Chapter 7, Problem 7.42P

Jets of high-speed steam are used in spray cleaning. Steam at 15.0 bar with 150°C of superheat is fed to a well-insulated valve at a rate of 1.00 kg/s. As the steam passes through the valve, its pressure drops to 1.0 bar. The outlet stream may be totally vapor or a mixture of vapor and liquid. Kinetic and potential energy changes may be neglected.

(a) Draw and label a ?owchart, assuming that both liquid and vapor emerge from the valve.

(b) Write an energy balance and use it to determine the total rate of ?ow of enthalpy in the outlet stream $\left(H_{\text{out}}={\stackrel{\dot{}}{m}}_1{\hat{H}}_1+{\stackrel{\dot{}}{m}}_{\text{v}}{\hat{H}}_{\text{v}}\right)$. Then determine whether the outlet stream is in fact a mixture of liquid and vapor or whether it is pure vapor. Explain your reasoning.

(c) What is the temperature of the outlet stream?

(d) Assuming that your answers to Parts (b) and (c) are correct and that the pipes at the inlet and outlet of the valve have the same inner diameter, would $△{\stackrel{\dot{}}{E}}_k$across the valve be positive, negative, or zero? Explain your answer. (Note: You don’t have to calculate the kinetic energies at the inlet and outlet to solve this problem, although you can if you have extra time on your hands.)