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AIR INFILTRATION "Used for hygienic purpose and for removing excess…
AIR INFILTRATION
"Used for hygienic purpose and for removing excess heat"
Factors:
- Air tightness/permeability
- Wind speed, direction inside/outside #### - Temperature that causes pressure difference
- Surrounding environment
Ventilation options:recycle:
Mechanical
Mechanical extraction of air -> rooms underpressure -> infiltration
Mechanical supply of air -> Overpressure inside the building -> exfiltration
Mechanical balance ventilation system avoid infiltration (annoying during winter and summer because of the incrementation of thermal load) and exfiltration (
Natural
Window opening,
NAD designed to provide a controlled amount of fresh air:
manually adjustable;
automatically controlled
Amount of N.V. variable with weather conditions
Hybrid
Air flow through cracks
a = flow exponent:
(
fully turbulent flow
) 0.5 < a < 1
(
laminar flow
): a ~ 2/3
Air permeability of building envelope
UNI EN 12207
Wind effect
Pressure difference across the walls due to the wind is a fraction (Cp) of wind kinetic energy
-1 < Cp < 1
Cp > 0 on windward side
K depend on terrain asperity and v_{ref} is v @ z = 10 m
Stack effect
From Stevin's Law
z_pn is the heigth to which \deltap=0
Modelling of air infiltration
Single-zone calculation model
Input:
Indoor-Outdoor
Temperatures
;
Local
wind
speed;
Local terrain and
shielding parameters
;
Mechanical ventilation
features (if present);
Number (nfp) and characteristics (k, alfa) of
flow paths
Imposing mass continuity (
for deltaT<20 it is equal to volume continuity equation
):
The only unknown term in the previous equation is Pin=f(z)
Particular solution for STACK EFFECT ALONE
where A is the area of the building envelope
Wind-Stack combined effect
