[0001] The invention concerns a jet ring of the type designed to surround the propeller
in a so-called propeller mixer.
[0002] Mixers of the above mentioned type include a driving unit, an electric motor, a propeller
and a driving shaft between those two. The propeller may have two or more blades and
can be arranged to be driven with varying speeds, all depending on the use.
[0003] Mixers are used within various fields for homogenizing and suspension maintenance
of liquids for different industrial processes, for liquid manure, for waste water
etc. The mixer can be arranged horizontally or vertically or with different angles
therebetween. It can also be arranged movable so that the flow from the mixer is brought
to sweep over bigger areas or directed towards certain spots within the liquid tank.
[0004] Another field where a mixer may be used is for creation of currents in a lake to
prevent formation of ice.
[0005] In order to create an effective mixing or flow it is normally a condition that the
propeller must be surrounded by a jet ring with a diameter somewhat bigger than the
diameter of the propeller and with a normally limited axial extension. If the jet
ring is eliminated, a radial flow would accur limiting the axial flow through the
machine as a part of the liquid amount would be sucked back into the propeller and
as a certain leakage would accur between the pressure and the suction side of the
propeller.
[0006] According to known technique the jet ring is cylindrically formed and provided with
a somewhat outwards bent collar to make the inflow easier. Such jet rings are shown
in the Swedish Design Registration No:s 34601 and 39392.
[0007] A disadvantage with these known designs is that the flow close to the ring may be
turbulent depending on a too sharp alteration. This causes so-called discontinuance
of the boundary layer with an often unstable blocking as a result. The flow through
the machine and thus the mixing ability then decreases.
[0008] Another disadvantage with an unsteady or failing flow (blocking) close to the ring
is that vibrations are easily amplified as the tips of the propeller blades will
operate under conditions for which they are not designed. If in addition the cylindric
part of the ring after the propeller is extended, there is a relatively large area
that is exposed to pressure pulsations causing strain in the material.
[0009] According to the invention the disadvantages mentioned above are reduced by help
of a specific design of the jet ring as specified in the claims.
[0010] The invention is described more closely below with reference to the enclosed drawings.
Fig 1 shows a side view of a mixer provided with a jet ring according to the invention.
Fig 2 shows the jet ring seen in axial direction and Fig 3 shows a diagram over the
fluid losses as a function of the ratio between the radius and the diameter of the
jet ring.
[0011] In the drawings 1 stands for an electric motor, 2 a driving shaft, 3 a propeller
with blades 4, 5 a jet ring with inlet and outlet rims 6 and 7 respectively.
[0012] In order to obtain an effective mixing a jet ring 5 must thus be arranged around
the propeller 3. In this way an axially directed jet is obtained which in its turn
brings parts of the surrounding fluid mass. The flow towards and into the propeller
has mainly a direction perpendicular to the axis of rotation.
[0013] In order to secure a good flow near the inside of the jet ring, the ring is designed
in a special way according to the invention.
[0014] The conventional, mainly cylindric ring is replaced by a funnel shaped ring having
a significant radial extension. The transition between its inlet rim, which in principle
is perpendicular to the axis of rotation and its outlet rim, which is axially directed,
takes place along an even curve having a relatively big radius.
[0015] The ratio betwen the axial and radial extensions of the jet ring may vary somewhat
depending on number of revolutions, type of mixer, the qualities of the liquid to
be mixed etc. It is important however, that the inlet rim is directed mainly radially
and that the transition from inlet to inlet takes place along an even curve having
a relatively big radius. The latter is of course dependant on the diameter of the
ring. The bigger the ratio is between the transition radius and the ring diameter,
the smaller the flow losses will be and thereby the disturbances (within certain limits).
A ratio between 0.1 and 0.3 is normally sufficient. Bigger ratios may cause manufacturing
problems.
[0016] According to the invention there is obtained a jet ring for mixers of different types
which in a simple and effective way increases the efficiency of the mixer and simultanously
adds to a diminishing of the vibration problems. The latter is further diminished
as the funnel formed ring is more rigid than the previously used tube form.
1 A device for mixers comprising a driving motor, a rotating driving shaft and a
propeller, the periphery of which is surrounded by a jet ring having a short axial
extention, characterized in that the jet ring (5) is funnel shaped having its bigger
diameter at its inlet rim (6) and its smaller at the outlet rim (7), the transition
between inlet and outlet rims (5) and (6) respectively, taking place along a smooth
curve with a radius (r) not being less than 10 % of the diameter (D) of the jet ring
(5).
2 A device according to claim 1, characterized in that the inlet part of the jet ring
(5) is so designed that its inlet rim (6) is directed mainly perpendicular to the
direction of the axis of rotation (2).