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Modern developments of high-performance industrial fans
Karadzhi Sergey
Ph.D., sv@air-g.ru
Brodach Marianna
Ph.D., brodach@abok.ru
Karadzhi Viacheslav
Ph.D., vg@air-g.ru
Modern centrifugal and axial impellers were developed on the
basis of domestic and foreign experience. Highly efficient centrifugal and
axial fans of different aerodynamic schemes for different practical
applications were designed on the basis of those impellers. Optimal
technological solutions allowed to master the serial production of the
developed fansin short time. Realized technical decisions will allow to reduce energy
consumption of ventilation systems of modern buildings and structures.
Keywords: centrifugal fan; axial fan; impeller; free impeller; tubular fan;
efficiency
Introduction
The main directions of the development of
general industrial fans are:
·
increase in energy efficiency,
·
expansion of the working area for
air flow rates at specified dimensions and speeds,
·
noise reduction,
·
optimization of design performance
in accordance with classes of tasks to be solved.
The most significant fact is that, firstly,
fan science has been developing for a long time and high levels of efficiency
have been achieved (up to 92%),therefore, every step in its development requires
more and more efforts. Secondly, today the development is, as a rule, at the
junction of directions, when it is necessary, for example, not only to reduce
the power consumption of the fan, but also to reduce its noise. Or increase the
pressure of the fan while increasing the temperature of the pumped medium. One
can cite other examples of concrete tasks at the junction of two or three
different branches of science. We strive to be at the level of modern
requirements and we are engaged in research, development and practical
implementation of new promising technical solutions in the field of ventilation
equipment. In this article, we will offer several technical solutions for
radial and axial general industrial fans from the spectrum of problems we
solve.
Centrifugal fans
Aerodynamic scheme “free impeller”
The last 20
years, the development of fans with the aerodynamic "free impeller" principle
has been developing. This principal is based on a scheme of a radial impeller
with backward curved blades and an inlet cone on the frame, without a housing. The
impeller can be located either directly on the shaft of the electric motor or
connected to it via a pulley-belt transmission. It should be noted that this scheme
has been used for a long time, for example, in roof fans. Today - this is one
of the most used schemes in supply and exhaust systems.
The outlet of the flow from the impeller
occurs in the volume of the casing of sufficiently large dimensions [1,2] and
the velocity head is lost, the impeller operation is estimated on basis of
static parameters. In accordance with modern energy saving trends, research and
development work is conducted in the direction of increasing the static
efficiency of such impellers by optimizing their geometry.
Currently the upper limit of the capabilities
of such an aerodynamic scheme seemed to be reached. The problem solving has an
optimum: to increase the static efficiency of the impeller, it is necessary to
reduce the angle of the blades at the outlet, however, in order to increase the
flow and pressure coefficients it is necessary to increase the angle of
installation of the blades at the impeller outlet and its width. A number of
other factors are also ofinfluence. The actual efficiency for static pressure corresponds to approximately
FEG71(Fan efficiency grade 71) see[3,4]. Obviously, further improvement of the "free
impeller" scheme will bevery difficult and may prove to be economically
unjustified, since it will not be possible to completely get rid of the lost
dynamic pressure at the outlet of the free impeller.
Regarding the modern impellers with backward
curved blades, it is worth mentioning the impellers of the companies
"Ziehl-Abegg", "EBM Papst", "Flakt Woods",
"Comfrey" (see, for example, [5]). These impellers were designed,
according the aerodynamic scheme of the fan "free impeller", for
application in supply and exhaust systems. In our country schemes such as
"free impeller" began to be used in the early 2000s. At the same
time, the most popular were centrifugal impellers with backward curved blades
having a flat front disc of an enlarged diameter (in comparison with the
blades) with a smooth turning radius at the impeller inlet (Figure 1).
Figure 1. The basic unit of the
"free impeller" scheme.
In this article, we want to demonstrate the
basics of aerodynamics of the best foreign and domestic centrifugal impellers
with backward curved blades. This to identify opportunities, generalize their
advantages and minimize shortcomings. Our experiments showed the dependence of
the aerodynamic performance and efficiency on the width of the impeller (Figure 2),
with other geometric parameters of the impeller also affecting its efficiency,
but the main effect is on the width. As the width of the impeller increases,
the flow in it loses stability, there is a flow separation near the front disc,
which leads to a decrease in the aerodynamic parameters and efficiency, volume
flow growth is disproportionate to the width of the impeller. The best impellers with a flat front
disc have optimum performance at widths of about (25 ... 28%) of the diameter
of the blade system. For smaller widths, efficiency also falls off rather
quickly, which is already associated with a suboptimal relationship between the
inlet diameter and the width of the impeller. For example, in the book [6], one can find
the results of work related essentially to the effects observed in a centrifugal
impeller with a change in its geometry.
