U.S. patent number 3,768,546 [Application Number 05/212,555] was granted by the patent office on 1973-10-30 for axial flow fan assembly.
This patent grant is currently assigned to Hudson Products Corporation. Invention is credited to Kelly V. Shipes.
United States Patent |
3,768,546 |
Shipes |
October 30, 1973 |
AXIAL FLOW FAN ASSEMBLY
Abstract
An axial flow fan assembly having two or more fans in series to
provide successive fan stages in which less than the total number
of fans are of the variable pitch type.
Inventors: |
Shipes; Kelly V. (Houston,
TX) |
Assignee: |
Hudson Products Corporation
(Houston, TX)
|
Family
ID: |
22791514 |
Appl.
No.: |
05/212,555 |
Filed: |
December 27, 1971 |
Current U.S.
Class: |
165/96; 165/299;
165/122; 165/900; 415/130; 416/127; 416/130; 416/157B |
Current CPC
Class: |
F04D
19/007 (20130101); F04D 27/002 (20130101); F04D
29/362 (20130101); F28B 1/06 (20130101); F04D
19/024 (20130101); Y10S 165/90 (20130101); F28F
2250/08 (20130101) |
Current International
Class: |
F04D
27/02 (20060101); F04D 29/32 (20060101); F04D
29/36 (20060101); F04D 19/00 (20060101); F01d
007/00 (); F28f 027/00 (); B60h 001/20 () |
Field of
Search: |
;415/198,199,130,127,149
;261/DIG.10 ;416/130 ;165/39,96,122 ;138/39 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Publication: Pub. in Aviation Week, 11-28-1960, "The Variable
Camber Propeller" Vol. 73, pages 50-51..
|
Primary Examiner: Raduazo; Henry F.
Claims
The invention having been described, what is claimed is:
1. An air cooler, comprising a tube bundle, and an axial flow fan
assembly mounted on one side of the tube bundle for causing air to
pass thereacross, said fan assembly including a fan ring, first and
second axial flow fans mounted coaxially within the fan ring and
relatively close together, means for rotating the first and second
fans, the blades of the first fan having a positive pitch which is
non-adjustable during rotation thereof and arranged to move the air
through the fan ring in a direction from said first fan to said
second fan, and remotely operable means for adjusting the pitch of
the blades of the second fan, during rotation thereof, within a
range between positive pitches gerater than and less than the
positive pitch of the blades of the first fan.
2. An air cooler of the character defined in claim 1, wherein said
remotely operable pitch adjusting means includes means for
adjusting the blades of the second fan to a negative pitch.
3. An air cooler, comprising a tube bundle, and an axial flow fan
assembly mounted on one side of the tube bundle for causing air to
pass thereacross, said fan assembly including a fan ring, first,
second and third axial flow fans mounted coaxially within the fan
ring, the first fan being relatively close to the second fan, and
the second fan being relatively close to the third fan, means for
rotating the first, second and third fans, the blades of the first
fan having a positive pitch which is non-adjustable during rotation
thereof and arranged to move the air through the fan ring in a
direction from said first fan to said second fan, and remotely
operable means for adjusting the pitch of the blades of each of the
second and third fans, during rotation thereof, within a range
between positive pitches greater than and less than the positive
pitch of the blades of the first fan.
4. An air cooler of the character defined in claim 3, wherein said
remotely operable pitch adjusting means includes means for
adjusting the blades of the second and third fan to a negative
pitch.
5. An air cooler, comprising a tube bundle, and an axial flow fan
assembly mounted on one side of the tube bundle for causing air to
pass thereacross, said fan assembly including a fan ring, first,
second and third axial flow fans mounted coaxially within the fan
ring, the third fan being relatively close to the second fan, and
the second fan being relatively close to the third fan, means for
rotating the first, second and third fans, the blades of each of
the first and second fans having a postivie pitch which is
non-adjustable during rotation thereof and arranged to move air
through the fan ring in a direction from the first fan to the
second fan and from the second fan to the third, with the pitch of
the blades of the second fan being greater than the pitch of the
blades of the third fan, and remotely operable means for adjusting
the pitch of the blades of the third fan, during rotation thereof,
within a range between the positive pitches greater than and less
than the positive pitch of the second fan.
6. An air cooler of the character defined in claim 5, wherein said
remotely operable pitch adjusting means includes means for
adjusting the blade of the third fan to a negative pitch.
