Compressed Air Is Cool

Recently, I had the pleasure of assisting an engineer whose company makes honing tools.  One particular item is a large, heat treated, alloy steel part.  When it comes out of the furnace, it’s over 1,700°F.  Needless to say, it has to cool off quite a bit before they can do anything with it.  He was doing the best he could with some large shop fans when he called me to pursue the possibility of doing it quicker with engineered compressed air products.  His goal was to cool the part to 600°F in fifteen minutes.

For those of you without an engineering background (or for you engineers who “phoned it in” during Heat Transfer and Fluid Flow), there are 2 components to convective heat transfer: temperature differential, and flow of the cooling medium.  Increase either or both of these, and you get better results.

In this particular case, the temperature differential between the part (1,700°F) and ambient (80°F) is already huge.  Now, a Vortex Tube device could cool the air an additional 50° or so, but that wouldn’t really have much overall effect.  That left us looking at increasing the air flow.

We considered several options, but the Super Air Knife’s 40:1 amplification ratio made it the obvious choice for maximizing flow, while minimizing consumption.   I didn’t do the CAD drawings (although CAD is my favorite video game), but I supplied my customer with the 3D models he needed to design his system.  He incorporated three 12” Super Air Knives, supplied with plumbing kits.  He mounted them with our Universal Air Knife Mounting Systems.  It made for a very nice looking little rig.

Based on the initial data, my heat transfer calculations were in the “close but no cigar” range – making some conservative assumptions, I figured it could take as long as half an hour to reach 600°F.  When all was said & done, though, the cooling performance was a great improvement over the fans, and even better than my calculations indicated it would be – the system actually cooled the part to 200°F in twenty minutes.  Which was curious, because I actually paid attention in, did well in, and, dare I add, even enjoyed HT/FF.

As I come to grips with my failure to know everything, I’m encouraged that the as-yet unknown variables were skewed in our favor.  Yeah; compressed air is cool.  Even cooler than I thought.

Russ Bowman
Application Engineer
EXAIR Corporation
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