Question

An electric immersion heater, 10 mm in diameter and 300 mm long, is rated at 550 W. If the heater is horizontally positioned in a large tank of water at 20 °C, estimate its surface temperatures. Estimate the surface temperature if the heater is accidentally operated in air at 20 °C.

Answer 1

Given is:-

Rating of immersion heater is

diameter of heater    length of the heater

(water or air)

From the rate equation, the surface temperature, Ts, is

eq-1

where h' is estimated by an appropriate correlation. Since such a calculation requires knowledge of
Ts, an iteration procedure is required. Begin by assuming for water that Ts = 64°C such that Tf =
315K. Calculate the Rayleigh number,

Using the Churchill-Chu relation

Substituting numerical values into Eq. (1), the calculated value for Ts in water is

Therefore the surface temperature is 64.8 degree celcius