The original Design of most contemporary reverse pulse jet collectors was developed in the early 70”s. This design was a breakthrough in dust collector technology. In 1978, several technologies were developed to remedy operating problems of Fabric Pulse jet collectors. The cause of these operating problems was a flaw in the design.

Cause of Operating Problems

The main flaw in the contemporary design of fabric pulse jet collectors was that during the cleaning cycle dust was driven from the cleaning bags at very high velocities, then driven through the adjoining bags through the filter cake and filter media. The bags became partially blinded. These velocities were typically 20,000 to 36,000 feet per minute. The permeability of the filter cake and dust imbedded in the media increased. The bag developed a coating to resist further penetration until the system stabilized.

These high cleaning jet velocities resulted in short filter element life, high pressure drops and high compressed air usage.

It is and was common for fabric pulse jet collectors to run at pressures between 4 and 6.5 inches, and air consumption of over 1.4 SCFM of compressed air per 1000 CFM of filtered air at 80 psig. As might be expected the heavier density dust an powders ran at the highest pressure drops.

NEW TECHNOLOGIES

Four new technologies were developed to combat these operating problems:

The cartridge collector with pleated filter elements.

The high ratio fabric collectors.

PTFE Laminated Fabric Filter Bags

Bag Diffusers

Cartridge Collector

The cartridge collector was immediately widely accepted because it solved a problem with venting electrostatic powder paint systems. Previously, fabric pulse jet collectors on this application gave unsatisfactory service. Within months of the introduction hundreds of collectors were sold and installed with spectacular results. The cartridge collector remedied some operating problems in the contemporary fabric pulse jet collectors. An example of this was the experience of Nordson Corporation of Ohio, who supplied powder pigment spray systems that sprayed powder on metal surfaces that were then cured to a hard coating. They vented the spray booths into Fabric Pulse jet collectors. On some pigments, they had bag lives of less than eight weeks and pressure drops of 6 to 8 inches w. g. Even at filter ratios as low as three to one. They were desperate for a new technology and were the first to embrace it.

Laminated Filters

In 1975 the Gore Corporation introduced PTFE membrane laminated bags. This prevented the dust driven from the bag in the cleaning mode from penetrating below the surface of the media through the filter cake. Hundreds of thousands of bags were successfully installed that eliminated many operating problems with contemporary designs. The laminated bags lowered the bag permeability, which sometimes limited filter ratio. Typically the bags cost 5-6 times more than conventional bags. Since then patents for this construction have expired.

Low Velocity Cleaning Jets and High Ratio Pulse Jet Fabric Collectors

In 1978, Scientific Dust Collectors introduced another breakthrough in Pulse Jet Dust Collector Technology. The venturies were removed from the bags and jet was changed so that velocity of the jet decreased by 60 to 80%. The velocity of the dust leaving the bag was also decreased by 60 to 80%. At the same time the volume of the air jet was increased by more than 3 times. As an adjunct of these changes, these collectors were operated filter ratios between 12 and 20:1 with pressure drops under 2 inches. In 1983 Carter Day Corp. introduced the same design with oval bags. Since then over 4,000 of these collectors operating at high filter ratios have been installed and are operating with those parameters.

These collectors must have special inlets which eliminate any upward velocity of dust laden air entering the filtration section of the collector. The lowering of jet velocities increases the collection efficiency on fine particulates. These may have difficulty falling into the hopper if the collector has a hopper inlet.

Bag Diffusers

These diffusers consist of light gage are perforated cylinders inserted into the bags below the venturies of existing collectors. They operate to improve dust collector operation by slowing the velocity of the jet as the cleaning air exits the filter bag toward adjoining bags. Because they too increase collection efficiency on finer dust, collectors with bottom inlets can may encounter limited effectiveness with this modification.

American Foundry Society Tests 1978

A test run by AFS on foundry dust showed that the penetration of the dust compared to a standard pulse jet was astounding:

STD Fabric Pulse jet at 4:1 filter ratio 800 x 10^-5 grains per cubic foot with 10 grain inlet

Cartridge Collector at 2:1 filter ratio 3 x 10^-5 grains per cu. ft with 10 grain inlet

Gortex laminated bags 10 x 10^-5 grains per cu.ft. with 10 grain inlet

Low Velocity Cleaning Jet Fabric Collectors* 10 x 10^-5 grains per cu. ft.

