Question

Take a glass window that is 2.5 m x 1.78 m and is 4.4 mm thick. If the outside air temperature is 44 ℉ and the inside temperature is 68 ℉, determine how long it would take to lose 2022 Btu of heat energy. Report your answer in minutes.

Answer 1

1 Btu = 1055.06

• Heat flow =  2022 Btu = 2022 x 1055.06 = 2133.33 KJ
• Area = 2.5 x 1.78 = 4.45 m²
• Thickness ( L ) = 4.4 mm = 4.4 x 10^-3 m
• Thermal conductivity of glass ( k ) = 1 W/m.K.
• Outside air temperature = 6.67 ^o C = 44^o F
• Inside air Temperature = 20^o C = 68⁰ F

Q = ( KAT ) / L

= 1 x 4.45 x ( 20 - 6.67 ) / ( 4.4 x 10^-3 )

= 13481.47525 W

Time required in heat flow = Total heat flow / Rate of heat flow

= 2133.33 x 10³ J / 13481.47525 W

= 158.24 seconds

= 2.637 Minutes .

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