Venturi effect at fan intake

[TAB TECH]

Could someone explain the method of testing (verifying) a venturi effect at the intake side of centrifugal fans?

[frank]

I don't think I understand you question about testing the Venturi effect, but a venturi is a tapering section within a ductwork or pipework length where the fluid flow is "squeezed" because the cross sectional area is reduced. This has the effect of increasing the velocity, as the same volume of fluid passing through the preceeding ductwork or pipework now has to pass through a smaller cross sectional area.

As P1V1 = P2V2, the corresponding pressure in the smaller cross sectional area will increase so the change in pressure can be measured. How this relates to your question, I'm not sure.

[abcdefg1675]

this is a simple one to answer. it works just like the expansion valve on an airconditioner.

in a Venturry, like in a carburator, gasses are compressed. then the throat of the venturry widens out, creating lower pressure and then fuel is drawn from the fuel bowl and mixed with the air.

on old model "T" fords, the venturry would frost up because of the venturry effect. when a gas is compressed, allowed to cool, and then decompressed, it cools (just like an airconditioner).

Im not shure why their would be a venturry on the intake side of a fan. i dont know anything about that. maybe to cool the air?

[RogGoetsch]

Let's see if I can remember this. Like Frank, I'm not sure what you're asking because it doesn't have much to do with the intake of a fan. But if you want to measure the Venturi effect, here's how:

In any fluid stream, there are two pressures present. The velocity pressure is the pressure exerted by the flowing mass in the direction of flow but it can't be measured alone. The second pressure is the static pressure which acts in all directions, and is the pressure that is exerted by the fluid on the walls of the duct or pipe.

The measurements need to be taken in a long enough section of tube or duct so that the flow stream is smooth, not turbulent. (Near the intake of a fan you will get interesting results but not because of the Venturi effect.)

If a sensing tube is placed in the stream parallel to the flow with an opening facing upstream, this probe will register the total pressure of the system, both velocity pressure and static pressure.

If another probe is inserted at right angles to the flow, this will register only the static pressure.

If the two probes are connected to a device (a manometer) so that the pressures oppose one another, the effect is to subtract the static pressure from the total pressure picked up by the first probe and the resultant reading gives the velocity pressure.

Now, as the cross sectional area of the duct or pipe is gradually narrowed and then gradually enlarged again to create a venturi, the same two measurements are taken within the restricted portion, and compared to the original readings. It will be discovered that the total pressure, picked up by the first probe is exactly the same in both sections, as Frank's formula shows.

The velocity pressure will have increased proportional to the increase in velocity and the static pressure will be found to have decreased by the same amount. This is the Venturi effect.

Of course, you can just measure the change in static pressure in the two sections, but the principle at work is of constant total pressure while changing the ratio of velocity pressure to static pressure.