Building envelopes

Airflow through intended and unintended openings and cracks in the building envelope (Section 7.8.2), and aerodynamic means of the large opening protection, described in Section 7.7 ... 

Outdoor air is generally less polluted than the system return air. However, problems with reentry of previously exhausted air occur as a result of improperly located exhaust and intake vents or periodic changes in wind conditions. Other outdoor contamination problems include contaminants from other industrial sources, power plants, motor vehicle exhaust, and dust, asphalt vapors, and solvents from construction or renovation. Also, heat gains and losses through the building envelope due to heat conduction through exterior walls, floor, and roof, and due to solar radiation and infiltration, can be attributed to effects from external sources. 

This section discusses primarily internal sources of contaminants and other occupational hazards related to the process or the building envelope. 

Heat losses and gains by heat conduction through the building envelope... 

There are two large openings in the upper part of the building Uqj, which is 4 m wide x 2 m high, and Uq2, which is 2 m wide x 1 m high. The lower-level of 5 m. Otherwise the building envelope is assumed to be airtight. This information is summarized in Table 7.29. 

A]R MOVEMENT AROUND BUILDINGS AND THROUGH A BUILDING ENVELOPE... 

This section will describe general features of airflow patterns and then present information on the dimensions and locations of recirculating (stagnant) zones around the building envelope, which determine wind pressures and contaminant dilution. This knowledge allows one to select the locations of stacks and air intakes and to calculate infiltration and natural ventilation rates. 

For a building with sharp corners, Cp is almost independent of the wind speed (i.e., Reynolds number) because the flow separation points normally occur at the sharp edges. This may not be the case for round buildings, w here the position of the separation point can be affected by the wind speed. For the most common case of the building with a rectangular shape, Cp values are normally between 0.6 and 0.8 for the upwind wall, and for the leeward wall 0,6 < C, < —0.4. Figure 7.99 and Table 7.32 show an example of the distribution of surface pressure coefficient values on the typical industrial building envelope. 

FIGURE 7.99 Example of surface pressure coefficient values for a typical industrial building envelope. 

In general, the air leakage rate through a building envelope is dependent... 

Pressure coefficients over the building envelope for the wind directions under consideration... 

The pressure change outside the building envelope with such an opening is illustrated in Fig. 7.103. The outside air static pressure, is greater than... 

When the gate is totally protected, there are no other openings in the building envelope and there is a balance between supply and exhaust ventilation systems, (Fig. 7.104). In this case the pressure difference across... 

Static pressure difference between two zones resulting from the wind effect on the building envelope (Fig. 7.108fe). 

FIGURE 11.31 Radiaiion fluxes at the buildirtg facade the solar radiation components (direct or beam, diffuse, and reflected radiation from the ground or other buildings) and the components of the radiation back from the building facade (reflected solar and thermal infrared radiation from the building envelope). 

A fictive sky temperature, dependent on ambient temperature, emissivity, and cloudiness, is introduced to account for the long-wave radiative heat exchange between the building envelope and the sky. 

Infiltration and ventilation are driven by pressure differences across the building envelope, which are caused by... 

Leaks in the building envelope, due to cracks in walls or joints, gaps in window and door frames... 

The absolute, barometric pressure is not normally required in ventilation measurements. The air density determination is based on barometric pressure, but other applications are sufficiently rare. On the other hand, the measurement of pressure difference is a frequent requirement, as so many other quantities are based on pressure difference. In mass flow or volume flow measurement using orifice, nozzle, and venturi, the measured quantity is the pressure difference. Also, velocity measurement with the Pitot-static tube is basically a pressure difference measurement. Other applications for pressure difference measurement are the determination of the performance of fans and air and gas supply and e. -haust devices, the measurement of ductwork tightness or building envelope leakage rate, as well as different types of ventilation control applications. 

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