Intelligent Compressed Air®: Compressor Motors And Controls

Use of compressed air has gone hand in hand with manufacturing for centuries. From manually operated bellows devices that stoked fires to generate the high temperatures needed for forging metals in ancient times, to the massive steam or oil driven compressors used in the 1800’s on projects like the Mont Cenis Tunnel drills, to the sophisticated electric-powered compressors used widely across modern industry, compressed air has actually been “the fourth utility” longer than the other three (electricity, gas, and water) have been in existence.

Diesel & gas powered compressors offer advantages like higher power ratings, portability, and freedom from reliance on local electric power grids, but most air compressors in industrial use are powered by electric motors. They’re plentiful, reliable, and easily adaptable to a range of control schemes that offer efficient operation across a wide variety of operations.

Which control method is right for you will depend on a number of factors specific to your operation. Here’s a brief run-down that may help you narrow down the selection:

  • Compressors in smaller facilities that supply intermittent loads like air guns, paint sprayers, tire inflators, etc. (like the one shown on the right) are oftentimes controlled via Start/Stop. This turns the compressor motor on and off, in response to a pressure signal. This is the simplest, least expensive method, and is just fine for smaller reciprocating compressors that aren’t adversely affected by cycling on & off.
  • Some compressors ARE adversely affected by Start/Stop control…like rotary screw models. These take a finite amount of time to start back up, which could allow header pressure to drop below usable levels. If they cycle too often, heat from the starting current can build up & overheat the motor. If that’s not bad enough, the screw elements & bearings of the compressor itself are oil lubricated…every time they start up, there’s a finite amount of time where metal-to-metal contact occurs before the oil flow is providing rated lubrication. With Load/Unload control, the motor turns continuously, while a valve on the intake of the compressor is cycled by the compressor discharge pressure: it opens (loads) to build or maintain pressure, and closes (unloads) when rated pressure is achieved. When unloaded, the motor uses about 1/3 of the energy it uses while loaded.
  • While turning down energy use to 1/3 of full load is a great way to cut operating cost while maintaining operational integrity of your compressed air system, and physical integrity of your compressor, it doesn’t necessarily make sense when demand may be low enough to be serviced by existing system storage over long periods of time. That’s where Dual/Auto Dual control comes in. It allows you to select between Start/Stop and Load /Unload control modes.  Automatic Dual Control incorporates an over-run timer, so that the motor is stopped after a certain period of time without a demand. This method is most often used in facilities where different shifts have substantially different compressed air load requirements.

When any of the above control schemes are used, they will necessarily rely on having an adequate storage capacity…the compressor’s receiver, and intermediate storage (like EXAIR’s Model 9500-60 60 Gallon Receiver Tank, shown on right) must be adequately sized (and strategically located) to ensure adequate point-of-use pressures are maintained while the compressor’s motor or intake valve cycle. Other methods use variable controls to “tighten up” the cycle bands…these don’t rely on as much storage volume, and in some (but not all) cases, result in higher energy efficiency:

  • A variation of Load/Unload control, called Modulation, throttles the intake valve instead of opening & closing it, to maintain a specific system pressure. This method is limited in range from 100% to 40% of rated capacity, though, so it’s fairly inefficient in many cases.
  • Slide, spiral, or turn valves are built in to certain compressor designs to control output by a method called Variable Displacement, which (as advertised) changes the physical displacement volume of the air end. When header pressure rises, it sends a signal which repositions the valve progressively, reducing the working length of the rotors. This allows some bypass at the inlet, limiting the volume of air that’s being compressed with each turn of the rotor. Since the inlet pressure & compression ratio remain constant, the power draw from the partial load is considerably lower…so it costs less to operate. The normal operating range for this method is from 100% to 40% of rated capacity, but when used in conjunction with inlet valve Modulation, it’s effective & efficient down to 20% of rated capacity.
  • Of course, the most significant advance in efficient control of rotating industrial equipment since Nikola Tesla invented 3-phase AC is the Variable Speed Drive. When the frequency of the AC power supplied to an electric motor is changed, the speed at which it rotates changes in direct proportion. By applying this type of control to an air compressor, the motor’s speed is continuously controlled to match the air demand. Energy costs can be greatly reduced, as this method allows efficient turn down to as low as 20% of rated capacity.

As mentioned a couple times above, multiple control schemes can be applied, depending on user specific needs. Adding accessories, of course, adds cost to your capital purchase, but discussions with your air compressor dealer will lay out the pros, cons, and return on investment. While we don’t sell, service, or even recommend specific air compressors, EXAIR Corporation is in the business of helping you get the most out of your compressed air system. If you’d like to talk more about it, give me a call.

Russ Bowman, CCASS

Application Engineer
EXAIR Corporation
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Intelligent Compressed Air®: Common Compressor Room Mistakes, And How To Avoid Them

While we don’t sell, install, or service air compressors, EXAIR Intelligent Compressed Air Products run on compressed air, so helping you get the most out of your compressed air system is important to us. Today, we’re starting where it all begins: the compressor room.

