Preferred numbers
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 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 |
Further 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