Abstract:
An energy efficient and compact axial fan with a housing comprising a fan assembly comprising a drive motor and a propeller coupled thereto. The inlet opening and/or the outlet opening of the fan housing are provided with a shutter assembly positioned adjacent the propeller and having a plurality aerodynamically shaped pivoted louvers working in tandem for less air obstruction and noise. The propeller comprises a plurality of radial blades comprising means for straitening the air flow and reducing the vortex flow of the propeller thereby permitting the shutter to be mounted adjacent the propeller to lessen the dept of the fan housing for making the fan more compact.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention disclosed herein relates generally to axial flow fans improvements and, more particularly to axial flow fans having a shutter operated by a centrifugal mechanism for maintaining a controlled environment in buildings such as poultry houses, greenhouses, steel-plants, foundries, etc.  
         [0003]     2. Description of the Related Art  
         [0004]     Axial flow fans are used to move high volume of air at low static pressure. An application in which they are beneficially used involves the ventilation of poultry house buildings which often are shaped like a tunnel. Intensive rearing of birds (typically more than 10 000) imposes severe demands on the fans which are provided for removing moisture, heat, carbon dioxide and other contaminants from said buildings.  
         [0005]     Hot summer days often cause indoor conditions to become much hotter than desired, temperatures above 25 degree Celsius have an adverse effect on production of said birds. The high temperatures can be controlled effectively with large exhaust fans and with water-wetted cooling pads. Typically eight to twelve ( 48 ″) propeller fans are installed in one end wall of the building and water-wetted cooling pads are installed on the opposite end wall of said building. When the fans work create a small negative pressure in the building causing outside warm air to pass across the water-wetted cooling pads causing some water to evaporate and extract some heat from the air, thereafter fresh air passes throughout the building cooling said birds.  
         [0006]     Most of the conventional fans for use in poultry house buildings applications are propeller type fans comprising a shutter assembly with gravity shutters or vanes extending horizontally across the intake or the exhaust of the fans. Said vanes do not open to a position where they are perfectly horizontal, i.e., the vanes have a slight downward angle thereto because of their weight. In additions the fans are too sensitive to added resistance therefore, a small increase of static pressure causes a substantial reduction of the air flow of the fans. As a result, when wind blows against the fans, the air flow decreases substantially and will not be sufficient to cool the birds which will suffer of heat stress.  
         [0007]     To overcome the detrimental effect of the insufficient airflow, a larger number of fans are used. However, the solution is not satisfactory because increases the capital cost and the operating cost due to an increase of energy consumption.  
         [0008]     In general, a conventional axial flow fan for agricultural applications includes a housing made of galvanized steel enclosing a drive motor and a propeller which is running adjacent to a venturi or orifice. The fan comprises a shutter assembly provided for preventing infiltration of outside air into the building and back flow through idle fans. Said shutter assembly typically is composed of a myriad of horizontally mounted pivoted vanes connected with cranks and at least one tie rod. As it can be imagined, the large number of vanes constitutes an obstruction to air flow and raise the cost of the fan, which is also increased by the cost of labor required to assemble the large quantity of vanes and elements connecting them.  
         [0009]     For maintaining a controlled environment on those poultry house buildings, the fans need to provide an appropriate and precise airflow. Any reduction in needed air flow can reduce bird performance and increases mortalities of the birds. In a conventional axial flow fan with gravity shutters used in a poultry house building dirt can build up on the shutters as fast as a ¼ pound per day and after few days of operation said accumulation of dust causes a reduction of at least 30% of the air flow of the fan and a reduction of air speed in the building of 30%. The lower air velocity causes a significant less cooling for birds.  
         [0010]     The shutters or vanes of the fans can be located at the fan intake where ambient air passes across the vanes with low turbulence or can be located at the fan discharge where air flow is more turbulent therefore causing more vibrations and noise. In north climates, for protection from stormy weather the vanes are mounted at the fan intake therefore they open inwardly the fan housing interior therefore the housing is deeper.  
         [0011]     In addition, in order to limit the vibrations and noise said vanes must be mounted away from the propeller, of at least of ⅓ of the diameter of the propeller. As a result, the fans are bulkier and heavier and the manufacturing and shipping costs are significantly higher.  
