If there’s one thing I’ve learned about compressed air, it’s that pressure is a lot like coffee. A little bit gets the job done, but too much just makes you jittery and costs you more than it should. The last step in our 6 Steps to Optimizing Compressed Air series is all about dialing that pressure back to where it actually needs to be. So why worry about pressure?
Compressed air is one of the most expensive utilities in your facility. For every couple of PSI you crank the system higher than necessary, your energy bill climbs right along with it. In most cases, your application doesn’t even need that extra pressure. It’s like using a fire hose to water a houseplant.
This is where pressure regulators come in. They’re simple devices, a knob, a spring, a diaphragm, but they’re doing some heavy lifting. Twist the knob, and the spring loads or unloads. That changes how much the diaphragm allows through, and suddenly you’ve got a steady, consistent downstream pressure without overfeeding your air tools or EXAIR products.
The real magic happens when you lower that setpoint. If your Super Air Knife is blowing water off parts just fine at 60 PSIG, why run it at 100 PSIG? Less pressure means less flow, and less flow means more savings. You’ll get the same result with a smaller demand on the compressor. That’s a win-win every plant manager can appreciate.
Now, before you start cranking down knobs all over the place, keep sizing in mind. Regulators need to be matched to the volume of air your application requires. If they’re undersized, you’ll experience droop, when the pressure drops off during demand spikes. EXAIR takes the guesswork out by offering properly sized regulators in kits with a lot of our products. We’ve already done the math so you don’t have to.
Turning down the pressure might be the last of the six steps to optimizing compressed air, but it’s one of the easiest changes to make and one of the fastest ways to save. A couple twists of the regulator could be all it takes to lighten the load on your compressor, cut operating costs, and keep your system running lean and efficient.
And if you’re not sure where to start, that’s what our Application Engineers are here for. Call us, chat with us, or shoot us an email. We’ll help you find the right pressure for your setup without the trial and error.
EXAIR is now partnering with EasyCAS by DirektIn software. This tool lets you actually measure and validate the savings you’re getting from steps like lowering pressure and implementing engineered solutions. No more guessing, you’ll have hard data showing how much air and money you’re saving.
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.
EXAIR Stainless Steel Super Air Knives are popular in food processing applications (left to right): removing excess moisture prior to flash freezing of fish fillets, preventing clumping while packaging shredded cheese, and (my personal favorite) ensuring a consistent and even glazing of fresh, delicious doughnuts.
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:
Super Air Knife Kits include a Shim Set, Filter Separator, and Pressure Regulator.
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 Visit us on the Web Follow me on Twitter Like us on Facebook
I find myself interchanging these terms; pressure drop and differential pressure. This is very common as both are determined by the change in pressure between two points. In this blog, I will cover the difference between these two terms in my view.
Pressure drop only occurs when the air is flowing. The higher the velocity, the more extreme the pressure drop will be. Velocity is created when the pressure changes. So, the higher pressure will go toward the lower pressure. But we wish for that pressure difference to be as low as possible. Pressure drop is always a loss, and you cannot regain that energy. Forms of pressure drop that can be found are like small diameter pipes or tubing; restrictive fittings like quick disconnects, and conditioning equipment like after coolers and air dryers. If too large of a pressure drop occurs, the pneumatic equipment will not have enough power to operate effectively and efficiently. I have another blog with a video that helps demonstrate this, “Pressure Drop and its Relationship to Compressed Air”.
Pressure Regulators “dial in” performance to get the job done without using more air than necessary.
Differential pressure can be static or flowing. It is very similar to pressure drop except that the energy is stored. The most common device that does this is the pressure regulator. You are able to reduce the pressure downstream to the point-of-use. This type of pressure reduction stores energy, and it will save you money, instead of wasting money. For every 10 PSI reduction in pressure, it will save you 5% in energy. With blow-off devices, you want to use the least amount of pressure to “do the job”. Over-using your compressed air is wasteful.
Here is a graph of a typical compressed air system. As you can see, the typical pressure drop from the air compressor to the point-of-use. So, if you can reduce the pressure drop through the system and optimize the differential pressure from the regulator to your point-of-use, you can optimize your system.
