Who are the physics police? And can they save compressor energy?

Read this Q&A to get very specific answers to some compressed air questions

By Plant Engineering February 7, 2024
Courtesy: CFE Media and Technology

Compressor insights

  • Air leaks in compressed air systems can lead to increased cycling of equipment, causing air compressors to run longer and requiring additional maintenance.
  • To learn more about energy efficiency in compressed air systems, a recommended resource is the educational webcast “Energy efficiency: Focus on compressed air systems,” along with a Q&A session featuring insights from Tom Taranto and Eric Bessey on topics such as optimizing loading and unloading of compressors, conducting field checks for system effectiveness, the cost-effectiveness of acoustic imaging compared to ultrasonic imaging, the role of accumulators in maintaining optimal pressure and considerations for selecting the most energy-efficient compressor type based on specific applications.

Air leaks cause equipment to cycle more frequently, thereby increasing the running time of the air compressors, which leads to additional maintenance requirements and possible increased unscheduled downtime.

Watch the educational webcast “Energy efficiency: Focus on compressed air systems” and then read this transcript of the Q&A for additional details. This has been edited for length and clarity.

Answering this compressor Q&A:

  • Tom Taranto, Principal Engineer and Owner, Data Power Services LLC, Baldwinsville, New York
  • Eric Bessey, President, TTed Solutions, Beaverton, Oregon

What’s the best method of loading and unloading units running in parallel to help conserve energy? 

Eric Bessey: The best way to do it is to not do it simultaneously. It’s typically inefficient to run more than one compressor at load-on-load manner at one time. The rule of thumb is try to baseload one of those compressors and then as efficiently as possible, load-unload the one compressor.

Then you might find that if you are loading and unloading two compressors, you might be able to get by with shutting one of them off and then the other one can just stay loaded longer. That often happens, but that won’t happen unless you have a management system of some sort. That’s the best way.

What field checks can be done to verify the system is operating effectively? Then what spot checks can you do to find potential issues?

Tom Taranto: As far as tracking operating efficiency overall, if you have a method of tying some information into your building management system, you could tie into questions like: How many compressors are running? When are compressors stopped? You bring in the pressure and see what is happening to the pressure, as the plant’s going through normal things. Then you look for those things that we’ve been talking about.

Are we applying excessively high pressure? Do we have too many compressors running versus the amount of air demand that they’re supplying? That is a good way to track and trend things. Then as you uncover potential issues, you say, “We have got a big air gulper over here and it takes a gob of air out of the system, drops the pressure, the compressor starts up. By the time the compressor starts, the event is all over, yet I’ve now got another compressor on our line running.”

It takes time for it to realize it’s not needed and get shut off. You start to look at what is going on in the system that you’re picking up the change in your trend information. Then you develop a targeted measurement plan to understand what’s going on and you can figure out how to make the system do a better job. It very might well be put storage at the big air gulper. That would be the process of looking at those things.

How expensive is acoustic imaging compared to ultrasonic or ultrasound imaging? 

Eric Bessey: Just like all things when they come out and they’re new and shiny and considered the best or better, they’re expensive. But at the end of the day, over time eventually the market takes over and things settle down. But typically speaking, the ultrasonic leak detector is less expensive than the acoustic. The acoustic uses more microphones, does a higher precision analysis with the acoustics levels themselves.

Then it splits them out into the frequencies and ultimately gives you sort of looks like thermal imaging on the screen. All that stuff adds up to money. Ultrasonic leak detectors can be had for reasonable money nowadays.

The thing that I don’t like about them specifically though, is that they don’t give you a good, pinpoint location of that leak. It gives you a good ballpark, but to find the hole on the putting green, so to speak, the acoustic imaging does do a better job at that.

What if you have an accumulator, isn’t it better to maintain the highest pressure possible? 

Tom Taranto: Let’s use this example with accumulator tanks: here’s one on the wet side of the dryer, one on the dry side of the dryer and then the pressure flow control. To have the most amount of pneumatic energy stored in the air receiver, you want the receiver to be at high a pressure as possible. If you have a pressure flow control to reduce the system pressure to the appropriate target, you’re not raising and creating artificial demand in the system by increasing the receiver pressure.

But you might also think about this as a revolving account. You’re going to put money into the bank account here. Let’s say that you could put $100,000 in your bank account in the case there’s some sort of emergency, but let’s say the biggest emergency that you ever have is a $10,000 emergency. Do you want to have $90,000 sitting in the bank account getting a pittance of interest rather than being invested? You’re losing money. You’re banking more money than you really need to have in the bank.

Well, the same thing can happen with the receiver. Let’s say your air compressor will go up to 150 psi and you put the receiver all the way up to 150 psi. Well, we know that the power or the consumption of the compressor has gone up because we’ve increased that pressure. But if during our biggest demand of that, when the pressure flow control valve opens, we drop down to within to 95 psi where our target pressure’s 90. Then guess what? We needed to have all that air in storage.

But if you see that your receiver’s pumped up to 150 psi, but it never drops down below 130 psi and your target for the system is 90 psi, then you’re storing more air than you need. You’re paying extra money for the power or the compressor of doing that. The process is to determine what is the lowest optimum pressure for the demand side, throttle it to that and then monitor your use of the stored air upstream. Have it come down in pressure and get just down within a few psi of the target pressure. Then you’ve got your system optimized both in terms of supply and demand.

Which compressor is the most energy efficient? 

Eric Bessey: Selecting which compressor type is a really loaded question. It depends on how you’re going to use a compressor. Variable speed drive (VSD) compressors are very good at part load. Baseload compressors can be all over the map. The best full-load compressor is the old reciprocating compressors, double-action reciprocating compressor water-cooled, but they’re just not around anymore as much.

It really varies. If you’re going to have a very large air demand, a centrifugal compressor multistage can make a very good baseload machine and then get yourself a bank of very efficient heart load compressors. They might be variable speed or variable displacement compressors.

Tom Taranto: That’s a really loaded question. It depends on the application of the compressor. Like axial flow machines, if you’ve got need for 10,000 hp air compressor, axial flow machines can be designed for that specific performance and be very efficient at that performance point. The same way with centrifuges. Centrifuges can be very efficient, if they never go into blow off.

But if you’re going into blow off on a centrifugal, you’ve spent all that energy and cost in terms of the power to drive it and now you’re just blowing it off the atmosphere. There’s more to the total efficiency application in a system than just the compressor efficiency. Regardless of the compressor type, if you do the thermodynamics, most of the energy consumed by a compressor is to overcome heat of compression.

The Physics Police — they’re nasty guys. You can’t break the laws of physics, just remember that.