Clive603 said:

Just recalled where the manual for my retired Atlas KE Vee twin compressor was hiding. These units came in 3 sizes and 15 versions. A quick look shows the single stage KE series are rated at 100 psi and the otherwise identical two stage KT versions are rated at 200 psi. Interestingly the output is given as piston displacement at specified run speed not directly as air delivery. I guess this avoids many potential specification mis-read problems as its then down to the user to figure out exactly how much air he (or she) gets at what temperature. My 3 hp KE2 was rated at 16.2 cfm piston displacement running at rpm. The lower pressure KE23 version running at rpm delivers 45 psi from 19.5 cfm piston displacement.

The KT2 version has two piston displacement ratings 7.32 and 9.75 cfm covering 2 HP 900 rpm and 3 hp rpm drive. The manual also lists a higher pressure 18 suffix version rated at 255 psi and 8.13 cfm piston displacement running at rpm.

Its interesting to see that specified running speed varies from 720 to rpm for various versions despite similar design. My mate Andy also got a KE2 from the same scrappy and ran his at 130 psi but the last 10 psi were very slow. I haver seen single stage units installed to deliver 150 psi. Hot air!

Clive

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Heating the air in compression can actually help. If you start with room temperature air and compress it, the heating makes it expand so it has a larger volume and lowered viscosity, making it push out the exhaust valve faster. If the tank is large enough that the air in it is reasonably cool, that air temperature defines the back pressure, not the air going through the exhaust port. If the heat conduction between the intake and exhaust ports is high enough to significantly heat the incoming air, then the compression stroke starts with a smaller charge and resultant loss in output. Cool in, hot out improves the throughput.

The manufacturer only listing displacement (swept volume) is just dodging the issue. A good spec sheet gives the actual output at various pressures. These compressors never deliver the full swept volume. Running at slow speeds and against low back pressure, they may come close, but all drop off when they have to work hard. Configured as a two stage where the low pressure cylinder was only working against about 15 PSI in the manifold between the cylinders, at 62 CFM swept volume and 100 PSI it actually delivered 56 CFM. That is about the best you could expect. I sized the orifice in the bead gun to match and sent the air through the cooler and to it, only feeding the 200 gal. tank when not blasting. With the large storage, other people in the shop never noticed.

Another factor usually neglected is the force needed to open an atmospheric intake valve. Running my Gardner Denver ACR as a vacuum pump, it would rapidly go up to 25 in hg gauge and stop. The approximately 4-5 in difference between that and atmospheric pressure was the differential needed to actuate the valve. That difference is also subtracted from the fill pressure when it is used as a compressor, although that is mitigated a bit because once the valve snaps open, it will tend to stay.

The viscosity of the air can have a surprising effect. I once repaired some huge heaters for an aluminum annealing oven big enough to handle truck loads. The operator commented that even though he had the blower motors rewound for the maximum performance possible, he had to start with the blowers off and heat the air until it got thin enough to avoid overloading them.

All these factors get in the equation, which isn't simple.

Bill

Heating the air in compression can actually help. If you start with room temperature air and compress it, the heating makes it expand so it has a larger volume and lowered viscosity, making it push out the exhaust valve faster. If the tank is large enough that the air in it is reasonably cool, that air temperature defines the back pressure, not the air going through the exhaust port. If the heat conduction between the intake and exhaust ports is high enough to significantly heat the incoming air, then the compression stroke starts with a smaller charge and resultant loss in output. Cool in, hot out improves the throughput.The manufacturer only listing displacement (swept volume) is just dodging the issue. A good spec sheet gives the actual output at various pressures. These compressors never deliver the full swept volume. Running at slow speeds and against low back pressure, they may come close, but all drop off when they have to work hard. Configured as a two stage where the low pressure cylinder was only working against about 15 PSI in the manifold between the cylinders, at 62 CFM swept volume and 100 PSI it actually delivered 56 CFM. That is about the best you could expect. I sized the orifice in the bead gun to match and sent the air through the cooler and to it, only feeding the 200 gal. tank when not blasting. With the large storage, other people in the shop never noticed.Another factor usually neglected is the force needed to open an atmospheric intake valve. Running my Gardner Denver ACR as a vacuum pump, it would rapidly go up to 25 in hg gauge and stop. The approximately 4-5 in difference between that and atmospheric pressure was the differential needed to actuate the valve. That difference is also subtracted from the fill pressure when it is used as a compressor, although that is mitigated a bit because once the valve snaps open, it will tend to stay.The viscosity of the air can have a surprising effect. I once repaired some huge heaters for an aluminum annealing oven big enough to handle truck loads. The operator commented that even though he had the blower motors rewound for the maximum performance possible, he had to start with the blowers off and heat the air until it got thin enough to avoid overloading them.All these factors get in the equation, which isn't simple.Bill

Rotary Screw Compressor Buyer's Guide

How to Pick The Perfect Rotary Screw Compressor

You work hard and expect the same from your tools and equipment.

