A refrigerator with an average COP of 2.8 is used to cool a well-insulated container whose contents are equivalent to 12 kg of water from 40°C to 10°C. When running, the refrigerator consumes 400 W of electric power. The length of time the refrigerator must run to accomplish this cooling is

Question

Quizplus · Accepted Answer

Solved by EES Software. Solutions can be verified by copying-and-pasting the following lines on a blank EES screen. 
COP=2.8
C=4.18 "kJ/kg.C"
m=12 "kg"
T1=40 "C"
T2=10 "C"
QL=m*C*(T1-T2) 
Win=0.4 "kW"
Win*time=QL/COP
time_min=time/60
"Some Wrong Solutions with Common Mistakes:"
Win*W1_time*60=m*C*(T1-T2) *COP "Multiplying by COP instead of dividing"
Win*W2_time*60=m*C*(T1-T2) "Ignoring COP"
Win*W3_time=m*C*(T1-T2) /COP "Finding time in seconds instead of minutes"

A Refrigerator with an Average COP of 2