Costs of Oversizing Compressed Air Systems

June 17, 2022
In this episode of the Food For Thought Podcast, Kaeser Compressors’ experts address why oversizing occurs and how an oversized system reduces reliability and increases maintenance and lifecycle costs.

A review of thousands of compressed air systems has reveal that, on average, users operate below 50% of system capacity. And, in fact, oversized compressors are among the most common problems in air systems.

Here to talk amore about that on the podcast today are Kaeser Compressors’ Wayne Perry and Neil Mehltretter. They’ll address why oversizing occurs and how an oversized system reduces reliability and increases maintenance and lifecycle costs.


Erin: You’ve talked before about air systems that operate below 50% system capacity. Can you elaborate more on that? What does it mean? And what is the significance?

Wayne: People oftentimes think that you know, "Oh, I need this much compressed air so I better put in a fudge factor," or "We're going to expand next year or the year after so I'm gonna buy the compressors for that expanded size and just run them at part-load capacity." And that's super inefficient. There have been a lot of cases that I've looked at where it would have made more sense for them to buy or rent a small compressor. The energy savings alone would have paid for buying another compressor when the plant expanded. It's also very hard on your equipment to run them at that part of you wind up getting a lot of snorkeling with the machine, and that wears out valves and switches and starting gear, and it's just something you really don't need to be doing.

Neil: I think Wayne really hit it on the head. You know, your biggest issues are gonna be lack of performance on your equipment. If you have the pulse of your compressed air system—if you're getting warnings, messages, reporting, those kinds of things—those folks are well established on what's happening inside the compressor room in the system. In a lot of cases, most folks aren't and so you close the door, and you never really hear from it again unless there's a problem.

With these systems that are below 50% capacity, you hear about them quite a lot. If you're the maintenance manager, the plant engineer or a production engineer, or the plant manager you know when the compressor is down, the whole plant is down. Typically, those are the systems that are below capacity or a operating capacity. Meaning that we have a system that the compressor is never able to operate in the best way possible. We tend to get more maintenance issues and shorter mean time between failure for these systems.

Erin: What are some key points our listeners can look for to tell if they're operating below capacity?

Neil: Wayne, I just touched on a little bit of that. If the folks on the plant floor are screaming that they don't have enough compressed air and the compressor is down again, that's typically a key point on whether you're operating the right system.

Other indicators of oversize would be having to go too frequently for shutdown calls or calling somebody in for those breakdown issues. Of course, over temperature is also an indicator. Too frequent cycling – that loading and unloading of your compressor. We typically say you're looking at 30 seconds on, 30 seconds off, that's a 50% duty cycle. That's still a lot of cycles within a five- or 10-minute period, so if you look at the load counter on your load's solenoid valve and you're into the millions, you know, that's a big indicator.

Load hours versus service hours is how you're going to tell if you're below capacity or not. If you have a variable frequency drive or a variable speed drive, that might not be a great indicator. You do have to kind of dial in there to see how that particular compressor or a system is actually operating.

Another key indicator is your specific power. A specific power is a kilowatt per 100 CFM delivered for the system. And if you're not certain on how to calculate that, you know, certainly there are compressed air professionals that can help you figure that out as well.

Kaeser Compressors provides products, services and complete systems for supplying production and work processes with quality compressed air. Learn more about Kaeser Compressors' products and services on their website.

Wayne: Neil, you touched on high temperature. But the other issue if your system is oversized is you may not be operating your compressors long enough to get them up to operating temperature. And if you're not, actually, well-flooded rotary screw compressors, you're gonna wind up with temperatures that are too cold, and you're gonna get water condensing in the oil separator system. It's gonna shorten the life of your oil or shorten the life of your barrel.

And in oil-free systems—which is a lot of food processing—you must look at what's going on with the temperature. Because when those machines unload, you don't really have air going through the air ends. Air going through the air ends stabilizes the temperature. If you've been running a compressor loaded and then suddenly you unload the compressor, the temperature in the air is going to go up for a while before it goes back down. You have to be careful with how you're handling oversized systems to keep the maintenance in line and keep the longevity of the machines to what they were designed to do.

Erin: If someone has identified that the system is oversized and they're not using it to its full capacity, what now? What are some of the steps to remedy the situation?

Neil: As a compressed air provider, there's a lot of different things that we can do to help. And one of the paramount ways is to figure out how much air you're really using now. How much are you using, how much do you actually need?

To measure or log, what we call an “air demand analysis,” is a great way to figure out, what you're doing today. You should log data for a longer-term period, i.e., a week, two weeks, etc. Ideally, if you're already dialed into the system, you're getting reporting, maybe it's weekly, maybe it's monthly, you really know what's going on. You know how much air you're using; you know how much power you're pulling into the system. First, you measure it and figure out how much you're using and how much you need.

