Wet Collector Underperformance

Equipment:
C5-2500, orifice scrubber style wet dust collector, rated for 2500 CFM, purchased to handle explosive aluminum dust particles.

Problem:
The dust was going right through the collector and packing into the fan / outlet compartment. Very little dust was collected in the dust collector sump.

Investigation and observations:
We requested pictures and system layout drawings (sketches were actually provided). From these we observed that the client did not describe the application accurately at the time of purchase.
1. The dust was produced from a spray coating operation. Therefore, it was fine powder type aluminum dust. Wet collectors are designed for metal dust 5 microns and larger, as generated from grinding and cutting operations.
2. The inlet was connected to a properly sized 8” duct but was over 20 feet long with three elbows. These units are designed for maximum 10 feet of duct directly to the collection point.
3. The client also decided to exhaust the discharge of the collector to the outdoors with another 20-25 feet of duct, and three more elbows. These collectors are designed for an open, unrestricted discharge on the top of the unit.
The result of this was a questionable capability of collecting the powder type dust. The biggest problem was that the collector performance was choked by far too much resistance to airflow in the installation. By doing this the air entered the collector with far too little volume to cause the necessary turbulent energy in the “omega” style baffles. The necessary wetting action of the dust particles was not taking place and filtering action was non-existent.

Solution:
1. We asked the client to place some of the collected dust in a closed jar with water. Then shake it and let it sit for a few minutes. If the dust settles, it can be collected. If it doesn’t, a different dust collection solution must be found.
2. A wet dust collector is very particular about the airflow through it. You need to be in a range of +/_ 10% of the rated CFM for that model dust collector. In this case, the minimum flow that could be tolerated is 2250 CFM. Conversely, with more than flow than the 10%, water gets drawn up too much and discharges out the unit. We recommended bringing the collector closer to the application and remove the duct on the outlet. Alternatively, add a booster fan to overcome the restriction.

Other Considerations:
A. If the excess resitriction is minor (within the 10% range) but dust is discharging at the top, You would add more water to the collector, in small increments, until the dust/water stops coming out the top. The added water compensates for the higher restriction. Then reset the float control to maintain this new water level.
B. In some cases the discharge is required to be exhausted by code. An example is beryllium. In such a case, do not attach a duct on the outlet. Instead build a capture hood over the top of the wet collector outlet, approximately 4-6 above the top, and duct that to the outside. Install a fan to give you 5-10% more air flow than what is running through the collector, to ensure no contaminents escape back into the room.
C. If you oversized the wet collector, Do not restrict the flow with a damper more than the 10% tolerance. Install a bleed-in on the inlet duct with an adjustable shut-off damper. Open the damper to the point where the collector performs properly. This is often a scenario when you size the collector for multiple collection points but don’t have them all installed until later.

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