Ask the Expert Question-and-Answer Archive

by Ron Joseph

September, 2001

Emissions From Wire Dipping Coating Operation

Q. I work for a University-based technical assistance service that helps businesses in our state. I'm working with a hanger manufacturer on an air emissions problem and would appreciate any suggestions might have to offer.

They recently received a notice of noncompliance for opacity violations at the stack from their bake oven. They use a diptank to coat hangers, which they then bake in an oven at about 350F. Details are provided below.

DNR has indicated opacity readings of as high as 30%. The stack, which I inspected up close on the roof, has a thick, black deposit that can be peeled from the metal. Emissions that I saw had a blue-grey tint.

My assessment is that the problem is a combination of suspended solids primarily from drippage in the oven, along with organics. The primary fronts of attack I have recommended are reducing drippage through a variety of means (listed below) and improving design/operation of the oven.

My questions are:

1. Sound like anything you've tackled before? Anything I'm missing?

2. Do you know of any applications where powder coating is has proven economically feasible on such low-cost, non-durable products as hangers? This would be a bigger investment than they are anticipating, but might be the better long-term investment. I suspect but don't have good data to verify, that powder coating would be too expensive for this application.

Background:

In operation for 30 years. Recently applied for renewal of operating permit. DNR identified opacity as a problem on recent inspection. Just received notice of noncompliance. DNR says opacity is as high as 30%.

Have performed no stack testing.

DNR grabbed sample of paint for testing.

Bake oven temp: 350 F

Approx time from racking of hangers to removal: 30 min Approx time in oven: 10 min.

Bake oven uses indirect heat. Combustion area is under the floor of the oven. Fired by natural gas. Apparent main areas for introducing air to oven is at ends, where hangers enter and exit. Oven has recirculating fans after burners.

Changed from solvent-based paints to water-based paints approx. 10 years ago. Says they have made no significant change to operation since, except to reduce HAPs in paint. Says smoke has always been there, but that DNR just noticed. Said they had smoke when they used solvent-based also.

Coatings used:
Wolke Paint Mfg.
LC HAP-Free Bakecote Blue; WC-2248-04 (mixed 5 parts paint to 1 part water)
LC HAP-free Bakecote Clear Gold; WC-2237-01 (mixed 1 part paint to 1 part water)
HAP-free Bakecote Chi-gold; WC-2255-05 (mixed 5 parts paint to 1 part water)
HAP-free Bakecote White; WC-2291-14 (mixed 6 parts paint to 1 part water)

Exhaust stack is coated with blackened hard coating that can be peeled away. They say they remove this every few months.

They shot-blast racks to remove excess paint once per shift.

Options

• Reduce drippage in oven:
  • Improve design of rack and hooks to pick up less paint in dip tank and shed paint more readily
  • Air knife to blow off excess paint
  • Vibrate hangers to shed excess paint
  • Decrease line speed to reduce carryout from dip tank and increase drippage and drying before oven
  • Increase travel time between drip tank and oven
  • Adjust paint mix (viscosity and constituents) to reduce drippage
• Improve design and/or operation of oven
  • Improve mixing of air within oven; place baffle(s) to eliminate short circuit between fans following burners and the intake for the exhaust duct near the entrance to the oven.
  • Reduce velocity / increase turbulence of air flow to reduce suspension of particulate Possibly increase temperature to improve combustion of organics (Laidlaw is concerned that higher temp will cause white hangers to lose gloss)
  • Review size of oven exhaust fan; oversized for application?

A. Thank you for your email. As it happens, you are the second person to write to me about this problem. Last week I corresponded with someone else in your state who also is working on what appears to be the same problem.

Based on your more detailed description I now have additional information that is helpful.

Without seeing the problem first hand it is difficult to do more than make some suggestions, but it seems that perhaps the oven temperature is too high. By lowering the temperature to just over 250oF you might prevent the evaporation of the semi-volatiles. In other words, some of the resinous material in the coating seems to be evaporating in the oven and then when the semi-volatile vapors come into contact with the cooler surfaces in the stack, they condense to form the "black deposit that can be peeled from the metal".

My guess is that by lowering the oven temperature you might well solve the problem, but the company will need significantly more time to cure the coating. There are several design options for accomplishing this goal, but they will almost certainly require capital expenditures. An alternative is to search for coatings that comprise different resins. Over and above this, there seems to be a serious problem with transfer efficiency from the dipping process, and while an improvement in the application process would , in my opinion reduce the emissions, it will not solve the problem.

Powder coatings may well turn out to be more cost effective because this process inherently has high transfer efficiency in excess of 95%, but you would need to do a complete cost effectiveness study before suggesting it as a viable option.

Unfortunately, as a full-time consultant there is only so much time I can spend on this problem on a gratis basis. (I spent well over an hour last week.) I would be more than happy to visit the facility for one day and sit down with all those involved with this problem and provide recommendations. Perhaps I have now provided you with sufficient additional information to solve the problem, but if the company can fund my visit, we might be able to find some more cost-effective solutions.