GB2235016A - Twin centrifugal fan units - Google Patents

Abstract

A twin centrifugal fan unit has first and second centrifugal fans (1, 2) having scrolls of opposite directions of curvature. The first and second centrifugal fans (1, 2) have a common scroll outlet connected to a common duct (5). A pair of flaps (21, 22) are mounted within the scroll outlet and are movable to close either the first or the second fan (1, 2). The first and second flaps (21, 22) are urged apart by magnets (23, 24). <IMAGE>

Description

TWIN CENTRIFUGAL FAN UNITS This invention relates to a twin centrifugal fan unit.

It is known to provide a twin centrifugal fan unit whose scrolls have opposite directions of curvature and whose scroll outlets are adjacent and communicate with a common duct.

An example of a known twin centrifugal fan unit is illustrated in Figures 1A and 1B in which the fans are denoted at 1 and 2 respectively. The scrolls of the fans 1 and 2 have opposite directions of curvature; the scroll outlets 3 and 4 of the fans 1 and 2 respectively are rectangular and are positioned so that they have one long side in common and communicate with the ducts 5 which, in the example of Figures 1A and 1B, is a four inch duct. It will be seen that the corner areas of the scroll outlets do not communicate with the duct 5 and are covered by a metal plate, as denoted by the masked areas 6 shown in Figures 1A and 1B and this generates noise.

It is desirable to provide a back draught shutter, preventing reverse air flow and in the unit shown in Figures 1A and 1B, this consists of a two-part shutter having two semicircular flaps 7, pivoted about a line which is normal to the axis of the duct 5 and in line with the junction line of the two scroll outlets 3 and 4. In normal use, one of the fans 1 and 2 is energised and the resultant air pressure swings both of the flaps 7 backwardly to permit air flow through the duct 5. It will be observed that even in their open positions the flaps 7 constitute a central obstruction as can clearly be seen in Figure 1B, and this itself increases noise. Only one fan is energised at any one time and consequently only the area corresponding to one of the scroll outlets 3 and 4 is in use at any one time, so that only half the inlet area of the duct 5 is in use at any one time.This permits the air output of the twin cylindrical fan shown in Figures 1A and 1B. When neither fan is energised reverse air flow blows the flaps 7 to the closed position transverse to the ducts.

It has been proposed, in a modified known construction illustrated in Figures 2A, 2B and 2C, that the scroll outlets 3 and 4 communicate with a five inch duct 10, which is reduced to the four inch duct 5 by a frustoconical shoulder 11. Back draught is prevented by a pair of shutters 12, pivoted to the junction between the outlets 3 and 4. When any one of the fans 1 and 2 is energised, it blows open the shutter 12 covering that outlet and it will be appreciated from Figure 2B that the upper fan 1 is assumed to be energised. In the event of back flow of air when neither fan is energised, the shutters 12 are both blown to their closed position by reverse air flow if any. As can be appreciated from Figure 2C, the shutter 12 do constitute a central obstruction as in the unit of in the unit of Figures 1A and 1B and, moreover, the area of only one scroll outlet is available at any one time.However, only one shutter 12 will be in its open position at any one time contrary to the impression given by Figure 2C. As the inlet of the five inch duct 10 is greater than that of the four inch duct 5, the masked areas 6 at the corners of the scroll outlets 3 and 4 are very much smaller. This reduces noise and permits a 20% increase output in comparison to the arrangement shown in Figures 1A and 1B.

Yet another known twin centrifugal fan unit is illustrated in Figures 3A and 3B and in this arrangement the centrifugal fans 1 and 2 have a common outlet 13 which leads to the four inch duct 5.

A single flap 14 is pivoted within the common outlet 13 on a common rear base and is swingable between the two positions illustrated to close off one of the two centrifugal fans. It will be appreciated that flap 14 will be swung by the pressure produced by the only fan in operation to close off the scroll of the other fan and it will also be appreciated that the entire area of the scroll outlet 13 of the twin centrifugal fan unit of Figures 3A and 3B is available when either fan is energised.

To prevent back draught, a pair of back draught shutters 7, as shown in Figures 1, are mounted within the duct 5 at the plane of junction of the scroll outlet 13 with the duct 5. The scroll outlet 13 must be rectangular in shape whereas the duct 5 is circular, resulting in masked areas 6 as in Figure 1B, but these areas are small relative to those shown in Figure 1B, reducing noise. However, the central obstruction provided by the back draught shutters 7 in Figures 3A and 3B is of the same significance as that of the unit of Figures 1A and 1B.

