Exhaust hood Aaberg

Simple exhaust hoods have a very short effective range and the hood must be placed very close to the contaminant source to be efficient, which may interfere with technological processes. This lack of direction of the flow may result in the use of excessive exhaust flow rates with large source-to-hood distances and this may result in a large amount of wasted energy. 

The two versions of the Aaberg exhaust system, namely an axisymmetri-cal version and a workbench version, both work on the same principle. In order to illustrate the principle of the Aaberg we describe the axisymmetrical version but the full theoretical, computational, and experimental basis is presented for both systems. 

FIGURE 10.79 Typical streamlines for the flow near the exhaust hood when there is (o) only suction. (b) some exhaust Bow, and (cl a large exhaust flow. (The flow is symmetrical about X = 0.) The shaded area represents the predicted effective capture region. 

FIGURE 10.80 A typical Aaberg ventilator unit. v hich is suspended above the floor white smoke is released on the floor beneath the venciiator. 

Finaliy, the same principle as described above applies to the bench version of the Aaberg principle, which we discuss in detail in the next section, 

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Mathematical Model for an Axisymmetric Aaberg Exhaust Hood... 

Plane jets could be used to create a closed volume in which a contaminant source could be placed. In some ways, these systems are similar to Aaberg exhaust hoods (Section 10.4.4). The objective is to use plane jets instead of walls around an exhaust opening to create a vortex which enhances the capture efficiency of the exhaust. 

G. R. Hunt. The fluid mechanics of the Aaberg exhaust hood. Ph.D, thesis, University of Leeds, 1994. 

It should be noted that when there is no jet reinforcement of the flow, i.e., the exhaust hood is used in its conventional mode, then in the two-dimensional form of the Aaberg principle the fluid flow velocity due to the exhaust decays approximately inversely proportionally to the distance from the exhaust opening. However, for three-dimensional exhaust hoods the fluid velocity outside the hood decays approximately inversely as the square of the distance from the exhaust hood. Thus in the three-dimensional conventional hood operating conditions the hood has to be placed much closer to the contaminant in order to exhaust the contaminant than is the situation for the two-dimensional hood (see section on Basic Exhaust Openings). Thus for ease of operation it is even more vital to develop hoods with a larger range of operation in the three-dimensional situation in comparison with two-dimensional hoods. 

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