Proper humidification makes new GM Corvette's paint shine

June 1, 2000
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General Motors manufactures its new Y-car project, the C-5 Corvette Targa, at its Bowling Green, Ky. facility. Each new Corvette Targa needed to roll off the assembly line equipped with a state-of-the-art paint finish, critical to its ultimate consumer appeal.

Each new Corvette Targa gets painted in Bowling Green’s 50,000-sq-ft cleanroom environment, ensuring a dirt- and contaminant-free finish. Strict GM Paint Group standards require that humidity inside the cleanroom be maintained at 50% at all times.

This was a daunting challenge for plant engineers in southwest Kentucky, where ambient humidity levels fluctuate widely over the course of the year.

After researching the humidification options, plant engineering supervisor Charlie Gries chose two 1,680-lb/hr, and one 1,020-lb/hr “Herrmidicool” systems from the Herrmidifier Co., Lancaster, Pa.

The systems employ more than 250 “no-drip, no-spit” atomizing heads that modulate based on the demands of the spray booth. The consistently small droplet produced by the atomizing heads, and more than 15 years of experience applying these systems, resulted in many repeat customers for Herrmidifier.

GM is such an example. The Herrmidicool system had been used successfully in air handler systems at another GM manufacturing installation, and came highly recommended by its contractors, George Koch & Sons Hvac, of Evansville, Ind.

To meet GM’s stringent standards, the Herrmidifier Company worked in cooperation with George Koch & Sons’ system engineer Irv Spindler, General Motors’ paint group supervisor Peter Gray, and Corvette plant engineering supervisor, Charlie Gries.

A state-of-the-art Herrmidicool air/water atomization humidification system for the new 112,000-cfm, in-duct air handler was installed in the plant’s new spray area. Two 40,000-cfm air makeup units in the main paint cleanroom were retrofitted with the humidification system.

In addition, an “Ecotec” water pretreatment system was specified to ensure that environmental conditions in the Corvette plant’s cleanroom, paint preparation, and paint spray spaces were maintained at optimal operation levels.

“This plant maintains a strictly controlled environment for each Corvette piece being painted,” said Gries. “We found that maintaining proper humidity in the paint spaces from start to finish dramatically improves overall paint quality.

“Our technical specifications mandate strict environmental control of the painting process in order to stabilize dirt problems and improve paint quality. Maintaining a constant 50% relative humidity in these areas is key.”

After tracking dirt and paint information over several years, GM engineers were able to determine that dirt problems diminished when relative humidity in plant paint spaces is maintained at a minimum level of 50%.

They found that moisture suppresses ambient static electricity, causing dirt in the environment to migrate during the paint prep and spraying processes. The Bowling Green Corvette plant, where this became especially critical, employed a new process of rack-painting parts prior to assembly.

They found that maintaining proper humidity in the paint areas only included half the story in achieving improved paint characteristics at the Corvette plant. The quality of the water used to humidify the environment in the plant’s prep and spray areas constituted the other critical link to improving the finish quality on the automobiles.

Herrmidifier engineers tested the water used at the plant and discovered it was high in calcium carbonates. They consequently advised Gries that high levels of dissolved solids in the water used to humidify the plant’s cleanroom environment could lead to undesirable dusting in the paint areas.

They recommended the addition of a water pretreatment system at the front end of the humidification installation, in order to ensure the elimination of potential dusting problems at the back end of the system.

Most hvac experts agree that water pretreatment enhances the performance of any humidification system. However, most don’t know that many existing water pretreatment options can promote dusting and premature system failure. For example, deionization and reverse osmosis, two widely accepted pretreatment options, can leave water in a corrosive state, according to Herrmidifier.

The water treated by these systems is unstable and can pull electrons from the metal surfaces to obtain equilibrium, resulting in corrosion and loss of metal integrity. Even with typical ion exchange, or water softening, mineral dusting exchanges for sodium dusting.

All three systems leave water in a charged state. Charged water droplets combine with ambient dust and find an oppositely charged surface, like a computer screen or electrostatic paint spray equipment.

Spindler alerted plant officials that many commercially available water pretreatment technologies are ineffective in minimizing dusting and could corrode the humidification system within a short time. Therefore, he specified Herrmidifier’s Ecotec water pretreatment system.

The system reduces harmful mineral levels while leaving water in a non-corrosive state, according to the manufacturer. The system installed at Bowling Green produces safe, healthy humidification by reducing dissolved solids in water, while lowering humidifier maintenance requirements and improving system longevity.

Ecotec also operates with less energy, at lower water and sewage costs than other pretreatment systems.

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