Reports

Proper humidification makes new
GM Corvette's paint shine.

Each new C-5 Corvette Coupe that rolls off the General Motors assembly line in Bowling Green, Kentucky features a state-of-the-art paint finish that is critical to the cars ultimate consumer appeal. Maintaining the environment necessary to achieve such a finish, however, was a daunting challenge for GM plant engineers at the southwestern Kentucky facility, where ambient humidity levels fluctuate widely over the course of the year.

After tracking dirt and paint information over several years, GM engineers determined that dirt problems are diminished when the relative humidity in plant paint spaces is maintained at a minimum level of 50 percent. Air moisture suppresses ambient static electricity, which causes dirt in the environment to migrate to pieces during paint prep and spraying. Now GM Paint Group standards call for maintaining humidity at 50 percent, plus or minus one percent, at all times. This is especially critical at the Bowling Green Corvette plant, which is employing a new process of rack painting parts prior to assembly.
"We have found that maintaining proper humidity in the paint spaces from start to finish can dramatically improve overall paint quality, " says Charlie Gries, plant engineering supervisor. "Our technical specifications now mandate strict environmental control of the painting process in order to stabilize dirt problems and improve paint quality. Maintaining a constant 50 percent relative humidity in these areas is the key. "
After researching the many humidification options available for the Bowling Green facility's 50,000 square-foot clean room, Gries chose two 1,680 pound/hour and one 1,020 pound/hour Herrmidicool systems from the Herrmidifier product line of Trion Inc. in Sanford, NC. With more than 250 no-drip, no-spit atomizing heads that will modulate based on the demands of the spray booth, these systems produce consistently small droplets. GM had already experienced success with the Herrmidicool system in air handler systems at another manufacturing installation.

Herrmidifier worked closely with Gries; with Irv Spindler, system engineer at George Koch & Sons; and with Peter Gray, General Motors Paint Group supervisor. The company installed a state-of-the-art Herrmidicool air/water atomization humidification system for the new 112,000-CFM in-duct air handler system in the plant's new spray area and retrofitted two 40,000-CFM air make-up units in the main paint cleanroom with the Herrmidicool system.

Maintaining proper humidity in the paint areas is only half of the story in achieving improved paint characteristics at the Corvette plant, however. The quality of the water being used to humidify the environment in the plant's prep and spray areas is the other critical link to improving the finish quality on these popular sports cars. In addition to the Herrmidicool humidification system, an Ecotec water pretreatment system was specified to further ensure that environmental conditions in the Corvette plant's clean room, paint preparation and paint spray spaces are maintained at optimal operation levels.
Herrmidifier's tests of the water originally used at the Bowling Green plant indicated high calcium carbonate content. High levels of these dissolved solids in the water used to humidify the plant's clean room environment were likely to lead to undesirable dusting in the paint areas. Pretreatment prior to humidification could eliminate potential dusting problems.

Most HVAC experts agree that water pretreatment can enhance the performance of any humidification system. However, many water pretreatment options can actually promote dusting as well as premature system failure. For example, deionization (DI) and reverse osmosis (RO), two widely accepted pretreatment options, can leave water in a corrosive state. The water treated by these systems is unstable and will actually pull electrons from the metal surfaces to reach equilibrium, resulting in corrosion and loss of metal integrity. Even with typical ion exchange, or water softening, mineral dusting is exchanged for sodium dusting. All three systems leave the water in a charged state. Charged water droplets will combine with ambient dust and eventually find an oppositely charged surface, such as a computer screen or electrostatic paint spray equipment. Ecotec's system offered GM's Corvette plant engineers a safe and stable alternative to conventional water pretreatment options.

Ecotec's system is the only humidification water pretreatment option available that reduces harmful mineral levels while leaving water in a non-corrosive state of equilibrium. The Ecotec system installed at Bowling Green produces safe, healthy humidification by reducing dissolved solids in water, while lowering humidifier maintenance requirements and improving system longevity. The humidification system's performance is thereby improved, dusting is eliminated, and equipment life is extended. The environmentally friendly Ecotec system also operates with less energy, and lower water and sewage costs than would be experienced using other pretreatment systems.

