Forced convection

Forced ventilation is used to remove/add heat in a space to maintain appropriate thermal conditions.

Sensible heat exchange:


Where Q s = sensible heating/cooling load(W)

V = airflow rate (m3/s) p = air density (kg/m3)

Cp = specific heat of air (J/kg K)

Ti = indoor air temperature (°C)

Ts = supply air temperature (°C)

Latent heat exchange:


Where Q i = latent heating/cooling load (W)

V = airflow rate (m3/s) p = air density (kg/m3)

Ifg = latent heat of water vapor at room air temperature (J/kg K)

Wi = indoor air humidity ratio (kgv/kga)

Ws = supply air humidity ratio (kgv/kga)

V must be the same for the sensible and latent heat exchange.

Forced ventilation is also used to maintain an acceptable indoor air quality.


Where Ci = contaminant concentration of indoor air Cs = contaminant concentration of supply air S = contaminant source

V = volume airflow rate

Example 6.1

An PSU classroom with 10 occupants has 4000 W sensible cooling load under summer design conditions. If Ti = 25 °C, Ts = 15 °C, Cs = 300 ppm, and SCo2 = 0.30 L/(min/person). determine the ventilation rate.


For comfort:


For air quality:


Select greater one to meet the demand for both comfort and air quality:

V = 1200 m3/h

Posted in Fundamentals of Heating. Ventilating, and Air-Conditioning