A class room that normally contains 40 people is to be air conditioned with wind

Question

A class room that normally contains 40 people is to be air conditioned with window air conditioning units of 5 -kW cooling capacity. A person at rest may be assumed to dissipate heat at a rate of about 360 kJ/h. There are 10 light bulbs in the room, each with a rating of 100 W.The rate of heat transfer to the classroom through 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 21 degrees celsius, determine the number of window air conditioning units required.

answer is 2 units, but how do you
set it up?

Explanation / Answer

In this problem, we define the system to be the air in the room. We can assume that the air behaves as an ideal gas. If the temperature remains constant, there is no change in the internal energy of the air. Since the volume of the room remains constant, there is no work done. Thus the first law reduces to the equation that Q = 0. We account for the different heat sources in the total heat rate, Q, as follows. 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 = (15,000 kJ/h)(kW/kJ•s)(h/3600 s) =4.167 kW, and (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 = 9.167 kW. The heat removal comes from the air conditioners which remove 5 kW each. Thus for the net Q to be zero we must have N air conditioners such that (5 kW)N = 9.167 kW. To satisfy this requirement, we must have two air conditioning units.

Related Questions

Navigate

• Browse (All)
• Browse A
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.