How to know the pitfalls of chasing higher pressure for better performance

In this excerpt of a webcast transcript, understand how compressor pressure and performance are related

By Plant Engineering January 10, 2024
Courtesy: CFE Media

Compressor insights

  • Eric Bessey discusses the relationship between airflow, pressure and power in air compressors, emphasizing that higher pressure doesn’t necessarily translate to better efficiency.
  • Tom Taranto expands on the concept, explaining how increased pressure leads to artificial demand, causing inefficiencies in the system. The discussion also touches on different control types for compressors, highlighting the variable speed control as an effective strategy and addressing the challenges associated with centrifugal compressors.

Compressed air leaks are not only a direct source of wasted energy, they also can contribute to system pressure drops, making pneumatic equipment function less efficiently and shortening its life cycle. Watch the educational webcast “Energy efficiency: Focus on compressed air systems” and then read this transcript for additional details. This has been edited for length and clarity.

Learn compressor pressure from:

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

Eric Bessey: Compressors range in all kinds of sizes. One thing in addition to the compressed air itself that air compressors produce is pressure.

We’re really talking about energy, which is a combination of the airflow itself and the pressure. Oftentimes, pressure is misunderstood to the point where more is better and that is it. The power to compress air to a pressure is really a function of the pressure ratio. That’s defined as the absolute discharge pressure, divided by the absolute inlet pressure.

I like to make the analogy of a bicycle pump. When I grew up, my dad made me pump up my own tire. I’m 8 years old and the pump weighs more than I do and I must press that piston down with the handle. I noticed as a very young kid that the higher the pressure gets, the harder it gets to inflate that tire. That’s the same thing that is exhibited with a compressed air system as power relates to pressure.

From that, you might deduce, “You know what? Maybe 120 psi is not as good as 100 psi. If I can get by with 90 psi in my system, why would I want to pump it up to 120 psi?” A general rule of thumb and again, this is just a rule of thumb, is about for every psi that I raise the pressure, it will cost me about half a percent in power.

Tom Taranto: The pressure is the same way. It requires more power to compress air to the higher pressure, but what happens on the consumption side?

Courtesy: CFE Media

Courtesy: CFE Media

In the upper left of Figure 2, we’ve got a 5/16-inch orifice, with an orifice coefficient of 0.61, which happens to be a sharp-edged orifice. If you machine all the leaks in your system to exactly be a sharp-edged orifice, a 5/16 hole at 100 psi upstream, is going to pass 100 cubic feet per minute (cfm) of air. That’s the airflow rate that’s going to go through that hole at 100 psi.

Now at the same time, if you lower the air pressure to 80 psi, that’s a 20-psi reduction and 1 psi is a half a percent. Right there, the compressor at your power is going to go down. That same hole of 5/16 inch now passes 20% less air 80 cfm. Why do you want to make your compressor work harder? And make everything in the system consume more air, when you could operate the system at 90 psi?

That is the effect of pressure in the air demand. Now also, we want to try to fix leaks. As we’re fixing leaks, what we’re trying to do is we’re trying to make the hole smaller. If we look at the bottom left of Figure 2, now we’ve got a hole of 9/32 inch, as opposed to 5/16 inch and 100 psi, that’s going to pass 80 cfm. You really want to do both.

Lower the pressure to the lowest optimum pressure necessary to support production. At the same time, make that hole that’s out there, which represents the entirety of the plant air demand, as small as possible. Now we are down to 66 cfm.

What is artificial demand? Artificial demand is the extra amount of air consumed by your system, because you’re shoving more pressure at it than the system really should have. If we look at the top two orifices, if the system can operate at 80, but we run it at 100 and it consumes 100 cfm. We’re driving 20 cfm of artificial demand into that system, because it’s demand that doesn’t do anything good for production.

It’s demand that simply is passed through the system because we’re applying a higher pressure and we’re not regulating the pressure to the leakage in the end uses. If you have an end uses that is regulated, and it requires 60 psi and you have a good quality regulator on it that holds it at 60 psi. That component of demand doesn’t have artificial demand associated with it because it’s being fed at the proper pressure.

The general solution to problems with compressed air is always increase the pressure. That drives an efficiency through your system, not only by making the compressor work harder, but also by creating artificial demand.

Eric Bessey: We’ve talked about airflow itself, pressure, power and really what does that mean in terms of what the result in power is? If I need 66 cfm at 80 psi, what will that cost me on the compressor? How do I predict what it might cost? These are all rules of thumb sort of things, but one thing that I learned up through the years, is that each compressor has what we call a performance profile.

Courtesy: CFE Media

Courtesy: CFE Media

Figure 3 shows a group of typical lubricant-injected rotary screw air compressors of different control types. It’s a busy screen, but it basically goes from the very worst type of control, which is that top shallow line called inlet modulation with some blow down. You can see from there that at 100% power or 100% flow rather, it requires 100% power. That makes sense. But if it’s not putting any air out at all, it might be as bad as 70% power doing nothing.

Well, it’s doing something but nothing beneficial, other than just spinning the power meter. Then you get to a little better improvement as you go down the line, until finally the very, very best, of course, is just to start it and stop it. That’s the little pancake compressor. But something that comes very close and has been in the market for several years now, is what we call variable speed control.

That’s that green one, it’s the second one from the bottom and that’s a really nice control strategy for a compressor that should be considered.

Moving on to a different type of compressor, what about centrifugal compressors? Many know that centrifugal compressors are very different in nature. They are not positive displacement compressors, but rather a centrifugal dynamic type of compressor. They look much like a hydraulic pump or a liquid pump.

Here we have on the right side, you see that typical, classic centrifugal pump curve that many of you are aware of. Well, that’s what an air compressor is, it’s just pumping fluid. It’s just that it’s in the form of a gas instead of a liquid. Then typically what they’ll give you when you purchase one of these compressors, is they will give you what’s called a throttle range. That’s in the middle of it just below the red line, in between the slanted line and the black.

That’s the allowable throttle range where the inlet throttle can pinch back the air, restricting the air to match what the true demand is, but it can only do it so much. It can only do it until the point where we go into what’s called blow-off or bypass mode. Consider that at 100% flow, it requires 100% power just like the screw compressor.

Then it can throttle back on its inlet to a point of about 75%, in this example, they’re all different. Below that point, the compressor really has no choice other than to either blow off the air that isn’t needed, or it can unload. This auto-dual unloading mode is rarely used. Typically, people will operate the compressor into what we call blow-off mode.

If I’m above 75%, I’m great, but if I’m below it, then I’m wasting air. I’m going to hand this over and we’re going to see how these different control types tie in.