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velocity cone design

velocity cone design

(OP)
A velocity cone (or dilution cone) is used to increase the velocity of exit stack gases by use of the venturi effect.  How does one design this kind of cone, and are there any text book references anywhere?

RE: velocity cone design

WHAT ARE YOUR GENERAL DIMENSIONS, PRESSURE, AND VOLUME? THAT YOU ARE WORKING WITH? I WILL LOOK FOR THE LINK ON AN AUTOMOTIVE FORUM I BELIVE IT IS ON HONDA-TECH.COM IN THE WELDING AND FABRICATION FORUM.  AUTOMOTIVE INDUSTRY USES VELOCITY STACKS OFTEN AND THERE IS A THREAD THAT I READ RECENTLY THAT SHOWS HOW TO MAKE THEM AND THE MATH TO USE TO CALCULATE.

RE: velocity cone design

(OP)
TEAMKLR2BAR,

Thanks very much for your response, I would be very interested in knowing more about that reference.

I cant give you many more details about the type of flow etc. I need this calculation to calculate exit velocities for a variety of stacks.  We perform environmental studies that include the exit velocity of industrial stacks.  Usually I just use the velocity cone exit diameter to calculate the velocity based on mean flow, but sometimes there is so much pressure build-up, that the gaz is forced out the underside.  In order to assure the government that the correct minimum exit velocity of the cone is met, I need to be able to prove that the velocity cone is effective.

Here is some data from a company I studied recently:

Velocity cone height: 4 ft
Diameter of stack: 34 in
Exit diameter of cone: 25.5 in
Gas flow: 12 388 cfm

Normally, this gives an exit velocity of 3493 fpm, but there is a some pressure overflow to the outside of the cone.

What Im looking for, is a way to design a velocity cone, to ensure that there is no pressure overflow, and thus to be certain of the exit velocity.

RE: velocity cone design

In the Uk we design the 'efflux' cone to give an exit velocity of about 15m/sec which equates to 2880fpm.

tests show that the natural buoyancy effect of the hot rising gas actually has more effect on the dispersal of the effluent gas, however the recommended is 15m/sec regardless.

The cone should be designed with a slow taper to reduce pressure drop and an arrangement as shown in the American Confereration of Industrial Hygienists should be used. (I think thats the right name for it anyway)

A drain arrangement needs to be incorporated in case rain gets down the flue.

One arrangement is to put a 'sleeve' around te exit cone or install a drain at the foot of the stack.

Friar Tuck of Sherwood

RE: velocity cone design

(OP)

Thanks for the interest,

Im looking for a design approach with exact figures.  I need to be able to prove that the angle and the exit diameter do not cause any overflow of air out from the bottom of the cone.  If there is overflow, then the exit velocity cant be properly estimated.

As for the exit velocity, the standard for the region is 15 m/s, just as you mentioned.

RE: velocity cone design

I dont know of any specific calcs to prevent 'downwash' as we call it.

All I can suggest is that you stick to the required minima values and place the exit cone as high as planning will allow. (usually we need to go about 3m above the roof level)

Good luck

Friar Tuck of Sherwood

RE: velocity cone design

(OP)

Yeah, in this region the government asks for an exit velocity of 15 m/s and a stack height of 5 m.  The problem is, even with these standards, I still run into stacks that have downwashing.  Because of the downwashing, the velocity of the exit gas is minimised, and there are contaminants dispersed too close to ground level.  Almost always, the fabricators of these cones use an exit diameter of 75% of the chimney diameter, and an angle of approximately 30°.  In spite of these very crude standards, I still find the occasional chimney with backwashing.  I`m sure of the chimney velocity because I mesure it with a pitot tube, according to established scientific procedures.

Thanks again

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