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Improving performance, reliability and efficiency in multi-compressor air systems

Nov. 1, 2022
In this episode of the Plant Services Compressed Air Podcast series, Neil Mehltretter and Werner Rauer from Kaeser Compressors explore how multi-unit controls are impacting maintenance and reliability manager.

Neil Mehltretter is Technical Director for Kaeser Compressors, and he has conducted and supervised thousands of industrial compressed air studies to help plant teams achieve significant energy savings and operational improvements. Werner Rauer is the Product Manager for Oil Injected and Oil Free Screw Compressors at Kaeser Compressors, and he's an active leader in the Compressed Air and Gas Institute, where he helped develop the widely used CAGI compressor data sheets. In this episode of the Compressed Air Podcast series, Neil and Werner explore control options for systems with multiple compressors.

PS: Here we are at the end of the second series together on compressed air systems, and again, today's topic is going to be multi-unit controls. Neil let’s jump right in and start with you. What would you say are the reasons or driving factors for adding separate controls in the system with multiple compressors?

NM: Typically pressure fluctuation is the primary function, you see it in the plant and it trickles back to the compressor room. In those general situations, production equipment probably isn't working well or working right, and so if you add a master controller with a single pressure signal for everything, that's going to ensure pressure stability.

Typically what happens is you have multiple compressors, those individual compressors are fighting each other, maybe they're coming on together or coming on in (quick) succession, and that really does not only waste a lot of energy, but it causes premature wear and tear (on the compressors), and your run time between failure is going to decrease. So, excess starting and stopping and idling, all of these things affect overall system performance.

Other reasons to add a master controller or a separate control would be addressing potential control gaps, or really what most people would say: the wrong compressor at the wrong time. So instead of having the 100 hp operate for the 400 CFM demand, you have a 500 hp compressor running for that same demand. If you have a different constellation of compressed air equipment, it makes it a lot easier if you have one master orchestrating what's happening in the facility. You also have that need to balance runtime among same-size compressors for scheduling service.

Those are really kind of the impetuses that we see in the industry, and I think Werner probably wants to go in a little bit more detail.

WR: Thanks, Neil. You mentioned the pressure, how important it is. A single separate control basically also adds its own system pressure signal, usually the transducer, and that's what it's using to orchestrate all of these compressors rather than relying on these multiple individual signals and logic, so we can just go up to one device and make those changes and improvements to your system.

Here's the reality where you have a bunch of compressors of different vintage, and you're asking yourself, how can I make them play nicely together and possibly get more information about how my system actually works all day and all week? Maybe it would be helpful to have a babysitter for my system, and that would be the one spot to tell me what energy I put into my system and how much do I get out of it that is useful for my plant. Or, what seems to be going on every time we have a shift change, lunch break? Or for example, last Saturday afternoon when nobody was supposed to be working here and we had a low pressure system alarm?

So yes, you can get all of this with one modern system controller that is able to connect all of your system components, not just compressors, and step into this age of Industry 4.0.

PS: OK, I liked your babysitter analogy, and I'm guessing just like there's many kinds of babysitters, Werner are there other different kinds of system controllers to look into?

WR: Oh, God, yes, there are many different types and providers of system controllers. And remember, I'm 36 years in the industry, so I'm going back to, let's say, in the 1980s, we were building very simple styles. We used selector switches, some pressure switches, gauges and timers. Later on it moved into PLC-based types with a single pressure sensor with some kind of a control scheme that typically was a cascade control. And then all the way up to nowadays, where we have the latest computer-based versions of these controllers.

The sophistication goes from manual operation to pressure and timer-based solutions to cascade pressure controls as I mentioned, and nowadays all the way where we have very sophisticated control algorithms using communication protocols and technologies that are available today (not in the ’80s) that also offer remote monitoring or control via the Internet or Internet Intranet connection. The range of support for those devices from the homemade PLC or the other software and then all the way to a complete package solution either from a compressor manufacturer or also third party providers. Some of those have superior support for future expansion and the software and operational changes that you will experience. Others have been abandoned, they no longer have any support for both hardware and software, so buyer beware.

Another type is a networking type where the compressor manufacturer dedicates one compressor to be the master controller, along with sometimes proprietary software. So be mindful of what you choose for the long-term growth of your company. A separate controller allows for more flexibility, easier upgrades and integration of a variety of different compressor types or models or brands.