The unit of Figures 3A and 3B has a higher output than that of Figures 1A and 1B and produces less noise, but it is nevertheless desirable that the noise output should be reduced further and the fan output should be increased.

According to this invention, there is provided a twin centrifugal fan unit comprising first and second centrifugal fans scrolls of opposite directions of curvature and having a common scroll outlet connected to a common duct, and a pair of flaps mounted within the scroll outlet and both movable to close either the first or the second centrifugal fan, the first and second flaps being urged apart.

If either fan is energised both flaps are moved to close off the other fan but there is no obstruction in the duct to restrict the flow of air. In the event that neither fan is energised, the flaps are urged apart so that the first flap closes off the first fan and the second flap closes off the second fan so that the first and second flaps also act as back draught shutters.

Preferably, magnets are provided on the facing surfaces of the first and second flaps and are arranged to repel each other and thereby urge the flaps apart.

A twin centrifugal fan unit in accordance with this invention will now be described, by way of example, with reference to the accompanying drawings of which: Figures 1A and 1B are respectively a perspective view and an end view of a centrifugal fan unit which has already been described; Figures 2A, 2B and 2C are respectively a perspective view, a partial sectional view and a partial end view of a second known twin centrifugal fan unit which has already been described; Figures 3A and 3B are, respectively, a partial sectional elevation and a partial end view of a third known twin centrifugal fan unit which has already been described; and Figures 4A, 4B and 4C are respectively a partial sectional elevation of the twin centrifugal fan unit in accordance with this invention when one fan is energised, a partial sectional elevation of the unit when neither fan is energised and a partial end view.

Referring to Figure 4A, 4B and 4C, the illustrated twin centrifugal fan unit in accordance with this invention comprises centrifugal fans 1 and 2 having a common scroll outlet 13, connected to a four inch circular duct 5 in known fashion.

Opposite the outlet 13, two flaps 21 and 22 are pivoted along adjacent parallel lines normal to the plane of the section of Figures 4A and 4B and are resiliently urged apart by facing permanent magnets 23 and 24 which are arranged to repel each other.

In Figure 1, fan 2 is energised whereas fan 1 is not energised (only one fan is ever energised at any one time), and the air pressure swings both flaps 21 and 22 together to close off the fan 1 by engagement of the flaps 21 and 22 at the top of the outlet 13, which is rectangular.

In the event that fan 1 is energised, the air pressure on the flaps 21 and 22 is such as to close off fan 2. In the event that neither fan is energised, the flaps 21 and 22 are urged apart so that they respectively close off fans 1 and 2, as shown in Figure 4B.

As is clear from Figure 4C, there are masked off areas 6 at the corners of the rectangular scroll outlet 13 which do not communicate with the duct 5 but they are smaller than the corresponding areas 6 of the twin centrifugal fan unit shown in Figures 1A and 1B. Moreover, there is no central obstruction as in the known twin centrifugal fan units of Figure 1 to 3. The overall effect is that the twin centrifugal fan unit in accordance with this invention and as illustrated in Figures 4A and 4B has a air flow output approximately 20% greater than that shown than the unit of Figures 1A and 1B and the unit is also relatively quiet.

Near their outer ends the flaps 21 and 22 carry outwardly directed dams 25 and 26 which never significantly obstruct the outlet 13. The dams 25 and 26 have the function of assisting opening of the scroll outlet of an energised fan. Without such dams, the reduced static pressure of the rapidly moving air stream tends to suck the flaps 21 and 22 into a semi-open position.

Claims (5)

1. A twin centrifugal fan unit comprising first and second centrifugal fans having scrolls of opposite directions of curvature and having a common scroll outlet connected to a common duct, and first and second flaps mounted within the scroll outlet and both movable to close either the first or the second centrifugal fan, the first and second flaps being urged apart.

2. A twin centrifugal fan unit as claimed in claim 1 wherein magnets are provided on the facing surfaces of the first and second flaps and are arranged to repel each other and thereby urge the flaps apart.

3. A twin centrifugal fan unit as claimed in claim 1 or claim 2 wherein the flaps are pivoted about adjacent parallel lines on opposite sides of a plane of symmetry from which the scrolls extend away.

4. A twin centrifugal fan unit as claimed in claim 3 wherein each flap carries at its end remote from the line about which it is pivoted a short dam extending normal to the flap, towards the associated scroll and parallel to the line about which that flap is pivoted.

5. A twin centrifugal fan unit substantially as hereinbefore described with reference to Figures 4A and 4B of the accompanying drawings.