A room contains 25 kg of air at 100 kPa and 10°C. The room has a 250-W refrigerator (the refrigerator consumes 250 W of electricity when running) and 1-kW electric resistance heater. During a cold winter day, it is observed that both the refrigerator and the resistance heater are running continuously but the air temperature in the room remains constant. The rate of heat loss from the room during that day is
A)3600 kJ/h
B)2700 kJ/h
C)4500 kJ/h
D)1000 kJ/h
E)1250 kJ/h

Question

Quizplus · Accepted Answer

Solved by EES Software. Solutions can be verified by copying-and-pasting the following lines on a blank EES screen. 
R=0.287 "kJ/kg.K"
Cv=0.718 "kJ/kg.K"
m=25 "kg"
P1=100 "kPa"
T1=10 "C"
time=30*60 "s"
W_ref=0.250 "kJ/s"
W_heater=1 "kJ/s"
"Applying energy balance E_in-E_out=dE_system gives E_out=E_in since T=constant and dE=0"
E_gain=W_ref+W_heater
Q_loss=E_gain*3600 "kJ/h"
"Some Wrong Solutions with Common Mistakes:"
E_gain1=-W_ref+W_heater "Subtracting Wrefrig instead of adding"
W1_Qloss=E_gain1*3600 "kJ/h"
E_gain2=W_heater "Ignoring the Refrigerator"
W2_Qloss=E_gain2*3600 "kJ/h"

A Room Contains 25 Kg of Air at 100 KPa