Abstract:
A fan is provided for use with a casing having a bellmouth and a conduit portion. The fan has a hub, a number of blades extending from the hub, and a shroud. In cooperation with the casing and the conduit portion surrounding the shroud, the shroud has means for generating a separation bubble effective to limit a recirculation flow.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to heating ventilation and air conditioning (HVAC) systems. More particularly, the invention relates to axial flow rotating shroud fans for such systems. 
   Fans are ubiquitous in HVAC systems. Many fan configurations exist. One group of fan configurations is axial flow rotating shroud or banded fans. These fans include a circumferential shroud at outboard ends of the blades. The blades and shroud of many such fans are unitarily molded of plastic to provide lightness and ease of manufacturing. Such fans are often situated closely downstream of a heat exchanger to draw an air flow across the exchanger (e.g. an air-cooled condenser). 
   A variety of efficiency concerns attend the design of fans and the associated environment (e.g., housings or casings). One area of efficiency loss results from the recirculation of air near the blade tips. The presence of the shroud may somewhat address this recirculation. Nevertheless, many additional mechanisms have been proposed to further reduce recirculation. For example, U.S. Pat. No. 5,489,186 shows one such proposal. 
   SUMMARY OF THE INVENTION 
   One aspect of the invention involves a fan having a hub and a number of blades extending from the hub. A shroud is unitarily formed with the blades and has first and second rims and inboard and outboard surfaces extending between the first and second rims. Along a portion of the shroud proximate the second rim, the outboard surface is radially inwardly convergent toward the second rim along a longitudinal span effective to generate a separation bubble to limit a recirculation flow. 
   In various implementations, the longitudinal span may be at least 3 mm. The first rim may have a first diameter and the second rim may have a second diameter greater than the first diameter. First and second such fans may be stacked with the first rim of the first fan received concentrically within a portion of the shroud of the second fan. The fan may be combined with a casing including a first portion having a rim having a third diameter less than the first diameter and a second portion at least partially surrounding the shroud. The casing first portion may be a bellmouth of essentially outward longitudinal concavity. The second portion may be a stack extending beyond the bellmouth. The fan may be oriented so that the shroud first rim faces upward. The stack may have a distal end beyond the shroud first rim. The casing first portion may be partially within the shroud. The outer surface of a portion of the shroud proximate the second rim may be radially inwardly convergent toward the second rim along a longitudinal span of at least 3 mm. A cross-sectional median of the portion of the shroud may be similarly convergent along such longitudinal span. The hub may carry a central metallic element having a central longitudinal aperture and a lateral surface. The hub may be unitarily formed with the blades and the shroud. The hub may be formed as an open cup having a sidewall and a base. The hub may further comprise a central boss for engaging a shaft mounting insert and a number of substantially radially-extending ribs extending from the boss to the sidewall and along the base and having rims. For each of the blades there may be a single associated one of the ribs aligned with the root of such blade. The hub may further include, for each of the ribs, a longitudinally and transversely-extending post formed in an outboard portion of the associated rib. Each post may have a cross-section characterized by a flat central web and first and second terminal protuberances. Inboard of the associated post, each of the substantially radially-extending rib rims generally increases in longitudinal position from inboard to outboard. Outboard of the associated post, each of the substantially radially-extending rib rims is of generally constant longitudinal position from inboard to outboard. The fan may further include a polymeric cover secured at an open end of the hub. The fan may be combined with a motor having a shaft having a portion coupled to the hub against rotation and a stator coupled to the shaft so as to drive the fan. 
   Another aspect of the invention involves a fan system having a motor. A fan is driven by the motor and has a hub, a number of blades extending from the hub, and a shroud. A casing has a conduit at least partially surrounding the shroud. Means are on the casing and shroud for creating a separation bubble between the casing and the shroud effective to limit a recirculation flow. The casing may have a downstream rim beyond a downstream rim of the shroud. The shroud may have a downstream rim portion with an outer diameter less than an outer diameter of an upstream portion, effective to permit a number of identical fans to be stacked in a partially nested configuration when off-motor. 
