EP1662149A1

EP1662149A1 - Axial fan inlet duct system with sound attenuation

Info

Publication number: EP1662149A1
Application number: EP04028226A
Authority: EP
Inventors: Dipti Datta
Original assignee: M&I Heat Transfer Products Ltd
Current assignee: M & I Power Technology Inc
Priority date: 2004-11-29
Filing date: 2004-11-29
Publication date: 2006-05-31

Abstract

A sound attenuating duct unit (55) for delivery of air to an inlet of a fan unit includes an elongate duct for transferring air to an outlet end adapted for connection to the inlet of the fan unit. This duct has a substantial bend formed therein and a splitter mechanism (76) is rigidly mounted inside the duct for providing sound attenuation. This splitter mechanism (76) is located upstream from the bend and extends longitudinally in the direction of airflow. The duct unit has a curved intake baffle (116) fixedly mounted in a lower end section of the duct (55) and this baffle (116) extends about and below a central axis (X-X) of the lower end section. The baffle (116) converges inwardly towards the outlet end (104) and is spaced from the central axis. Preferably, at least one vane member (132,134) is also mounted in the lower end section for turning the airflow to a horizontal direction and at least a portion of this vane member is curved in a vertical plane that is parallel to the central axis.

Description

This invention relates to sound attenuating duct units and suction boxes for transferring air or gases to a fan unit, such as an air supply unit for use in conjunction with a large air heater.
It is known to provide large utility and industrial boilers (steam generators) which are used for power and co-generation. These boilers can be oil fired with water tubes extending through the boiler and the water therein being heated by means of suitable air heaters. Large amounts of combustion air can be provided to these air heaters by means of a forced draft fan unit (herein sometimes referred to as a "FD fan"). This fan unit is powered electrically and can be arranged to rotate about a horizontal axis. It is known to deliver fresh air from the atmosphere through a long, vertically extending air duct that may include a splitter-type sound attenuating section. In a known air delivery system, the incoming air must change its direction through a substantial angle and must pass through a so-called trap section prior to moving horizontally into the fan inlet section.
Some known difficulties or deficiencies with the duct systems for delivering air to the FD fan include substantial power consumption for the fan, relatively high operational noise created in the vicinity of the fan unit, and duct vibrations.
In one conventional system for an inlet duct providing combustion air to an FD fan, there is a standard splitter silencer which has an open area across the transverse cross-section of the duct between 45 and 55% and which employs multiple splitters. Because of the configuration of these known silencers and because air follows the path of least resistance through an air duct, the flow through the passageways formed by the splitters is not uniform. There is in fact a biased flow in the center of the splitter silencer which results in increased pressure drop across the silencer that is directly proportional to the change in the face velocity. There can also be airflow induced vibrations in the inlet duct unit due to a sudden change in the velocity of air.
An example of an inlet duct system for a fan unit, which system also provides sound attenuation, is described in United States Patent No. 5,728,979 which issued March 17, 1998 to Air Handling Engineering Ltd. This sound attenuating duct unit is designed for use in a commercial or industrial building and has a box-like exterior housing that includes a top, bottom and end walls. An annular outlet opening is formed in one end wall for arrangement next to the fan inlet and this end wall is in a vertical plane. There are two rectangular main inlet openings located on opposite sides of the housing, these sides extending perpendicular to the aforementioned end wall. Interior walls are arranged in the housing and define airflow passageways, each of which is substantially curved in an axial plane. The interior walls can be constructed with perforated sheet metal and can contain sound absorbing material.
In more recent published U.S. patent application No. 2002/0182061 published December 5, 2002, there is described a large sound attenuating inlet structure for a turbine. The inlet structure comprises a vertically extending duct structure having an upper section with a plurality of vertically extending sides and a top cover. Air inlet openings are formed in the sides and an air outlet is formed at the bottom end for connection to the air intake housing of the turbine. An elongate, central airflow defining member extends downwardly from the top of the structure and has an initial conical section and a longer cylindrical section. Funnel-shaped interior walls can be provided in the inlet duct structure and these provide additional sound attenuation.
According to one aspect of the present invention, there is provided a sound attenuating duct unit for delivery of air or gases to an inlet of a fan unit, which unit may provide good sound attenuation while also providing good airflow characteristics and efficient airflow to the fan unit.
According to another aspect of the invention, there is provided a new sound attenuating duct unit for delivery of air or gases to a fan inlet, which may be constructed at a reasonable cost while providing the advantages of good sound attenuation and efficient air or gas delivery to the fan unit.
