What are the methods to calculate the required exhaust air flow rate?

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)

Air Cooling

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

Moisture

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)

Dehumidifying

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.