Electrode cooling

Another important parameter is the electric current for discharge. If the current density is too high, the quantity of the hard shell increases and that of the MWCNT decreases. To keep the arc discharge stable and the electrode cool are effective to increase in the product quantity of MWCNT. A considerable quantity of graphite is produced in the cathode deposit even under the most suitable condition to the synthesis of MWCNT. 

Fig. 9.4.10 Apparatus for the gas flow-arc plasma method. The apparatus is composed of two components. The upper part is a glass Dewar, which accumulates small particles in a cryogenic matrix on the trim cooled with liquid nitrogen (LN). Sorv, inlet of organic vapor Syr, syringe for transferring produced colloids under anaerobic conditions RP, rotary pump S, target sample. Lower part is for plasma discharge. A BN furnace has gas inlets (G) and is specially designed for Ar gas to flow in screwed stream hence the plasma is emitted in a jet flame due to a plasma pinch effect. The black parts are copper electrodes cooled by water. In order to maintain a constant spacing between the surface of sample and tbe upper electrode, the sample position can move vertically so that the current through the sample to the upper electrode is precisely controlled and constant. This is very important to produce powders with a narrow size distribution.

High tension electrode cooled with water... 

Elevated storage reservoir shall be located centrally/near to as many process units as possible which need water always. The bottom outlet valve shall be operated by a solenoid control, and it shall open when power fails. It shall supply water to emergency use points such scmbbers, cooling jackets, electrode cooling, etc. 

Fig. 2.1.3a-f. Various radiofrequency (RF) electrodes a single needle design (Cool Tip, Valleylab System, Tyco), b cluster-electrode (Cool Tip Cluster, Valleylab, Tyco) multi-tined electrodes (c LeVeen, Radiotherapeutics, Boston Scientific d Star-burst XL e perfused Talon needle, AngioDynamics-RlTA) f internally cooled, bipolar single needle electrode (Olympus Celon)... 

Grevstad, E. Paul and Raymond Gelting, L. Fuel cell electrode cooling system using a non-dielectric coolant. US Patent No. 3,969,145, July 13, 1976. 

Dissolve 100 g. of iron alum (ferric ammonium sulphate) in 300 ml. of water at 65°, Pour the solution, with stirring, into a solution of 25 g. of hydroquinone in 100 ml, of water contained in a 600 ml. beaker. The quinhydrone is precipitated in fine needles. Cool the mixture in ice, filter with suction, and wash three or four times with cold water. Dry in the air between filter paper. The yield of quinhydrone, m.p, 172°, is 15 g. It contains a trace of iron, but this has no influence upon the e.m.f, of the quinhydrone electrode provided that the washing of the crude material has been thorough. The quinhydrone should be stored in a tightly-Btoppered bottle. 

Electrolysis cell. This is shown in Fig. VI, 31, 1 and is almost self-explanatory. The cylindrical cell of Pyrex glass (6" long by 2 " diameter) is cooled by immersion in a cooling bath. The electrodes consist of two platinum plates (4 cm. X 2-5 cm. X 0-3 mm.), which are placed about 2 mm. apart. The temperature of the electrolyte is maintained at 30-35° by means of the internal cooling coil and also by immersion of the cell in ice-water. A current of 1 5-2 0 amperes is passed until the electrolyte becomes slightly alkaline, which normally takes about 20-50 per cent, longer than the calculated time on the basis of the current and the amounts of acid employed. It is advantageous to reverse the direction of the current occasionally. 

Westinghouse Electric Corp. initiated a program to develop air-cooled PAFC stacks, containing cooling plates at six-ceU intervals. Full size 100-kW stacks (468 cells, 0.12-m electrode area) were built, and a module containing four of these stacks was tested. An air-cooled stack operated at 0.480 MPa yielded a cell voltage of 0.7 V at 267 m A /cm (187 mW/cm ). Demonstration of this technology is plarmed for a site in Norway. 

The roof, in the form of a dome, is either comprised of refractory brick held in place by a water-cooled steel roof ring, or it may be composed of water-cooled panels. Sometimes water-cooled rings or glands are placed on the roof around the electrodes to maintain the refractory. On high power furnaces refractory is used around the electrodes to minimize the possibiUty of electrical short circuits. 

Eig. 3. Coasumable-electrode melting ia the vacuum-arc furnace (13). A, arc B, buttoa E, electrode I, iagot L, Hd M, water-cooled mold and P, pool of... 

A typical large three-phase ferroalloy furnace using prebaked carbon electrodes is shown in Eigure 4. The hearth and lower walls where molten materials come in contact with refractories are usually composed of carbon blocks backed by safety courses of brick. In the upper section, where the refractories are not exposed to the higher temperatures, superduty or regular firebrick may be used. The walls of the shell also may be water-cooled for extended life. Usually, the furnace shell is elevated and supported on beams or on concrete piers to allow ventilation of the bottom. When normal ventilation is insufficient, blowers are added to remove the heat more rapidly. The shell also may rest on a turntable so that it can be oscillated slightly more than 120° at a speed equivalent to 0.25—1 revolution per day in order to equalize refractory erosion or bottom buildup. 

Fig. 5. Kivcet process A, concentrate burner B, fume-laden gas C, lead tap D, slag tap E, 2inc tap F, water-cooled underflow wall G, electrodes H,...

In the electrothermic part of the furnace, electrical energy introduced via three carbon electrodes, keeps the bath molten and completes the lead oxide reduction. Fumes generated in the electrothermic section are oxidized in a post-combustion chamber by adding ambient air, before the vapor is cooled, dedusted, and released to the atmosphere. 

Electrode Walls. Development of durable electrode wads, one of the most critical issues for MHD generators, has proceeded in two basic directions ceramic electrodes operating at very high surface temperatures (>2000 K) for use in channels operating with clean fuels such as natural gas, and cooled metal electrodes with surface temperatures in the range 500—800 K for channels operating with slag or ash-laden flows. 

Although ceramic electrodes have received much attention (101), they have not been successful in channels operating with slag-laden flows, because of excessive electrochemical corrosion caused by the slag. Only weU-cooled metallic elements have been used successfully in slagging environments. 

The interelectrode insulators, an integral part of the electrode wall stmcture, are required to stand off interelectrode voltages and resist attack by slag. Well cooled, by contact with neighboring copper electrodes, thin insulators have proven to be very effective, particularly those made of alumina or boron nitride. Alumina is cheaper and also provides good anchoring points for the slag layer. Boron nitride has superior thermal conductivity and thermal shock resistance. 

Arc melting also can be used to consoHdate bars of metal that are pressed from powder or sponge and used as consumable electrodes in a low voltage, high current arc. The bar is suspended vertically and the molten metal falls from the bottom of the bar onto a water-cooled copper cmcible, from which it is removed as an ingot. 

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