Question

# a) When is a heat exchanger classified as being compact? Draw a two-shell-pass and four-tube-pass shell-and-tube heat exchanger and explain the reason for using many tubes passes.[2 marks] :) How

does a crossflow heat exchanger differ from a counter-flow one? What is the difference between mixed and unmixed fluids in cross flow? Use sketches where possible.[3 marks] In the heat transfer relation Q = UA × F × LMTD for a heat exchanger, what is the quantity F called and represents? Can F be greater than 1? [2 marks] Hot water at 100°C flows at a rate of 3 kg/s inside a seamless tube of54.8 mm inside diameter, 60.3mm outside diameter and exposed to the ambient air at 24°C. Determine the convection heat transfer coefficient at the inside wall of the tube.[4 marks] \text { - Water density }=909 \mathrm{~kg} / \mathrm{m}^{3} \text { - Dynamic viscosity }=173 \times 10^{-6} \mathrm{~N} \cdot \mathrm{s} / \mathrm{m}^{2} \text { - Water thermal conductivity }=685 \times 10^{-3} \mathrm{~W} / \mathrm{mK} \text { - Prandtl number = } 1.09 A single-pass crossflow heat exchanger is used to cool jacket water of a diesel engine from 90°C to 60°C, using air with an inlet temperature of 30°C.Both airflow and water flow are unmixed, as shown in Figure Q-1f. The water mass flow rate is 42,000 kg/hr, and the air mass flow rate is 200,000 kg/hr.Take: The specific heat of water c = 4.18 kJ /kg °C The specific heat at constant pressure for air1.02 kJ/kg °C The overall heat transfer coefficient U = 90 W /m² i.The rate of heat transfer from the water to the air. ii.The exit temperature of the water. ii.The logarithmic mean temperature difference in the heat exchanger. iv.The area of heat exchanger. V.Explain how to enhance the heat transfer in this heat exchanger while maintaining the volume unchanged.[2 marks]

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