         [0012]     The vibrations and noise of said vanes is caused by the interference between the vanes and the vortex flow produced by the propeller, said interference creates a large head pressure that may be prevented providing propellers comprising means producing less turbulent air flow and vanes designed to be less affected by the turbulent air flow. The applicant provides shutter assemblies with a plurality of aerodynamically shaped vanes mounted adjacent the propeller and extending vertically across the intake and/or the exhaust of the fan. The vanes comprise leading edges connected together by a continuous hinge, therefore the vanes work in tandem.  
         [0013]     As the vanes are vertically pivoted gravitational dust is prevented from accumulating thereon. As the vanes work in tandem their drag and noise is reduced. However, said vertical vanes have to be operated by an actuator as a centrifugal shutter device. When the fan operates any pressure and energy is required to keep the vanes in their wide open position.  
         [0014]     In contrast to prior art fans with shutters comprising a myriad of vanes, the fan of the present invention comprises few vertical pivoted vanes working in tandem therefore the fan is able to move consistently and quietly at least 20% more than the prior art fans. The poultry house fans may operate continuously generating also undesirable noise and vibrations. As such, the fans collectively represent a significant energy drain and source of noise therefore it is important to pay more attention to energy efficiency and reduction of noise level in the design of such fans.  
         [0015]     Vibrations and excessive noise of the fans mainly are caused by the air pushed by the propeller and passing across the vanes which oscillate. Another important source of noise is caused by the blades of the propeller which during its rotational movement over obstacles produce a pressure variation on the obstacle which results in noise emission.  
         [0016]     As the drive motor is energized, the movement of rotation of the propeller blades imparts to the air a great deal of centrifugal force generating a turbulent air flow that quickly and erratically disperses upon exiting the orifice or venturi that normally surrounds the propeller blades. The turbulent air flow not only comprises a velocity component that is parallel to the propeller blades rotation axis but also a swirl comprised of variable velocity components of various obliquities to the propeller blades rotational axis. Since the function of the propeller blades is to move air in a direction parallel to a rotational axis, velocities imparted in other directions represent degradation in efficiency, increase of the noise and cost to operate the fan.  
         [0017]     In particular, the air flow around the tip region of said propeller blades creates a very noisy and strong vortex trailing downstream from the blade tips. To reduce this vortex, it is important to have a very close and uniform clearance between the propeller blade tips and the venturi, so as to minimize the air “leakages” and back air flow.  
         [0018]     The applicant solves the problem of vortex production by providing a propeller comprising a hub and plurality of airfoil shaped blades comprising a plurality of fins positioned at the pressure side of the blades. For each propeller blade the applicant provides a radially slidably adjustable shoe comprising a fin extending circumferentially for preventing radial components of the air flow from moving radially toward the venturi, for guiding the air flow substantially axially toward the fan discharge and for preventing air from flowing back around the blades tips. This results in a somewhat more efficient fan capable of operating quietly at a relatively higher static pressure.  
         [0019]     The applicant provides also a propeller with a hub and a plurality of airfoil shaped blades comprising a plurality of fins and an outer duct or band secured to the tips of the blades for co-rotation therewith; the blades are pivotally attached to both the duct and the hub. As the duct is attached to the blades tips the creation of vortices and vibrations is reduced. As the vortex generation of the propeller is reduced, if not eliminated, noise is reduced, the problems of air flow interference with the vanes are minimized therefore the vanes can be mounted in proximity of the propeller of the fan. As a result, the novel fan is more compact than prior art fans and it can be delivered to the end user totally assembled. Heretofore, it was necessary to ship a large amount of “air” in the assembled fans at a cost that became significant especially with overseas shipments.  
         [0020]     The overall effect of the combination of the improved propellers and improved vanes working in tandem is a noticeable reduction of back pressure on the propeller and a noticeable increase of the efficiency and performance of the fan.  
         [0021]     A lower vortex emission also implies that a lower amount of the energy provided to the fan is being spent in the vortex production so that a greater amount of energy can be used to produce useful work in the fluid. Hence, the reduction in the noise level comes with an increase in the fan efficiency.  
         [0022]     Another problem encountered with axial flow fans used in poultry house buildings includes the drive belt. Over time, a typical V-belt will eventually wear and the tension transmitted by the belt will vary causing slippage and reduction of fan air flow. Therefore the applicant provides toothed gears and timing belts. End users never have to worry about belt slippage and reduction of the airflow of the fans.  
         [0023]     To solve some of prior art problems of the shutters, the applicant previously provided a fan described in U.S. Pat. No. 6,276,895 issued on Aug. 21, 2001, and its teachings are incorporated herein by reference. The fan described in that patent had an exhaust shutter assembly with vanes that did not have insulation or protection from the elements.  