Description
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 6 to 30 feet. The
amount of work to be performed by a fan is approximately a function
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
rotating 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 sufficient. This large spacing
between fan stages of these assemblies causes them to be quite
expensive, and, in some cases, to interfere with available head
room.
In a copending application Ser. No. 209,923, executed Dec. 17,
1971, entitled "Axial Flow Fan Assembly," and assigned to the
assignee of the present invention, there is disclosed a series
axial flow fan assembly which is considerably less expensive to
construct in that fan stages are considerably closer together than
heretofore thought possible, and preferably substantially adjacent
one another. More particularly, as explained in the copending
application, it has been found that when the blades of the
downstream fan have a greater average pitch angle or pitch than the
blades of the upstream fan, such a fan assembly is capable of
performing substantially the same useful work, and at substantially
the same noise level, as the above-described prior series fan
assemblies in which successive stages are widely spaced apart. As
also explained in such application, it has further been discovered
that additional advantageous results can be obtained when the
blades of the second fan are circumferentially staggered or offset
with respect to those of the first.
The environment in which fan assemblies are used often requires
that the amount of air flow generated by the assembly be varied in
view of changing conditions, such as a change in the ambient
temperature or, in the case of an air cooler, a change in the
temperature of fluid flowing through the tube bundles. More
particularly, there is a need for varying the amount of air flow
without shutdown of the operation of the fan assemblies. For this
purpose, it has been proposed to provide fan assemblies with
mechanisms for remotely adjusting the pitch of the blades of the
fan assembly during their rotation.
A mechanism of this type is shown, for example, in Petty U.S. Pat.
No. 2,826,395, wherein the blades of a fan assembly are caused to
rotate within sockets on the hub of the fan in response to a
controller, which may be at a remote location and actuated in
response to a signal indicative of the temperature of a fluid
flowing through a tube bundle. Thus, for example, upon a drop in
the temperature of such fluid below that for which the system is
designed, the mechanism may cause the blades of the fan to rotate
into positions in which their pitch are reduced, so as to in turn
cut down the amount of air flow across the bundle, and conversely,
upon an increase in such temperature, the blades may be rotated
into positions increasing their pitch so as to increase the amount
of air flow.
Although fans having mechanism of this type, and known as "variable
pitch" fans, serve an extremely useful purpose, they nevertheless
are considerably more expensive and space consuming than
conventional fan assemblies, known as "fixed pitch" fans, wherein
the pitches of the blades can be adjusted, if at all, only by
stopping rotation of the fan so that they may be manually rotated
to a desired position. In fact, these mechanisms represent a large
portion of the over-all cost of a single stage fan assembly.
Obviously, in a series fan assembly having two or more fan stages
with such mechanisms, they would represent an even larger
percentage of the over-all cost of the assembly.
An object of this invention is to provide a series fan assembly in
which the amount of air flow generated thereby can be varied
without interruption of serivce, but in which the additional
expense of providing this adjustment, as compared with a series fan
having "fixed pitch" fans, is more economically feasible than
heretofore thought possible.
This and other objects are accomplished, in accordance with the
illustrated embodiment of the invention, by a series fan assembly
in which less than all the stage fans are of the variable pitch
type. That is, at least one fan is of the fixed pitch and at least
another fan is of the variable pitch type, with additional fans, in
the case of three or more stages, being of either type, whereby the
amount of the air flow generated by the assembly is determined
solely by the one or more variable pitch fans. Although the
adjustment of the blades of one fan to a pitch different from that
of another may result in some loss of efficiency, and thus an
increased cost of operation, as compared with a fan assembly in
which all fans are of the variable pitch type adapted to be
adjusted to the same extent, this is ordinarily more than
compensated for by the reduced initial investment. Thus, in the
range of pitch adjustments which would ordinarily be required in
the operation of the variable pitch fan or fans of a fan assembly
for use as part of an air cooler, for example, there is little or
no loss of efficiency. Still further, as suggested by the
aforementioned copending patent application, the fan whose blades
are adjusted to the larger pitch during design conditions may be
disposed downstream of the other, and the fans arranged axially
close together, so as to further reduce the cost of the fan
assembly without substantial loss of efficiency, as compared with
conventional fan assemblies having widely spaced stages.
