EP2742245B1

EP2742245B1 - Centrifugal fan with reduced vibration and noise

Info

Publication number: EP2742245B1
Application number: EP12769489.1A
Authority: EP
Inventors: Piero COTROZZI
Original assignee: Enel Ingegneria e Ricerca SpA
Current assignee: Enel Ingegneria e Ricerca SpA
Priority date: 2011-08-11
Filing date: 2012-08-10
Publication date: 2016-09-14
Anticipated expiration: 2032-08-10
Also published as: EP2742245A2; WO2013021365A2; WO2013021365A3; ITFI20110178A1

Description

Field of the invention

In general the invention refers to the field of centrifugal fans, in particular to those used in smoke evacuation systems in thermal power plants. More precisely the invention concerns a centrifugal fan equipped with a device that is suitable for making the flow in outlet homogeneous, reducing vibrations and reducing the mechanical stress on the components hit by the flow of outlet fumes.

State of the art

Centrifugal fans for aeriform substances (especially air and smoke) are plant components that are widespread and applied in numerous fields. Their purpose is to provide a thrust to the fluid so as to overcome the hydrostatic losses that the flow generates in the circuits upstream and downstream of the fan.
There are centrifugal fans of all sizes. The greater sizes, used for instance in the smoke evacuation circuit of thermal power plants, are usually of the type having a double inlet. In this case the inlet of the fluid inside of the impeller occurs symmetrically from both sides of the volute housing and the impeller is usually divided into two adjacent semi-impellers.
In smoke evacuation circuits of thermal power plants a battery of such fans is usually provided, arranged in parallel, confluent through outlet sub-manifolds, generally having a square or rectangular section, into a main manifold, generally with a circular section, which conveys the smoke into a chimney.
It is known that centrifugal fans, even under normal operation conditions, produce high noise and vibration levels caused by the impeller which, by centrifuging the gas in the outer part of the volute, creates an unstable and uneven movement which propagates into the duct downstream of the outlet, where it may jeopardise the integrity and the regular operation of the surrounding components. The fluid dynamic pulses generated by the flow instability can create resonance phenomena in the structural parts of the fan or in the surrounding components, for example a duct or a dumper.
The large amount of variation of the thrust produced by this turbulent flow causes fatigue stress on the affected structures, wherein said stress is particularly dangerous also due to the low frequency of the phenomenon and therefore leads to the possible breaking of these components.
The problem of the pressure pulses downstream of the centrifugal fans has already been tackled. For instance, EP 1378668 describes a centrifugal fan having a double inlet comprising a stabilising element placed between the volute and the opening and essentially arranged on a median plane perpendicular to the flow direction so as to make the outlet flow homogeneous. EP 2182220 describes a centrifugal fan with a perforated plate which radially subdivides the space of the volute so as to form two separate channels projecting for a portion to inside the exhaust duct.

Subject of the invention

Subject of the present invention is to provide a centrifugal fan which has a homogeneous flow downstream of its outlet section, so as to eliminate or considerably reduce the non stationary turbulent phenomena that are responsible for breaking the components installed directly downstream of the fan, and to reduce the vibrations and the noise emitted during normal operation of the centrifugal fans according to the known art.
A particular purpose of the present invention is to allow the installation of the fan even in plants in which, for layout problems, the rectilinear portion between the fan and the exhaust manifold is reduced and an interception dumper has to be necessarily installed here.
This is achieved with a centrifugal fan according to the present invention whose essential characteristics are defined in claim 1. Further important characteristics are defined in the dependent claims.

Brief description of the drawings

The characteristics and advantages of the centrifugal fan according to the invention will be apparent from the following description of an embodiment thereof, given as a non-limiting example with reference to the attached drawings, in which:

figure 1 shows a schematic section view of a centrifugal fan with a double inlet according to the present invention;
figure 2 schematically shows the flow field downstream of a centrifugal fan according to the prior art;
figure 3 schematically shows the flow field downstream of a centrifugal fan according to the present invention;
figure 4 is a graph illustrating the transverse thrust on the louvers of the dumper of a centrifugal fan according to the prior art (A) and according to the present invention (B);
figure 5 shows a particular embodiment of a rods array in a centrifugal fan according to the invention.

