Compressed Air Best Practices - March 2008 - (Page 24)

| 03/08 Focus Industry FOOD PROCESSING | BY DOCTOR VACUUM One of the most perplexing topics in vacuum technology is the concept of flow inside a vacuum system. Vacuum flow is sometimes termed pumping capacity or the pumping “speed” of a vacuum pump. Manufacturers of vacuum equipment have their own special language regarding pumping capacity and new users will have a difficult time following nomenclature in and around what a vacuum pump can deliver to their system. One of the root causes of this confusion is that the term “cfm” is used interchangeably to describe every type of flow in a vacuum system. What we find is that the term cfm, while accurate, is not descriptive enough to be helpful in sorting this out. CO2 O kW Acfm and Scfm Flow from vacuum systems can be boiled down to two terms — scfm and acfm. There are other designations but these are the most essential and provide the essence of what we are seeking to understand. Delivered capacity of a vacuum pump is also described with these same two terms. The trick is to match the system demand requirement with the correct vacuum pumping capability. Correctly matching demand with supply will ensure that adequate vacuum will be attained and production equipment will function properly. We must understand the meaning of each type of flow before these two flow concepts can be matched. Scfm is straightforward and stands for, “standard cubic feet per minute.” In other words, it is the flow of one cubic foot of air per minute at standard conditions. This can be loosely described as the air that is drawn into a vacuum system from the ambient environment. Note that standard conditions of pressure, temperature and relative humidity vary from industry to industry and can significantly affect any conversion to volume flow. Other terms that are used loosely in place of scfm are mass flow, free air and standard air. Acfm stands for, “actual cubic feet per minute” and is the flow inside the vacuum system through distribution piping and at the inlet to the vacuum pump. This air is less dense than air at atmospheric pressure and becomes proportionately less dense as vacuum increases. A good way to illustrate acfm is to use a balloon as an example. A slightly inflated balloon is tied off and placed inside a vacuum chamber. As the chamber is evacuated, the balloon expands under vacuum even though it is sealed and no additional air is let in. As vacuum increases so does the volume of the balloon. Consequently, the air inside the balloon becomes less dense as the balloon volume increases. This is the nature of acfm — more volume and less density. This is exactly what happens at the inlet to a vacuum pump. As vacuum increases, inlet air becomes proportionately less dense. Each revolution of the vacuum pump draws less air than the last as vacuum increases. When the amount of air entering the system is equal Understanding the difference between acfm and scfm is critical to correctly size vacuum pumps 24 www.airbestpractices.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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