Constructional features

Fans have developed over a very long period of time and are therefore considered to be a “mature” product. As with automobiles, this means that there are remarkable similarities between the competing products of different manufacturers. Whilst the more cynical amongst us will put this down to blatant copying, it should also be recognised that once a buyer’s specification is sufficiently detailed and has been established for a length of time, then the resulting solutions will also be remarkably similar.

Thus, just as all “super minis” in the car world look much the same, so it is with Category 1 fans. Just as all medium sized saloon cars exhibit considerable similarities, so do Category 2 fans. It is only with purpose-made fans to Category 3, that real differences become apparent. Of course, the mass-produced fan can be customised and various extras can be added — just as cars having alloy wheels, leather seats, air conditioning, satellite navigation, etc, etc.

This Chapter cannot describe all the options which are available. To repeat, the fan industry is a mature one. Often the options are the sole means of differentiation. Thus they proliferate ad nauseam. Those that are most popular (or appeal to the author) are described in the next few pages.

Centrifugal fans can be manufactured to various casing thick­nesses and with various forms of construction according to us­age. Thus at one extreme they can be handling clean air whilst at the other, air or gas handled can be at a temperature well above ambient and/or may contain substantial quantities of moisture and/or solids. It may also be at high pressure such that loadings on the fan casing and the associated ducting system are much higher than usually expected for a HVAC fan.

Connection to the ducting may be via flexible connections, or alternatively may be directly connected. In the latter case the fan has to withstand additional loads due to the dead weight of these connections. Where gases, or the surrounding ambient atmosphere, are at a high or low temperature, additional load­ing can result from the effects of expansion or contraction.

To ensure that the buyer can choose an appropriate form of construction, and to assist him in either specifying or recognis­ing what he buys, ISO 13349, Section 5.3 gives a categorisa­tion which is outlined in Table 8.1. This in no way indicates any form of grading but reflects current practice. Category 1, (Fig­ure 8.1) is as valid for low pressure clean air applications as Category 3 is preferred for heavy industrial usage.



(see Figure 8.1)


(see Figure 8.2)


(see Figure 8.3)


Light HVAC Clean air

Heavy HVAC Light industrial

Heavy industrial



Light dust or moisture

Dirty air/gas containing moisture and/or solids or

High pressure or

High power

Casing features (typical)

Lockformed, spot-welded or screwed construction Cradle or angle frame mounting

Lockformed, seam welded or fully welded construction. Semi-universal construction with bolted on sideplate

Fully welded fixed discharge

Casing thickness

<0.0025 D

> 0.0025 D

> 0.00333 D

Note: D is the impeller nominal diameter in millimetres

Table 8.1 Categorisation according to casing construction and thickness

This categorisation is particularly appropriate for centrifugal fans, as the great majority of axial flow fans are supplied for clean air, albeit some handle small amounts of entrained mois­ture. Nevertheless, there is no specific restriction to centrifu­gals. The special features detailed in the subsequent Sections may be limited to specific types of fan, which will be identified

Constructional features

When appropriate. It is often difficult to differentiate between these special constructional features and the ancillaries de­scribed in Chapter 16.

A distinction has been made that constructional features are part of the basic fan as manufactured, whilst ancillaries are bolt-on “goodies” which may or may not be supplied. Readers can enjoy themselves looking for the undoubted anomalies which arise!

Cradle mounted fans (centrifugal — Category 1)

These are very light duty fans for clean air applications. They are normally manufactured from pre-galvanized sheet steel and are either of lockformed or flanged and spot welded con­struction. The bearings are usually of the ball race type, grease packed for life. The casing volute is often supported in a cradle which can be bolted on to give different angles of discharge.

Semi-universal cased fans (centrifugal — Category 2)

This is best understood by reference to Figure 8.2. It will be noted that the casing “snail” consists of a scroll plate seam welded to the volute sides. Mild steel fabricated sideplates are bolted on at an outer pitch circle diameter such that they can be assembled to any of the standard angles of discharge, (see Chapter 9).

Constructional features

Figure 8.2 Typical Category 2 fan

Constructional features

Fixed discharge cased fans (centrifugal — Category 3)

These fans are purpose made for a specific contract and have a fixed position for the casing outlet flange. They are usually of sheet steel welded construction and are most common for fans having impellers greater that 1000 mm diameter, (see Figure 8.3).

Horizontally split casings

Constructional featuresBecause of their size, fixed discharge fans may have to be split horizontally to facilitate transport and/or site assembly. The “split” comprises and angle flange terminating each half casing and these can then be bolted together (see Figure 8.4).

Constructional features

Figure 8.4 Typical large fan with casing split on horizontal centreline

Casings with a removable segment

Whilst a horizontally split casing facilitates transport and as­sembly, it may not be ideal for routine maintenance or for break­downs. For vertically up (0°), top horizontal (90°) or any angular (45°, 315° etc.) discharges, it may require that the discharge ducting also be disassembled before the impeller/shaft assem­bly can be removed for maintenance. A removable segment (see Figure 8.5) overcomes this difficulty. The segment should be larger across its extremities than the impeller diameter.

Constructional features

Figure 8.5 Typical large fan casing with removable segment

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