Episode 98: Two-Stage Compressors Finally Reach the Middle Market

Jason Reed

Welcome to the Big Dog Podcast. I'm Jason Reed.

Lisa Saunders

And I'm Lisa Saunders. Today we're talking about bringing the advantages of two stage compressors to the middle market. Jason, for a long time that phrase would have been a contradiction.

Jason Reed

It would have. If you ran a plant with a 30 or 50 horsepower machine, two stage simply wasn't on the menu. The major manufacturers didn't build a two stage rotary screw compressor below 125 horsepower. So small and midsize shops got single stage, whether their duty cycle called for it or not.

Lisa Saunders

So let's start with why anyone cares. What does two stage actually do differently?

Jason Reed

It comes down to how much squeezing happens at once. A single stage machine reaching 100 PSIG does it in one chamber, and that's a compression ratio of about 7.8 to 1. That's a lot of work in one step, and it brings heat and mechanical stress with it.

Lisa Saunders

Two stage splits that job. The first stage runs at about 2.8 to 1. Then the air passes through an interstage cooler that pulls out a good chunk of the heat of compression. The second stage takes that cooler air and goes another 2.8 to 1.

Jason Reed

Right, and 2.8 times 2.8 lands you right around that 7.8 overall. Same destination, easier route. Cooler air is denser, so the second stage does less work. The payoff is up to 15 to 20 percent more flow than a single stage machine of the same horsepower.

Lisa Saunders

There's a mechanical side too, and I think people overlook it. In a typical single stage airend, two discharge bearings carry the entire thrust load. In a two stage design you can have four discharge bearings on the first stage and three on the second. Seven bearings sharing the work.

Jason Reed

Less force on each bearing means less heat and less wear. For a maintenance manager, that's the part that shows up as fewer surprises. But here's what I'd ask you, Lisa. If two stage has been around for decades on big machines, why did it take this long to shrink it?

Lisa Saunders

A big piece of the answer is the motor. Most rotary screw air compressors use induction motors. They're reliable, and they're flexible. But the rotor lags behind the rotating magnetic field in the stator. That's the slip.

Jason Reed

And slip hurts you when you slow down. Put an induction motor on a variable speed drive, and as the speed drops, its efficiency falls off quickly. The drive also tends to hunt, bouncing around as it chases the demand.

Lisa Saunders

A permanent magnet motor behaves differently. The magnets are rare earth, at least twice as strong as what's in an induction motor. The rotor turns in step with the field, so there's no slip. Efficiency stays much steadier across the speed range, and the speed control is cleaner.

Jason Reed

And they can be 20 to 50 percent smaller than a comparable induction motor. That compact size matters when you're trying to fit more machinery into a package for a mid sized machine room.

Lisa Saunders

So today you can find variable speed two stage machines at 30, 50, 75 and 100 horsepower. There are also two stage oil free designs for places where contamination is a real risk. Think food and beverage, healthcare, electronics and semiconductor plants.

Jason Reed

Let's talk money, because that's where the argument gets settled. A two stage unit carries a premium of roughly 30 percent over a comparable single stage machine. And the cabinet is a bit larger because there are more components inside.

Lisa Saunders

Thirty percent sounds like a lot until you look at what the Department of Energy says about lifecycle cost. Buying the compressor is only about 12 percent of what it costs to own it over ten years. Electricity is roughly 76 percent. Maintenance is about 12.

Jason Reed

So you pay for the machine once, and you pay the power bill every month. Now here's a measured example. In one plant case, a single stage variable speed unit was replaced with a two stage variable speed unit. Total energy use dropped 27 percent, and measured efficiency went from 4.76 cfm per kilowatt to 6.1.

Lisa Saunders

I want to be careful with that one, because it's one plant. Savings depend on duty cycle, operating hours and your electricity rate. But when the plant runs steadily, a payback of two to three years on the premium is a reasonable expectation.

Jason Reed

There's also a sizing angle. If you get 15 to 20 percent more air per horsepower, you might replace a 300 horsepower single stage machine with a 250 horsepower two stage one. That shrinks the price gap by 15 to 20 percent right at the purchase.

Lisa Saunders

I like the Unverferth Manufacturing story for showing right sizing. It's an agricultural equipment manufacturer. They run two 150 horsepower two stage machines for full production. Then they added two 30 horsepower two stage units just to cover the much smaller weekend workloads.

Jason Reed

That's smart, because a big compressor running a light load is an expensive way to make a little air. Match the machine to the demand, and the energy goes where the work is.

Lisa Saunders

Now the honest part. Two stage isn't always the answer.

Jason Reed

No, it isn't. Its strength is continuous, steady base load with a lot of running hours. If your plant runs intermittent shifts, or demand swings widely, or the compressor sits lightly loaded for much of the year, that 30 percent premium is hard to justify.

Lisa Saunders

In that case a single stage machine wins. The first cost is lower, the footprint is smaller, and a good single stage control handles swinging demand well. If the unit runs only a small share of the time, there just aren't enough hours to earn the premium back.

Jason Reed

So the question to ask is simple. How many hours does it run, and how steady is the load? Pull your data before you pull a quote. An independent assessment, like the product neutral training CAGI's Compressed Air Challenge offers, is a good place to build that understanding.

Lisa Saunders

Well said. That's our time for today. Thank you for tuning in to the Big Dog Podcast. I'm Lisa Saunders.

Jason Reed

And I'm Jason Reed. Thanks for listening, and we'll catch you next time.