The Herrmidicool humidification system, coupled with the Ecotec water conditioning pretreatment technology installed at the Corvette plant, makes it GM's "cleanest" paint operation to date. "Our desire to create a strict clean room environment in our new paint spray areas was the biggest reason we specified the Herrmidicool humidification system. Maintaining the proper constant humidity level in the paint spray booths has enabled us to stabilize dirt problems and has helped us to improve overall product quality dramatically," says Gries. "The ecologically sensitive water pretreatment in the system has also eliminated the dusting and corrosion problems we saw at other plants, while reducing our energy and water consumption costs as well."

When the New York Times decided to upgrade their existing humidification system, they had several goals beyond the scope of humidification equipment. Their current humidification system had run for nearly 10 years on potable water. This water caused an added routine maintenance burden as scale was constantly forming on the atomizing heads and cooling coils. Atomizing head performance suffered severely causing standing water in the ducts leading to deteriorating conditions of the AHU itself.

When engineers from ECOTEC first met with the operating engineers at the New York Times, a list of goals was established:

1. Minimize Corrosion
2. Prevention of Scale at All Costs
3. Minimize Dusting
      A. Mineral Dusting
      B. Sodium Dusting
      C. Negative Charged Dusting
4. Minimize Maintenance
5. Reduce Operating Costs
6. Prevention of Biological Fouling and Microbial Growth
7. Maintain Proper Indoor Air Quality (IAQ)
8. Reduce the Negative Impact on the Environment

After careful review of all options, the New York Times selected an ECOTEC Model ECOHUMS-20000S. The size of the system was selected to handle their 40 gpm humidification load plus their 20 gpm process load, allowing the New York Times to take their existing R.O. system off line.

The incoming water travels through eight
stages of treatment as it prepares for service:

1. A pre-pressurization system insures 55 psi is available to the PWC/Nano system.

2. A Granulated Activated Carbon Filtration system removes sediment and is an absorbent for certain dissolved gasses and organic matter.

3. A 5 Micron Multi-Cartridge Filtration system is designed to remove small particles. It includes pressure gauges on the inlet and outlet ports providing for an easy service indicator.

4. A anti-scalant injection system was installed in this application for three reasons: One, to control the alkalinity; two, to control the high levels of iron; and finally, to extend the maintenance interval of the membranes.

5. Physical Water Conditioners treat the water based on the principle of nucleation enhancement under the influence of magnetic resonance. Calcium carbonate forms around the newly created nucleation center in the water and the previously dissolved calcium carbonate precipitates to the new seed center and surrounds the center to form the micro crystal or platelet. The viscosity and surface tension are also decreased.

6. Cross Flow Nano Membrane Filtration removes particulate in the 300 to 1000 molecular weight ranges or approximate weight cut-off of 150-300 Dalton for uncharged organic molecules, rejecting selected salts. Divalent and multivalent anions are preferentially rejected by the membrane while monovalent ion rejection is dependent on feed concentration and composition. Since monovalent ions pass through the membrane, they do not contribute to osmotic pressure and with the physical water conditioners ability to decrease surface tension. The membranes permeate flux is enchanced by the physical water conditioner’s improvement of boron retention and the flow rate as well as the reduction of scale formation at the solid liquid interface. At this point the treated water is distributed to four 500 gallons atmospheric storage tanks.

7. The seventh stage is ultraviolet (UV) treatment. The ultraviolet light is part of the electromagnetic spectrum between visible light and X-rays. The portion of the UV range between 200-300 nm is known to be germicidal. The mechanism of microbial destruction is accepted to be the alteration of the cell DNA, which is required for replication. A portion of the conditioned water from each tank is continuously re-circulated by a pump through the ultraviolet system and returned back to the tanks.

8. Finally, a duplex alternating variable speed type re-pressurization system increases the pressure to 80 psig for use by the humidification system. Between the pumps and the bladder-type storage tanks, a pH monitor and injection system constantly maintains the pH level of the conditioned water at 6.9.

With the successful installation of the ECOTEC PWC/Nano, the New York Times achieved their eight goals established at the beginning of the project.