GB2449688A

GB2449688A - An impeller / fan having radially extending blades and axially extending blades

Info

Publication number: GB2449688A
Application number: GB0710434A
Authority: GB
Inventors: Anthony Lawrence Croke
Original assignee: Greenwood Air Management Ltd
Current assignee: Greenwood Air Management Ltd
Priority date: 2007-05-31
Filing date: 2007-05-31
Publication date: 2008-12-03
Also published as: GB0710434D0

Abstract

An impeller 1 for an axial fan comprises a hub 2, a plurality of blades 3 extending radially outwardly from the hub, and a plurality of blades 4 extending axially rearwardly from the hub. The rearwardly extending blades 4 preferably extend backwards from the radially outer portion of the hub, and also extend partially into the hub. The axially extending fins 4 ideally protrude 5mm rearwardly of the hub. A lip 5 may be provided outside the hub at the rear so as to redirect axially flowing air radially outwards. Preferably the impeller belongs to an axial extractor fan assembly comprising a motor to drive the impeller. The arrangement expels air from behind the hub, thereby creating a circulation which cools down the fan motor. An independent claim relates to an impeller having a lip formed on the outside of the hub at the rear, and a plurality of blades located radially inwardly of the lip.

Description

--. . 2449688 Fan impeller The invention relates to an impeller for an

axial extractor fan and a fan comprising the impeller.

In-duct axial extractor fans are commonly found in commercial environments such as pubs and restaurants or around the home in kitchens and bathrooms. They are generally either fitted when the building is built or they can be installed later as part of home improvement projects or renovations or when replacing existing units.

In-duct fans are fitted into a duct which is fitted into a hole in the wall or ceiling. In wall mounted duct fans, the duct leads through the hole in the wal1,usually to the outside for venting the extracted air. Ceiling mounted duct fans are mounted in a hole in the ceiling and a duct (usually a flexible duct) carries the air to an outside vent.

There are general desires to make fans run faster and operate less noisily. However, when fans are made larger or run faster, more heat is generated. If damping systems are provided around the motor to reduce noise levels or if water protection is provided, the dampers and seals or baffles surround the motor and make it more difficult to vent the heat. Running the fan faster or using a larger motor will generate more heat which needs to be dissipated. This leads to an increase in operating temperature which can be dangerous and may reduce the efficiency of the fan motor.

In order to cool the motor in such circumstances, one solution is to generate an airflow past the motor so as to remove some of the heat. The main airflow generated by the fan impeller cannot be used for this purpose without the motor obstructing the airflow and reducing the efficiency of the fan. Also, in fans which are water protected or provided with vibration dampers, the motor is shielded from the main axial airflow.

One known solution is to create a separate airflow past the motor. This is typically achieved by creating a counter-current airflow in the opposite direction to the main airflow, but insjde the main flow, i.e. concentric to the main flow.

Specifically, the main airflow moves air from the front of the fan through the rear of the fan while the motor cooling airflow flows from the rear of the fan through the motor towards the impeller at the front of the fan.

In order to create this counter-current airflow, the impeller is used to create a negative pressure region at the front of the fan assembly so as to suck air through the motor towards the impeller. Known systems for doing this use air agitators on the inside of the impeller hub to expel air from the impeller hub into the main rearward axial airflow. A typical impeller of this type is shown in Fig. 5.

Fig. 5 shows an impeller 102. with a hub 102 from which radially extending blades 103 extend. When the hub 102 is rotated in the correct direction (anticlockwise as viewed from the front in Fig. 5), the impeller generates an axially rearwardly flowing air stream. The hub 102 is provided with an axially extending shaft 106 and bore 107. In addition, the hub 102 is provided with a number of radially extending agitating blades 150 which extend radially from the shaft 106 across the inside of the hub 102. As the hub 102 is rotated, these agitating blades 150 tend to move air within the hub 102 radially outwardly. Due to the curved shape of the hub 102, this air is then directed rearwaxdly. However, the air is only directed generally axially rearwardly. It is not expelled radially from behind the hub into the main rearwardly flowing axial air stream.

This type of. impeller induces sufficient airflow for many applications, but it does not provide a strong enough cooling current for more recent designs where motor size and speed are increased or vibration damping and/or water protection systems are provided around the motor.

The invention provides an improvement over such known systems by generating a stronger cooling current past the motor.

According to the invention there is provided an impeller for an axial fan comprising: a hub; a plurality of fins extending radially outwardly from the hub; and a plurality of fins extending axially rearwardly from the hub.

When the impeller is rotated in the correct direction, the radially outwardly extending fins generate the main rearwardly flowing axial air stream. At the same time, the axially rearward].y extending fins move air from behind the hub of the impeller and displace it radially outwardly into the main axial air stream. This creates a low pressure area behind the hub (radially inwardly of the axially extending fins) which in turn induces air flow towards the hub in the axial direction past the motor. In other words, as air is displaced away from the region behind the hub, air is sucked axially through the fan past the motor, thus cooling the motor. The axially extending fins can thus act as a centrifugal fan. This arrangement is much more efficient than the arrangement described above with air agitators as it expels a much greater quantity of air from behind the hub, inducing a greater axial air flow past the motor.