         [0024]     To prevent the problem of dust accumulation on shutters, the applicant previously proposed a fan with a shutter assembly having vertically pivoting vanes mounted across the fan housing. Reference of this fan may be found in my U.S. patent pending application Ser. No. 10/679,475 having filing date Oct. 7, 2003 and its teachings are hereby incorporated by reference. Other prior patents which are pertinent to the present application are: U.S. Pat. No. 4,217,816; U.S. Pat. No. 5,195,928; U.S. Pat. No. 5,288,202.  
         [0025]     The above patents have been fully described in my referred patent and patent application and show propeller fans comprising a centrifugal mechanism designed to operate positively the shutter vanes which are made of thin metal providing relatively poor insulating qualities, said vanes lack positive seals which do not permit them to be used effectively in severe and cold climate areas.  
       SUMMARY OF THE INVENTION  
       [0026]     The present invention relates to an axial flow fan that is particularly quiet in operation and effectively overcomes the prior art drawbacks.  
         [0027]     The first embodiment of the present invention provides an axial flow fan comprising a compact cylindrical housing having an air intake opening comprising a substantial large bell shape mouth, a drive motor for rotating a propeller secured to a fan bracket comprising four intersecting arms secured to four opposite sides of said input portion of said fan housing and to a bearing case for rotatably holding a rotating shaft to which are secured the propeller and a pulley coupled to said drive motor via a toothed belt.  
         [0028]     The drive motor is mounted on a pair of L shaped supports slidably mounted on said bracket such that the weight of the drive motor tightens the toothed belt. The fan housing further comprises a spaced apart outlet or discharge opening comprising a shutter assembly and a discharge cone.  
         [0029]     The propeller comprises a hub which is secured to said shaft for rotatably holding a plurality of airfoil shaped blades, each of which comprising a stem journaled into said hub. Each airfoil blade comprises a plurality of fins and a radial adjustable shoe comprising a circumferential extending fin configured for directing the air flow axially with reduced swirls. Said shutter assembly comprises a pair of vertically pivoted doors, semi-circular in form, positioned vertically across the outlet opening of the fan housing. The doors are coupled by a pair of gears for simultaneous movement in opposite directions, so as to move between open and closed positions to close or to open the discharge opening. The shutter assembly further comprises biasing means to maintain positively wide open the doors upon fan rotation. The shutter assembly is normally operated by a centrifugal mechanism secured to the aforesaid rotating shaft and positioned adjacent the propeller. The centrifugal mechanism comprises at least two centrifugal masses operatively connected to an axial sliding reciprocating actuator comprising operative biasing means to maintain positively closed the doors when the fan is not in use.  
         [0030]     When the drive motor is energized, the doors are quickly and quietly opened by the air flow produced by the propeller and when the drive motor is off, the doors are firmly closed by the operative biasing means.  
         [0031]     The second embodiment of the present invention provides a fan with a compact cylindrical housing similar to the one previously described but the centrifugal mechanism comprises a push rod secured to a bearing case comprising bearing means adapted to push and to pull the shutter doors via a pair or arms, causing their opening and their closing.  
         [0032]     The third embodiment of the invention provides a fan with a compact square shaped housing and a shutter assembly comprising a plurality of pair of vertical mounted vanes vertically extending across the outlet opening of the fan and having leading edges connected each other with a hinge. The vanes work in tandem and are operated by a centrifugal mechanism via a roller bearing connection. A propeller is provided with a plurality of airfoil shaped blades comprising an outer circular band bolted to the tip portions of said blades to lessen the known vortices, vibrations and noise. The fan housing is very compact and comprises an intake bell mouth having a round portion mounted coaxially to the outer circular band of said propeller for guiding smoothly the ambient air for best efficiency and low noise. The fan further comprises a discharge duct for protecting the shutter assembly and permitting moisture to be evacuated away from the fan housing.  
         [0033]     The fourth embodiment of the present invention provides a fan with a cylindrical housing comprising an intake opening and an output opening, a drive motor secured to a bracket rotatably supporting a hollow shaft on which are secured at least a propeller assembly and a centrifugal actuator. The propeller comprises a plurality of blades pivotally secured to a split hub, each of said blades is provided with a radial adjustable shoe mounted at the blade tip thereof. The discharge end of the fan is provided with a shutter assembly comprising a plurality of evenly angularly spaced pair of hinged vanes arranged to work in tandem. Thus provided, when the fan is on, all pair of vanes are able to rotate simultaneously and silently to their wide open position such as to assume a position parallel to the air flow. Each of the vanes comprises means for rotatably connecting them to the aforesaid centrifugal actuator.  
