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Metal Casting Technologies : March 2005
gravity from the tube (D). this weak solution flows down through the absorber, when it comes into contact with the mixed ammonia and hydrogen. This readily absorbs ammonia from the mixture and hydrogen is free to rise through the absorber coil to return to the evaporator. Hence the hydrogen circulates continuously between the absorber and the evaporator. The strong ammonia solution, produced into the absorber, flows down to the absorber vessel in order to pass to the boiler system, thereby completing the full cycle of the operation. 4.2 Pressure Swing Adsorption (PSA) system: The atmospheric air contains approximately 78% nitrogen, 21% oxygen and the balance is other gases. Oxygen is separated from air using PSA technology, which centers on a molecular sieve, known as zeolite. At high pressures, the sieve adsorbs nitrogen and at low pressures, it desorbs the same to the atmosphere. The oxygen generator consists of two tanks filled with sieve. As high pressure (about 5 kg/cm2) is introduced into the first tank, it passes through the sieve, resulting in adsorption of nitrogen from it. The remaining oxygen is piped to a buffer storage tank. Just before the first tank becomes completely saturated with nitrogen, feed air is redirected to second tank, which then repeats the above the process. The first tank is then vented to atmosphere, which allows the nitrogen to desorb from the sieve. To complete the regeneration of the first tank, a small amount of oxygen is used to purge it. This process is continued over the time. Under normal operating conditions, which include the use of clean, cold, dry air for separation, the sieve will last indefinitely. Productivity of a PSA generator is solely dependent on the purity of oxygen required. A generator can produce significantly more oxygen at 90% purity than it can at 95%, with a marginal increase in feed air. By means of a PLC or some other microprocessor-based controller, it is practical on larger generators to change the swing cycles. Purity and flow levels can be optimized based on the demand variables of the cupola. 4.3 Pressure Reducing Regulator is used to supply the air at a constant desired pressure to the cupola. In it, the down stream pressure acts on one side of a diaphragm while a preset spring is balanced against it on the other side of the diaphragm. The valve will remain open as long as the downstream pressure is too low to balance the constant thrust of the spring. The upper chamber of the regulator is vented to the atmosphere. 4.4 Recuperator is a device that recovers waste heat from the exhaust flue gas from the cupola. In cupola, flue gases are drawn off below the charge door and are connected with the combustion chamber of the preheating equipment. As the hot gases pass into the combustion chamber, they are mixed with air, supplied by a bypass line from the main blast pipe, resulting in liberation of heat from the combustion of the effluent carbon monoxide of the cupola. The total heat, as derived from the sensible heat of flue gas and the heat of combustion of effluent carbon monoxide, helps in heating up the cold blast within the metallic tubes of the recuperator, before it passes to the tuyeres. 5. BENEFICIAL EFFECTS OF THE MODEL 5.1 Humidity control: Moisture present in the blast results in water gas reaction when it comes in contact with hot coke in front of the tuyere: This strong endothermic reaction affects the temperature level of the combustion zone adversely, resulting in higher fuel consumption. Again the moisture within the blast takes away a considerable amount of heat, yielding in higher flue loss. In addition to it, partial pressure of water vapor in flue gas increases abruptly due to high presence of moisture content in com- bustion air. These water molecules in flue gas (resultant of combustion product and moisture in combustion air) start to dissociate at high flame temperature. The rate of this endothermic dissociation (H2O = H2 + O) solely depends on partial pressure of the component (here water molecule) and temperature. So the presence of moisture in combustion air lowers the net adiabatic flame temperature adversely, thereby requiring additional amount of fuel to compensate this effect of moisture. However it is pertinent to mention here that the optimal presence of moisture acts as beneficial reducing agent. The moist blast generates more reducing gas per unit volume than that of dry blast. The hydrogen generated acts as a reducing gas in addition to carbon monoxide and hence coke consumption would be reduced considerably. Like other metallurgical operations where carbonaceous fuel is in direct contact with materials being treated, to which it supplies heat by combustion with air, it is found that the moisture content of the air plays a vital role on the mechanical & metallurgical properties of the materials. In production of both the gray & white iron castings for malleabizing in cupola, this matter becomes obvious. Several experimental works have proved the effect of increased moisture content in cupola blast as follows: 1. Slowing down of melting rate; 2. Increase in manganese & silicon losses; 29 METAL Casting Technologies March 2005 H2O+C=CO+H2