Knowing how to utilize variable-frequency drives in paint application and curing equipment can boost productivity, save energy and increase your bottom line.

Figure 1. In a paint curing oven in which the fan is controlled by a variable- frequency drive, airflow is controlled by controlling the fan motor speed.

Energy savings is one of the key ingredients in the reduction of costs for any manufacturing operation. Variable-frequency drives (VFDs) provide the adjustable motor speed that can reduce energy consumption on paint curing lines.

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.



Figure 2. In an oven outfitted with damper controls, while the flow (cfm) decreases, the pressure increases on the primary side of the control damper. However, due to pressure loss across the outlet damper, the pressure downstream of the damper decreases. The pressure/flow condition of point 2 corresponds to approximately 23 BHP of energy required. The pressure/flow condition of point 3 corresponds to approximately 20 BHP of energy required.

An opportunity for energy savings lies in the use of a VFD control rather than outlet dampers. During the use of outlet dampers, the pressure within the system increases to restrict the amount of airflow needed for the process (figure 2).

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.



Figure 3. The Affinity Laws state that fan output (cfm) is directly proportional to the speed of the fan; static pressure is proportional to the fan speed squared; and fan-required horsepower is proportional to the fan speed cubed.. The Affinity Laws state that fan output (cfm) is directly proportional to the speed of the fan; static pressure is proportional to the fan speed squared; and fan-required horsepower is proportional to the fan speed cubed.

A more efficient system would employ a VFD to operate the fan motor. In a VFD system, the speed of the fan motor is reduced, thereby reducing the amount of airflow. Simply locking the outlet dampers at fully open position and supplying a VFD can retrofit an outlet damper system to a variable-speed system. Another solution would be to remove the outlet dampers, as even fully open dampers have some associated head drop across the device.

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.



Figure 4. In systems equipped with VFD control, to produce 50 percent airflow, only 12.5 percent of rated horsepower is required.

If 75 percent airflow was required, as in the previous example, pressure in the system would be reduced to approximately 50 percent, not raised 125 percent, as noted earlier. The motor does not work as hard, and energy is saved because the fan speed is reduced to produce a corresponding airflow (figure 4). According to the Affinity Laws of pressure, horsepower and speed, to produce 50 percent airflow, only 12.5 percent of rated horsepower is required. This can yield a tremendous amount of energy savings if the system is operated at 50 percent to 75 percent airflow for half or more of its operating period. Figure 5 summarizes the type of energy savings (reduced power consumption) that can be realized using a VFD.

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.



Figure 5. The shaded area between the curves represents the potential savings. The system curve is the theoretical Affinity Law energy required.

Because VFDs use power-switching devices, the theoretical curves can never be fully realized. The shaded area between the curves in figure 5 represents the potential savings. The system curve is the theoretical Affinity Law energy required. However, modern VFDs are efficient; therefore, the difference between the calculated theoretical and VFD curves is small.

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.



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