Infiltration and Exfiltration
Most building envelopes have purposely provided openings (i. e., doors, windows, vents, flues, chimneys, and other ducts) and unintentional openings (i. e., cracks, mortar joints, and gaps around closed windows and doors). Air leakage through unintentional openings in the building envelope result in exchange between outside and indoor air. Uncontrolled outside airflow through cracks and other unintentional openings is called infiltration; the uncontrolled indoor airflow through unintentional openings is called exfiltration. Air leakage through the building envelope is a measure of the air tightness of the building envelope. Air leakage through the building envelope has a positive effect by allowing for natural (free) building ventilation. On the other hand, infiltration increases heat losses (in winter) and gains (in summer) through the building envelope, and also may result in reduced control over contaminant movement within the building.
In general, the air leakage rate through a building envelope is dependent
• The sizes and distribution of leakage paths
• The flow characteristics of the leakage paths
• The pressure difference across the leakage paths
The flows through the openings in a building are not independent but are based on the mass balance across the whole building envelope. The flow rate through an opening depends upon the pressure difference across it. Normally, the pressure difference occurs due to the wind effect and a temperature difference between the indoor and outdoor air. Also, an imbalance in the mechanical exhaust ventilation system performance over the mechanical air supply (positive or negative pressure building) might be a factor influencing infiltration and exfiltration.
A typical envelope opening has a complicated shape and is often subject to unsteady flow conditions at its inlet and outlet.25 There are no simple analytical solutions for the flow through such openings. The most-used equation representing flow characteristics is the so-called power law:
Q = QA(AP/p)p, (7.237)
Where Q is the airflow through the opening, CdA is the effective leakage area, and p is a coefficient.
Experimental evidence regarding the power law is somewhat contradictory. A constant value of (3 = 0.5 is considered to give a good fit to experimental data by many authors.25 According to Awbi, (3 depends on the flow regime and has a value of 0.5 for fully turbulent flow and 1.0 for laminar flow.26 In practice the value of (3 tends to be between 0.6 and 0.7.
Limited information is available on values of for industrial buildings. The effective leakage area, CdA, can be determined by means of a building pressurization or depressurization test.27’28 A range of values of Cd for cracks formed around closed windows are given in Table 7.33. These should be used with a value of p = 0.67.
For practical situations Etheridge et al. suggest representing the typical unintentional opening by a long, narrow straight pipe or duct and describing the flow characteristics by a quadratic relation between Q and AP:25
AP = aQz + bQ, (7.238)
Where a and b are constants. The first term on the right-hand side represents turbulent flow and the second represents laminar flow. The values of a and b can be obtained from experimental tests on the openings, and some values can be found in Baker et al.29
TABLE 7.33 Values of CH for Windows6
For extremely narrow openings (cracks) with deep flow paths (such as
Mortar joints and tight-fitting components) the flow is laminar and the flow rate, Q (m3/s), can be described by the Couette flow equation:26
B = length of crack, m h = height of crack, m L = depth of crack in flow direction, m (X = absolute viscosity of air, Pa s
The pressure difference across the crack can result from the difference in temperature (air density) between the air inside and outside the building. Static pressure in the vertical column of air varies with height and can be described by the following equation:
G = gravitational acceleration, m/s2 p = density of air, kg/m3
Thus, static pressure, ph, at height h can be calculated as
Where p„ — static pressure at the reference height in undisturbed flow’ outdoors, Pa. Assuming that the air density does not change along the building height, Eq. (7.241) can be simplified to
Ph = Po-g? h-
When the air temperature inside the building is greater than the outside air temperature, air infiltrates through the lower openings in the envelope (the
Pressure difference between the outside air and inside air is positive) and exfil —
Trates through the upper opening (pressure difference is negative). The height of the neutral plane where the pressure difference across the crack due to stack effect equals 0 depends upon the crack’s size, location, and characteristics.
The stack pressure can be expressed relative to the lowest opening
Height,26, 30 relative to the static pressure at the floor level,31 or relative to the
Point with the minimum static pressure.32
Static pressure over the building surfaces produced by the action of wind is generally positive on the windward side and negative on the leeward side. Pressures on the other sides of a building are negative or positive, depending on wind angle and building shape. The pressure difference across the crack produced by the action of wind can be calculated using Bernoulli’s equation:7
• AP = P — Pr = pressure difference across the crack (between outdoors and indoors) at the height of the crack, Pa
• P0 — static pressure at the reference height in undisturbed flow outdoors, Pa
• Pt = interior pressure, Pa
• Cp = surface pressure coefficient
• v = wind speed at the datum level (usually the height of the building), m/s
• p = air density, kg/m3
Principles of wind speed and surface pressure coefficient evaluation were covered earlier.
General and local supply and exhaust ventilation systems can create negative, positive, or neutral pressure in the building. Static pressure created by a mechanical ventilation system inside the building does not change with height. The pressure difference across the crack due to the mechanical system’s performance does not change with height and depends on the fan’s performance curves and the crack’s characteristics. The pressure across cracks due to unbalanced ventilation system performance depends on the difference in supply and exhaust airflow rates, AQmcch, and the effective leakage area, CdA.
The airflow rate infiltrating and exfiltrating through each air leakage pass, Q„ due to the combined effect of wind, stack, and mechanical ventilation system performance can be calculated from the mass balance equation
^ Qi + AQmech = 0. (7.244)
The airflow rate Q, for each air leakage path is expressed with Eqs. (7.237), (7.242), and (7.243) using the information on effective leakage area, CdA, and a pressure difference across the path. The total pressure acting on an opening from the outside is the sum of the pressure due to wind, gravity forces, and mechanical ventilation performance, and the static pressure inside the building results from Eq. (7.244).
The graphs in Fig. 7.100a show the pressure distribution along the building height due to gravity forces using the method suggested by Titov.32 The reference point with a pressure equal to 0 is located in the upper point outside the building. The graph shape is not affected by the number of openings and their locations. The same approach can be applied to the case with temperature stratification along the room height. The graph in Fig. 7.100h illustrates the case for the two-zone model, and the graph in Fig. 7.100c illustrates a temperature gradient.
The graphs in Fig. 7.101a show the pressure distribution along the building height due to the wind effect (wind velocity does not change along the height). The reference point with a pressure equal to 0 is located at the point with minimum surface pressure coefficient, Cp. The graph reflecting the influence of the nonuniform velocity distribution along the height is presented in Fig. 7.101 b.
Height should be considered, because the lower part of the opening can provide infiltration and the upper part exfiltration
The principles discussed in this section enable the calculation of air infiltration/exfiltration, provided that the following quantities are known or can
• Wind speed and direction
• Internal and external air temperature
• Position and flow characteristics of all openings
• Pressure coefficients over the building envelope for the wind directions under consideration
• Supply and exhaust ventilation airflow rates
In practice it is difficult, if not impossible, to determine all these quantities accurately, and the following simplified calculation methods based principally on equations discussed in this section are used:
• Empirical methods6,7
• Simplified theoretical methods8,30,31,33
• Network models, primarily for multizone buildings10,34
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