Every interior space where people work, play or rest has a fresh air requirement due to the Carbon Dioxide we expend. For auditoriums the need is 15 Cfm (Cubic feet per minute of air) per occupant. In an office environment the requirement is 20 Cfm per occupant. In the summer air is hot and humid and when that air is used as ventilation air, the heat and humidity need to be removed. The sensible and latent heat of the air needs to be estimated in Btu/h (British Thermal Units per Hour) to determine the added amount of “cooling power” required to condition the space to the desired temperature setting.
Sensible Heat – Heat that causes a change in the level of a thermometer
Grains – 7000 grains equals one pound (of moisture)
Latent Heat – Heat energy absorbed or rejected when a substance is changing state and there is no change in temperature
(Refrigeration & Air Conditioning Technology 3rd Edition)
Two calculations are used:
Qs (Sensible Heat) = 1.085 x (Cfm requirement) x Delta of the design outdoor air temperature and the indoor temperature set-point = BTU/h
Ql (Latent Heat = 0.68 x (Cfm requirement) x Delta of the humidity ratio (in Grains) = BTU/h
Example: A simple office with 11 occupants, an outdoor design temperature of 88 degrees and a thermostat set at 68 degrees.
11 times 20Cfm per person = 220 Cfm
Qs = 1.085 x 220 x (88 – 68) = 4,774 Btu/h
Ql = 0.68 x 220 x (63 – 57) = 1,048 Btu/h
(The 63 and 57 are the grains per lb of dry air we extrapolate from our Psychometric Chart based on the outdoor and indoor temperatures of this equation)
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