Monday, August 10, 2015

FAQ: Does a dust buildup or “dust cake” on dust collector filters indicate that the system is not working properly?

Some people worry that their dust collection system is not performing properly when they open the collector and see dust on the filters. It is just the opposite: A controlled level of dust on your filters actually improves the filtration efficiency by helping to filter out more dust particles, thereby extending the life of the filters.

A dust collector automatically pulse-cleans its filters using very brief bursts of compressed air that blow the dirt off the filter surfaces and down into a collection device. Most dust collector control panels offer a choice of three pulse-cleaning methods:

  1. Continuous pulsing describes a dust collector that pulses at a set time period – for instance, every 10 to 15 seconds. Sometimes this method is absolutely required, but this pulse-cleaning method uses the most compressed air over the life of a dust collector and is very costly.
  2. On-demand pulsing only activates the cleaning system when the collector’s pressure drop reading goes above a preset high set point and pulses back to a preset low point on the collector’s control panel. This method typically conserves on compressed air as the collector only periodically pulses. In addition, most dust emissions pass through a filter when the reverse pulse occurs in a dust collector, so fewer pulses will equal low dust emissions for most dust collectors.
  3. Downtime pulsing is used while the dust collector fan is not running. Using downtime pulsing reduces dust re-entrainment and will return your filters to their lowest possible pressure drop reading.

To maintain a controlled dust cake on filters, it is recommended that you pulse your dust collection system as little as possible – avoiding continuous pulsing unless absolutely necessary. This approach will also save on compressed air, reducing the total cost of ownership to operate your dust collection system. Your manufacturing plant probably has a value placed on compressed air and you understand that excessive compressed air usage gets expensive.

If filters become totally blinded by heavy dust buildup, and/or the pressure drop reading has reached a level higher than the filter manufacturer’s recommended limit, it’s time to change the filters. However, even if this situation has not occurred, it’s a good idea to replace filters every two to three years at longest. An older filter may develop a hole or leak after time and will no longer deliver the guaranteed efficiency.

Request/download a comprehensive white paper on dust collector filter selection here.

Wednesday, December 10, 2014

Five Tips for Selecting Cartridge Dust Collector Filters - #2

#2 Test Your Dust

The collection and testing of dust samples is a long-established practice used by manufacturing professionals to make informed dust collection decisions. While dust testing has always been good practice, it is rapidly becoming a necessity in today’s regulatory climate.
An engineered test is performed at an official testing facility to confirm that this dust collector can withstand an explosive event.

There are two types of dust testing: (1) bench testing, which pinpoints physical properties of the dust, and (2) explosibility testing, which determines whether a dust is combustible.

Bench testing involves a series of tests that provide valuable data for filter selection. Particle size analysis reveals the dust’s particle size distribution down to the submicron range to determine the filtration efficiency needed to meet emissions standards. This test pinpoints both the count (the number of particles of a given size) and the volume or mass spread of the dust. Knowing both is important because many dusts are mixed.

A video microscope provides visual analysis of the dust shape and characteristics. Together with particle-size analysis, this tool is vital for proper filter selection. For example, a microscope may be needed to detect oil in the dust.

Moisture analysis equipment measures a dust’s moisture percentage by weight, providing information that can prevent moisture problems. A humidity chamber is used to see how quickly a dust will absorb moisture. This helps to identify hygroscopic (moisture-absorbent) dust which requires widely pleated filter cartridges, as these sticky dusts cause filters to plug. Additional bench tests can help to determine the optimal design of other dust collection system components. It is not necessary to bench-test dust 100 percent of the time, but if there is anything at all unusual about the process and/or the dust, it’s a good idea.

To determine whether a dust is combustible, it should undergo separate explosibility testing as stated in NFPA Standard 68. If a dust sample is not available, it is permissible to use an equivalent dust (i.e., same particle size, etc.) in an equivalent application to determine combustibility. But once the dust becomes available, it is still recommended that you go back and test the dust using either the 20-liter test method described in ASTM E1226-12a or the similar method described in ISO 6184/1.

