Project Description

Bellmouth Reducer

Bell mouth reducers have a smaller opening in the end of a duct system. The shaped of  bell mouth reducers looks like a bell shape. Fabric expansion joints are sometimes used in a duct system. We also supply ducting expansion joints. Bellmouth reducers are primarily used to return air, or extract air in a building ventilation systems. The Bell mouth reducer open area is normally double that of the duct area. Therefore, the air entering the bell mouth reducer is low. Thus reducing noise, turbulence, and pressure drop. The bell-mouth reducer shape allows the maximum amount of air to be drawn into the duct with minimum loss.

The purpose of bellmouth reducers is to lower the pressure drop at the intake into a duct from an open space. Also bellmouth reducers discharge from a duct into an open space. This is know as shock loss or more generally a fitting pressure loss. The fitting pressure loss for a duct intake from an open space is equal to the velocity pressure. Therefore, it is equal to the the square of the velocity. By increasing the area at the intake you can drastically reduce the pressure drop of the fitting. Also a bellmouth reducer is used to reduce shock losses at entry.

A bell-mouth reducers inlet duct is a form of convergent inlet air duct used to direct air into the inlet of a gas turbine engine. The area of a convergent duct gets smaller as the air flows into the engine. A bell-mouth reducer inlet duct is extremely efficient and is used where there is little ram pressure available to force the air into the engine. Bell-mouth ducts are used in engine test cells and on engines installed in helicopters.

How To Choose A Bell Mouth Reducer.

Know the velocity of air being used for the bellmouth reducers. Also know the well as the size of the duct. And the CFM being handled by the duct section. Next know that bell mouth reducers should be tapered at 45 degrees for smooth entry/exit of air. Also know about the equation Q=Av. Using all this information, how do find the maximum size of the bell mouth reducer?

How to find the maximum size of bellmouth reducers?

This depends on what you want the average inlet velocity to be. Moreover, it’s not so much about size, it’s more about geometry.

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.

If the entrance of the bellmouth reducers velocity then Q=Av gives us the area of the bell mouth reducer entrance/exit section. The other end is off course sized equal to the duct.

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