Some electronics are mounted in an enclosure with dimensions of 9 X 16 X 22 cm.

Question

Some electronics are mounted in an enclosure with dimensions of 9 X 16 X 22 cm. Power consumption by the electronics under steady conditions is 600 W. The temperature of the equipment cannot exceed 60 C. This is to be achieved by either blowing atmospheric air through the unit or by circulating water through the unit (a heat exchanger transferring energy from the air inside the enclosure to the cooling water). Determine the mass rate of flow (kg/s) and the volumetric flow rate (m3/s) if a. the coolant is air entering at 1 bar, 20 C with a temperature rise of 30 C (i..e the cooling air exits at 50 C). Ans: 1 m3/min b. the coolant is water entering at 1 bar, 20 C with a temperature rise of 40 C. Ans: 0.215 liters/min

Explanation / Answer

a) Energy input to air = 600 W

We have, 600 = m*Cp*(T2 - T1)

600 = m*1005*30

m = 0.02 kg/s

Density = P/RT = 1*10^5 /(287*(273+20)) = 1.19 kg/m3

Volumetric flowrate = m/density = 0.02/1.19 = 0.0168 m3/s = 0.0168*60 m3/min = 1 m3/min

b) For water, Cp = 4180 J/kg-K and density = 1000 kg/m3.

So, 600 = m*4180*40

m = 0.0036 kg/s

Volumetric flow rate = 0.0036/1000 m3/s = 0.0036/1000*60 m3/min = 0.0036/1000*60*1000 lt/min

= 0.215 litres/min

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