According to another aspect of the invention, there is provided an improved suction box apparatus for delivery of air or gases to an inlet of a fan unit, this apparatus having the capability of bending the direction of air or gas flow substantially between its inlet end and its opposite end, while providing efficient airflow characteristics for delivery of air or gas to the fan inlet.
According to one aspect of the invention, a sound attenuating duct unit for delivery of air or gases to an inlet of a fan unit having an axis of rotation includes elongate duct means for transferring air or gases from an inlet end thereof to an outlet end thereof adapted for connection to the inlet of the fan unit, the duct means having a substantial bend formed therein. Splitter means are rigidly mounted inside the duct means for providing sound attenuation and the splitter means extends upstream from the substantial bend. The splitter means includes at least one splitter member extending longitudinally in the direction of air or gas flow, containing sound attenuation material, and having sidewalls made of perforated sheet metal.
This duct unit is characterized by a curved intake baffle fixedly mounted in a lower end section of the duct means and extending about and below the central axis of the lower end section during use of the duct unit. The lower end section is located at the outlet end of the duct means and its central axis is substantially aligned with the axis of rotation during use of the duct unit. The intake baffle is formed by a baffle wall which converges inwardly towards the outlet end and is spaced from the central axis.
In a particular, preferred embodiment, the duct unit has at least one vane member for helping to control the direction of air or gas flow and turn the direction of flow from a substantially vertical direction to a more horizontal direction during use of the duct unit. This at least one vane member is disposed above the central axis of the lower end section and at least a portion of the vane member is curved in a vertical plane and is located in an upper portion of the lower end section.
According to a further aspect of the invention, a sound attenuating duct unit for delivery of air or gases to an inlet of an air supply fan having an axis of rotation includes an elongate first duct section for transferring air or gases from an inlet end thereof to an opposite end thereof and an elongate second duct section having an upstream end adapted for connection to the aforementioned opposite end and a downstream second end. Splitter means is rigidly mounted in the second duct section for providing sound attenuation and extends longitudinally in the direction of air or gas flow. The splitter means contains sound attenuation material and has sidewalls made of perforated sheet metal. A third duct section has a top opening adapted for connection to the second end of the second duct section during use of the duct unit and an outlet opening in a vertical side thereof adapted for connection to the inlet of the air supply fan unit. The third duct section during use thereof causes a substantial change in direction of the air or gas flowing through the duct unit. This duct unit is characterized by a curved intake baffle fixedly mounted in the third duct section and extending about and below a central axis of the third duct section. This central axis is substantially aligned with the axis of rotation during use of the duct unit. At least one vane member is provided for helping to control the direction of the air or gas flow and turn the direction of flow from a substantially vertical direction to a more horizontal direction during use of the duct unit. The at least one vane member is disposed above the central axis and at least a portion of the vane member is curved in a vertical plane and located in an upper portion of the third duct section.
In one preferred embodiment of the duct unit, there are two vane members and at last a portion of each vane member is curved and located in the third duct section, these members being spaced from one another and extending between opposite sidewalls of this duct section.
According to another aspect of the invention, a suction box apparatus for delivery of air or gases to an inlet of a fan unit having an axis of rotation includes duct means for transferring air or gases from an inlet end thereof to an opposite end thereof. The opposite end is adapted for connection to the inlet of the fan unit. The inlet end of the duct means faces upwardly and the opposite end is disposed substantially vertically during use of the apparatus. The duct means forms a passageway for air or gases that bends substantially from the inlet end to the opposite end. The duct means is adapted to accommodate a horizontal drive shaft for the fan unit, this drive shaft extending through a horizontal section of the duct means and extending to the opposite end during use of the suction box apparatus. The apparatus is characterized by a curved intake baffle fixedly mounted in the duct means within the passageway and extending circumferentially about and below a central axis which is co-axial with the axis of rotation during use of the apparatus. The intake baffle is formed by a baffle wall for directing the air or gas flow, this wall converging inwardly towards the opposite end and being spaced from the central axis.
In one preferred embodiment of this suction box, the intake baffle extends through an approximately semi-circular arc about the central axis and below the central axis and contains sound attenuating material.
Further features and advantages of the sound attenuating duct units and suction boxes of this invention will become apparent from the following detailed description of preferred embodiments taken in conjunction with the accompanying drawings.
In the drawings,

