Aaberg Exhaust Hoods

10.4.4.1 General

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.

In order to overcome this limitation in the use of conventional exhaust hoods, Aaberg exhaust hoods (or reinforced exhaust systems) have been devel­oped where the exhaust extracting contaminated air is reinforced by an in­jected supply jet flow that enhances the exhaust flow — considerably in comparison to other outlets. Through a balanced combination of the injection and exhaustion flows, the airflow toward the exhaust opening, in the region where there is the largest concentration of the contaminant, may extend over a far greater distance than is possible by using an exhaust alone.

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.

Jet

1 Jet

The design of the axisymmetrical Aaberg exhaust hood is very similar to a traditional flanged hood. However, it is fitted with a flange through which air can be ejected radially from a narrow slot (see Fig. 10.77). The dramatic effect of the blowing jet on the hood’s overall airflow can be ex­plained as follows: due to the friction developed at the radial jet/air inter­face an entrainment flow develops which, under the correct conditions, has the property of removing the clean air from in front of the hood (the recycled flow) as well as enhancing and concentrating the exhaust’s suc­tion in a zone along the hood’s longitudinal axis (the efficient flow). The flow in front of the exhaust opening is now directional and the process is capable of creating a larger fluid flow toward the exhaust opening at greater distances along the axis of the exhaust hood. Further, although re­placement air should still be supplied, the Aaberg exhaust works with sig-

Nificantly smaller quantities of air than do traditional exhausts. This, together with a higher concentration of pollutant in the exhaust air. makes the Aaberg process for limiting pollutant emissions more effective than traditional methods. The performance difference between a conven­tional exhaust hood and the Aaberg system can be seen in Figs. 10.78j and 10.786, which show the effects of smoke released at a distance five times the diameter of the exhaust inlet when there is no radial jet, and when rhere is a jet. When there is no radial jet the pollutant enters the en­vironment and only a few wisps of the contaminant are successfully ex­hausted. However, when the radial jet is present, virtually ail the contaminant is captured.

Figure 10.79a shows the typical streamlines for a conventional tube exhaust hood with no radial jet. At large distances from the exhaust opening the flow is approximately radial and behaves very much like a sink flow. The shaded area is

(ji When the radius of the exhaust hood is 0.15 m, the radius of the exhaust hood inlet is 0.037 m, and the width of the exhaust |et Is 8.0 mm. (a) Suction alone with average inlet velocity of 12.7 m s ‘. (b) Com­bined suction and Injection with the average inlet and exhaust velocities of 12.8 m s*1 and 7.7 m s_l. respectively. (Figures are courtesy of the Health and Safety Executive. Research Division. Sheffield. UK.)

The region where the fluid speed is greater than the random air speed that is gen­erated in a typical workplace. Thus al! the contaminants in the shaded region, the effective capture region, will be exhausted. For the same rate of exhaustion but with the inclusion of the radial jet, i. e., the implementation of the Aaberg princi­ple, Figs. 10.7% and 10.79c show the streamlines and the effective capture re­gion for increasing values of the momentum of the jet. The streamlines assigned a value of magnitude less than unit)’ all go to the exhaust outlet whereas all the other streamlines form a large recirculating region in the room. Thus the effect of the jet is to concentrate the fluid that flows through the exhaust outlet along the axis of the exhaust hood. Further, the distance from the exhaust hood that the ef­fective capture region extends increases with increasing momentum of the jet. The above discussion clearly explains the reasons for the fluid and contaminant flow observed in Fig. 10.78. It should be noted that in addition to the Aaberg principle being used for a local ventilation system it can be used in the form of a large ventilation unit (see Fig. 10.80). In this case the Aaberg exhaust hood is freely suspended with its axis pointing downward and the contaminant is placed on the floor. This situation has also been investigated by Hunt and Ingham.25“’’

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

10.4.4.2 Principle

Bench Aaberg Slot Exhaust

A schematic diagram of the version of the Aaberg slot exhaust (ASE) system is shown in Fig. 10.81. It consists of a horizontal bench to which a vertical flange is attached, housing a rectangular exhaust slot and jet nozzle. Figure 10.82 shows the two-dimensional geometry and the coordinate system of the ASE model.

