Dust Collection in Plasma and Wire Spray Operations (continued)
The application is extremely difficult. The collector, with a remote backward incline fan on the clean side of the equipment, pulls from two types of spray guns: plasma and wire.
Plasma generally comprises hydrogen and argon gas. A powdered metal is introduced to this gas, and the gas and powder are sprayed at a high velocity to form a coating on the roll. The plasma spray dust is spherical in shape and consistent in size at around 3 microns, making it fairly easy to capture in the dust collector.
The wire spray, on the other hand, is the real challenge. Various types of wires made of 316 stainless steel, MONEL® alloy, zinc, and other materials are fed into the spray gun two at a time. One is charged positively and the other is charged negatively. When they touch in the gun, they create an arc.
The wire spray gun also is fed with a compressed air line. The compressed air blows through these wires as they cross, creating an arc that blasts the wire onto the roll to form the coating. The wire spray is very fine, mostly submicron particulate and is inconsistent in shape, causing a difficult challenge for the dust collector.
A "clamshell" aspiration hood encloses a wire arc gun spraying a print roll. The hood travels with the spray gun and roll, aspirating the excess spray and providing a cooling function by pulling air across the roll.
Dust capture is aided by a hood and duct system designed by Underwood Air Systems of Atlanta. The hoods are designed to travel with the spray guns and rolls, and they not only aspirate the excess spray particulate but also provide a cooling function as they pull the air across the rolls (see Figure 2). There are two hoods, one for the plasma gun and one for the wire gun, and each is designed to handle 4,500 cubic feet per minute. The fan is sized to run at 9,000 CFM and pulls this total amount of air even if only one spray gun is operating.
With its 48 cartridge at 425 sq. ft. per cartridge, the system provides 20,400 total sq. ft. of media, which calculates to a 0.44-to-1 air-to-media ratio. In general, this is considered a very conservative air-to-media ratio, but it is appropriate for achieving the company's goals of extended cartridge life and low differential pressure.
"When we are running continuous, heavy loads, the collector reaches 3.5-inch water gauge differential pressure, which is where we have it set to begin its cleaning cycle. All it takes is a few pulses and it comes right back down to 1.5- or 2-in. w.g.," said Drozd. Cleaning is accomplished by pulse waves that emanate outward from the inner cone within the cartridges. this design provides uniform dispersion of back-pulsed air for cleaning with less wear and tear on cartridges.
The company has been saving about $4,000 to $6,000 annually in disposal costs, along with additional savings in reduced downtime and better energy performance. The cartridge estimated to perform for two full years in this operation. When filter replacement is needed, the collectors have patented cambar system that will allow fast cartridge changeout with no tools required.