Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to axial or mixed 1OrJl
blowers or suction fans and more particularly to such blowers and
fans which exhibit improved stability of operation.
It is known that conventional fans suffer from the
problem of stalling when the gas flow past the fan blades breaks
away from the surface of the blades and/or the housing surrounding
the fan. The fans and blowers according to the present invention
are designed to significantly reduce the liability of a stall con-
dition occurring over a wide range of possible operating condi-
tions.
Summary of the Invention
The present invention consists in a high pressure axialor mixed flow blower or suction fan, comprising: an elongate
tubular casing, an impeller disposed within said casing and having
a hub rotatable about a longitudinal axis of the casing, and also
having a plurality of impeller blades extending radially from said
hub, each impeller blade having a tip with a leading edge and a
trailing edge, a stationary center tube positioned within said
casing upstream of the hub and carrying a plurality of guide vanes
extending radially therefrom, the casing comprising a tubular inlet
portion the cross sectional area of which is constant or is reduced
gradually along its length and which extends at least from the
guide vanes in a downstream direction until it terminates in a
tubular intermediate portion of larger cross sectional area than
the inlet portion at its point of termination, the intermediate
portion in turn terminating in a tubular outlet portion which is
no larger in cross sectional area than the inlet portion at its
point of termination, the inlet portion and the intermediate por-
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tion being connected by an imperforate and substantially annular
first wall portion and the intermediate portion and the outlet
portion being connected by an imperforate and substantially annular
second wall portion, the impeller being so positioned that the
leading edge of the tip of each blade is positioned within the
intermediate portion while the trailing edge thereof is positioned
within the outlet portion, characterised in that a plurality of
fixed vanes are provided in, and are affixed to, the intermediate
portion of the casing, each vane being connected to at least one
of the wall portions.
The casing of the blower or fan is preferably circular
in cross sectional shape. The inlet portion is preferably frusto-
conical or cylindrical while the intermediate and outlet portions
are preferably cylindrical. The diameter of the outlet portion is
preferably slightly smaller than the minimum diameter of the inlet
portion. As is conventional in such fans the diameter of the out-
let portion should be only slightly larger than the diameter of the
circle swept by the rotating blades of the impeller.
The diameter of the intermediate portion is larger than
the diameter, or minimum diameter, of the inlet and outlet portions.
It is preferred that the diameter of the enlarged intermediate por-
tion be from 1.1 to 1.5 times the diameter of the impeller and most
preferably between 1.2 and l times that diameter.
The fixed vanes within the intermediate portion are pre-
ferably disposed radially of the hub and axially of the casing. It
is particularly preferred that these vanes narrow in a downstream
direction.
The commencement and termination of the intermediate
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portion are preferably both abrupt with the intermediate portion
being joined to the inlet or outlet portion by an annular wall por-
tion which lies in a plane at right angles to the longitudinal axis
of the Ean or blower casing. While it is not essential that the
intermediate portion terminates abruptly in a downstream direction,
it is essential that this termination takes place, in an axial
direction, between the leading and trailing edges of the impeller
blades.
The arrangement according to this invention may be used
with equal advantage in fixed or variable pitch fans and blowers.
In certain cases it is desirable to provide in the inlet
portion a plurality of radially directed guide vanes which are each
rotatable about its own longitudinal axis. These guide vanes pre-
ferably extend between the wall of the inlet portion and a cylin-
drical stator disposed coaxially with the casing. These guide
vanes are preferably used in axial flow fans and blowers in which
the blades of the impeller are not themselves rotatable about their
own longitudinal axes. These variable pitch guide vanes may be
positioned at angles relative to the longitudinal axis of the fan
of from + 45 to - 90. The angle is considered to be positive
if the
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vanes cause the incoming gas stream to be moved so as to
increase the angle through which the direction of movement of
the gas stream is changed by the impeller; it is considered
negative when that angle is reduced. A similar convention is
used when considering the fixed vanes in the intermediate
portion. In fans and blowers where the impeller blades are
of variable pitch such guide vanes are not generally required.
The minimum diameter of the inlet portion is preferably
slightly larger than the impeller diameter. The most
preferred minimum diameter of the inlet portion is from 1.01
to 1.1 times the impeller diameter. The stability of the fan
for any given minimum diameter of the inlet portion increases
as the axial distance between the leading edge of the
impeller and the terminal edge of the inlet portions
increases. This increased stability is however associated
with a slight loss of efficiency.
