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Air blowing back out of fan (the wrong way)

Air blowing back out of fan (the wrong way)

Air blowing back out of fan (the wrong way)

I have an inline fan that is being used for dust extraction. The fan is 1200mm wide with a 55kW motor (1400rpm). There is a cone/hood on the inlet side of the fan that is around 1m long and is slightly larger than the diameter of the fan (not sure exactly how much larger, around 400mm diameter overall I'm guessing). On the outlet side of the fan there is square ductwork that is 1400x1400mm that goes around 30m into a dust extractor.

The issue we are having is that at the inlet of the fan there is air blowing back out of the inlet the wrong way. It is happening around the outside edges of the hood. It isn't much air, but it is disturbing the dust in the room that has fallen to the ground and is actually making the entire situation worse.

I'm just a lowly electrical engineer and so I really need a bit of help with this one!

One last thing - if we open up the dust extractor hatches and remove the socks the blow back seems to improve.


RE: Air blowing back out of fan (the wrong way)

Needs drawing / photo, but might be that the fan is stalling?

That's one big fan and motor so how much air is being blown through it?

What is the flow on the fan data sheet? versus actual flow?

where are these hatches and "socks"?

Remember we can only see what you tell us or send us pictures / drawings.

Descriptions work when you've got something to look at at the same time.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Air blowing back out of fan (the wrong way)

and as LittleInch suggests, if your filters are plugged, the back pressure can blow back inside the fan shroud.

RE: Air blowing back out of fan (the wrong way)

As LittleInch said more details required, type of fan etc. By the way hope you have checked the direction of rotation in addition to closure of discharge damper or clogging of downstream equipment like filters etc.

Engineers, think what we have done to the environment !https://www.linkedin.com/in/goutam-das-59743b30/

RE: Air blowing back out of fan (the wrong way)

Datasheet of the fan is attached.

Rotation of the fan is correct and it doesn't appear to be slipping on the shaft (it's held in place by a grub screw into a keyed shaft).

The dust extractor that the fan is blowing air into contains a number of "socks". These are basically a long tube of dense wool like fabric that captures the dust particles as the air blows through them. These are in a large chamber that looks like a tank. The air blows into the bottom, through the socks and then the clean air blows out the other side and out through a hole in the top of the chamber. The hole in the chamber was made to be the same size as the ductwork (1400x1400). There is a sliding plate that you can move across the hole to dampen the air flow if you want.

I'm not on site today so I can't take any photos but I will try to get them to you soon. But in the meantime here is a sketch of the system:

RE: Air blowing back out of fan (the wrong way)

Something is off about your drawing. The dust goes through the fan? Your first picture shows a gap between the fan and duct. This must be some type of eductor. You need to set the gap between the motor and duct. The dusty air should get drawn in through the gap.

Also, the air entrance shouldn't be at the bottom. Those types of candle filters are meant to shed dust into the bottom collection cone where it can be drained off. If the air is blowing in from the bottom it will keep the dust suspended forcing the filter to handle all of it.

RE: Air blowing back out of fan (the wrong way)

If the sketch is real then probably the accumulated dust is blocking the discharge from fan.So the air is backflowing with dust. A photo of the installation will help in understanding.

Engineers, think what we have done to the environment !https://www.linkedin.com/in/goutam-das-59743b30/

RE: Air blowing back out of fan (the wrong way)

One thing you might consider is a pressure monitor (inches or mm water column pressure) as a gauge of when you need to shakedown your buildups.

We used air-centrifuge separation where production particulates (large) were involved,
but with the exhaust gas particulates, electrostatic precipitators requiring constant monitoring and automatic valving to divert flow to cleaned units.

The flour mills and such were a different story.

RE: Air blowing back out of fan (the wrong way)

So, this is apparently a new problem, i.e., when the system was brand new, it didn't do this. The question is then what changed"

You pumped a truckload of dust through the system; did all of it actually leave the system at the other end? Dust, particularly in the presence of high humidity or moisture, has a tendency to stick, particularly in bends, and given your supposed geometry, particularly there.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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RE: Air blowing back out of fan (the wrong way)

Nothing attached.

Also can't do much without some actual readings either flow or differential pressure.

So how is the dust extracted from the flyer?

What measurements have you got?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Air blowing back out of fan (the wrong way)

This is the sort of dust problem cyclonic separators were invented to handle. Just remember to empty the dust collection container before it fills.

It may be that the socks have plugged, allowing a significant settlement to build up in the duct, blocking the free flow of air even after the socks are removed.

Bring a good quality dust mask with a good number of replacement filters. Also, see about moving the fan to the outlet of the filter - then there won't be leakage from back-flow. There are airflow switches available to indicate if the airflow drops as well as differential pressure sensors to check if the filter is plugged.

Also - the attach process depends on copying and pasting the link that is generated in the upload process into the message, so the file is there, just no one knows what the internal name is, so you'll have to upload again and copy/paste the link.

RE: Air blowing back out of fan (the wrong way)

This one is strange. Typically the fan is on the exhaust of dust extractors. It's not ideal to have the dusty air pass through the fan because the velocity changes tend to separate the dust within the fan. I think this system is supposed to perform as some type of eductor according to OP's drawing. That explains the gap and the fan on the inlet to the system. We should probably wait for clarification at this point. If it is truly plumbed to the bottom of the hopper that will prevent settling and out 100% of the load on the filters which will result in rapid clogging.

RE: Air blowing back out of fan (the wrong way)

An unusual system layout. Typically, as stated, the fan would be on the extractor discharge. The inlet duct to extractor would typically be near top of extractor and tangential to the extractor axis to swirl the dust-laden air for cyclonic separation prior to entering the filters. Air blow-back at fan indicates the discharge pressure has increased and exceeded fan rating. If the system is actually built like the drawing, then the filters are probably prematurely clogging and causing the high back pressure.


RE: Air blowing back out of fan (the wrong way)

The symptoms point toward excessive static pressure across the fan. The guidance above recommending looking at the bags (sock) is a good idea.
Condensation can often cause dust to stick to the bag material. If you see this either launder the bags or replace them.
Check the bag cleaning device (this could be a bag shaker). If the bag cleaning device is not working the build up of excessive dust will increase the required fan head, and as propeller fans are rather sensitive to discharge pressure rise, airflow drops dramatically.

Installing a manometer to measure the pressure across the bags can be a useful troubleshooting tool.

A dust collector troubleshooting guide can be found here https://www.baghouse.com/2011/02/04/dust-collector...


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