Detailed description of the invention

With reference to figure 1, a centrifugal fan with double inlet (shown in a side section view), intended, in the specific example, to be installed in a smoke evacuation system of a thermal power plant, has been indicated with reference numeral 1. Reference numeral 2 indicates a dumper equipped with louvers 3 connected to the outlet opening 4 (indicated with a broken line) of the fan. Downstream of the dumper 2 a smoke exhaust manifold projects communicating with a chimney (both not shown).
The fan 1 is formed by a housing 5 made up of two flat side walls 6 (only one of which is visible in the section view) united with a back 7, in its turn made up of a curved plate with a spiral-shaped profile. Between the two side walls 6 an impeller 8 is positioned, which defines a central chamber 8a thereof wherein the sucked fluid is conveyed. The impeller comprises two lateral plates 11, to which a double ring of suitably shaped blades 9 is fixed, the rotation of which gives a radial thrust to the fluid coming out from the outer cylindrical surface of the impeller 8 and collected in an outer volume, named volute, indicated with reference numeral 10, enclosed in the housing 5 of the fan.
As generally provided in double inlet fans, the inner volume of the impeller 8 is divided in half by a central wall 13 arranged on the median plane of the impeller. Two semi-impellers are thus created, each equipped with an array of blades. This makes it possible to angularly offset the arrays of the two semi-impellers so as to obtain a more uniform flow at the outlet.
In order to provide a sufficiently wide outlet section 4, the side walls 6 of the housing 5 of the fan are spaced apart from the impeller 8, so that part of the fluid recirculates inside the housing 5 around the impeller itself. The housing 5 has an opening that is typically rectangular, and coincides with the outlet opening 4, from which the fluid comes out with an increased dynamic pressure with respect to the inlet, thus overcoming the hydrostatic losses of the remaining part of the circuit.
The back 7 radially delimiting the volute 10 is shaped so as to increase the distance between the periphery of the impeller and the volute itself starting from a minimum value at the beginning of the volute, to a maximum value at the end of the volute. The beginning and the end of the volute are joined through the opening 4. The beginning of the volute is joined through a profile, called tongue, indicated with reference numeral 12, which separates the flow directed towards the opening 4 from the small part which recirculates going back in the volute 10.
According to the invention, at the outlet section 4 there is an array of rectilinear elements 14, in the rest of the description called rods for the sake of simplicity, mounted transversally, with median lying plane perpendicular to the average direction of the smoke flow, indicated with the arrow F in figure 1. Since the outlet section and the ducts downstream generally have a quadrilateral section, and in particular square or rectangular section, the rods array 14 extends between two opposite sides of said section and the rods are substantially parallel to the axis of the impeller 8. Preferably, the median lying plane of the rods array 14 is perpendicular to the median plane of the impeller 8 and, in the example under consideration, it is perpendicular to the central wall 13 of the impeller.
In the array 14 the rods are distributed with an increasing spacing starting from the outer side of the volute towards the inner side. In the present embodiment the rods 14 are made up of tubular elements that are parallel to the axis of the impeller 8 and are positioned, at the opening flange, closer together in the centrifuge area, i.e. outside the volute, and more spaced apart in the recirculation area, i.e. inside the volute, close to the nose. The loss of static pressure consequent to the introduction of the array 14 is partially recuperated in the portion downstream due to the lower losses associated to a more uniform flow downstream of the array itself.
According to one embodiment of the invention, particularly suitable for centrifugal fans of large size, the rods array 14 is fitted with one or more elements 15 which extend from one side to the opposite side of the outlet section, perpendicular to the rods of the array and by these crossed or fixed to them, and arranged with regular and uniform spacing, so as to divide each rod into parts of equal length among each other. These perpendicular elements 15 further increase the stability of the rods array without affecting the array's positive effects and advantages described in detail in the following. A preferred embodiment is illustrated in figure 5, wherein only one element 15 perpendicular to the rods array 14 is present, positioned at the center of the outlet section in order to cut each rod in two equal portions.
From the tests carried out it has been found that the rods array 14 arranged in the aforementioned position has the effect of making uniform the smoke flow downstream of the outlet section 4 substantially reducing both the turbulence intensity and the fluid dynamic load flowing on the surfaces of the inner components.
Indeed, from a study of the behaviour of the flow field inside and in outlet from the fan it has been found that the fluid tends, due to centrifuge effect, to form layers in the outermost area of the fan volute. Such a phenomenon leads to an increase of the penetration of the gas jet inside the circular manifold suitable for collecting the flows coming out from the fan set, emphasising the effect of the impact of the aforementioned jet on the walls of the manifold. This behaviour, in addition to creating vibrations in the structure of the duct itself, also leads to considerable loads on the mechanical components which are hit by the fluid fillets with greater speed (concerning this see figure 4).
