Edited By Charles Bates
|Clamp-together ducting along with short pieces of flex hose are easy to disassemble and re-assemble when machines need to be moved. |
After a shop decides on mist collector location, either indoors or outdoors, and on the appropriate capture hood size, it is time to determine the pick-up point location on the intended machine tool. Once that is accomplished, ducting and piping needs must be addressed to complete the installation.
The most common method of picking up mist is to connect the mist collector to a machining center enclosure. Many machine tool manufacturers provide a knockout plate or cut out designed specifically for mist collector connection. However, the locations specified by the machine tool designer are not always the best location, especially on older machines.
When selecting where to connect a mist collector to an enclosed machining center, the purpose of mist collection must be kept in mind. It should keep mist contained inside the machining center by pulling a negative pressure on the machine’s enclosure. This prevents mist from leaking out the doors, around seals, and escaping through other open areas in the machine tool cabinet. Further, mist collection is for filtering the smallest and lightest amount of mist pulled into the mist collector. Cross-ventilation of the enclosed space makes this possible. So any open area — whether it’s always open such as a chip conveyor, or sometimes open such as door — will allow air into the machine and will sweep the inside of the enclosure free of mist.
The process is similar to how fresh air is drawn into a cave or mine: Before fresh air can move in, the old stagnant air has to be swept away. A vent hole is cut at the back of the cave, a fan pulls old cave air up and out of the vent hole, and fresh air enters through the entrance.
The location of the mist collector pick-up point on the machine tool cabinet needs to be well away from several other machine openings. These include openings for the chip conveyor, main access door, tool changer door, machine drive motors, and any other areas where plant air can enter the machine.
Good pick-up points are usually located high on the machine’s enclosure walls or on the top to prevent excess mist and chips from being picked up. Two common locations are along the back wall of the cabinet, as high up as possible, and on top of the cabinet, as close as possible to the rear left or right corner.
Pick-up point do’s and don’ts
• Don’t install a mist collector directly above the main access door or tool-changer door. Mist in the corners of the machine will not be picked up, and once these large doors are open, mist may drift out of the machine enclosure.
• Don’t install the mist collector where mist is directly thrown. If mist comes off the machine tool and impacts the rear right corner of the machine, install the mist collector to pull from the rear left corner of the machine to minimize the mist load to the collector. If the collector must be installed in an area that has a lot of mist thrown at the pick-up point, install deflectors between the mist source and pick-up point to knock down as much of the larger mist as possible.
• Do size the pick-up point so that the inlet velocity is less than 2,000 feet per minute.
• Do cut a small slot (1 in. by 6 to 10 in.) or two in the machine enclosure if it is too well sealed to produce the necessary cross ventilation air pattern.
The most common types of ducting used in mist collection include metal spiral, metal flanged, clamp together, PVC, and flex hose.
|A proper flex hose installation for mist collection.|
|A poor flex hose installation for mist collection (notice the sag).|
Most shops use metal spiraltype ducting because it is relatively inexpensive and all ducting contractors have experience with it. Also, metal spiral ducting comes in almost any diameter desired and in lengths to 20 ft and longer, so it minimizes the number of joints needed.
Metal flanged-type ducting’s use in mist collection is less common because it carries a high purchase price and high installation cost. The advantage is that each section of this type of ducting is caulked and bolted, which effectively prevents leaks. Shops should go with flanged ducting for mist-collection systems where any leak will be a problem.
Clamp-together-type ducting, such as Donaldson Torit Easy Duct, is popular because it is easy to work with. Duct sections come in 5-ft-long sections so one person can install it. Additionally, with a price falling between metal spiral and metal flanged ducting, the cost is minimal.
Clamped-together ducting minimizes leaking for operations running 24 hours a day, but if not installed properly, this type of ducting can develop leaks at clamp points. It offers advantages to shops that frequently move their equipment around also because it’s easy to disassemble and re-assemble once the equipment is in its new location.
PVC and CPVC are not commonly used in mist collection. PVC may look nice when it’s first installed, but it can carry a static charge and attract dust from the air if not properly grounded. Materials known to build up static charges are not recommended when handling mists that are flammable/explosive.
Most consider PVC for piping, not ducting. A 6-in. PVC pipe diameter is not the same as the diameter of 6-in. spiral duct. And since PVC is rarely used, collars to mount it to a machine enclosure and even to the mist collector are difficult to find or must be specially made.
Flex hose is used just about everywhere in mist collection – to connect a machining center to a mist collector or to connect to overhead ducting in a cellular or central system. Shops often incorporate flex hose where hoods need to be moved. They also use it with centrifugal mist collectors because flex hose is easy to maintain and because such collectors are typically located less than 10 ft away.
The only real problem with flex hose arises due to poor installation. If flex hose is not hung properly, a pool of liquid may develop and cause it to sag, eventually plugging up the hose and cutting off the airflow. Flex hose should always run vertically, not horizontally.
Do’s and don’ts of ducting
Always slope ducting toward the source of mist, toward the mist collector, or both if necessary. Mist will impact the cutting walls and will settle out of the air stream.
Flex hose lengths should never be longer than 10 ft. Use solid ducting as far as possible and run only the last few feet with flex hose. Flex hose requires more pressure drop due to internal resistance caused by the metal helix surrounding the hose.
Flex hose should be well supported and never installed horizontally. Liquid will build up, and as quantities increase, the flex hose will sag and blind off the airflow.
All ducting should be installed per local, state, and federal regulations. Proper duct hangers should be used, and if installing systems where the ducting could become 100 percent filled with liquid, such as with a firesuppression system installed using water, ensure that the ducting is installed to withstand the weight.
When using clamp-together ducting, install the clamps so that the clamp end is upward to minimize leaks.
When using slide-gate dampers, install them so that the damper is serviced from the top side of the duct. Slide-gate dampers that slide horizontally or downward will be a source of leaks.
Whenever drilling a hole in the ducting for taking airflow measurements, temperature measurements, static pressure readings or other purposes, locate the holes on the side or top of horizontal ducting to minimize the chance of leaks. Only drill holes to the size you need. Covering any hole with duct tape will not only prevent air from being pulled into the hole, it will make it easier to locate the hole in the future.
The duct conveying velocity for mist collection is important (see recommended duct-conveying velocities chart below).
|Mostly mist (most mist collection systems)||2,000 to 3,000 fpm|
|Mist with fine dust, such as with surface grinding||3,500 to 4,000 fpm|
|Mist with large dust, small chips||4,000 to 4,500 fpm|
Duct conveying velocities should be maintained throughout the duct system. Use the highest conveying velocity required for any source of mist and design the entire duct system to operate at that duct velocity.
Caution. Mist collectors are designed to filter mist, not handle large amounts of dust and chips. Use deflectors to prevent dust and chips from entering the ducting, or move the hood location to where dust and chips will not be picked up.
For additional ducting recommendations, see the Industrial Ventilation – A Manual of Recommended Practice, from the American Conference of Governmental Industrial Hygienists Inc.
Information for this article supplied by Donaldson Company Inc. Part 1 ran in the April, 2009, issue, p. 20. Look for Part 3 in the June issue.