Get up to speed on compressed air storage and master controllers

Four questions from the recent Plant Services webinar "Master Controllers: Your Compressed Air System's Watchdog."

Before retiring in 2016, Ron Marshall was the industrial compressed air systems expert at Manitoba Hydro, where he worked for 38 years. His efforts supported the organization's Power Smart Performance Optimization Program, and he now operates his own compressed air energy efficiency consulting firm. Neil Mehltretter oversees Kaeser Compressors' System Design and Engineering group, which has conducted thousands of industrial compressed air studies and helped users achieve significant energy savings and operational improvements.

During the live Q&A portion of the on-demand webinar "Master Controllers: Your Compressed Air System's Watchdog," Ron and Neil tackled several attendee questions related to hybrid permanent magnet motor technology. 

PS: How many compressors can a master controller handle?

NM: If you have more than three or four compressors in your compressed air system, you've done something right in regard to system design. Typically, you can have 10, 12, 14, or 16 compressors. There are probably controllers out there that are modular and could do more, but 16 is probably a good number.

You also have to look at what your overall flow profile is, the types of controls, and how it's going to interact with the master, as well as whether it's a centralized compressor station or decentralized. The number of compressors will help you decide which master controller to use.

PS: Can a master controller help with leaks?

NM: It certainly can in two ways. Most master controllers that are out there are going to have some kind of data storage or data analysis function, and that will help you build the system curve, but it also gives you an opportunity to really dial in and see what's happening on a daily, weekly, or monthly basis for your compressed air system.

If your selected controller has a data storage function, then you should definitely use it, because you're going to be able to identify off-peak times – specifically holidays, weekends, and off-shift – and that will give you an indication on what the lowest flows are. These data typically represent your leakage artificial demand and nonproductive demands.

PS: How much system storage capacity do you really need?

RM: That's a hard question to answer. Generally, for good compressor control, you need quite large storage, actually. I always tell people that if you're walking into the compressor room with somebody that's never been there before and they spy your tank and their eyes pop out, then it's probably big enough. If it's a tiny little thing, then it's probably not doing its job.

I would recommend – and we're talking about the trim unit here, the biggest unit that's varying to control the flow – you'd want something in the range of 5 to 10 gallons of storage for each CFM. So if the compressor is 400 CFM output, you'd be talking about between 2,000 and 4,000 gallons. Those are big tanks. The tanks don't have any large power cables going to them, but putting them in actually saves you power, so it's important to put proper storage to slow down the cycles of the compressors to allow the compressors to start and stop without experiencing low pressure.

PS: Do you find that storing at a higher pressure and then reducing the pressure at the end user can also contribute to energy efficiency and save energy?

RM: That's a tough one because running at higher pressure uses more energy. When you regulate down, you lose that energy; it's not a benefit. The best situation is to run as low as possible, as close as the end use requires. So if you're generating at a higher pressure, say 10 or 20 psi higher than you actually need to use, then you're consuming more energy to do that because the compressor consumes 1% more energy for each 2 psi in added pressure at the discharge.

Now, there are some situations where you need to run at higher pressure, and that's where storage comes into play. A storage receiver needs to have a buffer that needs to have some extra pressure in it to be able to store air, and that air might be used in a peaky load – say a transport system or something like that that would come on, use a lot of air for a short period of time, and then go off. If you stored at high pressure, you can use that stored air to supply that peaky load without starting another air compressor. Then that would save you the demand charges on your power bill because often a power bill is not only kilowatt hours but also it will be the rate at which you're using the power.

For some of my customers in Manitoba, 50% of their bill is heat demand. So if you can keep a compressor off by using storage then that would save you demand charges. The drawback is it consumes a little bit more energy, so it's a trade-off, and you have to do the business case. This is something a compressed air auditor could calculate for you, whether it is better to operate at a higher pressure to feed those large demands or to operate at lower pressure with lower kilowatt hours.

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