A certain brand of freezer is advertised to use 880 kW middot h of energy per ye
ID: 1521334 • Letter: A
Question
A certain brand of freezer is advertised to use 880 kW middot h of energy per year. Part A Assuming the freezer operates for 6 hours each day, how much power does it require while operating? Express your answer using two significant figures. Part B If the freezer keeps its interior at a temperature of -5.0 degreeC in a 20.0 degreeC room, what is its theoretical maximum performance coefficient? Part C What is the theoretical maximum amount of ice this freezer could make in an hour, starting with water at 20.0 degreeC?Explanation / Answer
b) The refrigerator is a Carnot Engine working in reverse. The COP is given by
COP = Tc/(Th - Tc ) , temperatures are in Kelvin scale
Th = 20 C and Tc = -5C
COP = (273.15-5)/((273.15+20)-((273.15-5)) = 10.726
c)
Operating power W = 400 W
time = 1 hour
water quantity is fixed, this is a control mass. As the water freezes it happens at constant pressure of P = 101 kPa
Th = 20 C = 293.15 K
Tc = 0 C = 273.15 K
W = 400 watts
W = -1Q2 / cp
1Q2 = - cp*W = -10.726 *400 = 4290.4 J
1W2= P m(v2– v1) = 101 ×m(0.0010891 – 0.001001) = m*0.0088 kJ = m*8.8 J
as.,m(u2– u1) =1Q2–1W2
m*(-354.09 – 62.98) = 4290.4 - 8.8m
m (-417.07) = 4290.4 - 8.8m
m = 10.5 kg of ice
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