How do I calculate tube and pipe size in my gas system given specific gas flow rates and pressure metrics?

Sizing tubes or pipes is easily accomplished with software based on the Colebrook Equation and the Darcy Equation.  The Colebrook Equation requires an iterative solution for friction factor thus lending itself to a computer application.  The input parameters are pipe or tube outside diameter, wall thickness, gas pressure, gas temperature, flow rates, tube or pipe roughness, gas viscosity, and gas density at atmospheric pressure.

For example, flowing 75 scfm of Argon gas (density = 0.1037 lb/ft3 and 0.0225 cP viscosity) through a 1 inch OD x 0.065 wall electropolished tube at 120 psig and 85 OF, the calculated pressure drop would be 2.7 psig per 100 feet of tube.

While the software is easy to use, some experience is required to provide the correct input and interpret the results.

Ralph Cohen, P.E., who provided this information, is an engineering consultant to customers for gas systems components and HVAC, specializing in semi-conductor and other industrial manufacturing facilities.   He has over 30 years in the field, and continues problem solving for clients through his consultancy.    More information on Ralph’s project work, publications, etc. can be found here RalphCohenConsultancy or call 971-227-8989.


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