Episode 35: Right-Sizing Centrifugal Compressors

Jason Reed

Alright, welcome back to The Big Dog Podcast, folks. I’m Jason Reed, and as always, I’m here with Lisa Saunders. Today, we’re getting into the nitty-gritty of right-sizing centrifugal compressors. Now, if you’re in steel, automotive, or machine-building, you already know these beasts are the backbone for high-volume, oil-free air. But, Lisa, I feel like people still underestimate just how critical it is to get the sizing right on these things.

Lisa Saunders

Absolutely, Jason. I mean, we’ve talked about rotary screws a lot on this show, but centrifugals are a different animal. They’re inherently oil-free, which is huge for industries where air quality is non-negotiable. But what really sets them apart is how sensitive they are to ambient conditions—temperature, humidity, even barometric pressure. That’s something I think a lot of folks overlook when they’re designing out a system.

Jason Reed

Yeah, and it’s not just a minor detail. Like, take a steel plant—on a hot, humid day, your compressor’s gonna put out less air at a lower pressure than it does on a cool, dry morning. If you don’t account for that, you’re either coming up short or, more likely, you’re way oversized. And that’s where the headaches start. I remember back on the shop floor, we had this one setup—management wanted to play it safe, so they went big. Too big. That compressor spent half its life short cycling, blowing off air, and the maintenance guys were pulling their hair out. I mean, it was just constant nuisance shutdowns and wasted power.

Lisa Saunders

That’s such a classic story. And it’s not just steel—automotive OEMs, too, they need those high volumes, but if you don’t size for your actual demand and the real-world conditions, you’re just asking for trouble. I always tell people: you’ve got to look at your coldest days for freeze protection, but most of your sizing effort should be on those hot, sticky days when your compressor is working its hardest. But, you know, you can’t ignore the average day either, because that’s where you’re running most of the time. It’s a balancing act.

Jason Reed

Exactly. And if you get it wrong, you’re not just risking downtime—you’re burning through energy and racking up maintenance bills. So, yeah, sizing matters. And it’s not as simple as just picking the biggest machine you can afford. There’s a lot more to it, and that’s what we’re gonna dig into today.

Lisa Saunders

So, let’s talk about the infamous “fudge factor.” I love that term. It’s basically this habit engineers and manufacturers have—adding an extra 10 to 15 percent to the calculated demand, just in case. And, Jason, I get why they do it, but it really complicates things, doesn’t it?

Jason Reed

Oh, for sure. I mean, it’s human nature, right? Nobody wants to be the one who undersized the compressor and brought the line to a halt. But here’s the thing: you size for your worst-case, hot-day scenario, then you tack on that fudge factor, and suddenly you’re running a compressor that’s way too big for what you actually need 90% of the time. And that’s where you get into trouble with short cycling and efficiency loss. I mean, we talked about this in our episode on compressor cycling—oversizing is a silent killer. You end up blowing off air, wasting power, and your operating point drops into the lower half of the turndown curve, which is just not where you want to be.

Lisa Saunders

Right, and the numbers back it up. Most plants are running at 65 to 100 percent of capacity, but even at 70 percent, you’re probably only using, what, 3,000 to 3,500 CFM out of a 4,750 CFM machine? That’s a lot of wasted horsepower. And if you’re running even lower, it gets worse. I always say, just because you can buy a bigger compressor doesn’t mean you should. It’s not like buying a pickup truck for the one time a year you haul a couch.

Jason Reed

(laughs) Yeah, that’s a terrible analogy, but I get what you mean. And honestly, a lot of folks think, “Well, bigger is more efficient.” But that’s only true if you’re running it at full load. Two compressors at half capacity are less efficient than one at full load, but if your demand drops below 50% for long stretches, a multi-compressor setup is actually the smarter play. You can shut one down, run the other at full load, and avoid all that blow-off and wasted energy.

Lisa Saunders

Exactly. I worked with a plant that switched to a base-trim-backup system—so, one big base load compressor, a trim unit with a variable-speed drive, and a backup. Their demand fluctuated a lot, and after the switch, they saw a big drop in energy use and way fewer maintenance calls. It’s just a more flexible, reliable setup. And you’re not stuck paying for capacity you only need a few days a year.

Jason Reed

Yeah, and it’s not just about efficiency. It’s about reliability, too. If you’re constantly short cycling, you’re wearing out your equipment faster. So, ditch the fudge factor, size for your real needs, and think about how your demand actually looks over the year. That’s where the savings are.

Jason Reed

So, let’s dig a little deeper into these multi-compressor strategies. The base-trim-backup approach isn’t just about efficiency—it’s about keeping your plant running, no matter what. Your base load compressor handles the steady, minimum demand. The trim unit—usually a rotary screw with a VSD—takes care of the ups and downs. And the backup? That’s your insurance policy. If something goes down, you’re not dead in the water.

Lisa Saunders

And the control strategies are just as important. Most operators stick with constant pressure control because it helps avoid short cycling, which, as we’ve said, is a maintenance nightmare. Surge anticipation is another big one—when the system pressure gets close to the compressor’s output, the control system opens the unloading valve to keep you out of that surge zone. It’s all about protecting your equipment and keeping things stable.

Jason Reed

Yeah, and honestly, this is where having a good local partner makes all the difference. I mean, you can read all the manuals you want, but nothing beats having someone on-site who knows your system and your facility. I’ve seen so many places benefit from a proper air audit—just having a pro come in, look at your demand profile, and help you right-size your setup. It’s not just about the compressor, it’s about the whole system working together.

Lisa Saunders

Totally. I remember a facility that thought they needed a massive new centrifugal, but after an audit, they realized they could get by with a smaller unit and a smarter multi-compressor setup. Their operating costs dropped, downtime went down, and they had way more flexibility. Sometimes you just need that outside perspective to see what’s really going on.

Jason Reed

And that’s why we always say—don’t go it alone. Work with your local compressed air professional, get that audit, and make sure you’re not leaving money on the table. Alright, that’s gonna do it for this episode of The Big Dog Podcast. Lisa, always a pleasure.

Lisa Saunders

You too, Jason. And thanks to everyone for tuning in. If you’ve got questions about sizing, multi-compressor systems, or just want to avoid those classic mistakes, reach out to your local experts—or drop us a line. We’ll be back soon with more real talk on compressed air. See you next time!

Jason Reed

Take care, everybody. Don’t let your compressors run you—run your compressors. Catch you on the next one.