US3830587A - Axial flow fan assembly - Google Patents

Abstract

An axial flow fan assembly in which two or more fans in series provide successive fan stages which are close together and in which the average pitch angle of the blades of one stage differs appreciable from that of the blades of a previous stage.

Description

Unite tates atet [191 Shipes et a1.

[58] Field of Search ..415/198, 199,219 R, 207, 415/208, 209, 210, 129, 130; 417/424; 138/39-40; 416/127 [56] References Cited UNITED STATES PATENTS 1,402,869 1/1922 Knutson 415/198 1,909,611 5/1933 Charavay 415/198 1 Aug. 20, 1974 2,313,413 3/1943 Weske 415/198 2,314,572 3/1943 2,466,827 4/1949 Roth 138/39 2,681,178 6/1954 Mart 26l/D1G. 11 2,908,335 10/1959 Petty 415/129 2,982,361 5/1961 Rosen 416/127 3,357,496 12/1967 Peterson 415/130 FOREIGN PATENTS OR APPLICATIONS 357,368 9/1931 Great Britain 415/198 411,839 7/1945 Italy 415/199 R 1,181,926 1/1959 France 415/130 77,645 1/1949 Czechoslovakia 415/129 Primary Examiner-Henry F. Raduazo Attorney, Agent, or Firm-W. F. l-Iyer; Marvin B. Eickenroht 5 7 ABSTRACT An axial flow fan assembly in which two or more fans in series provide successive fan stages which are close together and in which the average pitch angle of the blades of one stage differs appreciable from that of the blades of a previous stage.

3 Claims, 3 Drawing Figures AXIAL rLow FAN ASSEMBLY This invention is a continuation of my copending application, Ser. No. 209,923, filed Dec. 20, 1971, and entitled Axial Flow Fan Assembly" now abandoned.

This invention relates to axial flow fan assemblies for use in air coolers or other industrial environments. More particularly, it relates to improvements in fan assemblies wherein two or more fans are mounted in series to provide successive stages within a single fan ring.

When used in air coolers, fan assemblies are mounted either above or below tube bundles for performing useful work by causing air to pass thereacross. In these and other industrial environments, the fans may be quite large, ranging in diameter from six to thirty feet. The amount of work to be performed by a fan is approximately a functionn of the cube of its tip speed. At the same time, the noise generated by a fan is a function of its tip speed to the fifth power. Consequently, the problem of excessive noise is compounded as the work requirement on the fan assembly is increased. At the same time, of course, in response to public demand, our governmental bodies have, in regulating noise levels, adopted more stringent standards.

The historical purpose for using series fans has been to permit the useful application of more air moving power to a given piece of equipment than could be applied with only a single stage fan. In other cases, series fans have been used with the fan blades at somewhat lesser speeds so as to move the same quantity of air as a single stage fan, but with less noise.

However, in prior series axial flow fan assemblies of the latter category, the successive fan stages are of identical construction and axially spaced apart a considerable distance, in some cases as much as three fan diameters, because it has been shown that when such stages are not widely spaced apart, they do not perform as much useful work and are less efficient. This phenomenon may be explained by several theories, one of which is that a certain distance between stages is required to permit the kinetic energy imparted to the air molecules by the upstream fan to be converted to a pressure increase, and thus useful work. One authority, William C. Osborne, states, in his book entitled Fans, that unless the stages are widely spaced, the rotational velocity of the air leaving the upstream stage will modify operation of the succeeding stage or stages.

Regardless of the theory which is reasonable for this construction, the large spacing between fan stages of these assemblies causes them to be quite expensive, and, in some cases, to interfere with available head room. Thus, the long shaft on which the fans are mounted, the large size and/or number of bearings necessary to mount the shaft, and the considerable height of the fan ring all contribute substantially to a large capital investment.

The primary object of this invention is to provide a series axial flow fan assembly which is considerably less expensive to construct, but which is capable of performing substantially the same useful work at substantially the same noise level, as the above-described series fan assemblies.

This and other objects are accomplished, in accordance with the illustrated embodiment of this invention, by a series fan assembly in which it has been made possible to mount the fan stages considerably closer together than heretofore thought possible, and preferably substantially adjacent one another, by providing the blades of the downstream fan with a greater average pitch angle or pitch than the blades of the upstream fan. Thus, we have discovered that a series fan assembly of this construction operates without substantial loss of efficiency or work potential, as compared with the above-described prior series fan assemblies. At the same time, since the fans may be rotated at the same speed as the fans of prior assemblies, they have the advantage of the same low noise level. Of course, due to this close spacing of the stages, the over-all assembly is less expensive to build and consumes less space due to the shorter shaft, the smaller and/or lesser number of bearings, and the correspondingly shorter fan ring.