Figure 2. Dependence of the maximum
efficiency and the flow coefficient on the width of the impeller divided by
blade system diameter.
These results
were obtained from the results of a cycle of experiments to optimize the
geometry of the impeller, including the shape and relative dimensions of the
front disc, the shape of the blade and the angles of its installation at the inlet
and outlet of the impeller, the shape of the rear disc (Figure 3). In addition to the tests, estimates of the aerodynamics of impellers
were made for a deeper understanding of the influence of the basic geometric
dimensions and parameters.
Figure 3. The newly developed basic unit
of the "free impeller" scheme.
The smooth inlet cone, in combination with the
conical front disk of the impeller and the curvilinear shape of the sheet
blades, made it possible to obtain aerodynamic, power and noise characteristics
similar to the best "free impeller" schemes. It should be noted that
there isa significant difference: the developed aerodynamic scheme allows creating
impellers of greater width, providing a significant expansion of the
aerodynamic characteristics of the "free impeller" without loss of
pressure and, accordingly, the efficiency of the fan (Figure 4).
The developed scheme can be used without a noticeable reduction in efficiency
to impeller widths of at least 25% of the diameter of the blade system. At
smaller widths, the efficiency of the circuit decreases and it is necessary to
apply other aerodynamic parameters of the "free impeller" principle.
Figure 4. Dimensionless operating
characteristics of the developed "free impeller" scheme for two
impeller widths (here φ = L / (FU); ψ = ρU² / 2; U = πDn /
60; F = πD² / 4; D-diameter of the impeller blade system, m; n-impeller
speed, rpm; η - fan efficiency).
Volute casing
The need for
unification led us to inspect the performances of new impellers as part of a
fan with a volute casing. As the width of the impeller widens its working zone
by the volume flow rate of air, it is necessary to change the geometry of the
volute casing accordingly. For unification reasons, it was decided not to
change the front and rear walls of the volute casing. The effect of the change
in the spiral wall of the casing (its axial extension) proportional to the
width of the impeller was studied. The completed cycle of works allowed
designing two types of fan, differing in width (axial extension) of impeller
and the volute casing correspondingly. One fan made it
possible to obtainwide
aerodynamic characteristics corresponding best industrial centrifugal fans and
is distinguished by the reliability and stability of these characteristics. The
second fan has a wider impeller, a wider volute casing and its aerodynamic
performance is significantly wider, while maintaining high efficiency values (Figure 5).
An essential advantage of the second scheme is a high proportion of the static
pressure in the total pressure of the fan, which simplifies its matching with
the ventilation systems.
Figure 5. Dimensionless performance of
two fans with a volute casing, differing in the width of the impeller and
volute.
Tubular fan
For practical applications of considerable
interest are radial ducted direct flow fans with a cylindrical casing (for
example, [7]). In our country, fans of this type started to be studied in the
fifties [8], but unlike at the foreign market where they were widely spread are
called "tubular fan", they have not yet been applied. These fans are
close to fans with a volute casing, but smaller in size, having a larger static
pressure part in total pressure, higher static efficiency, strait direction of
flow, which in some cases simplifies the connection of the fan with the network
with lower noise levels. Such fans can successfully replace corresponding fans
with a volute casing. The impeller can be centrifugal, centrifugal with profile
blades, mixed flow, or mixed flow with profile blades. At the same time, the centrifugal impeller with
backward curved blades is characterized by higher pressures than the mixed flow
impeller, but the use of a mixed flow impeller makes it possible to expand the
performance zone (with other things being equal). The possibility of changing
the width of the centrifugal impeller in a large range allows you to combine
the advantages of mixed flow and centrifugal impellers. (Figure 6).
Figure 6.Centrifugalin-line fan with a
round casing. Performance data are given for two impeller widths.
As a result of a series of experiments, the
aerodynamic characteristics of the created fan were confirmed, providing a
sufficiently wide aerodynamic characteristic for air volume flow coefficient.