Best Mode of Carrying Out the Invention
Hereinafter given by way of example only are details of
a variety of embodiments of the invention described with
reference to the accompanying drawings in which :-
Fig. 1 is a diagrammatic longitudinal sectional viewthrough `one embodiment of a mixed flow fan according to this
invention.
Fig. 2 is a pressure/volume diagram for the mixed flow
fan of Fig. 1.
Fig. 3 is a diagrammatic longitudinal sectional view
through another embodiment of a mixed flow fan according to
this invention.
Fig. 4 is a pressure/volume diagram for the mixed flow
fan of Fig. 3.
Fig. 5 is a diagrammatic longitudinal sectional view
through a further embodiment of a mixed flow fan according to
this i.lvention.
Fig. 6 is a diagrammatic longitudinal sectional view
through a still further embodiment of a mixed flow fan
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according to this invention.
Fig. 7 is a pressure/volume diagram for the mixed flow
fans of Figs. 5 and 6.
In describing the various embodiments of this invention
the same numeral will be used to describe the same integers
in the different embodiments of the invention.
As is seen in Figs. 1, 3, 5 and 6 the fan 10 comprises a
casing 11 or circular cross sectional shape which surrounds
and is coaxial with an impeller 12 which comprises a hub 13
and a plurality of radially directed blades 14. The
cylindrical casing 11 is divided into three principal parts.
A frusto-conical inlet part 15 terminates in a cylindrical
imtermediate part 16 of enlarged diameter which in turn
terminates in a cylindrical outlet part 17. The intermediate
part 16 terminates and the outlet part commences intermediate
the leading and trailing edge of the impeller blades 14.
Upstream of the impeller 12, and coaxial therewith is a
centre tube 18. A plurality of radially directed guide vanes
19, each mounted on a spindle 20, are positioned about the
centre tube 18. Each vane 19 extends radially outwardly to
the inside surface of the inlet port 15 of the casing 11 and
is rotatable about its spindle 20 such that the angle of the
vane to the longitudinal axis of the fan may be varied.
The intermediate part 16 may include an array of fixed
vanes 21 which may be axially of the fan as in Figs 3 and 5
or at an angle to that axis as in Fig. 1.
In the arrangement shown in Fig. 1 the minimum diameter
of the inlet part 15 is equal to the diameter of the outlet
part 17, i.e. approximately equal to the diameter of the
impeller 12.
The intermediate portion includes a number of fixed
vanes 21 each of which extends at 45 to the axis of the
casing and is radially directed relative to the impeller.
The inlet portion 15 is extended rearwardly as
cylindrical wall 22 which continues the minimum diameter of
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inlet portion 15 and is connected to the radially inner edges
of the vanes 21.
Figure 2 is a pressure/volume diagram for the fan shown
in Fig. 1. Each solid line represents the measured P/V
relationship for the angular setting of moveable guide vanes
indicated. The dotted lines represent contours of equal
efficiency with the point of maximum efficiency marked with a
cross. It can be seen from Fig. 2 that the fan of Fig. 1
showed a smooth P/V curve for all vane angles tested showing
that the fan was not stalling at any point throughout its
full operating ranges.
Figure 3 shows another embodiment of the invention in
which the intermediate portion includes a number of fixed
vanes which extend axially of the casing and radially
relative to the impeller. The inlet portion 15 is extended
with the frusto conical wall 22 defining the minimum diameter
of the inlet portion.
In this arragnement an aperture was left in the fixed
vanes adjacent the leading edge of the intermediate portion
to increase interaction of circulating flow between each pair
of fixed stabilising vanes.
In can be seen from Fig. 4 that the above embodiment is
free from stall characteristics.
Figures 5 and 6 show further embodiments of the
invention.
Figure 5 shows an arrangement in which axially and
radially disposed vanes 21 are provided in the intermediate
portion. Figure 6 shows a very similar arrangement but
without the vanes. In each of these situations the inlet
portion stops abruptly at the start of the intermediate
portion.
Figure 7 compares the P/V performance of the fans of
Fig. 5 and 6 with the Fig. 5 fan results shown in dashed
lines and the Fig. 6 fan in solid lines. It can be seen that
while the removal of the fixed guide vanes has not induced
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stalling in the fan, it has substantially reduced pressures
at low volumes.
