Question

QUESTION 4 (25 marks) Water (Cr 4.208 kJ/kg.K) at flow rate of 5.11 kg/s s heated from 78°C to 98°C in an economizer inside a boiler. The boiler is a cross flow heat exchanger with single pass, shell fluid mixed and other fluid unmixed. The average water velocity in the 1.5 cm diameter (D) tube is 1.27 m/s. On the shell side, hot air (C,-1.0341 kJ/kgK) was used as the heating fluid with 7.3 kg/s of it entering the exchanger at 160°C. The overall heat transfer coefficient, U. for the process is 3198 W/m2K. The relevant figures for references are given in Appendices. a) Determine the number of tubes and the length of each tube (in unit m) for the cross flow heat exchanger with single pass, shell fluid mixed and other fluid unmixed (11 marks) (PO3/ CO3/C4) b) If the flow of the water is increased to 6.7 kg/s while the inlet temperatures of hot stream and cold stream remain the same, determine the outlet temperature of water (in unit C). (7 marks) c) Ifthe conditions in (a), which includes length and number oftubes, inlet and outlet temperatures of hot and cold streams as well as the overall heat transfer coefficient, converted to unmixed configuration, remain the same but the fluid at the shell side is calculate the rate of heat transfer in the heat exchanger (in unit kW) (3 marks) (РОЗС 03C4) d) Calculate the percentage of increase/decrease of heat transfer rate if the cross flow heat exchanger configuration is converted from (a) to (c). (2 marks) e) Based on the calculation in (d), justify whether it is to convert the original configuration of heat exchanger with single pass, shell fluid mixed and other fluid unmixed to shell fluid unmixed configuration. (2 marks) РОЗСОЗС5) END OF QUESTIONS

Answer 1

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