Air mass movement in various building situations
In general in can be remarked that the incoming air mass at the doorway is largely determined by three physical factors.
- Due to the laws of thermodynamics, the cold air moving into the interior mixes with the heated air that is present there. This process, like ohter laws of physics, tends to result in an equalisation of temperatures of differently out, a neutralisation of forces.
- The second physical factor to be taken into account i the wind pressure directed at the doorway and therefore also at the interior.
The higher the proportion of cold air streaming into an area in relation to the proportion of warm air streaming out at the doorway, the bigger the output required from an air curtain to ensure effective screening between the two climatic zones at the doorway.
BUILDING SITUATION A:
In an enclosed building with no connection to upper storeys or open windows/leakages, the neutral zone occurs at about 50% of the doorway height. At that point the incoming and outgoing air masses at the doorway take on equal cross sectional area proportions.
BUILDING SITUATION B:
In a building with two storeys connected by a staircase and other possible warm air exits as will, the neutral zione at the doorway shifts in proportion to the size of the leakages and/or open windows. These local conditions cause the cross-sectional area of incoming cold air to exceed that of the outflowing warm air, with the result that the neutral zone moves into the upper half of the doorway.
BUILDING SITUATION C:
In a multi-storey building with several levels connected by staircass and possibly several leakages and/or opened windows, an increase in warm air loss occurs in proportion to the size of the leakages and/or open windows. Teh neutral zone also moves further in fo the upper half of the doorway, which in extreme cases can mean that the proportion of outflowing warm air at the doorway approaches zero, leaving only an inflow of cold air.