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There are many methods available to calculate the required exhaust airflow rate, namely:
- Face Velocity Method
- Linear Extract
- Thermal Convection/Heat Load Method
- Appliance Power Input
- Air Changes
For some application we need to apply exhaust to control some factor like temperature (cooling or heating) and moisture so ASHRAE rates will not be enough
Air Heating
If air is used for heating, the needed air flow rate may be expressed as
qh = Hh / (ρ cp (ts - tr)) (1)
where
qh = volume of air for heating (m3/s)
Hh = heat load (W)
cp = specific heat air (J/kg K)
ts = supply temperature (oC)
tr = room temperature (oC)
ρ = density air (kg/m3)
If air is used for cooling, the needed air flow rate may be expressed as
qc = Hc / (ρ cp (to - tr)) (2)
where
qc = volume of air for cooling (m3/s)
Hc = cooling load (W)
to = outlet temperature (oC) where to = tr if the air in the room is mixed
If it is necessary to humidify the indoor air, the amount of supply air needed can be calculated as:
qmh = Qh / (ρ (x2 - x1)) (3)
where
qm = volume of air for humidifying (m3/s)
Qh = moisture to be supplied (kg/s)
ρ = density of air (kg/m3)
x2 = humidity of room air (kg/kg)
x1 = humidity of supply air (kg/kg)
If it is necessary to dehumidify the indoor air, the amount of supply air needed can be calculated as:
qmd = Qd / (ρ (x1 - x2)) (4)
where
qmd = volume of air for dehumidifying (m3/s)
Qd = moisture to be dehumidified (kg/s)
To specify minimum ventilation rates, refer to ASHRAE.1 Ventilation for Acceptance Indoor Air Quality.