At the same time, it was possible to regulate the working volume flow area by
changing the width of the impeller, without changing the fan casing. According
to aerodynamic parameters and efficiency, the fan replaces the corresponding
fans with a volute casing, but with higher static parameters. According to
aerodynamic and power characteristics, the fan is at the level of the best analogues
and exceeds in the width of the flow rate working zone.
Thus, the new centrifugal impeller has allowed
to a significant extent, to unify the technology and design of several types of
centrifugal fans, and to obtain new high-efficiency low-noise general
industrial fans for modern ventilation systems of energy-efficient buildings,
structures and technologies.
Axial fans
Unlike the radial fans calculation methods,
which are mostly based on semi-empirical and empirical dependencies and require
experimental investigation and refinement of each individual design, the
calculation methods of axial fans have theoretical design bases which allows providing
a set of accurate, specific design parameters.
One of such methods is the technique developed
in Central Aero-hydrodynamic Institute ( TsAGI) (for example [9,10]). This
technique allows determining all the optimal parameters, starting with the
maximum allowable diameter of the axial fan hub (the works of Brusilovsky I.V.
and Mitrofovich V.V.) and ending with the prediction of aerodynamic
characteristics (the works of Brusilovsky I.V., Gegin A.G., Kolesnikov A.V.,
Dovzhik S.A., and others). The developed technique also allows you to design an
axial vane diffuser under an existing axial impeller blade system.
Unfortunately, the forms of the blades
obtained as a result of calculations by this method as, indeed, for any other,
are quite complex and usually have the form of a saddle surface. This makes it
impossible to fabricate them with simple technological operations such as
bending and rolling, not to mention profile blades. For this reason, some
manufacturers either manufacture sheet blades with forms just like the
calculated geometry (with several bends or along a cylindrical surface). Some
users acquire blades of known companies specialized in axial blades
manufacturing with known characteristics in a wide range according to angles of
installation, number of blades as well as changes when pruning blades and
installing on hubs of different diameters. It is obvious that the blade rings
made by geometry only approximate to the optimal and have much reduced levels
of efficiency and pressure. Impellers with several types of blades used for all
applications cannot have high efficiency in all designs due to too wide unification.
In addition, axial vane diffuser has to be coordinated with impeller geometry
if not, most part of dynamic pressure associated with swirling the flow at the
outlet of the impeller will be lost and consequently with increased pressure
coefficients, the efficiency will decrease.
To solve problems in which high flow rates,
medium and low-pressure coefficients are required at high efficiency values,
fan blades with sheet blades were designed with observance of the calculated
geometric parameters corresponding to the optimal aerodynamic scheme (Figure 7).
The technology of such blades available in applications has been developed
which allows creating dimension rows of fans.
|
|
Figure 7. Axial fans according to the
scheme "impeller + axial vane diffuser" and scheme "impeller"
with sheetblades of spatial curvature.
Figure 8. Dimensionless aerodynamic
characteristics of a medium-pressure fan according to "impeller + axial
vane diffuser" scheme.
Figure 9. Dimensionless aerodynamic
characteristics of a low-pressure fan according to the "impeller" scheme.
A wide dimensions range and the possibility to
use fans with or without axial vane diffuser, as well as, in some cases, the
creation of a complete system with axial guide vanes for regulation, allow
solving a wide range of problems with high efficiency levels (Figure 8
and Figure 9). Such fans can successfully compete with
the best samples of other companies. High levels of efficiency allow them to be
used in the construction of ventilation systems of energy-efficient buildings
and structures.
References
1. Sadi O., Kremer P. Einfluss von Klimakastengeraten auf
das Verhalten von Laufradern. HLH,
BeratendeIngenieureSondertell, October 2004.
3. ISO 12759-2010. Fans-Efficiency classifications for fans.
4. GOST 31961-2012.
Industrial fans. Energy efficiency indicators.
5. ZIEHL-ABEGG. https://www.ziehl-abegg.com/gb/en/product-range/ventilation-systems/centrifugal-fans.
7. Twin City Fans and Blowers. http://www.tcf.com/products/inline-centrifugal-fans/tsl---tubular-inline-centrifugal-fan-airfoil.
10. Mitrofovich V.V. Definition of limiting design parameters of axial fans
with a high static efficiency, "Industrial aerodynamics", Issue 4
(36), Moscow: Mashinostroenie, 1991.
Pages 34 - 38