Question

You are responsible for designing a new heat exchanger to reheat a 100 kmol/hr water stream producing superheated steam. You can assume the heat exchanger operates at steady state, and there are negligible pressure losses throughout the system (it’s new, so there’s no fouling). The water enters the heat exchanger at 300 K and 5 bar and is heated to 575 K. Estimate the heat exchanger duty in kW.

Answer 1

Water will be subcooled at 300K (300-273.15)=26.85 deg.c at 5 bar pressure. Saturation temperature corresponding to 5 bar from steam tables is 153 deg.c. So sensible heat will have to be supplued to rise the temperatute of water from 26.85 deg.c to 153 deg.c to convert into saturated liquid

Sensible heat= 1000 mol/hr*18*4.18*(153-26.85)=949152.6 Joules=949.5126 Kj/hr

at 153 deg.c and 5 bar satruated enthalpy is 643 Kj/kg

Enthalpy of super heated steam at 575K= 775 Kj/Kg

Enthalpy change= 775-643 Kj=132Kj/Kg

total flow rate = 100kmol/hr =100*18 kg/hr

total enthalpy change= 1800* 132 Kj=237600 Kj/hr

total hwat duty=237600+949.5126=238549.5 Kj/hr

1Kj/hr= 0.00028 KW

238549.5 Kj/hr =238549.5*0.00028=66.8 Kw