I\'m having trouble with this problem, can anyonehelp? A classroom that normally

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I'm having trouble with this problem, can anyonehelp?
A classroom that normally contains 40 people is to beair-conditioned with window air-conditioning units of 5 kW coolingcapacity. A person at rest may be assumed to dissipate heat at arate of about 360 kJ/h. There are 10 lightbulbs in the room, eachwith a rating of 100 W. The rate of heat transfer to the classroomthrough the walls and the windows is estimated to be 15,000 kJ/h.If the room air is to be maintained at a constant temperature of 21degrees C, determine the number of window air-conditioning unitsrequired. I'm having trouble with this problem, can anyonehelp?
A classroom that normally contains 40 people is to beair-conditioned with window air-conditioning units of 5 kW coolingcapacity. A person at rest may be assumed to dissipate heat at arate of about 360 kJ/h. There are 10 lightbulbs in the room, eachwith a rating of 100 W. The rate of heat transfer to the classroomthrough the walls and the windows is estimated to be 15,000 kJ/h.If the room air is to be maintained at a constant temperature of 21degrees C, determine the number of window air-conditioning unitsrequired.

Explanation / Answer

Define the system to be the air in the room. If thetemperature remains constant, there is no change in the internalenergy of the room. Since the volume of the room remainsconstant, there is no work done. Thus the first law reducesto the equation that Q = 0. We account forthe different heat sources in the total heat rate, Q, asfollows. The heat input comes (a) from the 40 students,Qs = (40 people)(360 kJ/h•person)(kW•s/kJ)(h/3600 s) = 4 kW
(b) heat transfer through the wall, Qw = (15,000kJ/h)(kW/kJ•s)(h/3600 s) =4.167 kW (c) heat added by the light bulbs, Qb = 10(100 W) =1000 W = 1 kW. Thus the total heat input is 4 + 4.167 + 1 = 9.167kW. The heat removal comes from the air conditioners which remove5 kW each. Thus for the net Q to be zero we must have N airconditioners such that (5 kW)N = 9.167 kW. To satisfy thisrequirement, we must have 2 air conditioningunits. /

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