US1781155A - Propeller fan - Google Patents

Description

Nov. 11, 1930. s. M. ANDERsoN 1,781,155

PROPELLER FAN Filed May 21. 1928 Patented Nov. 11, 1930 UNITED vAN'r COMPANY, or HYDE PARK, cHUsE'r'rs 'rRorEtLEnv-IANF Application led Hay 21.,. i1328i'- g erialllolf 5 increases with its speed, it is advantageous to drive a fan of given size at as high a speed as possible consistent with the strength of the fan and the power .of the driving unit. However, in existing types of fans, even though high speeds and large outputs may be obtained, a practical limitation of speed is imposed by the factor of noise. It has been found that above a critical tip speed of about four thousand feet per minute, the noise increases with the speed to an intolerable intensity. If the fan is used in connection with a heating or ventilating system, the noise is particularly objectionable, not only because the ducts serve to conduct the sound to all parts of the building, but they also act somewhat in the nature of sounding boards to intensify the noise produced by the fan.

The object of the present invention is to eliminate or reduce the noise of a prop'eller fan to permit operation at higher speeds, whereby consequently greater outputs may be obtained.

I have discovered that the principal source of noise in the ordinary propcller fan resides in the obstruction of air flow in the inlet by the supports used for mounting the fan bearings or the driving motor. These obstructions ordinarily are in the form of struts disposed in the inlet and in the path of air flow to the blades of the fan. In passing 'a support, the air currents necessarily divide and they do not again converge for an appreciable distance beyond the inner surface of the obstruction, thereby forming a dead spot or eddy region, which comprises a zone of decreased fluid pressure, usually of gencrally triangular shape. Since the supports are placed in close proximity to the blades, in order to reduce the length of the fan shaft, these zones vare intercepted by the rotating blades. Each blade in entering and leaving such a zone of decreased pressure sets up a Wave which manifests itself as sound energy. Below a certain critical speed, the sound energy appears to be insuflicient to be ob- 'ectionable when| attenipts :made tol mcreasethe :Operating speed above the criticalvalue, thenoiseincrea'ses lin -intensity'and in greater propoii'tion :than the vspee'd 'until the noise eventuall becomes the v'principal limiting factor inft e o eration of the fan.

VVith, the above state object in view, the present invention consists in the provision of supporting ,arms arranged at the inlet and of such size, shape, and disposition that the dead spots or regions of reduced ,pressure do not extend into the path of the moving fan blades. In other words, the supporting means which form necessary obstructions in the inlet are so arranged that'the converging air streams shall -]oin .and completely restore uniform pressure conditions before entering the moving blades. The blades therefore engage the fluid which is at the same pressure at all points and the fluctuations which would be caused by the Varia- ,tions in the fluid pressure are eliminated.

In the accompanying drawings illustrat- 111g what is now considered the preferred form of propellerfan, Fig. 1 is a rear end v elevation of the fan; Fig. 2 is a top plan view with the thimble shown in section and one of the supporting arms omitted; and Fig. 3 is an enlarged section on line 3-3 of Fig. 2.

The illustrated embodiment of the invention comprises a fan having a thimble 4 connecting with a converging inlet chamber G to which is attached a flange 8. The thimble, the inlet Chamber, and the flange form the fan casing and are preferably constructed of Sheet metal. The fan blades 10 rotate in the thimble 4 and to that end are attached to a hub 12 which is mounted onthe end of a motor shaft 14. The motor 16, indicated as an electric motor of conventional form, drives the shaft 14 and is so mounted with relation to the fan blades that the inwardly protruding end of the shaft 14 upon which the fan is mounted is relatively short. The bearings for the shaft 14 are contained in the motor frame in the usual manner.

