Dust collectors consume electrical energy the whole time they are running, but the largest portion of the electrical load goes to the fan motor that moves the air through the system. A variable frequency drive (VFD) can minimize the energy used by the fan motor.
Energy consumption is directly proportional to the volume of air (or CFM) that the motor is moving through the system. Dust collectors are variable systems. Their resistance to airflow (pressure drop) changes over time, according to how loaded the filter cartridges are with dust.
Without any intervention, in the early stages of the filters’ life when the static pressure across them is low, the fan will move more air than needed. This consumes unnecessary energy and also causes air to hit the filters at a high velocity, which reduces filter life.
In the late stages of the filters’ life when they are loaded with dust particles, the airflow becomes restricted and the fan has to work harder to keep the airflow high enough to capture the dust particles. This increases the static pressure, also referred to as pressure drop, which is measured by inches of water. At this point, the airflow needs to be adjusted to avoid excessive energy usage. This can be accomplished manually or by installing a variable frequency drive (VFD).
Manual Airflow Adjustment
Dust collectors typically use a damper at the outlet of the fan motor to mechanically vary the system’s static pressure. One option to alter the airflow is to manually adjust these dampers. When the filters are new, the damper can be closed more to achieve the desired airflow. As the filters become dirty, the damper can be opened more to increase airflow.
This figure illustrates the typical relationship between a constant speed fan and energy usage when using an outlet damper to mechanically control the system’s static pressure.
Energy Control Device
A better option is to use a variable frequency drive to electrically control the fan speed. A VFD is an electrical device that automatically manipulates the frequency and power consumption supplied to the fan motor. Routine human interaction is no longer required. The VFD, equipped with an airflow or static pressure sensor, will automatically sense changes in airflow and pressure drop, and will adjust the fan speed to return the system to optimal airflow. Operators achieve significant electrical savings over the long term because the amount of energy needed to operate the fan motor varies with speed.
When the filters are new, the drive decreases fan speed to obtain the desired airflow. When the filters become loaded with dust, the drive speeds up the fan to keep the airflow consistent. The electrical control is much more efficient than human intervention in keeping the desired airflow and minimizing the electrical energy consumed.
Adjusting the frequency of the incoming power is an effective way to change the fan motor speed since their relationship is directly proportional. For example, a VFD can change a motor that runs at 3,600 RPM at 60 Hz to run at 1,800 RPM at 30 Hz. The fan draws only the amount of power required for the specific fan speed. For example, a fan that runs 25% slower would use 42% of the power that would be required for full speed. The same fan running 50% slower would use 12% of the full speed power.
The bottom line is that VFD’s enable users to save an average of 30% on their energy costs to operate the dust collector. Also, maintenance and operation costs are reduced because fan speed adjustments don’t require human intervention.
The figure to the right illustrate the multiple relationships that define the amount of energy being used at different speeds. These graphs were acquired from the 26th edition of the Industrial Ventilation manual of recommended practice for Design, figure 7-9b). Variable frequency drives have been proven to save an average of 4” w.g. of static pressure over the life of the filters. The additional capital cost savings possible by installing a VFD on a dust collection system will vary with different applications. However, the return on investment is typically under a year.
Consider this example
You have a dust collector with a 50 HP motor running at 85% efficiency, 460 Volts, 60 Hz frequency, 57 full load amps current and 1500 RPM. If the VFD operates 24/7, the collector would use 38.6 kW power at full load. If the electricity rate is $.10 per kWh, it would cost $33,776 per year to operate the collector. By installing VFD (estimated cost $11,000), the same dust collector would cost $17,012 per year to operate, saving $16,763 per year. The VFD would pay for itself in just under 8 months.