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 one embodiment of the present
invention and supported above the bundle for drawing air upwardly
thereacross;
FIGS. 2 and 3 are elevational views of a portion of air coolers
having fan assemblies constructed in accordance with additional
embodiments of the invention;
FIG. 4 is a graph showing a curve illustrating the amount of air
flow which must be generated by a fan assembly in order to condense
a process fluid in an air cooler, such as that shown in FIG. 1, at
different ambient temperature conditions; and
FIG. 5 is another graph showing curves illustrating in solid and
broken lines, respectively, the power required to so condense the
fluid by a fan assembly in which both stages are variable pitch
fans and a fan assembly in which one stage is a variable pitch fan
and the other is a fixed pitch fan.
With reference now to the above-described drawings, the air cooler
shown in FIG. 1, 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 series 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. 1, the blades of both fans
are adjusted to positive pitch 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." However, it is contemplated that, as
discussed to follow, the blades of one and possibly both such fans
may be adjusted to cause a reversal of air flow - i.e., in a
direction downwardly through the fan ring.
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 21b.
The lower or upstream fan 19 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 19a are
releasably secured in the hubs, which enables the average pitch on
each blade to be adjusted as desired, depending on operating
conditions. However, such adjustment requires that the fan first be
stopped, and, in this sense, the fan 19 is of the fixed pitch
type.
The upper or downstream fan 20 also includes a hub 27 fixed to
shaft 21 and having a plurality of sockets 28 extending radially
therefrom to receive the inner ends of blades 20a. However, the hub
27 includes a mechanism (not shown) such as that shown in U.S. Pat.
No. 2,826,395, which is remotely operable for causing the blades to
rotate in their sockets, and thus adjust their pitch, in response
to a signal indicative of a condition in the cooler. In this sense,
the fan 20 is of the variable pitch type. As disclosed in such
prior patent, there is a pressure responsive operator at the upper
end of the hub 27 which is adapted to receive a signal, which may
represent the temperature of process fluid in the bundle 16. This
signal may be produced by a transducer 30 in a conduit connecting
the outlet end of the bundle with the operator. The operator is
also adapted to receive power fluid supplied through a conduit 31
for operating the mechanism, and thus rotating the blades 20a, in a
desired manner responsive to such signal. Thus, for example, upon
an increase in the temperature of the process fluid, the mechanism
may cause the pitch of the blades 20a to increase and thus increase
the amount of air flow across the bundle. On the other hand, upon a
decrease in the temperature of the process fluid, the blade pitches
may be decreased to decrease the amount of such air flow.
The hubs 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. Also, the blades of the fans are shown in
so-called design or 100 percent air flow position, wherein the
positive pitch of those of the downstream fan 20 is greater than
that of the blades of the upstream fan 19, the particular pitches
and the axial spacing of the blades being determined in accordance
with the copending application. As will be apparent from FIG. 1,
the blades of the two fans are staggered or circumferentially
offset from one another, the extent of such staggering being
determined in accordance with the teaching of the copending
application.
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 blades 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 mentioned, during operation of the fan assembly, the
pitch of the blades of the fan 20 may be adjusted so as to change
the amount of air flow through the fan ring and thus across the
tube bundle 16, and, in some cases, the direction of such air flow.
As previously mentioned, the blades 20a of the fan 20 are shown in
FIG. 1 adjusted to a positive pitch greater than the positive pitch
of the blades 19a of the fan 19, for generating 100 percent air
flow during conditions for which the assembly is designed.
Adjustment of the blades 20a from a positive pitch equal to the
positive pitch of the blades 19a to a feathering point will usually
not decrease the amount of air flow upwardly through the fan ring.
In other words, within this range, the amount of air flow through
the fan ring will be determined by the pitch of the blades 19a.
However, in the event it is desired to reduce the amount of air
flow through the fan ring, the pitch of the blades 20a may actually
be adjusted to a negative pitch so as to oppose the direction of
air flow generated by the blades 19a. In fact, it is contemplated
that the blades 20a of the variable pitch fan 20 may be adjusted to
negative pitches greater than the positive pitch of the blades 19a
so as to actually cause air flow in a direction downwardly through
the fan ring.
As previously mentioned, under ordinary design conditions, air flow
through the fan ring will be in an upward direction and in an
amount determined by adjustment of the blades 19a to a positive
pitch greater than that of the blades 20a. According to the present
invention, it is preferred that, as illustrated in FIG. 1, the
variable pitch fan 20 be on the downstream side of the fixed pitch
fan 19, because it is known that the amount of power required to
rotate a fan is dependent upon the extent to which its blades are
pitched - i.e., the greater the pitch, the greater the power
requirements. Consequently, it is possible to take advantage of the
lower power requirements when the blades of the variable pitch fan
are adjusted to lesser pitches during changes from design
conditions.
