Fires - Smelting Process

First we must be careful to find out if the fires are a result of ignitions by a spark. All these acids mixed with carbon can spontaneously ignite, especially if the precious metals include catalysts, such as vanadium, like they use in catalytic converters. Or, they get fires when they are in the ten hour mode? My calculations indicate that during the ten hour precious metal mode they are basically diluting the exhaust to meet discharge requirements into the atmosphere.  To discharge from the hood at 125 degrees they either need a lot of dilution air or water vapor. For 30 hours a week, they use it as a trash burner. Here again they seem to be trying to hide the true nature of the operation. In the trash burning 30 hours they probably have wild swings in emissions which might ignite poorly combusted components and would be ignited by sparks in the baghouse. I would hate to live downwind during either operation. You need to find out the following.

1.  Do the fires occur while operating?
2. Do fires occur when they are shut down?
3. What are the temperatures in the ducts over a 60 hour period?
4. If they run 8 hours a day do they run 2 hours on smelting mode and six hours trash mode?
5. Do they insulate the duct?
6. What is the charge to the smelter? Coins? scrap gold, scrap silver? perhaps catalytic converters?.
7. It seems like they would have a severe corrosion problem and preheating of ducts should be considered.

We can put out sparks with Quencher^tm spark arrester but we cannot remedy poor control.

The ten hour mode is the ten hours per week when they are smelting. The 30 hour mode is when they are trash burning. Get a piece of bag to examine. It will be obvious if it has been chemically attacked. You should be able to tear or burst a piece with pliers and a vise. 

For more on spark arresters, see  QUENCHER spark arrestor

Wood Fired Boilers

There are many conditions that lead to fires in wood fired boilers. The engineering solution requires recognizing the factors that contribute or cause these problems. Most are related to the combustion process and that the wood composition varies widely:

1. The emissions from the boiler consist of dust and gases that are not completely burned. These can be ignited by sparks coming from the boiler. The loading of these pollutants can vary widely. Gases may continue burning with a flame, and, sparks may be in the process gas stream. 
2. If we install a spark arrestor, such as a QUENCHER, before the dust collector, it will cause the gas pollutants to burn producing heat, carbon dioxide and some small fraction of water vapor. The QUENCHER will also prevent any sparks from entering the dust collector as the hot ciders will be immediately cooled to the gas temperature, as the air goes from laminar flow to turbulent flow and back to laminar flow. We now have cooled dust and ash entering the dust collector. The pulsing action of the cleaning system will fan any red-hot cinders if they are present. For reasons beyond the scope of this report, the dust cake will be dense and subject to ignition. Fortunately, conditions must be in a narrow LEL/UEL range to start a fire. With conventional pulse cleaning systems only a fraction of the filter cake on the bags is functional and the rest of the bags are plugged. 
3. We would modify the cleaning system with modifications that would enable the full surface of the filter media to be active. This would mean that the inventory of flammable dust would be reduced by over 95% so there would be no combustible dust between cleaning pulses. Surprisingly, the cleaning frequency would also be reduced. To accomplish this, we would need information regarding the design of the existing dust collector.

When completed this would be the best design, and, barring unforeseen circumstances such as power failures at inopportune times, the system should perform flawlessly for years. The only risk is that the filter bags could be attacked chemically. 

More information on... retrofitting existing dust collectors 

Pleated Bag, Cartridge Dust Collectors

Pleated bags and cartridge collectors have been applied to collecting spice dust with mixed results. The first cartridge collectors were offered in 1976. They were an immediate success on applications where contemporary fabric pulse jet collectors were marginal. (e.g. Welding fume, arc furnaces with low temperature canopy hooded vent systems, and electrostatic powder coating vent systems.)

The 1978 American Foundry Society's test, at an automotive foundry, was to ostensibly find out if collectors could be used for recirculation provided an impetus to the cartridge collector's wide acceptance. The test report was widely circulated on a system that had shot blast and sand recovery systems. The results were very outstanding for dust emissions:

Standard Pulse Jet Collectors Penetration 600 to 800 x 10^-5 grains per cu. ft.

Shaker Collectors Penetration 60 to 80 x 10^-5 grains per cubic ft

Pleated filter / Cartridge Collectors Penetration 4 to 4.5 x 10^-5 grains per cu. ft.