This situation has also been experimentally investigated by Braconnier et al.,-7 Pedersen,28,29 Fletcher and Saunders’0 and Hollis. n All of these investiga­tions show that as the ratio of the momentum of the exhaust to the inlet flow, increases, the efficiency of the collection of the contaminant increases. Here I is defined as Uj b/ujS where It, and U, are the initial average velocities of the air at the exit of the jet exhaust and the entrance of the slot inlet, respectively, and B And S are the widths of the slot and jet nozzles, respectively. See Fig. 10.81.

The investigators found that the best operating conditions of the Aaberg in disturbed surroundings was when 1 was approximately 0.6. Further, when the momentum rate, i, becomes too large, then the fluid flow becomes un­steady and highly three-dimensional, with large swirling flows being generated at some distance from the exhaust opening. This flow enhances the level of the turbulence, reducing the capture efficiency by enhancing the diffusion of the contaminant into the induced jet flow.

Axisymmetrlc Aaberg Exhaust Hood

The Aaberg reinforced exhaust system was first studied in the 1940s,32 then more extensively in its axisymmetrical form in 1965, but it was not until

Y

FIGURE 10.82 The geometry and coordinate system of the ASE model.

The 1980s that researchers experimentally attempted to combine the injection and the exhaustion principle; see Hyldgard33 and Hogsted.34 Pedersen and Nielsen35 measured the centerline fluid velocity for three types of Aaberg flows, whereas Fletcher and Saunders used a laser Doppler velocity (LDV) flow analyzer to measure the fluid speeds.

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.

10.4.4.3 Applicability of Sources

In general, the Aaberg principle is suitable for all open “processes” which demand an open work area. These processes avoid the use of closed or partially closed systems, for instance a conventional hood with specific walls around the pollution source. In particular, the principle is very suitable for welding (i. e., spraying particle sources) or hot sources of contaminant. Some examples of open
processes where the Aaberg principle may be of use are as follows: removal of pollution by solvents in printing shops and spraying particles and solvents in color spraying units in small car repair and wood spray shops. A good descrip­tion of the use of an Aaberg hood in a welding shop may be found in Pedersen.28

10.4.4.4 Specific Issues

It should be noted that although optimum operating conditions for the Aaberg exhaust system might be accurately found under ideal working condi­tions, the following points should be considered:

Cross-drafts;

Finite size of the room;

Moving sources of contaminants and moving workers;

Moving particles;

The flow becomes fully three-dimensional, i. e., the flow becomes unstable, when the momentum ratio is too large. This is typically when I s 0.6;

Noise levels;

Hysteresis/short circuiting (see Pedersen28) namely I — 0.1;

In addition to the momentum ratio I, numerous other geometrical aspect ratios should be investigated.

10.4.4.5 Design Equations and/or Parameters

In the work of Hunt and Ingham25*26’36 it was found that the experimental predictions, the LDV measurements, the potential flow predictions, and the full turbulent fluid flow calculations all give results that are in extremely good agreement when the Aaberg principle is used both in the axisymmetrical and bench configurations.

Thus it is recommended the simple potential flow model be used to obtain a first estimate for the optimization of the effective capture region in any par­ticular application. Once this has been achieved, the equipment should be built to this specification but with sufficient flexibility to adjust it to obtain the practical optimum effective capture region.

Potential Flow Model for a Bench Slot Exhaust

Outside the jet and away from the boundaries of the workbench the flow will behave as if it is inviscid and hence potential flow is appropriate. Further, in the central region of the workbench we expect the airflow to be approxi­mately two-dimensional, which has been confirmed by the above experimen­tal investigations. In practice it is expected that the worker will be releasing contaminant in this region and hence the assumption of two-dimensional flow appears to be sound. Under these assumptions the nondimensional stream function ‘P satisfies Laplace’s equation, i. e.,

^ + ^ = 0, (10.110)

Ax2

Where

And U and V are the nondimensional X and Y components of the fluid veloc­ity, respectively, where we have nondimensionalized as follows:

X = x/p, Y = y/p, R = R/p, S = S/p, Hs = Bs/p, ^ U; = «,/ (m/p)

(10.112)

1/2

3 Bu0p cr

A V A 2m

(T is an empirical constant which has been taken to be 7.67,2′ and 2

Erfc(z) = 1 — erfc(z) = 1

‘7*

Sin

(A-l)f

A-2

A(A — 1)R

7T 2‘

(A-2)

Sin

(A-2)J

2! sin

26 TT

1 —

‘ +

A-l

A R

10Y X-l

Y1/7erfc

DY,

Where M is the volumetric flow rate of the exhaust and P is the height of the jet nozzle above the bench surface, equal to A + hs. For a turbulent jet, at the outer edge, we obtain25

|/.