The particular curve shown in FIG. 4 is representative of the
amount of air flow relative to design air flow which must be
generated across a tube bundle, such as that shown in FIG. 1, in
order to condense a vapor at 190.degree. F with ambient air less
than 90.degree. F. Thus, the term "design" is used to designate the
anticipated condition, wherein the ambient temperature is
90.degree. F and the blades of the fans are adjusted as described
to generate 100 percent air flow across the tube bundle through
which the vapor is conducted. As shown in the curve, and as is well
known in the art, as the ambient temperature decreases, the
percentage of design air flow required to condense the vapor
decreases. This decrease in air flow is, of course, effected by an
adjustment of the blades 20a of the fan 20 to a pitch for reducing
the amount of air flow which would be generated by the fan 19
alone. This adjustment is, of course, automatically responsive to a
signal produced in transducer 30 in response to a decrease in the
outlet temperature of the process fluid, which of course results
from a decrease in ambient temperature.
The solid and broken line curves of FIG. 5 compare the power
required by a fan assembly constructed as shown in FIG. 1, and a
fan assembly (not shown) forming part of the same air cooler of
FIG. 1, but wherein both fans are variable pitch type - i.e., of
the construction of the fan 20. As will be apparent from FIG. 5,
the latter fan assembly would be more efficient when the air cooler
is operating at an ambient temperature less than approximately
62.degree. F, at which point the illustrated fan assembly of the
present invention begins to subtract from the amount of air flow
due to the fixed pitch fan 19 alone. However, this invention takes
advantage of the fact that during the majority of the time of its
use, the air cooler will be operating at an ambient temperature
greater than the point of divergence of the two curves, and in an
even greater percentage of the time, during such time that the
power requirements of the comparative fan assemblies are either the
same or not substantially different.
For example, for the years 1951 through 1960, the northern and
southern U. S. cities of Philadelphia and Miami have average
temperatures 15.degree. to 40.degree. F only 12 pecent of the time,
average tempertures between 40.degree. and 65.degree. F only 26
percent of the time, and average temperatures between 65.degree.
and 90.degree.F during the remaining 62 percent of the time. As
shown by the curve of FIG. 4, it is, of course, during this latter
temperature range that the two fan assemblies would operate with
approximately the same efficiency.
By striking an average, it has been found that the fan assembly
constructed in accordance with the present invention will consume
59 percent of the designed power rate over the year, while a fan
assembly having variable pitch fans at both stages will consume 48
percent of the designed power rate during the year. Based on
typical figures of a 25 horsepower requirement per fan (at design
level) and power costs of $60.00 per horsepower per year, power
costs per fan for the fan assembly of the present invention will be
$885.00 per year, as compared with the power cost per fan of the
fan assembly having variable pitch fans at both stages will be
$720.00 per year.
On the other hand, in a typical fan assembly wherein both fans are
14 feet in diameter and have six blades each, the cost of the fan
assembly of the present invention would be approximately $3,540.00,
as compared with a cost of approximately $4,774.00 for the fan
assembly having variable pitch fans at both stages. Thus, taking
the difference in power costs between the two fan assemblies, it
would take approximately 71/2 years of savings and operating
expenses to pay the capital investment difference between the fan
assembly of the present invention and the comparative fan
assemblies.
It may be found that the difference in operating costs of the
above-described fan assemblies may be reduced even further. Thus,
several such fan assemblies are normally required in a given
installation, and, in accordance with well known practices, it may
be possible to effect a desired reduction in the amount of design
air flow, responsive to a change in operating conditions, by
shutting off one or more of the assemblies. In this event, the
blades of each of the downstream fans of the fan assemblies
remaining in operation would be adjusted toward a feathering pitch
to a lesser extent than would be required if the same amount of air
was to be moved with all the fan assemblies remaining in service.
As a result, the power required to operate the fewer number of fan
assemblies would be less than that required to operate all of
them.
By way of example, assume that the air cooler of FIG. 1 has four
fan assemblies, and that, in the exemplary system for which the
curves of FIGS. 4 and 5 are applicable, the ambient temperature
drops to 30.degree. F. From FIG. 4, it is seen that, at this
temperature, the required air flow is 48 percent of design, or 192
percent (4 fans .times. 48 percent) for all four fans; and from
FIG. 5, it can be seen that the total power requirement is 248
percent (62 percent .times. 4 fans). If one fan assembly is cut
out, the required percentage of design air flow through each of the
remaining three is 192 percent/3 or 64 percent; and, from FIG. 4,
it can be determined that this is the equivalent of all four fans
operating at an ambient temperature of 57.degree. F. Referring then
to FIG. 5, it will be seen that the power requirement of each of
the three assemblies is 40 percent of design, and thus equivalent
to 160 percent (4 .times. 40 percent) of design, as compared with
192 percent with all four fans operating.
