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# How Does One Measure Enthalpy?

05/19/2011 10:12 AM

I want to measure the efficiency of a fan or air compressor. Reply#1 of http://cr4.globalspec.com/thread/31413/Centrifugal-Air-Compressor-Efficiency-Calculation
provides a formula and states that all required information can be measured. I can measure most of the terms required except enthalpy, so how does one measure enthalpy?

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#1

### Re: How does one measure enthalpy?

05/19/2011 10:15 AM

One looks it up in tables. Try "Thermodynamic Properties of Fluids" by Mayhew & Rogers, or just about any ASHRAE psychrometric chart.

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#2

### Re: How does one measure enthalpy?

05/19/2011 12:02 PM

I conclude then that one does not measure enthalpy directly. Instead one will need to measure a dry bulb temperature and then also a wet bulb temperature (to determine % Relative Humidity). If the the inlet air pressure to a fan or air compressor is at sea level then I can use a sea level Psychometric Chart, but for discharge enthalpy, where the discharge air pressure is somewhat variable (it would depend on the actual discharge air pressure) then I would need to find (if one exists) a psychrometric chart that has pressure as a variable, and a high enough pressure value for the outlet of an air compressor.