The reasons for this astounding increase in collection efficiency was not discovered for another ten years.

1. The media was similar (identical permeability and collection efficiencies before formation of a filter cake) to the felted media but thinner.
2. The ejection of the dust towards adjacent rows of filtering elements was halted. During cleaning the gas and dust leaves the filter media perpendicular to the surface. In a pleated filter element, the dust is hurled toward the surface on the other side of the valley in the pleat. But this surface is also pressurized, blowing dust and gas towards the middle of the pleats. The dust laden gas is slowed down and directed toward the wide part of the pleat. The agglomerated dust then falls into the collection hopper below.

Read more on different dust collectors ...  QAM dust collectors

The belief was that more filter media (and associated filter ratio) made the selection more conservative. This is generally true with pulse jet fabric collectors with high velocity cleaning jets in that it extends bag life. The fact is that the opposite is true in cartridge collectors because of bridging across the pleats. If the media is not cleaned, bridging occurs. As dust accumulates in the valley of the pleat, it bridges. During the cleaning cycle the cleaning air looks for the easiest path from the inside to the outside of the filter cartridge. That path is above the bridge. What contributes even more to this phenomenon, is the fact that a certain volume of air can only clean a certain amount of media. Because the cartridge may contain huge amounts of media that cannot be cleaned, the media not cleaned plugs. In most designs running at filter ratios of less than 2:1, an operating cartridge may contain 10 to 40 pounds of dust. Table 1 illustrates the area of media that can be cleaned with various orifices and /or converging diverging supersonic nozzles.

Table 1

Orifice or      ON Line        OFF Line        ON Line           OFF line

Nozzle dia   Sq.ft.(Orifice)  sq.ft. (Orifice)  sq.ft. (Nozzle)  sq.ft. (Nozzle)

0.25 in.           5.5              6.8                 8.8                   11

0.312 in.         8.6             10.7               13.75                 17.2

0.375 in.       12.3             15.4               19.8                   24.7

0.50 in.         22               27.5                35                     43

0.75 in.         49.5            61                   79                     98.7

1.0 in.           88             110                  132                   165

1.5 in.         198             247                  376                   395

Advanced Pleated filter technology has allowed us to increase filter flow per size of cartridge. Formerly the limitation on application was to be able to use all of the filter media to filter the dust. Even spreading the pleats widely apart was only an improvement rather than a solution. When a pleat with conventional media is placed under pressure across the filter element, the pleat collapses and squeezes together so that media on the filtered side and is no longer in service. This squeezing has several direct and indirect effects:

1. The pressure drop goes up and squeezes even more of the media and the additional pressure drop disables a larger percentage of the media in the filter element. 
2. This decrease in effective media decreases the quantity of dust that can be stored in the element between cleanings. To compensate for this effect the cleaning frequency is increased to keep the pressure drop stable. 
3. Since the dust penetration through the filter element is a direct function of the cleaning frequency, the collection efficiency will  be reduced  by up to 90%, especially with applications with varying dust loading. 

The latest advanced technology, we have developed, is the media that is applied has sufficient resiliency (or springiness) to prevent any squeezing or pinching of the pleats. The new media allows the cartridges even to recover from failures of the cleaning system where a presumably plugged filter element can recover completely within a few off-line cleaning cycles. Another innovation is a tandem pleat with a stiff backing, to prevent pinching. This in effect allows us to have a permanent re-cleanable filter that can be washed manually in a laundry tub. Be the first in your company to take advantage of this technology. We can usually supply retrofit cartridges to bring an older conventional dust collector into the 21st century. This approach eliminates disposal problems, lowers operational costs.

Analysis of 2 tandem cartridge (Torit style) design

This typical contemporary design can be analyzed.

Valve: 0.75 inches Cartridge media (two cartridges) 450 sq. ft.

Approximately 550 grains per sq. ft. of dust loading

From Table 1; 49.5 sq. ft. cleaned on line 61 sq. ft. off line

Plugged media area 400 sq. ft. / 389 sq. ft (on-line / off-line cleaned),

When media is plugged, 550 gr./ sq. ft x 400 sq. ft. = 220,000 grains is imbedded in cartridge.

