A wide, uniformly pleated filter allows the collected dust to release from the filter, keeping the resistance lower through the filter for a longer time. When pleats of the filter media are tightly packed, the reverse-pulse cleaning system of the dust collector will not eject the dust that has settled in between the pleats; tightly packed pleats increase the resistance of the air through the filters and diminishes airflow.
There are two basic categories of media commonly used in pleated cartridge filters. The choice is usually driven by dust type, operating temperatures, and level of moisture in the process.
- Nonwoven cellulosic blend media is the most economical choice for dry dust collection applications at operating temperatures up to 160 degrees Fahrenheit (71 degrees Celsius).
- Synthetic polyester media or polyester-silicon blend is a lightweight, washable media that can handle dry applications with maximum operating temperatures ranging from 180 degrees Fahrenheit (82 degrees Celsius) up to 265 degrees Fahrenheit (129 degrees Celsius). These filters are washable and can recover from a moisture excursion, but they are not intended for wet applications.
Standard and nanotechnology filter media treated with a flame retardant are recommended for applications considered a fire risk. Conductive or antistatic filters may be used where conveyed dusts generate static charges that require dissipation. Cartridge filters with antistatic media can also be used in explosive dust applications, making it possible to conform to NFPA requirements and lessen the risk of ignition sources due to static electricity charges.
High-efficiency dust collection systems also use self-cleaning mechanisms that regularly pulse dust off the filters, allowing units to run longer between filter change-outs. When a layer of nanofibers is applied on top of the base filter media, it promotes surface loading of fine dust and prevents the dust from penetrating deeply into the filter’s base media. This translates into better dust release during cleaning cycles and lower pressure drop readings through the life of the filter.
Combustible Dust Explosions
A dust explosion occurs when a confined and concentrated combustible dust cloud meets an ignition source. Many solid food and beverage ingredients produce explosive dusts including sugar, starch, flour, spices, tea, grain, and proteins. Good housekeeping and installing a well-designed dust collection system can prevent airborne dust from building up in the work environment, on electrical equipment, and on other areas where dust can accumulate, such as false ceilings.
These measures help negate the risk of a primary or secondary explosion. The primary explosion is the first point where an explosion occurs and is usually an isolated incident. A secondary explosion occurs when the primary explosion pressure disturbs the dust collected in the areas mentioned above, creating a far more extensive and potentially deadly explosion.
Regulations Governing Combustible Dust
In the U.S. there are three key entities involved in combustible dust issues, each with its own area of responsibility.
NFPA sets safety standards regarding combustible dust, amending and updating them on a regular basis. Most insurance agencies and local fire codes state that NFPA standards shall be followed as code. Exceptions would be where the authority having jurisdiction, such as Factory Mutual or local fire marshals, specifies an alternative safety approach, which might be even more stringent.
OSHA, together with local authorities, enforces the standards published by NFPA. OSHA’s Combustible Dust National Emphasis Program (NEP) outlines policies and procedures for inspecting workplaces that create or handle combustible dusts.
U.S. Chemical Safety Board (CSB) is an independent federal agency responsible for investigating industrial chemical accidents. The CSB conducts thorough investigations of combustible dust explosions, sifting through evidence to determine root causes and then publishing findings and recommendations.
Relevant NFPA Standards
In trying to sort through the list of combustible dust standards, a good starting point is NFPA 652, the Standard on the Fundamentals of Combustible Dust. This covers the requirements for managing combustible dust fires and explosions across industries, processes, and dust types. The owner or operator of any facility where combustible dust exists is responsible for conducting a dust hazard analysis to identify hazards, create a plan for managing hazards, and provide training for anyone affected by hazards.