One of the advantages to compressed air operated equipment is the ability to precisely “dial in” the performance by regulating the supply pressure. Consider an EXAIR Super Air Knife, for example. The flow & force can be adjusted from a “breeze to a blast” and any point in between, via a point-of-use Pressure Regulator. I know of users who operate them with a supply pressure as low as 5psig (that’s the “breeze”) and as high as 120psig (that’s the “blast”), depending on the requirements of the application.
For a wide variety of typical industrial blowoff applications, a supply pressure of 80psig is a good place to start. So, it stands to reason that the compressed air header pressure will have to be at least 80psig. If the piping/distribution system is sized properly to carry the total amount of air flow you need to the points of use, though, it doesn’t need to be an awful lot higher than 80psig…and that’s a good thing. Here’s why:
Any fluid encounters friction as it flows through a pipe (or hose or tube) which causes a drop in pressure along every bit of the length of flow. The larger the pipe (or hose or tube) the lower the friction and hence, the lower the pressure drop. Now, that’s only important if you care about how much you’re spending on running your air compressor(s). Consider this:
We’ve got a customer that puts our Model 110042 42″ Aluminum Super Air Knives on machinery they make & sell to their customers. This Air Knife will use 121.8 SCFM when supplied at 80psig with the stock 0.002″ thick shim installed, and does the job quite well, most of the time. Some specific applications, however, need higher flow & force from the Air Knife, so our customer offers, as an option, the Super Air Knife with a 0.004″ thick shim installed. Since this doubles the air gap, it also doubles the air consumption. They’d plumbed the supply line to the Air Knife per the recommended in-feed pipe sizes from the Installation & Maintenance Guide:
Since the drop was less than 10ft long, they used a 3/4″ pipe, which was fine…until they installed the 0.004″ thick shim, which meant the air consumption doubled, to 243.6 SCFM. To get that much flow, at 80psig to the Air Knife, they had to increase their header pressure to 110psig, from the 90psig level at which they had been running. This was well within the operating parameters of their air compressor, but it made the compressor work harder, so it used more energy…and cost more to run. In fact, every 2psi increase in compressor discharge pressure results in a 1% increase in operating horsepower (source: Compressed Air & Gas Institute Compressed Air Handbook, chapter 4, page 8).
So, by increasing the discharge pressure by 20psi, the compressor’s power draw (and hence, operating cost) went up 10%. Now, I never found out what size their customer’s compressor was, but I DID look up prices for SCH40 black iron pipe, and for an 8ft length, the 1″ pipe was only $10-15 more than the 3/4″ pipe they were using. Since 243.6 SCFM is roughly 60HP worth of a typical industrial air compressor load (industry thumb rule says they use about 1HP to make 4 SCFM), we can assume that it’s at least a 75HP compressor. Using the following formula to calculate the operating cost while it’s drawing 80% of full load (while making a few reasonable assumptions):
Cost ($) = bhp x 0.746 x # of operating hours x $/kWh x % time x % full load bhp
motor efficiency
bhp = motor full load horsepower (frequently higher than nameplate HP but we’ll use nameplate 75HP to be conservative)
0.746 = conversion from hp to kW
# of operating hours (assume a month’s worth, 8 hours/day, 5 days/week, 4 weeks/month=800 hours)
$/kWh (assume $0.08/kWh)
% time = percentage of run time at this operating level (assume 85% of the time)
% full load bhp = brake horsepower as percentage of full load bhp at this operating level (assume 60HP load, 85%)
Motor efficiency = motor efficiency at this operating level (assume 95% fully loaded)
75HP x 0.746 x 800 x $0.08 x 0.85 x 0.85 = $2,723.29
.95
An additional 10% power draw changes the % full load bhp to 95%, and the cost for monthly operation is:
75HP x 0.746 x 800 x $0.08 x 0.95 x 0.85 = $3043.68
.95
That’s an extra $320.00 spent on running the compressor (per month) at 110psig discharge pressure, instead of an extra $15.00 spent on a larger pipe (one time cost) to run it at 90psig.
This is just one example of the effect of “artificial demand”, which is, essentially, wasted energy due to running your system at a higher pressure to compensate for undersized lines, leaks, intermittent high loads, etc. In addition to helping you specify the right supply line size for your compressed air operated products, we can assist with leak detection, intermediate storage, regulating supply pressures for differing loads, and replacing inefficient devices with engineered products. If you’d like to talk about any, or all, of that, give me a call.
Russ Bowman, CCASS
Application Engineer
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