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For industrial use, there are four main types of compressors.  Choosing which is right for your application depends on a number of factors;

  • How your plant uses air.  (i.e. constant use or intermittent)
  • Your plant’s overall volume required
  • Noise considerations
  • Shift to shift variations in use.

We don’t mean to overwhelm you.  Usually one or two from the above list will be your overriding factor(s).

         Ex. A typical factory may require roughly 500 SCFM of air to adequately supply two shifts per day.  This would suggest a 150 Hp single speed rotary screw (the most popular compressor type) as having enough extra capacity to give the compressor an acceptable duty cycle (operating loaded vs running unloaded) and still allow for plant growth.

         If, however, the plant wants to add a smaller 3rd shift, they will waste a lot of capacity running the compressor.  They might consider a variable speed compressor that operates at different horsepower’s meeting the demands of the plant without wasting a lot of air.  This is much more efficient and can save quite a bit on electrical costs.

         While the cost for a variable speed compressor is substantially higher, the energy savings down the road can provide a short payback period.

Types of Compressors

The Oil-Flooded Rotary Screw Compressor or simply “rotary screw” has become the industry standard.  With an output in the 4.25 scfm/hp range, it provides a good compromise of physical size, maintenance and initial cost. 

         The name “rotary screw” is derived from the fact that the air producing elements of the compressor are formed from two interlocking screws.  As they turn together via gearing, they move air from one end of the chamber to the other.  The oil provides a seal and increases the efficiency dramatically. 

ÿ=ߟ‘÷◊ÿ–ÿ“Ÿ”Ÿ‘Ÿ’ŸIn this design, the motor runs constantly.  When no more air is needed, the inlet is shut off and the air end spins without moving air.  A compressor operating in this state is said to be operating “unloaded”.  While the work the motor performs in this state is lower (remember- less work = lower amps = less electricity used), it still draws current and the costs of operating excessively in the unloaded state do add up.  Add to this that the rotary screw likes to operate “loaded” and you can see that using an overly large compressor will waste lot of money not to mention the possible need for more frequent repairs.  Obviously, proper sizing is important.  Sizes of rotary screws range form 10 Hp (40 SCFM) and up.

A subset of the rotary screw is the “oil free” design.  The rotors in these are designed to even tighter tolerances and 2 and 3 stage (2 or 3 consecutive air ends) versions are not uncommon.  These designs are to overcome the lower efficiency of the design.  Consequently, oil free compressors are very expensive compared to a standard screw compressor.  Why have them?  There are industries, such as the food industry, that cannot tolerate ANY oil in the air stream.  This makes their use mandatory.

As mentioned in the example above, there is another version of the oil flooded rotary screw that is gaining popularity and that is the variable speed compressor.  There are several ways to accomplish this feat.  The most popular is to vary the motor speed.  In other words, as you slow down or speed up the air end, you will produce ?a≥more or less air.  You may ask yourself how this can be done with precision.  Well, since a rotary screw is a fixed volume pump, each rotation produced a precise amount of air not unlike a piston pump.  This means that for a given speed of rotation, the air end (pump) produces a predictable amount or air. 

So What?  This means that the compressor can, via controls, calculate exactly how fast to run itself to keep up with demand and run at its most efficient speed and efficiency is the name of the game here.  This is where you can save money!

Now the only thing that remains is to determine the best way to vary the motor speed.  Some manufacturers use electric inverters that vary the frequency (cycles per second) going to the motor and this in turn, varies the speed.  One disadvantage of this technology is that running a motor very slowly produces excessive heat.  Designing a motor that can withstand the extra heat production is expensive and it also means that in most cases, the range of operating speeds is 40% of rated output.  In other words, a compressor capable of 500 SCFM output can throttle back to about 200 SCFM.  This is decent but there is another way to accomplish this with an even greater output range called “Switched Reluctance”.  This technology allows for an output range of 5 to 1 or 20% of rated output.  This means that that same 500 SCFM machine can now throttle back to 100 SCFM.

Rotary Screw Partial Load Capacity Controls

Before the advent of variable speed rotary screws there were systems in place that provided a mechanical means of cutting back on the output (and the electricity consumed!) of  the compressor.  They are referred to collectively as partial load capacity controls. There are two well know types of capacity control air ends generally available to provide this output/energy match.  Both work on the same principal.

A Little Background:

The output on a rotary screw air compressor is determined by three main factors. 1) The diameter of the rotors, 2) the length of the rotors and 3) The RPM’s that the rotors are turned at. So if we choose not to vary the RPMS of the air end rotors we must change one of the other two parameters. Since the diameter cannot be changed then we need to effectively change the rotor length. This is accomplished not so much by changing the actual length of the rotor itself, but by changing how much of the length of the rotor actually compresses air.

