Airflow through Large Openings and Gates

When the building envelope has high openings (e. g., gates or large glazing), the pressure difference across such openings changes with height. In some situ­ations, airflow through a part of the opening enters the building and the air­flow through the rest of this opening exits the building. In certain cases this can be neglected, and the pressure difference across the whole opening is as­sumed to be equal to the one at the center of this opening. However, for open­ings such as high gates, such simplification results in significant error.

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

Airflow through Large Openings and Gates

The inside air static pressure, P0, at the lower part of the opening, h > b0, and is lower at its upper part, h < h0. Thus, air is infiltrating through the lower part of the opening and exfiltrating through its upper part.

The airflow rate through an opening area with a height of dh can be cal ­culated as

DG = Cd(2Phy’HP0M-P0)"2dh. (7.245)

The infiltrating and exfiltrating airflow rates can be calculated using the fol­lowing equations:

Cj — I dG (/.246)

подпись: cj — i dg (/.246)R h

In

Gexf=PW (7.247)

Jh

Let’s consider airflow through the gate under the influence of buoyancy forces and wind (Fig. 7.103). The gate has width b and height hg and is lo­cated on the upwind face of the building. The gate pressure loss characteristic does not change with height. Also, assume there are no apertures or cracks in the building envelope, other than the gate.

The mass balance equation for the airflow through the gate can be de­scribed as

Gexf + Ginf = 0. (7.248)

At the height ha of the boundary between the infiltrating and exfiltrating flows, air pressure across the gate, Pout — P0, equals 0, and thus

P0 = Apg/>0 + Cgatc^. {7-249)

Assuming (Pout/Po)173 “ 1 ^ the equation allowing calculation of the pres­sure inside the building can be presented as

P0 = y°APg + Cgate^. (7.250)

A properly designed air curtain protects the building from outside airflow through the gate. Only air entrained by the air curtain enters the building through the gate.

Total gate protection occurs when the air flow rate exiting the building through the gate, Gex equals the air flow rate, Ga, supplied through the air curtain slot:

Gex = G0 (7.251)

When the gate is totally protected, there are no other openings in the

Building envelope and there is a balance between supply and exhaust ventila­

Tion systems, h0 = hgate (Fig. 7.104). In this case the pressure difference across the gate is influenced only by the buoyancy forces, and the airflow through the gate can be evaluated using the following equation:

^ex — ^d-^ot^gate^KPout — Po)Pout]> (/.252;

Airflow through Large Openings and Gates

IHI FIGURE 7.104 Schematics of airflow through the gate with an air curtain, for a building having no other openings.

Where: Cd is the pressure loss coefficient for the gate; Cd typically has a value between 0.2 and 0.3 (it can be assumed equal to 0.25). For more information about gate protection with air curtains, see Section 7.7.

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