Kitchen cooker hood ducting question

[swilli88]

Hi

replacing an integrated cooker extractor hood. The new one wants a 125mm extractor ducting but the old one was 110mm. Am I ok to just reduce down and run with the old ducting. It’s straight out the wall no run on the duct really at all. Will the impact of reducing be severe for this one?

 

[Mike13]

about 60% of the flow rate, but if it is restrictive enough then the extra pressure drop across the fan will make it even less

 

[swilli88]

I’ll run with it for now and see how it goes. The bigger size would need a whole new hole in the house!

 

[Wayners]

Makes hood noisy reducing pipe size.
Also because its a centrifugal fan vs standard fan, it creates an air pressure which through the reduced gap causes strain on the motor.

 

[RandomGrinch]

The reduction in area, is only 20% between the two sizes.
There won't be any reduction in flow rate, although there will be an increase in the air velocity through the narrowed section, hence some (maybe unnoticeable) increase in noise on a short pipe run.

Basically, go for it!

 

[swilli88]

The reduction in area, is only 20% between the two sizes.
There won't be any reduction in flow rate, although there will be an increase in the air velocity through the narrowed section, hence some (maybe unnoticeable) increase in noise on a short pipe run.

Basically, go for it!

That’s the spirit! that’s what I did anyway thanks

 

[jj4091]

The reduction in area, is only 20% between the two sizes.
There won't be any reduction in flow rate, although there will be an increase in the air velocity through the narrowed section, hence some (maybe unnoticeable) increase in noise on a short pipe run.

Basically, go for it!

I tend to have the same opinion as you with respect to the effect it has. There seems to be differing opinions, but one has to wonder where a 40% reduction figure is arrived at.

 

[Mike13]

Flow rate through ducting and piping depends upon the ratio of the two radii raised to the 4th power, go online and look it up on any of the online piping and ducting professional websites. I forgot it was radius and used 4th power of the diameter instead, so it will be much worse than I suggested:

Flow Rate Vs. Pipe Size

According to Poiseuille's Law, the flow rate through a pipe at constant pressure and temperature varies with the fourth power of the pipe radius.

sciencing.com

 

[jj4091]

That is talking about the flow of liquids, if you do similar research for air it tells you that the pressure through the reduced section just increases rather than the flow rate decreasing.

 

[bernardgreen]

Even when turbulence is a factor, you can still use Poiseuille's equation to get a reasonably accurate idea of the how flow rate changes with pipe
diameter.

Turbulence will have a significant effect where the length of straight ducting is small compared to the total length.

 

[RandomGrinch]

Flow rate through ducting and piping depends upon the ratio of the two radii raised to the 4th power, go online and look it up on any of the online piping and ducting professional websites. I forgot it was radius and used 4th power of the diameter instead, so it will be much worse than I suggested:

Flow Rate Vs. Pipe Size

According to Poiseuille's Law, the flow rate through a pipe at constant pressure and temperature varies with the fourth power of the pipe radius.

sciencing.com

I'm afraid you are making an incorrect assumption.

The Hagen–Poiseuille equation is relevant when the pipe length is significantly larger than the pipe diameter.
It is relevant for designing long pipe or duct runs and is used to determine the pressure drop, due to the viscosity of fluids.

In this case...

It’s straight out the wall no run on the duct really at all.

.... without any significant back pressure, flow rate in = flow rate out, accompanied by an increase in air velocity through the narrowed section - although there will also be added turbulence to the air flow.

... And here's an online calculator (some assumptions have been made - please correct me if I'm wrong! )...

I make that a pressure difference of only
0.044 Pa (4.4x10^-7 bar) between the two pipe diameters.

 

[Mike13]

That makes a pressure change of close to 40%, doesnt it. the flow does the same with gases, just the boundary conditions and typical pressures that change

 

[RandomGrinch]

That makes a pressure change of close to 40%, doesnt it. the flow does the same with gases, just the boundary conditions and typical pressures that change

Yes, a pressure increase of, well, closer to 67%.

That's still not flow rate though...

about 60% of the flow rate,

A 67% increase on a pressure differencial of 0.06 Pa, is still next to nothing...

 

[Mike13]

That depends upon what the fan can supply doesnt it. A bit different if its a 2000 bar pump or a pancake fan that only puts out a few millibars at best

 

[RandomGrinch]

That depends upon what the fan can supply doesnt it. A bit different if its a 2000 bar pump or a pancake fan that only puts out a few millibars at best

Yes, it would depend on what the fan can supply.

My IKEA UTDRAG at maximum efficiency has a static air pressure of 232 Pa and an air flow of 194m³/h.
Yes, that is only 2.32 mbar.

I may be very wrong, so please let me know why, but if I understand you correctly..

about 60% of the flow rate, but if it is restrictive enough then the extra pressure drop across the fan will make it even less

I forgot it was radius and used 4th power of the diameter instead, so it will be much worse than I suggested

You are suggesting that the flow rate would reduce from 194m³/h to 116m³/h (or more),
Due to an extra (from my previous example) 0.044 Pa of pressure (when static pressure is 232 Pa) caused purely by the viscosity of air travelling through a short 110mm pipe compared to a 125mm pipe.