Figure 1 is a side elevation for a prior art system for delivering combustion air to a large utility or industrial boiler or steam generator, this view including a long duct section for delivery of air to a forced draft fan and a duct section connecting the fan unit to a bottom end of the boiler;
Figure 2 is a schematic side elevation of a prior art sound attenuating duct unit providing with hatching to indicate total pressure readings at various locations in the duct unit;
Figure 3 is a schematic vertical cross-section taken generally along the axis of rotation of the drive shaft of the fan illustrating a sound attenuating duct unit constructed in accordance with the invention;
Figure 4 is a side elevation providing details of a preferred construction of sound attenuating duct unit constructed in accordance to the invention;
Figure 5 is a perspective view of a lower portion of the sound attenuating duct unit of Figures 3 and 4, this view showing the lower portion from above and from the fan end;
Figure 6 is a schematic side elevation of the prior art sound attenuating duct unit of Figure 2, this view indicating the velocity of air flow by means of hatching at various locations in the duct unit;
Figure 7 is a schematic perspective view of the prior art sound attenuating duct unit of Figure 6, this view showing two longitudinally extending sides of the duct unit;
Figure 8 is a schematic perspective view similar to that of Figure 7 but illustrating a sound attenuating duct unit constructed according to the embodiment of Figure 3;
Figure 9 is a schematic side elevation of the prior art duct unit of Figure 6, this view illustrating the negative pressure distribution at various locations by means of hatching;
Figure 10 is a schematic side elevation similar to Figure 9, this view illustrating the velocity of air flow at various locations;
Figure 11 is a schematic perspective view of the lower portion of a duct unit taken from above and from the fan side, this view illustrating the total negative pressure distribution in this portion of the duct unit (that incorporates aspects of the invention) by means of hatching;
Figure 12 is a schematic side elevation of the lower portion of the duct unit shown in Figure 11, this view illustrating the velocity of air flow at various locations; and
Figure 13 is a schematic outline in perspective of the lower end section of the duct unit of Figures 11 and 12, this view illustrating the velocity of air flow across a transverse, vertical cross-section of the duct unit near the outlet end by means of the same hatching used in Figure 12.