       OBJECT OF THE INVENTION  
       [0034]     It is an important object of the present invention to teach certain additional unique improvements upon the fans taught by the applicant in U.S. Pat. No. 6,276,895 and co-pending U.S. patent application Ser. No. 10/679,475.  
         [0035]     A further object of the invention is to provide a high efficiency fan comprising a shutter assembly comprising vanes operated by a centrifugal mechanism.  
         [0036]     A further object is to provide a maintenance free axial flow fan suitable to work at higher static pressure to move efficiently, quietly and consistently large quantities of air.  
         [0037]     Another object of this invention is to provide an axial flow fan that is strong, compact and durable, comprising few parts that can be quickly assemble.  
         [0038]     Another important object of this invention is to provide a low noise fan with a positive drive comprising a synchronous drive belt.  
         [0039]     Other features, advantages and the manner in which the foregoing objectives and advantages of the invention may be best achieved will be more fully understood from the following description when read in conjunction with the accompanying drawings which illustrate the preferred embodiments of the invention by a way of example  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0040]     A more particular description of the invention, briefly summarized above, may be added by reference to the embodiments thereof which are illustrated in the appended drawings and described herein. It is to be noted, however, that the appended drawings illustrate only some embodiments of the invention and are therefore not be considered limiting of its scope, because the invention may admit to other equally effective embodiments.  
         [0041]      FIG. 1A  shows a fragmentary rear view (intake side) of the axial flow fan  10  according the first embodiment of the present invention.  
         [0042]      FIG. 1B  is an enlarged front view of the frame assembly  25 .  
         [0043]      FIG. 2  is a longitudinal cross-sectional plan view of the axial flow fan  10  taken along line  2 - 2  of  FIG. 1A  illustrating the shutter assembly  39  in open position.  
         [0044]      FIG. 3A  is a vertical cross-sectional view taken along line  3 - 3  of  FIG. 1A  illustrating the shutter assembly  39  in open position.  
         [0045]      FIG. 3B  is an enlarged plan view of the support  46 , gears  48 A and  48 B.  
         [0046]      FIG. 4  is a front view of the propeller  20 A.  
         [0047]      FIG. 5  is a section along line  5 - 5  of  FIG. 4  showing the guiding fins  35 F and  38 F.  
         [0048]      FIG. 6 . is a view partially section of bearing housing  23 , bearing  22 , shaft  21 , hub  36 .  
         [0049]      FIG. 7  is a plan view partially in section of the centrifugal mechanism  33  with the shutter assembly  39  in a closed position.  
         [0050]      FIG. 8  is a plan view partially in section of the centrifugal mechanism  33  with the shutter assembly  39  in an open position.  
         [0051]      FIG. 9A  is a fragmentary plan view of the axial fan  10 ″ according the second embodiment of the present invention. The doors  41 ″ and  42 ″ are shown in open position and the motor  27  is not shown for clarity.  
         [0052]      FIG. 9B  is a fragmentary plan view in larger scale of the rotatably connection of the doors  41 ″ and  42 ″.  
         [0053]      FIG. 9C  is a side elevation of  FIG. 9B   
         [0054]      FIG. 10  is a fragmentary plan view partially in section of the axial flow fan  10 ″. The doors  41 ″ and  42 ″ are shown in their closed position. The drive motor  27  is not shown for clarity.  
         [0055]      FIG. 11  is vertical section view of the axial flow fan  10 ″ taken along line  11 - 11  of  FIG. 9A .  
         [0056]      FIG. 12  is a front view (discharge side) of the axial flow fan  300  according to a third embodiment of the present invention.  
         [0057]      FIG. 13A  is a sectional view of the axial fan  300 , taken along line  13 - 13  of  FIG. 12  with the pair of vanes  341  open.  
         [0058]      FIG. 13B  is a sectional view of the fan  300  taken along line  13 - 13  of  FIG. 12  with the pair of vanes  341  closed.  
         [0059]      FIG. 14  is an elevation of propeller  20 B.  
         [0060]      FIG. 15  is a section of  FIG. 14  along lines  15 - 15  of  FIG. 14 .  
