Compressed Air Best Practices - September 2008 - (Page 54) | 09/08 Compressed Air Industry FOOD PACKAGING & PROCESSING | REAL WORLD BEST PRACTICES UTILIZING VENTURI VACUUM GENERATORS Basic Vacuum Pressure Measurement Units NEGATIVE GAUGE PRESSURE PSIG ABSOLUTE PRESSURE PSIA INCHES OF MERCURY IN HG 0 14.7 ATMOSPHERIC PRESSURE AT SEA LEVEL 0 -1.5 -3.0 -4.5 13.2 11.7 10.2 TYPICAL POROUS VACUUM LEVEL 3 6 9 -6.0 -7.5 -9.0 -10.5 8.7 7.2 5.7 4.2 TYPICAL NON-POROUS VACUUM LEVEL 12 15 18 21 -12.0 -13.5 -14.7 2.7 1.2 0 PERFECT VACUUM (ZERO REFERENCE PRESSURE) 24 27 29.92 Central Vacuum System If the vacuum using processes are all located in an area which appears to be more effectively better served by a separate central system, an evaluation is in order. Items to consider are: p Are you tying into an existing central vacuum system with available capacity or do you need to add a new system or expand the existing system? The cost and risk of effective piping to the process areas eliminating reduced performance due to restrictive piping valves, filters, leaks, fittings, etc. Can the central vacuum system react quickly enough to optimize the production cycle time? p p When conditions call for replacement of a large group of Venturi vacuum generators, the result can be very energy effective — for example: At a recent plant utilizing 30 Venturi vacuum generators of various brands, they were flowing 150 cfm of compressed air (32 kW) to remove 200 cfm of evacuation air to an 18" HgV on a continuing basis with a relatively porous material. These units were replaced with a 10-hp singlestage, rotary screw vacuum pump (7.8 kW) at 18" of vacuum. The actual input power fell to about 5 kW where it remained most of the time. In this case, the average electric power rate was $.095 kWh and the operating hours were 6,240 per year; a net savings of 27 kW x .095 x 6240 = $16,000 per year with a $15,000 cost of the rotary screw pump system. 54 www.airbestpractices.com http://www.airbestpractices.com
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