Figure 1 illustrates a known system for delivering combustion air to a boiler unit by means of a standard forced draft fan located at 10. Fresh outside air is drawn into the inlet end of the fan by means of a long, generally vertical inlet duct 12. As illustrated, this inlet duct has a straight upper section 14 followed by a short, sloping section 15 and a long, vertical intermediate section 16 that extends down to a transition section 18. The transition section widens the air passage substantially to a width W which in one embodiment is about thirteen feet. Connected to the bottom of the transition section is a known, splitter silencer unit indicated generally at 24 and explained in more detail hereinafter with references to Figures 2 and 6. The splitter silencer has four vertical exterior sides, including opposing sides 25 and 26. Located below this silencer unit is another transition section which acts to narrow the passageway formed by the duct means or duct unit. Located below the transition section is a trap section 32 which forms an almost 90° elbow for turning the airflow through a substantial angle so that it becomes generally horizontal as it enters the fan unit 10. The trap unit 32, which also acts as a form of suction box, has a low level region 33 which collects any water that might come down the inlet duct 12 so that it can be drained out and will not enter the fan.
Connected to the outlet side of the fan unit is an elbow section of duct 34 wherein the pressurized airflow from the fan turns a sharp 90° and becomes an upwards flow through an elongate connecting duct 36 which increases in width to a transition section 40. The section 40 has a substantially wider upper end 44 as compared to its bottom end and connects the duct 36 to an open bottom of the boiler unit indicated generally at 46. This boiler unit includes a standard steam coil air heater 48 (SCAH) and can also include a regenerative air heater (RAH) of known construction. If desired, there can also be an economizer 50 located at the top of the boiler unit. As is well know, the boiler unit includes coils which can be used to produce steam, with the water in the coils being heated by hot air from combustion at the air heaters.
Aspects of the present invention are directed to both an improved sound attenuating duct unit to replace the inlet duct 12 (and similar prior art inlet ducts not illustrated) and the splitter silencer 24 as well as the trap section 32, and also to an improved suction box apparatus which can take the place of the illustrated, known trap section 32.
With reference to Figure 2, this figure illustrates schematically the sound attenuating duct unit for delivery of air in the system of Figure 1 and, in particular, the splitter silencer. The known splitter silencer has a plurality of standard, generally flat air stream splitters 52, each of which contains a standard form of sound attenuating material such as fiberglass bats or mineral wool. The flat, vertical sides of each splitter are formed of perforated sheet metal in a well known manner and the vertical length of each splitter is the same. The top ends of the splitters are aligned in a horizontal plane and the same is true of the bottom ends. The splitter silencer of this type typically has an open area as seen in horizontal cross-section of between 45 and 55%. Since the incoming air flow follows the path of least resistance, the flow across the splitters 52 is not uniform and there is a bias flow in the center of this silencer unit 24. This results in a change in the face velocity which is directly proportional to the pressure drop across the splitters. There is also a sudden change in the flow of velocity, particularly in the region of the trap section 32 which may cause flow induced vibrations.
Turning now to a preferred form of sound attenuating duct unit for delivery of air or gases to an inlet of an air supply fan unit, this duct unit being constructed in accordance with the invention, reference will be made to Figure 3 to 5. This duct unit indicated generally at 55 includes an elongate duct or duct means indicated generally at 56 for transferring air or gases to the inlet of the fan which can be either a FD (forced draft) fan or a ID (induced draft) fan. The duct unit can include an elongate first duct section 58 for transferring air or gases from an inlet end thereof indicted generally at 59 to an opposite end thereof indicated generally at 60. As can be seen clearly in Figure 8, the first duct section can have a rectangular horizontal cross-section formed by four exterior walls 61 to 64. If desired, two opposing walls of the duct or all four walls of the duct can be lined with sound attenuating material, this material being covered by perforated sheet metal in a manner known per se. The duct unit 55 also has an elongate second duct section 68 having an upstream end located generally at 70 adapted for connection to the aforementioned end 60 of the first duct section and also having a downstream second end 72. Although the second duct section as illustrated is shown as straight but tapered in the direction of air flow indicated by the arrow A, it will be appreciated that the second duct section can be made so that it bends through a smooth gentle curve between its first and second ends, if desired. Generally, the amount of this bend would be substantially less than 90 degrees. Preferably, the amount of any bending in the second duct section is less than 45 degrees, more preferably, less than 30 degrees.
In the illustrated preferred embodiment, splitter means are rigidly mounted in the second duct section 68 for providing sound attenuation and, in the illustrated embodiment, the splitter means extends substantially the entire length of the second duct section. Depending upon the amount of sound attenuation required, however, the splitter means can be made shorter than the length of the second duct section and, if it is made shorter, its longitudinal position in the second duct section can vary. The illustrated preferred splitter means includes a single splitter member 76 and sound attenuating material can extend substantially the entire length of the splitter between an upper end 80 and a bottom end 82. The preferred sound attenuating material comprises mineral wool which is wrapped in Mylar™ sheeting which acts to prevent the mineral wool from being pulled from the interior of the splitter by the airflow in the duct unit. Instead of mineral wool, it is also possible to use fiberglass batting which can also be covered by protective sheeting, if desired. Preferably the sheet metal sides of the splitter 76 are made of perforated 16 gauge galvanized steel. This perforated metal forms first and second longitudinally extending sides 84, 86 which are flat in the case where the second duct section 68 is a straight section. A semi-cylindrical nose portion 88 of the splitter can be made of imperforate sheet metal and it can be reinforced and strengthened by means of an internal wall 90 extending from one side to the opposite side of the splitter. A smaller, semi-cylindrical tail section can be provided at the downstream end 82 of the splitter. Preferably, the splitter member 76 is slightly tapered in the direction of air flow as illustrated. In a preferred embodiment, the amount of taper corresponds substantially to the amount of taper of the second duct section 68 so that the flat splitter walls 84, 86 are substantially parallel to respective opposing walls of the duct section. Again, if the second duct section bends through a smooth curve, the splitter member will be bent in an amount corresponding substantially to the bend in the second duct section. Preferably the splitter member 76 is located centrally in the second duct section so as to divide the airflow passageway in this section into two, substantially equal smaller passageways 92 and 94.
