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Q: What is MERV?

A: MERV (Minimum Efficiency Reporting Value) is the industrial filter efficiency rating system from ASHRAE 52.2 standards. MERV ratings evaluate a filter’s efficiency based on particle size. Previous standards measured efficiency based on particle weight.

Why the change in standards? More manufacturers today need advanced filtration technology like Ultra-Web to capture lighter weight submicron particulate (0.3 -1.0 micron). MERV ratings that evaluate efficiency based on particle size pinpoint a filter’s efficiency in capturing smaller dust particles with much greater accuracy.

Q: EPA standards specify PM10 and PM2.5. What do these numbers mean? Do my Donaldson Torit collectors comply with the standards?

A: In 1990, EPA set requirements for particulate matter emissions, concluding that particulate matter with an equivalent diameter of 10 microns or smaller (PM10) presented a major threat to the nation’s air quality. In 1997, EPA toughened the standards to 2.5 microns or smaller (PM2.5), stating that studies showed major health and visibility concerns with the finer particulate (PM2.5).

Since nature doesn’t produce particles smaller than 2.5 microns, they argue emissions smaller than 2.5 microns are man-made. PM10 and PM2.5 emissions requirements vary by region, but are always at least equal to or below federal standards. By federal standards, “major” sources have the potential to emit 70 to 100 tons of PM10 per year. Small shops can still be affected depending on what they process. Shops processing rubber or hazardous compounds like lead are required to permit regardless of emission volume. Obtaining and keeping a permit requires periodic emissions monitoring that you can perform at your site. Donaldson Torit filter media is capable of meeting PM2.5 emission guidelines. As an example, Ultra-Web efficiency is rated at MERV 13, 14 and 15. This rating means that Ultra-Web captures 90% of the particles in the 0.3 to 10 micron size.

Note: MERV stands for Minimum Efficiency Reporting Value as measured by ASHRAE Standard 52.2 – a test that measures particle size efficiency and expresses efficiency as a function of particle size. 52.2 classifies these sizes into 12 ranges. If this seems complicated, don’t worry – the easy way to remember is that the higher the MERV rating, the more efficient the filter media. Of course, you always have to balance the efficiency with airflow restriction or pressure drop….you don’t want to sacrifice one for the other….it’s a balance thing. For instance, a very highly rated filter, say MERV 17, sounds great because it’s in the HEPA range – but depending on what you’re filtering, it will fill up fast and you’ll be replacing it often.

For more information about PM10 or PM2.5 refer to the EPA’s website, www.epa.gov, or refer to your state’s Pollution Control Agency website.

Q: What is the advantage of filter cartridges in a horizontal position compared to vertically hung?

A: Horizontal vs. vertically-hung cartridges: The major advantage to horizontal is the airflow pattern within the collector. Horizontally mounted cartridges allow the tubesheet to be positioned in vertically, which allows the air to enter in a downward pattern. The inlet airflow then assists with cleaning.

When the filters are pulse-cleaned, the particles are carried downward into the hopper, decreasing reentrainment and making it easier to maintain a lower operating pressure drop. The housing design for a vertical cartridge demands that the inlet be located in the hopper with an upward airflow. This design then demands that the released particulate must pass through the upward airflow, which in turn carries back to the filter. This arrangement results in a higher operating pressure drop that may require prolonged or frequent cleaning -- increasing compressed air usage and a higher stabilized pressure drop.

Some designs attempt to overcome this airflow problem with high side inlets and staggered inlet baffles, but this creates turbulence in the hopper and increases filter abrasion. Fluent® Flow Modeling Software (CAD) clearly indicates that cross-airflow patterns cause significant hopper sweeping and create an upflow air pattern into the collector and filters. This can cause major dust re-entrainment and difficulty pulse cleaning, resulting in higher pressure drop and shorter filter life.

Donaldson pioneered dust collectors with cartridge filters starting in 1973 with the vertical cartridge arrangement and tried several design iterations. Through our FLUENT® and CAD modeling, lab tests, and extensive field experience, we created & patented another enhanced technology in 1982: the Downflo® design with horizontally-mounted cartridges. In 2000, we enhanced Downflo collector performance even further with oval-shaped cartridge filters, which handle more airflow without increasing velocities.

Q: How important is sizing the ductwork for my dust collection systems?

A: Very. The dust collector is the main ingredient in getting cleaner air for your shop, but if you can’t get the dirty air into the collector efficiently, you risk high energy usage, degraded collector performance, and unsafe conditions in the plant. Ductwork is used to carry the dirt & air from the collection point to the collector. It must be sized properly to create a "carrying velocity" to keep dirt moving through. Duct sizing is based on 2 variables: the particulate being collected and the volume of air being moved. The heavier the dirt, the higher the air velocity needed.

EXAMPLE: Steel grinding dust conveyed @ 3500 feet per minute (velocity). A typical grinder uses 500 cubic feet of air per minute (cfm) to control dust. To move this much air, select an appropriate size of ductwork – in this example, a 5-inch diameter duct yields just over 3500 feet per minute for 500 cfm air volume.

If the ductwork is too small, two problems can occur: (a) Higher pressure loss (delta P) and higher energy usage because of higher velocity. (b) Increase delta P will reduce air volume entering the collection hood, degrading the performance of the dust collector.

If the ductwork is too large, two problems may happen:

(a) If the volume of air is not increased, dirt will not be carried through the ductwork and material will collect inside.

(b) If the particulate is heavy, the ductwork will become heavier than the structure support can carry and may crash to the floor.

Bottom line: correct sizing of the ductwork is a key factor in the dust collection system.

Learn about Donaldson Torit’s Easy Duct. Easier installation -- no welds, no rivets, no screws.