Steam enters a counter flow heat exchanger dry saturated at 10 bar and leaves at
ID: 707479 • Letter: S
Question
Steam enters a counter flow heat exchanger dry saturated at 10 bar and leaves at 350°C. mass flow of steam is 800 kg/min. The gas enters the heat exchanger at 650 °C and mass flo rate is 1350 kg/min. If the tubes are 30 mm diameter and 3m long, determine the number o tubes required. Take for steam: tsa-180°C (@i 0 bar); Cps?2. 71 kJ/kg°C, h.-600 w/m2°C Take for gas: Cpe 1 kJkg C, he 250 W/m2 C 3. 4. What is the significance of Reynolds No. in determining the heat transfer coefficient in forced convection? 5. What is the significance of Nusselt No. in determining the heat transfer coefficient in forced convection? 6. Prove that Prandtl No and Reynolds No. is a dimensionless quantity. 7. Write a short note on occurrence of radiation heat transfer in a process equipment.Explanation / Answer
Ans 3
Steam inlet temperature at 10 bar
Tsi = 180°C
Steam outlet temperature
Tso = 350°C
Mass flow rate of steam
ms = 800 kg/min x 1min/60s = 13.33 kg/s
Cps = 2.71 kJ/kg-C
Gas inlet temperature Tgi = 650°C
Gas outlet temperature = Tgo
Cpg = 1 kJ/kg-C
Mass flow of gas
mg = 1350/60 = 22.5 kg/s
Heat given by gas = heat absorbed by steam
mg x Cpg x (Tgi - Tgo) = ms x Cps x (Tso - Tsi)
22.5 x 1 x (650 - Tgo) = 13.33 x 2.71 x (350 - 180)
14625 - 22.5Tgo = 6141.131
Tgo = 377.06 °C
Heat duty Q = 6141.131 kW
LMTD for counter flow
= [(650-350) - (377.06 - 180)] / ln (300/197.06)
= 244.94 °C
Overall heat transfer coefficient
1/U = 1/hs + 1/hg
= 1/600 + 1/250
= 0.005666
U = 176.47 W/m2-C
Area of exchanger
A = Q/UxLMTD
= 6141.131 * 1000 / 176.47 * 244.94
= 142.07 m2
Number of tubes = A/(3.14*D*L)
= 142.07/(3.14*0.030*3)
= 502.7
N = 503