A counter-flow heat exchanger is stated to have an overall heat transfer coefficient of 284 W/m²·K when operating at design and clean conditions. Hot fluid enters the tube side at 93°C and exits at 71°C, while cold fluid enters the shell side at 27°C and exits at 38°C. After a period of use, built-up scale in the heat exchanger gives a fouling factor of 0.0004 m²·K/W. If the surface area is 93 m2, determine

(a) The rate of heat transfer in the heat exchanger

(b) The mass flow rates of both hot and cold fluids. Assume both hot and cold fluids have a specific heat of 4.2 kJ/kg·K.

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