         [0061]      FIGS. 16A and 16B  show a detail of the geared hinged connection  341 G of the pair of geared vanes in the closed and open configuration, respectively.  
         [0062]      FIG. 17A  is a front view of the air output end of the axial flow fan  400  according to a fourth embodiment of the invention illustrating the shutter assembly  439  in closed position.  
         [0063]      FIG. 17B  is a front view of the air output end of the axial flow fan  400  according to a fourth embodiment of the invention, illustrating the shutter assembly  439  in open position. The fan bracket  25 ″ and the motor  27  are not shown for clarity.  
         [0064]      FIG. 18  is a sectional view of the fan  400  taken along line  18 - 18  of  FIG. 17A .  
         [0065]      FIG. 19  is a sectional view of the fan  400  taken along line  19 - 19  of  FIG. 17A . The drive motor  27  is not shown for clarity. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0066]     The invention will be now described by way of example with reference to the accompanying drawings in which like reference numerals are used throughout the various views to indicate identical elements.  
         [0067]     Referring now to the drawings in detail, and initially to  FIGS. 1A, 1B ,  2 ,  3 A,  4 ,  5  and  FIG. 6  thereof, an axial flow fan assembly  10 , constructed in accordance with a first preferred embodiment of the present invention, is illustrated.  
         [0068]     The axial flow fan  10  is a high efficiency fan suitable to move efficiently large quantities of dusty and corrosive air in “hostile environments” such as agricultural, chemical and industrial buildings. The fan is preferably a belt driven propeller fan including a compact housing  11  having a cylindrical cavity defining an air flow passageway axially extending about a central axis CL 1 .  
         [0069]     Housing  11  comprises an air intake end  12  with a mounting flange  12  F, a spaced apart air output or discharge end  13  with a flange  13 F for fastening with mechanical fasteners  14  a discharge cone  15  equipped with a safety guard  16  positioned at the cone exit face. The guard  16  is made of heavy galvanized steel wire having large openings to allow free airflow therethrough.  
         [0070]     The air intake end  12  includes a removable streamlined bell mouth  17  secured with mechanical fasteners  17 B thereto. The bell mouth  17  comprises a curved wall defining a convex interior surface, preferably elliptical in cross-section, such to direct smoothly ambient air into the fan housing  11 .  
         [0071]     Further said bell mouth  17  is provided with a safety guard  18  secured to housing  11  with quick release clips (not shown). The safety guard  18  is positioned at the inlet area of the bell mouth  17 . It is preferably made of galvanized circular wires comprising large size openings, to allow free air flow therethrough.  
         [0072]     The bell mouth  17  can be made of one piece with the fan housing  11  by a conventional rotational moulding technique or may be formed of fibreglass, etc. The space designed  19  in  FIG. 1  is a display space and accommodates a label which may have imprinted thereon the trademark of the fan.  
         [0073]     For corrosive environments the fan housing  11  may be formed of a corrosion resistant material such as fiberglass or may be made of light gauge stainless steel rolled from a flat plate into a cylindrical shape to form the hollow body and the flanges  12 F and  13 F which extend radially and circumferentially along the hollow body ends. The flanges  12 F and  13 F reinforce substantially the structure of the housing  11  and provide a suitable anchor for the installation of the axial flow fan  10  to the building.  
         [0074]      FIGS. 4, 5  and  6  show a propeller  20 A secured to shaft  21  that is journaled on a double row angular contact ball bearing  22  fitted into a bearing case  23 . The ball bearing  22  is a permanently lubricated type of bearing and comprises a pair of adjacent inner rings that are fitted over the shaft  21  and an outside ring that is fitted into the bearing case  23  and is axially secured on the shaft  21  with snap ring  23 S. Said bearing case  23  is normally made of extruded aluminum and comprises a plurality of radial extending lugs  23 L fastened with mechanical fasteners  24  to a frame  25  or fan bracket that is made of four intersecting arms extending radially toward a central locus including the bearing case  23 . The frame  25  comprises a pair of vertical arms  25 V and a pair of horizontal arms  25 R made of streamlined aluminium extrusion comprising airfoil shaped wings  25 W. The arms  25 V and  25 R are welded at each respective ends to plates  25 P which are fastened with mechanical fasteners  26  to four opposite sides of the air intake end  12  of the fan housing  11 .  
