Air Infiltration into Buildings by Natural Means

Air Infiltration

Air entering a building can be due to:• Infiltration through cracks round windows,

door, etc• Air flow through ventilation openingsCaused by wind-flow or variation between the

internal air temperature and external temperature producing what is termed ‘stack effect’

• The actual amount of air flowing into the building will depend upon the pressure difference between the inside and outside.

• Pressure difference is produced by the action of the wind flow around the building.

• The pressure is caused by a difference in the density of the internal and external air.

• This difference in pressure being due to the phenomenon that the warm air is less dense than cold air.

Calculation of Infiltration

2 methods• Calculations based upon the leakage

characteristics of the structure and pressure differences (crack method)

• Calculations based upon an assumed number of air change per hour

Pressure due to Wind Flow

• The relationship between wind pressure and wind velocity can be obtained from Bernoulli’s equation:

Wind Pressure, Pw = ρv2/2

where ρ = air density; v = wind velocity BRE gives Pw = 0.613 v2 Pa

where v is m/s

Air Flow Through Openings

• From experiments it has been found that the rate of air flow is approximately proportional to the square root of the pressure difference .

• IHVE Guide gives the expression as:V=0.827 A (∆p)0.5

where V = rate of air flow (m3/s); A = area of opening (m2) and ∆p= pressure difference across the openings.

Infiltration does not take place through a single opening but that it is more likely that there will be a number of openings.

Where the openings are in parallel, the above equation change to the form:

V=0.827 (∑A) (∆p)0.5 m3/sAnd where the openings are in series:V=0.827 [A1A2An/(A1

2 + A22 + An

2)0.5] (∆p)0.5 m3/s

• Since there was a relationship between wind pressure and velocity Pw = ρv2/2, then it follows that the pressure difference can be expressed as a function of the wind velocity squared.

• Also shown that that the flow of air through the openings is proportional to the squared root of the pressure difference, then by combining these two relationship it is possible to obtain an expression for the flow of air in terms of the wind velocity i.e: V = constant x A x v

• ASHRAE expressed as: V = Eav where V = air flow; A = area of inlet; v = wind

velocity; E – effectiveness of openings, having a value between 0.5 and 0.6 for the case where wind is acting at right angles to the face of the openings.

• Where the wind is blowing 450 to the inlet face then ASHRAE propose that the value should be reduced by 50 %.

Exercise

• A building is subjected to a wind velocity of 4m/s with the wind blowing at right angles to one face. A total inlet area is 2m2 is situated on the windward face and a total outlet area of 2m2 is on the leeward face. Calculate the ventilation rate. Assume E = 0.6

• A building is situated in a city centre and is 20m high. A total inlet area of 1m2 is situated on the windward face and a similar outlet area on the leeward face. Calculate the ventilation rate given the pressure difference as 11 pascal.