It is also known that the blades of an individual fan have an optimum average pitch for accomplishing useful work under given conditions. This average pitch will usually range from about 10 to about 20 with respect to the horizontal, and when it substantially exceeds 20, the fan is generally found to be less efficient with little or no increase in air flow. We have found, however, that in the novel series fan assembly of the present invention, the average pitch of the blades of the downstream fan may exceed 20 to a considerable extent without appreciable loss of efficiency in the over-all fan assembly. Thus, the average pitch of the blades of the downstream fan does not place a serious limitation on that of the blades of the upstream fan i.e., it does not place a practical upper limit on either the average pitch of the blades of the upstream fan or the difference in average pitch of the blades of the two fans.

One theory by which the surprising results of the present invention may be explained is that the air stream leaves the upstream fan in modes of high and low velocity, and, since it has not traveled the distance heretofore allowed in prior assemblies to permit its kinetic energy to be fully converted to velocity pressure, it is at a velocity at which it may be picked up by the blades of relatively large average pitch. Regardless of the theory, however, the results have been demonstrated for various operating conditions.

Its also known that each fan imparts a certain amount of swirl to the air, so that with series fans rotating in two stages, the swirl at the outlet of the downstream fan is the sum of the two. It has been proposed to increase the efficiency of prior series fan assemblies by straightening out this swirl with fixed vanes between fans. In accordance with the present invention, its proposed to realize a similar or greater increase in efficiency without the use of fixed vanes, and thus eliminate their cost and headroom, by increasing the average pitch of the blades of the downstream fan. Furthermore, we have found that optimum results are obtained when the blades of the downstream fan are so arranged circumferentially with respect to those of the upstream fan as to avoid the most turbulent part of the air stream from the upstream fan.

We have also found that although the average pitch of the blades of the fans for producing optimum results are best determined by trial and error, which is in any case rather standard practice in the fan industry, such pitches may at least be approximated in accordance with certain design criteria, as set forth to follow. We have further arrived at additional criteria which is useful in setting the blades of the two fans in optimum circumferential relationship.

In the drawings, wherein like reference characters are used throughout to designate like parts:

FIG. 1 is an elevational view of an air cooler which has been broken away in part to show a tube bundle and a fan assembly constructed in accordance with the present invention and supported above the bundle for drawing air upwardly thereacross;

FIG. 2 is a top plan view of the fan assembly; and

FIG. 3 is a graph showing a curve illustrating the results of tests comparing work and the efficiency of a series fan assembly so constructed.

With reference now to the above-described drawings, the air cooler shown in FIG. I, and designated in its entirety by reference character 10, includes a tube bundle 11 mounted on vertical columns 12 above the surface 13, and a tandem fan assembly 14 mounted above the tube bundle by means of a transition 15. As shown by the broken away portion of FIG. 1, the bundle 11 includes a plurality of heat exchange tubes 16 extending laterally between headers (not shown) at opposite ends of the bundles for conducting a process fluid to be cooled across the air stream induced in an upward direction by means of the fan assembly. Side walls 17 extend along opposite sides of the tube bundle from one header to the other so as to confine air flow to the bundle.

The fan assembly 14 includes a cylindrical fan ring having upstream and downstream series fans 19 and 20, respectively, providing successive stages mounted for rotation coaxially thereof. More particularly, the fans are of such diameter as to cause the tips of their blades 19a and 20a to move closely and concentrically within the fan ring. Also, and as shown in FIG. I,-the blades of the fans are pitched to cause air to move upwardly through the fan ring, and thus upwardly across the tube bundle in response to rotation of the fans in clockwise direction (looking downwardly). It is in this sense i.e., direction of air movement that the lower fan 19 is called upstream and the upper fan 20 is called downstream.

Both fans are mounted on a shaft 21 which extends vertically and coaxially of the fan ring. The lower end of the shaft is driven by a motor 22 mounted on a motor support 23 suspended from the tube bundle or other portion of the air cooler in any suitable manner. The motor drives a belt within a belt guard 24 disposed about the lower end of the shaft for rotating the fans at a desired speed. The shaft is mounted for rotation at its upper end by means of a bearing 21a supported in the fan ring 18 by radial struts 2111.

Each fan includes a hub 25 fixed to shaft 21 and having a plurality of blade sockets 26 extending radially in equally spaced apart relation. The inner ends of the blades are releasably secured in the hubs, such as shown in US. Pat. No. 2,908,335, dated Oct. 13, 1959, which enables the average pitch on each blade to be adjusted about its spanwise axis as desired, depending on operating conditions.

