Question

An electric resistance heater is employed around the periphery of the tube that provides uniform heating. Water is heated inside the circular tube of 10 cm internal diameter and 5 m length from 30c to 90c. The outer surface of the heater is well insulated such that all the heat is transferred to water. Determine the power rating of resistance heater if the water flows inside the tube at a rate of 0.2 kg/s. Properties of water at mean temperature are: = 989.12 kg/m3, = 577 x 10-6 n-s/m2, k = 0.640 w/m-k, pr = 3.77.

Answer 1

Water is to be heated from 10°C to 80°C as it flows through a 2-cm-internaI-diameter, 13-m-Iong tube. The tube is equipped with an electric resistance heater, which provides uniform heating throughout the surface of the tube. The outer surface of the heater is well insulated, so that in steady operation all the heat generated in the heater is transferred to the water in the tube. If the system is to provide hot water at a rate of 5 L/min, determine the power rating of the resistance heater. Also, estimate the inner surface temperature of the pipe at the exit.