It Looked Good On Paper

“It looked good on paper…” – this has been the Call to Retreat for many a good-intentioned project, and it’s been on my mind as I prepare for a little construction undertaking this weekend. A few of my fellow Cub Scout leaders and I are going to let our 9 & 10 year-old Scouts build a bridge. See, when we have our Blue and Gold Banquet in a few weeks, the 10 year old Webelos II Scouts will be “crossing over” to Boy Scouts, and the common ceremonial prop we use to do that is a small bridge – their Cub Scout leaders remove the blue loops from their epaulettes before they go across, and their new Boy Scout leaders put the green loops on when they get to the other side. It’s been almost a year since my oldest crossed the bridge, and my pride over that still hasn’t subsided enough to keep me from mentioning it again!

Anyway, we drew up a nice little sketch of this nice little bridge, we’ve dug out our saws and hammers, all of our cordless drills’ batteries are on their chargers, and our materials list is ready for the hardware store, so we’re as prepared as we’re going to get. Until, of course, we get started, and realize what we forgot. We actually did plan for this contingency, though…Mr. Rob and I are the “Second Trip To The Hardware Store” team.

Every once in a while, though, it’s good to have a reminder that there are certain immutable physical principles that bear out from the drawing board to the plant floor. Yesterday, I had the pleasure of assisting a customer with a Super Air Knife system installation. Here’s how it went:

They had recently purchased (2) 54” Super Air Knives, and had installed them to blow debris from sheets of plywood. This should be a slam-dunk: you simply can’t find a more “textbook” application for a Super Air Knife. Unfortunately, the air flow rate wasn’t sufficient to produce good results, so I asked about the compressed air supply. They had 60 feet of ¾” pipe supplying these two units from their 80 PSIG air header, but were getting only 30 PSIG at the inlet to the Super Air Knives. Now, I knew instantly from looking at the Super Air Knife Installation Manual (registration required for access) that this was undersized, since the recommended size for that length of piping, and that amount of Air Knife, would be 1-1/4”. When I calculated the compressed air pipe pressure drop I used these parameters: a flow rate of 313 SCFM (total flow rate of two 54” Super Air Knifes at 80 PSIG supply pressure), 60 ft of ¾” pipe (0.82” ID), and 80 PSIG initial pressure. The results were a pressure drop of 50.45 psi, which is pretty darn close to their 80 PSIG header pressure, minus the 30 PSIG they were seeing at the Air Knives!

I guess the moral to this story – from my perspective, anyway – is that “it looks good on paper” is a harbinger of doom if a trip to the hardware store is involved. When it comes to applying the laws of physics, it comes down to the accuracy of your calculations. As we’re reminded in this  video, the laws of physics are absolute, and, as the engineer in the video says, “if you make physics mad, physics will hurt you!”

Russ Bowman
Application Engineer
EXAIR Corporation
(513)671-3322 local
(800)923-9247 toll free
(513)671-3363 fax

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