Some of the mistakes that are commonly made in the compressor room are by design, and others are operational. My colleague Tyler Daniel wrote a great blog on design considerations recently, so I’m going to focus on the operational aspects, which include maintenance…and maybe some minor design stuff:

  • Poor ventilation: Air compressors get hot. They’ve got a lot of moving parts, and many of those parts are moving under a great amount of force (pressure is literally defined as force per unit area), and at a high rate of speed. Add in the heat of compression (it takes energy to compress air, and that energy has to go somewhere, something another colleague, John Ball, explains here), to all that friction and you come up with a TREMENDOUS amount of heat. An industry thumbrule, in fact, states that over 2500 Btu/hr of heat is generated, PER HORSEPOWER, by a typical industrial air compressor. If the compressor room isn’t big enough, you’ll need an exhaust fan capable of removing all that heat.
  • Lack of filtration: Take a good, full breath in through your nose, right now. Did you smell anything unpleasant or irritating? I hope not…clean air is a “must” for your lungs (and the rest of your body), and the same is true for your air compressor (and the rest of your compressed air system). Keeping up with the maintenance on the intake filter is literally “starting where it all begins”…from the 1st paragraph.
  • Not removing moisture: Water & water vapor will have an adverse effect on many components of your compressed air system: it’ll cause rust in iron pipes, damage the seals in air cylinders, motors, tools, etc., and if you use it for blow off or conveying, it’ll contaminate your product. We’ve writtenagain and again…about the importance of dryers, and which type might be best for you.
  • Tolerating leaks: The compressor room is loud, so leaks are going to be pretty big before you can hear them. And to add insult to injury, the vibration of a running compressor makes the compressor room a prime location for them to occur. Even one small leak that you couldn’t hear in a quieter area will cost you over $100 over the course of the year, and maybe only take minutes to fix. Good news is, even if you can’t hear them, they ALL make an ultrasonic signature, and we’ve got something for that.
EXAIR Model 9061 Ultrasonic Leak Detector “finds them all, big or small!”
  • Ignoring maintenance. If you don’t schedule planned maintenance, your equipment will schedule corrective maintenance for you…oftentimes at greater expense, and with no regard to your schedule.
    • Moving metal parts that make metal-to-metal contact (or that have very tight spacing tolerances) HAVE to be lubricated properly. If you run low on oil, or let it get dirty or emulsified, severe damage will follow. Keeping an eye on the oil level, and changing it (and the filter) at the manufacturer’s recommended intervals, is critical.
    • Emulsified or otherwise contaminated oil can damage seals, gaskets, and o-rings. That’s obviously a big problem for the compressor, and when it carries over into the header, it’s a big problem for pneumatic cylinders & tools as well. Periodic sampling & analysis of your oil can provide timely notice of issues that can be corrected before they become catastrophic failures.
    • Depending on the type of compressor, and its drive system, the manufacturer’s maintenance recommendations may also include:
      • Checking coupling or belt alignment of the drive.
      • Checking bolts for loosening due to vibration (a “necessary evil”, especially with reciprocating compressors).
      • Adjusting the pistons to maintain valve plate clearance.
      • Tightening or replacing the mounts & vibration pads.

If you’d like to find out more about how EXAIR Corporation can help you get the most out of your compressed air system, give me a call.

Russ Bowman, CCASS

Application Engineer
EXAIR Corporation
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Image courtesy of PEO ACWA Some rights reserved Creative Commons Attribution 2.0 Generic (CC BY 2.0)

Air Compressor Motors and Controls, Working Together.

One of the most important aspect of an efficient compressed air delivery system is effective utilization of compressor controls. The proper use of compressor controls is critical to any efficient compressor system operation. In order to reduce operating costs, compressor controls strategies need to be developed starting with minimizing the discharge pressure. This should be set as low as possible to keep energy costs to a minimum.

The compressor system is designed with maximum air demand in mind. During periods of lower demand compressor controls are used to coordinate a reduction in output that matches the demand. There are six primary types of individual compressor controls:

  1. Start/Stop – This is the most basic control. The start/stop function will turn off the motor in response to a pressure signal.
  2. Load/Unload – The motor will run continuously, but the compressor unloads when a set pressure is reached. The compressor will then reload at a specified minimum pressure setting.
  3. Modulating – Restricts the air coming into the compressor to reduce compressor output to a specified minimum. This is also known as throttling or capacity control.
  4. Dual/Auto Dual – On small reciprocating compressors, this control allows the selection of either Start/Stop or Load/Unload.
  5. Variable Displacement – Gradually reduces the compressor displacement without reducing inlet pressure.
  6. Variable Speed – Controls the compressor capacity by adjusting the speed of the electric motor.

All of these controls then control the compressor motors and they have several different starting methods.

There are several types of modern motor starters:

Full Voltage Starters: The original, and simplest method.  These are similar in theory to the old knife switches, but the operator’s hands aren’t right on the connecting switch.  Full line voltage comes in, and amperage can peak at up to 8 times full load (normal operating) amperage during startup.  This can result in voltage dips…not only in the facility itself, but in the neighborhood.  Remember how the lights always dim in those movies when they throw the switch on the electric chair?  It’s kind of like that.

