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I am currently designing the intercoolers of a 5 stage centrifugal compressor (i.e. designing 4 intercoolers) by trying to follow the example given in "Process Heat Transfer" by D.Q Kern (Example 9.2 page 196, section titled "Gases").
My understanding of the process of compression and intercooling of air: the humid air (at 1atm) is compressed - reducing the volume whilst increasing the temperature and pressure in accordance to the pressure ratio. This flow is then sent to the intercooler where it is cooled to the original inlet temperature (assuming 100% efficiency and no pressure drop in the intercooler for simplicity). However, because of the change in volume (as a result of the compression) reducing the temperature in the intercooler means that a portion of the original water vapour will become liquid and "drop out". Thus resulting in a new (lower) humidity entering the 2nd stage of the compressor. A very simple explanation but I think it is right - please correct me if I am wrong!!
Now referring back to the solution of example 9.2 in Kern. I follow all the steps up until the calculation of the "Specific volume of atmospheric air = (359/29)(555/492)14.7/(14.7-0.8153)=14.8 ft3/lb" i.e the specific volume of the air entering the 2nd stage.
My understanding of the numbers (since I cant find the equation using just symbols anywhere!!) is that 29 = MW air. 555/492 = inlet temp/ref temp in oR. 14.7 = atm pressure in psi and 0.8153 = sat partial pressure of water at inlet temp in psi.
I understand this is a rather trivial humidity question - but I have been looking at it for days and cant get my head around where the 359 comes (or infact what the equation is?!!) Perhaps it is the imperial units that are confusing me!!
Any help understanding this last equation would be much appreciated!!
I have attached the problem for you information.
Kern problem.pdf 180.46KB
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