EP0148140A2

EP0148140A2 - Impeller for aeration of water tanks

Info

Publication number: EP0148140A2
Application number: EP84850375A
Authority: EP
Inventors: Hjalmar Fries
Original assignee: Flygt AB
Current assignee: Xylem Water Solutions AB
Priority date: 1983-12-19
Filing date: 1984-12-04
Publication date: 1985-07-10
Also published as: EP0148140A3; SE8307006D0

Abstract

The invention concerns an impeller for mixing purposes.

The impeller has a leading edge angle which decreases from the hub and towards the tip, the tip vortexes then diminish and a more congruent division of the vortexes over the entire impeller blade is obtained.

Description

When aeration of a liquid and simultanous mixing with an impeller, it has shown out that the two activities influence on each other.
It is ofter a wish to keep the size of the air- or gas bubbles to avoid coalescense. Creation of vortexes at the impeller may, if it is extensive and cnncentrated, increase the coalescense as the bubbles go towards the rotation centre of the whirls as there the density of the liquid decreases. On the other hand, the bubbles may have a negative effect on the impeller if-they concentrate on its suction side and create a discontinuance, thus obtaining that the flow gets.so unfavourable that the impeller looses its suction ability. Even here a coalescense takes place on the suction side and this is the same phenomenon as at the free vortex. The air or the gas concentrate at areas having a low static pressure.
These phenomena bring about that areas where air is brought in must be separated from aeras where the impellers are situated and this could mean that bigger tanks or more expensive constructions must be used. The invention solves these problems..
The purpose of the invention is thus to give the impeller such a shape that its tip vortexes are suppressed and replaced by a vortex pattern which gradually leaves the impeller from the hub and outwards. The low pressure zone on the upper side of the impeller which normally is subject to concentration of air, is also given such a shape that impeller sections of different radii cooperate as little as possible.
The invention also has an impeding influence on the suction of air from the water surface. Air that is sucked in at the blade tip when it passes near the surface can not move inwards along the blade surface depending on the plane design of the impeller. This means that the impeller may be mounted near the surface and nevertheless obtain a controlled operation.
The results obtained are especially important at waste water plants, where the water and the sludge must be aerated and moved simultanously to avoid sedimentation. The line of the lifting force for the impeller is given a backward swept extension and at the same time the blade surface is divided, preferably in the form of elliptical functions, so that the angle of the leading edge of the blade succesively decreases relative the circumference from the hub towards the blade tip.
Fig 1 shows a conventional plane impeller and its typical tip vortex.
Fig 2 shows a backward swept impeller blade according to the invention and how its vortex pattern leaves the blade from the centre and outwards to the tip. The vortex vr the vortex pattern is a result of an overflow from the underside of the blade to its upper side and depending on the relative movements of the chord elements caused by the backward sweeping, the overflow will not be concentrated to the tips.
In order to obtain the ability of self cleaning, mixer impellers have already been manufactured having a backward swept leading edge. These impellers have however not shown to give the above described effects as the trailing edge has not accompanied in the backward sweeping. A contineus underpressure area having a radial extension has therefore occurred on the suction side.
An elliptical area division which is swept backwards so that the angle between the leading edge and the circumference decreases towards the tip, gives automatically the above described geometry of the trailing edge. Small variations from the elliptical area division may be made, without any major deterioration of the wishe< results. This may be dictated from a physical point of wiew or other constructive wishes. Typical is however, that a relatively small part of the blade area is divided towards the tips and that the sesulting traction centre of the blades thus lies at around 70% of the radius. This is a known proportion in the aviation field, but has never been utilized in a self cleaning impeller for mixing.
A further possibility to counteract air intake at the suction side of the blade is to gradually transfer the profiles of the impeller axially, so that the tips will be situated downstream the hub. In this way even the blade area may be corrected to essentially be a tangent to a straight cylindrical impeller shaft and thus obtain a simple design.

Claims

! An impeller for mixing purposes, characterized in that the angle of the leading edge with the circumferance decreases from the hub and towards the tip and that the impeller blades area has an essentially elliptical division.
2 An impeller according to claim 1, characterized in that also the trailing edge describes a back-ward going curve with reference to the rotation direction of the impeller.
3 An impeller according to claims 1 and 2, characterized in that the profiles of the impeller is axially displaced, so that an impeller blade shaft directed backwards in the rotation direction essentially is a tangent to the blade surface.
4 An impeller according to claims 1 and 2, characterized in that the profiles of the impeller blades are axially displaced downstream, succesively from the hub towards the tips.