Consider a large classroom on a hot summer day with 150 students, each dissapati

Question

Consider a large classroom on a hot summer day with 150 students, each dissapating 60W of sensible heat. All the lights, with 6.0kW of rated power, are kept on. The room has no external walls, and thus heat gain through the walls and the roof are negligible. Chilled air is available at 15 degrees C, and the temperature of the return air is not to exceed 25 degrees C. Determine the flow rate of the air, in kg/s that need to be supplied to the room to keep the average temperature of the room constant.

Explanation / Answer

150* 60W + 6kw = 15kW = 15000 J/s that must be removed to keep the temperature constant. Dry air around that temperature has a specific heat capacity of around 1005 J/(kg*K). So, consider we need to remove 15000J in the process of heating up from 15 to 25 (equivalent to 10 K difference), one kg of air would take 10*1005=10050 J of energy So we need 15000/10050 = 1.4925 kg Since the 15000J is per second, and it asked for kg/s, no further conversion is needed. 1.4925 kg/s Double check: 1005 J/(kg*K) * (25-15)C * 1.4925 kg/s = 15000 J/s

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