An economizer is to be purchased for a power plant. The unit is to be large enough to heat 7.5 kg/s of pressurized water from 71 to 182°C. There are 26 kg/s of flue gases (cp = 1000 J/(kg K)) available at 426°C. Estimate (a) the outlet temperature of the flue gases and (b) the heat transfer area required for a counterflow arrangement if the overall heat transfer coefficient is 57 W/(m2 K).
GIVEN
Counterflow heat exchanger - flue gases heating water Water flow rate ( m w)= 7.5 kg/s Water temperatures
? Tw,in = 71°C
? Tw,out = 182°C Gas flow rate ( m g) = 26 kg/s Gas specific heat (cpg) = 1000 J/(kg K) Gas inlet temperature (Tg,in) = 426°C Overall heat transfer coefficient (U) = 57 W/(m2 K)
FIND
(a) Outlet gas temperature Tg,out
(b) Heat transfer area (A) required
SKETCH
PROPERTIES AND CONSTANTS
the specific heat of water at the mean temperature of 126.5°C (cpw)= 4240 J/(kg K)
The heat capacity rates are
(a) A heat balance yields
Solving for the outlet gas temperature
(b) for counterflow
The rate of heat transfer is
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