Chapter 8, Problem 8.53P

Heated air required for a food-drying process is generated by passing ambient air at $\text{2}0°\text{C}$ through long, circular tubes $\left(D=50mm,L=5m\right)$ housed in a steam condenser. Saturated steam at atmospheric pressure condenses on the outer surface of the tubes, maintaining a uniform surface temperature of $\text{1}00°\text{C}$ .
(a) 1f an airflow rate of $0.01kg/s$ is maintained in each tube, determine the air outlet temperature $T_{m,o}$ , and the total heat rate q for the tube.
(b) The air outlet temperature may be controlled by adjusting the tube mass flow rate. Compute and plot $T_{m,o}$ , as a function of $\stackrel{\dot{}}{m}$ for $0.005\le \stackrel{\dot{}}{m}\le 0.050kg/s$ . If a particular drying process requires approximately $1kg$ of air at $\text{75}°\text{C}$ , what design and operating conditions should be prescribed for the air heater, subject to the constraint that the tube diameter and length be fixed at $\text{5}0\text{ mm}$ and $\text{5 m}$ , respectively?