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Is LPM the same as Nl/min

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  • Is LPM the same as Nl/min

    Is liters per minute the same as normal liters per min. If not can this be converted; 2.1 kPa and 20 LPM to Nl/min?

  • #2
    Re: Is LPM the same as Nl/min

    Originally posted by Unregistered Bill G. View Post
    Is liters per minute the same as normal liters per min. If not can this be converted; 2.1 kPa and 20 LPM to Nl/min?
    Maybe, maybe not.

    The word "normal" means the volume has been adjusted from volume at actual temperature and pressure conditions to the volume the substance would occupy at 0 C, 101.325 kPa (1 standard atmosphere).

    The difference will depend on what the substance is, particularly whether gas or liquid, and how much the temperature and pressure differ.

    For gases, the volume is strongly dependent on temperature and pressure. For liquids, there is a much smaller dependence on temperature, and a VERY small dependence on pressure.

    You don't mention the substance, or the temperature. The pressure is extremely low compared to atmospheric pressure. Is it gauge pressure, or really near vacuum conditions?

    We obviously need more info to help you convert.

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    • #3
      Re: Is LPM the same as Nl/min

      The system is for a machine tool spindle assembly which is lubricated by an oil over air delivery system. The system air pressure is metered down to 2.1 kPa and the flow meter reads in LPM (liters per minute) which is at 20 LPM. With these two figures I need to be successful in obtaining a tolerance of 20 - 50 Nl/min. How can the two readings be converted to Nl/min?

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      • #4
        Re: Is LPM the same as Nl/min

        Originally posted by Unregistered View Post
        The system is for a machine tool spindle assembly which is lubricated by an oil over air delivery system. The system air pressure is metered down to 2.1 kPa and the flow meter reads in LPM (liters per minute) which is at 20 LPM. With these two figures I need to be successful in obtaining a tolerance of 20 - 50 Nl/min. How can the two readings be converted to Nl/min?
        Given that wide tolerance, I recommend not overthinking this. Even when the relationship is 1:1, 20 LPM is on the ragged edge of 20 - 50 LPM. Go to 25 LPM and call it good.

        However, as to the conversion, air obeys the ideal gas law pretty well. Atmospheric pressure varies above and below standard on a day to day basis. Let us suppose it is 101 kPa in your facility on a given day. The gauge pressure is 2.1 kPa over, or 103.1 kPa absolute. Let us also suppose the air temperature is 25 C (it must also be worked as absolute by adding 273.15 K to it).

        25 LPM corrected to "normal" would be
        25 lpm x (103.1 kPa/101.325 kPa) x 273.15 /(273.15 + 25) = 23.3 NL/min

        Not a huge change, but noticable. Worse, it will vary day-to-day with atmospheric pressure and temperature on your shop floor. Work some "worst case" numbers, get deeper into the middle of your spec range, and manage actual flow. At least, that is my recommendation.

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        • #5
          Re: Is LPM the same as Nl/min

          So, if I measured 80 L/min from a compressed air source with 5.5 to 7 bar of pressure (550 to 700 kPa) at 25C on a day with air pressure let's say 101 kPa (absolute pressures 651 to 801 kPa), the NL/min would be as follows:

          80 Lpm x (651 kPa/101.325 kPa) x 273.15 /(273.15 + 25) = 470.9 NL/min for 5.5 bar

          80 Lpm x (801 kPa/101.325 kPa) x 273.15 /(273.15 + 25) = 579.4 NL/min for 7 bar

          This seems high. Please let me know if I misinterpreted your previous post.
          Thx!

          Comment


          • #6
            Re: Is LPM the same as Nl/min

            Originally posted by Unregistered View Post
            So, if I measured 80 L/min from a compressed air source with 5.5 to 7 bar of pressure (550 to 700 kPa) at 25C on a day with air pressure let's say 101 kPa (absolute pressures 651 to 801 kPa), the NL/min would be as follows:

            80 Lpm x (651 kPa/101.325 kPa) x 273.15 /(273.15 + 25) = 470.9 NL/min for 5.5 bar

            80 Lpm x (801 kPa/101.325 kPa) x 273.15 /(273.15 + 25) = 579.4 NL/min for 7 bar

            This seems high. Please let me know if I misinterpreted your previous post.
            Thx!
            What exactly are you measuring? The measured flow in compressors is almost always "free air delivered," that is air at current ambient pressure and temperature as discharged, not air at pressure. It needs some modest adjustment to normal depending on current conditions being slightly off normal, but it is not (normally) measured at the compressor's operating pressure.

            Your calculation would only be correct if the flow were measured on the high pressure side at 6.5 bar.

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            • #7
              Re: Is LPM the same as Nl/min

              Thanks for your help on this.

              Our compressed air system is a closed system to operate pneumatic valves on a piece of equipment with a minimum flow rate of 50 NL/min with pressure requirements of 5.5 to 7 bar. We connected the line to a calibrated flow meter (that only measures L/min) with 90 psi line pressure before discharge. At the discharge point, where air enters the flow meter, based on your comment, I assume the pressure would indeed drop to ambient when measured as it just blows through.

              Am I correct to say then that flow in L/min would be lower if measured at 90 psi pressure, but that it would result in the a NL/min (when corrected for that pressure) that is basically the same as when measured at ambient pressure (as apparently we did) with correction for ambient pressure?

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