Werner Rauer is Kaeser Compressors' product manager for rotary screw compressors. He has a degree in mechanical engineering and has been with Kaeser USA for more than 30 years. Rauer is a frequent speaker and writer on compressed air system technology and system improvement, and currently chairs the rotary positive engineering committee for the Compressed Air & Gas Institute.
Neil Mehltretter is a Certified Energy Manager (CEM®). As Kaeser’s Engineering Manager he has conducted and overseen thousands of industrial compressed air studies and helped users achieve significant energy savings and operational improvements. Mehltretter is AIRMaster+ certified, has completed the Compressed Air Challenge curriculum, and is a Master Certified System Specialist through Kaeser’s Factory Training Program.
During the live Q&A portion of the webinar “Compressed Air: Applying Compressor Efficiency Ratings to Real World System Design” (now available on demand) Rauer and Mehltretter tackled several attendee questions related to compressed air system design and efficiency metrics.
PS: Neil, earlier in your presentation, you discussed the idea that by implementing multiple compressors in the compressed air system, you might be able to achieve lower maintenance costs overall than with a single compressor. Could you speak a little bit more about how you can achieve it?
NM: If you have a single compressor that is running the entire time, maybe 8,760 hours per year, you will probably have more than one maintenance interval, and you will probably have two required service visits. Now let's talk a little bit about what that duty cycle is. Let's say that the duty cycle on that large machine is 50% – call it 1,000 CFM capacity – so your demand is only 500 CFM. Regardless of what the demand is, the service hours are what you're basing your service visits on; if you're hitting 8,760 hours a year, then you have to get service performed twice typically.
Let's talk specifically about the multi-unit fix, the 50%. If you have two machines installed, your average demand is only 500 CFM. So guess what? You really have "100% duty cycle" on one of two machines. Now you've split the overall service hours in two, and that's where you're getting the big benefit in regard to maintenance intervals.
PS: When does it make sense to use a flow controller? In general, is there a rule of thumb where you'd recommend that a plant or an organization adopt a flow controller method?
NM: The biggest bang for your buck with the flow controller is dropping the artificial demand of the plant. Most facilities are going to say, "You know, my drop dead pressure is 95 lb," but it's going to depend on who you talk to. If you talk to the plant manager, he might say 115 lb. If you talk to the maintenance supervisor, he might say 105 lb. But if you talk to the guy that's using the equipment, and he's going to say 95 lb, and he also will say, "When I hit 95 lbs, I can't make this particular product at all. My scrap rate increases by 15%." It all depends on who you talk to, and what the comfort level is with the operators.
In general, the flow controller is going to be effective in any installation in regard to dropping the overall system pressure. But the question is, "What's the generation pressure?" because that's what the customer is paying for, so there's a balance. If you have to run your compressors at a generation pressure of 115 lb to get 90 lb out of a flow controller, then maybe that's not the best application for it. When I'm looking at an application of a flow controller, I'm looking at what can I do because I have a very high intermittent demand event that I have to overcome.
What we typically do is increase the storage volume, creating that control buffer with the storage volume and the flow controller so that when this demand event occurs, it's all hands on deck. You have the buffer from storage dwindling down very quickly to that set pressure of the flow controller, and the end use doesn't see any difference. The end user says, "Oh, it's 90 lb. Everything's fantastic." But meanwhile, the pressure at the tank is going from 105 lb down to 95 lb really quickly, and maybe you need to start loading another compressor to feed that tank to fill that void.
The best possible scenario would be that the storage volume handles everything, and you don't need to start another compressor. You're not operating at the highest possible generation pressure, and you're not starting another compressor. This means that you have two parts of the savings, demand and overall generation.