General notes on exhausting


Pipe dia mm

Mouth square

C-

100

100

75

200

200

748

Detailed designs for hoods to suit most applications may be found in the standard design manuals produced by machinery manufacturers and also in Industrial Ventilation published by ACGIH®.

A point to note is that, in all dust extract work, the hood should be fitted to enclose the source of the dust as much as possible, whilst in fume extract the hood should be reasonably close to the area of evolution. These considerations should be clear from a basic study of air flow into exhaust openings.

In dust extract, the principle is to so design the hood that the particles are thrown from the point of generation directly into the throat of the hood. Grinding wheels, as an example, may be re­volving with a peripheral velocity of 30 m/s.

For grinding wheels the throat velocity (see Figure 3.53) should be about 5 m/s to 5.5 m/s with a duct velocity from 17.5 m/s to 20 m/s. Normal duct sizes vary from 75 mm to 180 mm diameter and are generally standardised by manufacturers for their own types and size of wheels.

General notes on exhausting

Figure 3.53 Grinding wheel hood showing throat and maximum enclosure

Overall system resistances for complete dust extract systems are typically in the range of 1000 Pa to 1500 Pa although exten­sive systems may reach higher values.

The cutters on wood working machinery, such as planers and moulders must be hooded so that the chips are thrown directly into the throat of the hood.

The air velocity into the opening of the hood around the cutters may be from 5.5 m/s to 8 m/s. The duct velocity is typically from 20 m/s to 22.5 m/s on chips and from 16 m/s to 17.5 m/s on saw­dust. The duct connections to each hood range from about 75 mm to 180 mm diameter. System resistances are typically from 1250 Pa to 1500 Pa but for larger more extensive systems, could be higher.

For extract from spray booths the velocity into the open side of the enclosure may be from 0.5 m/s to 1 m/s with a general aver­age of 0.75 m/s to 0.8 m/s.

In fume hoods over appliances the velocity into the actual open­ing may be as low as 0.25 m/s up to 1 m/s. The duct connection to the hood may have a velocity of 7.5 m/s to 10 m/s with the main ducting sized for 12.5 m/s to 15 m/s. Plant overall resis­tance with fume discharged direct to atmosphere can be as low as 250 Pa to 325 Pa. However, the addition of fume collection equipment to give a clean discharge to atmosphere can add considerably to this figure.

For further information on dust and fume hoods referto Chapter 21, Section 21.7.

Sizing of extract grilles for HVAC plant

The sizing of extract grilles is very similar to the method de­scribed for those used for supply air. Again, due to the wide range available it is recommended that the manufacturer should be consulted. In general, noise becomes an important factor and the “throw” does not arise. The maximum velocity of the entering air is suggested to be as follows:

• Boardrooms and private offices 3.5 m/s

• General offices 5.0 m/s

• Industrial applications 7.5 m/s

The velocity of the air entering the grille is affected by the vane angle. For straight vane grilles, multiply the selected velocity by the factor in Table 3.5 to obtain the upstream velocity after the vanes.

Vane angle degrees

0

10

20

30

40

45

Factor

1

0.98

0.94

0.86

0.76

0.71

Table 3.5 Factors for straight vane grilles

For most straight vane grilles it is usual to allowa mean factor of

0. 85. The resistance is assumed to be equal to the velocity pressure at the air entry. Sizing is based on the upstream veloc­ity (i. e. immediately after the grille).

Posted in Fans Ventilation A Practical Guide


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