Reduced Voltage Starters: These are electro-mechanical starters.  Full line voltage is reduced, commonly to 50% initially, and steps up, usually in three increments, back to full.  This keeps the current from jumping so drastically during startup, and reduces the stress on mechanical components…like the motor shaft, bearings, and coupling to the compressor.

Solid State (or “Soft”) Starters: Like the Reduced Voltage types, these reduce the full line voltage coming in as well, but instead of increasing incrementally, they gradually and evenly increase the power to bring the motor to full speed over a set period of time.  They also are beneficial because of the reduced stress on mechanical components.

The Application Engineering team at EXAIR Corporation prides ourselves on our expertise of not only point-of-use compressed air application & products, but a good deal of overall system knowledge as well.  If you have questions about your compressed air system, give us a call.

Jordan Shouse
Application Engineer

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Compressor Photo Credits to Bryan Lee, Creative Commons License

Which Condensate Drain Is Best For Your Compressed Air System?

In a perfect world, your air compressor’s intake would be free of dirt, oil, and water. Proper maintenance (i.e., periodic cleaning and/or changing) of the intake filter will keep most of the dirt out. Oil and water vapor will pass right through…but that’s not the end of the world (however imperfect it may be); they’re easy to take care of later in the process.

Once these vapors have been compressed (along with all that air that was drawn in), it’ll go into the receiver (usually via an aftercooler in industrial compressors) where it cools down, and that vapor condenses. If it’s left alone, a couple of things can happen:

  • Standing water in the bottom of a steel tank will cause corrosion. This can be carried into your compressed air distribution system. Over time, it will also rust through the reservoir. You don’t want either of these things to happen.
  • Eventually, it’ll take up enough space that your reservoir’s capacity will effectively shrink. That can cause your compressor to cycle rapidly. You don’t want that either.

Even the smallest of compressors will have manual drain valves on the bottoms of their reservoirs. Users will simply blow down the gallon or so tank every so often and go about their business. The small amount of electrical power that the compressor will use to recharge those tanks makes this a perfectly acceptable practice.

In the perfect world I mentioned above, the large reservoirs on industrial air compressors could be drained of condensate in the same manner. There are a few challenges to periodic manual draining:

  • You could do it on a schedule, but varying levels of humidity mean different accumulation rates of condensation. Weekly blowdowns might be OK in the winter, but you may need to do it daily in the summer. And a couple days a week in the spring or fall. It can be a real chore to keep track of all of that.
  • A practiced operator may develop the skill to shut the valve immediately upon the last drop of condensate passing. More often than not, though, you’re going to lose some compressed air doing it manually.
  • File this under “don’t try this at home (or anywhere, really)” – an unfortunately all-too-common practice is to just leave a manual drain cracked open. It works, but it wastes compressed air. On purpose. There’s too much accidental waste to give this any further discussion. Just don’t do it.
  • Plain old forgetfulness, someone going on vacation, or even leaving the company could result in someone else noticing the compressor is frequently cycling (because the reservoir is filling with water…see above), and realizing nobody’s drained the tank in a while.

Again, these manual drains are quite common, especially in smaller air compressor systems…and so are the above challenges. I may or may not have personal experience with an incident similar to that last one. Good news is, there are automated products designed to prevent this from happening to you:

  • Timer drains are popular and inexpensive. They operate just as advertised: a programmable timer opens and closes the drain valve just like you tell it to. They don’t do anything at all to address the first two challenges above: they might blow down for longer than needed (and waste compressed air) or not long enough (and allow water to build up in the reservoir.) They come in two primary configurations:
    • Solenoid Valve: the timer energizes the valve’s coil to open the valve, and a spring shuts it when the timer runs out. Strainers will prevent blockage, and will need periodic maintenance.
    • Ball Valve: the timer operates an electric actuator to open & close the valve. The full port opening of the ball valve means a strainer is usually not necessary, so these are less maintenance intensive.
  • Demand (AKA “no waste” or “zero loss”) drains are actuated by the condensate level in the reservoir. They don’t discharge any of the reservoir’s compressed air, because they close before the last bit of water exits. There are a few common options to choose from:
    • Mechanical float drains can be internal or external…the latter is more common for use with air compressor reservoirs; the former is fairly standard with point-of-use filters (more on that later). When the liquid level rises, the float opens the drain; when liquid level drops, the float closes the drain…easy as that. They CAN be susceptible to clogging with debris, but many have screens to prevent or limit that.
    • Electronic types use a magnetic reed switch or capacitance device to sense the condensate level…so they require electric power.
    • These cost more than the timer types, though, and they’ve got a number of moving parts, so they can find themselves in need of repair. Inexpensive and user-friendly rebuild kits are oftentimes available, and many of these come with alarms to let you know when to use that rebuild kit.

Whether you have a manual, timer, or demand drain, keep in mind that some moisture can still be carried over, and rust/scale can still form in pipelines. Good engineering practice calls for point-of-use filtration, like EXAIR’s Automatic Drain Filter Separators and Oil Removal Filters. If you’d like to talk more about getting the most out of your compressed air system, give me a call.

Russ Bowman, CCASS

Application Engineer
EXAIR Corporation
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