As previously described, and as will be apparent from FIG. I, the hubs 25 of the fans 19 and 20, and thus the planes of the inner sides of the fans themselves, are substantially adjacent one another, whereby the axial distance between the fans is at substantially a minimum. In any event, as compared with prior series fan assemblies, wherein the successive stages may be axially spaced apart three fan diameters or more, the fans 19 and 20 shown in FIG. I are spaced apart only a fraction of their diameters. In actual practice, we have successfully tested series fan assemblies constructed in accordance with the present invention wherein fans 14 feet in diameter were spaced apart from the center of one hub to the center of the other hub a distance of only 20 inches.

Thus, as will be apparent from FIG. 1, the fan ring 14 need be of height not substantially greater than the vertical axial thickness of the fans 19 and 20, whereby it consumes a minimum of head room. Also, the shaft 21 need be substantially no longer than that required to extend between the fans and the motor drive, plus the axial thickness of the blades. Still further, generally only one bearing 21a of relatively small size is required as support for the fans adjacent the entrance of the fan ring, whereas a fan assembly having the fans spaced three or more diameters apart would obviously require two or more relatively large bearings.

As shown, each blade tapers inwardly in a radially outward direction, and has a cross-section which is of generally air foil shape. In some cases, the opposite surfaces of the blades may twist to some extent, so that the pitch, or angle which the active or upper blade face forms with a horizontal plane perpendicular to the axis of the shaft, may vary to some extent along the length of the blade, and it is in this sense that the term average pitch is used herein. However, as is well known in the art, this variance is generally relatively small and thus insignificant insofar as design considerations are concerned.

As previously described, the average pitch of the blades of the upstream fan 19 is less than the average pitch of the blades of the downstream fan 20. Also, as best shown in FIG. 2, and for reasons previously mentioned, the blades of the two fans are staggered or angularly spaced apart in a circumferential sense. The respective average pitches of the two fans will depend on that required to accomplish the optimum useful work in a particular installation, and, as previously noted, although final adjustment in this reqard is normally obtained by trial and error, we have arrived at certain design criteria by which these angles, as well as the staggering of the blades of the fans, may at least be approximated.

In accordance with well known criteria, the work down by the fan assembly 14 is a function of the pressure drop (in inches of water) of the air across the tube bundle i.e., the difference between its pressure measured on its lower side and its upper side in causing a predetermined volume of air to move across the bundle. This requires consideration of several factors, including tip speed of the fan blades, size of the blades, pitch of the blades, spacing between the tubes of the bundle or free area, etc., which will be explained to follow, is also observed in the design of the fan assembly of the present invention, with such additional considerations as are required in view of the novel construction of this fan assembly.

Although the fan assembly 14 is illustrated as part of an air cooler, it will be understood that it may instead be used in other environments in which air is to be moved in order to perform useful work, such as in ventilation, other types of heat transfer, combustion processes, and the like. Thus, no novelty is claimed in the construction of the described air cooler itself, other than that which results from the novel construction of the fan assembly 14.

As previously noted, the curve on the graph of FIG. 3 illustrates the results of tests comparing the work and efficiency of a series fan blade assembly having the pitches of the blades of its fans adjusted in accordance with the present invention with that of a fan blade assembly having the pitches of its fan blades otherwise adjusted. Thus, the coordinates plotted on the graph represent work (measured static pressure raised to the 1.6 power) and efficiency of fans in which the average pitch (in degrees) of the blades of the downstream fan is indicated above that of the blades of the upstream fan. As will be observed from the curve, within a certain range, the useful work as well as the efficiency of the assembly adjusted to follow the teachings of the present invention were greater than that of the assembly otherwise adjusted, including by interpolation an adjustment to provide the blades of both fans with the same average pitch. Outside of this range, where the differential between the average pitches of the fans became quite large, the efficiency decreased without an increase in useful work, indicating, of course, one end of the range. Although not of precise definition, a range so obtained will assist a person skilled in this art in arriving at desired pressures and efficiency for a given installation.

In this test, each fan was 14 feet in diameter, had six blades, and was rotated at a tip speed of 5,630 feet per minute. The centers of the hubs for the fans were spaced apart 20 inches.

As previously mentioned, one theory for the surprising results of the present invention is that with the fan stages close together, the kinetic energy imparted to the air by the first stage has not been fully converted to a pressure increase. The design criteria for use in approximating the pitches of the blades of the stages assumes that full conversion takes place at a distance of 1.5 diameters from the first or upstream stage, and that, when the fan stages are spaced a lesser distance apart, as in the present invention, the extent of conversion is directly proportional to this distance divided by 1.5 fan diameters. This assumption is based on an analogy of the air flow encountered in the use of the present invention to the known characteristics of fluid flow through nozzles and orifices.

This design criteria involves application of the following equations:

VPO [Vaa/ 1 m (1) VP, [Van/66.651 (pa/p Va, 66.65 VP, (3)

Va, 66.65 VP (SP/2) [W Z'A ti -arctan (sin do) 0zarctan Vaa (Sin a) w a1-arctan VMVaOpa (7) R 0.67 [AS/D] wherein:

As Axial spacing between successive fan stages, ft.

