AIRFLOW NEAR EXHAUSTS

7.6.1 Introduction

The pollutant-capturing efficiency of local ventilation systems depends on hood design, the hood’s positioning near the source of contamination, and the

Exhaust air flow. The type and the size of the hood depends on the type and geometry of the pollution source and its characteristics (see Chapter 10 for de­tails). Contaminant movement in the source vicinity is specific to the source type. Pollution sources can be classified (see Section 7.2) as

• A nonbuoyant (diffusion) source

• A buoyant (heat) source

• A dynamic source

The first type of source is characterized by contaminant diffusion in the room in all directions due to the concentration gradient in all directions (e. g., emission from a painted surface). The emission rate in this case is significantly affected by the intensity of the ambient air turbulence and air velocity. With the second type of source, contaminants move in the space primarily due to heat energy as buoyant plumes over the heated surfaces. The third type of source is characterized by contaminant movement in a space with an air jet (e. g., a linear jet over the tank with push-pull ventilation) or particle flow from a grinding wheel. In some cases these factors influencing contaminant distribution are combined.

The geometry of the contaminant source can be compact or linear. The source geometry affects the hood geometry: round, rectangular, or slot.

Hoods are either enclosing or nonenclosing. Enclosing hoods provide bet­ter and more economical contaminant control because their exhaust rates and the effects of room air currents are minimal compared with nonenclosing

Hoods. For more detail regarding nonenclosing hoods, see Chapter 10.

For nonenclosing hoods, the airflow rate that allows contaminant cap­ture is called a target airflow.1 The target airflow rate q$ is proportional to some characteristic flow rate Q0 that depends on the type of contaminant source:

<70 = kq, (7.205)

Where

If is a dimensionless coefficient depending on the hood design

Q is the characteristic airflow rate depending on the contaminant source

For a nonenclosing hood with a nonbuoyant contaminant source, the charac­teristic airflow can be calculated using the following equation:

Q=V0-A0, (7.206)

Where

V0 — average air velocity in the hood opening that ensures capture velocity

At the point of contaminant release, m/s

A0 = hood opening area, m2

For a buoyant source q can be equal to the airflow in the convective plume at the hood suction cross-section. For a dynamic source q can be equal to the airflow rate in the jet.

An exhaust airflow rate lower than results in reduced contaminant — capturing effectiveness. An exhaust airflow rate greater than q§ results in ex­cessive capturing effectiveness (Fig. 7.81).

(a) (b) (c)

FIGURE 7.81 Hood performance for different exhaust airflow rates, (o) Target airflow rate q = q* (b) Target airflow rate q < q*. (c) Target airflow rate q > q*.

Capture Velocity

The capture velocity is the air velocity at the point of contaminant generation upstream of a hood. The contaminant enters the moving airstream at the point of generation and is conducted along with the air into the hood. The concept of capture velocity is primarily used by designers to select a volumetric flow rate for withdrawing air through a hood in the case of a nonbuoyant contam­inant source. Ranges of capture velocities for several industrial operations are listed in Table 7.21.2 The values for capture velocities are based on successful experience under ideal conditions. For the given hood, if design velocities any­where upstream of the hood are known [v = f (q0, x, y, z)], the capture veloc­ity is set equal to vc at the point (x, y, z) where contaminants are to be captured and q0 is found. To ensure that contaminants enter an inlet, the transport equations between the source and the hood have to be solved.

Airflow near the hood can be influenced by drafts created directly by the supply air jets (spot-cooling jets) or by turbulence of the ambient air caused by the jets, upward/downward convective flows, moving people, and drafts from doors and windows.

Process equipment may be responsible for other sources of air movement in the room. For example, high-speed rotating machines such as pulverizers, high-speed belt material transfer systems, falling granular materials, and com­pressed air escaping from pneumatic tools all produce air currents.

These factors can significantly reduce the capturing efficiency of local ex­hausts and should be accounted for by the correction coefficient on room air movement, Kr > 1, in Eqs. (7.205) and (7.206). For example, Eq. (7.206) is replaced with

<?o = Krkq0 . (7.207)

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