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|Powder Coating Manual -"The Powder Booth"------ (Part - 37)|
System Air Flow
The containment air flows through the cartridges carrying powder and depositing it on the cartridges. The compressed air blow pipe discharges on a sequenced basis and the powder falls to the base of the collector.
Combination Cyclone/Cartridge Module Booth
There are booths available that combine some of the advantages of the cyclone system with the advantages of the module reclaim system. The theory with this design is to give the operator flexibility and efficiency in one booth.
The cyclone system can be used for short runs of various colors and the overspray can be captured for reuse or scrap. For larger volume colors, a cartridge module can be used to take advantage of the higher reclaim percentage or one module can be used to collect small volumes of various colors.
For installations that use only one booth this is an excellent alternative. The high efficiency of the module is available as well as the ability to collect any number of colors for reuse without the need for additional equipment.
Each type of recovery system has advantages depending on the application. The major selection criteria are available space, color change requirements, airflow characteristics, powder utilization, capital equipment cost, and operating costs.
Available Space - One important criteria when selecting a powder system is the available floor space and height restrictions. The cartridge recovery system is typically the most compact design in floor space requirements, however, due to the collector and blower being mounted directly to the booth, each color to be recovered will require its own module, adding to the cost and space requirements. The cyclone system requires more headroom because the cyclone and cartridge collector stands much higher than the booth. The floor space required for this system is greater because the cyclone and final filter are remotely mounted using interconnecting ductwork. The twin air belt system does not require the headroom of either the cartridge or cyclone system because the floor can be closer to the ground than other systems. The floor space required for the filter belt design can be greater than a cartridge system due to the remote reclaim system. However, the reclaim system can be located in an open area away from the booth providing layout flexibility. Color Change Requirements - The actual time required to change colors in any of these booths depends on the quality of finish required, how many people work on it, number of powder guns, the size of the booth and the amount of duplicate equipment for dedicated colors. The cost and time required to color change can vary drastically between the different recovery systems. To reclaim each color in a cartridge booth, a dedicated collector is required. With a cyclone system or a filter belt booth using the cyclone or mini-cyclone, multiple colors can be reclaimed without any additional equipment. An average cartridge system takes 30 to 60 minutes to change, the cyclone system takes 30 to 90 minutes and the twin air belt takes 60 to 120 minutes to change over.
The other important factor to consider on color change is the number of colors and the interval between color change. If the time between color changes is long and the number of colors is few, the cartridge module system makes sense because it will provide the highest potential reclaim percentage. When the time between color changes is short, or there are a lot of colors, then the cyclone system becomes more practical.
The cartridge system is less practical when there are many colors required due to the added cost in dedicated color modules. The cyclone system is better when many colors are run for short intervals because there is no need for additional recovery equipment. The twinair- belt system is difficult to color change, making it impractical for most multi-color systems.
Lines with two or more booths can change colors with a minimum of production loss. One booth can be prepared for another color off-line while the second booth is in production.
Powder Utilization - Although there is some powder loss in a cyclone system, a well-designed system can provide 70% to 90% material utilization. Typical utilization for the filter belt using a mini-cyclone is 85-95%. In a cartridge module system, there is virtually no powder loss due to the nature of the recovery design. These systems typically provide from 90 to 98% overall material utilization. Always keep in mind in any economic evaluation that there is a certain amount of loss on racks and in handling. Also, testing has shown that the fine particles (less than 20 micron) of powder do not contribute much to the film. So as the percentage of fines increases, the material will become less efficient. In some operations, it is necessary to periodically dispose of the powder in the cartridge module due to accumulation of fines.
To avoid excess accumulation of fines, the overspray should be consumed fast enough to maintain a low level of powder in the cartridge collector. In order to consume the over sprayed powder fast enough to avoid accumulation the first pass transfer efficiency (FPTE) must be near 50% or better. If the percent of overspray exceeds 50% the powder level in the collector will continue to rise when fresh material is added.
Capital Equipment Cost - The cyclone system and the cartridge system are close to the same cost until you add additional cartridge modules. For one color operation, the cartridge module is the lowest cost. Additional modules will add significantly to the cost. The twin air belt system is the most expensive type of system. System costs depend heavily on the number of colors required.
Operating Cost - The operating cost for a cartridge system can be higher than both the cyclone and twin air belt booths due to the amount of compressed air used in the collector unit and more frequent replacement of cartridges. Compressed air is required to fluidize the powder in the bottom of the collector and provide the necessary pressure for back pulsing. The cartridges tend to wear a little faster because they are subjected to a higher volume of powder and more frequent back pulse at higher pressure.
In a filter belt system, you should also consider the electrical requirements for the addition of the reclaim exhauster and the belt drive. The selection of a recovery system must consider all of the system requirements. All recovery systems have advantages and limitations.
The primary advantage of the cyclone system is the ability to run many colors without adding additional reclaim equipment. It is best suited to shorter runs of smaller volume. The disadvantages are a slightly lower material utilization percentage and some additional safety considerations.
The primary advantage of a cartridge module system is the highest possible material utilization percentage. All of the overspray is collected for reuse. The disadvantage is the need for a module for every color that is reclaimed for reuse. The primary advantage of the filter belt booth is the potential for high transfer efficiency. This is due to the broad surface of the belt for exhaust air. The broad surface area allows the volume of air required for containment to spread out and moved at a very slow velocity so that it does not interfere with powder deposition.
Another option that is part of the consideration is spray-to-waste. In some operations with short runs of many colors it is not practical to invest the time and money into reclaim. Spray-to-waste can be done with a batch booth, cartridge booth or cyclone system. The powder is captured and disposed of instead of being recycled into the system. Spray-to-waste is strictly an economic decision. The cost of equipment and labor for recovery is compared to the cost of the scrap powder. A system that does not reclaim powder can be color changed faster than a system that reuses overspray. The guns and feed system must be cleaned the same as the reclaim operation but the booth interior does not have to be cleaned as thoroughly and the recovery system does have be switched.
If automatic guns are used for spray-to-waste, multiple feed hopper may be required. Automatic guns are usually fed from a large feed
hopper that is very difficult to color change. If the cost of dedicated feed hoppers is not practical, two 80 lb. canisters can be used, one for
half of the automatic guns and one for the other half. This size of canister can be quickly emptied, dismantled and cleaned.
Your eyes can deceive you. Don't trust them
The squares marked "A" and "B" are the same shade of grey. Don't agree? Try these methods to prove for yourself.
1. Try putting the image in Photoshop.
Using the eyedropper tool you can determine that the RGB values of the grays in both square A and square B are same.
2. Cut out a cardboard mask.
By viewing patches of the squares without the surrounding context, you can remove the effect of the illusion. A piece of cardboard with two circles removed will work as a mask for a computer screen or for a printed piece of paper.
3. Print the image and cut out the squares.
This is another way to isolate the patches form their surrounding context. Cut out each square along the edges. Remove them. Hold them side by side. Please note that some printers, especially laser printers, have "enhancement" processing that increases the contrast of edges. This can cause the printed squares to have slightly different values of gray.
4. Get a photometer.
5. Still don't believe? click here
The visual system needs to determine the color of objects in the world. In this case the problem is to determine the gray shade of the checks on the floor. Just measuring the light coming from a surface (the luminance) is not enough: a cast shadow will dim a surface, so that a white surface in shadow may be reflecting less light than a black surface in full light. The visual system uses several tricks to determine where the shadows are and how to compensate for them, in order to determine the shade of gray "paint" that belongs to the surface.
As with many so-called illusions, this effect really demonstrates the success rather than the failure of the visual system. The visual system is not very good at being a physical light meter, but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view
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