Compressed Air Best Practices - March 2008 - (Page 25)

Focus Industry FOOD PROCESSING | 03/08 | to the amount of air being drawn out, the ultimate vacuum level is attained. If more air is let in, vacuum will decrease, and if less air is let in, vacuum will increase. Other terms that are used for acfm are volume flow, expanded air, inlet air and aspirated air. Note that many vacuum pump manufacturers rate vacuum pump capacity in acfm. Know Your Flow If applications are rated in scfm and most vacuum pumps are rated in acfm (or vice versa), we must convert between the two to match application demand to vacuum pump capacity. Without too much detail, the conversion is scfm x P1/P2 = acfm where P1 is the starting pressure in absolute terms and P2 is the target pressure in absolute terms. A simple example is an application that generates 100 scfm at 20" HgV. The conversion to acfm is 100 x 29.92/9.92 = 302. With this formula, it is evident that as vacuum increases, so does the volume flow requirement. To attain 29" HgV with this same 100 scfm starting point requires a vacuum pump capacity of at least 3,252 acfm or over 200 hp. Conversely, attaining only 5" HgV would require a minimal 120 acfm or about 5 hp. The bottom line is that there is a cost associated with attaining higher vacuum. It is important to make sure the requirement for higher vacuum is real or that the payback in production throughput is greater than the additional energy cost for operating larger vacuum pumps. This illustrates why the term cfm is not descriptive enough. The example shows that matching a 100 “cfm” vacuum pump with a 100 “cfm” application can get you into trouble if you make invalid assumptions regarding scfm or acfm. Keep in mind that in this example we did not consider temperature. As inlet air temperatures increase or decrease, acfm will increase or decrease. In addition, stock vacuum pumps are typically rated for pumping air. If the inlet gas stream is a mixture of gases other than air there are other factors to consider before deciding on the right pump. There are additional flow designations other than cfm that can be found in production vacuum system specifications. Some of the more frequent ratings are in cubic meters/hour, cubic meters/minute and liters/second. In these cases, you will have to perform conversions that match vacuum pump capacity with application demand. Be sure to know which side of the vacuum pump these ratings are based on and you will be well on your way to making the right decisions. For more information contact Dan Bott, Dan Bott Consulting LLC, tel: (251) 609-1429, email: dan@dbott.com, www.danbottconsulting.com “Many . vacuum pump manufacturers rate vacuum pump capacity in acfm” www.airbestpractices.com 25 http://www.danbottconsulting.com http://www.airbestpractices.com

Table of Contents Feed for the Digital Edition of Compressed Air Best Practices - March 2008

Compressed Air Best Practices - March 2008 Contents From the Editor Utility-Air News The Compressed Air Audit of the Month: Saving Energy with Blowers in Canada Do You Provide World-Class Service? Managing Rotary Screw Air Compressors at Midwest Bakery The Difference Between Acfm and Scfm for Proper Vacuum Sizing Best Practices — Multi-Module Process Air Dryers The Vacuum Formation Process in Building Compressed Air Filter Elements Solberg Manufacturing Real World Best Practices: Measuring Pressure Dewpoint in a Compressed Air System Resources for Energy Engineers: Training Calendar & Product Picks Wall Street Watch Advertiser Index Job Market

Compressed Air Best Practices - March 2008

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