*These are New Technology collectors as described below.

This test was run in 1978 prior to the introduction of the new technology low velocity jet high ratio dust collectors but other independent tests showed the performance shown above.

Later tests of New Technology cartridge dust collectors, such as ULTRA-FLOW, revealed that they operate at 2 x 10^-5 grains per cu.ft. but at 6-8:1 filter ratio.

Determination of Flow ratings of New Technology versus Contemporary Designs

The flow capacity of a bag or cartridge in a reverse jet self cleaning collector is limited by the volume of the reverse cleaning jet. It is obvious that if I have 50 CFM flow in the filter element and the reverse jet volume is 40 cfm, the flow in the bag will not be reversed and no cleaning will take place on line. If the bag has ten sq. ft. of media or 100 sq.ft. of media it will not clean on line. However if I take this reverse jet and increase the flow to 200 CFM (50 CFM x 4) it will clean on line easily. If we temporarily accept the premise that you need four times the cleaning flow to maintain equilibrium, adding more square feet to the filter element will not increase filtering flow capacity unless you also increase the reverse flow volume. Another limitation is the operating permeability of the filter media and dust cake as covered in the first part of this memo.

Another limiting factor in filtering volume through collectors is the well- known “can velocity” parameter. This effect is most pronounced on very light density dusts such as paper dust, feathers, grain, dried organic fertilizers, dust from recycling etc. Dried manure is similar to grain and is susceptible to “can velocities” New technology collectors have special inlets where there is no vertical component to the dust laden gas as it enters the filter compartment of the dust collector.

If we consider the conventional designs where they have basically a reverse air volume of 250-350 cfm, and 20,000 to 30,000 FPM cleaning jet velocities, specifying a low filter ratio is excellent engineering.

However, specifying the new advanced technology results in better filtration, lower operating costs and longer filter life.

The better engineering approach is to specify dust collectors by considering the capacity and design of the cleaning system. This can be accomplished by taking the compressed air flow capacity of various sizes of diaphragm valves and multiplying by the number of valves, if orifices are installed in the pulse pipes: (This assumes that the total area orifice opening are maximum at 85% of throat area) If the pulse pipes have converging diverging nozzles installed the cleaning jet volume is increased by 50% with same increase is filtering flow in the collector.

¾ “ valve orifices 688 CFM nozzles 1032 CFM

1 “ valve orifices 1224 CFM nozzles 1836 CFM

1 ½ “ valve orifices 2754 CFM nozzles 4406 CFM

Example of comparing two collectors

Standard Design Collector with 81 bags 9 -¾ inch valves 10 sq. ft. per bag

9 x 688 CFM = 6,192 CFM 6192/1000 sq. ft. = 6.192 (nominal filter ratio) based on cleaning volume with no regard to can velocity. For low density dusts ratio must be lowered for lower density dust.

Advanced Technology Collector with 81 bags with 9- 1 inch valves with nozzles, and special inlet to eliminate can velocity considerations.

9x 1836 CFM = 16,524 CFM 16524/1000 sq. ft = 16.524 (nominal filter ratio)

The net cleaning jet velocity should be specified as no more less than 9,000 fpm and gross cleaning velocity no more than 15,000 fpm for dense dusts.

Upward component flow air entering compartment should be limited to 150 fpm for lower density dusts and 350 FPM for higher density dusts.

Specifying collectors based on filter ratio alone penalizes the supplier who provides more cleaning capacity with the larger more expensive cleaning components, and better engineered cleaning systems, depriving the client of superior performance at lower cost.

When to specify Fabric or Pleated filter elements

The other decision is between pleated filters and unpleated filters. The selection for pleated filters is confined to dust that has a thin filter cake. If the cake is deep bridging will take place in the valleys of the pleats rendering the media below the bridges uncleanable.