For over 50 years, the orifice and venturi have been an integral part of the cleaning system for the reverse-pulse jet baghouse dust collectors. The venturi-based cleaning system allows dust to be filtered at a nominal air-to-cloth ratio of 5:1.
By Brian Mathews
This cleaning system also has inherent problems that cause a significant decrease in efficiency over time. Not only is the cleaning system unstable in cleaning of a single bag, the cleaning of the single bag affects the neighbouring bags.
During the cleaning cycle, not only is a vacuum created at the top of the bag, but a region of excessive flow. This excessive flow is so substantial that as it expands beyond the limits of the bag and hinders the filters within a close proximity. In a generic baghouse, the nominal bag spacing is 1 in. between the filters (or 5-1/2 in. between centres).
At this spacing, once the flow is above 790 CFM the surrounding bags are affected. Looking back at the results, the peak flow of 3391 CFM is over four times the amount air allowed by the surrounding media!
On the other hand, the patented converging and diverging nozzles cleaning system from Scientific Dust Collectors (SDC) allows for dust to be filtered at a nominal air-to-cloth ratio of 10:1. SDC also utilizes a wider bag spacing of 3 in. between the filters (or 7-1/2 in. between centres). If the generic baghouse were to utilize a similar spacing of 3 in. between filters, once the flow is above 2060 CFM the surrounding bags are affected.
During a typical “generic” baghouse application, the cleaning is accomplished by back-flushing the filter with compressed air that travels through an orifice hole in a purge tube (blow pipe) and through a venturi.
The typical venturi is approximately 6 in. long, has a throat diameter of 1-3/4 in., and placed 2 to 3 in. away from the outside of the purge tube (blow pipe). As the compressed air leaves the orifice, it becomes a jet of air traveling at the speed of sound, Mach 1.0.
This jet of air expands under the Law of Conservation of Momentum at a cone angle of about 15° until it is stopped by the throat of the venturi as shown (Figure 1). This same concept holds true for both 4-1/2 in. and 6 in. diameter bags.
This cleaning system allows dust to be filtered at an air-to-cloth ratio of 5:1. That means for every 5 CFM of airflow, there is 1 square foot of media used to filter the air. This has been the industry standard for over 50 years. Just because this cleaning system has been standard, does not mean that it is flawless. There are several inherent problems with the orifice and venturi cleaning system.
First, due to the proximity between the orifice and throat of the venturi, not enough clean air is induced during the cleaning cycle, and as a result, air is pulled in through the bag from the dirty air plenum to compensate.
Second, due to the energy contained within the compressed air and the close proximity to the orifice, as the jet expands and passes through the venturi at high velocity, it overwhelms the surrounding media with air and can create a puffing effect. Lastly, due to the high velocity of air after the venturi, the induced air is forced out of the bag creating another vacuum further down the bag. As a result, there is a large section of the top of the bag that is unusable for repetitive cleaning of the dust.
Looking at the results of tests conducted by SDC, the generic baghouse is still almost one-and-a-half times greater than the maximum flow allowed by the surrounding media. In fact, for the generic baghouse cleaning system not to affect the neighbouring bags, the bag spacing would have to be 9-1/2 in. (or 14 in. between centres)!
The nozzle-based cleaning only reaches 1436 CFM, well under the maximum flow. This wider spacing promotes dust release while also protecting the surrounding media from blinding. As a result of efficient dust collector design and patented nozzle cleaning technology, SDC is able to operate at higher air-to-cloth ratios, provide better and more complete cleaning of the filter, and longer filter life.
As a recommendation, when quoting baghouse, either buying or selling, remember to ask for wide bag spacing. It is the correct thing to do.
Brian Mathews is Project Engineer at Scientific Dust Collectors.