A counter-flow heat exchanger is attached to a stenter for waste heat recovery.
Given data: \[\begin{array}{rl} \bullet & \text{Ambient temperature, \( T_1 = 30^\circ C \)} \\ \bullet & \text{Temperature of exhaust from stenter, \( T_2 = 150^\circ C \)} \\ \bullet & \text{Temperature of exhaust at exit of heat exchanger, \( T_3 = 100^\circ C \)} \\ \bullet & \text{Specific heat of exhaust gas, \( C_p = 0.42 \, \text{kcal} \cdot \text{deg}^{-1} \cdot \text{kg}^{-1} \)} \\ \bullet & \text{Specific heat of air, \( C_a = 0.24 \, \text{kcal} \cdot \text{deg}^{-1} \cdot \text{kg}^{-1} \)} \\ \end{array}\]