In the embodiment of the invention illustrated in FIG. 2, the fan
assembly has three stages, including not only the fixed pitch fan
19 and the variable pitch fan 20 downstream thereof, but also an
intermediate or second stage fixed pitch fan 19'. As shown in FIG.
2, this second stage fan 19' may be of identical construction to
the first fixed pitch fan 19, and in accordance with the teachings
of the copending patent application, the blades of the second fan
19' are substantially adjacent to and adjusted to a positive pitch
greater than those of the first fan. As in the case of the first
and second fans of the FIG. 1 embodiment, during design conditions,
the blades of the fan 20 are substantially adjacent those of the
fan 19' and adjusted to a positive pitch greater than the positive
pitch of the blades 19a' thereof. Still further, and as also shown
in FIG. 2, the blades of the second fan 19' are staggered or
circumferentially offset with respect to the first fan 19, and the
blades of the third fan 20 are staggered or circumferentially
offset with respect to the blades of the second fan 19'. The
difference in pitches between adjacent fans, as well as the
staggering or offsetting of their blades, may be determined in
accordance with the teachings of the copending patent
application.
The operation of the fan assembly of FIG. 2 will be obvious in the
light of the foregoing discussion of the operation of the fan
assembly of FIG. 1. Thus, for example, upon a drop in ambient
temperature, the pitch of the blades 20a of the variable pitch fan
20 would be reduced so as to reduce the amount of air flow through
the fan assembly. Thus, it is contemplated that the variable pitch
mechanism of hub 27 of the fan 20 would permit the pitch of the
blades of the fan 20 to be adjusted over a range from a positive
pitch greater than the positive pitch of the blades 19a' to a
positive pitch less than the positive pitch of the blades 19a'.
The embodiment of the fan assembly illustrated in FIG. 3 also has
three fan stages, and similarly to the fan assembly of FIG. 2, the
fan 19 of the lower or first stage thereof -- i.e., upstream with
respect to the direction of design air flow -- is of the fixed
pitch type which may be identical to the fan 19 of the assembly of
FIG. 1. Still further, the second fan 20 downstream of the first
fan is of a variable pitch type which may be identical in
construction and operation to the variable pitch fan 20 of the FIG.
1 fan assembly. Thus, the fan 20 is substantially adjacent the
fixed pitch fan 19, and, under design conditions, its blades are
adjusted to a pitch greater than the positive pitch of the blades
19a of the fixed pitch fan 19, and are further circumferentially
offset or staggered with respect thereto.
However, the third fan 20' of the FIG. 3 fan assembly is of the
variable pitch type, which may be identical in construction to the
variable pitch fan 20 of each of the FIGS. 1, 2 and 3 embodiments.
Although the fan 20' is axially spaced from the second fan 20 a
greater distance than the distance between the fans 19 and 20 (in
order to accommodate the upward extension of the hub 27 of the fan
20), the fans 20 and 20' are nevertheless relatively close
together, in comparison to adjacent fans of prior series fan
assemblies. In accordance with the teachings of the copending
application, under design conditions, the positive pitch of the
blades 20a' will be greater than the positive pitch of the blades
20a, and the blades 20a' and 20a are circumferentially offset or
staggered with respect to one another. More particularly, similarly
to the blades of the fan 20, the blades 20a' of the variable pitch
fan 20' are adjustable, depending on system conditions, over a
range between a pitch greater than the pitch of the fan blades 19a
and a pitch less than the pitch of the fan blades 19a.
It is contemplated that a reduction in the amount of air flow
through the fan assembly of FIG. 3 may be effected by adjustment of
the pitch of the blades of one or both of the variable pitch fans
20 and 20'. Thus, the mechanisms for so adjusting the pitch of the
blades of each such fan may be responsive to a single transducer,
such as transducer 30 shown in FIG. 1, or separate transducers, in
which event each separate transducer may be responsive to a
different signal. Also, the power fluid for operating each such
mechanism may come from the same or different sources.
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.
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.
* * * * *