How to size a bell mouth intake/supply?

[Mech110]

I know about the velocity of air being used, size of the duct, CFM being handled by the duct section. Also the fact that the bell mouth should be tapered at 45 degrees for smooth entry/exit of air. I also know about the equation Q=Av. Using all this information, how to find the maximum size of the bell mouth? I am confused because using this information given cross-sectional area of the duct and hence the area of the bell mouth at the duct side. How to find area at the larger end (where the air enters/leaves) the system? Could any one help with an example?

Thanks to all in advance.

 

[MFJewell]

You could use CFD to check the flow conditions for your exact case, but I believe a general rule is 2:1. The only bellmouth I have used is this one on the suction piping at a sump. It was a standard reducer that I had my installer remove material from.

 

[LittleInch]

"how to find the maximum size of the bell mouth?"

Depends on what you want the average inlet velocity to be.

Typically anything up 4-5m/sec is possible or as low as 0.1m/sec if you just want the air to "drift" in or out. Therefore there is a large range of end area which is up to you and your particular requirements and issues.

Be aware that in reality unless you make a big effort, the velocity across the end area won't be uniform and could easily be 2-3 times as much in the centre as the outer edge.

Entry air is usually a lower velocity as the pressure drop is more critical than exhaust air where you are providing the motive power.

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

 

[Mech110]

@LittleInch "Depends on what you want the average inlet velocity to be." Yes thats what I am asking for. Ok we know te average velocity. 2.5 m/s. (And I suppose its the velocity with which air flows in the duct..not the entrance velocity. Correct me if I am wrong).

So we have the average velocity...How to proceed now?

 

[LittleInch]

No, I was talking about the average velocity at the plane where it enters the bellmouth, not the average speed in the duct.

Many inlets are just the same size as the duct but results in a bit of noise and some losses / turbulence as it enters the duct.

2:1 sounds like a good initial size ratio as it gives you 4 x the area so on average 1/4 of the velocity.

but as said, inlet or out let velocity is up to you depending on your particular circumstances - noise, mixing with the ambient air, entry or exit losses, room available for the bellmouth etc.

E.g. I look at my cooker hood and the area ratio is about 20 because I don't want high inlet velocities at the plane of the hood but a small discharge pipe. but the exit expansion is much smaller.

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

 

[capliard]

A bellmouth is used to reduce shock losses at entry. It's not so much about size, it's more about geometry. Say your fan is 54'' inches diameter, then contact supplier asking for assorted inlet.

Check-out McPherson Mine Ventilation Chapter 5 in the appendix(free download on the net) for Shock Losses calculation

Attached : Typical Geometry

Ingenieur Minier. QuTbec, Canada.

 

[Mech110]

Thank you all! got it now! if it is the entrance velocity then Q=Av gives us the area of the bell mouth entrance/exit section. The other end is off course sized equal to the duct.

 

[lukaiENG]

The purpose of a bell mouth is to lower the pressure drop at the intake into a duct from an open space (or discharge from a duct into an open space). In Capliard's terms a 'shock loss' or more generally a fitting pressure loss.
The fitting pressure loss for a duct intake from a plenum is proportional to the velocity pressure and therefore proportional to the the square of the velocity. By increasing the area at the intake you can drastically reduce the pressure drop of the fitting.
Consult your ASHRAE handbook for reference fitting pressure loss data for more details.