Such examination has revealed the device’s working characteristics,
which are especially valuable for low NOx blue burners. In addition to
burners, applications include vacuum cleaners and low-pressure compressors.
High requirements due to the steep incline of the pressure-volume-curve
at the working point are claimed by the market at a high static pressure,
including a compact, low-cost construction. For innovative burner blowers,
special demands are not only made on the absolute size of the static blower
pressure and the volume flow, but also on the path of the curves and their
stability, as well as the flow profile at the outlet cross-section. The
steep incline of the characteristic curve—pressure against volume
flow—is the most important component to consider for current low
NOx blue burners. The steeper the curve, the less the combustion air volume
changes when the counter-pressure in the combustion chamber varies. The
increase of the counter-pressure at the starting explosion of the burner
should not cause any temporary air deficiency during the combustion, as
this would generate toxic carbon monoxide and soot particles.
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Figure
1. Hybrid blowers act as a centrifugal fan, cross-flow
fan, and regenerative blower due to a separating wall,
suction plate with an opening, and the application specific
flow islands. |
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HG Principle of Activity and Arrangement
The hybrid blower is a special design that simultaneously incorporates
the principle activity of a conventional centrifugal fan and cross-flow
fan. It also is a regenerative blower by means of a centrifugal fan wheel
with forward-curved blades and a specially designed inlet air duct.
Apart from the fan wheel and the housing, the most important components
of a hybrid blower are composed of a separating wall, a suction plate with
an opening, and the application specific flow islands (see Figure 1). In
the development phase, the construction of the inlet air duct is preferably
of a movable design that allows a variable adjustment of the separating
wall.
Through the inlet’s implemented separating wall, the suction area
of the fan wheel is divided in two areas, with the function of a centrifugal
fan representing area 1 and a cross-flow fan representing area 2. The suction
air of area 1 is accelerated by the fan wheel and then thrown out to the
outside. As Figure 2 shows, a part of the exhaust flow from area 1 then
flows through the wheel into area 2, forming a second working current,
which results in a flow circulation. Thus, the hybrid blower practically
works as a multistage blower.
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Figure
2. Flow circulation is achieved as exhaust flow
is pushed through the wheel into a second working
current. This technology achieves the function
of a multistage blower in addition to a fan.
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Construction Parameters
A development study revealed that there are a great number of geometric
parameters affecting the blower characteristic curve to different extents.
The main components and geometric parameters are:
- housing with a variable
spiral outline
- blower wheel
- inlet opening (size, shape, position)
- separating wall (shape, inclination,
position)
- 3-D discontinuous step
(the inside of the inlet)
- flow islands
It is possible to achieve a steep characteristic curve
with an unusually high pressure number up to more than 15 in the
completely throttled
flow by reasonably combining the above parameters.
Comparison Against the Technology
Due to the unique performance and the special arrangement, a blower
characteristic has achieved an equal curve to that of a multistage
regenerative blower.
Figure 3 shows four blower curves for the same fan wheel, however,
in a housing of a conventional design (B6, 7-degree housing)
and a HG version
with two different positions of the separating wall.
The hybrid blower technology stands out against conventional
centrifugal blowers because of the following technical features:
- an increased incline
of the blower curve
- a high specific performance density
- compact design
- distinctly lower sound emission due to the lower operating
speed
- high-pressure
control comfort
To date, the HG technology has been approved by burner
manufacturers and is currently being applied in several cooking and
home appliances.
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Figure
3. The graph depicts four blower curves results
for the same fan wheel. The fan wheels, however,
were equipped in a conventional design housing,
(B6, 7-degree housing), while the two hybrid
blowers were equipped with two different positions
of the separating wall.
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This
information is provided by Ernst Clausen and Dr. Yingan Xia,
PUNKER GmbH & Co.
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