Variable Frequency Drives (VFD)
Understanding your dust collector and system as a whole.
All dust collectors in operation today are operating in what is called a Variable System. This is a system where resistance to air flow, also called the “pressure drop,” changes over time. The change in pressure drop is due to the aging of the dust collector filters. When dust collector filters are new, the amount of force needed to draw air through them is low. As the filters age, dust particles become imbedded in the filter media causing the amount of force needed to pull air through the filters to increase.
If nothing is done to correct for the varying pressure drop across the filters, the collector will perform in the following fashion. In the early stages of the filter life when the pressure drop across them is low, the fan will move more air than is required. This produces negative effects on energy consumption and also causes a higher velocity of air entering the filters. When high velocity air enters filters, the dust particles are embedded deeper and filter life is reduced. At the end of the filter life the fan will most likely not be moving sufficient amounts of air to maintain good capture velocity, and dust and fumes may not be collected at a satisfactory level. A graphical representation shown below in figure 1.
Dampers and Dust Collectors
The most common method of controlling the phenomenon of varying pressure in a dust collection system is the use of an outlet damper. Outlet dampers are a mechanical device used to vary the static pressure in a given system. Closing an outlet damper adds artificial pressure to the system, or “damper pressure”. When filters are new, the damper is more closed, adding a high amount of damper pressure to compensate for the low filter pressure. As the filters age, the damper is opened, reducing the amount of damper pressure as the filter pressure increases. Adjusting the damper in this way provides a constant flow throughout the life of the filters. This method is labor intensive, however, prone to human error, and offers no energy saving benefits. It is analogous to driving your car with the parking brake on. You want to go 50 mph, but due to the break you have to provide the same amount of gas that you would to go 80 mph without the break. The representation of an outlet damper collector system is shown below.
What are VFDs?
VFD stands for variable frequency drive. VFDs are electrical devices that can manipulate the characteristics (frequency, voltage, etc.) of the power being supplied to another electrical device. Typically the supplied device would be an electric motor and the characteristic being modified would be the frequency.
Adjusting the frequency of the incoming power is an effective way to change the speed at which a motor runs. The relationship between frequency and speed at which a motor runs is directly proportional. A VFD can change a motor that runs at 3,600 RPM at 60 Hz to run at 1,800 RPM at 30 Hz.
Why would I want or need a VFD on my dust collection system?
VFDs are always the smart choice and a great investment for any dust collection application. They provide several advantages to outlet damper control. The first is that VFDs take the human interaction out of fan adjustment. As the dust collector’s filters begin to plug, the velocity of air entering the collector drops. The VFD can sense this decrease in flow via a pressure transducer located in the dirty air inlet duct. The VFD will automatically increase the speed of the fan and return the system to the optimal operating point of flow and air to cloth ratio.
This is beneficial to dust collector users because it decreases the amount of labor needed to maintain the collector since no one has to adjust an outlet damper routinely. The VFD also insures the collector is constantly operating at the optimum air to cloth ratio. There is no possibility for human error where the outlet damper may be opened too far and the air to cloth ratio is askew, causing premature filter failure.
The most attractive benefit to dust collector owners is the long term energy cost savings. Because fans are variable torque devices, the amount of energy needed to operate them varies with speed. The amount of power the fan draws actually varies the cubic ratio of the change in speed, e.g., a fan with a 25% reduction in speed consumes only 42% of the full speed power, and the same fan with a 50% reduction in speed consumes only 12% of the full speed power. In many cases the utility company offers rebates that cover 50% of the cost of the VFD.
This benefit turns into money in the bank. The customer can save an average of 30% on their energy cost to operate the dust collector. This savings, combined with incentives and rebates from local utility companies, typically returns the cost of investment in is less than a few years.
VFDs are an excellent addition to dust collector applications. The advantages are:
- Reduced maintenance cost due to automatic speed adjustment.
- Increased filter life as a result of the collector always operating at the optimum air to cloth ratio.
- Substantial cost savings associated with reduced speed operation. Quick return on investment.
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