Reducing Solvent Emissions in Painting Operations
Crenlo Inc. is a metal fabricator that manufactures electronic and electrical enclosures, and off-road vehicle cabs. During the manufacturing process, materials are sheared, welded, painted and then assembled. Crenlo has two primary painting lines: the enclosure line and the cab line.
In the enclosure line, parts are manually painted in spray booths using electrostatic spray guns. Both smooth and textured finishes are produced. This line changes color over 16 times per day and uses approximately 100 gallons of paint daily. Paint is delivered to the spray booths in reusable five-gallon pails. When the pails are empty, they are sent to a pail-washing system developed by Crenlo.
In the cab line, cabs are manually painted using conventional air-atomizing spray guns. Most of the paint for the cab line is pumped to the spray booths directly from 55-gallon drums. However, some colors are applied at each spray booth using pressurized pots. Color changes are not as frequent on this line as in the enclosure line.
Paint used in the two lines are thinned with solvents that release volatile organic compounds (VOCs) as the paint cures. Additional VOCs are released as fugitive emissions when solvents are used to clean equipment. Solvent contaminated with paint during cleaning processes is shipped off-site for distillation and then returned for reuse at Crenlo. In 1992, Crenlo released over 130,000 pounds of VOCs, as reported on its Toxic Release Inventory (TRI) Form R.
Incentives for Change
Crenlo was interested in reducing its solvent emissions in order to:
- Meet anticipated lower allowable VOC emission levels in the future.
- Address employee health and safety concerns.
- Achieve Minnesota-50 Project goals. By participating in this project, Crenlo made a voluntary commitment to achieve a 50 percent reduction in emissions of targeted priority chemicals by 1995.
The intern project focused on three options for reducing VOC emissions:
- Improve the design of an existing paint pail-washing system to reduce the amount of cleaning solvent used.
- Evaluate the feasibility of using a high-volume, low-pressure (HVLP) paint spray system.
- Evaluate two types of low-VOC coatings: high-solids and waterbased paints.
1. Paint Pail-Washing System
One source of VOC emissions at Crenlo is the paint pail washer. The old pail washer operated like a dishwasher with two cleaning stages followed by a solvent rinse. The solvent remained in a shallow tray on the bottom of the washer until it was sprayed into the pails. The tray exposed a large surface area of solvent to the air each time the washer was opened. Approximately 13 gallons of new solvent were added to the washer each day.
The intern helped construct a new pail-washing system that was designed to minimize the surface area of solvent exposed to the air. The new system holds the solvent in an enclosed container under the washer until it is needed, rather than holding it in a shallow tray within the cleaning compartment. A funnel in the bottom of the washer directs the solvent back into the container where a recirculating pump system directs it back into the washer. The new design allows less solvent to evaporate into the atmosphere, and reduces the amount of new solvent added to the washer. The intern also developed a list of suggested procedures for improving the handling of paint pails and solvents.
2. HVLP Spray System
Paint consumption tests were run on the tractor cab line, which compared Crenlo’s existing conventional spray system to the HVLP system. Tests were performed on identical parts that were sprayed with the same coatings by the same operators. The average amount of paint used to cover each cab was measured for each system and results were compared.
3. Low-VOC Coatings
High-solids and waterbased paints were evaluated as substitutes for the solvent-based paints. All paints tested on production parts required approval from the finishing engineer and the customer to ensure that product specifications were met. In the tractor cab line, four high-solids paints were tested on production cabs as replacements for the current paint. High-solids and waterbased paints were also tested on nonproduction panels as potential replacements for solvent-based paints used in the enclosure line. Line speeds, part configuration and operating procedures were kept similar to current practices.
1. Paint Pail-Washing System
Although the intern did not measure the amount of solvent used in the new washer, it was estimated that solvent use would be reduced by nearly 50 percent or 1,600 gallons per year. This would save Crenlo nearly $2,000 annually through reduced solvent purchase and reclamation costs. The redesigned pail washer was constructed for $2,200 and is currently in use at Crenlo.
2. HVLP Spray System
The HVLP spray guns required 10 percent less paint to cover the same parts as the conventional spray system due to reduced overspray. At this reduced rate, Crenlo would reduce its use of thinned paint in the tractor cab line by 2,100 gallons, saving $37,000 annually. The finish applied with HVLP guns exhibited a uniform dry-film thickness. Crenlo has purchased 14 HVLP guns and pressurized pumping equipment for its cab line at a total cost of approximately $24,000.
3. High-Solids, Low-VOC Coatings
Crenlo has converted approximately 90 percent of its coatings from conventional to high-solids paint in the cab line. Converting to a high-solids system will save approximately $37,000 annually and reduce annual VOC emissions by 45,000 pounds. No additional equipment will be required for the conversion.
In the enclosure line, reduced VOC emissions and cost savings could result by converting to either the high-solids or water-borne paint systems. At a negligible cost for conversion, using high-solids paints in the enclosure line would reduce VOC emissions by an estimated 70 percent (40,000 pounds) and save $86,000 annually with an almost immediate payback.
Converting to a waterbased painting system for the enclosure line would cost an estimated $132,000, but would reduce VOC emissions by an estimated 70 percent. This would also result in annual savings of about $120,000 annually, with a payback of approximately one year. Tests revealed that the transfer efficiency of waterbased paint was 52 percent, followed by high-solids paints at 39 percent, and solvent-based paints at 32 percent. However, tests showed that the waterbased paints tended to blister on the nonproduction panels, and the high-solids paints caused excessive paint thickness. Further testing for each system will be required to address these concerns.
Approximately $162,000 in total annual cost savings could result from using the new paint-pail washer and converting to an HVLP spray system and low-VOC coatings, assuming Crenlo selected a high-solids paint system. The one-time cost for these modifications is estimated at $26,000.
The above changes could also reduce annual VOC emissions by 109,000 pounds. Additional low-cost ways to further reduce VOC emissions include improving procedures for solvent handling and equipment cleaning.
Since the project ended, Crenlo has decided to implement both water-borne and high-solids paint systems on the enclosure line. Six stainless steel pumps have been purchased to pump the water-borne paint. In addition, another paint pail washer is being built for the cab line.