Problem 110
Water is to be boiled at sea level in a 30 -cm-diameter mechanically polished AISI 304 stainless steel pan placed on top of a \(3-\mathrm{kW}\) electric burner. If 60 percent of the heat generated by the burner is transferred to the water during boiling, determine the temperature of the inner surface of the bottom of the pan. Also, determine the temperature difference between the inner and outer surfaces of the bottom of the pan if it is \(6 \mathrm{~mm}\) thick. Assume the boiling regime is nucleate boiling. Is this a good assumption?
Problem 119
Saturated ammonia vapor at \(25^{\circ} \mathrm{C}\) condenses on the outside of a 2 -m-long, \(3.2\)-cm-outer-diameter vertical tube maintained at $15^{\circ} \mathrm{C}\(. Determine \)(a)\( the average heat transfer coefficient, \)(b)$ the rate of heat transfer, and \((c)\) the rate of condensation of ammonia. Assume turbulent flow and that the tube diameter is large, relative to the thickness of the liquid film at the bottom of the tube. Are these good assumptions?
Problem 129
Saturated ammonia vapor at \(25^{\circ} \mathrm{C}\) condenses on the outside surface of 16 thin-walled tubes, \(2.5 \mathrm{~cm}\) in diameter, arranged horizontally in a \(4 \times 4\) square array. Cooling water enters the tubes at \(14^{\circ} \mathrm{C}\) at an average velocity of \(2 \mathrm{~m} / \mathrm{s}\) and exits at \(17^{\circ} \mathrm{C}\). Calculate \((a)\) the rate of \(\mathrm{NH}_{3}\) condensation, (b) the overall heat transfer coefficient, and \((c)\) the tube length.
Problem 130
Saturated steam at \(270.1 \mathrm{kPa}\) condenses inside a horizontal, 10 -m-long, \(2.5\)-cm-internal-diameter pipe whose surface is maintained at \(110^{\circ} \mathrm{C}\). Assuming low vapor velocity, determine the average heat transfer coefficient and the rate of condensation of the steam inside the pipe. Answers: 8413 $\mathrm{W} / \mathrm{m}^{2} \cdot \mathrm{K}, 0.0608 \mathrm{~kg} / \mathrm{s}$
Problem 131
When boiling a saturated liquid, one must be careful while increasing the heat flux to avoid burnout. Burnout occurs when the boiling transitions from _____ boiling. (a) convection to nucleate (b) convection to film (c) film to nucleate (d) nucleate to film (e) none of them
Problem 132
Heat transfer coefficients for a vapor condensing on a surface can be increased by promoting (a) film condensation (b) dropwise condensation (c) rolling action (d) none of them
Problem 133
At a distance \(x\) down a vertical, isothermal flat plate on which a saturated vapor is condensing in a continuous film, the thickness of the liquid condensate layer is \(\delta\). The heat transfer coefficient at this location on the plate is given by (a) \(k_{l} / \delta\) (b) \(\delta h_{f}\) (c) \(\delta h_{f g}\) (d) \(\delta h_{\mathrm{s}}\) (e) none of them
Problem 134
When a saturated vapor condenses on a vertical, isothermal flat plate in a continuous film, the rate of heat transfer is proportional to (a) \(\left(T_{s}-T_{\text {sat }}\right)^{1 / 4}\) (b) \(\left(T_{s}-T_{s a t}\right)^{1 / 2}\) (c) \(\left(T_{s}-T_{\text {sat }}\right)^{3 / 4}\) (d) \(\left(T_{s}-T_{\text {sat }}\right)\) (e) \(\left(T_{s}-T_{\text {sat }}\right)^{2 / 3}\)
Problem 135
Saturated water vapor is condensing on a \(0.5 \mathrm{~m}^{2}\) vertical flat plate in a continuous film with an average heat transfer coefficient of $5 \mathrm{~kW} / \mathrm{m}^{2} \cdot \mathrm{K}$. The temperature of the water is $80^{\circ} \mathrm{C}\left(h_{f g}=2309 \mathrm{~kJ} / \mathrm{kg}\right)\(, and the temperature of the plate is \)60^{\circ} \mathrm{C}$. The rate at which condensate is being formed is (a) \(0.022 \mathrm{~kg} / \mathrm{s}\) (b) \(0.048 \mathrm{~kg} / \mathrm{s}\) (c) \(0.077 \mathrm{~kg} / \mathrm{s}\) (d) \(0.16 \mathrm{~kg} / \mathrm{s}\) (e) \(0.32 \mathrm{~kg} / \mathrm{s}\)
Problem 137
An air conditioner condenser in an automobile consists of \(2 \mathrm{~m}^{2}\) of tubular heat exchange area whose surface temperature is $30^{\circ} \mathrm{C}\(. Saturated refrigerant- \)134 \mathrm{a}\( vapor at \)50^{\circ} \mathrm{C}\( \)\left(h_{f g}=152 \mathrm{~kJ} / \mathrm{kg}\right.$ ) condenses on these tubes. What heat transfer coefficent must exist between the tube surface and condensing vapor to produce \(1.5 \mathrm{~kg} / \mathrm{min}\) of condensate? (a) \(95 \mathrm{~W} / \mathrm{m}^{2}, \mathrm{~K}\) (b) \(640 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (c) \(727 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (d) \(799 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\) (e) \(960 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\)
