to operate in the right hand 50% of the curve. So your linked fans quote a range of 400 - 520 m3/hr for the high speed case

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[lilliput1]

You must flow the codes on kitchen hood exhaust system design.