A(X- 1/

(10.113)

For a free jet,

(10.114a)

For a wall jet,

(10.114b)

M

Where erfc is the well-known error function. In the case of the free jet, the solution for the Aaberg exhaust system can be found by solving the Laplace equation by the method of separation of variables and assuming that there is no fluid flow through the surface of the workbench. At the edge of the jet, which is assumed to be at 0=0, the stream function is given by Eq. (10.113). This gives rise to ^(R, 6) = F(R, 6)

( I — / 1 A’ T) A « R sin A 77 2 Sin A L-e J _ V J —

— ( > (A-l) F-fl _ )

Sin

(10.115) The exhaust opening is modeled as a finite-sized slot with a uniform ve­locity distribution. The workbench and the vertical wall below the exhaust slot form a streamline of fluid flow through which the fluid does not cross and, therefore, along this line we have = 0. Between the slot and the jet, the vertical wall is also a streamline and from the dimensionalization given

In Eq. (10.112) we have f = 1 on this part of the boundary. Thus we have the following boundary conditions:

Ty(0, Y) =0

0 (X — 1 1 + A(X-1./

H)/S

Ґ(X,0)

0 < Y < °° 0 < X < H H < X < H + S, H + S < X < 1 X > 1 R = L » 1

(10.116)

V(X, Y) = F(R, 0)

9 "? 1 / ^

Where is a large value of (X — + Y") ” at which the asymptotic solution (Eq. (10.115)) may be considered appropriate. The potential flow resulting from the solution of the Laplace equation (Eq. (10,110)) subject to the boundary condi­tions (Eq. (10.116)) may be easily solved using boundary integral techniques, e. g., the boundary element method,37 or the Schwartz-Christoffel transformation tech­nique. However, although these methods give a more elegant mathematical solu­tion of the above problem they are more complex to use than the more straightforward finite-difference approach. In this method the solution domain is divided into a system of rectangular meshes, see Chapter 13 for more details, and an approximate solution may be found at all of the mesh points.

Turbulent Flow Model for a Bench Slot Exhaust

The mathematical model presented in the previous section has been devel­oped under the assumptions that the flow induced by an Aaberg exhaust hood is inviscid and potential and that turbulent effects have been limited to the flow in the jet. However, the typical experimental operating conditions of an Aaberg ex­haust hood lead to Reynolds numbers in the order of 103 to 104. Thus the fluid flow in the jet and in the region surrounding the exhaust inlet are very likely to be turbulent. However, in the region of practical interest, i. e., the region of the flow where there is likely to be large amounts of contaminant, the airflow created by the Aaberg exhaust hood is a convergent flow and therefore in this region we ex­pect a low level of turbulence.

Hunt38 and Kulmala39 have solved the full turbulent fluid flow for the Aaberg system using the K-e turbulent model or a variation of it as described in Chapter 13—the solution algorithm SIMPLE, the QUICK scheme, etc. Both commercial software and in-house-developed codes have been employed, and all the investigators have produced very similar findings.

Mathematical Model for an Axisymmetric Aaberg Exhaust Hood

A similar mathematical model to that just described for bench slot ex­hausts can again be used, but in this case the Laplace equation should be em­ployed in a cylindrical coordinate system (see Fig. 10.83), namely,

= 0

___ cot0 S’P, 1 D2V

,2

No. n:

Thus the Laplace equation (Eq. (10.117)) has to be solved. subject to the following boundary conditions:

V(R,0) = 0

J—‘R2 0 sR<S

(10.123)

LUj

( V /

WIR, Ф) = g(R, В) R = R

Where R„ Is a sufficiently large value of R at which the asymptotic solution (Eq. (10.115)) may be assumed to be appropriate. The solution to this prob­lem may be found using a finite-difference method as described in Chapter 13 or see Hunt and Ingham.25’26’36

As with the two-dimensional workbench problem, the numerical solution of this problem can be found by solving the full turbulent fluid flow equations using the methods described in Chapter 13.

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