Using your dust sample, the lab will start with a screening test to determine whether the dust is combustible. If the dust is not combustible, testing will stop there. If it is combustible, the lab will conduct further testing on dust cloud parameters to pinpoint the Kst (defined as the deflagration index of a dust cloud, or rate of pressure rise) and Pmax (the maximum pressure in a contained explosion).

Explosibility testing is performed primarily to help determine what explosion protection or prevention equipment is needed on your dust collector and related components. But it can also play a role in filter selection. For example, testing for the minimum ignition energy can help determine if conductive filters are needed.

This tip is an excerpt from the white paper "Five Tips for Selecting Cartridge Dust Collecting Filters" by Rick Kreczmer of Camfil APC. The full article can be downloaded here.

Thursday, November 20, 2014

Five Tips for Selecting Cartridge Dust Collector Filters - #1

#1 Understand the different measures of filter efficiency and how to apply them.

There are two methods that dust collector filter manufacturers typically use to express filter efficiency. Gravimetric analysis is based on particle capture by weight: For example, filter efficiency might be stated as 99.995 percent on particles of 0.5 μm or larger by weight. Filter efficiency may also be expressed as a Minimum Efficiency Reporting Value or MERV, based on a scale from 1 to 16, with MERV 16 being the highest efficiency.

The MERV scale was developed for the HVAC filter market and does not take into account the way a dust collector operates (i.e., it pulse-cleans filters periodically when a dust cake builds up). Though MERV and gravimetric efficiency ratings are useful tools for comparing filters, you should not rely on these measures alone to determine efficiency. It is more relevant to make sure you are satisfying OSHA or EPA requirements for filter performance – OSHA if you are discharging the air and recirculating it indoors downstream of the collector, the EPA if you are discharging the air outside or into the environment.

Mass density efficiency, defined as the weight per unit volume of air, is the best predictor of a filter’s OSHA compliance. For example, OSHA might require that emissions will not exceed 5 milligrams per cubic meter at the discharge of the dust collector. Similarly, the EPA doesn’t care about percentage efficiency claims: They want to know that emissions will be at or below required thresholds, typically stated as grains per cubic foot or milligrams per cubic meter.

Concerned about which efficiency measure(s) to use? If so, you are not alone. But there is a way to cut through the confusion and make sure your bases are covered: Require your filter supplier to provide a written guarantee of performance stating that the filters you select will satisfy applicable OSHA or EPA requirements. It is worth noting here that ASHRAE is developing a new standard specifically for measuring performance of dust collector filters. Finalization of that standard, however, is at least a couple of years away.

This first tip is an excerpt from the white paper "Five Tips for Selecting Cartridge Dust Collecting Filters" by Rick Kreczmer of Camfil APC. The full article can be downloaded here.

