Blowing outlets

When air is discharged from an outlet, the perimeter of the airstream is slowed down by contact with the surrounding air, which is induced onto the primary airstream as secondary air. The jet in consequence expands with distance from the outlet.

As stated previously, an unconfined jet of air in free space has an included angle of about 3°, but due to the induction of sec­ondary air, its “spread” is increased to around 14-16°. Ameri­can tests on air blasts from circular outlets are shown in Figure 3.22, the results being plotted for the percentage of the initial velocity on the centre line at distances measured in diameters.

Blowing outlets

Figure 3.22 Diffuser fitted to a centrifugal mine fan

•tmosphate

Negativ« pressure here

О T

In the 1950s, Sturtevant Engineering Company made tests on three blast outlets of virtually the same cross-sectional area:

229 mm diameter

203 mm square

And 254 mm x 165 mm

When plotted on a basis of initial velocity on the centre line

Against distance m results were virtually on the same Voutlet area m2

Blowing outlets

Distance from opening in diameters Figure 3.23 Circular blast outlets

Curve and were in reasonable agreement with the American tests shown in Figure 3.23.

Hence, provided the long side of the rectangle is not more than

1.5 x the short side, the chart may be used by taking the cross-sectional area of the square or rectangular outlet and converting it into the corresponding equal area circular outlet.

Table 3.2 is a numerical equivalent of Figure 3.23.

Percentage of initial centre line velocity

Distance m Diameter m

Distance m )Area m2

90

3.0

3.38

80

4.4

4.95

70

6.25

7.05

60

8.5

9.6

50

11.0

12.4

40

14.5

16.4

30

19.0

21.4

20

24.0

27.1

10

31.0

35.0

5

36.0

40.6

Table 3.2 Circular blast outlets For example:

For 50% of the initial centreline velocity, the distance in metres from the outlet

= diameter of outlet m x 11 = Voutlet area m2 x 12.4

Slot width mm

Blowing outlets

Blowing outlets

0 0.123 0.29 0.375 0 5 0.625

Equivalent diameter metres

Tests have also shown that the ratio of centreline velocity to av­erage velocity is about 3.0 irrespective of outlet size, shape or initial velocity between 10 and 50 diameters from the outlet. In industrial ventilation, maximum velocity is usually the important factor from the viewpoint of draughts on persons. More recent data has suggested that the “blow” is to much greater dis­tances, but practical experience suggests otherwise. It may be that this data is based on theory unsupported by actual site tests.

Narrow slot outlets require a different approach as the rate of fall in velocity with distance is greater. Figure 3.24 shows the equivalent diameter in metres against slot length for various slot widths, and is based on American data.

For example a slot on 762 mm x 76 mm is equivalent in perfor­mance to a 216 mm circular outlet. Its throw may then be deter­mined from Figure 3.23 in the normal way. No practical confir­mation has been made for all the combinations and it would be wise to restrict its use to slots of less than 76 mm long. Figures 3.25 and 3.26 show tests on a 914 mm x 38 mm slot, which sug­gest some caution.

Oscar Faber and John Kell used multiple nozzles to introduce ventilating airfrom high level in the original ventilation system at

Blowing outlets

Diameters on USA Slot chart

Figure 3.25 Slot blast outlet

Blowing outlets

Figure 3.26 914 mm x 38 mm slot showing throw

Blowing outlets

— Noise __ _____ __ Noise. Noise ______________________

Reasonable increases excessive

Rapidly

Figure 3.27 Faber’s tests on round nozzles

The construction of the Earls Court exhibition main hall in Lon­don (311500 m3and 23000 people maximum).

The published tests results for which this nozzle scheme was designed are shown in Figure 3.27. These results are in general agreement with the practical experience of many engineers.

With Faber’s design of nozzle fixed on the end of a short duct from a main duct, the static pressure required, as measured in the branch duct is given by:

P (Pa) = 0.535 (vel m/s at nozzle mouth)2 Equ 3.33

At an area ratio of 0.535 the value of K^ 1.06. Readers may like to do the mathematical manipulation to justify the formula.

Punkah louvres

Another application of nozzles is in the cabin ventilation of ships where even today, Punkah louvres are used. These have the advantage that they can be swivelled to vary the direction of blow, to suit the particular preferences of the occupiers (see Figure 3.28).

Blowing outlets

A standard range has been developed over the years in accor­dance with Table 3.3.

Dia of outlet mm

Dia of “ball” mm

M3/s at 125 Pa

K

25

50

0.007

1600

37.5

75

0.016

700

50

100

0.028

400

62.5

125

0.049

228

73

150

0.064

175

Table 3.3 Performance details for Punkah louvre range

The makers claim that the pressure loss Pa = (7n3/s xkY

Posted in Fans Ventilation A Practical Guide


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