Optimizing factors such as a dust collection system's hood and ducting can enable the use of smaller, more energy-efficient motors.

For environmental and worker safety reasons, among others, effective dust collection is a must. Mechanical and media filtration dust collectors are designed primarily for "dry" industrial dusts, but some tolerate a minimal amount of liquid contaminants. The most common mechanical dust collector is the cyclone collector. Typical media filtration collector types include media fan/filter, cartridge filter and bag house.

Cyclones are the oldest type of dust collection device. They operate by spinning the collected material within the device using centrifugal force to direct the dust to the outside wall of the separator. Gravity and mechanical internal deflectors direct the dust-laden air in a downward spiral and discharge the dust through the cone bottom. Cyclones can also be used as pre-separators for heavy concentration levels.

A self-contained media fan/filter unit is the simplest form of industrial filtration device. It works by passing contaminated air through a pre-filter, which collects larger particles, then through a primary filter where smaller particles can be captured. The clean air is then returned to the workplace.

Continuous cleaning cartridge filter units use pleated paper or polyester cartridge filters. The filter elements are cleaned on-line during the dust collection process. Cartridge units offer high filtration efficiency and are capable of trapping up to 99% of sub-micronic materials and virtually 100% of larger dust particles.

A bag house dust collector is a tubular bag device that, like cartridge filter units, can be cleaned on-line during the dust collection process. This results in a relatively constant airflow, as well as energy savings. Bag house dust collectors are highly efficient in the collection of fibrous and other large-size process particulate at relatively high concentration levels.

The design and location of a dust collection system's hood, ducting, collector and fan can collectively add sufficient static pressure requirements to the point where larger, more expensive-to-operate motors are necessary to maintain effectiveness. However, optimizing these areas can make it possible to use smaller, more energy-efficient brake horsepower motors. The electrical savings potential for a simple dust control system can be at least $2000 per year, and the savings increase significantly for larger systems.

Hood Design/Location

When a duct is under suction with a plain end opening acting as a hood, air is pulled into the opening from the front, sides and even behind the opening. The plain end, also known as a raw edge orifice (see Figure 1), has a lot of resistance associated with the opening. Specifically, this resistance measures 0.93 velocity pressure (VP). Ideally, air should be pulled in from the front. Placing a flange around the plain opening also allows air to be drawn from a further distance, which lowers the resistance to 0.50 VP. The perfect hood, in terms of energy savings, is a bell mouth-shaped fitting with a resistance factor of 0.04 VP (see Figure 2).

The VP in a duct comes from the actual velocity in the duct; 4005 feet per minute (fpm) velocity equals 1.0 VP. Energy is required to make the contaminated air flow into the hood opening, and this is always calculated with a factor of 1.0 VP (duct). When multiplied by the actual duct VP, this 1.0 VP will equal the static pressure (SP), which is measured in inches water gauge (wg, defined as pressure equal to that which is exerted by a column of water of the same height). Static pressure is a measure of the differential air pressure between the air pressures inside a duct and the ambient air pressure outside the duct. As static pressure increases, airflow decreases, and larger motors are required to maintain effectiveness.

The hood static pressure equals the hood VP multiplied by the actual duct VP, plus the energy loss to make air flow into the hood. The static pressure requirement of a bell mouth-shaped hood is 1.04 in. wgSP. In comparison, the plain opening design requires 1.93 in. wgSP, for an increase of 0.89 in. wgSP.

Further energy savings result when the collection hood is located as close as possible to the point of dust generation. This reduces the volume of air required to collect the dust. If the dust generated is 12 in. from the hood opening, a volume of 1000 cu ft per minute (cfm) might be required. But, if the hood opening is 24 in. away, the required cfm volume increases as the square of the distance to 4000 cfm.

Duct Design

The air velocity required to carry collected dust through the ducting is another important consideration. If the collected dust can be conveyed at 3500 fpm when VP = 0.76, it is disadvantageous to convey the dust at 4500 fpm when VP = 1.26. At 3500 fpm, the duct friction factor (the friction created by the duct itself) is 0.018 VP per foot of duct (0.018 VP x 100 ft = 1.8 VP x 0.76 = 1.37 in. SP). At 4500 fpm, however, the duct friction factor is 0.02 VP per foot of duct (0.02 x 100 ft = 2.0 VP x 1.26 = 2.52 in. SP). The slower speed saves 1.15 in. of wgSP.

