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Volumetric flow at the top middle and bottom of a 10ft long bag for all systems at 100 psig

Need To Know: Effective, Efficient Baghouse Cleaning, with Less Energy

March 11, 2013
Understanding reverse pulse-jet technology New converging/diverging nozzle Reducing puffing effects

There cannot be many metalcasting processes that are less exciting than collecting dust, but that’s no reason it cannot be done more effectively. A new nozzle technology makes it possible to reduce compressed-air pressure in reverse pulse-jet baghouse dust collection, its developers say, and therefore to cut energy requirements, and still to disperse the particulates effectively.

Reverse pulse-jet baghouse technology has not changed much in recent decades. These systems involve a blowpipe positioned just above the bag, with an orifice and a Venturi scrubber at the top of the bag/cage. The process uses bursts of 100-psi compressed air, and back-flushes the filters sequentially during the cleaning cycle.

Scientific Dust Collectors’ engineering manager Brian Mathews explained in a recent report that the cleaning cycle is the most important part of dust collection because of its effect on the system’s efficiency and media life. Despite its prevalence, reverse pulse-jet technology has deficiencies, which SDC aimed to correct with the converging/diverging nozzle-based cleaning system it introduced more than 30 years ago.  This “Original” supersonic nozzle eliminated the Venturi and added a cleaning system that improved cleaning, filter life, and lowered pressure drop.

Cleaning system comparison of generic baghouse with Venturi to SDC Nozzle.
Volumetric flow at the top, middle, and bottom of a 10-ft. long bag for the original SDC nozzle at 100 psig, and for the Next Generation nozzle at 80 psig.

Ten years ago, SDC introduced a new converging/diverging nozzle to correct some of the remaining inefficiencies of the reverse pulse-jet baghouse. This Next Generation nozzle improved on SDC’s previous supersonic nozzle design, but Mathews explained that the generic orifice and Venturi system actually draws in a vacuum of air at the top of the bag during the cleaning pulse. The developer produced a separate report to compare cleaning effects at the top of the bag with an orifice and Venturi system, and the SDC nozzle cleaning system. It concluded that the generic orifice and Venturi system does not allow the top 18 in. of filter to be used for repetitive cleaning, rendering useless that section of media.

The Original design has operated successfully at higher air-to-cloth ratios and with a manifold pressure of 100 psig, so the developer investigated the minimum manifold pressure needed for the Next Generation nozzle to achieve comparable cleaning flow at the bottom of the bag as the original supersonic nozzle? The answer is 80 psig, and a comparison of the results are shown in the nearby graph.“The significant increase in cleaning at standard pressure, and similar cleaning achieved at a lower pressure is the result of our many years of research,” Mathews noted. “In the Next Generation nozzle, we have a better understanding of supersonic flow that has allowed us to redesign and fine tune the many facets of the internal converging/diverging nozzle. Also, we were able to review the entire process from start to finish in the nozzle and eliminate flow reducing manufacturing and fabrication issues.”

A 'gentler' cleaning pulse

Among the results, SDC said reducing the manifold pressure from 100 to 80 psig makes it possible to deliver a “gentler” cleaning pulse, and still achieving the same volumetric cleaning at the bottom of the bag. Lowering the manifold pressure slows the supersonic pulse, which slows the air as it enters and provides more balanced cleaning throughout the entire length of the bag.

Another benefit to slowing the cleaning air is reducing the “puffing” effect — a particle-emissions problem that occurs during the cleaning pulse. When the system pulses the bags to remove the dust particles, the particles accelerate and pass through the pores of adjacent bags into the clean air stream. Dust in the clean-air stream is drawn out of the collector to the fan and/or stack. By reducing the manifold pressure to 80 psig, the cleaning pulse slows to roughly 180 FPS, which further decreases puffing.

Along with filter replacements, one of the most significant long-term costs for dust collector maintenance is compressed-air usage. The U.S. Dept. of Energy reported 10- 30% of electricity is consumed for compressed air in a typical industrial facility. Reducing compressed-air supply pressure by 20 psig can reduce annual compressed-air cost by more than 25%, according to Mathews.

The Next Generation nozzle is a flexible design, according to the developer, capable of adapting to almost any dust-collection application. While the Original supersonic nozzle is a one-size-fits-all design, the new nozzle cleaning system can be retrofitted to any dust collector, clean any diameter filter bag, and operate at any pressure above 80 psig, SDC said, and still provide superior pulse-jet cleaning. It also serves as a direct replacement for the company’s Original nozzle.

The advantage of SDC’s Next Generation nozzle is that it establishes a more balanced cleaning system that supplies a significant amount of induced air into each bag, Mathews concluded, using less compressed air and therefore less energy.