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MAINTENANCE OF DUST CONTROL SYSTEMS
With the advance of the colder winter days, it is timely to consider what maintenance work should be carried out on your Dust Control System, to ensure trouble free operation during a time period when no one likes the idea of service work, under the unpleasant winter conditions.
It is probably safe to say that a dust control system is probably the one system that receives the least attention when it comes to maintenance or service. The following outline is designed to help you and your personnel understand and service this equipment at the least cost in material and downtime.
By itemizing the system into components you can program service work to each component when time and conditions best suit your needs. For instance, schedule outside work on a warmer Saturday when the plant is not operating. If you plan your work in such a manner, you will be able to do a thorough job and keep your costs to a minimum.
Remember always, to turn off your power supply to the system when working on the equipment, ensuring that someone else cannot start up the system while you are working on it.
a. Filter Tubes (or bags, envelopes, etc.) A filter type of dust collector works on the principle of separating the dust particles from the air by filtering it through the cloth.
In reality, the cloth actually forms a holding framework for dust particles that build up on its wall. This built-up dust cake then becomes the filter. Cloth alone would not clean the air sufficiently and would pass larger sizes of particulate, whereas the dust cake will keep back microscopic sizes of particles while still passing the air.
It is therefore important to realize that a dust collector is doing its job properly when 1/8" to 1/4" of dust clings to the filter wall.
If the air flow speed through the filter is too high for the condition, particles will become lodged in the fabric weave. This is known as "binding". These particles will not be released when the filters are regenerated through the shaking cycle. As more and more particles become lodged, the air speed through the remaining cloth openings increases, and this cycle accelerates until the pressure drop through the dust collector rises dramatically and the system looses it suction.
(This is why the "cloth ratio" or "cloth velocity" is so important in selecting and sizing a proper dust collector.)
If your filter tubes have been "binded", they should be replaced with new ones, because cleaning will only do a partial job in removing the lodged particles. Normally, filters can be reused time and time again if they're cleaned, and tears repaired as necessary, and then replaced.
Here's what we recommend: Buy and keep one complete extra set of filters on hand. Then once a year, schedule a maintenance day and replace all the filters with the new ones. Have the old ones cleaned, checked, and repaired as required, replacing the one or two that might be needed to complete the set. Then a year later repeat the above program. This way you will reduce your labour and replacement costs to an absolute minimum.
If your dust collector is equipped with spring hangers (as shown in the example on the left), you are always assured of proper tension in your filters. If not, you need to be careful to install your filters with tension as illustrated. The tube in the center is properly installed. The tube on the right will plug up with dust material bridging above the necked down portion just above the baffle plate.
b. Shaker Mechanism Depending on the type and size of your baghouse, its shaker mechanism will consist of pushrods, bearings, brackets, hanger springs, gear reducers, V-belts, and/or drive chains. Servicing this system consists of inspection, repair or replacement of broken or worn components, and lubrication.
Starting with the drive unit, check the pulleys, V-belts and bearings. The V-belt should be free of checks or breaks and be properly tensioned to prevent slippage. (See section of Fan V- following.) Check any mounting brackets, bearing housings, and grid hangers for missing or loose bolts, lock washers, and nuts. Remember that although the filter shaking sequence is only for a short time period, its action subjects the components to large stresses and vibrations. Check components for fatigue cracks and failures.
Lubricate all grease nipples with a light weight, winter grade general purpose grease. (This grease weight is good all year round for this application.) Each fall you should drain the oil in the gear reducer (if your system is so equipped) and refill it with No. 10 grade mineral oil to the proper level. The time-length of the shaking sequence is so short that you need never go to heavier oil; the gear reducer will never have the chance to run warm or hot.
c. Baghouse Casing or Housing The forces that the baghouse casing or housing are subjected to are; the vibrational forces imposed during the shaking operation, those due to the airflow pulsations, vibration caused by the fan, if mounted to the baghouse, and the weather forces if the unit is outdoors.
Inspect your housing for any fatigue failures. If your baghouse is rectangular or square, inspect the corner seams for cracks. If you detect any such cracks they should be repaired or reinforced before further deterioration occurs. If you re-weld such cracks be aware of the extreme fire or explosion danger.
When we weld on existing units, we always ensure that the hopper is empty, the bags are shaken and are as clean as is possible, no sawdust or shavings are piled up in any corners, and the inside of the hopper, ductwork, or wherever possible is hosed down with water so that all fine dust is contained. Even then, we proceed only with appropriate fire extinguishing gear at hand.
As a final check, see if your access panel or vent fits its frame properly so that no air is leaked. Any warm air lost in this manner has to replaced inside your shop with cold outside air, at a cost!
What about the heat loss? If your dust control system is airtight, then all the air that is taken out of your building will be returned. it will, however, be colder than when it was taken out of the building. This temperature drop will depend upon the amount of outside ductwork, the volume of airflow, the size of your dust collector, and the temperature difference between the outside and your shop inside.
How much heat is lost? You can quickly calculate how much heat is being lost by the following formula:
where: H = BTU/Hr. heat loss
CFM = airflow
= Main duct cross-sectional area (in sq. ft.) X 3500
Temp. Diff. = inside temp. - temperature of returning air in degrees F.
for example, if your main duct going to the dust collector is 18" dia. (1.767 sq. ft.), and your shop inside temperature is 70 degrees F. with your air returning at 55 degrees F., your heat loss is:
H = 1.767 X 3500 X 15 X 1.08
= 100,189 BTU/hr.
Should you be considering the application of insulation to your dust collector and outside ductwork, you could compare this cost versus the cost of the heat loss.
If you are not returning any air and the outside temperature was -30 degrees F., your heat loss to warm up the replacement air would be:
H = 1.767 x 3500 x 100 x 1.08
= 667,926 BTU/hr.
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