WO1995005542A1

WO1995005542A1 - External fan duct system

Info

Publication number: WO1995005542A1
Application number: PCT/US1994/009357
Authority: WO
Inventors: Glenn E. Warnaka; Robert Robey
Original assignee: Applied Acoustic Research
Priority date: 1993-08-16
Filing date: 1994-08-15
Publication date: 1995-02-23

Abstract

A system of the invention comprises a fan (20) through which a fluid passes, at least one fluid intake port located upstream from the fan, a fan housing (21) enclosing both the fan and the at least one fluid intake port, and an external duct (25) for directing fluid through the fluid intake port and thereafter to the fan. The duct has one end thereof attached to the fan housing at the fluid intake port with the end of the duct furthest away from the fan having a contoured configuration. The duct is divided into multi-cellular cross-sectional regions (41, 42, 43, 44, 45, 46, 47, 48, 49).

Description

EXTERNAL FAN DUCT SYSTEM

BACKGROUND

This invention relates to the use of an external duct system to promote the reduction of pressure loss at the inlet of a fan, thus resulting in, among other advantages, the fan's smoother performance. The system also serves, surprising, to reduce the power used by the fan's motor to operate the fan.

This invention relates to the reduction of undesirable back pressure in a fluid inlet, and in particular a centrifugal fan and to the reduction of the power required to drive such a fan. In particular, this invention is advantageously utilized to promote smoother performance in fans, and particularly large fans, that are utilized in industrial and utility applications.

Techniques and systems are known for guiding the flow of fluid such as air into the rotating blades of a fan. Most typically, stationary inlet guide vanes located within the housing of the fan may be utilized for such purposes. Such vanes are generally curved to guide the air into the rotating blades of the fan, and as a result such vanes are expensive to produce. It is further believed that such inlet vanes should be located as close as possible ahead of the rotating blades of the fan, so that the effect of turning the flow into the blades will not be lost. One disadvantage, however, to the use of such vanes is that it is believed that such stationary guide vanes will result in a pressure loss at the inlet of the fan. For example, see R.C. Binder, Advanced Fluid Dynamics and Fluid Machinery. Prentice-Hall, 1951, wherein he provides calculations for the pressure lost in inlet guide vanes.

With a significant amount of pressure loss at the fan inlet the fan will tend to run rough and unsteady. It, therefore, would be advantageous to have a system of reducing the amount of pressure loss at the inlet of a fan. There is a need for such a system that can be implemented on a wide variety of fluid intake devices and, most particularly, on a centrifugal fan, particularly larger fans utilized in industrial, such as power generating, applications. There also is a need for a system for reducing the pressure loss and improving the air flow on a centrifugal fan which can be implemented economically. Finally, it would be obviously advantageous to reduce the power needed to drive such a fan.

It is accordingly one object of the present invention to provide a system for improving the flow of air into a fluid intake device such as a fan, and accordingly reducing the pressure loss at the inlet of the device, in a manner which is improved over the prior art techniques.

SUMMARY

By this invention, there is provided a system to reduce the pressure loss at the inlet of a fan and the power consumed by a fan.

According to the invention, there is provided an apparatus, system, and method for improving the flow of air into a fan which also surprising serves to reduce the power consumed by the fan. In the method of the present invention, an external duct system, that is, a duct system that is located outside the fan housing and adjacent to the fan inlet, directs air through the fan housing and into the rotating blades. It is believed that the feature of being external to the housing of the fan is one aspect which structurally distinguishes the present inlet duct system over those of the prior art. More significantly, the present arrangement is distinguishable over prior art arrangements in that prior art inlet duct or vane arrangements known to the applicants are located much closer to the rotating fan and/or vane blades. Moreover, it is very surprising that the inlet duct system of the present invention actually reduces pressure loss at the fan inlet, with the result that the fan runs smoother with the virtual elimination of buffeting and pulsating sounds in the fan. Finally, it is very surprising that the power used to drive the fan is reduced significantly when the duct arrangement of the present invention is utilized in conjunction with such a fan.

