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3. (a) Explain what logarithmic mean temperature difference (LMTD) is and its significance. [5 marks] (b) In a shell and tube heat exchanger the hot fluids enters at 450°C and leaves at 250 °C. The cold fluid is pressurised water (Cp = 4.2 kJ kg-1 K-1) and enters at 30°C and leaves at 150 °C. (i) For the same surface area and heat transfer coefficient, determine the percentage increase in heat transfer for a counter-flow heat exchanger compared with a paralier-flow heat exchanger [13 marks] (ii) If cold water flow is 6 kgs-1, calculate the rate of heat transfer. [2 marks] (ii) If the overall coefficient of heat transfer is 450 W m-2K-1. calculate the required exchange area for a counter-flow heat exchanger. [2 marks] (iv) If the tubes are 3 m long and 25 mm (thin walled), calculate the number of tubes required. [3 marks]