Baghouse Design

[djplatz]

I have a question on general baghouse design.
At the plants i have been to the reverse air fan (for cleaning the bags)suction comes off the ID fan inlet (suction) flue. Is there a reason that the reverse air fans don't take their suction form the ID outlet (discharge) flue?

 

[ventguy]

I'm not understanding your question. What do you mean by ID? and do you mean duct when you say flue? What is the baghouse filtering?



Peter Ott

http://www.utieng.com

 

[insult2injury]

FYI ventguy: ID = induced draft

I2I

 

[CountOlaf]

I don't exactly get what you're saying, and I'm also not familiar with this operation, but I would think it has to do with positive vs. negative pressure. If you're on the ID inlet side, you're taking air in a negative pressure zone, and of course you'd be in a positive zone on the downstream side of the ID fan.

So, if you connected to say the positive side of the ID fan with the "bag cleaning duct", and it wasn't being utilized or something, the ID fan would continue to force air through this "bag cleaning duct" at all times. Again, I don't realy understand the operation, but presumably this would be a bad thing. Whereas if you're on the negative side of the ID fan, and the bag cleaning operation was off-line, at least the connected duct would be under negative pressure (i.e., no chance of flue gas leakage).

This is similar to why it is best to have exhaust fans for buildings on the roof/outside, as opposed to in the space say near the source of contaminant. The overall fan pressure rating is the same either way, but there's more potential for leakage into the space with positive pressure ductwork.

Anyway, I could be way off base on this.

 

[mizzoueng]

hmmm, normally you pulse the bags with compressed air to knock the remaining ash and dust to the bottom of the bag and then remove the bags when they are full to empty.

Now if when you remove the bags you tie the cleaning line to the bags to empty them, you would want that line to be a negative pressure line. This would be the only reason you would tie this duct into the ID suction line. The greatest suction on the ID fan line would be right at the ID fan inlet. This could be around -20 in w.g.

If you tied into the discharge of the ID fans you would have some complications making a bag claenout. You would have to turn the bag inside out and then blow the air into the inverted bag. This would also mean that at some point before you clean the bag, there would be a chance that the ash/dust would be exposed to the environment while inverting it. With a pure suction system you just hook the bag to te suction line and allow it to filter into a hopper. While still connected to the hoppers, pull the bag back out and then disconnect it.

Both systems will probably require you to investigate your ID fans and see if you have enough HP and fan capacity.