Originally Posted by RJB324

We are having a significant issue with solid leakage from our baghouse into our vacuum blower causing major maintenance headaches. I have 2 questions:

1. Anyone have a good chart on air to cloth ratio for certain materials? I believe we might have too high of an air to cloth ratio (currently around 5).

2. We do not see any dropped cartridges in the baghouse. So even not seeing that would a poor air to cloth or high pressure drop still lead to carryover to the vacuum blower? Or would there have to be a down cartridge or a ripped cartridge?

Thanks in advance.

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Vacuums, the stuff of pressure gradients.

Greetings and salutations from my corner of the slowly defrosting eastern wilderness @1140 feet above mean sea level.

A vacuum leak is exactly like water leaks, electricity leakage and static electricity build up.

electricity has a perfect conduit to a ground in water.

All three of them are the lazy "opportunists" in physics.

Water will find the easiest way to get some where with gravity.

Water under pressure will seek a leaking pipe joint or bad soldering job or may break a pipe to overcome the resistance of the restricting pipe.

Electricity will find an "earth ground" as close to its exit point as possible, meaning you as a conduit for static electricity as it is lazy unless it is controlled.

Example:

A cow at water drinking fountain is a conduit to ground for stray electricity as it needs water to drink and sustain itself.

Water succumbs to the effects gravity because it has mass and that is why a water leak stays on the ground.

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What type of blower is it ? positive displacement, regenerative,, or paddle fan,

What is the maximum and minimum Hg reading allowed?

What is the "Denier rating" of the fabric weave?

Where are the vacuum gauges located?

You cannot depend on restriction indicators with the red and green pop ups.

If the bag house was properly sized to the blower that should not be a problem.

Is a reverse air blast used to purge the bag house fabric or air cartridges to clean it or is gravity the cleaner?

If you reuse your air cartridges rather than replacing them that is a no no and a bad idea to begin with as it can and will damage the filter fabric.

Are you careful about housekeeping when changing the air filters-meaning do you wipe down and vacuum around the mounting plates prior to installing the new cartridges- but if you do not use new cartridges its a lost issue.

"A vacuum" which is a pressure gradient is part of the six laws of gasses in a pipe;

As long as their is adequate resistance to suction a vacuum is created by the fan or blower creating the pressure gradient.

The minute there is a tear in the fabric or a hole in one of the filter cartridges the vacuum becomes very lazy and will follow this path and that is why you have dust in your blower housing

You have a leak in the suction side and that is the unavoidable fact in this case.

A cartridge filter could have a leak and be unseen, a sealing gasket could be torn or be worn out from compression.

A hem tear in a the fabric or a pin hole is another possibility.

The pressure gradient created by the blower did the one thing it is capable of doing by itself ; it escaped the vacuum restriction created by the system.

The easy ones to spot are the worn out bad door gaskets to the cartridge cabinets.

I will never understand why a gravity cyclone with a dump gate is never installed prior to the filter system and bag house in any of these installations and sometimes a second cartridge house has to be added-it was in our case instead of a prefiltration cyclone to eliminate heavy fugitive dust loads to aid in filtration.

A vacuum leak is a simple thing to find with a flashlight and stethoscope;

The other way is using low pressure (5 PSI) back through the piping prior to the blower to locate the offending leakage points with a smoke generator stick.

You should also have a Delaval milk house vacuum gauge kit to test for vacuum leaks as a matter of necessity.

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If you are using a positive displacement blower it will have been damaged by the dust and fines and will need to be replaced.

lzaharis ■

Dear Rob,

The allowable air to cloth ration (m3/min)/(m2) = (m/min) is mainly depending on the material as well as on the fabric itself in terms of pressure drop and efficiency.

A filter load of 5 m/min seems very high.

You have established material in the blower. That proves that material (which material?) has either passed the filter assembly, directly through the filter elements or by passed.

Their must be traces of those leaks either by adhered dust or blank spots.

Put on your boiler suit, grab your tools and start the field research.

Do your best

Teus ■

5m/mn is extremely high for any type of filter and material.

Could you give us more specific for filter ( filter bag type, area...) and handle material ?

Best regards,

Dantherm Filtration. ■

Does your "vacuum blower" = exhaust fan?

What units for air-to-cloth ratio?

What kind of dust?

With cartridge filters (or even filter bags) and if all the filters are in place, you should not see any leakage which would cause you problems, unless there are holes in the filters.

Michael Reid ■

Originally Posted by RJB324

So wouldn't a high air to cloth ratio cause with a high loading ratio cause poor filtering of the dust? Or even if I had an air to cloth ratio of 10 to 1 no dust would pass through the filters if there were no tears in the filters or leaks where the filters hit the housing?

This is a pneumatic conveying system of fine calcium phosphate dust mixed with plastic dust.

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The cartridges and bag house filters are the problem, not the blower and the blower is damaged now.

If the bag house and filter system was sized properly for fugitive dust and was purchased as sized with no short cuts- meaning buy less system that specified, no it would not be a problem.

Any fugitive dust with solids is a headache as it can rip fabric.

If the blower has no relief valve to bypass the bag house it will continue to increase the pressure gradient until something gives and its either the cartridges and then the filter fabric and its a lost cause with no solution except:

either adding filter and baghouse capacity in parallel with asecond blower to cut the preloading of filter fabric loading in half or simply installing a big enough cyclone with a dump gate at the same time of repairs.

lzaharis ■

To control any further confusion, please describe your system more precisely.

-air displacement of vacuum blower

-operating vacuum

-vacuum pump type

-Filtertype

-filterfabric

-air pulse cleaning yes/no?

-measured pressuredrop

-particle size distribution

Higher air to cloth ratio worsens the filter problems by increasing the filter pressure drop and the operating vacuum of the blower.

The filter plate can distort then and the filters do not seal anymore.

Information that is more specific is required; otherwise, we keep on discussing things with no structure.

Cheers

Teus ■

My 10 cents says the cartridges are overloaded & the blowdown operation is causing distortion at the sealing face to allow passage of displaced dust. ■

Hi,

I used to work in a cement industry, and for normal materials with low abrasiveness, air-to-cloth of max. 1.5m/min was used. For abrasive materials like fly ash and clinker, we used a ratio of 0.8m/min

Branco ■

RJB,

In properly designed and built bag type dust collectors, a high air to cloth ratio will result in dust leakage only if the resulting high pressure drop is ignored and eventually results in bag failure. Normal pressure drop should be about 4 inches of water. There should be an alarm set at about 10 inches indicating the need for bag replacement. Most commonly made bags start to fail when the pressure drop reaches 1 psi.

For very fine dusts, such as a few microns in particle size, the recommended air to cloth ratio is about 1. But in addition, the material that is used for bags should be membrane type such as Goretex. Such a material prevents micron size dust particles from penetrating through the bag and escaping.

To prevent damage to the Vacuum Blower if it is Roots type, my recommendation is to install a guard filter at the Bag Filter outlet. This filter can be an inexpensive cartridge type without any blow-back provision.

Best regards,

Amrit Agarwal

Consulting Engineer

Pneumatic Conveying Consulting

Email: polypcc@aol.com

Ph and Fax: 304 346 5125 ■

RJB324,

Please give answers to Teus's questions. Otherwise the discussion goes around and around....

Michael Reid. ■