The axially extending fins can be positioned at any radial position according to convenience. However, for best results, the fins are positioned at the radially outermost portion of the hub (e.g. on an outer rim) as this allows for more fins and/or a greater contact area between the fins and the air.

This allows the fins to displace a greater quantity of air.

The hub functions merely as a support for the fins and it may take any suitable form. For example it could be a simple cylinder or it may have a more aerodynamic profile such as a hemisphere or a cone so as to facilitate air movement through the fan. The hub may be solid, but preferably it is hollow.

A hollow hub uses less material and is therefore lighter.

With this arrangement, the rearward side of the hub presents only a radially outer rim on which to provide the axially extending fins.

The axially extending fins may extend into the interior of the hollow hub so as to agitate air out from the inside of the hollow space. The distance which the fins extend into the hub will vary depending on the geometry of the hub, the size of the fan and the speed of operation. In a preferred embodiment of a 150 mm diameter duct fan with a hemispherical hub, the fins extend at least 5 mm into the interior of the hub.

However in larger fans (e.g. up to about 300 mm), the fins may extend up to about 20 nun into the interior of the hub.

To increase the negative pressure behind the hub and thereby create a stronger cooling airflow past the motor, the axially extending fins must extend rearwardly of the hub so that air is forced directly out past the hub and into the main axial airflow. Preferably the fins extend at least 5 mm clear of the rear of the hub. The distance which the fins extend rearwardly of the hub will depend upon the size of the motor and the intended speed of operation as well as the purpose of the fan (i.e. the rate at which the fan is intended to move air in its main axial airflow). Longer fins have more surface area in contact with the air and are therefore capable of moving a greater quantity of air from behind the impeller.

Longer fins can therefore increase the power of the centrifugal fan. . Preferably a lip is formed on the outside of the hub at the rear of the hub for directing axially rearwardly flowing air radially outward. As the lip aôcelerates the main axial airflow and deflects it radially outward, the pressure in the axial air stream is decreased, creating a low pressure region radially outwardly of the axially extending fins. Thus, the pressure gradient across the axially extending fins is increased. This has the effect of strengthening the radially outward airflow from behind the hub and thus increases the suction power of the centrifugal fan. This increases the rate at which air is sucked past the motor and therefore further increases the cooling of the motor.

Any lip-like obstruction which diverts the axially flowing air radially outwards will achieve this effect. However it is also desirable to minimise the disruption of the axially flowing air so as not to limit the flow rate of the main axial flow. In other words, it is desirable to maintain the efficiency with which the impeller can move air through the fan. In the most preferred embodiments therefore, the lip has a smooth surface which is Continuous with the surface of the hub. This allows the axially flowing air to be smoothly redirected radially outwardly without greatly disturbing the flow and causing minimal turbulence.

The dimensions of the lip (the height above the axially extending fins and the angle of the lip) will depend upon the size of the motor, the speed of the fan, the amount of air to be shifted by the fan in the main axial airflow and the amount of cooling air which needs to be drawn past the motor. Air which is redirected radially outwards by the lip reduces the efficiency of the main airflow while increasing the amount of cooling air which is drawn past the motor. A balance therefore has to be struck between these two factors for a particular fan. Another factor influencing the respective airf lows is the length of the axially extending fins. In a preferred embodiment of a 150 mm diameter duct fan, the radially outermost part of the lip is separated from the radially outermost part of the axially extending fins by about 3mm.

The lip on its own serves to create a significant low pressure area in the region of the rim and thus induces a radially outward airflow from behind the hub into the main axially flowing air stream. Therefore according to another aspect, the invention provides an. impeller for an axial fan comprising a hub, a lip formed on th outside of the hub at the rear of the hub for redirecting axially rearwardly flowing air radially outwardly and a plurality of fins radially inwardly a of the lip. Preferably the fins extend axially rearwardly from the hub, but the lip can be beneficial in creating a low pressure area and thus a radially outward airflow even if the fins do not extend rearwardly beyond the rear of the hub.

Preferably the impeller forms part of an axial fan, particularly an axial extractor fan or an in-duct type axial fan.

A preferred embodiment of the invention will now be described by way of example only, and with reference to the accompanying drawings in which: Figs. 1 to 4 show a fan according to the invention.

Fig. 5 shows a known impeller having agitating members.

Figs. 1 to 4 respectively show a cross section, a perspective rear view, a perspective front view and a rear view of an impeller for an axial extractor fan according to a preferred embodiment of the invention.

