Carbon dioxide at 427°C is to be used to heat 12.6 kg/s of pressurized water from 37°C to 148°C while the gas temperature drops 204°C. For an overall heat transfer coefficient of 57 W/(m2 K), compute the required area of the exchanger in square feet for (a) parallel flow, (b) counterflow, (c) a 2-4 reversed current exchanger, and (d) crossflow, gas mixed.
GIVEN
• CO2 heating water in a heat exchanger
• CO2 temperatures
? Tg,in = 427°C
? Tg,out = 427 – 204 = 223°C
• Water temperatures
? Tw,in = 37°C
? Tw,out = 148°C
• Water flow rate ( m w)= 12.6 kg/s
• Overall heat transfer coefficient (U) = 57 W/(m2 K)
FIND The area (A) required for:
(a) parallel-flow (c) A 2- 4 reversed current exchanger
(b) counterflow (d) Crossflow, gas mixed
ASSUMPTIONS
• In configuration (c), the gas is in the shell side
SKETCH
PROPERTIES AND CONSTANTS
the specific heat of water at the mean temperature of 92°C (cpw) =4205 J/(kg K)
(a) For parallel flow,
The rate of heat transfer is
Solving for the transfer area
(b) For counterflow,
Similarly
(c) The counterflow LMTD must be corrected for this configuration
F ? 0.97
(d) The counterflow LMTD must be modified using for crossflow, gas mixed. For P = 0.28, Z = 1.84 ? F = 0.93
COMMENTS
A simple counterflow heat exchanger requires the least transfer area for this case.
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