The oil-sealed liquid ring vacuum pump (LRVP) deserves a place at the table – the router table, that is. LRVPs for nested parts routers can provide a totality of advantages that include:
• Powerful depth of vacuum and hold-down force
• Low maintenance
• Convenient, self-contained
• Sawdust tolerance
• Long pump life
• Steady performance
• Quiet operating level
Liquid ring vacuum pumps are rotary positive displacement machines that use oil (sometimes a custom-formulated sealing oil) to create the vacuum that is used to hold down wood in place as it is shaped and cut. LRVPs get their name from the liquid ring that is created when centrifugal force holds the seal liquid against the inner wall of the pumping chamber.
Today’s higher speed machining puts a substantial force on the parts, thus requiring tremendous vacuum hold-down. Hold-down force depends on a number of variables, such as the type of router tools and the force generated by the depth of cut and feeding speed. As a rule of thumb, the average hold-down force on a single piece should be around 2000 pounds. Because there is a continual leak of air through the fiberboard, a table with a fiberboard of 4 foot by 8 foot usually uses a vacuum system that can deliver a minimum of 300 ACFM and as much depth of vacuum as possible.
These parameters make the oil-sealed LRVP a powerful solution. There is a wide range of product lines with ACFM capacities ranging from under one hundred into the thousands. High-efficiency, single-stage liquid ring vacuum pumps are capable of deep vacuum levels up to 29 inches HgV, with maximum efficiency throughout the vacuum range.
But the most significant advantage of the oil-sealed liquid ring vacuum pump over other pump types is its steady performance in a sawdust-heavy work environment. Woodworking may be the most difficult application for vacuum pumps. With a nested parts router, cutting and drilling puts the vacuum system right in the middle of a sawdust storm.
Carryover of wood dust into the vacuum pump and possible rupture of the filter element can increase pressure loss, resulting in a reduction of pump capacity and vacuum level. If the vacuum level is 24” Hg and there is a pressure drop of 2 inches Hg, the pump capacity loss is 33 percent. If the pressure drop is 3 inches Hg, the pump capacity loss is 50 percent. Because of the heavy dust load, the pressure drop can increase rapidly.
Sawdust ingestion can break vacuum pump types such as the lubricated rotary screw and dry rotary vane, which are totally dependent upon the functionality of the inlet filter. The design of the LRVP allows it to handle soft solids and entrained liquids or vapors without compromising the pumps’ mechanical integrity or efficiency. This is because there is no metal-to-metal contact between the rotating parts and the casing, eliminating the need for internal lubrication.
Rugged liquid ring pump systems provide years of trouble-free operation. Unlike other types of pumps, LRVPs don’t need to be rebuilt or replaced on a regular basis. The ingenious design contributes to this, since the pump has only one moving part, and no metal-to-metal contact means the pump is wear-free. Grease-lubricated bearings are located external to the pumping chamber, negating the damaging effect contaminated lubricants can have on the bearings. The pump runs at a low operating temperature. Also, because oil is used instead of water for the seal liquid, the corrosion and scale buildup associated with water is completely eliminated.
Liquid ring design benefits the work environment itself through low operating noise level. Most liquid ring pumps operate at speeds of 1,800 RPM or less. For this reason, and because the pumps have no metal-to-metal contact, they are exceptionally quiet, with noise levels in the 68 - 80 dBA range. “Quiet running” is frequently mentioned by end users as one of the most popular features of the pump.
The oil-sealed LRVP is also adaptable to variable frequency drives (VFDs), which allow motors to adjust speed to better match the required demand at that point in time. VFDs offer two advantages. The first is reduced energy consumption (50% turndown on power) when the pump is not in use. Second, VFDs help eliminate part slippage. As leakage increases from cutting, the vacuum pump speeds up to maintain constant pressure, eliminating slippage and scrap.
Oil-sealed liquid ring pumps are low maintenance. Following an established schedule of preventive and predictive maintenance per IOM can help them run for years without trouble. The systems have a long oil life, with a changing interval usually about every 10,000 hours, or annually. At 30,000 hours, or about five years, mechanical seals and bearings should be replaced (by an authorized distributor or properly trained service technician).
Preventive maintenance can be enhanced through “adders” that help safeguard the system, including:
• Inlet filter: high-efficiency oil filter; 5 microns or finer is typical
• Vacuum relief valve: to prevent over-pressurization of the system
• Spin-on oil filter: a secondary filter that helps protect the heat exchanger from overheating issues
• Exhaust drip leg: helps scavenge oil back into the system when running a shallow vacuum
To reduce carryover concerns, leave the system running with the valve to the router table closed for at least 30 minutes every day. This is called dead-heading the machine. It helps recirculate the oil and allows the separator element to clear. In between cutting cycles, close the vacuum isolation valve to the table to allow the vacuum system to run at a deeper level. This will allow the system time to recover the oil, and there should be less oil carryover.
Achieve a potentially deeper vacuum level with the following tips:
• Check density of the fiberboard - less porous material can result in a deeper vacuum level
• Check thickness of fiberboard - thicker fiberboard can result in a deeper vacuum level
• Optimize patterns to minimize open table space
• Turn off any unused zones
• Check condition of gasket material at the router table
• Check quality of piping connections from the table to the vacuum pump
• Ensure proper operation of the table in tandem with the system
Many factors go into the selection of an oil-sealed liquid ring vacuum pump. Your table manufacturer or authorized distributor has the expertise to help size the system that best fits your woodworking application.
Source: Anthony Korol is with Dekker Vacuum Technologies Inc. For information call 888-925-5444 or visit DekkerVacuum.com.
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