Question

please do 11.33

11.32 A single-pass, cross-flow heat exchanger uses hot exhaust gases (mixed) to heat water (unmixed) from 30 to 80°C at a rate of 3 kg/s. The exhaust gases, hav- ing thermophysical properties similar to air, enter and exit the exchanger at 225 and 100°C, respectively. If the overall heat transfer coefficient is 200 W/m2.K, estimate the required surface area. 11.33 Consider the fluid conditions and overall heat transfer coefficient of Problem 11.32 for a concentric tube heat exchanger operating in parallel flow. The thin-walled separator tube has a diameter of 100 mm. (a) Determine the required length for the exchanger. (b) Assuming water flow inside the separator tube to be fully developed, estimate the convection heat transfer coefficient.

Answer 1

(a)

$L=130m$

(b)

$h=1985.5W/m^{2}K$

The detailed solution is given below:

A Tem => (195)-(20) In (195220) = 76.85K (al # The length of the heat exchangers q= UX X ATEM UX (A xDxL) x A Tem (627.45X10) (200 wl?x)x(Txo.lm)x (76-8SK) & L=1130m =La q UX TXDX ATm B. # The regnolds olds number: > Red = 4x mic 4x (3kg/s) Ax (o.lm)x (505x10-6N.s/me) TX DX U 75638 >2000 - The flow is turbulent. Als # The Dites Boelke egestion : Lo flow is fully developed Herbert → Nup = 0.023 x Rep x An [oon=Oot, for = 0.023x (75638)419x (3.58124 for heating 306.40

h= NUD XK. Now, x (306.40 ) x (0.648 Wm.k) Coolm) 1985.5 w/m?K