The motor and its cooperating parts including the bearings for the motor and fan shaft are supported by means of a plurality of supporting arms 18. The arms are shown as being three in number, although for large installations, a greater number might be used.v

Each arm 18 isof rectangular' cross section and is supported edgewise with relation to the inlet. he arms 18 are radially disposed. Each arm is formed a't its inner end with a right angle projection 20 which `is secured to a cleat 22, the cleat in turn being bolted to the i ring 24 forming part of one of the end ieces of the motorv frame. The outer end o each 'arm 18 is secured to the flange 8 by means of the motor supports 18 are brought into relatively close proximity to thev rotatin'g` fan blades. This proximity of the arms' to the blades necessitates a careful constructionof the arms in order that the operation of'the fan at high speed may not give rise to 'objectionable noise. i s

The construction and arrangement ofthe arms 18, especially with regard totheir dimensions and location. will now be described in order .that the invention may be 'applied to a fan of any size. The greater portion of the air flowsinto the fan along the lines indicated in Fi 2,8 which are' of theoretically hyperbolic orm. Viewed from the end 'of the fan,

V as in F ig. 1, the inflow of air from the surrounding space is substantially vradial, so that the radially extending arms offerv only the obstruction of their thickness' to the incoming air. Referring to Fig. 3, which is an enlarged section of one-of the supporting arms, two currents of air' a; and y are shown flowing on opposite sides of the supporting arm. Thesev currents are divided by the arml and after' flowing beyond it, y converge into a single stream. The distance from the edge' of the 'arm to the pointof convergence eomprises a dead space or eddy space in which the fluid is at a decreased pressure; Thev length of this' dead space is a matter of considerable importance because briefly stated, the invention contemplatesspacing of the supporting arms 18 at a distance from the fan blades which is greater than, the dead space formed by the passage of air by the arms. ,This dead space for all values of Velocity likely to be met in practice, is substantially independent of the air Velocity and is a function only of the physicaldimensions and arrangement of the supporting arm. In the present case, employing the preferred form of `flat rectangular radially disposed arm, the length of this dead space has been found to beapproximately 1.5

1381,35 i i w times the'thickness of the arm (the width of.

the arm being several times the thickness). In Fig. 3,th'e thickness of the arm is in'dicated by L, the width by B, and `the length of the dead space' by D.

As stated above D =1.5L. Whether the value of D as given represents the actual distance Within which the opposite 'streamsv completely converge is diflicult, if fnot impossible, to determine, but I have determined experimentally that if the separation betwee the arm and the fan blade is at least equal to 1.5L for an armof the Shape described, the convergence is sufficiently complete to avoid` anyappreciable generation of noise. i p

In 'the construction of the fan, the distance from the fan blade to the arm measured .along any stream line should therefore be greater than the length of the dead space on the inlet side of the supporting arm.` For the three stream lines shown in' Fig. 2, the separations are indicated by the lengthsof lines ab, od, and ('f. Inasmuch as the air streams indicated by ab'and cd are obstructed only by the arm 18,'the only condition to besatisfied is that each of these lengthsshall be greater than D, the length of the dead.v

space, or in other words, thatab and od shall eachv be greater than `1.5L. For the stream line ef, it .will be noted that the air is obstructed not only by the thickness of the arm 18, but also by ing arm of the The effect of these increased obstructions is to lengthen the dead space and it is for this thethickness of the upstandcleat 26, and bylthe bolts 28. v

reason that the cleat 26 is preferably mounti ed well out on the flange at a considerable vconical inlet chamber 6. The conditions to be satisfied are that ef shall be greater than 1.5L, and that the line e'f, measured ,from

the cleat` to the blade, shall be greater than 1.5 times the value of L increased by the hickness of the cleat and the protruding o ts.

the linear speed ofv -radial distance from the outer edge of the I Itwill be seen that stream lines which pass the inner end 20 of the supporting arm, as" y er an increased ends lof the blades,l if permitted to cut through the 'zones of decreased pressure would tend to set up noises of the same pitch as the outer ends'of the^blade, the energy generated as sound is practically negligible because of the smaller linearv Velocity. Since the .sound energy increases greatly with an bodying the features of the present invention, if rectangular supporting arms are employed as herein speclfically illustrated and described, it is necessar onl that the arms shall be arranged radial y an shall be spaced from the fan blades a distance at least 1.5 times the thickness of thesupporting arm. This thickness should include any cleats 'or attaching means which are disposed in 'a manner to obstruct the inflow of air. Although'the preferred form of the invention has been illustrated and described, it is within the scope of the invention to modify the preferred form except as limited by the appended vc'laim. For example, although radial supporting arms have been shown and are preferred, the arms may if desired deviate from a radial arrangement but in that case, the flow of air will not be directly past the thin edge of the arm and allowance should be made for an increased effective thickness of the arm because of'the greater obstruction of the flow of air. Moreover, the arms are not necessarily of rectangular cross section. Arms of different cross section may advantageously be employed and the length of the dead space for other shapes of arm may be readily determined by experiment. In any case, the arms should be arranged so that the dead space extending from the obstruction does not reach into the path of the moving fan blades.