The present external duct system can be utilized with existing fan configurations. With regard to the length of the duct system, and more specifically the length of the partitions that are incorporated in the duct, they are preferably, but not in every instance, of a length no greater than about the duct's largest cross-sectional measurement. For example, if the duct is circular the length of its partitions will generally be no greater than the duct's diameter.

As indicated, the duct is always externally located relative to the housing of the fan. Moreover, the duct is preferably multi-cellular in cross-section. Preferably, the cells are constructed between radial walls, i.e. partitions, directed from a central axis of the duct to the circumferential wall of the duct. Moreover, in order to facilitate the reduction of the flow resistance of the duct, the open end or mouth of the duct, that is, the end of the duct that is farthest away from the fan, may be contoured into a smoothly convergent structure. Preferably, the duct end may be shaped in the form of a smoothly convergent structure such as a tapered or a bell end. The purpose of having such a tapered or bell-shaped mouth is to provide a smoother transition for the flow of air and to reduce the turbulence at the interface of the moving and still air. At the same time, the bell-mouth structure also reduces the build-up of a pressure wave at the end of the duct. The pressure wave also reduces the flow into the duct, and when it is reduced the resistance to flow through the duct is diminished, and a smoother, more even flow results. Other contoured configurations, as will be described in more detail below, are also contemplated for the open-ended mouth of the duct.

The invention is now further described with reference to the accompanying drawings.

DRAWINGS Figure 1 is an pictorial illustration of the prior art relationship between the inlet guide vanes to an axial fan.

Fig. 2 is a sectional view of the method and structure of the present invention as seen through the axis of the duct system of the present invention.

Figure 3 depicts, looking into the inlet area of a fan, one embodiment of an air inlet section of an external fan duct showing a multi-cellular cross-sectional arrangement according to the invention. Figure 4 is an elevational view of a fan which may utilize the air inlet according to the invention. Figure 5 is a diagrammatic elevation of another embodiment of a fan according to this invention illustrating the relationship of the external inlet duct of the present invention to a fan. Figure 6 depicts, looking into the inlet area of a fan, another embodiment of an air inlet section of an external fan duct showing another form of a multi-cellular cross-sectional arrangement according to the invention.

Figure 7 is an elevational view of an alternative inventive external duct structure showing the compartments or cellular structures to the external inlet duct of a fan which further depicts the flared duct end according to the present invention.

Figure 8 is an end view of the compartmental or cellular arrangement shown in Figure 7.

DESCRIPTION A prior art duct system is diagrammatically represented in Figure 1. A fan means, generally depicted by 10, has a housing 11. Air will flow into housing 11 via duct 12 in the direction as indicated by arrow 14. Inside the duct 12 there are located a plurality of variable inlet vanes 13 which are designed to direct air flow into fan blades 15. As shown in Figure 1, inlet guide vanes 13 are located immediately ahead of fan blades 15. In addition. Figure 1 shows a further prior art embodiment wherein exit guide means 16 are located behind fan blades 15.

