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ACBs Versus Compressed-Air-Using “Self-Cleaning” Filter Systems

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COMPARE SELF-CLEANING FILTERS

Air-Cleaning BlowersTM (ACBs) require no pressurized air for cleaning.  In contrast, Compressed-Air-Using “Self-Cleaning” Filter System (SCFs) can not function without a directly connected supply of compressed air, often using it in large quantities.

Self Cleaning Filters and Fans must capture and collected all the debris that they remove from the air.  ACBs do not need to remove accumulated debris because they do not collect it in the first place.

ACBs cost less to buy compared to other self cleaning fans with filters.  Since they are much simpler and smaller, they usually cost less, often much less, to install, and ACBs take up less space, again often much less.  In addition, ACBs generally require less ductwork to install and clean than SCFs.

THE ACB DIFFERENCE

Since ACBs are compact, they can frequently be installed much closer to the point of use.  The complications of SCFs often require that they be centrally, and remotely, located.  The required ductwork also adds to the work their blowers must do and, consequently, to the consumption of electricity.

Even though they self-clean for a while, self cleaning filters’ internal filter media eventually has to be replaced because the pores in the material become too clogged to blow out sufficiently.  ACBs never incur the costs of buying, installing and disposing of filter elements because they do not contain any filter media.

In most cases, ACBs simply discharge the dust and other particles that they remove back where they came from, usually outdoors or into the dusty environment surrounding the application.  Since whatever air goes into an enclosure ultimately comes out and back into the original dusty atmosphere, the ACBs do not make any net change to that environment—it retains its original amount of air and dust.

In contrast, since SCFs collect the dust, they usually remove a portion of the dust from the dusty environment.  However, that amount can be negligible versus the total amount of dust in the facility in which they are installed so that they make little net change in the overall environment, and their benefits are limited only to the application toward which they are directed.

ENERGY EFFICIENCY

Electrical consumption per CFM produced generally runs lower, often far lower with ACBs than it does with SCFs, and new designs of ACBs are pushing their required electrical consumption down still further.

Image provided by Better Plants, U.S Department of Energy

By their very nature and their design requirements, self cleaning filters have a difficult time approaching the energy efficiency of ACBs.  The principal issue is that they must trade off between the costs of cleaning the filters and running them significantly clogged: 

SCFs that use compressed air often, if not usually, designed to run with high pressure losses.  Since letting the compressed air constantly is costly and could damage the media, most systems let debris accumulate in the filter media until that accumulated dust reaches a pre-set level of clogging (pressure drop).  Reaching that point triggers the compressed air to clean the media.  A common set point is in the range of 4″ w.g. (water gauge, which is equivalent to about 1250 Pa—pascals). When it reaches that point, the compressed air comes on and runs until the pressure loss reaches another set point, often in the range of 2″ w.g. of pressure loss.

AIR FLOW

To put these pressure losses in perspective:  A new fan and filter element usually rates its capacity in airflow at 0.5″ w.g. (or even lower).  Since an doubling in back pressure from clogging can increase electrical consumption by as much as 50% if the airflow is kept constant, the electrical cost of running a self-cleaning air filter of these types can become expensive quickly as the debris accumulates in the filters.  Also worth noting is that, since the lower trigger point is often about 2″ w.g., the filter elements never reach their original rating point again (about 0.5″ w.g.) once the initial dirt loading has brought the pressure loss up to the lower trigger point. 

In other words, once the elements accumulate enough clogging debris to reach the bottom of their operating range,  by design, it will not run at maximum efficiency until the elements are replaced. The only way to reduce the pressure loss of ~2″ w.g. is to buy new filter cartridges or bags and pay someone to replace them and someone else to dispose of them.

The dust blown out of the SCFs’ cartridges or bags by the compressed air, generally goes into some kind of recipient that someone must empty and, in doing that, incur the risks of any hazardous particles in the filter element.  In addition, the user has the costs of disposing of this debris, which can be high if by accumulating and concentrating it, the user is creating a quantity that OSHA or others consider dangerous-enough that it must be disposed of in some special way.

In contrast, since ACBs do not collect or concentrate the debris in one place, they do not normally require handling or disposal of collected waste.

THE SELF CLEANING FILTER COST DIFFERENCE

ACB-based systems generally cost less to buy and install than SCFs, and far less to maintain.  Part of the reason is that SCFs need elaborate control systems which can be costly to buy and require training to operate and repair.  Some manufacturers of the systems even recommend VFDs to control the motors, at significant additional cost.  The VFDs are needed to slow the fans of the SCFs when their filter elements are clean and to speed them up enough to drive the same airflow through when the filters are dirty. ACBs, in contrast, usually have controls limited to an on-off switch; why? Because ACBs do not need operational controls since their inputs do not have to vary (for instance, as they clog) in order to maintain their consistent outputs—constant airflow, air pressure and electrical consumption.

One benefit of the consistency of output by the ACBs is the ease of sizing the units.  By using an Air-Cleaning BlowerTM, a designer knows that his/her system will provide consistent performance.  Hence, he/she can avoid the problems caused by fluctuations such as having to oversize the unit so that it will produce the minimum air needed by the application during the time when the filter media is most clogged.  By their very nature, SCFs must generate variable airflows, air pressures and energy consumption.

If the materials being removed from the air have a value, or if there is another reason to collect them, ACBs can be adapted to drive them to a recipient for collection.  These recipients can take many forms, and they can usually be made on premises and adapted and configured for the available space.  As a result, their installed costs run lower than the recipients provided with SCFs by their manufacturers.