It should be understood that the first duct section 58 can be longer than the second duct section 68 and, in their preferred use, both sections extend substantially vertically. For ease of illustration, only a portion of the first duct section 58 has been shown in Figures 3, 4 and 8. In fact, the first duct section 58 can be as long as the duct sections 14, 15 and 16 illustrated in Figure 1. It should be understood that it is possible to replace the single splitter shown with two or three splitters that are evenly spaced across the horizontal width of the duct section 68 although a single splitter is preferred. Also, instead of the single splitter 76 or several splitters in the section, it is also possible to construct a duct unit or suction box incorporating aspects of the invention which employs a multiple splitter arrangement as shown in Figures 2 and 7.
The duct unit 55 also includes a third duct section 100 having a top opening 101 located at a top end 102, which is adapted for connection to the second end 72 of the second duct section during use of the duct unit. The third duct section also has an outlet opening 104 in a vertical end thereof adapted for connection to the inlet of the air supply fan unit 10. The third duct section 100 during use thereof causes a substantial change in the direction of the air or gas flowing through the duct unit 55. In fact, in the illustrated preferred embodiment, the change of direction is 90 degrees with the air flow being redirected from downwards vertical flow to horizontal flow. Also, the preferred third duct section includes a trap portion 106 which serves a similar trap function as the trap section 32 of the prior art inlet duct system described above. The trap portion also forms a suction box for the air supply fan. The third duct section has a horizontal portion 108 that is connected to the downstream end of the trap portion and is adapted for connection to the inlet of the air supply fan unit 10. It will be understood that the trap portion 106 has a bottom 110 located below the horizontal portion 108 during use of the duct unit. In this way, any water collecting in the bottom of the trap portion is held there and is not drawn into the horizontal portion and the fan. It will be understood that the length of the horizontal portion 108 can vary depending upon the particular system requirements and this portion can be shorter or longer than the illustrated portion 108. Also, in a known fan drive system for the fan unit 10, the motor rotating the fan is spaced some distance away from the fan and is operatively connected to the fan by means of an elongate drive shaft indicated in dash lines at 112 in Figure 3. Preferably, this drive shaft which can define the axis of rotation of the fan extends along a horizontal central axis of the horizontal portion 108 and is covered by a tubular drive shaft cover 114.
Located in the trap portion is a curved intake baffle 116 which is fixedly mounted in the third duct section. For example, it can be mounted by means of radially extending struts or straps 119 indicated in dash lines in Figure 5. These struts are arranged and distributed so as not to interfere with the airflow through the duct section. The support struts can be cross-bars (for example, ¼ inch by 4 inch flat bars) spaced at 90 degrees apart. The baffle extends about and below a central axis of the third duct section, this axis indicated at X in Figures 4 and 5. This central axis is preferably substantially aligned with the axis of rotation of the fan during use of the duct unit. This intake baffle may contain sound attenuating material such as the aforementioned mineral wool (or fiberglass batting) which is wrapped in and protected by Mylar™ sheeting. It is also possible to construct this intake baffle with no sound attenuating material using imperforate sheet metal. The purpose of this intake baffle is to direct and funnel the airflow that is being redirected in the third duct section into the fan unit. It is desirable that this airflow enter the horizontal portion 108 in a smooth flowing manner and in a manner which avoids unnecessary turbulence. The intake baffle also assists in distributing the airflow more uniformly across the width of the horizontal portion.
The preferred trap portion or suction box 106 has a rounded, semi-cylindrical bottom 120 which extends below the horizontal portion 108. The horizontal portion 108 is cylindrical in shape and open-ended. The drive shaft for the fan can extend through the trap portion to an electrical drive motor (not shown) located outside of the duct unit. The hole through which the drive shaft extends in the side of the trap portion is suitably sealed in a known manner. The preferred trap portion has opposed vertical sidewalls 122 and are spaced apart a distance greater than the internal diameter of the horizontal portion 108 and an end or back wall 124. Fixedly mounted on the back wall is an interior air directing cone 126. It has a conical axis that is co-axial or at least substantially co-axial with the central axis X. This cone can be made of imperforate sheet metal and the cone merges at its apex with the tubular drive shaft cover 114. Thus, the drive shaft for the fan passes through the center of the cone as well. The cone 126 tapers in a direction towards the outlet opening 104 of the third duct section.
Returning to the intake baffle 116, the preferred baffle has a partial frusto-conical shape and converges inwardly towards the outlet opening 104 in the vertical end of the third duct section. In one preferred embodiment, this baffle is made of perforated sheet metal, such as 16 gauge galvanized steel, on both sides, if it contains sound attenuating material. The illustrated preferred baffle has a convexly curved inner surface 128 and a radially outer surface 130 which is concave.
The preferred duct unit 55 also has at least one curved vane member and, more preferably two curved vane members 132, 134, which are provided in and mounted in the third duct section 100 for helping to control the direction of the air or gas flow and turn the direction of flow from a substantially vertical direction to a more horizontal or substantially horizontal direction during use of the duct unit. At least one vane member and preferably both vane members 132, 134 are disposed above the central axis X in an upper portion of the third duct section 100. The two vane members 132, 134 are spaced from one another and extend between opposite vertical sidewalls 122 of the third duct section. Each of the vane members has a leading upstream edge 136 which can be located adjacent a top end of the third duct section and each vane member preferably curves downwardly and inwardly towards the outlet opening through an arc of at least 30 degrees. Each preferred vane member as shown in Figures 3 to 5 is curved in a vertical plane that extends parallel to the axis of rotation of the fan unit. It will be appreciated that with the combination of the elongate splitter member 76, the curved intake baffle 116, the interior cone 126 and the curved members 132, 134, a generally streamlined flow can be provided in the region of the trap portion or suction box and there is a relatively large open area for the air to flow through the duct unit into the fan. This streamlining of the airflow and the control of velocity of the airflow eliminates flow induced vibrations and smoothes airflow. This in turn results in minimal noise coming from the duct unit. Furthermore, the preferred duct unit 55 can provide more uniform loading of air to the fan blades for better fan performance. It is believed that a preferred embodiment of the duct unit can provide pressure drop savings compared to the above described prior art in the range of 1-1.5 inches WG and the noise attenuation provided can be up to 5 dB better than the noise attenuation provided by some conventional units.