NM: Tom, you know, as an ’80s kid, I I've got to get a shout out to Adventures In Babysitting. It really reminds me of that. You know, who's going to show up in the driveway? So great analogy there, Werner and Tom.

We get some very specific requests from customers, and not only do they want a master controller to orchestrate what's going on with the compressors, but they also want to handle many other facets of your compressed air system. If you're in the market for a master controller, make sure that you do your research and find one that's going to work for you today, but can also grow with you tomorrow. That's really one of the things that I'd like to stress.

PS: Neil, maybe we can ask you to follow up on that. How do system level controls help achieve better efficiency, maybe that in part matching your system to the right controller.

NM: Yeah, absolutely, I kind of alluded to this before, using the wrong machine at the wrong time. But what about using the right machine at the right time? If you've heard me talk about master controllers before, you've heard me say many systems grow over time, and maybe that's a euphemism for, “yeah, we started out with a 20 (hp), and now we have a 50 (hp), and then we needed a 200 (hp).”

You know, these things happen and it's easy to forget or not even know which compressor is best to run at any given time. In a lot of cases you know, if you're the maintenance supervisor or you're in in charge of the compressors, you just want to make sure that they're on and ready to go, right, because that's the most important thing. You have to have pressure, which means you have to have flow. But do you know which one should be run at which time? Why not let that master controller choose which is the best compressor for that current demand?

Because demands change! You know, we see thousands of air audits a year, and a lot of the majority of them vary from shift to shift, weekend to weekday, peak / off-peak. Some of them are automated and they're pretty much stable all the time, but this load optimization that's available from master controllers nowadays is fantastic. They're going to also reduce idling and runtime, so not only is it going to choose the right units to run when, but it's also going to make sure that they idle out and shut down. You're going to hear us talk a lot or if you listen to any other industry leads about key performance indexes, we call them KPI's, it's a fun term for compressed air. One of the big ones is specific power, so kilowatt per 100 CFM, it's just like miles per gallon for your car. And you also have that cost of compressed air, which is dollars per thousand CFM. If I was a production manager or plant manager or you know owner of a company, these are values that I'd want to know.

A master controller is going to trend these values over time and then allow you to look and see where your efficiency gains can be made. So not only is a master controller going to strive for the most efficient, it can give the user trend data to confirm that.

WR: These programmable modern system controllers, they have the ability to know the compressor in great detail. As a minimum: the performance at full load, idle, or whatever speed if it's a variable frequency drive and at whatever pressure the compressor is operating as directed by this master controller, and then be able to calculate the specific power that Neil mentioned to keep the energy cost at a minimum but still provide what is needed, but at the lowest energy and maintenance cost. I know I repeat myself: the key is to know when to turn a compressor off, when it is not needed, and selecting the right compressor that is needed at that particular moment in time. I compare this ability very often to a chess computer. It knows each chess piece and the capabilities individually. It looks at the current situation, and then it calculates and chooses the best move forward at every step of the game.

PS: Interesting analogy with chess then too, and for a lot of our primary audience, which is maintenance and reliability folks, asset managers, I think they are playing this constant chess game with not only balancing energy efficiency against reliability, but trying to figure out which of the most critical machines to keep running so that machines that might happen to fail don’t take the majority of the production line down. Werner, could you talk a little bit about how multi-unit controls factor into the job of the maintenance and reliability manager and technician?

WR: Sure, reliability is always a little bit of a difficult one, but with those master controllers that have networked compressors, not just compressors, also dryers and filters, etc., where you can check differential pressure for maintenance, there is a single point where you get all the relevant maintenance reminders as well as alarms or warning messages at a single point. Those messages, they can either be displayed locally and/or emailed or messaged to whoever is in charge of that specific message. So different messages go to different people, maintenance versus catastrophic event. Warnings allow time for maintenance scheduling, while the alarms typically require immediate action or planning for a shutdown or repair in the near future.

These controllers would also allow for continued messages, but as well send actual performance data (like I mentioned the Delta-P on filters etc. or rising temperatures) into the cloud where there could be a service provider that uses appropriate algorithms to look at the data for that specific component and then use that to actually prevent such messages to appear, meaning the catastrophic ones, by projecting potential maintenance or even failures for the future. That in my book certainly accounts in the reliability department.

Listen to the entire interview

One more thing about the actual controlling part that can provide: pressure stability is something I would put under reliability as well. Bring on backup units automatically. That's reliability. No human interaction. Reduce cycling and wear by selecting the right compressors for the ongoing air demand that would be about maintenance.