The two known systems are the spiral turn valve system (seen largely on Gardner Denver and Sullair Air compressors and the poppet valve system seen largely on Comp Air and Quincy Air Compressors.

The Spiral valve system works by having a series of holes down the center of the air end rotor housing with a spiral valve directly above the holes. The spiral valve will then move back and forth opening and closing these holes as necessary to match plant needs.  As more holes are “opened”, the less air that can be compressed as we have effectively “shortened” the available sealed compression area of the rotors. This means less air is compressed AND we have used LESS electricity to compress it!  Closing more holes means more sealed compression area, more compressed air MORE energy used to compress it.

The same theory works for the poppet valve system, the poppet valve system generally has a series of four poppet valves. These valves are mounted in the air end rotor housing and again open and close as needed which effectively changes the length of the compression area. Again, an open valve means less sealed compression area so less air compressed and less energy used to compress it. A closed valve means more sealed compression area, more air compressed air more energy spent to compress it.

As far as overall efficiency goes both systems greatly reduce the overall energy requirements. In the Spiral or turn valve system, some of the compressed air is trapped in the closed pockets which are openings in the cylinder. As the rotor edge passes, some of the trapped high pressure compressed air leaks back to the trailing low pressure cell. This has a negative effect of approximately 4% on efficiency at higher loads. In the poppet valve system the valve closes the opening with no significant cavity to hold high pressure air to leak back and reduce efficiency.

A widely misrepresented sales tactic for those selling the spiral turn valve system is to state that since the spiral valve circuit has more openings that is a more precise control. This is factually inaccurate. The spiral valve system and the poppet valve system both have a very high degree of accuracy, for the poppet valve to duplicate a setting in a spiral valve with more ports it is only necessary to hold the poppet valve open or closed for a shorter or longer period of time to duplicate the exact spiral valve setting. So in basic terms if the poppet system needs to duplicate a spiral valve setting where two ports are opened in the spiral valve, the poppet valve circuit needs one poppet valve to stay open twice as long. This would replicate the exact setting of the spiral valve.

All of this taken in stride, both systems work very well. We also need to point out that how you control these systems is key, some systems work on air pressure differential and some on electronic control. Electronic control does hold a much tighter pressure control (within 1 to 2 psi) over air differential pressure which holds about a 6 to 10 psi spread, so be sure to check into how these systems are controlled. This alone could have the most effect on which system performs the best.

Two Stage Oil Flooded Rotary Screw

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A two stage rotary screw compressor works on the same principal as a single stage rotary screw air compressor. The difference is that there are literally two air ends used off one drive motor. The first stage air end is generally very large and compresses the air to around 50 PSI. The second air end is smaller and takes the air pressure to the desired higher rating of 100 psi or so.  Compressor companies generally advertise a 12% to 17% energy savings generated by this method of compressing air. It is important to note that those savings are best achieved when the compressor is operating between 90% and 100% of full load, as the compressor comes down to operating points less than full load then the performance curve changes, and the advantage diminishes, and all but disappears at points below 90% of full load.

Another important fact to consider on oil flooded two-stage units is that it is difficult to attain a steady, high(er) efficiency (than a single-stage) within the two stage unit. This is because modern two stage units do not "intercool" the compressed air between the stages. In the old days of piston compressors it was very common if not always the case, that the compressed air from the first or low pressure stage was then run through an intercooler to drop the temperature before final compression in the second or high pressure stage.

In a modern rotary screw air compressor, it is an engineering challenge to intercool; this is primarily because oil is used in the rotors to seal the chamber for more efficient compression. If compressor companies would institute inter cooling to get the efficiency higher for the second stage, then by the process of inter cooling they would also remove the oil used for sealing. The result of no oil in the second stage would cancel out any efficiency gained, and decrease bearing life. No one to date has manufactured a system that can intercool the first stage and re inject oil into the second stage. All of that being said if someone would design an intercooled system that would re inject the oil into the second stage.

Reciprocating (Piston) Compressors

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The technology behind reciprocating compressors has been around in one form or another for several centuries!   Pistons have been used for everything from steam engines to vacuum pumps not to mention that thing that sits between you and the front bumper of your car (Corvair drivers excepted!).  As a matter of fact, a popular  portable air compressor in the 1960’s and 70’s was a 4-cylinder Jeep engine converted such that 2 of the pistons now pump air as the other two run via special carburetion.

h,œ¨ß©™´£´•¨¶¨ß¨®¨The technology is so simple and efficient that you find piston compressors used for everything from the tiny “road emergency” compressors that take 20 minutes to fill the average car tire and run off of your cigarette lighter to the big box store “specials” meant for home use to true industrial compressors that runs into hundreds of horsepower.  The pistons can be so large in these behemoths that you almost crawl into them!

