Can I determine pressure loss by only using cfm & psi at 2 locations?

  1. I work for a manufacturing company. We want to make some improvements to our compressed air system, but we want to establish a baseline of our current efficiency for cost justifications, etc. One planned improvement, is redesigning our current piping layout. We spoke with a consultant who wants to sell us several gauges to measure cfm and psi at various locations in the facility. He claims by measuring cfm and psi at the header nearest compressor, we will be able to determine pressure loss from header to point of usage. Is this possible or is he just trying to sell product? Are there not other variables that need to be considered (ie: temperature, distance in pipe, number of fittings, types of fittings, pipe diameters, etc.) ? I realize we only need ballpark figures for justification, but can we measure pressure loss by only knowing cfm and psi at two given locations?
     
  2. jcsd
  3. SteamKing

    SteamKing 8,591
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    If you can't measure pressure loss with two pressure gauges, it's time to start over.
     
  4. I’m sorry, I am not an engineer and know little about fluid dynamics, so I am having trouble with this. We are trying to determine pressure loss due to the facilities current piping layout (ie: undersized pipe diameter, multiple fittings, switchbacks, etc.) Let’s say I place gauge at a 4” compressor header (Gauge 1), and a gauge on a 1” line 300ft downstream (Gauge 2) at point of usage (sandblaster). Assuming we are at steady state, static pressure at Gauge 1 and Gauge 2 should be equal. Now if I start the sandblaster I will exhaust the compressed air through a 1” line at Gauge 2, and create a transient system. Is it as simple as taking pressure at Gauge 1 minus pressure at Gauge 2 to determine Pressure Loss? Pressure Loss = Total Pressure1 – Total Pressure2. Are there gauges that measure total pressure? Total Pressure = Dynamic Pressure + Static Pressure.
     
  5. SteamKing

    SteamKing 8,591
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    If you are using compressed air in your system, the friction produced by the flow of the air thru fittings, valves, lengths of pipe, etc., will produce pressure drops in the system. Allowing the pressure to exit thru a sand blaster will create a temporary transient for a brief period after opening the valve, but any transient effects should die out after a certain interval.

    I don't know how complicated your system is, and I don't know the qualifications of your consultant. If you want to improve your current piping layout, then the current layout should be analyzed by a qualified engineer. IMO, the engineer will provide his own tools, gauges, whatever is necessary for him to do his job. I would recommend that your management and the engineer understand beforehand what the engineer will do, what results he is to furnish, and what additional equipment or information he requires before the project commences.
     
  6. The dynamic pressure is related to the volumetic flow rate - ie cfm.

    You can pre-analyze your system with some charts.

    http://www.engineeringtoolbox.com/horsepower-compressed-air-d_1363.html
    as it says add 10 to 15% fro friction of the air compressor

    http://www.engineeringtoolbox.com/compressed-air-pressure-loss-d_1014.html
    for a pressure loss in pipe. For each reduction or elbow, I think you add about 10 feet of straight pipe but I am not sure of that figure.

    http://www.engineeringtoolbox.com/air-line-length-recommended-sizes-d_1537.html
    for recommended sizes of pipe.

    ( right now the site is slow for some reason - just wait it out )
     
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