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Buyer's Tips - The Dreaded @If you're in the market for an industrial vacuum, here is where we discuss performance claims. |
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Buyer's Tips - The Dreaded @If you're in the market for an industrial vacuum, here is where we discuss performance claims. |
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Marketing being what it is, there is a tendency by some vendors to "emphasize the positive" and even "exaggerate beyond reality". Performance claims for every product in existence are boastful, but it seems that we see such foolish extremes in the vacuum business. Here's a reality check! Almost everyone in this business states performance figures at either end of the performance spectrum, because those figures will be the largest for each scale. For example, when making volume performance claims, many companies use the open condition volume; that is the volume achieved by a vacuum producer when pulling 0 vacuum. And when making vacuum performance claims, they use conditions found at zero flow. Combined, the two figures are meaningless. Its a little like measuring the mileage of your car by running it in the driveway. Worst of all are those companies who get confused and start to use the two performance claims together with the @ symbol. Let's show you what we mean. The following is a chart developed from the published figures for a vacuum producer used in a popular shop-vac type vacuum. It's the model 116472-00 made by AMETEK (who supplies the little high speed centrifugals for almost every canister type unit made in the USA!). You'll note that the maximum flow for this model is 112.0 cfm (the red line) and the maximum vacuum is 106.6 water lift (the blue line).
This chart uses "orifice size" to simulate different levels of performance. Note, that where they come together at the 0.75 inch orifice, the performance is roughly 60 cfm @ 60" H20 (we use the @ symbol instead of "at"). This is about where a little shop-vac type will run with a 12 foot long hose, yet many vendors of these units will state the unit performance is 112 cfm @ 106" H20. Not really the same thing at all - especially as the "exaggerated" rating takes 5 times more horsepower! Let's look at another common "exaggeration". Almost all the industrial manufacturers state their performance levels as Inlet CFM (ICFM) @ a certain vacuum level. At least this is stated correctly, in that the performance can be can be read off of a curve similar to above. There are two problems with this: First, the flow performance is overstated because to be "correct" and comparable between different systems, air flow should be indicated as Standard CFM (SCFM). If you've read the previous two pages, you're ready for the following vacuum conversions: ICFM = (SCFM x 29.92) / (29.92 - Vacuum in "Hg. ) So if a system's performance is stated as 500 icfm @ 12" Hg., then SCFM = 500 (30 - 12) / 30 = 300. There is a definite difference between 500 and 300. (We used 30 instead of 29.92, because this ain't rocket science here). Now, both numbers are correct, and we have no problem with using ICFM as a performance criteria, because most of the manufacturers do the same thing. But understand that only 300 scfm will enter the vacuum hose, and that's the figure that will determine the pickup velocity. Make sure that if performance is stated as "CFM", you must find out what kind of CFM it is - preferably by asking to see a curve. Below is a curve for the 15 Hp DuroVac. You won't often see performance curves displayed like this, but it clearly shows why these vacuum producers are called "positive displacement". You can see by the Red line, that the Inlet CFM remains roughly equal across its range, between 2" Hg and maximum 12" Hg. The vacuum pump draws roughly the same volume of air through it per revolution. You can also see that the Standard CFM - the air that is drawn into the hose - is reduced by roughly half as the vacuum increases toward 12" Hg.
Second, some manufacturers are starting to "confuse on purpose" ICFM and SCFM at a certain vacuum rating. So where they mean 500 ICFM @ 12" Hg., they state 500 SCFM @ 12" Hg. Remember this: if two manufacturers are recommending machines with similar seeming performance, but two very different horsepower ratings, get the curve and do some quick math! Because the answer is either a new discovery in mechanical efficiency, or too much "emphasis on the positive".
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Industrial Vacuums Home Page  DuroVac Industrial Vacuums 15 Iris Crescent, Brampton, ONTARIO L6Z 3H8 1- 888- 330- 3365 PHONE: 905-624-4003 FAX: 905-624-4641
See also: DuroVacTM and PowerLiftTM are trademarks |
