Cleaner Production

A pollution prevention audit was carried out to improve the environment and efficiency and working conditions in the painting areas.30 The objective of this program was to reduce the quantity of wastes and costs of painting by a combination of improvements to the technology and good housekeeping. The overall aim was to improve the quality of coating, to reduce the amount of paint raw material and to reduce the quantities of wastes.

The existing painting method was compared with two more advanced painting technologies. The transfer efficiencies for the different methods are as follows:

1. Air-atomized spray (conventional), 30 to 50%

2. Airless spray, 65 to 70%

3. Pressure-atomized electrostatic spray, 85 to 90%

Cost Saving in 2007 USD/yr

Total savings Capital investment Payback period

251,000 USD/yr 47,000 USD 2 months

In conventional spraying, compressed air is used both to atomize the paint and to carry it to the surface to be painted (Figure 1.5). With airless spraying the paint is pumped under high pressure to a small jet where the high velocity is sufficient to induce atomization. The lack of any expanding compressed air stream eliminates unwanted spray mist, reduces the loss of paint by overspray, and most of the paint adheres to the work surface (Figure 1.6). With pressure-atomized electrostatic spray, paint is delivered at high pressure as before, but it is fed to an insulated nozzle. An electrostatic charge of about 100kV is applied to this nozzle. The charging of the paint particles assists the atomization and causes them to repel each other. Additionally, the charged paint moves along the field lines to the earthed work piece. As the electrostatic field lines envelop the object the paint particles cannot fly straight past, but "wrap" themselves uniformly around the surface. It is this effect that gives the high paint efficiency and reduces waste (Figure 1.7). Note that electrostatic hand spray guns require a small mains transformer and a much reduced current to avoid accidental electrical shock. Comparison of raw material consumption and waste quantities of the different methods are as follows14:

Air-Atomized Spray Airless Spray Pressure-Atomized Electrostatic Spray

Wastes (kg) 2400 1400 500

Compressed air FIGURE 1.5 Air atomized paint spraying.

- Paint
FIGURE 1.6 Airless or high pressure paint spraying. (From Wang, L.K. et al. Case Studies of Cleaner Production and Site Remediation, Training Manual DTT-5-4-95, United Nations Industrial Development Organization, Industrial Sectors and Environment Division, Vienna, Austria, April 1995.)

FIGURE 1.7 "Wrap around" effect of electrostatic paint spraying. (From Wang, L.K. et al. Case Studies of Cleaner Production and Site Remediation, Training Manual DTT-5-4-95, United Nations Industrial Development Organization, Industrial Sectors and Environment Division, Vienna, Austria, April 1995.)

FIGURE 1.7 "Wrap around" effect of electrostatic paint spraying. (From Wang, L.K. et al. Case Studies of Cleaner Production and Site Remediation, Training Manual DTT-5-4-95, United Nations Industrial Development Organization, Industrial Sectors and Environment Division, Vienna, Austria, April 1995.)

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