Duct Collection System

[remp]

Hi, Im looking at designing a new Dust Colleciton System, with an extract fan, turrit dust collecor unit and circular ductwork to 4 machine outlets points, approx 4" per outlet.
I want to make it as efficient as possible.
Not all machines will be operational at once. I would like to put the fan motor on a VFD but do I have to watch the minimum velocity in the ductwork?..... so am I allowed to ramp down the fan if some rooms are not working and their dampers are closed? Are you allowed to close the dampers if you want to maintain min velocity in the duct????

Should I put velocity sensors at each outlet and make and automatic open./close damper for each machine? What velocity or pressure drop/length shoud I be sizing my duct on?

I have never before done one of these but I work in a site with many of theese systems that are over 20 years old, big motors (90kW) and in some instances only one machine hooked up and the rest dampered off beacuse of noise issues and not needed etc...
I would really appreciate your help..... any pointers anyone?

 

[jkauwale]

Hi Remp,

The primary method of sizing ducting serving a dust collection system is to maintain a certain minimum velocity through the duct. This velocity will allow the particles to be suspended in the air and it will also allow the particles to be transported to the collector.

The minimum velocity that you size your duct will depend on the size of your particles. For woodworking and other light particles (not metals), 3,500 to 4,000 FPM is typically used.

I would suggest you run different scenarios. (1) Machine on, (2) Machines on, (3) Machines on and (4) Machines on. Then determine what the velocities would be in the duct and the total pressure drops, in order to get a better understanding of how to size your ducting and fan.

A VFD would be beneficial in being able to control fan air flow and velocities. I think it is a good idea.

Justin K, P.E.

http://www.engproguides.com

 

[Compositepro]

A VFD is probably a bad idea for saving energy. You will not be able to slow the blower much or else you will not generate enough suction pressure. Closing the unused dampers will save energy. If you really want to save energy use small dust collectors where needed. Obviously a 90 hp blower is "outside". You are probably losing more energy by exhausting the heating and air conditioning than the motor uses. There are many things to consider, but VFD=energy saving not always true.

 

[berkshire]

If you have several machines where you want to damper off the unused machines, then you should also have a spring loaded damper at the end of the run to keep the duct velocity up, or you will have drop out in the larger sections of duct.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[Compositepro]

Berkshire makes a very good point, but that will also use more energy.

 

[urgross]

If you place a motor operated damper on each duct with contact to the machine control circuit, the damper would open when the machine turns on. Place a dP on each duct to a machine. The transport velocity given by the ACGIH would correspond to a static in each duct, just as with balancing any supply or exhuast system using a VFD against a dP sensor. The VFD would operate to maintain minimum static setpoint for each transport velocity. I think that the idea of smaller dust control units on each pice of machinery may be better from an O&M standpoint, especially if downtime of all equipment for a single exhuast fan is critical.