         [0075]     A drive motor  27  is fastened with mechanical fasteners  27 B to a pair of L shaped support plates  28  which are slidably mounted and secured with mechanical fasteners  28 B to the frame arm  25 V. The drive motor  27  is provided with a pulley  29  over which passes a drive toothed belt  30 . The weight of the motor  27  pulls the belt downwardly tensioning it automatically. The motion of the pulley  29  is transmitted to the toothed belt  30  and to fan pulley  31  which is keyed in one end of the shaft  21  and locked in place by lock nut  32  which in the meantime locks the inner rings of the ball bearing  22  against shoulder  21 S of the hollow shaft  21 .  
         [0076]     A centrifugal mechanism  33  is secured with set screws  34  to said shaft  21  and is positioned adjacent to the propeller  20 A.  
         [0077]     Referring to  FIGS. 4, 5  and  FIG. 6  it is depicted the propeller assembly  20 A which comprises four airfoil shaped blades  35  having a stem  35 S rotatably secured to a hub  36  comprising half portions  36 A and  36 B held together with screws  37  for locking said airfoil shaped blades  35  at the designed pitch.  
         [0078]     The blades  35  extend radially outwardly from the hub  36  to the blade tips. Each of said blades comprises at least a pair of guiding fins  35 F and a radial adjustable shoe  38  comprising a guiding fin  38 F positioned on the front face of said propeller blades  35  such to guide the air flow and reduce the tip blade clearance with the housing  11 , so that less air will flow back through said propeller blades tips.  
         [0079]     Shape and dimensions of blades  35  and shoes  38  have been thought to ensure a uniform air flow with reduced whirls and dispersions, contributing to increased fan efficiency and reduction of vibrations and noise.  
         [0080]     The fan  10  further comprises a shutter assembly  39  or back draft damper fastened with mechanical fasteners  40  to flange  13 F.  
         [0081]     The shutter assembly  39  comprises a pair of vertically pivoted doors comprising a master door  41 M and a slave door  42 S extending vertically across the discharge end  13 . The doors  41 M and  42 S are semi circular in form, parallel with one another, their adjacent leading edges are secured with mechanical fasteners  43  respectively to shafts  44 M and  45 S which are pivotally mounted at their opposite ends to a top bearings  46  and to a bottom bearing  47  which are made of plastic and are fastened with mechanical fasteners  40  to top and bottom portion of the flange  13 F. The shutter doors are gearingly coupled by means of meshing gears  48 A and  48 B which are keyed to the top ends of respective shafts  44 M and  45 S, and are protected with a guard  49 G as depicted in  FIG. 3 A and 3B .  
         [0082]     In addition, the applicant provides an extension biasing means  50  connected to a support  51  secured to the fan housing  11  and to a crank  52  which is secured to the bottom end of the shaft  44 M. When the doors  41 M,  42 S are in their closed position said extension biasing means  50  urge said doors against a seal  53  that is secured to the flange  13 F. When the doors are in their open position the extension biasing means  50  lock said doors one against the other to prevent vibrations.  
         [0083]     The master door  41 M as well the slave door  42 S are made of aluminium or galvanized steel or of one-piece of roto-moulded plastic comprising an internal cavity filled with at least ¾″ thick high grade insulated material. As the shutter doors  41 M,  4  are insulated and biased against the seal  53  they will effectively block heat transfer. In addition, each door  41 M and  42 S comprises at least two guiding fins  41 F and  42 F positioned symmetrically relative to the propeller&#39;s rotational axis CL 1 . The above depicted doors  41 M,  42 S are positioned adjacent said propeller  20 A and help on straightening any swirls and circular motion of the air flow.  
         [0084]     As best seen in  FIG. 7  and  FIG. 8 , a centrifugal mechanism  33  is provided to operate said doors  41 M and  42 S The centrifugal mechanism  33  comprises a steel hollow guide  54  fixed with a pair of set screws  34  to the shaft  21 , a bifurcated collar  54 C comprising at least a pair of lugs  54 L clamped with bolts  55  to the guide  54 , a reciprocating actuator member  56  made preferably of one piece of moulded plastic comprising at least a pair of lug  57 L. The reciprocating actuator member  56  is slidably mounted on said hollow guide  54 . Said lugs  54 L and  57 L are pivotally connected with mechanical fasteners  58  to a pair of links  59  and  60  which are symmetrically spaced apart in relation to the propeller&#39;s axis of rotation CL 1 . The links  59  are preferably made of flat aluminium bar and the links  60  are preferably made of steel. The links  60  comprise centrifugal masses  61  fastened with mechanical fasteners  62  on the outermost free ends thereof. The links  59  and  60  are pivotally connected to each other with mechanical fasteners  63  and are able to swing equally in unison toward and away from each other in response to the speed of rotation of the shaft  21  such as to cause axial movement of the reciprocating actuator member  56  toward the master door  41 M or away from it. The reciprocating actuator member  56  is thus displaced axially by the centrifugal effect of the weight of the links  60  and centrifugal masses  61  causing a corresponding rotation of the doors  41 M and  42 S from the closed configuration of  FIG. 7  to the wide open configuration of  FIG. 8 .  
