42 - Evaporation

An aqueous solution at 15.5^oC, and containing 4% solids, is concentrated to 20% solids. A single effect evaporator with a heat transfer surface area of 37.2 m\(^2\) and an overall heat transfer coefficient of 2000 W/m\(^2\).K is to be used. The calandria contains dry saturated steam at a pressure of 200 kPa(g) and the evaporator operates under a vacuum of 81.3 kPa. If the boiling point rise is 5 K, calculate the evaporator capacity. (AU-May-2017)

Hot exhaust gases of a stationary diesel engine are to be used to generate steam in an evaporator. Exhaust gases (\(C_p=1051\) J/kg,^oC) enter the heat exchanger at 550^oC at a rate of 0.25 kg/s while water enters as saturated liquid and evaporates at 200^oC. The heat transfer surface area of the heat exchanger based on water side is 0.5 m\(^2\) and overall heat transfer coefficient is 1780 W/m\(^2\)^oC.. Determine the rate of heat transfer, the exit temperature of exhaust gases and the rate of evaporation of water. (AU-Nov-2016)

A feed of 3000 kg/hr of a 1.2 wt% NaOH salt solution at 311 K enters continuously in a single effect evaporator and is being evaporated to 3.0 wt%. The evaporation is at atmospheric pressure and the area of the evaporator is 72 m\(^2\). Saturated steam at 383.2 K is supplied for heating. Since the solution is dilute, it can be assumed to have the same boiling point as water. The heat capacity of the feed can be taken as 4.10 kJ/kg,K. Calculate the amounts of vapor and liquid product and the overall heat transfer coefficient. (AU-Nov-2015)