The front wall of a building is to be 8-inch thick poured concrete, faced on the

Question

The front wall of a building is to be 8-inch thick poured concrete, faced on the outside with 4-inch thick brick and finished on the inside with a 1-inch thick layer of plaster. Both surfacing materials are to be applied directly to the concrete as shown below. Given: h [BTU/hr ft2 degree F] = 0.5(1 + v0.8) [where v is air velocity in feet/second] Inside air velocity = 1 mph [miles per hour] Outside air velocity = 20 mph [miles per hour] Inside air temperature = 70 degree F Outside air temperature = 20 degree F Thermal conductivity of concrete = 0.60 BTU/hr ft degree F Thermal conductivity of brick = 0.36 BTU/hr ft degree F Thermal conductivity of plaster = 0.95 BTU/hr ft degree F Calculate the heat loss per unit area per day. Calculate the inside plaster surface temperature, T1, in the sketch.

Explanation / Answer

hi=0.5(1+1^0.8)=1 btu/hr ft^2 degree F

h0=0.5(1+20^0.8)=5.99

a)Q/A=T1-T2/Rth

Rth=1/hi+L1/k1+L2/k2+L3/k3+1/h0

      =1/1+0.0833/0.95 +0.333/0.36 +0.666/0.60 +1/0.60

      =3.29

Q/A=70-20/3.29=15.1975 btu/hr ft^2 ans

b)Q/A=70-T1/Rplaster

Rplaster=0.833/0.45=0.185

15.195=70-T1/0.185

t1=68.607 degree F

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