         [0085]     A compression biasing means  64  is housed into the hollow guide  54  with one end biased to the inside front wall of the reciprocating actuator member  56  and the other is biased to one end of the shaft  21 , as pictured in  FIG. 7  and  FIG. 8 . The reciprocating actuator member  56  comprises a front operative end  56 F adapted to frictionally engage and rotate an operating cam  65  that is fastened with mechanical fasteners  66  to the master door  41 M and is positioned slightly below the center axis CL 1  as shown in  FIG. 3A  therefore to ease the opening and closing of the doors  41 M,  42 S.  
         [0086]     With reference to  FIGS. 9A, 9B ,  9 C,  10 , and  FIG. 11 , a second embodiment of the present invention is shown and described.  
         [0087]     The axial flow fan  10 ″ of the present invention is similar to the fan  10  herein above described and comprises similar components as the fan  10  of the first embodiment of the invention set above. However, the housing  11 ″ is more compact than housing  11 , a centrifugal mechanism  33 ″ (similar to the  33 ) is rotatably connected to the shutter assembly  39 ″ which comprises a pair of vertically pivoting doors  41 ″ and  42 ″ opened and closed by said centrifugal mechanism  33 ″ that is mounted to one end of a fan hollow shaft  21 ″ opposite to the propeller  20 A and comprises an axial sliding reciprocating actuator member  56 ″ mounted on a hollow guide  54 ″. The reciprocating actuator member  56 ″ comprises a compression operative biasing means  64 ″ and an operative rod  201  adapted to pass through the fan hollow shaft  21 ″. The operative rod  201  is threaded and fastened with nut  202  to one end to said reciprocating actuator member  56 ″ and at the other end is fitted to a ball bearing  203  which is housed into a case  204  comprising two lateral portions  204 L connected with pivots  205  to a pair of arms  206  operatively connected with pivots  207  to forks  208  which are secured to a pair of vertically pivoting shutter doors  41 ″ and  42 ″ which are pivoting around a stationary rod  209  which extends across the discharge opening  13 ″ of fan housing  11 ″ and comprises opposite ends secured to top and bottom supports  211  and  212  secured with mechanical fasteners  213  to flange  13 F″ of the fan housing  11 ″.  
         [0088]     When the drive motor  27  is energized, the centrifugal force acting on the centrifugal masses  61  causes the reciprocating actuator member  56 ″ and the rod  201  to be displaced towards the shutter doors  41 ″ and  42 ″, then said rod  201  pushes the bearing  203  and the arms  206  causing the forks  208  to rotate doors  41 ″ and  42 ″ around the stationary rod  209  to a wide open position.  
         [0089]     When the drive motor  27  is turned off the compression biasing means  64 ″ pull the operative rod  201  and closes the shutter doors against seal  214 .  
         [0090]     The shutter doors leading edges may be connected with a continuous “Geared Hinge”  341 G which can be made from extruded aluminium. Each door comprises a cavity preferably filled with insulation material, as shown in drawing  FIGS. 16A and 16B .  
         [0091]     With reference now to  FIGS. 12, 13A ,  13 B,  14  and  FIG. 15 , a third embodiment of the present invention is illustrated and described. The fan  300  of the present invention comprises a square box housing  311  preferably made of sheet metal rolled with a roll-bending machine. The fan housing  311  comprises an air intake opening  312  provided with flange  312 F to which is fastened with mechanical fasteners  317 B a bell mouth  317  and a discharge opening  313  with a flange  313 F. A propeller  20 B is mounted in said fan housing  311  adjacent said air intake opening  312  and is secured to a hollow shaft  21 ″ that is press-fitted into the inner rings of bearing  22 . Said bearing  22  comprises an outside ring that is fitted into a bearing case  23  that is secured to a bracket  25 ″ (similar to bracket  25  of  FIG. 1B ) for supporting a propeller  20 B mounted on one end of the hollow shaft  21 ″. The bracket  25 ″ is secured with mechanical fasteners  25 B″to four opposite sides of the air intake opening  312  of the fan housing  311 .  
