GB2334756A - Fan unit with two fans, guide vanes and tapering duct - Google Patents

Abstract

Two centrifugal or mixed-flow fans 10,11 are mounted in series within a housing 12 for driving air through the housing from an inlet to an exhaust, for driving air in a ventilation system. A tapered duct 17 within the housing reduces the gas flow crossectional area from the exhaust of the upstream fan to the inlet 14 of the downstream fan. Several guide vanes 18 extend radially inwards from the tapered duct to reduce rotational flow of air within the duct. The inlet 21 to the guide vanes 18 is angled to improve the exhaust flow from fan 10. The fans are substantially identical and driven independently by substantially identical electrical motors. Only one fan is driven at any time and the non-driven fan windmills due to the air flow thus forced through it. The air flow through the housing is substantially the same whichever fan is driven (fig 7).

Description

FAN UNIT The present invention relates to a fan unit of the type used in ventilation systems in buildings, for example in extractor systems for toilets.

In the construction and refurbishment of buildings it is common that contractors are issued with equipment specifications to ensure that the all work undertaken meets the required standards including supplying equipment which meets specified performance criteria. In the case of a number of ventilation applications, such as toilet extractor fan units, it is conventional to specify that a twin fan unit be provided where only one fan at any one time is driven. If the duty fan fails for any reason automatic change-over is effected and the standby fan cuts in. Duty sharing between the fans is also required and it is usual for the fans to be driven by separate motors.

Contracts usually specify that centrifugal (or radial) fans, rather than axial fans, be used and that the flow rate and pressure provided by the unit be the same whichever fan is driven.

Known twin fan units use two centrifugal fans in parallel, the fans having separate intakes and exhausts.

The intakes may be coupled upstream to a common intake duct and/or the exhausts may be coupled downstream to a common exhaust duct, depending on the application of the twin fan unit. An example of a known unit of this type having a common exhaust duct is shown in Figure 1. Often the fans have scroll, or spiral, casings to direct the exhaust flow. Since only one fan is driven at any one time, if the fans have a common exhaust duct as in Figure 1, the exhaust of the driven fan tends to flow backwards through the standby fan, causing a loss of pressure and consequent loss of efficiency. In order to maintain efficiency, a damper mechanism must be provided which hinges to cover the outlet of the standby fan. This disadvantageously increases the complexity and cost of the fan unit. The parallel configuration of the two fans also disadvantageously results in the unit having a large radial cross-section.

Single fan units are known in which a centrifugal fan is mounted within a housing for driving air or gas along the housing. These are conventionally termed tubular fans or duct fans.

The invention provides a fan unit comprising two fans spaced from each other and mounted within a housing for driving gas through the housing, the gas flow passing through both fans in series; and a taper means for reducing the cross-sectional area of the gas flow and a plurality of guide vanes mounted with the housing between the exhaust of the upstream fan and the intake of the downstream fan.

Preferably both fans are centrifugal fans or mixedflow fans (a hybrid between a centrifugal fan and an axial-flow fan). Centrifugal fans produce higher pressure and make less noise than comparable axial-flow fans.

By using both guide vanes and a taper arranged axially between the two fans it has been found that the performance of the twin fan unit is substantially constant regardless of which of the two fans is driven. The fans are advantageously substantially identical fans and are preferably driven at substantially equal power.

The fan unit is primarily intended to drive air in a ventilation system, but in other applications may drive any gas.

One embodiment of the invention will now be described by reference to the accompanying drawings in which: Fig. 1 is a side view of a prior-art, parallelconfiguration, twin scroll fan unit; Fig. 2 is a longitunal section of a twin fan unit according to an embodiment of the present invention; Fig. 3 is a transverse section of the twin fan unit of figure 2 along line B-B showing the downstream fan mounting; Fig. 4 is a plan view of a guide vane of the embodiment of figure 2; Fig. 5 shows an exploded end elevation of the taper pieces of the embodiment of figure 2; Fig. 6 shows an end elevation of the taper pieces assembled to form a taper and the relative position of the guide vanes; Fig. 7 shows the results of experimental tests giving ISO certified performance data of the embodiment of figure 2 as plots of total pressure increase versus flow rate; and, Fig. 8 shows experimental results of tests of the embodiment of figure 2 as plots of pressure versus volume.

