However, one of finishing's more significant variables, spray booth ventilation, remains very difficult to control. While some enterprising technicians have attempted to monitor and control booth ventilation, few if any have achieved long-term success. Dry filter booth exhaust airflow varies much more than water-wash booth airflow. Resistance to airflow in filter-type booths increases as the arresting filters load with overspray. Because the tubeaxial propeller fans in widespread use are relatively inefficient air movers, the exhaust airflow steadily decreases as overspray accumulates in the booths' arresting filters. In comparison, properly maintained water-wash booths have relatively stable airflow. Traditional airflow control methods do not facilitate reliable monitoring or the effective real-time control of a dry filter or water-wash spray booth's actual exhaust airflow rate.
Compliance coatings and environmental rules have changed, making dry filter booths - once largely relegated to off-line and touch-up operations - the booth of choice for many industrial finishers. They are not plagued with nozzles and pumps that may become plugged with paint sludge nor do they contain hundreds of gallons of water laced with potentially hazardous paint-related chemicals. Unfortunately, dry filter booths do not provide a stable exhaust airflow environment. The red line on Figure 1's chart accurately depicts a typical crossdraft industrial dry filter booth's exhaust airflow variations over a complete filter loading cycle. In a typical dry filter booth it is common for the exhaust velocity to vary from 15 percent to greater than 60 percent over a single filter load cycle.