Therefore, 220,000 / 7000 gr./lb. = 31.5 Ibs per tandem set weight.

Another factor is that the cleaning action is generally initiated by a pressure switch. The most prevalent pressure switch setting is about 3 1/2 inches. For most applications the pressure should be about 3/4 -11/2 inches WC above the initial pressure drop. Typically, initial pressure drop through the cartridges is 0.3 - 0.5 inches of water column. At 3 1/2 inches WC, over 80% of the available media is generally plugged and the cartridge must be cleaned three times more frequently than if the switch were set in the proper range.

Dust Collector Selection

The best desiqn for a pulse jet collector with fabric media on these applications are those offered by ULTRA-FLOW/QAM; with low jet velocities and higher filter ratios. The characteristics of these designs are listed below:

Average velocity at bag opening    10,000 feet per minute

Bag opening (no venturi)               4"diameter

Jet volume                                  740 CFM

Bag diameter and length              4 inches x 96 inches

Bag area                                    10 sq.ft.

Filter volume rating per bag          190 CFM

Nominal filter ratio                       20 FPM

Average pressure drop                 2.5 inches water column

Average Air Consumption             0.5 SCFM/1000 CFM of flow

Average dust penetration             5 x 10^-4 gr. / cu. ft. (at 5 gr./ cu. ft. load)

Of course, as stated above pleated filter elements can be a collection option. However on many applications, such as low density spice dust, the operating cake can be relatively thick as much as 1/8th of an inch thick and the pleats bridge and cause operating problems.

Advanced Technology Cartridge Collectors :

You will note that our collector is somewhat different in appearance and design than those offered by the other bidders. However we are convinced that our design has a much more suitable dust service than the other offerings. We will briefly outline these differences and the advantages for your service.

1) Cartridge Mounting You will note that in our CV Series, cartridges are mounted vertically instead of horizontally. This allows the dust to fall unobstructed into the hopper. We have noted that at the trade shows several of the suppliers exhibiting their horizontally mounted cartridge dust collector units have a common problem. On these units, when you look through the Plexiglas view ports, there is a pile of dust on the upper surfaces of every cartridge except the top one in the vertical rows. This dust blocks the media and provides an undesirable inventory of dust. This is especially serious when handling dusts that are toxic or flammable.

2) Air to cloth ratio; It is often forgotten by cartridge dust collector specifiers and designers that the quantity of media that can be cleaned by a reverse jet pulsed cleaning system is a function of the reverse air volume of the cleaning jet. While the design of the cleaning system is relatively complex, we can relate this to the valve size. The other pertinent relationship is that the maximum air flow through a filter (or set of filters in the case of a tandem filter) is related to the maximum compressed air flow in the valve or orifice (in case of a blow pipe unit). Both of these relationships are listed below. (Note these values change depending on the efficacy of the cleaning system design).

Orifice         Flow in      Media Area

or valve dia.   scfm        cleaned

1/4”               90           12-17 sq. ft.

3/8”             200           30-40 sq. ft.

1/2”             360           50-70 sq. ft.

3/4”             810          115-155 sq. ft.

1”              1440          205-265 sq. ft.

1 1/2”         3240         450-619 sq. ft.

2”               5760        820-1100 sq. ft.

What happens to the media not cleaned? It bridges and plugs. Once plugged and bridged, the dust in the folds of a cartridge filter element cannot be removed by the cleaning system. The cleaning air will look for the path of least resistance so it renews some of the media until it reaches the areas listed above. In the typical tandem design with one 3/4” valve to clean a tandem set of two cartridges and with 530 sq. ft. of total media, approximately 135 sq. ft. of media get cleaned and 395 sq. ft are plugged. The uncleaned media eventually holds about 20 lbs of dust. The dust piling on the upper surfaces may amount to another 4 or 5 pounds.

3) Cleaning Controller Many designers advocate and supply a pressure switch to start the cleaning system pulse timer. Although this is a possibility for controlling the cleaning, the difficulty lies in determining the proper pressure drop at which to activate the controller. Selecting an arbitrary number such as 3.5” w.g., the dust build up in between the pleats will be four times as much, similar to the calculations above.

4) Clean Side Access If the power to the controller or the compressed air system drops below the necessary cleaning pressure, the pleats will bridge and the cartridge filter elements will not recover by reverse jet cleaning. Cartridges can be recovered to near new condition by blowing them clean with a modified blow gun.

