Consider entropy balance equation for human engines. Humans loose heat at the ra

Question

Consider entropy balance equation for human engines. Humans loose heat at the rate

of 80 W from the body which is uniformly at 37 C; they breathe in air at the rate of 360 LPH

(standard temp and pressure) with inlet air 25 C, 0.98 bar and exhaling air at 37 C, 1.02 bar. A)

Calculate entropy generated within human body in W/K. b) If life span is 75 years for 70 kg

humans and if the humans generate entropy at this constant rate how much entropy is generated

over lifespan in kJ/k and c) Express (b) in kJ/(K kg of human body). Apparently the answer for (c )

is believed to be true to all living systems (rats, cats, digs,

Explanation / Answer

A)

Entropy loss = Q/K = 80 / (37+273) = 0.258 W / K

Change in entropy due to breathing = m*[Cp ln (T2 / T1) - R ln(P2 / P1)]

= 360/(60*60)*1.2*10^-3 *[1005 ln((37+273)/(25+273)) - 287*ln(1.02 / 0.98)]

= 0.00338 W / K

Entropy generated within body = 0.258 + 0.00338 = 0.2614 W /K

B)

75 years = 75*365*24*60*60 sec

total entropy generated = (75*365*24*60*60)*0.2614 J / K = 618223.9 kJ / K

C)

618223.9 / 70 = 8831.77 kJ / K-kg of human body

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