Philosophy of design
1. Component and Fabrication costs
This line of collectors, Torit being the most popular, was developed for low initial cost, and to develop the replacement cartridge market. Donaldson has the lowest costs of production of any supplier. The replacement cartridges are at least 25% lower cost than the nearest supplier. They can sell cartridges at competitor’s costs and still have a good profit margin. With high volume customers, they use this ability to remove serious competition in a big account. For the other components such as valves and timers, they have the same advantage based on their purchase quantities. They buy in quantities of 25,000 to 50,000 and the competition buys in lots of 100-1,000.
On this line of collectors with the nominal 12 inch by 26 inch cartridge, they build the collectors in modular form so that there are two types of modules. There are two end modules and any amount of middle modules to come up with any size collector. The modules are bolted together so they match up and assembly is always the same. They can build modules in lots of 25 or more and assemble modules to make a finished collector. This lowers cost more than 35% compared to other suppliers with the same size and design collector. The modules are bolted together and they can ship collectors in two weeks with the first week devoted preparing the paper work. Since they use multiple inlets and outlets, it is hard to make a mistake in manufacturing that cannot be fixed by adding or removing modules.
2. Engineering
The holes in all of the flanges are gang punched with dies punched so they always match in spacing both for modules and hoppers. They promote their multiple hoppers to reduce engineering. The horizontal cartridge designs are designed for lowest possible headroom.
3.Shipping Costs
All assemblies are selected to be shipped with standard trucks and railroads without special permits.
Limitations of this design approach
This design requires special inlet and outlet manifolds. If cost of these manifolds are included, the cost of the system can be higher than competition. The configurations available are either right hand or left hand. Other competitive designs are more adaptable. Because of the bolted construction the tolerances between parts can produce misalignment when the cartridges are installed. One unit which we examined, which was a single cartridge instead of a tandem unit, we found that tolerances were such that the seals were not functioning. One side of the seal was bottomed and the other side was open so that dust continually leaked around the cartridge.
Capacity of cleaning system is limited by size of cleaning valve for the tandem set. This capacity is a function of the compressed air flow in the valve. As listed below the maximum filter flow rating for the tandem set is 810 CFM. The quantity of unplugged media whether fabric, cellulose or other media is also a function of valve size:
Valve size Filter flow Media Cleaned On line/off line
1/2 inch valve 360 CFM 23 sq.ft./ 28.5 sq.ft.
3/4 inch valve 810 CFM 49 sq.ft./ 61.2 sq.ft.
1 inch valve 1440 CFM 87 sq. ft./108.8 sq. ft.
1 1/2 inch 3240 CFM 193 sq ft. / 241.2 sq. ft.
Analysis of Two tandem cartridge design
This design can be analyzed as follow:
Valve: 0.75 inches to dean two cartridges
Maximum filter flow on line 810 CFM
Cartridge media area (two cartridges) 450 sq. ft.
From Table above 49 sq. ft. cleaned on line / 61.2 sq. ft. off tine
Cleaned area 49.5 sq. ft / 61 sq. ft. Plugged 400 sq. ft. / 389 sq. ft,
Cartridge weight is 40 pounds
Approximately 550 grains per sq. ft. of 60 Ib. per cu. ft. dust are collected when media is plugged. 550 gr/ sq. ft x 400 sq. ft. = 220,000 grains.
220,000 / 7000 gr./lb. = 31.5 lbs per tandem set.
Total weight of cartridge set is 40 Ib. + 31.2 Lb. = 77 Ib.
Cleaning System Actuation (Recommended)
Another factor is that the cleaning action is generally initiated by a pressure switch. The recommended prevalent pressure switch setting is about 3 1/2 inches. For most applications the pressure should be about 3/4 to 1 1/2 inches w.c. above the initial pressure drop. Typically initial pressure drop through the cartridges is 0.3 -0.5 inches of water column. Therefore, at 3 1/2 inches w.c. pressure drop, less than 17 sq. ft. of the media is available because the dust bridges across the pleats rendering the rest of the media in a condition where it cannot be cleaned by the reverse jet flow. The cartridge must be cleaned three times more frequently than if the switch were set in the proper range. This also adds about three pounds to the operating weight.
Compressed air consumption
Compressed air usage is a function of the square of the operating pressure drop. The usage is also related to the dust loading and the fineness of the dust. With a typical dust from material handling operations such as sand or rock dust and 5 grains per cu. ft., the air consumption per thousand CFM of filtered air would be as follows:
Operating Pressure drop at 85psig and 1000 CFM of filtered air
1.5 inches 0.2 SCFM
3.5 inches 1.1
4.5 inches 1.8
Filter life
The filter life is related to the cleaning frequency (or compressed air consumption) so a collector running at 1.5 inches of water will last five times as long as one running at 3.5 inches all other factors remaining the same.
Efficiency
Over 95% of the dust penetration through the filter comes immediately after each cleaning cycle. The dust penetration is related to pressure drop. With the same conditions of loading described above, the efficiency and penetration at the three pressure drops follow:
Operating pressure drop Penetration Efficiency
1.5 in. w.c. .00005 grains/cu.ft. 99.9990%
3.5 in. w.c. .00027 99.9946%
4.5 in. w.c. .00045 99.9910%
Conclusion
This design has some serious limitations, some of which can be remedied by the QAM retrofit design which will reduce pressure drop, increase efficiency and extend cartridge life.
For more information ... Retrofitting existing cartrdige dust collectors
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