Monday, June 8, 2009

Cleaning with pulse jet baghouses

Scientific Dust Collectors (SDC) describes the improved cleaning benefits available for reverse-pulse jet baghouses used in dust collection systems

Heavy duty reverse pulse-jet dust collector Heavy duty reverse pulse-jet dust collector

The reverse-pulse jet baghouse has been around for more than 45 years. In a generic model, the cleaning system consists of a blowpipe that is located typically approximately 3 inches above the bag, with an orifice and a venturi, which is located at the top of the bag or cage. The system uses bursts of compressed air at 100psi and back flushes one row of filters at a time during the cleaning cycle.

This cleaning system has some deficiencies, which is why SDC introduced its converging/diverging nozzle based cleaning system in 1981. The key to the performance of this cleaning system is the scientific design of the cleaning nozzle. This patented technology eliminates the venturi at the top of the bag and provides more induced cleaning air into the filter media than other available systems.

Using the original nozzle design, tests showed a 40% increase in cleaning air at the bottom of an 8ft long bag when compared with the venturi-based cleaning system. This significant increase in cleaning flow allows the bags to clean better, enabling SDC to operate successfully with fewer filters and provide the customer with a longer bag life than generic baghouses.

Now the company has improved the nozzle design to achieve an even better filter cleaning system and has conducted extensive tests to show SDC’s nozzle allows more cleaning air to be induced into the bag than a generic system.

The company carried out comparisons of a generic cleaning system, consisting of an orifice and venturi, with a system using its own nozzles; it says a 56% improvement in bag cleaning was achieved with the SDC system compared with venturi-based systems.

Crucial element

The cleaning cycle is the most important element of the dust collector because it controls both the system’s efficiency and media life.
To clean a row of bags in a reverse- pulse jet baghouse, the flow of filtered air must first be momentarily stopped and then back flushed to knock the dust cake off the outside of the filter media. A venturi at the top of the bag or cage is used to accomplish this task. The typical venturi is 6 inches long with a hole measuring 0.75 – 1 inch in diameter.

While the reverse jet with venturi does provide some advantages when compared with other methods, such as reverse air or mechanical shaker cleaning systems, the venturi creates undesired effects during the cleaning cycle. Not only is a vacuum created due to the close proximity of the blowpipe to the venturi, but also an area of significant flow is created directly after the venturi.

This contrast in positive and negative flow is detrimental to the media and as a result, there is a large section at the top of the bag that is unusable for repetitive cleaning of the dust.

Flow variations

During the cleaning cycle, the venturi creates four distinct zones (see Figure 1). The first zone from 3 –7 inches is under negative flow, or a vacuum, pulling in the necessary air
to compensate for the small area available to induce air before the flow enters the venturi. The second zone, from 8 – 12 inches, just below the end of the venturi, is under very high positive flow. A significant amount of air is being forced out of the bag creating a “puffing” effect. The air is propelled at such a force that it embeds into the surrounding media.

The third zone, from 13 – 17 inches is also under a negative flow and more air is being pulled in to compensate for the amount of air forced out in the second zone. In the fourth zone, from 18 – 25 inches, the system finally reaches a stabilised pressure flow.

In comparison, the new SDC nozzle allows for consistent positive flow cleaning throughout the entire bag (see Figure 1). The red zone represents when the venturi cleaning flow is negative and the blue zone is when the venturi cleaning flow is positive. By comparison, the green zone is the SDC nozzle cleaning action, which is uniform.
The disadvantages of the venturi-based cleaning system far outweigh its advantages. Over time, as the dust affects the pores of the media and clogs them, the amount of
air able to aspirate will decrease, causing the cleaning system’s performance to decline.

Inefficient performance

As can be seen in Figure 2, with a system using a venturi, the top 18 inches of the bag cannot be used for repetitive cleaning. Over an 8-feet long bag, for example, that amounts to almost 20% of the media. This is the reason why the air-to-cloth ratio for this type of cleaning system has to be reduced.

For these reasons, SDC maintains that the reverse-pulse jet cleaning system without a venturi offers the best solution in dust collection. SDC’s patented converging-diverging nozzle based cleaning system, which eliminates the venturi, is able to induce enough air to consistently clean the entire bag. Figure 3 illustrates the cleaning flow during operation of one valve. It shows results at the top, middle and bottom of a standard 8ft long bag. As the graph shows, the cleaning results of a venturi-based cleaning system, SDC’s old nozzle cleaning system and SDC’s new nozzle cleaning system.

Thorough cleaning

In looking at the graph, it is clear that both SDC nozzles outperform the generic venturi cleaning system to a significant degree. In fact, the venturi system actually draws in air at the top of the bag during the cleaning pulse, as discussed earlier.

The advantage of using SDC’s nozzle is a more even cleaning system that supplies a significant amount of induced air into each bag for thorough bag cleaning. SDC cleans the entire bag – top, middle and bottom – with considerably more cleaning air than the generic venturi based baghouse. This allows the cleaning system to operate effectively and efficiently with fewer filters, fewer valves and using less compressed air to clean the entire collector.

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