Question

50 The emissivity of galvanized steel sheet, a common roofing material, is s = 0.13 at temperatures around 300 K, while its absorptivity for solar irradiation is aS = 0.65. Would the neighborhood cat be comfortable walking on a roof constructed of the material on a
(
a. If a dryer is designed to operate with an electric power consumption of Pelec = 500 W and to heat air from an ambient temperature of Ti = 20°C to a discharge temperature of To = 45°C, at what volu- metric flow rate 'e should the fan operate? Heat loss from the casing to the ambient air and the surround- ings may be neglected. If the duct has a diameter of D = 70 mm, what is the discharge velocity Vo of the air? The density and specific heat of the air may
day when GS = 750 W/m2, T00 = 16°C, and h = 7

W/m2 · K? Assume the bottom surface of the steel is

insulated.
be approximated as p = 1.10 kg/m3 and cp

J/kg · K, respectively.

Answer 1

Since the power is already given together with the initial and final temperature, the heat by convection and conduction should be determined.
Heat by convection can be determined using
qh = hAΔT
and the conduction is
qc =kAΔT
Which can be combined to determine the flow rate of air using
mcpΔT
and the mass flow rate can be converted to volumetric flowrate using the specific gravity of the air.