A prior art twin fan arrangement is shown in Figure 1. Two radial fans 1, 2 with scroll casings are mounted side by side in a housing 3. The intakes 4, 5 of both radial fans are perpendicular to the exhaust flow. Either fan may be driven, but only one fan is driven at any one time. The scroll casings direct air from either fan to a common outlet port 8. A damper mechanism 9 in the outlet port is necessary to prevent flow from the outlet of the driven duty fan flowing backwards into the outlet of the standby fan.

Figure 2 shows a twin fan unit according to an embodiment of the present invention. This type of unit is suitable for use in building ventilation systems. The direction of flow is shown by the direction of the arrow F. In the embodiment identical radial fans 10 and 11 are arranged co-axially within a duct or housing 12, which has square cross-section. The housing may be of other shapes depending on the intended application of the fan unit and other components it is to be fitted to. The intake of the downstream, or rear, fan 14 is axially spaced downstream of the exhaust of the upstream, or front, fan 13. Both the front fan and rear fan 10 and 11 are mounted within the centre of the housing 12 by support arms 15. Preferably each fan is a mounted within the housing by four support arms which are bolted at their inner ends symmetrically around a fan casing (such as a motor casing) and are attached at their outer ends to the centres of the sides of the housing. Separate electric motors (not shown) are provided for independently driving each fan. In the preferred embodiment of the invention both fans are driven by a single power supply electrically connected to each motor. A control mechanism (not shown) regulates the power supply to the fans resulting in only one fan being driven at any one time. The standby fan is allowed to freewheel, or windmill, during operation of the driven, or duty fan.

A housing front plate 16 blanks off the radiallyouter portion of the air intake to the housing and has a short frusto-conical nozzle at its centre to match the size of the intake of the front fan 13, preventing any flow from bypassing the front fan.

The outlet of the front fan occupies substantially the whole cross-sectional area of the housing. An approximately frusto-conical portion, or tapered duct, or taper, 17 gradually reduces the cross-sectional area of the air flow-path between a point downstream of the outlet of the front fan and the intake of the rear fan.

Additionally a rear fan intake nozzle 14 may be fitted at the intake of the rear fan to further restrict the crosssectional area of the air flow-path to match the rear fan intake.

Four static guide vanes 18 extend longitudinally within the housing between the outlet of the front fan and the intake of the rear fan. The guide vanes act to reduce the rotational component of the exhaust of the front fan and result in a substantially smooth axial flow being presented to the intake of the rear fan. The guide vanes and taper are described in more detail below.

A housing end plate or fitting 19 is provided to allow connection of the fan unit to further ducting or to terminate the fan unit on the exterior of a building.

Figure 3 shows a section of figure 2 along the line B-B, and shows that one end of each support arm 15 is rigidly fixed to the rear fan casing and the other end of each support arm is fixed to the housing by damping means in order to reduce any vibration of the housing caused by vibration of the fan. The front fan is supported in a similar manner.

Figure 4 shows a side view of a guide vane. Each guide vane 18 is stamped or cut from a sheet of, for example, metal, and bent to form an obtuse angle between two sections 20 and 21. The downstream section 20 of the guide vane is substantially trapezoidal, with a sloping outer side 22 designed to correspond to the slope of the taper 17 in order that the downstream section of the guide vane fits flush against the length of the taper 17.

The upstream section 21 of the guide vane is shaped to fit between the inner surface of the housing and the periphery of the front fan, with a small clearance between the fan and the inner edge of the vane. The outer edge of the section 21 of the vane fits flush against the inner surface of the housing.

Each vane is secured at is outer edge to the housing, and/or to the taper, for example by welding or rivetting or any convenient manner. Each vane extends substantially radially inwardly from the inner surface of the housing.

The downstream section 20 extends longitunally parallel to the housing axis and the upstream section 21 is at an angle to the housing axis as seen in figure 2. This improves the exhaust flow path from the front fan.

The downstream sections of each guide vane extends substantially radially inwardly from the taper, the vane extending into, and obstructing rotational movement of, the flow from the front fan outlet.

