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Rotary air compressor and vortex tubes questions?

  1. Jun 27, 2008 #1
    Hello, below are some specs I was provided of a micro screw air compressor, its physical dimensions are 4x4x4 and it seems pretty impressive for its size. However I am trying to understand a few things about the data that was provided.

    I am provided 6 charts on power/airflow/psi etc, each chart rates a specific discharge pressure in relation to increased hp and rotor speed and power consumption. The 6 charts are power/airflow/hp specs for: 7.3bar, 8.3bar, 10.3bar, 12.3bar, 14bar and 15bar.

    Each chart measures:
    A:Tip_Speed B:Rotor_Speed C:Discharge_Pressure D:FAD E:Power F:Specific_Power:

    Question 1:
    As rotor speed, HP, power and CFM increase, how come the discharge pressure (bar) doesn't increase or decrease accordingly? Chart 1 rates rotor speed at 4,900 RPM thru 22,000 RPM (accordingly power consumption, CFM increase with RPM), but the pressure (bar) remains constant at 7.3bar. How is that everthing else increases but the bar stays the same?

    Charts 2 thru 6 all reflect the same thing in described in Chart 1 but rated for the different bar measurements, as everything else increases the discharge pressure remains the same (above: 8.3bar,10.3bar,12.3bar,etc).

    Question 2: The final column is "specific power" defined as: kw/m3/min. Can someone explain this? To me it looks like kilowatt/meters cubed/minute. If I am correct, then the relationships don't make sense to me, one example is: kw/m3/min=9.514. What does this mean?

    Question 3: Is determining PSI is as simple as: PSI = bar x 14.504, so 7.3bar = 105.8 PSI ?

    Question 4: Compressors typically have tanks on them to sustain/build pressure. What if a compressor is used without a tank, simply feeding the outlet (discharge pressure) valve directly to the device its meant to power (in this case a vortex tube)? Is a tank required to sustain the pressure or can a compressor that is capable of 100 PSI be able to sustain that pressure on its own, without a tank? The inbound air to the compressor is going to be standard room air pressure.

    Question 5: If a compressor is pushing 100 PSI at 2.5CFM, and a vortex tube requires 100 PSI for 20CFM...is the vortex tube going to work to capacity considering the intake PSI is met via the compressor? Because the compressors 2.5CFM is considerably less than the vortex tube's CFM - will there be an impact on the vortex tubes capability?

    Thanks for your time and your responses.
    Last edited: Jun 27, 2008
  2. jcsd
  3. Jun 28, 2008 #2
    moe in wichita ks
    hi iam new here. i think iam going enjoy this site.
    i used the tubes you are talking about. in a machine shop. most of these tubes need a LOT air. and dont forget that compresed air is very expenseve. now remember i do not have a higher education, it was all could do to pass high school, and iam a very bad speller. my guess is that you will not get the performance you need. if you could get 17 or 18 cfm you might get by. these tubes need cfm more than psi. good luck.
  4. Jul 1, 2008 #3


    User Avatar
    Gold Member

    moe7404 said exactly the right thing in answer to Q5.

    I have waited to see if someone would answer your questions, and have been surprised at no response, so I looked at your profile and found all your other post, after reading them, my guess is that you are trying to design a cooler for a laptop computer (or any other). If I'm right let me know and I'll throw out some things for you to learn about.

    For now I don't have time to answer the questions above, but do have a couple of links that might provide some answers to some of your questions.


    Lots of stuff here to spin your brain:approve:


    The air motor guide will answer some of your compression questions, and some of the Power, CFM requirements. The air motor can also be viewed as an air compressor, the same as an electric motor can be a generator.

    Study the Sandia site and you can start to understand that going small is not a big thing.:cool:

  5. Jul 1, 2008 #4
    Thanks for responding, I too was wondering why no one was doing so. I suppose questions 2 & 4 are all I need answered at this point.

    My application is really about cooling with a vortex tube, but since that seems to be the easy part, the issue becomes getting the right kind of compressor to power the vortex tube. I am currently looking at compressors from this company: www.elgi.com - mainly at their Rotary air end compressors. Thus far they are the only ones I have found who make a compressor that meets my needs, or comes close to it. However, they are quite expensive...retailing over $400 US. Though, it still might be the best bang for the buck considering the units size versus output versus cost.

    I went through a spell where I was attempting to make my own compressor but that was a fruitless endeavor. However, I do have specific needs that are prioritized as follows:

    1. low power consumption (0.1 to 0.2 kW)
    2. can sustain 80-120 PSI (question #4?)
    3. physical size - smaller footprint the better
    4. noise level - how quiet can it be

    Now that I know CFM from the compressor (not just PSI) greatly affects the performance of a vortex tube, I suppose CFM should be on that list too.
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