Fig. 2 is a sectional view of the method and structure of the present invention as seen through the axis of the duct system of the present invention. Fan means 20 is comprised in part of housing 21, within which there is located internal fan duct 22 and variable inlet vanes 23 which leads into the fan interior. Attached to the exterior of housing 21 adjacent to the inlet area is the external duct 25 of the present invention. The depicted duct 25 of Figure 2 is constructed around fan shaft 26, which extends outwardly from the interior of the fan assembly. Fan shaft 26 is protected by shaft guard 26a. The external fan duct of the present invention is divided, has, radiating outwardingly from central portion 27 a plurality of straight partitions 27 which serve to divide the interior of external duct 25 into a plurality of pie shaped compartments, cells or chambers. It is preferred that the external duct of the present invention is divided into chambers by a plurality of stationary internal partitions; however, the exact configuration of the interior compartments or chambers is not essential to the present invention, as long as they are not constructed in a manner or to a degree that serves to obstruct the flow of air into the interior of the fan. The partitions should, however, be straight and typically are located relatively far, that is, for example, from about 12" to about 60" or more, in front of the rotor of the fan. Additionally, the number of internal compartments in the duct of the present invention should be as few as possible so as to reduce obstructions to the flow and to reduce pressure loss at the inlet of the fan. However, there should be a sufficient amount of compartments to provide for the smooth directional flow of air into the fan. Typically, the number of compartments in an external duct may range between about two and twelve compartments in each duct, and therefore the number of partitions used to form such compartments will be one or more, although it is understood that the above figures may vary depending on such factors as the type and size of the fan, the characteristics of the fluid being moved and so forth. These parameters are discussed in more detail below. Figure 3 depicts, looking into the inlet area of a fan, one embodiment of an air inlet section of an external fan duct showing, looking horizontally into the air inlet, a multi-cellular cross-sectional arrangement according to the invention. In Figure 3, there is shown an external fan duct 40 illustrated in section. The interior of the fan duct 40 is configured into a multi-cellular or compartmental arrangement 41, 42, 43, 44, 45, 46, 47, 48 and 49 through the employment of intersecting vertical partitions 50 and 51 and horizontal partitions 52 and 53. The central axis rotating shaft 54 of the fan is located in the central cellular region 45 and extends outwardly from the interior of the fan (not shown) . Shaft 54 may or may not extend outwardly all the way through the external duct. Figure 4 is an elevational view of a fan that may be utilized in conjunction with the external duct system according to the invention .

Referring to Figures 3 and 4, the vertical walls 50 and 51 and horizontal walls 52 and 53 are located in the external inlet duct 55 to the housing 56 for a fan 57. The fan, on which the external duct system of the present invention may be suitable employed is diagrammatically illustrated in Figure 4 and is typically a centrifugal fan with blades that rotate on shaft 54. As illustrated in Figure 4, there are inlets 58 and 59 to either transverse end of the fan 57 and the outlet 60 is tangentially arranged. In the illustrated embodiment of Figures 3 and 4, the compartmentalized external ducts of the present invention are located at the air inlets 58 and 59. The shaft 54 is suitably mounted in bearings 61 spaced to either side of the housing 56, and, in the depicted embodiment, extends beyond the outer edges 70, which in the depicted embodiment are flared, of external duct 55. Another configuration of a fan structure utilizing the external duct means of the present invention is shown in Figures 5. In Figure 5, there is depicted a fan 71 mounted on a shaft 72. In the depicted embodiment fan 71 is a centrifugal fan which operates to drive air tangentially outwardly in the direction of arrow 79 from a fan housing 78. An interior inlet fluid duct construction 73, which is located within housing 78. The external duct configuration 74 of the present invention mates with the interior inlet duct 73. As indicated, in the configuration illustrated construction 73 is essentially part of the housing configuration 78 which surrounds fan means 71. The external inlet duct of the present invention is a circular, compartmentalized duct which mates with the interior fluid inlet 73 of the fan housing and may be affixed to fan housing 78 or inlet 73. At the inlet to the fan housing 73 are radially arranged shutters 75 which are operated by a rod 76 to open and close and thereby control the amount of air passing into the fan 70. Upstream of the shutters 75, that is, in the direction toward the interior of duct 74, is a cage 77 which serves as a protection to the fan inlet. The rod 76 passes through the cage 77 suitably so as to operate the shutters 75.

Figure 6 depicts, looking into the inlet area of a fan, another embodiment of an air inlet section of an external fan duct showing another form of a multi-cellular cross-sectional arrangement according to the invention. The radial walls 81 are arranged between a circumferential inner wall 82 and the circumferential outier wall 84. It is this circumferential outer wall 84 which will preferably have its edge contoured, such as in a bell mouth shape, as further described herein. Radial walls 81 appear to be spoke-like when viewed in cross-section, Between the outer wall 74, inner wall 72 and radial walls 71, there are constituted a multi-cellular or compartmentalized construction 83, 85, 86, 87, 88, 89, 90 and 91. The compartments, when viewed in cross-section, form regions which are pie-shape type 5. configurations for the inflow of air to the fan inlet duct 73 (as depicted in Figure 5) .