NM: One thing I'd like to hammer home and I think we've already talked about it is maintenance is typically the one thing that we put off, whether it's for the compressed air system or you know other utilities within the plant that whether it's because of added costs or having to shut the equipment down which could stop or slow production, that's the biggest thing that we have to worry about inside a facility. Many customers choose to run to fail, or they don't choose that but that's what happens. It's not something we'd suggest, but that's the reality. Having a master controller act like a Big Brother or a parent constantly reminding the user what's going on – maybe it's somewhat annoying, but when used appropriately, it can really be a game changer. So, if you plan to do maintenance, you're planning to have a reliable system and that master controller, it can be the conduit that you need to kind of break through.

PS: Werner, let me take a deeper dive into some of the technology specifics of compressors, specifically variable speed drive compressors. These are becoming a lot more common these days. What would you say are some of the considerations for integrating one or more of these VFD units into a system?

WR: That's a great question, and just right down my alley since I love VFD machines. A lot of people have been truly convinced by all the talk over all these years that variable speed anything, including compressors, is the absolute best. Others again are reluctant due to concerns about extra cost, complexity and questionable reliability in their minds. So the bottom line is that it should be the actual system requirements that should be driving you to the ultimate solution, maybe with or without, or even multiple variable speed.

So nowadays those drives are very compact and sophisticated, and they can easily be integrated into systems with a modern master controller by networking, all digital. Gone are the days where we had to have analog-to-digital converters and high/low handshake signals. Today: all digital and very precise. The system can be completely controlled by one master, and that includes even different size VFDs, not just more than one but also different size as long as that controller is fully aware of the capabilities of each of those compressors.

So here are two of the main reasons to go with either one or more VFDs. Number one: very stable or consistent system pressure that is mostly required by the application or processes in the plant and that would mean without any elevated pressure like we use on flow controllers. They just want the minimum pressure and done by variable speed. Multiple VFD compressors are used if a rather large compressed air demand range is required and it varies so much that just one large VFD will either not be able to cover that range or there's a need for backup compressor which in some cases is also VFD so that would make it too small a machine.

We have a number of systems with Kaeser here that are running three or even four VFD compressors at a time, but the system controller is running only the number that is required to fulfill the current demand, and it also considers the varying efficiency of each one of those compressors over their entire control range. Sometimes we call it also the digital twin, which is rebuilt inside the controller. So one of our recent system that has three 1,000 CFM VFD compressors, whereas the demand can be anywhere from 300 to more than 2,200 CFM for not just the blip of a second, but for a sustained time period. You have to cover them all with actual compressors in the most efficient way. The master controller must be able to operate just one by itself and regulate two or three in parallel and always running them at the optimal specific performance. If they're the same type or the same machine they would run pretty much at the same speed when in parallel mode. If that is either too high or too much, or in particularly too low in terms of flow, it will either turn another one on or off to get back into the sweet spot of each of those machines. So please consider those applications. Those are rare.

Another scenario that happens sometimes is that you have a VFD trim solution for high demand production times and then you have a smaller one for second or third shift in sometimes even the weekend where just the small machine is running, you don't want to run the 300 hp VFD for a 30 hp weekend. So last but not least, please be mindful that VFD compressors have higher running hours, which means service hours, that could lead to higher maintenance costs, lifecycle costs in particular if the compressor is oversized. We have a whole webinar just on that topic, turning a unit off when not needed or outside its best performance is of course a major benefit of such a master controller.

PS: Sometimes a maintenance manager has to have nerves of steel to make sure you don't oversize because they want to make sure that you do have the capacity you need to keep the lines going.

NM: Yeah, that's a great point, Tom, and I think that dovetails into the point I'd like to make is, if you want to know if your system will benefit from a variable frequency driven compressors is to do an air analysis and that's going to help you avoid that oversizing. If you have flow and pressure data, boom, you know you can kind of figure out, hey, will my system benefit from a VFD? what size VFD do I need? And you know another plug for that master controller or system controller: most of these devices out there can actually provide you with a data set that you can look at and make that determination.

PS: Neil, you mentioned a couple of key data points and often times maintenance managers can be awash in various kinds of machine health data from temperature and pressure up to vibration of rotating machines. When it comes to compressors and these multi-unit control systems, what kind of information do you see these controls providing that's useful to the maintenance and reliability folks?