The impeller 1 has a hub 2, radially extending fins (or blades) 3 and axially extending fins (or blades) 4. The radially extending fins 3 extend generally radially outwardly from the hub 2 and are set at an angle to the axial direction so as to force air axially rearwardly when the impeller is rotated in the correct direction (anticlockwise when the impeller is viewed from the front, e.g. in Fig. 3). The fins 3 may have any desired shape, e.g. straight or curved (as shown) and any length according to the specific use. The axially extending fins 4 extend generally axially rearwardly from the hub 2 and are set at an angle to the circumferential direction so as to force air radially outwardly when the impeller is rotated in the correct direction (anticlockwise when the impeller is viewed from the front, e.g. in Fig. 3).

Again, the fins 4 may be straight or curved and may be of varying lengths depending on the specific application.

The hub 2 is generally hemispherical and has a lip 5 on its outer surface in the region of the rearward rim of the hub 2.

The lip 5 projects radially outwardly from the hub 2 and has a smooth outer surface continuous with the outer surface of the hub2.

As can be seen most clearly in Fig. 1, the axially extending impeller fins 4 extend some distance into the inside of the hollow hub 2 and some distance rearwardly of the hub 2.

The axially extending fins 4 form a centrifugal fan which, when rotated, expels air from within the hub 2 and from the region behind the hub 2. This air is forced radially outwardly into the rearwardly flowing airstream created by the radially extending impeller fins 3.

In use, the impeller 1 is mounted on a motor (not shown) in a fan housing (not shown). The impeller has a centrally located axial shaft 6 extending rearwardly from the centre of the hub 2. The shaft 6 contains a bore 7 into which the spindle of the motor can be inserted. The bore 7 and the spindle may be bonded together by adhesive or they may be otherwise engaged (e.g. by interlocking) for simultaneous rotation. Thus the impeller 1 is fixedly mounted to the spindle of the motor and can be driven by the motor. The motor is typically mounted in a cylindrical housing (not shown) to form an axial fan unit which is in turn mounted in a duct in a wall or ceiling.

For comparison, a known impeller is shown in Fig. 5 which shows a rear perspective view corresponding to Fig. 2. The known impeller 101 has a hub 102 from which radially extending blades 103 extend. The radially extending fins 103 are identical to those shown in Figs. 1 to 4 and, when the hub 102 is rotated in the correct direction (anticlockwise as viewed from the front in Fig. 5), the impeller generates an axially rearwardly flowing air stream.

The hub 102 is hollow and it is provided with a shaft 106 and bore 107 in the same way as the impeller of Figs. 1 to 4. In addition, the hub 102 is provided with a number of radially extending agitating blades 150 which extend radially from the central shaft 106 across the inside of the hub 102. As the hub 102 is rotated, these agitating blades 150 tend to move air within the hub 102 radially outwardly. Due to the curved shape of the hub 102, this air is then directed rearwardly.

However, the air is only directed generally axially rearwardly. It is not expelled radially from behind the hub into the main rearwardly flowing axial air stream. Therefore the agitating blades 150 do not create a very strong negative pressure behind the hub 102 and therefore do not induce a significant cooling airflow forward through the fan, past the motor.

Claims

Claims: 1. An impeller for an axial fan comprising: a hub; a plurality

of fins extending radially outwardly from the hub; and a plurality of fins extending axially rearwardly from the hub.
2. An impeller as claimed in claim 1, wherein the axially rearwardly extending fins extend at least 5 mm rearwardly of the hub.
3. An impeller as claimed in claim 1 or 2, wherein the axially rearwardly extending fins extend rearwardly from the radially outer portion of the hub.
4. An impeller as claimed in claim 1, 2 or 3, wherein the axially extending fins extend inside the hub.
5. An impeller as claimed in claim 4, wherein the axially extending fins extend at least 5 mm inside the hub.
6. An impeller as claimed in any preceding claim, wherein a lip is formed on the outside of the hub at the rear of the hub for redirecting axially rearwardly flowing air radially outwardly.
7. An impeller as claimed in claim 6, wherein the lip has a smooth surface which is continuous with the surface of the hub.
8. An impeller as claimed in claim 6 or 7, wherein the radially outermost part of the lip is separated from the radially outermost part of the axially extending fins by at least 3 nun.
9. An axial fan comprising an impeller as claimed in any preceding claim.

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10 - 10. An axial fan as claimed in claim 9, further comprising a motor arranged to drive the impeller.
11. An axial fan as claimed, in claim 10, wherein the motor is mounted in a fan housing.
12. An axial fan as claimed in claim 11, wherein the fan housing is generally cylindrical.
13. An impeller for an axial fan comprising a hub, a lip formed on the outside of the hub -at the rear of the hub for redirecting axially rearwardly flowing air radially outwardly and a plurality of fins radially inwardly of the lip.
14. An impeller as claimed in claim 13, wherein the fins extend axially.