   Another aspect of the invention involves a fan having a hub, a number of blades extending from the hub, and a shroud. In cooperation with a casing having a bellmouth and having a conduit at least partially surrounding the shroud, the shroud has means for generating a separation bubble effective to limit a recirculation flow. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partially exploded view of a pair of electric fan units. 
       FIG. 2  is a view of a fan assembly of one of the units of  FIG. 1 . 
       FIG. 3  is a partial longitudinal sectional view of the fan assembly of  FIG. 2 . 
       FIG. 4  is a front end view of an insert of the fan assembly of  FIG. 2 . 
       FIG. 5  is a longitudinal sectional view of the insert of  FIG. 4 , taken along line  5 — 5 . 
       FIG. 6  is a view of a hub of the fan assembly of  FIG. 2 . 
       FIG. 7  is a longitudinal sectional view of an upstream portion of a shroud of the fan assembly of  FIG. 2 . 
       FIG. 8  is a longitudinal sectional view of stacked fan assemblies. 
       FIG. 9  is a partial longitudinal flow diagram of air flow through the fan assembly of  FIG. 2 . 
       FIG. 10  is an enlarged view of an upstream portion of the flow diagram of  FIG. 9 . 
   

   Like reference numbers and designations in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
     FIG. 1  shows a pair of electric fan units  20  mounted from a casing component  22  of an HVAC system. Each fan unit includes an electric motor  24  having a shaft  26  with a portion protruding from the housing or case  28  containing a stator (not shown). In operation, the motor shaft is driven about a common central longitudinal axis  500  of the fan unit to drive an air flow  400  in a downstream direction along a flowpath (e.g., from a condenser (not shown) directly below/upstream). The fan unit further includes a fan (impeller/rotor) assembly  30  mounted to the protruding portion of the shaft. 
   In the exemplary embodiment, each fan unit  20  is mounted to the casing assembly by a pair of mounting brackets  32 . In the exemplary embodiment, each fan assembly  30  is concentrically mounted within an annular cylindrical casing conduit segment shown as a stack  40  extending from a proximal end at a flat casing wall  42  to a distal end (e.g., a downstream rim  43 ) carrying a grille  44 . Other configurations are possible. 
     FIGS. 2 and 3  show further details of the exemplary fan assembly  30 . The fan assembly  30  includes the combination of a unitarily-formed molded plastic component  50  ( FIG. 2 ) and a metallic insert  52  ( FIG. 3 ). The metallic insert is at least partially embedded in a hub portion  54  of the molded component. Blades  56  radiate outward from inboard root ends at a sidewall  57  of the hub. The molded component further includes an annular shroud  58  at the blade outboard ends. The metallic insert includes a central longitudinal aperture  60  for receiving the protruding end of the motor shaft. The exemplary central aperture  60  extends between first and second end surfaces  62  and  64  of the metallic insert and consists essentially of a right circular cylindrical bore  66  ( FIG. 4 ) coaxial with the fan axis and a slot-like keyway  68  extending radially outward from at least a portion of the bore. The keyway receives a portion of a key  70  ( FIG. 1 ) of which a second portion is similarly received in a keyway in the shaft to lock the metallic insert to the shaft against relative rotation. A screw, bolt, or similar fastener  71  ( FIG. 1 ) may have a threaded shaft extending into a threaded aperture in the motor shaft and a head bearing against (e.g., via a washer) the front surface  62  to prevent unintended longitudinal ejection of the fan. 
   The metallic insert  52  has a lateral surface characterized by four facets  72  ( FIG. 4 ) defining a square cross-section. The square cross-section may correspond to bar stock (e.g., brass) from which the insert is cut. In the exemplary embodiment, to improve longitudinal engagement between the insert and the molded component, there may be one or more recesses  74  ( FIG. 5 ) in the lateral surface. An exemplary recess comprises a near-right annular channel having a circular cylindrical base  76  and a pair of near-radial sidewalls  78  and  80  with slightly radiused transitions. Additionally, the exemplary embodiment includes a pair of blind threaded bores  82  extending longitudinally inward from the front surface  62 . The bores  82  are off-center and aid in fan extraction from the motor/shaft as is discussed in further detail below. 