NM: Yeah, it’s a great question. I think you pointed out some of the things that as a manufacturer we would certainly like to see. The big thing is compressor health, right? Are there any alarms? Are there any messages or warnings that's going on with the units and that would then trigger some kind of action. But in general, like I just mentioned, you know that continuous supply side audit – great set of information so you can see, OK, well, you know I have 1, 2, 3, 5 (etc.) different units. How are they actually operating, what's that flow profile?

These master controllers will also calculate your energy usage, so then you can see how efficient you are, so that key performance index kilowatt per 100 CFM trend that over time. The baselining existing conditions is a must. Once a master controller goes in, you can see OK where are we at? The interesting thing is most of these you can just flip a switch and turn it back off and you can really see how the system was performing before.

One of the things that we do with every system that we put in is ensure that when the master controller is off, you can revert back to as efficient a system as possible without that master, so that you don't have the same thing we talked about before, where the compressors are fighting each other. In general, it helps to validate any upgrades that you do for management review. So for me, that's pretty key.

Many masters out there can provide you with a simple table for energy efficiency, so you know that could be days, weeks, months or even years. You know, for me as a data nerd, it's pretty great. You know, I just plug in a couple days. How did I do last week? OK, the system’s efficient from a load optimizations standpoint, very little idle time, specific power of the station is pretty good, the cost of dollars per thousand CFM is within a range that we would expect.

That data can also provide you with utility incentives if you have to do that, or if you're company is ISO 50001. You know, these master controllers should be able to give you that information as well. There's a lot of data out there, and from a system design standpoint and a system efficiency standpoint, you're going to get that. But you know, as you pointed out, temperatures of the equipment, temperatures in the room, those things are pretty important as well. And that trend data you know is second to none.

WR: What type of information? In my mind, there's virtually no limit of what can be provided as long as your system has the sensors or the controls that are connected, and they are able to pump that data to the master controller and/or into the cloud for ongoing analysis or for future use, that's it. It's more about, what do you really need or even want to know for the purpose of running your business, not just your compressor system but your business. Like, how your compressed air usage syncs up with your production and so on. Nowadays it's about conservation of energy and other resources and running a reliable system to achieve sustainability goals.

PS: Well, maybe we can close on a higher level question too, Werner. In your opinion, what is the biggest benefit of multi-unit controls from the customer’s point of view?

WR: We ask many customers this question. There are the obvious energy cost benefits, remote alarms and messaging, management reports and other functions. But the most important thing is that people don't want to babysit compressors all day long in order to have a reliable compressed air supply and focus on what they do best, which is their business. Too many people are still forced to physically go to the compressor room to turn machines on, or to look up those messages and then see what's going on at the time, and then act on that. Having just the one box that turns machines on and off and maintains plant pressure is the biggest benefit to many plant operators: set it and forget it. Just like a utility, like your power and water, it's just there and no sleepless nights or worries.

For the future, there will be more and more sensors, meaning the price is coming down and then they will be added either within the compressor, dryers, filters, or even adding to your system additional ones. For example, let's mention compressed air quality sensors. We just went to that conference and there was a nice display of, for example simple pressure dewpoint is basic, that has been around for years, but the total air quality system monitor for compressed applications that justify the cost and provide the required documentation of the proper and reliable compressed air supply, like in electronics, pharmaceutical, food and beverage applications. The machinery will provide more and more measured data that will be collected and analyzed to predict a future event. This is going on, also, new sensors are being integrated and they will reduce or eliminate even catastrophic failures that affect your actual business and your bottom line. All of this with just one box, no matter how small or large your business is or your system is.

NM: Let's face it, we're glued to technology whether you have one phone, two phones, whatever it might be. I was joking with Werner, we were both working on the presentation today, the podcast,  and we were watching TV at my house, and the kids were doing their homework, and I was working on this, and we're watching The Voice. So, you know, perhaps it's a crutch. You know, we're all in the same room, we’ve got to interact. But, you know, think about this. There's a lot of data that you want and you need for your business and that can be provided from a master controller pushing it to your office computer or cell phone.

You've got a busy day, you've got a lot of things ahead of you, you want to know what's happening in the compressor room, but you don't have the time necessarily to go look at it. You can get reports. You can get alarms. It's a lot of things that you can get. Or, like Werner said, this data can be pushed to a third party service provider. There's a lot of powerful stuff that's out there and you know we're really just scratching the surface of what a master controller can do.

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