In Figure 7, there is illustrated a multi-cellular arrangement for an external inlet duct 108 of the present invention wherein the compartments 109, 110 and 111, 0 respectively are flared at the upstream ends 112, 113 and

114. The upstream ends are located at the inlet 115 to the duct. Between the cellular inlets 112, 113 and 114, there are partitions 116 and 117. Partition 116 is vertically arranged and partition 117 is horizontally arranged. The 5 flared or curved sections 112 and 114 are not extreme and conform to a construction to facilitate air flow into the duct 108.

Figure 8 is an end view of the compartmental or cellular arrangement shown in Figure 7. Referring to both 0 Figure 7 and Figure 8, different flair formations 119, 120,

121, 122, 123 and 124 are located around the perimeter of the inlet 115. The flared formations 112 and 114 are almost square in cross-section as are the sections 120 and 123. The flared formations 119, 121, 122 and 124 are circular sectors, i.e., pie-shaped sections. The central cross-sectional multi-cellular area 113 is a square configuration.

In the depicted embodiments, the radial spokes 81 or the partitions 116 and 117 are configured so as to minimize drag on air flow through the duct. EXAMPLES

The external duct system of the present invention was installed on a centrifugal fan having a capacity of 180,000 cubic feet of air per minute and a 60 inch diameter inlet. The external duct was located approximately two feet ahead of the rotor of the fan, and was divided into 6 internal compartments having a circular sector shape. Flow and power measurement were measured before and after the external duct of the present invention was installed. It was surprisingly discovered that there was a significant improvement in flow as measured by the pressure drop in front of the fan as per the following TABLE:

TABLE 1

PERCENT AIRFLOW PERCENT PRESSURE DROP REDUCTION

25% 69%

50% 67%

75% 62%

100% 58%

These results are very surprising in as much as, according to the authority cited above, it would have been expected that the pressure drop across the fan would have increased because of the presence of a duct, internal or external.

The external duct system of the present invention was installed on a second centrifugal fan being utilized in a power plant. The fan had a capacity of 450,000 cubic feet of air per minute and a 72 inch diameter inlet. The external duct was located approximately three feet ahead of the rotor of the fan, and was divided into six internal compartments having a circular sector shape. The current required to run the motor driving the fan was measured before and after the duct of the present invention was installed. It was surprisingly discovered that less current was required to operate the drive motor after the duct was installed. With the power plant operating at 90% load, the amount of current required to drive the structure was 5% less with the duct system of the present invention installed than without it. With the power plant operating at full load, the amount of current required to drive the fan was 6%% less with the duct system of the present invention installed. In addition, reading were taken of the vibration velocity of the fan in order to measure the structural vibration of the fan. A series of 26 readings measured at the fan's bearings were taken prior to the installation of the duct system of the present invention. The average reading for the fan was 0.061 ips, with the lowest reading being 0.051 ips. A reading was taken after the installation of the duct of the present invention. This reading indicated a vibration velocity of 0.029 ips, which represented a 53% reduction in velocity from the average reading and a 43% reduction in velocity from the lowest reading taken previously. ^" This reduction in velocity should therefore serve to improve the life of the fan system that is modified as per the present invention.

The large fans on which the inlet duct system of the present invention may be installed are typically, but not necessarily always, of the centrifugal type and generally run at constant speed. The air flow into such fans is regulated by inlet damper vanes. The vanes are positioned at the face of the fan and behind the inlet duct system of the present invention; that is, the vanes are positioned between the fan rotor and the inlet duct system of the present invention.

When no air flow into the fan is required, the inlet damper vanes are closed. When a maximum air flow is desired, the vanes are opened by rotating them approximately 90°. Hence, the air flow can be regulated between zero and the maximum capacity of the fan. Data taken from the fan indicates that, in all cases, the inlet damper vanes need be adjusted to a less open condition when the inlet duct of the present invention was installed than without it. This is another indication that the device of the present invention increases the air flow into the structure.

TABLE 2 set forth below illustrated forced draft fan data taken both before and after the inlet duct of the present invention. The motor used to drive the fan had 3500 horsepower and a voltage of 4160. The capacity of the fan was 455,000 cfm.

