Pulse jet collectors use bags supported from a metal cage fastened onto a tube sheet at the top of the collector as shown in the figure below. Dust-laden air enters the collector and flows from outside to inside the bag. The dust cake deposits on the outside of the bag and is cleaned by short bursts of compressed air injected inside the bag.
The burst of air sends a shockwave down the bag causing it to flex, breaking and releasing the dust cake. The compressed air must be clean and dry or moisture can build up on the bags, hindering the bag cleaning efficiency. Pulse jet collectors are not required to be compartmentalized, which allows for continuous bag reconditioning without process upset.
Pulse jet collectors are the most common type of baghouse and have been in use since the late 1950s. Pulse jet collectors use a pulse of compressed air (60–90 psig) for filter conditioning. The pulses are triggered based on differential pressure between the clean and dirty air plenums.
Adequate compressed air volume, pressure, and quality are critical for cleaning pulse jet collectors. For suitable volume and pressure, the design must accommodate for periodic demand usage on each baghouse without interruption. Typical best practice installations have demand storage or small receiver tanks incorporated into the design to ensure that a proper amount and pressure of the cleaning cycle air is delivered to the baghouse for each cycle. In some instances, it is necessary to have an independent storage to ensure proper function even during loss of electrical power.
Compressed air quality standards according to the International Organization for Standardization (ISO) 8573 [ISO 2017] are also critical in order to maintain expected moisture and oil contaminations. Foreign fluids can cause loss of function involving caking and fabric deterioration. Class 2 or 3 ISO 8573 are typical relevant design standards for pulse jet collectors.
Reconditioning or cleaning of the filter bags allows the dust collection system to maintain pressure drops and operate at designed airflows. The advantage of using pulse jet collectors is high product recovery and high collection efficiency. They also enjoy high flexibility of application with many inlet design options. Their limitation is that their performance varies with temperature and moisture.
Due to more frequent cleaning intervals, these collectors provide more complete bag cleaning than the previously discussed collector styles. Thus, the air to cloth ratios can be higher— typically, suppliers specify air to cloth ratios of 6:1 or higher. However, ratios of 4:1 should be used for applications involving abrasive minerals. High air to cloth ratios can cause high air velocity impingement on the bags, resulting in dust re-entrainment on neighboring bags after cleaning and low bag life.
Bags are typically made from nonwoven felt or polyester material. Woven materials are not used in pulse jet collectors because they require a buildup of a permanent dust layer to provide efficient air cleaning. Since pulse jet collectors aggressively clean the bags, excessive penetration of dust particles through the woven fabric can occur. Pleated bags can also be installed in place of traditional cylindrical bags. Pleated bags can offer higher cloth area in the same space as a traditional bag but at a higher initial cost.
Pulse jet collectors are more cost effective than collector styles discussed previously such as mechanical shaker collectors. They can operate with higher air to cloth ratios, have no moving parts to maintain, and involve lower capital costs.
Reference
NIOSH Mining Program Report of Investigations, «Dust Control Handbook for Industrial Minerals Mining and Processing»
