Episode 69: Turndown Capability: Why Bigger Range Isn’t Better

Jason Reed

Welcome back to The Big Dog Podcast. I'm Jason Reed, here with Lisa Saunders, and today we're talking about turndown capability. Which sounds like one of those terms people toss around in meetings like everybody already knows what it means. [chuckles]

Lisa Saunders

[warmly] Yeah, and a lot of folks kind of nod along because, sure, it sounds intuitive. Demand goes up, demand goes down, so you want the compressor to follow it. Fair enough. But the real question is what that actually looks like in a plant.

Jason Reed

Right. Because in the real world, compressed air demand is almost never perfectly flat. Maybe first shift is full production, second shift is lighter, third shift is basically cleanup and a skeleton crew. Or you've got one line that runs hard on Monday and then slows down by Thursday. Happens all the time.

Lisa Saunders

Seasonality too. Some facilities are slammed for part of the year and then much quieter the rest of it. And even inside one day, demand can bounce around just because operators are using tools differently, automation cycles are changing, or certain equipment only runs in batches.

Jason Reed

Exactly. So if your air use is moving around, you start looking for a compressor that can move with it. That's where turndown capability comes in. Simple definition: it's the compressor's ability to reduce or increase output to match system demand without just burning power for no reason.

Lisa Saunders

And that's the important part. Not just following demand, but following demand efficiently. Because a compressor that's technically responding, but doing it badly, isn't really helping you all that much. It's just surviving the swings.

Jason Reed

Good way to put it. I mean, if you've got a unit that can run across a broad range, that sounds great on paper. It feels like insurance. You think, okay, demand's unpredictable, this machine will cover me.

Lisa Saunders

And to be fair, it does solve a real problem. We're not saying variable demand is fake or overblown. Most plants do have it. So wanting turndown is not some bad idea. It's a practical reaction to what people see every day on the floor.

Jason Reed

But here's the part people skip over. More turndown is useful. It is not automatically the best fix. Those are two different statements.

Lisa Saunders

Yes. Because if the strategy becomes, let's just buy one machine and make it handle everything from really low demand to peak demand, now you're asking a lot from one compressor. Sometimes too much.

Jason Reed

And that usually leads to one of two problems. Either you're wasting energy at part load, or you're setting yourself up for reliability issues. Sometimes both, if we're being honest.

Lisa Saunders

Also, wider turndown can make people feel safer than they actually are. Like, we've got plenty of range, so we're covered. But if the system is poorly matched, oversized, or controlled the wrong way, you can still have unstable pressure, excessive cycling, moisture issues, maintenance headaches... all the fun stuff.

Jason Reed

Yeah, that false sense of security is expensive. And I know we say this a lot, but compressed air problems rarely stay in the compressor room. They show up in production, in scrap, in downtime, in operators complaining that tools feel weak.

Lisa Saunders

Or in electric bills. Let's not forget that one. Equipment cost is only a slice of what you spend over the life of a compressor system. Electricity is the big piece, so inefficiency adds up fast.

Jason Reed

So the setup for this whole conversation is pretty simple. Demand variation is real. Turndown capability matters. But if you treat wide turndown like a cure-all, you're probably gonna make decisions that cost more than they save.

Lisa Saunders

And in the next part, we'll get into the common control methods, where they help, where they don't, and why a lot of plants are better off thinking in terms of system design, not just one compressor with a big operating range.

Jason Reed

Alright, let's walk through the usual control approaches, because this is where the conversation gets real. Start with start-stop. Pretty much what it sounds like. Compressor runs, demand drops, it shuts off. Pressure falls, it starts again.

Lisa Saunders

Simple, but not always kind to the motor. If demand fluctuates a lot, that unit can start too often, and motors have limits on starts per hour. Blow past that enough times and you're asking for trouble.

Jason Reed

Next is load-no-load. Instead of shutting off, the compressor idles when it isn't making air. Sounds better, and sometimes it is better, but unloaded doesn't mean free. It can still pull about 35 percent of full-load power while producing nothing.

Lisa Saunders

Which is why people get surprised by the energy bill. They think, well, it wasn't loaded much. Okay, but it was still consuming power. And with oil-lubricated rotary screw compressors, unload conditions can create another issue: you lose the heat of compression, water can condense out in the oil, and that can damage the machine.

Jason Reed

That's one of those details people miss. They look at controls as an efficiency discussion only. It's also a machine health discussion.

Lisa Saunders

Then you've got modulation. That adjusts the inlet valve in proportion to demand. It's generally most effective from about 40 to 100 percent of capacity, and it can hold a tighter pressure band than load-no-load, something like 3 to 5 PSIG instead of around 10 PSIG.

Jason Reed

But there's still an efficiency hit. Energy loss, pressure drop, small operating inefficiencies. So, better control in some situations, yes. Perfect answer, no.

Lisa Saunders

And then the one everybody wants to jump to: VSD, variable-speed drive. [excited] Best turndown of the bunch, soft starts, very tight pressure control, around 2 PSIG. For variable demand, it can be a strong option.

Jason Reed

But not for everybody. Usually makes the most sense in a certain operating window, roughly 20 to 80 percent of capacity. And for oil-free units, not below 40 percent. So again, not magic.

Lisa Saunders

Right, and this is where people go off track. They say, fine, let's just buy one bigger compressor with a wide range and cover everything. Don't do that. Oversizing is a leading cause of compressor failure, and it's a lousy substitute for proper system design.

Jason Reed

Yep. If you're oversizing one machine to handle your lowest lows and highest highs, chances are it'll spend too much time operating in the wrong part of its range. That's bad for efficiency, bad for reliability, and bad for your maintenance team.

Lisa Saunders

A stronger long-term approach is often a multi-compressor setup. One base-load unit handles the minimum steady demand. That machine either runs hard or turns off. Then a trim compressor, often a rotary screw with a VSD, handles the ups and downs above that base load.

Jason Reed

And then you keep a backup unit on standby in case the base or trim machine goes offline. Ideally, that backup is sized to step in properly. That's the part that can save your neck when something fails on a Tuesday morning and production doesn't care that maintenance is still troubleshooting.

Lisa Saunders

Exactly. A multi-compressor system gives you range, but also stability and redundancy. You can rotate hours between machines, reduce maintenance frequency, spread wear more evenly, and avoid scrambling for emergency rental equipment.

Jason Reed

And from an energy standpoint, two smaller compressors are often better than one oversized machine when demand moves around. If demand stays really high and steady, a single fixed-speed unit may be most efficient. But most plants aren't that steady. That's the catch.

Lisa Saunders

One more thing: if you're building a system like this, controls matter. Cascading units and load sharing get more important, especially if a VSD is involved. So this is usually where getting a compressed air professional involved is worth it.

Jason Reed

Bottom line, don't confuse wide turndown with a complete solution. Match the control method to the demand profile, don't oversize out of fear, and think in systems, not just single machines.

Lisa Saunders

That's the takeaway. Jason, good one today.

Jason Reed

Appreciate it, Lisa. And we'll keep digging into the stuff that actually helps on the plant floor. Thanks for listening.

Lisa Saunders

See you next time. Bye, Jason.

Jason Reed

See ya, Lisa.