You are to design the air flow rate for the cooling system for a computer and th

Question

You are to design the air flow rate for the cooling system for a computer and the diameter of the fan casing. You plan for the computer to require 60 W when it is under full load but the temperature of the air exiting the computer must not be more than 65*C and, in order to keep the noise down, the exit velocity of the air must not be more than 100 m/min. You must design for the worst situation in which you expect the computer to operate in places where the environmental temperature is up to 47*C and the average atmospheric pressure is 66 kPa. Treat air as an ideal gas with a constant specific heat. You may also assume that kinetic and potential changes are negligible.

Explanation / Answer

computer delivers 60 W of power

rate of cooling = 60 J /s

aira can be heated to a maximum of 65 -47 = 18 deg C

air specifc heat = 1.005 kJ /kg-K

let m the mass of air required to take the energy

heat abosrbed by the air   = m*1.005e+3 *18

                                            = 60 J

m = 3.32e-3 kg

volume of the air flow rate = 3.32e-3/1.127 = 2.95e-3 m3 /s

we need to blow 2.95e-3 m3 of air per s

air speed = 100 m/min = 5/3 m/s

if a is the area of the fan then

A*5/3 = 2.95e-3

A = 0.59 sq.m

   = 59 sq.cm

diameter of the fan d = 8.67 cm

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