As illustrated in Figure 5, the preferred intake baffle 116 extends through an approximately semi-circular arc about the central axis X and is located below the central axis. The intake baffle is formed by a baffle wall which, as indicated, converges inwardly towards the outlet end of the duct unit and is spaced from the central axis X.
Turning now to construction details for an embodiment of the duct unit incorporating the invention, these details being illustrated in Figure 4, it will be seen that the second duct section 68 wherein the splitter member is mounted, can be made from several duct components connected end to end. As illustrated, there are three duct components 140 to 142 with the duct component 140 at the top and the duct component 142 at the bottom end. These components can be made as separate, manageable units in the factory and then transported separately by truck or train to the installation site where they are connected together. The splitter member 76 can also be split into three parts, one for each of the duct components 140 - 142, and these parts can be mounted in their respective components at the factory. The parts of the splitter are connected together when the duct components are connected. The duct components 140 - 142 can be connected together by bolts and nuts using connecting flanges such as adjacent flanges 144 and 146. In order to allow for thermal expansion of the overall duct unit, an expansion joint 148 of standard construction can be provided between the second duct section 68 and the third duct section 100. The external walls of the duct unit 55 can be fabricated from ¼ inch A-36 carbon steel plate which can be stiffened on at least two sides by means of 3"x3"x1/4" angle members such as the angle members 150, 152. The first duct section of the duct unit 55 can also be constructed using a series of duct components connected end to end. Only one of these duct components is illustrated in Figure 4 at 154 for ease of illustration. The length of these components can be similar in length to the components 140 to 142.
Each of the duct components 140 to 142 is preferably provided with opposed, perforated interior walls indicated at 156, 158 in Figure 4. Depending upon the amount of sound attenuation required for the particular duct unit, these interior walls can be provided on just two opposing sides of each duct component as shown or on all four sides of each duct component and sound attenuating material is provided behind each of these interior walls. The acoustic material preferably is a minimum 4" in thickness. In the illustrated preferred embodiment, two opposing interior walls are made of 16 gauge galvanized steel perforated with numerous small holes distributed over the surface of the sheet metal in a manner known per se. As in the splitter 76, the sound attenuating material can either be fiberglass batting, for example, ¾ pound fiberglass or mineral wool, and a Mylar™ film is arranged between the perforated sheet metal and the sound attenuating material to prevent erosion due to the air flow. Also, an interior support structure which connects the interior and external walls can be provided by criss-crossing twelve gauge formed channels that are joined by welding (for example) to the sheet metal panels. These channels are placed a maximum of two feet apart. This interior structure can also help to hold the sound attenuating material in place.
Also illustrated in dot dash lines in Figure 4 is a supporting framework 160 rigidly supporting the duct unit in a generally vertical position. There can be two to four steel support posts that are firmly mounted in the ground or in a concrete base (not shown). These posts can, for example, be made of interconnected steel angle members or they can be steel tubes. Each pair of posts can be rigidly connected to one another by means of crossing steel connectors 164, the ends of which can be welded to the posts. In one embodiment, each connector 164 is formed from two 8"x6"x7/16" angle members connected to each other by means of bolts or welding and suitable connectors, such as steel brackets, can be used to join the steel exterior of the duct unit to the framework 160.
In order that there will be no gaps or leaks between the duct components 140 to 142, 154, a 1/8^th inch neoprene gasket, which forms an airtight seal, can be arranged between the connecting flanges. In one preferred embodiment, the connecting flanges are formed by 3" x 3" x ¼" angle members.
Velocity and pressure tests have established the advantages of sound attenuating duct units and suction box apparatus incorporating one or more aspects of the invention as compared to prior art sound attenuating duct units. Referring first to Figure 2, the hatching used in this figure illustrates the total pressure readings found in the illustrated prior art duct unit as a result of applicant's testing and computer analysis. The scale on the left side of Figure 2 indicates the amount of total negative pressure (Pa) indicated by the various hatchings used on a scale of 0 to -1500 Pascals. In the prior art duct unit of Figure 2, the total negative pressure is fairly uniform and low in the straight upper portion of the duct unit. However, in the narrower air passageways in the region of the splitters (although not clear from the hatching in Figure 2), because of the biased flow in the central region of the splitter silencer (as indicated in Figure 9), there is an increased pressure drop across the splitter silencer that is directly proportional to the change in the face velocity of the airflow. In other words, the total negative pressure is higher in the central region of the splitter. Also, in the prior art, in the region indicated at 280, the total negative pressure is quite high along the top of the horizontal portion leading to the fan. There is also a very low total negative pressure region at 282. This is not the case in the improved duct unit 55 of Figure 3 wherein it has been found that the total pressure distribution in the horizontal portion 108 is generally more uniform. This is suggested by the total pressure illustration of Figure 11 (described in more detail below). This is a desirable condition as it results in more uniform loading of air on the fan blades for better fan performance.