D Fan Diameter, ft. R Fraction of kinetic energy converted to a pressure increase SP Static pressure differential imposed on the air flow stream separate from the fan, inches of water Va Axial component of air velocity leaving first stage and entering second stage, ft/sec Va Axial component of air velocity leaving second stage, ft/sec Vaa Average air velocity in the axial direction,

ft/sec Vb Blade Velocity at tip, ft/sec VP Velocity pressure into first fan stage, inches of water VP Velocity pressure leaving first fan stage, inches of water 0 Pitch of blades of first fan stage, degrees 0 Pitch of blades of second fan stage, degrees 0 Swirl angle (degrees) of air entering first fan stage, with respect to the axial direction a, Swirl angle (degrees) of air entering second fan stage, with respect to the axial direction pstd Standard air density, 0.075 lbs/ft (sea level,

F) pa Actual air density, lbs/ft. To illustrate application of these equations, assume that:

SP 0.542 inches of water Vb ft/sec Vaa 24.85 ft/sec As 1.33 ft D 14 ft pa pstd 0.075 lbs/ft and that the first stage must generate a velocity pressure equivalent to SP/2, or 0.5 (0.542) 0.271. With respect to the first stage:

VP =[24.85/66.65] X (0.075/0.075) =0.l39, and VP =0.139 +(0.542/2) 0.410, so that VP 66.65 (0.410)" 42.67 ft/sec. Since no swirl has been imparted to the air stream prior to reaching the first fan stage, 01 is zero, and:

0, arctan [42.67/110] 21.2".

With respect to the second stage:

R 0.67 (1.33)/l4 0.064, so that Va 66.5 [0.139 0.27 1 (0.936) 0.271 1 54.3

ft/sec., and

The previously mentioned design criteria for determining the circumferential stagger or offset between blades of the two stages involves the application of the further equations:

and

166.5 (0.410) [110 (42.67) 0.075 and 36O (As)Vb 36O (tan aQAs 2N 1rVa D n-D From the foregoing it will be seen that this invention is one well adapted to attain all of the ends and objects hereinabove set forth, together with other advantages which are obvious and which are inherent to the apparatus and method.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

The invention having been described, what is claimed is:

l. A series axial flow fan assembly, comprising a fan ring having an inlet and outlet and a portion therebetween which is of constant inner diameter, a shaft extending coaxially within said fan ring portion, first and second axial flow fans each having blades of equal outer diameter and a hub carrying the blades and mounted on the shaft for rotation in a direction to cause air to move from said first fan to said second fan in a direction from said inlet to said outlet, and means for adjusting the pitch of each blade about its spanwise axis, the blades of the second fan being adjusted to an average pitch greater than the average pitch of the blades of the first fan in order to accomplish optimum useful work, the blades of the second fan being circumferentially offset with respect to the blades of the first fan, and means for rotating the fans at a desired speed, the axial spacing between said fans being sufficient to permit the blades of each to avoid interference with the blades of the other, when the blades of the fans are so adjusted, but considerably less than the minimum axial spacing that would be necessary for the fans to accomplish the same useful work with their blades having the same average pitch and rotated at such desired speed.

2. An assembly of the character defined in claim 1, wherein said fan ring portion is virtually free of obstructions between said second fan and said outlet.

3. An assembly of the character defined in claim 1, wherein the planes in which the inner sides of the blades of the fans rotate are substantially adjacent one another.

Claims (3)

1. A series axial flow fan assembly, comprising a fan ring having an inlet and outlet and a portion therebetween which is of constant inner diameter, a shaft extending coaxially within said fan ring portion, first and second axial flow fans each having blades of equal outer diameter and a hub carrying the blades and mounted on the shaft for rotation in a direction to cause air to move from said first fan to said second fan in a direction from said inlet to said outlet, and means for adjusting the pitch of each blade about its spanwise axis, the blades of the second fan being adjusted to an average pitch greater than the average pitch of the blades of the first fan in order to accomplish optimum useful work, the blades of the second fan being circumferentially offset with respect to the blades of the first fan, and means for rotating the fans at a desired speed, the axial spacing between said fans being sufficient to permit the blades of each to avoid interference with the blades of the other, when the blades of the fans are so adjusted, but considerably less than the minimum axial spacing that would be necessary for the fans to accomplish the same useful work with their blades having the same average pitch and rotated at such desired speed.

2. An assembly of the character defined in claim 1, wherein said fan ring portion is virtually free of obstructions between said second fan and said outlet.

3. An assembly of the character defined in claim 1, wherein the planes in which the inner sides of the blades of the fans rotate are substantially adjacent one another.

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