Preferred numbers

22.4.1 General

Standardisation is necessary if products are to be sold across national frontiers without problems of installation and opera­tion. This applies in particular to dimensions to ensure that products can be purchased to a standard with the knowledge that it will fit in place of the same basic product manufactured by a different company or indeed manufactured in a different coun­try. The general principle can also be applied to output ratings and in turn standard ratings can be related to a set of standard

Principle dimensions. The electric motor industry was one of the first to adopt standard dimensions and ratings for a wide range of products and this is universally accepted world-wide. The same basic principle has more recently been extended to other products, for example gearboxes of particular interest to fans. There is also considerable standardisation throughout in­dustry on parts that are used in the make up of fans and a good example is the Standards applicable to fixings such as bolts and screws. With respect to the principle dimensions of fans, a pre­ferred number series forms the basis.

The problem of deciding what range of sizes to produce for any particular design ought to be considered at an early stage of its life cycle. Unfortunately, in the past, this was frequently not the case, and manufacturers often produced unique sizes to suit a favourite customer, without consideration of its effect on the competitive advantages it might have for other applications.

The demand of consumers for an infinite range of choice has to be balanced against any increase in the cost of production, stocking and distribution, which might result from the introduc­tion of too many types and sizes.

Long experience within the industry has shown, however, that sizes following a geometric progression can satisfy demand, as they are in harmony with the fan laws.

Preferred number series

Preferred numbers used for fans are therefore based on a geo­metric series where each number has a common ration be­tween it and the adjacent number. This suits both large and small numbers in a particular range. Table 22.7 shows pre­ferred numbers in the R20 and R40 series. These series give roughly a 12% and 6% increase respectively to the next highest number in the series.

R20

R40

R20

R40

R20

R40

4.5

4.5

45

45

450

450

4.75

47.5

475

5

5

50

50

500

500

5.3

53

530

5.6

5.6

56

56

560

560

6

60

600

6.3

6.3

63

63

630

630

6.7

67

670

7.1

7.1

71

71

710

710 J

7.5

75

750

8

8

80

80

800

800

8.5

85

850

9

9

90

90

900

900

9.5

95

950

1000

1000

Table 22.7 Preferred numbers in R20 and R40 series

As previously stated, the fan laws indicate that a geometric pro­gression in sizes is to be desired whilst BS 2045 1965 and PD 6481 1977 favour Preferred Numbers. These are arbitrarily rounded off values derived from a geometric series having one of the common ratios shown in Table 22.8.

I -■ Series

R5

R10

R20

R40

R80

Ratio

Яfф

1^ОФ

20VTц

AoM

=

1.58

1.26

1.12

1.06

1.03

Table 22.8 Renard series ratios

Preferred numbersFurther information is given in IS0497. Subsequently, Euro­vent produced their document 1/2 for the sizes of fan circular flanges and this first formally recognised the R20 series for a range of standard light duty fans. This was then adopted inter­nationally as ISO 6580.

More recently a Standard, ISO 13350 covering fan sizes, circu­lar flanges and rectangular outlet/inlet flanges, has been pro­duced, again based on an R20 series and covering fan sizes from 100 mm to 2000 mm. Customers are urged to specify this Standard, so that the situation current in the motor world, will eventually be replicated in the fan world. A level playing field may then be achieved and competitive products will be fairly compared.

These sizes are now widely used in Germany for both axial and centrifugal fans. They are also dominant in the UK axial fan market.

100

112

125

140

160

180

200

224

250

280

315

355

400

450

500

560

630

710

800

900

1000

1120

1250

1400

1600

1800

2000

Table 22.9 R20 series for fan sizes and flanges as standardized in ISO 6580 and ISO 13350

By using the rounded numbers, the divergence from the theo­retical value nowhere exceeds 1.22% as can be seen by refer­ence to Table 22.10. The similarity with the standard frame sizes of electric motors, although the latter have used a frame 132 from the R40 series in preference to the adjacent sizes from the R20 series, is striking.

Renard series

Percentage difference

(rounded)

Between R20 series

Basic logarithm

Calculated value

R20

And calculated values

2.00

100.00

100

0

Theoretical values

Renard series (rounded)

Percentage difference between R20 series and calculated values

Basic logarithm

Calculated value

R20

2.05

112.20

112

-0.18

2.10

125.89

125

-0.71

2.15

141.25

140

-0.88

2.20

158.49

160

+0.95

2.25

177.83

180

+1.22

2.30

199.53

200

+0.24

2.35

223.87

224

+0.06

2.40

251.19

250

-0.47

2.45

281.84

280

-0.65

2.50

316.23

315

-.039

2.55

354.81

355

+0.05

2.60

398.11

400

+0.47

2.65

446.68

450

+0.74

2.70

501.19

500

-0.24

2.75

562.34

560

-0.42

2.80

630.96

630

-0.15

2.85

707.95

710

+0.29

2.90

794.33

800

+0.71

2.95

891.25

900

+0.98

3.00

1000.0

1000

0

3.05

1122.0

1120

-0.18

3.10

1258.9

1250

-0.71

3.15

1412.5

1400

-0.88

3.20

1584.9

1600

+0.95

3.25

1778.3

1800

+1.22

3.30

1995.3

2000

+0.24

Table 22.10 Calculation of R20 series

Posted in Fans Ventilation A Practical Guide


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