As an example of a commercial construction, the present invention has been applied to a forty inch propeller fan adapted for operation at a tip speed of 9,000 feet per minute, the incoming Velocity of air being approximately 1,000 feet per minute. In this case, the supporting arms 18 are .5 of an inch thick and about 3 inches wide, that is, L equals .5 of an inch and B equals 3 inches. The length D of the (lead space behind each arm is by calculaton and experiment approximately .75 of an inch, this Value being 1.5 times L. This calculation indicates that the supporting arms 18 may be placed as close as .75 of an inch from the fan blades. A spacing as close as this, however, is unnecessary and it is therefore apparent that a separation (along the line cd, for example) of about 4 inches is mechanically feasible and is ample for prevention of noise. Moreover,

. this separation will afford protection against generation of noise by the passage of air by the supporting cleats. Arms of this size have been found to be sufliciently strong to support the motor and fan rigidly against the axial thrust of the blades and the motor torque.

It will be appreciated that a propeller fan which satisfactorily operates at a tip speed of 9,000 feet per minute will deliver a greatly increased output for a given size over one which is limited to. a speed of onl about 4,000 feet lper minute. The success o the present invention in eliminating noise depends largely upon the fact that the dimensions of the dea space are substantially 'independent of the Velocity. Although prior types of fans o erating at low speeds have been free from o jectionable noise, this freedom from noise follows not from proper construction and design of the supporting means, but rather from the fact that at low speeds the generated sound energy is insuflicient to be noticeable. The present invention difl'ers from former constructions in thatthe source of sound is almost completelyremoved, whether the fan be Operating at low speeds or at speeds which are considerably higher than those obtainable with ordinary types of fan.

Since it is common practice to construct propeller fans with any number of blades, it will be understood that the invention is not limited to a construction having only two blades, as shown in the drawings.

Although the present invention has been found to remove the principal source of objectionable noise, it will be understood that care must be taken to eliminate all extraneous sources such as unbalanced motor armatures, loose bearings, and the like. Other sources of noise consist in a swishing which is an inevitable consequence of settin air in motion and a ringing sound due to vi rat-ion of the blades after the m'anner of a bell. The *swish is ordinarily not obj ectionable either in pitch or intensity. The bell action appears to give a sound of nearly constant itch at all speeds, and it ma be reduced or e iminated by padding the lades with some absorbent substance such as felt. The noise which is avoided by the present invention differsV from other noises and may be readily distinguished as a hum which increases both in pitch and intensity as the fan increases in speed. It it by far the most objectionable noise to which operation of a fan gives rise, and'when it is eliminated, as by the means described, the fan may be said to have virtually noiseless characteristics.

Although the invention has been specifically described as embodied in a construction employing a motor directly supported upon the fan casing, it will be understood that it is within the scope of the invention to support other apparatus in'the path of the inflowing air. For example, it is within the scope of the invention to provide a support of any kind for the fan shaft, such, for example, as a bearing only, the fan being driven by a motor mounted at a considerable distance from the blades.

Having thus described the invention, what is claimed is:

A high speed propeller fan comprising fan m' with respect to the inflow ofva'r.

'blades, a thmble,'"converging inlet chamber connected with the thimble, a 'flange et- 'tached to the inlet lchamberl and arranged in w a plane rpendicular to the axis of the 3 blades, a en shaft, a motor, and a -plurality of flatsup ortng arms,A each connected at one end to t e motor-and at Athe other end to the flange beyond the periphei')n of' the inlet chamber, the arms beingerranged edgewi'se In testimonyfwhereof I havesignedl m name to this specification. i

' t SAMUEL M. ANDERSON.

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