TABLE 2

LOAD MOTOR AMPS INLET VANE POSITIONS, %

MW BEFORE AFTER CHANGE % BEFORE AFTER

130 180 178 -1% 23 7

230 200 198 -1% 24 22

360. 260 250 -3.8% 39 37

430 325 310 -4.6% 54 48

480 417 390 -6.5% 85 72

In addition, typically under high operating loads and when the temperature of the ambient air is relatively high, fans may not be able to draw sufficient air to reach their capacity. When these conditions occur, the fans may go into a condition known as centrifugal stall. A centrifugal stall condition in a fan is similar to a wing stall in an aircraft and occur because the blades of the fan are not fully loaded by the air. This condition causes an air pulsation within the fan, and the pulsation generates a low frequency noise that is extremely annoying to communities near the fan. The low frequency noise produced by the rotating stall rattles doors and windows as well as dishes and pictures hung on walls, etc. of homes near the structure. Since the inlet duct system of the present invention was installed, no rotational stall has been observed. This observation again shows that the present invention increases and improves the air flow into fans.

Laboratory testing was conducted in order to confirm the information found with larger fans utilized, for example, in the power industry. In such tests, a small centrifugal fan with a 9 3/16 inch diameter rotor and an 8 inch inlet diameter were utilized. These tests confirmed that there was a large air flow at the rim of the inlet duct which were not present before the external duct of the present invention was installed. The same improvement in air flow at the rim of the inlet duct was also observed in the large fans in the field. The following factors were found to be important in improving the flow:

(a) The contoured mouth served to smooth the flow at the entrance; (b) The partitions reduced the "swirl" or rotation of the flow, and, hence, eliminated much of the wasted flow energy so that it could be used for productive flow;

(c) The extension represented by the length of the short duct separated the contoured mouth and partitions from the near-field turbulence of the fan so that the interaction of the local air entrance effects are separated from the local turbulence caused by the fans. This appeared to be a particularly important factor in the larger fans; and (d) The contoured mouth and the partitions acting together seem to produce a synergistic effect of creating a large flow at the periphery of the fan. This large peripheral flow does not seem to occur without the presence of both the bell mouth and partitions. Laboratory tests were also performed on other parameters to determine their importance to the present invention. The effect of the number of partitions to which the internal duct was divided was determined. When the duct, fitted with a bell mouth, Was divided into two and four circular segments, there was a discernable improvement in the flow. When the duct was divided into six circular segments, the flow improved still more. When the duct was divided into eight circular segments, the flow was approximately the same as, but somewhat more disturbed, than when six segments were employed. When the duct was divided into twelve circular segments, the flow was good, but not quite as good as when the duct was divided into eight circular segments and the flow was more perturbed by the presence of the partitions. With the duct divided into eighteen and twenty-four circular segments, the flow became more degraded by the presence of the partitions. However, all test conditions show a large flow at the rim of the duct. However, the tests indicate that as more and more partitions are added inside the duct the point will be reached where the flow is obstructed to the degree that adding further partitions is not practical. In addition, tests performed on inlet ducts approximately 8 diameters in length, with partitions of varying sizes, indicated that the effect of partitions between 1/4 diameter and 1 diameter long had nearly the same effect of improving the flow as when the partitions extended the entire length of the duct.

It has been discovered that fans run smoother and quieter after the inlet duct system of the present invention was installed, and, as indicated, there was a marked decrease in structural vibration. In addition, the use of the duct system of the present invention has been shown to increase the amount of flow through the fan, which, in a power plant installation, for example, would enable the practitioner to increase the pressure at the boiler. This would better enable a power plant to utilize air pollution devices, such as scrubbers, which cause an increase in back pressure, and which generally, without the duct system of the present invention, inhibit the plants from coming up to full capacity.

Further tests were conducted utilizing the duct system of the present invention in conjunction with an axial flow fan, specifically a 17 1/2 inch diameter axial flow ventilation fan. The duct system utilized was made from cardboard. The length of the duct was 12 inches and there were 6 partitions, each 8 inches deep, extending internally from opposite sides of the duct. The duct therefore was divided into 12 multi-cellular regions. The velocity pressure measured at the edge of the fan before the duct system was installed was 0.02 psi; the velocity pressure measured after the installation of the duct system was 0.10 psi. This five-fold increase in flow velocity pressure was accompanied by an increase of 0.06 psi in stagnation pressure and a 0.02 psi decrease in static pressure due to the installation of the duct system.