Wednesday, November 12, 2014

Meeting OSHA IAQ standards when Recirculating Air From Dust Collectors

A crucial concern with any recirculating dust collector is to ensure the system has adequately removed the dust to protect workers' health. To do so, several factors must be evaluated.
The first step is to ascertain the allowable indoor limit for the dust being captured. OSHA has set an indoor limit of 5 mg of nonspecific or nuisance dust (< 10 microns in size) per cubic meter of air. Toxic dusts, such as silica, carry an indoor limit of only 0.05 mg per cubic meter - 100 times stricter than the allowable threshold for nuisance dust.
While OSHA guidelines must be met, plants should follow the guidelines published by the American Conference of Governmenlal Industrial Hygienists (ACGlH). The guidelines in this manual are often just a little tighter than those OSHA has adopted, and by meeting these guidelines, the plant engineer can ensure regulatory compliance (see Table 1).
Dust TypeTW, mg/m3
Graphite, except fibers2
Particles not otherwise classified
Silica - amorphous
Diatomaceous earth (uncalcined)
Precipated silica10
Silica, fume2
Silica, fused0.1
Silica, crystalline cristobalite0.05
Talc, with no asbestos fibers2
Zinc oxide
Table 1 - ACGIH recommended indoor air quality limits
Next, select a particulate removal system that meets IAQ requirements. Whatever brand or type of equipment is used, obtain a guarantee from the manufacturer for the maximum emissions rate (milligrams of dust per cubic meter of air) for the equipment over an 8-hr TWA.
Do not accept efficiency stated as a percentage, even if the manufacturer states 99.99% efficiency. OSHA only cares that the quantified amount of dust in the air is below established limits. If the established limit is an average of 5 mg/cu m, the manufacturer must provide a guarantee of something less than that, preferably at least half of the limit.
In most cases, a high efficiency cartridge dust collector will be the system of choice. These systems can be designed to produce emissions well below OSHA limits, except with the most hazardous dusts.
Today's cartridge collectors can be used for a wider range of processes than in the past, thanks to the advent of new media, improved pleat spacing, and increased application knowledge.
Pulsejet baghouses often have barely acceptable emission performance. Recirculation directly off a baghouse is generally not recommended unless a very high efficiency media is used.
To determine the best collector design for a recirculating application, dust testing is strongly recommended, especially wherever toxic dusts are involved. A qualified test laboratory can perform a series of bench tests on a representative dust sample to determine its characteristics - which can influence collector design.
For example, a particle size analyzer reveals the particle size distribution of the dust, down to the submicron range. This information can be very helpful in detetmining the filtration efficiency needed to meet indoor emission standards.
Additional tests can provide a visual analysis of the dust, determine its specific gravity, identify moisture content, absorbency, abrasiveness, and other characteristics. These properties aid the engineer in selecting filter media, hardware, and other components based on scientific analysis rather than guesswork.
A full-scale dust testing apparatus can be brought onsite to test actual conditions
Figure 1 - A full-scale dust testing apparatus can be brought onsite to test actual conditions.
In some cases. after bench testing is completed, full-scale testing, using one or more actual dust collectors. may be needed (Fig. 1). Full-scale testing is sometimes used to predict the behavior of an unusual or difficult dust. It can help ensure compliance with strict emission standards for processes that generate toxic dust and fumes - for example, the cutting or welding of galvanized material.
Tests can be run using either real-time or accelerated testing that simulates actual operating conditions.
Many performance variables can be evaluated in this manner - including different media types, filter configurations, air-to-cloth ratios, temperatures, airflows and dust loading conditions. Plant engineers may view the testing and make changes in a "what if" context to evaluate the impact of different variables.

Friday, November 7, 2014

History Proves Combustible Dust Can Not Be Ignored

Imperial Sugar Investigative Photo by US CSB
Combustible dust is a very real issue in manufacturing today.In many conversations on this topic, the 2008 Imperial Sugar explosion is brought up. This massive explosion and fire occurred on February 7th of that year, in a 91- year old plant near Savannah, GA.  The fires took over a week to put out and in the end 14 people had died and 38 others were injured, with many having life threatening burns. 

Investigations by OSHA and the Chemical Safety Board (CSB)* found that this tragic incident had been the result of several issues. These included, but were not limited to poor house keeping, lax safety standards, and improper dust collection systems. 

This video by the CSB, uses 3-D animation to detail their finding in the "how and why" of the Imperial Sugar Refinery explosion. 

* The CSB is an independent federal agency charged with investigating serious chemical accidents. The agency's board members are appointed by the president and confirmed by the Senate. CSB investigations look into all aspects of chemical accidents, including physical causes such as equipment failure as well as inadequacies in regulations, industry standards, and safety management systems.The Board does not issue citations or fines but does make safety recommendations to plants, industry organizations, labor groups, and regulatory agencies such as OSHA and EPA.