Once paint has been applied to the product, the paint must be cured in a curing oven (figure 1). The hot air used to cure the product must be free of outside contaminants that could affect the finish. Once heated, this clean air is forced down toward the product at a constant rate. The balancing of airflow during the curing cycle is accomplished with pressure sensors feeding a setpoint controller. The setpoint controller output moves actuators connected to balancing outlet dampers that automatically control exhaust-air volume or booth-differential pressure.
As indicated in the figure, as less airflow is required by the system, the outlet dampers must close to a level that restricts the output. If 75 percent airflow were required, the pressure in the system ahead of the damper would rise to approximately 125 percent. The fan, which continues to operate at rated speed, now has to work harder for less airflow output to overcome the loss of head pressure across the damper.
This control method is sometimes referred to as “riding the fan curve.” As shown in figure 2, outlet damper control and riding the fan curve results in a small reduction in brake horsepower (BHP) at the reduced flow rates. This method of control can be compared to driving a vehicle with one foot on the accelerator and the other foot on the brake. It is literally supplying energy (kW) to the fan to develop pressure, only to bleed the pressure off with the head loss across the restricting outlet damper.
The electrical signal (0 to 10 VDC, 4 to 20 mA, etc.) that was used to control the damper position now can be used as the speed reference signal for the VFD. With variable-speed operation, each speed represents a different fan curve. Running a fan at reduced speed produces a new fan curve or “map” roughly parallel to the full-speed design curve. From the Affinity Laws (also called Fan Laws), it is known that fan output (cfm) is directly proportional to the speed of the fan (figure 3). Static pressure is proportional to the fan speed squared, and fan-required horsepower is proportional to the fan speed cubed.
In regard to power consumption, the use of a VFD can provide savings over outlet dampers. Power consumption savings can be dramatic too, with VFD retrofits of paint spray booths that utilize inlet guide vanes. One automotive manufacturer saved 56,200 kW-hr per year by installing a 45 kW (60 hp) AC drive. The manufacturer also realized more stable process control, less CO2 emissions and improved paint quality.
Because VFDs are electronic, they can simultaneously accept electric signals from sensors, mathematically process those signals like a computer, and control the speed of a motor. This is done via proportional integral derivative (PID) control.
VFDs also can be used to control variable-speed conveyors such as those in a product paint-curing oven. Many processes can be optimized if the correct temperature and speed are used.
Overall, the payback in installing or retrofitting VFDs to your ovens can be seen in energy savings, improved process control and quality. Check with your local electric utility company, as some offer rebates.