Pressure Drop Chart
In a simple statement, a pressure drop loses energy while differential pressure stores energy for later use. EXAIR offers a variety of efficient, safe, and effective compressed air products to fit within the demand side. This will include the EXAIR Super Air Knives, Super Air Nozzles, and Safety Air Guns. If you wish to go further in optimizing your system, an Application Engineer at EXAIR will be happy to help you.
Pressure drop is an unavoidable occurrence in compressed air systems. It’s caused by restrictions or obstructions to flow in your system, and that includes…well, everything:
No matter how big your header, drops, supply lines, etc. are, pressurized fluid encounters friction with the inside diameter of the conduit through which it flows.
Odds are, your header has at least a few elbows, wyes, tees, reducers, etc. Individually, the restrictions from these are usually quite small, but when you look at a system full of them, they can add up.
The type of piping your header is made of matters as well. Iron pipe WILL rust, which roughs up the inside wall of the pipe, and increases friction. Copper and aluminum aren’t near as bad, but there’s no such thing as a zero coefficient of friction.
Filters force the air flow through very small passages, torturous paths, or directional changes to remove particulate, moisture, and oil/oil vapor.
While not a restriction or obstruction, leaks in your system DO let out perfectly good compressed air before it can be used, so they can be included in our discussion.
Before you go off and redesign your air distribution header or remove your filters (DON’T do that!), it’s important to point that, historically, the highest pressure drops occur at or near the points of use:
Undersized hoses. The friction mentioned in the first ‘bullet’ above is compounded by increasing length, and decreasing diameter, of your air operated products’ supply lines. If your product’s performance is suffering, look up its rated air consumption and compare that to the flow rating of the length & diameter of the supply line.
Quick connect fittings. The push-to-connect types are particularly notorious for this…the air has to flow around the plug that stops flow when it’s disconnected. You can either replace them with threaded fittings, or if you still want the convenience of the quick connect, consider bushing up a size or two. A 3/8 NPT push to connect fitting will flow twice as much as a 1/4 NPT, and a 1/2 NPT will flow over three times as much as a 1/4 NPT fitting. In the EXAIR R&D room, Efficiency Lab, and shop, we actually use 3/4 NPT quick connects for a wide range of testing, demonstration, performance, etc.
Leaks. Even if they’re not big enough to cause a pressure drop, they’re still wasting compressed air. And if they ARE causing pressure drops, please stop reading this and go fix them, right now. Yeah; it’s that important.
Back to back Elbows, Tapered fittings, clean outs and ball vales all cause friction in the line resulting in Pressure loss.
Now, there are culprits on the supply side too: after coolers, dryers, and system filters can all contribute to pressure drops if they’re improperly sized, or, more often, improperly maintained. For troubleshooting, your first and best shot is to have pressure gauges at strategic locations…you can’t manage what you don’t measure. And not managing it can get costly:
Let’s say your compressor discharge header pressure is set to 100psig, but an undersized hose is only letting you get 65psig to an air operated product that really needs 80psig. You can increase your header pressure to 115-120psig to “push” more air through that hose, but keep in mind that all your other unregulated loads will get that pressure increase as well: pneumatic cylinders would operate faster, impact drivers will generate more torque, blow off devices will use more air (and get louder), etc.
Even if those things weren’t a problem, it’s going to cost you more. For every 2psi increase in your compressor’s discharge pressure, its power consumption increases by 1 percent. So, for the 20psi increase, it’s going to cost you about 10% more to operate that compressor. A larger diameter air hose, on the other hand, is a one time investment that doesn’t affect the rest of your compressed air system.
If you haven’t fixed the leaks I mentioned above yet, increasing your supply pressure will increase the leakage flow rate and, especially if the leak’s in a hose or hose fitting, it can tear that opening wider, compounding the leakage flow rate further.
EXAIR Corporation is keen on making sure you get the most out of our products, and your compressed air system. If you’ve got questions, we’ve got knowledge, and a wealth of resources to help…give us a call.
