A magneto-hydrodynamic pump or a stirrer is a device which creates a flow in liquid metal, using electromagnetic induction, thus enabling pumping of the metal in a predetermined direction. Although MHD pumps are highly sought after in metallurgy, only a few companies produce them.
SIA ” Lējumi un Būve” produces different types of MHD pumps and stirrers and offers a lot of unique and effective technologies for metal casting and melting industries. Products can be made as per request and technical drawing of a customer.
MHD stirrers are produced with different dimensions for casting furnaces and mixers containing different volumes of the molten metal.
Volumes: 5-10 t, 15-25 t, 40-60 t, 70-95 t, 100-145t.
Voltage: 220-380 — 400-440 V; frequency — 50-60 Hz;
Productivity at stirring and pumping of the molten metal varies from 10-40 t/h to 600-900 t/h
The MHD stirrers with plain linear inductors are widely used to intensify the melting of aluminium alloys on the base of primary | aluminium and their decontamination from sodium and oxide dopants, to produce alloys of Al-Si, Al- Mn-Mg, Al-Si-Mg, Al-Mn, Al-Cu.
The application of the MHD stirrer with plain linear inductors allows:
- To enhance the productivity of the casting furnace by strengthening the processes of heat/mass transfer;
- To reduce the time of melt preparation (by 20-50 %)
- To reduce fuel consumption for melting and preparation of the alloy (by 20%);
- To provide high homogeneity of the chemical composition of the molten melt (5%) and equalizing of the meit temperature throughout the furnace volume (by 5-10 C);
- To decrease the number of alloy composition corrections during its preparation;
- To decrease the metal loss (by 15-25%).
As per standard MHD Stirrers and pumps product has such technical specifications:
The length of the active zone of such MHD stirrer is 600 -1360 mm and determined by the type and size of the MHD stirrer.
- Active power — Pact 35-130 kW
- Reactive power — Preact 320 — 1100 kvar;
- current 100 — 250 A;
- the non-magnetic gap (between the molten metal and the inductor) is about 50 — 80 mm.
- Water cooling of the inductors winding;
- the water flowrate is (0.7-2.3)10 ³m/s;
- water pressure is (2 5)10 ³Pa.
MHD stirrers with plain linear inductors are used mostly in single-section (a casting furnace or a mixer) or double-section (a charging chamber + a section for alloy preparation) furnaces. Such stirrers could be used in furnaces of different design.
The advantages of the MHD stirrers if compared to the known devices applied in industry are:
- a high reliability due to the absence of moving mechanical elements in the melt;
- convenience in assembling and disassembling of the equipment;
- simple control, a possibility to control the productivity and strength of stirring from 0 to 100%;
- a possibility to stir the aluminium melt without breaking the film on its surface;
- a possibility to intensify the processes of heat/mass transfer at all stages of the aluminium alloy preparation and refinement in furnaces;
- a possibility to strengthen additionally the melt stirring within the bath volume by changing the travelling magnetic field. direction that changes the metal flow direction to opposite (back and forth motion);
- a possibility to prepare alloys and alloying compositions at temperatures lower than usual 100-150 C;
- heating of the pumped melt by inductive currents at metal melting (within the MHD stirrer. channel) and at its circulation in the furnace bath; no additional heating sources of the flow path are needed;
- a high opportunity for unification of different design versions of the stirrer.
The application of the MHD stirrers with plain linear inductors allows:
- to enhance the productivity of the 25-t- melting aggregate to 130 t/day;
- to decrease Si consumption needed to prepare Al-Si alloy by 1.5 kg per 1 t of the alloy;
- to reduce hydrogen content by 0.2 cm ³/100 g of the metal, aluminium oxide by 0.02%, sodium by 5,10 ⁴% (in mass);
- to reduce metal losses at its oxidation by 0.5%;
- to reduce the time of decontamination to 20 min, with: the energy consumption for decontamination being 2-3 kW/h per 1 t of the alloy;
- to considerably improve the structure of metal ingots;
- to enhance the rate of dissolution for Cu, Si, Mn in aluminium; to distribute them uniformly over the cross-section of ingots, to enhance the yield of systems Al-Cu, Al-Si, Al-Mn.
The MHD stirrer with plain linear inductors consists of:
- an inductor with a frame
- a flow path (channel)
- systems of electric power supply and controla condenser set.
The inductor is designed to induce a traveling magnetic field and consists of a magnetic circuit with slots, in which the coils of the winding made of copper pipes are placed, through which cooling water runs. In some cases, a high temperature winding wire could be used as a winding.
The TMF inductors with a magnetic circuit. where the slots are filled with three-phase coils, embracing the yoke of the magnetic circuit, can be used.
The inductors are arranged on the frame welded to the wall of the furnace body. The metal flow path (the channel) of special shape; made of a heat-resistant material (heat-resistant concrete, borosilicicated graphite, silicicated graphite, silicium carbide, etc.), is produced in the furnace wall: it is resistant against aluminium and its alloys.
Fig. 11 and Fig. 12 illustrate the MHD stirrer set with a frame, fastening elements and the colling system.
The MHD stirrer is operated by a system of electric power supply and control, consisting of
- an electric board
- acondenser set
- acontrol panel
- temperature probes installed in the inductor
- a water pressure gauge
- a metal leakage gauge.
The system of power supply and control allows to operate the MHD stirrer following the programs, providing different directions and duration of stirring cycles under hand-held and automated regimes as well as under computer control.
The MHD stirrer is AC power supplied (50 … 60 Hz), voltage 220/380 or 400/440 V. The MHD Stirrer has a multi-layer electric isolation.
To protect the personnel from shock by the electric current, to prevent the device from intrusions of extraneous bodies and water, the — inductor and the condenser set are JP 21 made, the control panel and electric board JP 54, meeting the requirements of ST SEV 778-77.
On demand of the customer, the electric equipment is supplied from the producers of the companies specified by the customer.
The MHD stirrer should be used (employed) in closed rooms in the zones with moderate climate. As to vibro-acoustic characteristics, it Is | classified (the noise level does not exceed 50 DB). The stirrer does not create radio noise.
The power to the inductor, condenser set, to the electric board and the control panel is supplied according to the attached scheme of the electric circuit.
MHD plain linear pumps
Plain liner pumps with two inductors are used as MHD plain linear pumps for aluminium. Fig. 21 illustrates a principal schematic of such a pump. MHD plain linear pumps are arranged at the both sides of a rectangular-shaped metal flow path made of a material resistant against molten aluminium. The inductors could be arranged parallel to the bottom of the metal flow path, embrace the metal flow path from two sides or arranged at certain angles to the bottom or side wall of the metal flow path.
How does it work?
1. MHD stirrer operation is based on the travelling magnetic field interaction with the electrically conducting medium that induces electric currents in the medium.
2. These induced electric currents interact with a primary travelling magnetic field and induce an electromagnetic force, which drives the molten metal into the direction of the traveling magnetic field
3. As a result, the molten metal moves as a jet along the flow path and enters the furnace bath, initiating a strong flow.
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