In a power plant, pipes transporting superheated vapor are very common. Superheated vapor is flowing at a rate of 0.3 kg/s inside a pipe 5 cm in diameter and 10 m in length. The pipe is located in a power plant at 20°C and has a uniform surface temperature of 100°C. If the temperature drop between the inlet and exit of the pipe is 30°C, and the specific heat of the vapor is 2190 J/kg•K, determine the heat transfer coefficient as a result of convection between the pipe surface and the surroundings.

Question

Answer 1

The convection heat transfer coefficient heat transfer between the surface of a pipe carrying superheated vapor and the surrounding is to be determined.

Assumptions 1 Steady operating conditions exist. 2 Heat transfer by radiation is not considered. 3 Rate of heat loss from the vapor in the pipe is equal to the heat transfer rate by convection between pipe surface and the surrounding.

Properties The specific heat of vapor is given to be 2190 J/kg ? °C.



Analysis The surface area of the pipe is



The rate of heat loss from the vapor in the pipe can be determined from



With the rate of heat loss from the vapor in the pipe assumed equal to the heat transfer rate by convection, the heat transfer coefficient can be determined using the Newton’s law of cooling:



Rearranging, the heat transfer coefficient is determined to be



Discussion By insulating the pipe surface, heat loss from the vapor in the pipe can be reduced.