   In an exemplary process of manufacture, insert precursors are cut from square-section bar stock. The cutting (which may include one or more stages such as rough cutting and surface milling) essentially defines the end surfaces and the principal portion of the lateral surface. The cut precursor may be fixtured (e.g., in a lathe or similar tool) and the central bore  66  drilled and the channel  74  cut. The precursor may then be refixtured for milling the keyway  68  and again refixtured for drilling and tapping the bores  82 . 
   After the insert has been formed, it may be registered in a portion of a die (not shown) for molding the molded component  50 . The die may be assembled and plastic (e.g., glass-reinforced polypropylene) injected to form the molded component. The exemplary molding nearly entirely embeds the insert within the hub. In the exemplary embodiment, webs  84  and  86  ( FIG. 3 ) of the molded material extend along outboard portions of the insert ends  62  and  64 , having apertures therein to expose the channel at both ends and bores at the end  62 . The apertures advantageously extend sufficiently radially beyond the channel to permit engagement of the fastener  71  to the end  62  (e.g., by accommodating a washer) and engagement of a shoulder on the motor shaft with the upstream end  64  so as to longitudinally clamp the insert (e.g., via direct compressive contact). With the motor preinstalled in the appropriate environmental structure, the combination of the molded component and insert may be installed to the shaft (e.g., by sliding the insert over the shaft  26  and key  70  and installing the fastener  71  and/or by press/interference fitting). Thereafter, a cover (e.g., also molded plastic such as unreinforced polypropylene)  88  ( FIG. 3 ) may be placed over the hub (e.g., via snap fit within a perimeter of the hub). 
   Further details of the fan hub  54  are shown in  FIG. 3 . The hub includes a base wall  100  extending radially outward from a central boss  102  accommodating the insert  52 . In the exemplary embodiment, the base wall  100  is near an upstream end of the boss, with an upstream end portion of the boss including the web  86  protruding slightly upstream of the upstream surface of the base wall. The base wall has a rounded transition shoulder  103  with/to the sidewall  57 . A circumferential array of web-like radially-extending ribs  104  extend from proximal roots at the outer surface or periphery  105  of the boss  102  to distal ends at the sidewall  57 . Each rib  104  has a downstream end surface  106  diverging from the root outward. In a relatively outboard location (e.g., at about ⅔ of the radial span of the ribs  104 ) each rib has a post portion  107 . The post portions  107  have a pair of circumferentially/transversely extending portions  108  ( FIG. 6 ) having rounded circumferential end protuberances  110 . In the exemplary embodiment, downstream post surfaces  112  ( FIG. 3 ) are substantially flush to a downstream rim  114  of the sidewall  57  and a downstream rim  116  of the shroud  58 . The posts  107  divide the associated ribs  104  into inboard and outboard portions  118  and  119 , respectively. Along the outboard portion  119  the end  106  is just slightly recessed relative to the rims  114  and  116 . The end  106  is more substantially recessed along a major portion of a radial span of the inboard rib portion  118 . In the exemplary embodiment, the posts provide gates for the introduction of molding material. The post downstream surfaces  112  maybe engaged by mold-ejection pins to eject the molded component from the mold. The surfaces  112  also provide abutment surfaces for associated feet  120  of the cover  88 . Cover tabs  122  engage the inner surface of a downstream portion of the sidewall  57  in a snap fit/detent relation. 
     FIGS. 3 and 7  show further details of the shroud  58 . The shroud  58  has generally inboard and outboard surfaces  126  and  128  ( FIG. 3 ). The shroud  58  has a substantially longitudinally extending first portion  130  extending upstream from the rim  116  ( FIG. 3 ) for a length L 1  which may be a major portion (e.g., 80–90%) of a shroud height H 1 . The shroud then has an outwardly flaring portion  132  ( FIG. 7 ) transitioning from outwardly concave to outwardly convex and extending essentially to a maximum radius location  134 . Extending upstream therefrom is a tapering portion  136  along which the inboard surface is substantially longitudinal and the outboard surface is inwardly-convergent at a half angle θ 1 . Exemplary θ 1  values are 5–30°, more narrowly 12–25°. A sectional median is convergent at a half angle θ 2 . Exemplary θ 2  values are 3–15°, more narrowly 6–12°. 