The upstream and downstream sections of each guide vane subtend an obtuse angle, such as approximately 1600.

Advantageously the angle between the upstream and downstream sections is 164 . Thus, if the downstream section 20 is aligned with the centre line of one face of the housing, the upstream section of the guide vane is displaced from the centre of the side of the housing where the corresponding support arm is attached.

The four guide vanes are oriented such that each guide vane is at 90" to the vanes at the centres of the adjacent sides of the housing. The angle subtended between the upstream sections of guide vanes at opposite sides of the housing is, therefore, approximately 40 , and preferably 32".

Figure 5 shows an exploded end elevation of the way in which four taper pieces 23 form the taper 17. Blanks to form the taper pieces 23 may be stamped from sheet metal.

Each end of each taper piece 24 is folded upwardly from a trapezoidal centre section. A tab 25 extends from one end 24 of each taper piece which allows adjacent taper pieces to be fixed together by joining the tab 25 of one taper piece to the end 24 of the adjacent taper piece by rivets or any other suitable fixing means.

Figure 6 shows an end elevation, from the upstream end of the housing, of the assembled taper 17 and shows the positions of the guide vanes 18 relative to the nonuniform shaped taper 17. The taper is of square section at its upstream end 30 and of octagonal section at its downstream end 32. The downstream end approximately matches the circular intake 34 of the rear fan, as shown in figure 6.

Figure 7 illustrates the results of experimental tests on the fan unit of figure 2. In the figure, fan 1 denotes the front fan and fan 2 the rear fan. The figure plots total pressure increase across the fan unit against flow rate through the fan unit in each of four different experimental arrangements. In all cases, the power input to the duty fan is the same, and is constant.

In Test l, fan 1 was driven and fan 2 was not. In Test 2, fan 2 was driven and fan 1 was not. As shown in figure 7, in each case almost identical performance, in terms of air flow, was observed. In each case the standby fan was able to freewheel, or windmill, as air was forced through it by the driven, duty fan.

In Tests 3 and 4, the vanes were removed from the fan unit. The taper was retained in Test 4, when fan 2 was driven, almost identical performance as in Tests 1 and 2 was obtained. But in Test 3, when fan 1 was driven, significantly worse air flow (about 328 less airflow) was observed.

It is believed that the reason for the degraded performance is that when fan 1 is driven, its exhaust tends to be very turbulent and to have a large rotational flow component. This airflow enters fan 2 and causes fan 2 to windmill rather in efficiently. By contrast in Test 1, where the exhaust from fan 1 was prevented by the vanes from rotating, fan 2 can windmill much more efficiently.

Fan 2 therefore obstructs the airflow more in Test 3 than in Test 1.

Similar effects were observed if the tapered duct was removed as well as the vanes. When fan 1 was driven, the airflow through the housing was less than when fan 2 was driven.

When fan 2 is driven, performance is not greatly affected by the presence or absence of the vanes or the tapered duct. It is believed that this is because the air flow entering fan 1 is smooth in either case.

Regardless of the absolute efficiency of the twin fan unit when each fan is driven, an important feature of the embodiment of figure 2 is that substantially identical performance is obtained whichever fan is driven, with the same power supplied to each fan. In many applications this is a significant advantage. It should be noted however that two fan and motor units of the same specification may produce slightly different air flow performance when connected to identical power supplies, because of variations due to manufacturing tolerances.

Performance differences of 5 or 10% are not unusual in practice. Such variations should be allowed for when considering references herein to the effect that substantially identical performance may be obtained from each of two fans connected to similar power supplies.

Simply placing two fans in series such that the air flow to the second fan is provided from the outlet of the first fan does not, therefore, result in suitable performance. Without the guide vanes and cone the presence of the rear fan in the turbulent flow produced when the front fan is driven results in a marked loss of performance and significant difference in throughput dependent on which of the front or rear fans is driven.

The design of the guide vanes and taper should, therefore, be directed to achieving equal, and optimum, performance of both the front and rear fan regardless of which fan is driven.

