A 7-cm-external-diameter, 18-m-long hot-water pipe at 80°C is losing heat to the surrounding air at 5°C by natural convection with a heat transfer coefficient of 25 W/m2·°C. Determine the rate of heat loss from the pipe by natural convection, in kW.

Explanation:

Given:

D = 7 cm

= 0.07 m

A = pi × r ^2

= pi × (0.07/2)^2

= 0.00385 m2

T1 = 80°C

T2 = 5°C

h = 25 W/m2·°C

Q = A × h × delta T

= 0.00385 × 25 × (80 - 5)

= 7.22 W

= 7.22 × 10^-3 kW.

one "light year" is the distance that light travels, through vacuum, in one julian year.   that's 365.25 days, of 24 hours each.