   Extending outward/downstream from the flat wall  42  of the casing  22  is a bellmouth  146 . The exemplary bellmouth  146  is radial outwardly concave and downstream convergent to near longitudinal at a downstream rim  148 . The bellmouth is partially concentrically received within the upstream portion  136  with an exemplary radial separation S 1  and an exemplary longitudinal overlap S 2 . Exemplary S 2  is 0–10 mm. An exemplary separation between the shroud outboard surface  128  and stack inboard surface  160  is shown as a constant S 3  along the shroud lint portion  130  and a minimum value S 4  at the shroud maximum radius location  134 . 
     FIG. 8  shows how the fan assemblies can be stacked for shipping. In the stacking, a downstream portion of the shroud  58  of a first of the two stacked fans is telescopically received within an upstream portion of the shroud of a second fan. The hub base  100  of the second fan is concentrically received within a downstream portion of the hub sidewall  57  of the first fan. The dimensions may advantageously be such that there is contact both between: an inboard portion of the hub rim  114  of the first fan and an exterior surface of the hub shoulder  103  of the second fan; and an exterior portion of the shroud rim  116  of the first fan and the shroud interior surface along the outwardly flaring portion  132  of the second fan. This double engagement (which may be present under unloaded conditions or under very slightly loaded conditions (e.g., weight loading of an exemplary 2–10 stacked fans)) may provide exceptional stability and damage resistance for transportation of stacked fans. In stacking, an exemplary overlap S 5  may be an exemplary 10–15% of H.  FIG. 8  further shows the fan as having an exemplary maximum exterior diameter D 1 . Exemplary D 1  values are 0.5–1.0 m. Exemplary heights H are 0.08–0.15 m. An exemplary hub internal diameter D 3  at its downstream rim  114  is 0.25–0.35 m. An exemplary characteristic (e.g., mean or median) thickness T 1  along the portion  130  may be 2–4 mm. An exemplary thickness T 2  at the location  134  may be similar. An exemplary thickness T 3  at the upstream rim (or very close thereto) (e.g., within 0.5 mm or 11.0 mm) may be equal to or less than half of T 2  (e.g., 1.0–2.0 mm). The convergence may occur over a longitudinal span (e.g., between the location  134  and the upstream rim) in excess of 150% of T 2 . 
     FIG. 9  schematically shows the flow  400  forming from the casing interior  401  upstream of the bellmouth  146  downstream to an exterior  402 . This flow is through a central annular region  403  of the shroud. This flow may be characterized by a plurality of annular flow lines (lines when viewed in section) of which a single line  400 A is shown. Through an annular outboard shroud region  404 , between the inboard region  403  and the shroud inboard surface  126 , is a recirculating flow passing back down through a region  406  between the shroud outboard surface  128  and the stack inboard surface. This recirculating flow is illustrated by a number of exemplary flow lines  408 A,  408 B, and  408 C. The innermost of these flow lines  408 C is seen passing essentially along the shroud inboard surface. The presence of the outwardly flaring and inwardly tapering portions  132  and  134  helps create an annular separation region/bubble  410  ( FIG. 10 ) between the recirculating flow and the adjacent portion of the inboard surface  126 . Within this separation region, there may be a recirculating flow characterized by flow lines  412 . The effect of this separation bubble is that for a given physical radial spacing S 1  there may be a much smaller radial sectional span S 6  (and thus sectional area) for the main recirculating flow to pass. It is desirable to avoid collision between the shroud and bellmouth due to vibration, air disturbance, foreign object impact, and the like. This argues in favor of a large spacing (e.g., S 1  for the illustrated embodiment). However, minimizing loss due to the main recirculating flow argues against a large spacing. The presence of a separation bubble  410  thus helps achieve the anti-interference benefits of a large spacing with the efficiency benefits of a small spacing. 
   One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, when implemented as a reengineering or remanufacturing of an existing electric fan, details of the existing fan may influence details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.