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This may be a silly question, but I'm not a mechanical engineering. What's the best way to calculate the energy required to reduce the pressure of a gas stream below atmospheric pressure? Obviously you could work out the energy for isentropic expansion and apply an efficiency factor, but this seems like it may be inaccurate. I know my stream's mass flow rate, temperature and desired final pressure.

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Different techniques have been developed to evaluate the efficiency of energy use by a vacuum pump. Most vacuum pump manufacturers catalog their test results, including brake horsepower (actual hp) and cfm vs. vacuum level. Fairly accurate evaluations of power needs can be made from such information sources. For example, the relative efficiency of different pumps can be obtained by calculating the cfm of free air removed per horsepower. Or input horsepower can be compared to the "fluid power horsepower" delivered, which is proportional to the product of gauge vacuum and air flow rate. All comparisons must be made at the same specific vacuum level, usually at 20 in. Hg or above.

e.g. check this source http://www.tuthillvacuumblower.com/dam/357.pdf

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