Although the use of either vanes or a taper alone achieves better results than using neither, experiments suggest that both are required to provide substantially equal flow whichever fan is driven.

Figure 8 shows the results of Tests 5, 6 and 7, which show the reduction in performance (efficiency) of the twin fan unit of figure 2 compared with the use of a single fan in the same housing, with the other fan and the vanes and taper removed.

Test 6 shows the performance (pressure vs. volume at constant power) of a single fan in an unobstructed housing (i.e. the ideal performance of the fan) and tests 5 and 7 show the performance of the twin fan unit of figure 2 with, respectively, fan 1 and fan 2 driven as the duty fan. It can be seen, as in figure 7, that the same performance is obtained whicher fan is driven in the twin fan unit, but figure 8 shows that the twin fan unit achieves slightly poorer performance than either fan 1 or fan 2 operating alone in an unobstructed housing. Thus figure 8 quantifies the obstruction to the airflow produced by the vanes, the taper, and the windmilling, standby fan in the twin fan unit. This reduction in performance is most noticeable at high flow rate and low pressure but is acceptable under all conditions.

Whilst only the use of four guide vanes has been described, the choice of the number of guide vanes is based on an assessment of performance and cost. It was initially expected that a much greater number of guide vanes would be needed, but the inventor has found that, surprisingly, only a small number of vanes produces very good performance. In particular, tests with 6 vanes and 4 vanes (as in the embodiment of figure 2) produced very good results. The number of vanes may be less than 14, or preferably between 1 and 10 to produce good performance.

In keeping the cost of the unit to a minimum it is advantageous to keep the number of guide vanes to a minimum.

Whilst the preferred embodiment has been described with an angular taper formed of four taper pieces, the main requirement is that the taper gradually decreases the cross-sectional area from that of the outlet of the front fan to that of the intake of the rear fan. The taper or cone may be linked to the outlet of the front fan and/or to the inlet of the rear fan by non-tapered duct portions.

The taper or cone may be angular or rounded, straight or curved. In practice, the shape of the taper is likely to reflect the shape of the housing, for ease of construction. For example, a circular, truly frustocomical, taper would be conveniently used in a circular housing.

Whilst the housing of the twin fan unit has been described as rectangular or square in cross-section, it may be of any cross-section. However, in practice the ducting into which such units are fitted is normally of square, or rectangular, cross-section.

Although the foregoing description has considered only one fan being driven, in some applications it may be advantageous to drive both. The guide vanes and taper improve the performance of the rear fan by smoothing the gas flow entering it when the front fan is driven. If both fans are driven, better performance therefore results than if two fans in a housing are driven without the guide vanes and taper between them. In a further aspect the invention therefore provides an effective twin fan unit in which both fans are driven.

Claims (12)

1. A fan unit comprising two fans spaced from each other and mounted within a housing for driving gas through the housing, the gas flow passing through both fans in series; and a taper means for axially reducing the crosssectional area of the gas flow and a plurality of guide vanes arranged between the exhaust of the upstream fan and the intake of the downstream fan;

2. A fan unit according to claim 1, wherein both fans are centrifugal fans or mixed-flow fans.

3. A fan unit according to any preceding claim, wherein the taper means reduces the cross-sectional area of the flow from the upstream fan substantially to match the intake of the downstream fan.

4. A fan unit according to any preceding claim, wherein the guide vanes extend substantially radially inwardly from the housing, an upstream portion of one or more vanes being positioned near the exhaust- of the upstream fan and a downstream portion of one or more vanes extending towards the downstream fan.

5. A fan unit according to any preceding claim, wherein the guide vanes extend along the length of the taper means.

6. A fan unit according to any preceding claim, having 10 vanes or fewer.

7. A fan unit according to any preceding claim, having four guide vanes.

8. A fan unit according to any preceding claim, wherein only one fan is driven at any time.

9. A fan unit according to claim 8, wherein the undriven fan is allowed to rotate freely.

10. A fan unit according to any preceding claim in which the two fans are substantially identical.

11. A fan unit according to any preceding claim in which only one fan is driven at a time and substantially equal performance is obtained whichever fan is driven.

12. A fan unit substantially as hereinbefore described with reference to figures 2 to 8.