The easiest thing is if the system is using a lot less air than you're capable, like Wayne said, you put in that fudge factor. I looked at the design and somebody else looked at the design, and everybody had a fudge factor. We see this quite a lot; there's a 30 or 40 CFM system, which is a 10-horsepower compressor, we may have a 50-horsepower compressor in there, or a 100-horsepower compressor in there. And in a lot of cases, things change, maybe the production was downsized, or they changed production equipment. So they're not using as much compressed air as you were before.

The idea would be, can we use a smaller compressor for that same demand? And if that's the case, then great, what's the ROI on that? Or how much are we spending today? And it's not just what we're spending on energy, but it's also what we're spending on maintenance for this large, oversize, maybe unreliable compressor to then look at something that's right-sized.

That's one way we can really look at how it's operating, what it is we need to do.

Wayne: As Neil said before, if you're running 30 seconds loaded and 30 seconds unloaded, that's a 50% duty cycle, and you're cycling way too often. One of the ways, if you've already got the machines in, and you can't afford to buy a smaller compressor, a less expensive approach, even with that 50% duty cycle, what you wanna do is spread that out. Instead of being 30 seconds loaded and 30 seconds unloaded, it'd much better to be 3 minutes loaded and 3 minutes unloaded. That's still a 50% duty cycle. The way you can do that is by putting in storage.

I’ve talked to people who got a 240-gallon tank; and I’ve talked to people who got a 2000-gallon tank. But you can get your service provider to come in and do the calculations on how big an air receiver you would need to lengthen out that cycle time.

And the other thing that affects cycle time is your load and unload pressure set point. If you're loading at say 100 pounds and unloading at 105 pounds, you may wanna spread that out to loading at 100 pounds and unloading at 110 or maybe 115. That puts your compressor to put in more air into the system, you know, at a higher pressure, gives is some time to use that stored energy out of your storage and the piping system so it allows you to be unloaded for a longer length of time. But a good service provider can sit down with you and run the calculations and figure out a way to do that.

The other thing about putting in storage is you never have to rerun the motor every single time because you don't have to change the oil every single time. There's no filters on every single time for a change. So once you get it in, other than a drain on it, you know, it's pretty much maintenance-free in there. And it's a good solution to help you stretch out those duty cycles and keep the machine warm, and also give you some better energy efficiency with those longer duty cycles.

Neil: Wayne hit it on the head. When I first learned more about compressed air, that was one of the first things that I internalized from what Wayne said was, I'll sell you a maintenance contract on a receiver any day. Because like you said, there's really nothing to it but it provides such a big bonus in regard operation for your compressed air system.

Erin: When planning a new plant, or expanding an existing system, do you have any advice on how to plan for future growth without buying too big of a system?

Wayne: Oh, absolutely. We call an optimum system a split system. And a split system is where you have multiple smaller machines that you can bring on in steps instead of one big machine. I always try my best to discourage people from buying one big machine to serve their plant. That's a very inefficient way to do it Because if that machine runs at part load, it's a big machine running inefficiently. If you've got, say three or four machines. You divide by four machines, each 25% of your anticipated demand instead of one big machine. Then you've got some of the machines running fully loaded, which, if it's making compressed air, that's the most efficient design running point is fully loaded, the other one is off. You may have two of them running fully loaded, one of them off, and one of them trimming. You've got this inefficient part-load operation on a much smaller machine, you're wasting much less electricity because it's a 25% of your capacity machine that's loading and unloading.

Always split this up and put in multiple small machines instead of one big machine.

The other thing I tell people is if you anticipate any growth, put in your biggest pipe you can right off the bat. Put in enough pipe size to carry what you think you're going to need in 10 years. Go ahead and put that pipe in. It's just the cost of the material. The cost of the labor, you're going to be paying anyway to put in the pipe so you might as well go ahead and upsize the pipe. That larger pipe provides additional storage, just like a receiver tank, lengthens out your load cycles, improves the efficiency of the operation, also greatly reduces, you know, pressure drop by putting in a big pipe. And it also decreases turbulence in the pipe so that if you ever get any water or other contaminants in the pipe, chances are they're gonna stay in the bottom of the pipe because you don't have this turbulence slowing them all around the pipe. That's my recommendations to anybody who's designing a system is just look at using multiple smaller machines instead of one big one to meet your compressed air needs.

Neil: Yeah, absolutely. I think what we've seen over time is that even systems that split; where's that split? If the specification calls for 2000 CFM, is it better to have two, 1000 CFM compressors and a third one as a backup? Or do we wanna split that, say two, 500 CFM compressors and maybe one a little bit larger compressor, maybe 1000 CFM? Those are the things that you really need to think about because what ends up happening in specified jobs as we've seen is that we're always oversized.

We’ve seen it so many times where customers had the opportunity to purchase trim compressors, and said, "You know what? We're just gonna go for those baseline compressors first." And those baseline compressors are 1000, 1500 CFM, and they bought three of them. Well, you only need one. So I would have been better off buying two smaller machines to get that project done.

