The most commonly used cleaning method is the pulse-jet or pressure-jet cleaning. Baghouses using pulse-jet cleaning make up approximately 40 to 50% of the new baghouse installations in the U.S. today. The pulse-jet cleaning mechanism uses a high pressure jet of air (compressed air-induced pulse) to remove the dust from the bag. Bags in the baghouse compartment are supported internally by rings or metal cages. Bags are held firmly in place at the top by clasps and usually have an enclosed bottom (the bag is sewn closed at the bottom). In another design, a snap ring is sewn into the top of the bag which fits into the tube sheet opening. The cage slides inside the bag and the top of the cage sits on the tube sheet (see Figure 1). Dust-laden gas is filtered through the bag, depositing dust on the outside surface of the bag. Pulse-jet cleaning is used for cleaning bags in an exterior filtration system (See Figure 2).
Fig. 1. Snap-ring bag design for pulse-jet systems
Fig. 2. Typical pulse-jet baghouse with pulsing air supply
The dust is removed from the bag by a blast of compressed air injected into the top of the bag tube. The blast of high pressure air stops the normal flow of air through the bag filter. However, during pulse-jet cleaning, the flow of dirty air into the baghouse compartment is not stopped. The air blast develops into a standing or shock wave that causes the bag to flex or expand as the shock wave travels down the bag tube. As the bag flexes, the cake fractures, and deposited particles are discharged from the bag (Figure 3). The shock wave travels down and back up the tube in approximately 0.5 seconds.
Pulse-jet units are usually operated in a “non-dust cake” mode. Bags are pulsed frequently to prevent the formation of a thick cake and to keep the unit from having a high pressure drop across the dust cake and felted filter. However, sometimes a dust cake is desired in cases where woven bags are used in a pulse-jet baghouse.
Fig. 3. Pulse-jet cleaning
The blast of compressed air must be strong enough for the shock wave to travel the length of the bag and shatter or crack the dust cake. Pulse-jet units use air supplies from a common header which feeds pulsing air through a separate blow pipe located above each row of bags in a compartment. Pulsing air is directed into the bags through nozzles or orifices located on the blow pipe (Figure 4). A diaphragm valve on each blow pipe provides the very brief pulse of compressed air. The opening and closing of the diaphragm is controlled by an electrically operated solenoid valve.
Fig. 4. Pulse-jet cleaning system
In some baghouse designs, a venturi sealed at the top of each bag (see Figure 5) or just inside the top of each bag is used to create a large enough pulse to travel down and up the bag. Vendors using venturis in pulse-jet units claim that the venturis can help increase the cleaning pressure, and thereby improve bag cleaning. In other pulse-jet designs, venturis are not used, but the bags are still cleaned effectively. The importance of the venturis is debatable. The use of venturis has in some cases directed an increased air flow to a specific spot on the bag, and actually caused the bag to wear a hole very quickly. The critical factor to providing thorough bag cleaning is to make sure that the blow pipe and nozzle are properly aligned above the bag tubes.
Fig. 5. Venturis used with pulse-jet cleaning
The bag cleaning by the pulse occurs in approximately 0.3 to 0.5 seconds. The pressures involved are commonly between 60 and 100 psig (414 kPa and 689 kPa). Some vendors have developed systems to use a lower pressure pulsing air (40 psi). Most pulse-jet baghouses use bag tubes that are 4 to 6 in. (10.2 to 15.2 cm) in diameter. The length of the bag is usually around 10 to 12 ft (3.05 to 3.66 m), but can be as long as 20 ft (6.1 m). The shaker and reverse-air baghouses use larger bags than the pulse-jet units. The bags in shaker and reverse-air units are 6 to 18 in. (15.2 to 45.7 cm) in diameter and up to 40 ft (12.2 m) in length. Typical design parameters for pulse-jet cleaning are given in Table
Pulse-jet cleaning parameters
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