Question

Question 4: (70 Marks) To recover some of the waste heat in the exhaust of a gas turbine, a heat exchanger with unmixed fluids is used. Hot exhaust gases, at Thi-425°C, flow through tubes, and the cold intake air flows across these tubes at Tci= 25°C, and Tco-210°C, The heat exchanger is designed with mass flow rates m-c 10 kg/s, heat transfer coefficients hh hc150 W/m2.K. Neglect the wall thickness of the tubes and calculate: 1. 2. The heat transfer area of this exchanger by using both NTU and LMTD methods While maintaining the same inlet temperature find the new outlet temperatures after doubling the flow of i) The hot fluid. i) The cold fluid. 3. Plot the heat exchanger performance for 1 and 2 and show the effects of both i and ii

Note: Please always outline your assumptions, draw a sketch of the problem and provide a

detailed solution.

Answer 1

Q.4 Exhaust Ga ven- A ssumpton- exhaust, CPh. Γ.og K <p rok NTU Me hod ecssAcul Heat tset Rate Maximum Possible. Heaf yans 一一210-25-0:463- 42S-2S Heot oyBoth fuid Unmixed Ceoss fo -CNT -78 exp NTU0-22 0.22 0.4625 Cy 0.93 NTU o, 93

selving for NTU i n loxi.ooSX'ơ. 62-31m So LMTD Method Oy Hees, So. AT, - 2 S2. 9s-2s 227.5 215 sTar,-227-35-25 - 221.4118 C en/227.ss 2I S 2_ doubled m=2"X hot fuid 0.93 ) when , mass Cspaity Ratio, c

0、78 0.22 - (0.465x0.927* E= 0.5238 326.6932℃ Th ( 0.5373 c = 10x 1,08 0.8654 20 I.oos -s- 25)

8 X370 - 3 1 2 NTU Effect:- when, we deuble the mess Ho gmcrease ï) u shan , we double Hue mass flow Rafe of