Turning now to the velocity illustration of Figure 6, a velocity scale from 1 to 54 m/s is illustrated on the left side of Figure 6. Again, the hatching indicates that the velocity of the air flow is fairly low and uniform in the upper section of the prior art duct unit of Figure 6 and this is true as well in the duct unit of the invention. The velocity is also fairly low through the region of the splitters 52 or the single splitter 76. However, there is a substantial difference in the velocity readings in the horizontal portion that leads to the fan unit. In particular, there are much higher velocity readings in the region 286, in the order of 54 m/s extending along the length of the horizontal portion, both below and above the drive shaft 116. Tests have shown that in one of the duct units incorporating one aspect of the invention, the velocity readings in the horizontal portion 108 are also high, for example, in the range of 53 - 84 m/s as illustrated by the hatching in Figure 12. The controlled velocity in applicant's duct units helps to eliminate or reduce flow induced vibrations as well as eddies in the flow and this results in less noise being produced from the duct unit and lower pressure loss.
Turning now to the pressure distribution illustration of Figure 9, a pressure scale extending from 0 to -4600 kg/m² is shown on the left side of Figure 9. As illustrated by the various hatching on Figure 9, the negative pressure is reasonably low and uniform in the upper portion of the prior art duct unit. The uniform, low negative pressure readings extend down to the upper ends of the splitters. However, in the prior art duct unit of Figure 9, the negative pressure readings in the narrow passageways between the splitters varies across the width of the duct unit and the negative pressure remains low in the region 290 below the splitters before it increases to about -550 kg/m² at 292. The negative pressure then becomes quite high, in the range of -2850 kg/m² in the horizontal portion of the duct unit. In a duct unit incorporating a single elongate splitter member, there is a more uniform pressure distribution in the transverse direction in the splitter region. With this preferred duct unit 55, it may be possible to decrease the pressure drop in the range of 0.95 to 1.5 inches WG.
Turning to the velocity illustration of Figure 10, a velocity scale extending from 10 to 80 meters per second is shown on the left side of Figure 10. The hatching provided on Figure 10 indicates the velocity is substantially uniform and low through the upper region of the duct unit, through the splitters and in the transition section below the splitter and this is true in the duct unit of Figure 8 as well. In the prior art duct unit of Figure 10, there is a large region 300 where the velocity of the air is quite high, being in the range of 70 to as much as 80 metres per second.
With respect to schematic Figures 11 to 13, in addition to illustrating total pressure and velocity profiles by means of hatching, these figures also illustrate an alternate form of vane members. In particular, in this embodiment of the inlet duct unit, there are two relatively long vane members 302 and 304, each of which has a long straight section 306 and a much shorter, curved portion 308, 309. As will be apparent in the drawings, there is no single elongate splitter member 76 in this embodiment, at least not in the location indicated in Figure 3. Instead, in the tapered duct section 68 there are the two long, straight vane sections 306 which are spaced evenly across the width of the duct unit in this region. Of course, above these vane members 302, 304, there can be arranged a splitter attenuator either in the form of a single, elongate splitter member, similar to the splitter member 76, or a series of parallel splitter members of the type illustrated in Figure 2. The curved portions 308 and 309 can be constructed in a manner similar to the short curved vane members 132, 134 described above. It will be appreciated as well that, if desired, the long vane members 302, 304 can be constructed as sound attenuating splitter members themselves, in which case the sheet metal exterior is made with perforated sheet metal in the known manner and the sheet metal covers sound attenuating material which can be of the types already described above.
Turning now to the pressure profile illustrated in Figure 11 by the hatching, it will be seen that the total negative pressure in the tapered duct section 310 which leads to the suction box and which contains the straight sections 306 is fairly uniform and is in the range of -2800 Pascals to -3200 Pascals. However, in the suction box, the total negative pressure decreases generally and in the horizontal portion 108 is in the range of -2600 to -2800 Pascals.
Turning now to the velocity profiles of Figures 12 and 13, it will firstly be seen that between and along the two straight sections 306 of the vane members, the velocity of the air flow does change significantly. For example, in the region 310 on one side of the vane member 304 and along an upper portion thereof, the velocity is in the 5 to 11 meters per second range. In between the two vane members, there is an area 312 wherein the velocity is in the range of 32 to 42 meters per second, a much higher velocity. The air flow then increases in the region 314 (just above the curved portions 306, 309) to the range of 53 to 63 meters per second, reaching a fairly high velocity in the range of 63 to 74 meters per second in the region 316 located on the convex side of the curved portion 309. Located on the convex side of the curved portion 308 is a medium velocity region 318 where the velocity is in the range of 32 to 42 meters per second. The velocity becomes much higher in the horizontal portion 108 and is, for example, in the 74 to 84 meters per second range in regions 320 and 322. The air flow velocity is somewhat less in the regions 324 where it is in the 63 to 74 meters per second range in this example. Of course, velocity ranges will vary depending upon various parameters including the size and horsepower of the fan unit, the actual dimensions of the inlet duct unit, etc.
Figure 13 illustrates the airflow velocity profile across a transverse cross-section of the horizontal portion 108 of the suction box. As can be seen from the hatching, the velocity of the air is reasonably uniform across the width and height of the annular duct in this region. Thus, in the area indicated at 330, the air flow velocity measured between 74 and 84 meters per second and this area covers most of the transverse cross-section. There is a horizontal band at 332 of lower velocity air flow in the range of 63 to 74 meters per second, this band being located above a central airflow defining bullet at 340. The bullet 340 can be a smooth extension of the central, conical airflow defining member 126 but may be of increased diameter compared to the adjacent cylindrical section of the member 126 (as shown in Fig. 12).
The more controlled and uniform velocity of the air as it approaches the fan in the present duct unit helps to eliminate flow induced vibrations and eddies in the flow and this in turn results in less noise being created and lower pressure drop.
Accordingly, there has been provided by the sound attenuating duct unit for delivering air to a fan described herein, substantial advantages which can result in operational savings and, in the case of sound attenuating duct systems, a significant reduction in noise output. It will be readily apparent to those skilled in the air handling art that various modifications and changes can be made to the present duct unit and the present suction box apparatus without departing from the spirit and scope of this invention. Accordingly, all such modifications and changes as fall within the scope of the appended claims are intended to be part of this invention.

