Costs of compressed air quality

Neil Mehltretter

NM: Justin gave us some great how-to, and then we talked really about the standards on how to get to what is really required. But most customers ask, “What's the application? Is this a food application, you know, incidental contact, are we packaging something?” Or it could also be, let's say, a paint application, where you're very concerned with condensate water, oil, etc., that's getting in the line, and that's how you decide which type of air treatment you need.

Do you need a dryer? What kind of dryer is that? Is it a refrigerated dryer, which is nominally 38, 40-degree pressure dew point? Or is it a desiccant dryer, which would be, you know, zero-degree Fahrenheit pressure dew point, or lower? Most folks run at minus 40-degree pressure dew point with those desiccant dryers, and then you'll need the filtration associated with it. So, you know, it really runs the gamut on what's required for each application, and that's the great part about being in this business, is even when you do talk to the same customers, you don't see the same things.

PS: Justin, sounds like it's possible to have too little air treatments in your system, so let me ask about the possibility of too much air treatment. Can you have too many dryers or filters? At some point, does extra treatment do more harm than it does good?

JA: Oh, yeah, definitely. You can have too much air treatment. It would be a problem just for your operating costs and also your initial costs. If you're a general plant, and you only need ISO Class 4 for water content, but you have a desiccant dryer, you're going to have a much higher operating cost, as having a desiccant dryer, you're using your process air to regenerate the desiccant bed, and that process air is very expensive to make. Also, the capital cost of the desiccant dryer is higher. So, you have two facets there, operating cost and your initial cost, which is going to play both into having higher operating costs by having more air treatment and you don't really need that.

It plays into pressure drop, too, if you have too many filters in different places when you actually really don't need that many filters to meet your air requirement, you have a higher pressure drop, and that pressure drop is going to coincide with a higher energy usage in your compressor.

Justin Aycock

There's also an issue with oversizing dryers. If a dryer is oversized, and you don't have the new energy saving methods, you're missing out on opportunities to save a little bit of money. For example, if you have a system that only needs 100 SCFM of air, but you have a dryer that's rated for 200 SCFM after applying all the correction factors, and the dryer is non-cycling, then you're going to be using that dryer flat out – it's just going to keep running.

But, if you had an energy-saving dryer, like a cycling dryer, then it would just cycle on and off, depending on the demand, so then you could potentially save up to 50%, because the dryer is rated for 200 SCFM, but you only need 100 SCFM of air, then that's about 50% of energy savings that you could have.

This would also be the same for sizing desiccant dryers and membrane dryers, and even more so, because they use purge air, which is, again, like I said, it's inefficient and it costs a lot of money to generate. So, sizing your system, sizing your air treatment equipment for the system, will give you good money-saving benefits.

NM: I absolutely agree, Justin had some really fine points in there. It's a hard question to answer, because you do talk to customers who are very conscientious about either meeting a specific pressure dew point, or ensuring they don't have any water in the lines. And we do run into customers who, instead of putting all their refrigerated dryers in parallel, they might put two large dryers, one in series with the other. So like Justin said, if you have a hot gas bypass valve dryer which is running fully loaded all the time, and you don't have any energy-saving potential for that dryer, you're really missing the boat in regard to energy savings. And if you put those two in series, well, then now, you're spending 200% of your energy cost, and really, you could be spending 50% of that, complete, if the dryer has an energy-saving potential.

There's things that we always talk about and try to explain to customers for different applications, but it's also peace of mind, because for them, it may be, “I have a million dollars' worth of product that's out there and I want to make sure that it's going to be clean and dry all the time.” There are different ways that we can go about ensuring that the air treatment is always providing the correct pressure dew point, and if there's any alarms, indications, etc., that we can shut down different lines from redundancies. So, like Justin said, CapEx, you could be spending more money than you need, especially if you've been, for a lack of better term, burned before, where you had a refrigerated dryer in the system, and maybe that refrigerated dryer wasn't sized for summer conditions. All of a sudden, you get water down the line, and a knee jerk reaction to say, "I'm getting rid of that refrigerated dryer, and I'm putting in a desiccant, and I'm going to make sure that I don't have any moisture, and my production's fine." Well, again, with the CapEx, you're spending more money on the desiccant dryer and then operating costs, you're spending more money on that as well.

There are things too, like maybe you have a paint booth, and that's a small portion of what you run. Well, you can have a point-of-use desiccant dryer for that, and then run the rest of the facility on a refrigerated dryer. So, there are different things that we can do for various compressed air systems to save energy, save money, and then still build a system that's reliable for the customer, for their needs.

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At higher pressures, you will also have higher flows, so every use that's out there, if it's unregulated, it's going to use more air if you're running at a higher pressure. So, if I was able to run the entire plant at, say, 90 pounds, I'm going to have a significant savings running at 90 pounds than I would 100, and that's that artificial demand. The interesting thing with artificial demand is that all your tools, they may seem like... let's say you have an impact. My impact runs better at 120 pounds. Well, it will, but it's not going to run as long as it would if you were running at 90 pounds, so the longevity of the tools, you're going to have to replace those pieces of equipment faster – those are some other hidden costs that you might have.

