Originally Posted by nugget

Dear Sirs,

Does anyone have any experience on hot deep galvanizing process dust filtration? We have a bag filter system which is blocked quickly because of the sticky dust on the process. I wonder what is the tricky points on hot deep galvanizing bag filters ? I have to check them one by one and discuss with the manufacturer of the filter unit.

Thank you for your help at advance...

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It is good that you are checking them for holes in any case.

one option is to add another bank of filters but saying that-

If the filters are blocked quickly it is doing two

the two jobs it is required to do very well.

1. filtering.

2. delivering the required amount of Hg pressure gradient

for the bag filters.

3. eliminating galvanizing dust into the atmosphere.

a. the gaskets on the bag house doors have not failed

and providing the proper sealing to allow the suction system

of the bag house to work at its most efficient generated Hg pressure gradient.

The best solution for dust in many cases such as this

is a passive cyclone or a pair of passive cyclones in line

plumbed one behind the other "in line" then plumbed into

the bag filter or "plumbing them side by side" to split the flow

half in one cyclone and half in the other and then returning

to the bag filter in one common duct prior to your bag filters

which would allow the heavier dust to drop out by gravity

and the suction will carry the remaining dust into the second

passive cyclone and then the exhaust leaving the second

cylone will have even less dust going into the bag filter

and the end result will be less plugging period and longer

service life of the bag filters.

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Not knowing how much if any floor space inside your plant you have for a single larger cyclone or pair of smaller passive cylones is an issue as well or if your dust collector is out side next to the building etc.

Are your bags purged by gravity or by reverse air blast?

The simpler the better as a primary cyclone and another bag filter would be in order

in this case.

Perhaps a passive cylone with a wet collector sump would be a better option as is

done on pulverised coal boilers for collecting bottom ash? ■

Originally Posted by nugget

Dear Izaharıs,

Thank you very much for your help.. Thw problem we have is hot deep galvanizing process takes part open to atmosphere and we cann ot close it. So on the suction channel both fresh air and water vapour and dusts are both sucked. So on the way to the filter unit water vapour condense and dust gets humid. So at the bag filter we have a mud instead of dust on the bags. So this mud blocks the filter unit and suction decreases. Filter unit has jet-pulse cleaning system but for a dust like a mud (I mean sticky) cleaning system does not make sense..

Cyclone is normally good for unsticky dusts, decreases the dust load of the filter unit, I agree. But in this system it also will cool down the air and make the vapour condense and dust will be mixed with water drops. So I do not think that cyclone system will be good.

For wet scrubber it can be good to collect the sticky dust with water. But we have the bag filter right now and we can not throw it away so I have to make it effective in anyway...

So I wonder if someone had an experience on hot deep galvanizing process dedusting with bag filter..And send me advices and experiences on this process..

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perhaps the answer lies in a very small drum vacuum filter with a small wet scrubber which would solve both issues for you. ■

Originally Posted by nugget

Mr. IZaris,

You are right a wet scrubber will be a good solution but the area that our plant is located and the plant conditions does not let us to use a wet scrubber as it needs a waste water treatment. So I have to find a solution with the bag filter in anyway..

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You have four options then:

1. Continue at a slower rate of production to reduce bag house loading the most cost effective option as no money is spent.



2. Add a pair of passive cylones in line one behind the other prior to the bag filters

most cost effective method as no heavy dust will exit if the cylones are properly

sized (larger first and smaller second) or (larger twin cyclones piped in parallel).

3. Very large cyclone that slows the air down due to its oversize surface area size and allows huge amounts of dust to settle out into removable bins prior to the air stream entering the bag filter.

4. A vacuum drum filter with a diatomaceos earth cover using a sand filter to dry the water exiting the drum filter. you may not need water treatment of any kind with the diatomaceos earth anyway. ( the diatomaceos liner is very good filtration system. ■

Depending on the available space maybe a balloon flue could be considered to drop the air speed and allow some of the dust to drop out of suspension and be collected by a chain conveyor running the length of the flue. ■

Nugget,

The problem is that the fumes, zinc oxides, zinc chlorides and zinc ammonium chloride are ultra-fine, sub micron and are hygroscopic. Cyclones will not remove any of these fine fumes. Wet scrubbers would need to have high pressure differential and consume a lot of energy. They also create a secondary wet waste problem.

Bagfilters are the only way. Here are some essential design features:

The bagfilter casing needs to be insulated and heated to keep the inside above acid dew point (trace heating applied to the lower half of the bagfilter hopper). This also aids in preventing corrosion of the casing.

The compressed air used for filter cleaning should be dried with a refrigerant dryer.

Make sure there is no atmospheric air allowed to leak in anywhere. Pay special attention to door seals and gaskets.

Filter bags need to be polypropylene and have hydrophobic treatment to repel moisture and aid dust cake release. Filtering velocity should be below 1 metre per minute (air-to-cloth ratio less than 3:1).

Michael Reid. ■

The air which traps the fumes and carries it to the baghouse is ambient air, with entrained atmospheric moisture. You can't avoid this. You can reduce the amount of air by building a better enclosure but the baghouse will always see ambient air.

The most efficient way of protecting the filter bags is to insulate the baghouse and heat the hopper. This is commonly done in this country (Australia) to keep the internals above acid dew point. Depending on the size of the hopper, only a few kW is needed. Just heating the entering air will not make sense unless you also insulate.

A dessicant dryer will work just as well for the compressed air.

1.25 m/minute is a bit high but you can probably live with that if you do as I suggest. Merely increasing the cloth area will not solve your problem.

Michael Reid. ■

Originally Posted by Michael Reid

Nugget,

The problem is that the fumes, zinc oxides, zinc chlorides and zinc ammonium chloride are ultra-fine, sub micron and are hygroscopic. Cyclones will not remove any of these fine fumes. Wet scrubbers would need to have high pressure differential and consume a lot of energy. They also create a secondary wet waste problem.

Bagfilters are the only way. Here are some essential design features:

The bagfilter casing needs to be insulated and heated to keep the inside above acid dew point (trace heating applied to the lower half of the bagfilter hopper). This also aids in preventing corrosion of the casing.

The compressed air used for filter cleaning should be dried with a refrigerant dryer.

Make sure there is no atmospheric air allowed to leak in anywhere. Pay special attention to door seals and gaskets.

Filter bags need to be polypropylene and have hydrophobic treatment to repel moisture and aid dust cake release. Filtering velocity should be below 1 metre per minute (air-to-cloth ratio less than 3:1).

Michael Reid.

I am currently working on the same issue with a Galvanizing Baghouse and would like to know how to determine the internal temperature required to keep it above the dew point? The baghouse design specs indicate a max operating temp of 150F. The air into the baghouse ranges from 80 to 100F. If I keep the baghouse at around 140F would that do the trick? ■