Claims

A sound attenuating duct unit for delivery of air or gases to an inlet of an air supply fan unit having an axis of rotation, said duct unit comprising:
an elongate first duct section for transferring air or gases from an inlet end thereof to an opposite end thereof;

an elongate second duct section having an upstream end adapted for connection to said opposite end and a downstream second end;

splitter means rigidly mounted in said second duct section for providing sound attenuation, said splitter means extending longitudinally in the direction of air or gas flow, containing sound attenuation material, and having sidewalls made of perforated sheet metal; and

a third duct section having a top opening adapted for connection to said second end of said second duct section during use of said duct unit and an outlet opening in a vertical end thereof adapted for connection to said inlet of the air supply fan unit, said third duct section during use thereof causing a substantial change in direction of the air or gas flowing through said duct unit;

said duct unit characterized in that a curved intake baffle is fixedly mounted in said third duct section and extends about and below a central axis of said third duct section, this central axis being substantially aligned with said axis of rotation during use of the duct unit, and at least one vane member is provided for helping to control the direction of said air or gas flow and turn said direction of flow from a substantially vertical direction to a more horizontal direction during use of said duct unit, said at least one vane member being disposed above said central axis and at least a portion of said vane member being curved in a vertical plane and located in an upper portion of the third duct section.
A sound attenuating duct unit according to claim 1 characterized in that there are two of said at least one vane member and at least a portion of each vane member is curved and located in said third duct section, said two vane members being spaced from one another and extending between opposite sidewalls of said third duct section.
A sound attenuating duct unit according to claim 1 or 2 characterized in that said intake baffle extends through an approximately semi-circular arc about said central axis and below said central axis and contains sound attenuating material.
A sound attenuating duct unit according to any one of claims 1 to 3 characterized in that said at least one vane member curves downwardly and inwardly towards said outlet opening through an arc of at least 30 degrees.
A sound attenuating duct unit according to any one of claims 1 to 4 characterized in that said intake baffle is located in a trap portion of said third duct section which forms a suction box for said air supply fan.
A sound attenuating duct unit according to claim 5 characterized in that said third duct section has a horizontal portion connected to one side of said trap portion and adapted for connection to said inlet of the air supply fan unit, said trap portion has a bottom located below said horizontal portion during use of said duct unit, and an interior air directing cone with a conical axis substantially co-axial with said central axis is fixedly mounted in said trap portion, said cone tapering in a direction towards said outlet opening of the third duct section.
A sound attenuating duct unit according to any one of claims 1 to 6 characterized in that said curved intake baffle has a partial frustoconical shape and converges inwardly towards said outlet opening in said vertical end of the third duct section.
A sound attenuating duct unit for delivery of air or gases to an inlet of a fan unit having an axis of rotation, said duct unit comprising:
elongate duct means for transferring air or gases from an inlet end thereof to an outlet end thereof adapted for connection to said inlet of said fan unit, said duct means having a substantial bend formed therein; and

splitter means rigidly mounted inside said duct means for providing sound attenuation, said splitter means located upstream from said substantial bend and including at least one splitter member extending longitudinally in the direction of air of gas flow, containing sound attenuation material, and having sidewalls made of perforated sheet metal;

said duct unit characterized in that a curved intake baffle is fixedly mounted in a lower end section of said duct means and extends about and below a central axis of said lower end section during use of said duct unit, said lower end section being located at said outlet end of said duct means, said central axis being substantially aligned with said axis of rotation during use of the duct unit, said intake baffle being formed by a baffle wall which converges inwardly towards said outlet end and is spaced from said central axis.
A sound attenuating duct unit according to claim 8 characterized in that at least one vane member is provided for helping to control the direction of air or gas flow and turn said direction of flow from a substantially vertical direction to a more horizontal direction during use of said duct unit, said at least one vane member being disposed above said central axis and at least a portion of said vane member being curved in a vertical plane and located in an upper portion of said lower end section.
A sound attenuating duct unit according to claim 8 or 9 characterized in that said intake baffle extends through an approximately semi-circular arc about said central axis and below said central axis and contains sound attenuating material.
A sound attenuating duct unit according to any one of claims 8 to 10 characterized in that said intake baffle is located in a suction box portion of said duct means and said suction box portion forms a duct trap having a bottom located below said inlet of the fan unit during use of the duct unit.
A sound attenuating duct unit according to claim 8 characterized in that two, substantially parallel, curved vane members are provided for helping to control the direction of air or gas flow and turn the direction of flow from a substantially vertical direction to a more horizontal direction during use of said duct unit, said two vane members being disposed above said central axis and at least a portion of each vane member being curved in a vertical plane and located in an upper portion of said lower end section.
A sound attenuating duct unit according to any one of claims 8 to 12 characterized in that an air directing conical surface is formed in said lower end section and has a conical axis substantially co-axial with said axis of rotation during use of the duct unit, said conical surface tapering in a direction towards said outlet end of the duct means.
A sound attenuating duct unit to any one of claims 8 to 13 characterized in that said at least one splitter member comprises a single, substantially straight splitter member located centrally in a tapered section of said duct means, said tapered section extending substantially vertically during use of the duct unit.
A suction box apparatus for delivery of air or gases to an inlet of a fan unit having an axis of rotation, said apparatus comprising:
duct means for transferring air or gases from an inlet end thereof to an opposite end thereof, said opposite end being adapted for connection to said inlet of the fan unit, said inlet end facing upwardly and said opposite end being disposed substantially vertically during use of the apparatus, said duct means forming a passageway for said air or gases that bends substantially from said inlet end to said opposite end, said duct means being adapted to accommodate a horizontal drive shaft for said fan unit, said drive shaft extending through a horizontal section of said duct means and extending to said opposite end during use of the suction box apparatus;

said apparatus characterized in that a curved intake baffle is fixedly mounted in said duct means within said passageway and extends circumferentially about and below a central axis which is coaxial with said axis of rotation during use of the apparatus, said intake baffle being formed by a baffle wall for directing the air or gas flow, said wall converging inwardly towards said opposite end and being spaced from said central axis.
A suction box apparatus according to claim 15 characterized in that said intake baffle extends through an approximately semi-circular arc about said central axis and below said central axis and contains sound attenuating material.
A suction box apparatus according to claim 15 or 16 characterized in that at least one vane member is mounted in said duct means within said passageway for helping to turn the direction of air or gas flow from a substantially vertical direction to a substantially horizontal direction during use of said suction box apparatus, said at least one vane member is disposed above said central axis and extends across said passageway in a direction perpendicular to said central axis, and at least a portion of said one vane member is curved in a vertical plane which is parallel to said axis of rotation.
A suction box apparatus according to claim 15 of 16 characterized in that two vane members are mounted in said duct means within said passageway for helping to turn the direction of air or gas flow from a substantially vertical to a substantially horizontal direction during use of said suction box apparatus, said two vane members are disposed above said central axis and extend across said passageway in a direction perpendicular to said central axis, and at least a portion of each vane member is curved in a vertical plane which is parallel to the axis of rotation.