5) Seals and cleaning system design. Many suppliers have inadequate seals and poorly designed cleaning system. The cleaning jet grows at a relatively narrow angle until it encounters a venturi wall, an orifice or a wall of filter media. Many collector cleaning systems locate the blow pipes (or valve) too close to the opening in the cartridge. The net result is that the media nearest the pulse pipe or valve is not cleaned. The additional storage of dust in the pleats is often considerable. There are some suppliers who use inadequate or improperly designed seals. If seals leak premature cartridge replacement and high pressure drop operation will become the norm. A proper dust tight seal needs to maintain proper pressure on the sealing surface. Too high of pressure will harden the seals and cause leaks. Too low a pressure will allow dust to pass through, when the pressure drop exceeds the pressure on the seal. Our ULTRA-FLOW normal cartridge life can exceed 30 months.

Conclusion

The best dust collection choices are a fabric collector with low jet velocities and a manifold heater. The next best selection, only with light density powders, is cartridges or pleated bags with very wide pleats and a manifold heater. We trust that you will take the above into consideration when evaluating your needs for a cartridge dust collector. Advanced Technology cartridge collectors have been supplied since 1987 with hundreds of installations on a wide range of applications including a large portion on a brace of blast and grinding applications. Their performance has been outstanding with high efficiency, low pressure drop and long cartridge life being the normal expectancy.

Read more on ...  Quality Air Management dust collectors

Major Design Flaws and New Technology Baghouse Dust Collector

Design flaw #1 for conventional designs:

Conventional designs with cylindrical bags propel the dust from the rows of bags in process of being cleaned toward the adjoining rows in the filter mode. This high speed jet (between  350 and 400 ft/sec) drives the dust through the filter and filter cake, partially blinding the bags and reducing dust holding capacity by 80-90 percent with dense dusts. To operate at reasonable pressure drops, the potential filtering capacity of the bag is reduced by up to 80%. This high velocity dust also raises outlet loading above 100x10^-4 grains per cubic foot.

The new technology design reduces the exit velocity from the bag to between 190 and 250 ft/sec depending on gas density. This keeps the permeability of the media plus filter cake to a few percentage points higher than a new bag. It typically holds several times more dust between cleanings, even at filter ratios of 15 to 20, compared to conventional designs.

Design flaw #2 for conventional designs:

The filtering capacity of the filter element is limited by the reverse air volume generated by the cleaning system. The reverse air volume is also based on the diameter of the venturi at the entrance of the bag. This, for a four inch by 1.875 diameter throat bag is only 20% of the area of the opening at the top of the bag.

The new technology removes the restrictive venturi used in conventional designs and opens up the opening by 4 to 5 times. This increases the cleaning volume while reducing the pulse jet speed by 3 to 3.5 times. Half of the bags are removed and replaced with new bags and cages with the venturi eliminated. The rest of the bag openings are plugged and no longer used.

Other considerations

When these changes are made, the fine dust which formerly bled to the outlet is collected on the bags and ejected to the hopper. Because it is so fine, the vertical flow entering to the bag compartment, from a hopper inlet, would prevent this dust from falling into the hopper. This is the effect of upward “can” velocity.

The retrofit design removes half the bags from the collector. The dusty air enters from the bottom and also through the opening in the center of the bag compartment. This reduces the upward can velocity coming from a hopper inlet to a level 70 - 80% less than before the modification. Now the fine dust falls into the hopper unimpeded. It is equivalent to putting a high inlet in the center of the collector.

95% of the time, the collector will pass the initial engineering review. A report will be issued for your approval, before any fabrication of components begins.

Normal compressed air requirements, for contemporary designs, is 0.9 to 1.2 SCFM of compressed air per 1000 CFM of filtered air. Predicted for advanced technology designs is (0.328 x (0.9 to 1.2) =0.3 to 0.4 SCFM per 1000 CFM of filtered air.

Based on an average system requirement of 10 inches water column, a two inch reduction in pressure drop across the dust collector would reduce power consumption in the exhaust fan by 20%.

Read more about ... Baghouse retrofits
Read more about ... New advanced technology baghouse dust collectors