plating, anodizing, & finishing Q&As since 1989
Fume Scrubber Sizing for Aluminum Bright Dip
How do I size a fume scrubber for a bright dip tank?, and what is the best material to build it since you are washing NOx fumes?Guillermo Luna
- Mexico City, Mexico
Bright dip is probably the most noxious and difficult plating process tank to exhaust.
It should have a minimum of 250 CFM per SQ ft of area of solution. If the area of the tank has any cross drafts, 250 might not be enough.
Little things make a lot of difference. The ratio of length to width, type of hood, exhausted on 1,2,3, or 4 sides. Things like a pvc backsplash behind the tank and the tank on either side significantly aids in the fume capture. Wings as little as 3 inches significantly aid in the capture. A small cover that covers as little as 6 inches of the front of the tank significantly reduces the effective area of the tank and can easily be moved just prior to moving the rack into the tank. Push air will cut the required exhaust 20% IF it is properly designed, maintained and operated.
The hood needs to be able to resist hot fumes. depending on how it is designed, distance from the edge of the solution and etc., it can be made out of heavier than normal pvc on the part that is next to the tank. 316 SS is also common. CPVC will withstand the temp and the chemical, but is hard to weld and costs more.Polypropylene welds do not like hot nitric acid, so I personally would not use unless I had plastic welding repair of good quality in house.
The scrubber can be made out of pvc, but should have a neutralization system on it to kill the nitric acid before it can go up the stack.
I normally recommend that a person buy a scrubber that is rated at 2,000 CFM larger than calculated so it can be sped up by changing pulleys. The speed up costs more in electricity as fans become less efficient above a given volume for that fan. The media in the scrubber is normally rated fairly tightly, so a large increase or decrease in CFM from rated lowers efficiency. It is better to slow down a unit than to have bought one that is just a little bit too small as the fumes from the bright dip will rust nearly any metal half way across the process floor. Metal buildings do not like bright dip. It is a vile , nasty process that produces beautiful work.James Watts
- Navarre, Florida
My boss, Frank Kraft, tells me he met you last weekend at the Albright & Wilson Bright Dip Anodize Seminar. He said you should contact them & they will help you design the fume scrubber.David A. Kraft
- Long Island City, New York
Up until recently I was an advocate of Nitric based bright dips. I had my platers take them out and switch to Phosphoric based dips and they love me for it. I'd much rather overcome the slight drop in effectiveness than the dangers associated with those nasty concoctions.
Mabank, Texas USA
As the man says, 250 cfm per sq. foot of tank area is a must.
However, bright dip fumes (eg. dipping in a sheet of aluminum) can suddenly really increase in volume. Excess airflow may NOT be necessary but a good hood design is a must.
To capture the 'sudden' cloud of fumes, some people have fully enclosed the bright dip tank (stainless) and even even had 'curtains' at each end. This shrouding goes up, say,6 feet or more and the actual fumehood slots are at high level and on the very, very (flat) top.
In this manner, one encloses and surrounds the expanding fume cloud and allows it be efficiently removed without using too much airflow nor requiring a big scrubber, either.
The above design was used in the late 70's by Johnson Matthey in Toronto. They had an overhead track. Fume capture was excellent.
The essence of all good exhausting of fumes is CAPTURE!
I assume you have a smaller operation. Consider l) having the liquid level in the tank always under 6". Shroud, if feasible, the sides of the tank (i.e., extend the sides using PVC sheet)by around l2". Ensure that the hood has the normal low level slot AND a higher level slot. Ensure, too, that the slots are adjustable.
If the hood is, say, 4 feet away from the end of the tank, then push:pull is probably necessary. If properly designed, the fumes will 'roll' under the push air curtain until they reach the hood.
PVC is eminently OK. Don't use PP due to stress cracking. Use s.s. if you want to. Assuming that you are also doing some (sulfuric?) anodizing, why not use a common scrubber for both? The NOX problem is apparently not nearly as bad and vicious as when one is dipping brass into nitric.
There was a case of one of these scrubbers where Na0H was sprayed into the scrubber. To my utter horror when visiting this Company, the 'operator' was instead just using plain water and yet the scrubber still met air management requirements! If you want further details, let me know.
Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).
The CFM per square foot recommendations listed above (250 )are good rules of thumb for good conditions. But more exhaust volume may be required to keep your building joists from disappearing in certain applications. Best rule of thumb for any ventilation design: enclose, baffle, and/or shield around the process (tank) as much as possible to contain and capture the fugitive emissions.
As far as the scrubber design: leave it to the manufacturers. I've seen many companies buy used scrubbers not designed for NOx only to see an ugly brownish/orange plume exiting the stack. Neighbors don't usually like that.Rick Hall
- Forest City, North Carolina
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