Outside air filters into a conditioned space, even though the space may be slightly pressurised by an excess of air supplied over air extracted. Infiltration is principally due to:
(i) wind pressure, particularly on tall buildings,
(ii) stack effect, and
(iii) the entry of the occupants of the building, who also introduce dirt.
It is generally almost impossible to calculate, in advance of construction, the infiltration rate for a building. The only practical way of establishing the natural rate of infiltration for a building is to measure it, after construction. Even then the results will only be relevant to the particular set of circumstances prevailing at the time of measurement.
The product of the area of an opening, the mean theoretical velocity of airflow through it, and a factor to take account of frictional resistance, ought to give the rate of air flow through the opening. The theoretical velocity, v in m s_1, depends on the pressure difference across the opening, Ap in Pa (see section 15.5), given by
Ap = 0.6v2 (7.40)
In the case of a building this is complicated by two factors. First, the pressure is developed by two independent influences, wind effect and stack effect. Secondly, the air that enters through one opening must leave the building through another, so airflow through two openings in series must be considered. There is the further difficulty that, for a real building, the actual areas of inlet and outlet openings are not known with any certainty since they depend on the quality of the building structural components and the workmanship in construction.
Pressure differences across a building, due to wind effect, are tabulated and a method is presented in the CIBSE Guide A4 (1999) for determining the natural infiltration due to wind effect. The difficulty is in deciding on the areas available for infiltration through the building fabric.
A theoretical equation for the natural airflow rate through a building by stack effect is derivable from first principles. Two columns of air having a common height of h metres are considered. The column within the air conditioned building is cooler (at temperature tr) and therefore heavier, and the column outside the building is warmer (at temperature t0) and therefore lighter. This is satisfactory for small buildings having clearly defined, simple inlets and outlets, but it is of little use for high-rise buildings because of the complication introduced by the presence of multiple potential inlets/outlets in the form of windows, one above the other.
In the United Kingdom, where reasonably good standards of building construction prevail, the common practice is to assume half an air change per hour for natural infiltration in summer and one air change in winter for office buildings in a city. Some designers assume no infiltration at all in summer. This is not very scientific but appears to be adequate because the sensible gains that result from these assumptions do not represent a very large proportion of the total (about 2 per cent or 3 per cent). The latent gains are larger and account for about 25 per cent of the total latent gains but this is usually not critical, in the
UK, for commercial office block air conditioning because of the comparative unimportance of relative humidity in human comfort.
For industrial air conditioning it is possible that infiltration could be significant and assumptions of the sort mentioned above should not be made. This is also particularly true for air conditioning in tropical climates where the outside moisture contents are often very high in summer design conditions. A wrong choice of infiltration rate could then give a significant error in the latent heat gain calculation. For such applications reference should be made to the CIBSE Guide A4 (1999) and to the ASHRAE Handbook, Fundamentals (1997a).
Infiltration air change rates are much more critical in winter and the recommendations of the CIBSE Guide should be followed for the UK. This is notably true for entrance halls and lobbies, where infiltration rates can be very high in winter.
Heat gains by natural infiltration may be calculated using the following equations:
Qsi = 033nV(to-tr) (7.41)
Qn = 0.SnV(go-gr) (7.42)
TOC o "1-5" h z where <2si = sensible heat gain by natural infiltration W
Qu = latent heat gain by natural infiltration W
N = air change rate by natural infiltration h-1
V = room volume m3
T0 = outside air temperature °C
Tr = room air temperature °C
g0 = outside air moisture content g kg-1
Gr = room air moisture content g kg-1
Posted in Engineering Fifth Edition