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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Five Things To Know About Single Acting Reciprocating Compressors

With the development of highly efficient air compressors, there’s still a place for the most basic design: the single acting reciprocating compressor.  When the piston moves out of the cylinder, it draws air in, at atmospheric pressure.  When it moves in to the cylinder, it reduces the volume that air occupies, increasing its pressure.  These machines are durable, effective, relatively inexpensive, and pretty easy to maintain.  Here are a few interesting things to know about them:

1. Popularity. Because of the simplicity of their design, they’re the most common air compressor in the 10HP and under sizes.  You can get them from a number of sources, and they’re not going to set you back as much as some other types.
2. Oil free air (part 1) While the most basic design uses oil to lubricate the piston rings in the compression cylinder, oil-less reciprocating compressors have cylinders with very smooth (and hard) bore surfaces, like nickel or chrome plating. A series of guide rings around the whole circumference of the piston prevent metal-to-metal contact, eliminating the need for liquid lubrication in the compression cylinder.
3. Oil free air (part 2) If oil in your compressed air is a problem, an oil-free (as opposed to oil-less) compressor is another option. While an oil-less compressor doesn’t use lubricant for the piston movement, an oil-free compressor’s moving parts are oil lubricated, but that oil is kept away from the compression cylinder(s) with connecting rod(s) so that the oil is confined to the lower moving parts…the crankshaft and bottom ends of the connecting rods, and away from the pistons & compression cylinders.
4. Foundation. Reciprocating machinery, as the name implies, has parts that move back and forth. The sudden reversal of direction of heavy metal pistons & rods, dozens of times a minute, means that their operation is inherently unbalanced. This out-of-balance condition, though, can be absorbed by properly securing the compressor to a properly prepared foundation.
5. Higher pressures. If your facility’s compressed air usage primarily entails pneumatic tools, cylinders, and blow off devices like air guns, the system header pressure is likely maintained at around 100psig. While a one-stage reciprocating compressor is usually rated for discharge pressures up to 125psig, a second stage can increase that to 175psig. Multi-stage compressors are used for applications that require up to 3,000psig compressed air. Examples of these are scuba breathing air, pneumatic excavators, and my personal favorite: ballast tank blowing air, used to surface a submarine.

4-stage reciprocating compressors charge 3,000psig air tanks that are used to rapidly push water from a submarine’s ballast tanks to create positive buoyancy.  Because keeping your ‘diving-to-surfacing’ ratio at 1:1 is important.

At EXAIR Corporation, helping you get the most out of your compressed air system is important to us.  If you’ve got questions about how to do just that, give me a call.

Russ Bowman
Application Engineer
EXAIR Corporation
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Air Compressor Motors And Controls

Electric motors are by far the most popular drivers for industrial air compressors.  Indeed, they are the prime movers for a great many types of industrial rotating equipment.  In their simplest form of operation, rotary motion is induced when current flows through a conductor (the windings) in the presence of a magnetic field (usually by electricity inducing a magnetic field in the rotor.)  In the early days, you’d start one up by flipping a big lever called a knife switch.

Example of a knife switch

These are cumbersome and inherently dangerous…the operators literally have their hand(s) on the conductor.  If the insulation fails, if something mechanical breaks, if they fail to make full contact, electrocution is a very real risk.  Over time, motor starters came in to common use.  Early in their development, they were more popular with higher HP motors, but soon were made for smaller motors as well.

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 me a call.

Russ Bowman
Application Engineer
EXAIR Corporation
Visit us on the Web
Follow me on Twitter
Like us on Facebook