The duct utilized in the present invention should be comprised of a hard, smooth structural material such as steel or another suitable metal, structural plastic, plastic composites, etc. A suitable material will combine necessary structural strength with suitable environmental qualities. The surface of the duct material should be hard and relatively smooth in order to facilitate the flow of air into the fan. Additionally, the material of the duct and partitions is made as thin as possible so as to reduce obstruction to flow. The duct preferably should not contain any obstructional restriction including wiring, electrical hardware and the like, and should be free of passive, sound- absorbing liners which could obstruct the flow. Thus, the material of the duct should be hard, smooth material such as metal or plastic which could further facilitate flow through the duct. The external duct system of the present invention will be made up of the external duct itself (including the flared outer end utilized in the preferred embodiment) , the partitions specified above, and, typically, a shaft guard, various service channels, accommodations for the bearing pedestal support structure and the motion of the inlet vanes when such extend into the external duct. As indicated, the entire duct system can encompass much of the pedestal which encompasses the shaft and bearings. With particular reference to centrifugal fans, the external duct system will be configurated to cooperate with the internal fan duct, which, as is indicated above, is part of the original centrifugal fan and is built into the fan housing. In this regard, the diameter of the external duct system will be made equal to, or slightly larger than, the diameter of the internal fan duct so as not to restrict the flow in any way.

As indicated, the partitions may provide service channels so that cooling water, lubricating oil, electricity, control features and so forth my be supplied to and from various functions of the fan.

The shaft guard and bearing housing may take the form of an aerodynamic center body, if desired. In such cases, the front of the shaft guard may be fitted with a dome, cone, or any other aerodyna ically shaped body to improve the flow of fluid through the duct.

As indicated, the configuration of the free end of the duct, that is, the end further most away from the intake of the fan, will be contoured. For example, the free end can be an elliptical, parabolic, exponential or straight flared. Most preferably, the free end of the duct will have a bell mouth configuration, which is preferred as it is effective and more economical to fabricate than the other contours specified. The external duct system of the present invention may be utilized in a wide variety of fans, but is particularly suitable for use in large fans which are used in industrial applications. The system is advantageous in that it improves the flow of air or other fluid through the fan. The external duct will have a compartmentalized or multi- cellular configuration which will be square, round or other cross-sectional shapes so as to facilitate the mechanical configuration and flow of fluid through the fan. As indicated, the length of the external duct of the present invention is generally no greater than the width of the duct, although greater lengths may be suitable for certain applications. In general, however, the external length of the inlet duct should be as short as possible. This minimizes problems related to lost pressure and, hence, efficiency, in addition to avoiding interference problems with other nearby machinery, and, of course, reducing cost.

Although the invention has been described with regard to air flow for a fan, it is clear that other configurations could be applicable.

The application of the invention is also applicable to internal combustion engines which are stationarily mounted and air conditioning fans, to name but a few uses. However, it should be noted that the applications of the present invention may include both stationary and moving noise sources. For example, the external duct system of the present invention can be used, when appropriately modified, in conjunction with the radiators and/or air intakes, of large trucks, construction equipment, automobiles, generators, air compressors, axial fans, turbines and the like. Many additional examples may be relayed which can be adapted to the external duct system of the present invention.

Many other forms of the invention exist, each differing from the other in matters of detail only. The invention is not to be limited by the particular embodiments disclosed. The invention is to be determined in terms of the scope of the following claims.

Claims

What is claimed is:

1. An apparatus comprising a fan means through which a fluid passes, at least one fluid intake port located upstream from said fan means, a fan housing enclosing both said fan means and said at least one fluid intake port, and an external duct means for directing fluid through the fluid intake port and thereafter to said fan means, said duct means having one end thereof attached to the fan housing at the fluid intake port with the end of the duct means furthest away from the fan having a contoured configuration, and wherein the duct means is divided into multi-cellular cross- sectional regions.