This particular case in point that I'm talking about, we ended up putting in a 50 horsepower compressor for the overnight because they had some constant processes, a little bit of leakage. And the ROI for a 50-horsepower compressor was like six months. That's how much they were paying for an oversized compressor, let alone that was just energy, it had nothing to do with the maintenance costs, the larger machine, or the downtime they were experiencing because of the machine not being able to maintain temperature and frequent shutdowns.

That's a big thing that we see on our end. And it is something that we want to make sure that folks out there know it's definitely a possibility. Look at those things, try to figure out what your duty cycles might be of your equipment, which your off-shift might be, variable frequency drive, or variable speed drive machines, great trim compressor. You do have to worry about, you know, where you size those medium-load units so you don't have any kind of control gaps. But you know, that's really a great opportunity. So, you know, I could talk all day about system sizing, but I think we covered kind of the bases there.

Wayne: A couple of other points I'd like to make. You talked about variable speed machines. And with variable speed machines, if you grossly oversize the variable speed machine where, you know, running down, you know, below 50% much of the time, then again, you have the same concerns you would have with a load-unload machine. You're not gonna build enough heat in there to keep the thing at the proper operating temperature. And that's real critical with rotary-screw compressors. I mean, if you're talking about a piston compressor, 30% duty cycle is wonderful because it gets to sit there and run and cool 70% of the time, and just builds heat 30% of its time. Rotary screws are totally different. They're really designed to run 24/7 at 100% capacity, that's where they're happy. That's where all the temperatures even out. It's running at its designed temperature, it's keeping the water out of the system, and you're not hammering the valves.

Another thing that you're not doing is if you're loading and unloading rotary screw compressors, or centrifugal compressors if you're in that range, then you've got coolers to be concerned about, and if you hit those with temperature, because you're underloaded, and then you suddenly unload and you're not putting any air through it to be cool, but you're still blowing cooling air across those coolers, they're going through thermal cycling. And that expands and contracts the metal of the cooler and can cause premature cooler failures.

In addition to, you've got valves that will fail early because they're being hammered a lot of times. I've gone into some places where the machine is a year old, and you look at the cycle count, on a lot of the controllers you could go up and scroll through the menu and figure out how many times it's loaded and unloaded, and I've seen 1 million and 2 million cycles in a year of operation. I mean, those valves are going back and forth 1 million or 2 million times in that year. They're certainly gonna wear out a lot quicker than if you had, you know, some of your machines running flat out only one small one trimming, it is not gonna trim as much as a big one would. Oversizing just kills me.

I was giving a presentation to a group of engineers, an engineering company. And I said, "Okay, you guys do design-build, right?" They all said “Yeah." And I said, "Okay, what happens is, you're gonna assign one of your junior engineers the project of going through and counting all of the areas in the facility and figuring out how much compressed air this uses." So he's gonna look at all these tools and their duty cycles, he's gonna figure out an average compressor load, and then he's going to say, “Okay, maybe I better go above the average.” And if this junior engineer figures out what he thinks it is, and then goes, "Boy, if I'm wrong, I'm in big trouble. You know, if this plant doesn't have enough compressed air, they're gonna come back on us, and I'm gonna be in big trouble."

So he had the fudge factor, maybe 20% or 30%. Then he goes and takes it to his boss. And his boss looks at it, pats him on the back, "Great job, you've done a great job." And he looks at this and goes, "Boy, if this guy's wrong, I'm in big trouble." I don't want the client to call us back and say, "We don't have enough compressor." So he adds the fudge factor.

Then I turned to the audience I was talking to, I said, "Okay, who's the junior engineer?" This one guy raised his hands. And "I've been asked to do all that." I said, "Who's the senior engineer that had zero fudge factor?" Raise your hand. I got a laugh from everybody because it was exactly the way they design that and they'd come up with a more than adequate compressed air system through a screen field operation. The engineering company doesn't have to pay the power bill, they don't have to pay the maintenance bill, and they're leaving the customer with, you know, an inefficient system. Customers no longer run out of air, they've got plenty of air.

And my point to this engineering company was split this up, put a master controller in there, go ahead and be safe, put as many compressors in there as you want, but have a master controller decide which of these compressors are gonna be running as baseload machines, which are gonna be trim machines, how are they gonna do this efficiently from an energy standpoint and from a maintenance standpoint. But that's a tough sell to a lot of people. Putting in multiple machines generally is gonna be a little more expensive than putting in one or two big machines just initially. It's gonna be much less expensive in the long run, but initially, it's gonna cost a little more money.

Erin: Wayne and Neil, you definitely gave both myself and the rest of our audience a lot to think about and consider. So I want to thank you both for joining me today on this special bonus episode of the "Food For Thought" podcast.

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