How To Calculate Ventilation Requirements For Sizing A Canopy.
To size extract ventilation for a kitchen extraction canopy we use the Thermal Convection Method as outlined in DW172 Specification for Kitchen Ventilation Systems.
Firstly, we reference a drawing of the kitchen or an equipment list and get the surface area and fuel of the appliance. The size is calculated in m² and the fuel will be specified as electric, gas or solid fuel. Depending on the fuel the appliance will have a specific coefficient applied to it.
We get the size of the appliance and then multiply it by it’s specific coefficient which gives us a theoretical extract flow rate for each appliance. These are totaled for all appliances which gives us a total theoretical extract flow rate. We then apply a canopy factor to it (this is dependent on the type of canopy being used), so we take the theoretical flow rate and multiply by the canopy factor which gives us our specific extract flow rate.
Fresh air is then sized to replace between 75-95% of the extract flow rate. This is done to keep the kitchen under a negative pressure so as smells can’t permeate to seating areas etc. It is also to maintain quality air in the kitchen and dilute the biproducts of gas combustion.
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