2. An apparatus as claimed in claim 1 wherein the duct means has a substantially circular cross-section.

3. An apparatus as claimed in claim 2 wherein the length of the duct means is no greater than its largest cross-sectional measurement.

4. An apparatus as claimed in claim 2 wherein the duct means contains from about 2 to about 12 cellular regions.

5. An apparatus as claimed in claim 2 wherein the multi-cellular sections are formed by radial walls directed from a central axis of the duct to the circumference of the duct such that the multi-cellular regions are arranged about the central axis of the duct.

6. An apparatus as claimed in claim 5 wherein the radial walls forming the cells of the duct are constructed to minimize drag on flow through the duct.

7. An apparatus as claimed in claim 2 wherein the multi-cellular sections are formed by intersecting straight partitions that extend between opposite points on the circumference of the duct.

8. An apparatus as claimed in claim 1 wherein the end of the duct means furthest away from the fan has a bell shaped mouth.

9. An apparatus comprising a fan means through which a fluid passes, at least one fluid intake port located upstream from said fan means, a fan housing enclosing both said fan means and said at least one fluid intake port, and an external duct means for directing fluid through the fluid intake port and thereafter to said fan means, said duct means having one end thereof attached to the fan housing at the fluid intake port with the end of the duct means furthest away from the fan having a configuration in the shape of a bell shaped mouth, and wherein the duct means has a substantially circular cross-section and is divided into multi-cellular cross-sectional regions which are formed by radial walls directed from a central axis of the duct to the circumference of the duct such that the multi-cellular regions are arranged about the central axis of the duct and wherein further the length of the duct means is no greater than its largest cross-sectional measurement.

10. An apparatus as claimed in claim 9 wherein the duct means contains from about 2 to about 12 cellular regions.

11. An apparatus as claimed in claim 9 wherein the radial walls forming the cells of the duct means are constructed to minimize drag on flow through the duct.

12. An apparatus as claimed in claim 1 wherein the fan means comprises a centrifugal fan.

13. A method of reducing the pressure loss in a fan having a fluid intake port encased by a fan housing, comprising directing fluid first through a multi-cellular duct means located external to the fan housing and thereafter into said fluid intake port, said duct means having one end thereof attached to the fan housing at the fluid intake port with the end of the duct means furthest away from the fan having a contoured configuration.

14. The method of claim 13 wherein the duct means has a substantially circular cross-section.

15. The method of claim 13 wherein the fan is a centrifugal fan.

16. The method of claim 13 wherein the duct means contains from about 2 to about 12 cellular regions.

17. The method of claim 13 wherein the multi- cellular sections are formed by radial walls directed from a central axis of the duct to the circumference of the duct such that the multi-cellular regions are arranged about the central axis of the duct.

18. The method of claim 13 wherein the end of the duct means furthest away from the fan has a bell shaped mouth.

19. The method of claim 13 wherein the fluid is a gas.

20. The method of claim 19 wherein the fluid is air.

21. A duct means for directing fluid to a fan means, said duct means suitable to be placed external to said fan means, said duct means having one end thereof with a contoured configuration, and wherein the duct means is divided by at least one internal partition into multi- cellular cross-sectional regions.

22. The duct means as claimed in claim 21 wherein the duct means has a substantially circular cross-section.

23. The duct means as claimed in claim 22 wherein the length of the duct means is no greater than its largest cross-sectional measurement.

24. The duct means as claimed in claim 22 wherein the multi-cellular sections are formed by radial partitions directed from a central axis of the duct to the circumference of the duct such that the multi-cellular regions are arranged about the central axis of the duct

25. The duct means as claimed in claim 21 wherein the length of the at least one internal partition is no greater than the duct's largest cross-sectional measurement.

26. The duct means as claimed in claim 21 wherein the multi-cellular sections are formed by intersecting straight partitions that extend between opposite points on the circumference of the duct.

27. The duct means as claimed in claim 21 wherein the contoured end of the duct means has a bell shaped mouth.