         [0092]     The propeller  20 B best seen in  FIG. 14  and  FIG. 15  comprises four airfoil shaped blades  35 ″ surrounded by an outer band  35 D comprising a radial extending portion  35 R positioned adjacent a plate  318  that is secured to the fan housing  311  to prevent back airflow.  
         [0093]     A centrifugal shutter mechanism  33 ″ is mounted and secured with set screws  34  to the hollow shaft  21 ″ opposite to the propeller  20 B. It is provided for operating a shutter assembly  339  comprising a plurality of pair of vertical pivoted hinged vanes  341  working in tandem that are vertically mounted across the discharge opening  313  of the fan  300 . The vanes  341  are operatively connected to an axial sliding reciprocating spreader bar  342  operatively connected to an operative rod  201  connected to the actuator member  56 ″. The leading edges of the vanes  341  are pivotally connected with a continuous hinge  341 H as best shown on drawing  FIG. 12  or a continuous geared hinge  341 G as shown in  FIGS. 16A and 16B . The continuous hinge  341 H comprises a stationary rod  209 ″ fastened at each opposite ends with mechanical fasteners means  341 B to top and bottom flange  313 F of the fan housing  311 . It is to be understood that the pair of vanes  341  that can be mounted at the fan intake  312  and may be shaped in such a way that when they are in open position their trailing edges will be biased one against each other forming an aerodynamic profile like for example NACA  27 - 212  such to reduce pressure losses thereof.  
         [0094]     With reference now to  FIGS. 17A, 17B ,  FIG. 18  and  FIG. 19 a  fourth embodiment of the present invention is illustrated and described. The axial flow fan  400  of the present invention comprises a cylindrical shaped housing  411  comprising an air intake end  412  and an air output end  413 . The fan housing  411  is preferably made of one-piece of rolled steel or aluminium sheet metal. The air intake end  412  is provided with a bell shaped mouth  17  secured with mechanical fasteners  17 B thereto and is provided with a bracket  25 ″ (similar to the bracket  25  of  FIG. 1B ) secured with mechanical fasteners  25 B″ to four opposite sides of said air intake end for supporting a propeller  20 A best shown in  FIGS. 4, 5  and  6 ).  
         [0095]     A shutter assembly  439  is mounted at the fan discharge end  413  and comprises a plurality of even angularly spaced pair of vanes  414  comprising leading edges connected to each other with a hinge  414 H comprising four intersecting rods including a horizontal stationary rod  209 ″R and a stationary vertical rod  209 ″V having opposite ends fastened with mechanical fasteners  413 B to four opposite sides of the flange  413 F of said fan housing  411 . The stationary rods  209 ″R and  209 ″H are connected to a central rim  415  near their middle thereof.  
         [0096]     A centrifugal actuator  33 ″ (identical to the one afore depicted) is rotatably connected to the shutter assembly  439  and comprises a reciprocating actuator  56 ″ provided to open and close simultaneously all vanes  414  which are connected with pivoted means (similar to the ones of  FIGS. 9B, 9C ) to a rigid frame  417  comprising a central bearing case  418  comprising a ball bearing  419 . The opposite ends of said frame  417  are operatively connected to the vanes  414  via forks  420 , pivots  420 P, arms  421  and pivots  421 P. The frame  417  is rotatably connected via the ball bearing  419  to the operative rod  201  that is connected to the actuator member  56 ″.  
         [0097]     The centrifugal mechanism of the present invention permits to operate the shutters at relatively low fan speed. In the cooling system the total air flow volume of the fan shall match the system air volume requirements of the cooling system that vary over time with the temperature and other parameters. In this regard, it is usually advantageous to vary and reduce the air flow of the fans, so that the power requirements of the fans may be reduced.  
         [0098]     While the foregoing is directed to various embodiments of the present invention, other and further embodiments may be devised without abandoning the spirit of the invention. For example, the various embodiments of the invention can be included in combination with each other to produce other variations of the disclosed embodiments.  
         [0099]     For operating the shutters the applicant shows a centrifugal mechanism but any other device may be adapted to work with the novel aerodynamic pair of vanes working in tandem. Accordingly, the disclosed and illustrated embodiments herein should be considered as exemplary rather than restrictive of the invention which is defined in the accompanying claims.