Elements of ducting include straight duct, fittings like elbows and entries, and special fittings. The design of these components also affects static pressure and, therefore, energy requirements. For example, ducting with a well-designed branch entry of 30° has a duct friction factor of 0.18, whereas a 45°-branch entry has a 0.28 factor. At 4000 fpm, VP = 1.0 (1.0 x 0.28 = 0.28 in. wgSP) for the 45° branch. In comparison, the 30° branch entry only requires 1.0 x 0.18 = 0.18 in. wgSP, for a savings of 0.10 in. wgSP.

In addition, duct elbows with a 1.5 diameter radius can have a duct friction factor of 0.24, and a 2.0 duct elbow radius can have a factor of 0.19. Using the same factors as above, the 2.0 diameter radius will save 0.05 in. wgSP. The total savings gained from well-designed ducting for a simple dust collection system is 1.15 + 0.10 + 0.05 in., for a total of 1.30 in. wgSP.

Dust Collector Operation

Additional energy savings can be obtained by using a Photohelic^® gauge to control the pulsejet cleaning cycle in place of the traditional Magnehelic^® gauge. A Magnehelic gauge simply measures SP in inches wg. In a system with this type of gauge, the compressed air is always on.

A Photohelic gauge is a Magnehelic gauge with the added benefit of electrical contacts that control the pulsejet cleaning cycle, which gives operators the flexibility to set the high and low static pressure points at which the unit pulses. If a dust collector is set to operate with nominally dirty filters at 4 in. wgSP instead of the more common 5 in. wgSP, a savings of 1.0 in. wgSP can be achieved. Controlled cleaning with a Photohelic gauge not only saves compressed air and its associated energy costs, it also extends filter media life.

Fan Ducting

Air has weight, and if the ducting in and out of the fan is not properly designed and installed, the weight of the air loads the fan unevenly. This can cause the fan to pulse, or surge, and adversely affect performance. Fan pulsing must be overcome by adding extra static pressure. If the SP goes up on a fan performance curve and is not accounted for, cfm performance will go down. As cfm performance lessens, the hood performance will suffer and possibly allow contaminants to enter the workers' breathing zone.

One relatively common design includes the installation of a two-diameter 90°-radius duct elbow right at the fan inlet. This only serves to add 1.0 in. VP and (with inlet velocity at 4000 fpm) 1.0 in. wgSP. To improve efficiency, fan ducting designs should incorporate 7-10 duct diameters of straight ducting between the elbow and the fan inlet.

Potential Savings

The accumulated SP losses from poor design on a small cfm system can add approximately 4.20 in. SP (see Table 1). If a small dust collection system has a system static pressure (SSP) of 9.0 in. wg, poor design can add an additional 4.19 in. SSP, for a total of 13.19 in. wg. Assuming 2000 cfm, the brake horsepower (BHP) requirement for the 9.0 in. wg system would be 5.43. At 13.19 in. SSP, 7.65 BHP would be required.

Assuming the use of motors having the same efficiency and operating 8760 hours per year at a cost of $0.11 per kilowatt-hour, the annual operating cost of the larger 7.65 BHP motor would be $6706. In comparison, the annual operating cost for the smaller 5.43 BHP motor in a simple dust control system would be $4760, for an overall electrical energy cost savings of approximately $2000 per year.

The energy cost savings potential increases in direct proportion to the size and complexity of the dust collection system, and will depend on individual dust collection situations and requirements. For this reason, it is recommended that manufacturers consult an expert to evaluate their specific dust collection system requirements and the design approaches that will make the most sense in terms of economics and energy efficiency.

For more information regarding dust collection systems, contact United Air Specialists, Inc., 4440 Creek Rd., Cincinnati, OH 45242; (513) 891-0400 or (800) 252-4647; e-mail sales@uasinc.com; or visit www.uasinc.com.

Links

• United Air Specialists, Inc.