The neat thing that you can do these days is if you have a master controller, which has all your pieces of equipment, all your compressors on really one pressure setting (most people say, "Oh it's a glorified pressure gauge." Well, that's true. Really you can see, "Oh, it's 98 pounds, right now, or 98.3. Oh, it's 97. What's going on?"), is it gives you an opportunity to drop that pressure one psi at a time. What we usually tell operators is once that master controller is in, start dialing that pressure down one or two psi a week, and then see what happens. You know, usually you'll have an operator or somebody say "Oh, well, that's too low." And then, you know, "Oh my gosh. You know what? We were running at 125 pounds before, and now we're running at, you know, 95. We're saving, you know, for every 2 psi reduction, we're saving 1% in energy, and that's pretty huge.”

The other thing, too, is that you will have a pressure drop between the operation, or the generation, of the compressed air and that point of use, and that's an interesting point that I think most people don't think about: How do I make sure, if I'm operating in the compressor room at 95, that I get that out at the point of use? So that's one other thing that we have to take into consideration, and that's that distribution network.

PS: Okay. Well, Justin, if I could target the next question at you, how would compressed air pressure specifically affect air quality?

JA: Compressed air dryers are sized based on pressure and temperature, and the filtration is usually sized based only on pressure. So, you know, if your pressure goes up, the capacity of your air treatment goes up. When plants say they need more pressure and they ramp up their compressor, this leads to the air treatment to have more capacity, so you're going to have insurance that your air treatment is going to work well, because you're increasing the capacity of the equipment. But as Neil said before, whenever you turn up the pressure, you're going to increase the cost. The rule of thumb is every 2 psi increase correlates to about a 1% increase in energy cost, so you do need to have the pressure at the lowest possible amount, just so you can save some money.

Some of your pressure problems could be due to poor distribution, undersized air treatment equipment, or even lack of maintenance. In cases of facilities that are using more compressed air than their air treatment is capable of successfully treating, you're going to have contamination in the form of solid, moisture, or oil, and that can end up at the point of use. One way to avoid this is you can install an air-main charging valve. This valve operates based on maintaining operating pressure on the compressed air generation side of the system, so this will ensure that you're getting your proper air quality, and you'll protect the air treatment equipment. Stable pressure at the point of use is also critical, so adding FRLs to maintain that pressure is also beneficial.

PS: Neil, your thoughts on Justin's response?

NM: Yeah, you know, Justin's points were fantastic. When it comes to air quality and operating pressure, one thing that we do stress is velocity in the piping. When we talk about expanding systems, and maybe we have a 50 horsepower today, and now we're adding another 50 horsepower to the system, what's that distribution pipe network? Is that distribution pipe going to be able to handle the additional flow without, let's say, considerable pressure drop? And what ends up happening is most folks overlook that, or aren't aware that that could be a problem, and now we have, let's say, 500 CFM going through a system that we expected 200 CFM. So, the velocities, as Justin mentioned, they could be relatively high, and that means that we don't have enough contact time for air treatment. It also means that as we get faster in the distribution network, over 30 feet per second, then we start to see turbulence effect going on.

And so, normally speaking, when we're talking about drops in points of use, as you go faster, you're going to be potentially pushing, if there's any liquid water in the system, which it certainly could be if you have high velocities, you're pushing that water down to the point of use. Those are things, obviously, we would like to avoid, so that's what we say when you design the system, you're looking at 15 feet per second in the compressor room, 30 feet per second in the distribution piping, and 45 feet per second at the point of use, and that can really help ensure that you have the air quality that you started with, right? Because we're designing the system, we're making sure that everything works, but that piping aspect is really important.

PS: Speaking of piping, let's close out today's podcast by considering piping further. I read an article recently about how that piping itself can affect the quality of the air. Can you talk a little bit more about how that works?

NM: Yeah, sure. You know, the material type of the piping can make a difference. Number one, we're always looking at a leak-free system. You know, when we install something, we want to make sure that once it's up and running, pressure-tested, holding pressure, that it holds pressure indefinitely. One of the interesting things for me is that over time, when we look in a compressor room, we may have a mixture of equipment manufacturers. Maybe we're adding equipment, we have an expansion in the plant, so we need more compressors or more dryers, and so we may have a mismatch of equipment.

That's fantastic, but we may not be upgrading that system pipe We go into plants and we see piping that may be endemic from the start-up of the facility, and some of these facilities are 30, 40, 50 years old. I've seen certainly some receivers that are older than I am. Which, you know, as long you're having the inspections done, that's necessarily fine, but I think the thing that we tend to forget is that it's typically the low-cost materials that are being installed. And so, you see some black iron and you see some galvanized, and those tend to rust and scale. If we have that issue we just talked about, with velocities not being considered, pressure drops not being considered, so, you know, if you have moisture coming out, that can start to degrade the pipe internally, and then you get rust and scale buildup, and you have significantly more pressure drop, which like Justin and I both said, that's going to increase your overall operating cost.

The biggest things for customers is production. If you use, inferior materials, or materials that are going to rust and scale, now you're going to have the potential at that point of use, and then you're going to have a higher scrap rate, or more downtime, because now you have production equipment that is affected. So that's really I think the biggest thing for me. Copper, stainless steel, aluminum – those are really the best materials to choose for ensuring air quality in your system.

And the other thing, too, is that I mentioned regulated uses versus unregulated uses. You know, having a master controller is a great way to do that, but you can also put in a flow controller to maintain system pressure downstream, or really at points of use, depending on how big those demands are. That can also help with maintaining the operating pressure as low as possible, and potentially not affecting this air quality situation that we had in regard to the piping.