¨ß©™´£´•¨¶¨ß¨®¨Given their efficiency of about 5 SCFM per horsepower, you’d think everyone would have them.  Well, there are a couple of problems with recips (as they’re called).  First, they are noisy! So much so that great consideration has to be given to the location of the larger units to ensure compliance with OSHA regulations.  Second, they have an incredible number of moving parts (compared to a rotary screw that has very few moving parts).  They have valves that generally must be replaced yearly and overhauls are VERY expensive sometimes running ½ or more of the cost of a new machine.  Third, in the larger sizes, they must be run continuously by shunting the excess output to the atmosphere to avoid having to start the motor more than absolutely necessary.  This is required due to the motor’s inrush current that can cause penalties to be added to your electric bill.

On the other hand, for the businesses with smaller requirements, recips can be a viable alternative because of the ability to take advantage of their good features of high output efficiency and turning off the motor between cycles.  As a matter of fact, recips can overheat if run too much at full load so manufacturers generally do not recommend running them continuously unless designed specifically for it.

So, is a reciprocating compressor for you? Well, if you fall into the category of smaller business and have either intermittent or low volume requirements, they are well worth considering. Also, where rotary screws require a clean air supply, recips are rather forgiving when used in very dirty conditions.  In any case, get the industrial duty versions if this is for daily use; they are better built and will generally be cheaper in the long run due to lower maintenance costs as well as increased lifetimes.

Centrifugal Compressors

Today, if you want a really big compressor, the way to go is the centrifugal.  Centrifugals are available from about 200 Hp on up to several thousand Hp.  Centrifugals are for rather specialized applications.  First and foremost, they need an application that requires a constant or near constant flow of air.  This is because, unlike a rotary screw, you cannot unload them.  Instead, like a large reciprocating compressor, you must shunt the unused air out to the atmosphere.  Obviously, this is very wasteful and if they are put into the wrong application, you will find a ∞£±¨ß©™´£´•¨¶¨ß¨®¨substantial increase in electrical costs. 

Centrifugals are a variable volume design of pump.  A home sump pump is another example of this type of pump.  Consequently, their speed can only be changed within a small range lest their output drop to near zero.  Centrifugal compressor air ends are designed to produce a particular output volume and pressure at a particular speed of rotation (conversely, a rotary screw air end is routinely operated at different speeds and horsepower’s to provide a wide range of outputs for a given air end).  This design feature offers very high efficiency on the order of 5 SCFM per Hp.  This efficiency is only fully utilized when the plant’s uses are considered as outlined above.

Rotary Vane (Hydrovane) Compressors

It is a wonder that rotary vane compressors aren’t more popular than they are.  They combine the best features of the other designs and to top it off, they are quiet, as low as 67 dB.  Although only available in the smaller sizes (generally 5-50 Hp), they are well worth considering. 

The hydrovane design operates on the same principle as the air motor.  A circular wheel (rotor) is fitted with multiple “vanes” that sweep air through as it turns.  These vanes are spring loaded and by putting the rotor within an enclosure that is off center, compression is achieved with each turn of the rotor.  Each vein starts in the low pressure area in an extended state.  As the rotor turns, the veins are pushed in toward the short side of the cylinder thus forcing the air into a smaller volume.  The result is compression of the air!

The simplicity of the design gives it several attributes besides low noise.  They have few continuously moving parts.  They can operate unloaded as in the case of rotary screws.  Their efficiency is decent, about 4 SCFM per Hp and they can operate well in less than ideal conditions as can R»¨ß©™´£´•¨¶¨ß¨®¨the reciprocating compressors.  The Hydro vane can also work in extremely dirty environments. First the vanes will push any ingested dirt in second the controls on a vane compressor are hydraulic which means they are sealed oil filled lines. This means that dirt and contaminants are far less likely to get in a plug up the controls as can easily happen on an air controlled system. For the smaller user that requires a continuously running a machine, the rotary vane design, or hydrovane, is an excellent choice.

We have really only scratched the surface of air compressor design.  The purpose has been to show and explain the most common designs you are likely to encounter.  The pictures that accompany the text show just a few of the possible variations of design within each type.  Many open designs also come with sound proofing cabinets.  These cabinets are attractive and make for a neat installation but can hinder maintenance and repairs especially if your compressor room is small.  Take this into consideration when planning your layout.

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