Through the FLUENT simulation code the flow field inside the fan and in the relative opening duct was investigated. The calculation domain took into account the fan (including its inner components) and part of the exhaust duct where the interception dumper is installed, the latter being formed by two louvers and positioned at a close distance from the outlet of the fan itself.
Moreover, the analysis of the simulation has highlighted the presence of recirculation areas inside the exhaust duct (shown in figure 2), which in an alternating manner are formed and dissipate generating a strong instability in the flow coming out from the fan.
The efficiency of the rods array 14 in making uniform the smoke velocity at the fan outlet has been numerically verified by means of the Fluent code. In particular this method was applied first to a centrifugal fan operating according to the prior art and then to one operating according to the present invention.
The analysis of the results of the simulation has showed that the rods array 14 arranged at the opening of the fan according to the present invention is effective for making uniform the flow field in outlet from the fan and for inhibiting the formation of recirculation areas in the exhaust duct, between the impeller and the dumper louvers (compare figures 2 and 3). In particular, a substantial reduction has been found for the standard deviation of the velocity at the opening from the outlet duct of the fan; an estimation carried out on a section of the outlet duct of the fan, arranged at around 3 m from the opening, has showed values of 72% for the fan without rods array and of 22% with the introduction of the array.
As a further test of the effectiveness of the proposed solution, the transverse thrust that the flow generates on the dumper louvers has been analysed. It has been found that the introduction of the array reduces both the average value of the force which discharges on a louver of the dumper and the width of the oscillations due to the fluctuations of the flow; moreover, a more even redistribution of the forces acting on the two louvers of the dumper was observed (see the graph of figure 4, where with A1 and A2 the curve of the transverse thrust on the right and left louver of the dumper in a conventional fan is shown, whereas B1 and B2 show the curve of the transverse thrust on the right and left louver in the fan according to the invention. The terms right and left as used here are related to the average direction of the smoke flow according to the arrow F of figure 1).
The mounting position of the rods array 14 according to the present invention is at the outlet section of the fan. Practically this can be obtained positioning the rods array directly at the opening of the volute housing of the fan or pre-mounting it on a frame having the same dimensions as the exhaust duct to be placed against the outlet of the fan.
The use of the rods array according to the present invention is also particularly advantageous in all those cases in which, where little space is available for the plant, the sub-manifold connecting the outlet of the fan to the main smoke exhaust manifold towards the chimney must be made with a length that is not sufficient for ensuring a homogenisation of the flow velocity field of the smoke flow coming out from the fan and before input into the main manifold. The installation of the rods array at the fan outlet eliminates asymmetry in the smoke flow from the outlet of the fan and therefore it is possible to reduce the useful length of the sub-manifold without drawbacks.
Although in the present description we have referred to a double inlet centrifugal fan, the present invention should also be considered applicable to single inlet centrifugal fans.
Variants and/or modifications can be made to the centrifugal fan according to the present invention without for this reason departing from the scope of protection of the invention itself as defined in the following claims.

Claims

A centrifugal fan (1) comprising a housing (5) delimiting a volute (10) and housing an impeller (8) to convey a fluid in said volute and through an outlet section (4) thereof, characterized in that a rods array (14) is provided at said outlet section (4), said rods array (14) lying on a plane orthogonal to the fluid flow direction (F) and having the rods distributed with an increasing spacing starting from the outer side of said volute (10) towards the inner side.
The centrifugal fan (1) according to claim 1, wherein said outlet section (4) has a quadrilateral shape and said rods array (14) extends between two opposite sides thereof.
The centrifugal fan (1) according to any one of the previous claims, wherein the rods of said array (14) have tubular shape.
The centrifugal fan (1) according to any one of the previous claims, wherein said rods array (14) is mounted in said outlet section (4).
The centrifugal fan (1) according to any one of the claims from 1 to 3, wherein said rods array (14) is pre-mounted on a frame having the same dimensions as said outlet section (4) and applicable to the outlet of said fan.
The centrifugal fan (1) according to any one of the previous claims, further comprising one or more elements (15) extending between two opposite sides of said outlet section (4), perpendicular to said rods array (14), so as to divide each rod in portions of equal length among each other.