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Old Jul 9th 2013, 10:51 AM   #1
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Join Date: Jul 2013
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heat exchanger exergy analysis

i am having a problem with heat exchanger (tube and fin) exergy analysis...

the problem which i am having looks like this:

in one of the cases air (mass flow of air: m,a=0,71 kg/s) is entering the heat exchanger with temperature T,aIN=21C and relative humidity RH,aIN=57%, air is cooled to the state where it has temperature T,aOUT=19C and RH,aOUT=64%. Water entering the heat exchanger has T,wIN=7C and on the outlet T,wOUT=14,91C, mass flow of water is 0,043 kg/s.

for exergy calculation:
air inlet entalpy and entropy h,aIN=43,55 kJ/kg and s,aIN=0,1552 kJ/kgK. Air inlet conditions are the same as dead end state conditions. Air outlet enthalpy and entropy are h,aOUT=41,50 kJ/kg and s,aOUT=0,1487 kJ/kgK.
water enthalpy and entropy: h,wIN=29,52 kJ/kg; s,wIN=0,1064 kJ/kgK; h,wOUT=62,68 kJ/kgK;s,wOUT=0,2231 kJ/kgK. Water enthalpy and entropy at dead end state (21C): h,w0=88,18 kJ/kg and s,w0=0,3107 kJ/kgK.

calculated exergies, air: ex,aIN=0 kj/kg;(ex,aIN*m,a): Ex,aIN=0 kW, ex,aOUT=0,1330 kJ/kg, Ex,aOUT=0,0944 kW
calculated exergies, water: ex,wIN=1,4348 kJ/kg, Ex,wIN=0,0627, ex,wOUT=0,2675 kJ/kg, Ex,wOUT=0,0117 kW

if i calculate second law efficiency ( outlet exergies/inlet exergies), i get efficiency: 1,69 !?

i do not know where could be a mistake... enthalpy and entropy values seems to be correct... any ideas?

Thanks a lot!
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