When it comes to air compressors, nothing outworks a rotary screw compressor. These workhorses are designed to run 24 hours a day, 7 days a week, and 52 weeks a year - for many years.

If you're serious about your work, upgrade to a rotary screw air compressor!

Why Go Rotary?

1. Get the Job Done Faster
   You'll never have to wait for your compressor to play "catch up" again. With a 100% duty cycle,
   they are designed to run continuously without overheating - saving you time (and a headache).
   
   
2. Use it Longer
   Because the internal rotors are oil-sealed, parts last years longer and maintain the same performance throughout the compressor's life.
   
   

   The internal screws
   don't touch - meaning no
   friction, less maintenance, and
   fewer
   replacements on your investment.
   
   

3. Runs Quieter
   If you run a business, don't subject your employees (or customers) to uncomfortably loud and potentially unsafe environments. Rotary screw compressors run quietly, so they can be mounted close to
   workspaces
   without requiring hearing protection.

4. Cost and Energy Savings
   Although the upfront cost may be higher than reciprocating compressors, what you'll get is longer working life, increased capabilities, and the dependability only superior rotary screw technology can provide.

   Over time, rotary screw compressors pay for themselves
   by running cleaner and more efficiently, meaning you'll have to
   spend much less on filters, replacement parts, and accessories
   - all while enjoying a longer compressor life.
   
   With reciprocating (piston) type air compressors, the cost of energy required to run it can exceed the purchase price in
   one year, while rotary screw compressors will amount to roughly 70% of the purchase price in about
   10 years!
   
   Many of our rotary screw models are even Energy Star rated, using 20-30% less
   energy than federal standards.

   
   

How They Work

Want more information on Double-stage screw compressor? Feel free to contact us.

The rotary screw compressor gets its name from the way they are powered.


As air enters a sealed chamber, it is trapped and squeezed between two (helically-opposed) rotating screws. As the screws inter-mesh, they reduce the volume of trapped air and deliver it compressed, at the proper pressure level.

Rotary Screw Considerations

• Horsepower (HP)
  - One HP is the force needed to lift 550 lbs. one foot per second.
  
  
  
• Cubic Feet per Minute (CFM)
  - CFM is the
  amount
  (volume)
  of air velocity that passes through a medium. Think of a water hose where you can measure the quantity of water that is possible to pass through in one minute.
  
  
  
• Pounds per Square Inch (PSI)
  - PSI is the measure of
  pressure
  that a compressor can produce (in pounds) per square inch of space.
  
  
  Going back to our water hose example, PSI would be the amount of pressure at which the water (or air) comes out of the hose. Without a nozzle, it may trickle out, while one with a fine tip can blast grime out of concrete.
  
  
  If you ever had a kink in a line, then you would experience no CFM, while pressure behind the kink (PSI), would be building and possibly rupture the line.
  
  
  It's important to pay attention to both CFM and PSI requirements of air equipment so you choose a powerful enough compressor for the job.
  
  
  
• Electrical Phase
  - You need to choose single-phase or three-phase. Single-phase electricity is found in residential settings while three-phase electricity is most typically found in industrial locations & settings.
  
  Consult with your electrician and local codes to determine which type you have.
  
  

Choose Your Speed for Even More Savings

Variable Speed Drive (VSD) rotary screw compressors change the frequency and voltage feeding the motor, increasing its efficiency by calculating air compression needs instantly. Simply put, it only uses the energy required for the task at hand and doesn't overwork your machine by wasting power.

VSD compressors work best when sized correctly and are used for the correct applications. Keep in mind that these units are more sensitive to dirty work environments and should be properly protected.

Should I Buy?

If you're tired of your piston compressor not working as hard as you do, then do yourself a favor and upgrade. With a Rotary Screw compressor, you'll get speed, longevity, quieter jobs, and energy savings over time. For a little more, you can get a VSD compressor that maximizes efficiency even further.

Take your compressor needs to the next level and check out our Rotary Screw selection.

NEXT: Shop All Rotary Screw Air Compressors

By Melissa C.

Air Compressor Expert

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