Perforated plate pressure drop calculations?

[brazilianjj]

I'm trying to figure out how to calculate the pressure drop of a perforated plate!
Could you provide me with assistance or information?

My current project is the design of a biotrickling scrubber. It is essentially a biological scrubber using lava rock with nutrient rich solution to degrade foul air (predominantly hydrogen sulfide).

The tower I'm designing is about 22 feet in diameter and the lava rock packing is 12 feet high. The flow rate is approximately 20,000 scfm. In order to insure that the foul air is distributed evenly across the bottom of the lava rock packing, I'm planning to install a perforated pvc plate beneath the lava rock packing. The rule of thumb to create an even air distribution is to create a 1inch w.c. pressure drop across the perforated plate. This would in effect create a back pressure.

Do you know what formula I could use to calculate the pressure drop? The surface area of the plate is fixed, but I'll have to vary the number and diameter of the holes on the plate.

 

[ChasBean1]

See ASHRAE Applications, Ch. 51, Eqn. 9. This is from the orifice equation with conversion factors and built-ins to simplify:

Q = 2610 * A * (dP)^.5

Q is flow in cfm, A is area in ft2, dP is in inches w.c.

So my guess is 7.66 square feet net (total open area of all your holes). If this works out for you, please re-post as I'd be interested to know! Good luck, -Chas

 

[brazilianjj]

using that formula, the ratio of passage area to total area is 7.66/379.94 = 0.02. So the holes cover about 2% of the total area... it doesn't seem like much of an opening.. I'll have to double check.,

 

[butelja]

The following link from the Industrial Perforator's Association should be exactly what you are looking for.

http://www.iperf.org/pressure.htm

 

[ChasBean1]

I agree it sounds small, but this could be because your face velocity for 20,000 cfm across 380 square feet is only 53 feet per minute. I can burp faster than that. I stand by my verdict! (look, it's alot easier to add holes later than to try to remove them...)

 

[newhouse]

Dear brazilianjj:

Please also consider the mass tranfer for this problem. Further, the L/D ratio you propose will make distribution of lean solvent difficult. If pressure drop is the critical economic parameter and you must have such an odd shape commercial vessel, you will be plowing some very difficult ground for design of the liquid distributor at the top of this vessel. It will require likely a sealed chimney tray with both chimneys and tray perferations. You have a tough problem.

 

[62hog]

There is a great shareware program called Engineering Power Tools. You can search the net on sites such as C/Net or ZDNet. It has an area in the Mechanical tools which will calculate PD through a perf plate.