What is a fan?
We have seen from Section 1.2 that fans are built in all shapes and sizes. They run from the very lowest to high speeds. Their performances are just as different. Whilst it may be obvious, let us therefore have a general definition, on which hopefully we can all agree, of what we are talking about. That enshrined in Eurovent 1/1 and ISO 13348 is as follows:
“Afan is a rotary-bladed machine which receives mechanical energy and utilizes it by means of one or more impellers fitted with blades to maintain a continuous flow of air or other gas passing through it and whose work per unit mass does not normally exceed 25 kJ/kg.”
All very interesting, you may declare. But what exactly does it mean and why the need for an upper limit to the work per unit mass? The definition which follows is coloured, of course, by the texts which the author has read, and by his experiences over the years:
“Afan is a rotary-bladed machine which delivers a continuous flow of air or gas at some pressure, without materially changing its density”.
The words have been carefully chosen. Our sort of fan is not something for old-fashioned ladies to hide behind — thus the requirement for rotary motion. The flow is continuous into, through and out of the unit. Thus we can distinguish a fan from a positive displacement machine with pistons, vanes or lobes where the flow pulsates. A maximum pressure rise or density change has to be included to differentiate between fans and compressors. ASME, in its performance test Code PTC11 says that the boundary is “rather vague”.
AMCA/ASHRAE in Standard 210/51 state that “the scope has been broadened by eliminating the upper limit of compression ratio”. Nevertheless, a boundary exists somewhere.
ISO/TC117 has proposed that a maximum absolute pressure rise of 30% should be adopted. This equates to 30 kPa when handling standard air. For any others not yet fully metricated, this is about 120 ins water gauge. However, there are machines which we would recognise as fans developing pressures up to 240 ins water gauge or 60 kPa. Equally there are machines recognizable as compressors developing less than 6 kPa.
The prime function of a fan is, therefore, to move relatively large volumes of air at pressures sufficient to overcome the resistance of the systems to which they are attached. Afan’s aerodynamic performance in terms of the pressure it generates as a function of flowrate, and how efficiently this is done, is what differentiates one fan type from another.
For any specific duty of flowrate and pressure rise, an infinite number of fans of varying types could be offered. Figure 1.58 shows an end elevation of their impellers. Apart from these variations in impeller design, the units could be of small diameter running at high rotational speed or conversely largerfans at low speeds. The selection of an appropriate fan will be influenced by space availability, driving method, noise limitations, aerodynamic and mechanical efficiency, mechanical strength and even, alas, capital cost and lead time. The manufacturer invited to tender may not have the optimum design within his manufacturing programme and this will lead to less than ideal solutions.
[ Higher Specific dlameterf«—| Increasing Pressure
Figure 1.58 End elevation of impellers showing variation with flowrate and pressure
It will be noted that Figure 1.58 essentially indicates a continuous range of aerodynamic designs from lowflowrate/high pressure through to high flowrate/low pressure. There is a continuing increase of inlet area available to the air from the narrow centrifugal fans through to the propeller fans where the total swept area is open to the flow. Whilst the main generic types may be identified as shown in Figure 1.59, there are in fact no definite boundaries between the types and there are many intermediate types which have been designed or are possible.
There is, as has been previously stated, a variety of fan designs, but practically and for the sake of Fans & Ventilation, we may identify five generically different types (Figure 1.59) characterised by their impellers and the flow through them:
A) Propeller or axial flow where the effective movement of the air is straight through the impeller at a constant distance from its axis. The major component of blade force on the air is directed axially from the inlet to outlet side, the resultant pressure rise being due to this blade action. There is also, of course, a tangential component which is a reaction to the driving torque and the air, therefore, also spins around the impeller axis. Suitable for high flowrate to pressure ratios.
B) Centrifugal or radial flow where the air enters the impeller axially and, turning a right angle, progresses radially outward through the blades. As the blade force is tangential, the air tends to spin with these blades. The centrifugal force resulting from the spin is thus in line with the radial flow of the air, and this is the main cause of the rise in pressure. According to the blade inclination or curvature, there may also be an incremental pressure rise due to the blade action. Suitable for a low flowrate to pressure ratio.
C) Mixed or compound flow where the air enters axially but is discharged at an angle between say 30° and 80°. The impeller blading extends over the curved part of the flow
1 Fluid 2 Blade 3 Casing 4 Inlet 5 Outlet
Figure 1.59 The five main generic fan types
Path, the blade force having a component in the discharge direction as well as the tangential component. The pressure rise is thus due to both blade and centrifugal action. Intermediate in flowrate and pressure rise between the centrifugal and axial.
D) Tangential or cross flow in which a vortex is formed and maintained by the blade forces and has its axis parallel to the shaft, near to a point on the impeller circumference. The outer part of this vortex air is “peeled” off and discharged through an outlet diffuser. Whilst similar in appearance to a centrifugal impeller, the action is completely different, an equal volume of air joining the inward flowing side of the vortex. Thus air has to traverse the blade passages twice. Suitable for very high flowrates against minimal resistance.
E) Ring-shaped in which the circulation of air or gas in a toric casing is helicoidal. The rotation of the impeller, which contains a number of blades, crates a helicoidal trajectory which is intercepted by one or more blade, depending on the flowrate. The impeller transfers energy to the air or gas and is usually used for very low flowrates.
Posted in Fans Ventilation A Practical Guide