“I’ll get you my pretty, and your little dog too!”

“This EXAIR 42 inch Super Air Knife has ¼ NPT ports, but the Installation and Operation Instructions recommend feeding it with, at a minimum, a ¾ inch pipe…”

If you’re a movie buff like me, you probably recognize 75% of those quotes from famous movies. The OTHER one, dear reader, is from a production that strikes at the heart of this blog, and we’ll watch it soon enough. But first…

It is indeed a common question, especially with our Air Knives: if they have 1/4 NPT ports, why is such a large infeed supply pipe needed? It all comes down to friction, which slows the velocity of the fluid all by itself, and also causes turbulence, which further hampers the flow. This means you won’t have as much pressure at the end of the line as you do at the start, and the longer the line, the greater this drop will be.

If you want to do the math, here’s the empirical formula. Like all good scientific work, it’s in metric units, so you may have to use some unit conversions, which I’ve put below, in blue (you’re welcome):

dp = 7.57 q^{1.85} L 10^{4} / (d^{5} p)

where:

dp = pressure drop (kg/cm^{2})1 kg/cm^{2}=14.22psi

q = air volume flow at atmospheric conditions (FAD, or ‘free air delivery’) (m^{3}/min)1 m^{3}/min = 35.31 CFM

Let’s solve a problem: What’s the pressure drop going to be from a header @80psig, through 10ft of 1″ pipe, feeding a Model 110084 84″ Aluminum Super Air Knife(243.6 SCFM compressed air consumption @80psig)…so…

q = 243.6 SCFM, or 6.9 m^{3}/min

L = 10ft, or 3.0 m

d = 1″, or 25.6 mm

p = 80psig, or 94.7psia, or 6.7 kg/cm^{2}

1.5 psi is a perfectly acceptable drop…but what if the pipe was actually 50 feet long?

Again, 1.5 psi isn’t bad at all. 8.2 psi, however, is going to be noticeable. That’s why we’re going to recommend a 1-1/4″ pipe for this length (d=1.25″, or 32.1 mm):

I’m feeling much better now! Oh, I said we were going to watch a movie earlier…here it is:

If you have questions about compressed air, we’re eager to hear them. Call us.