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Metal Casting Technologies : March 2005
3. PROCESS DESCRIPTION This model involves the following steps of operation: 3.1 An compressor produces high pressure air to feed it into the air chiller where necessary heat is absorbed from the air stream by the ammonia/air system of vapor refrigeration unit, resulting in condensation of moisture, present in the air. The condensate water will be drained automatically by the leveltrol, which is actuated intermittently on getting requisite signal from the level height of the condensed water, as to be trapped at the bottom of the air-receiving tank. 3.2 From the air-receiving tank, the high pressure air with optimal moisture content enters the twin bed Pressure Swing Adsorption (PSA) unit in the following sequence: 3.2.1 Compressed air is fed into the first molecular sieve bed, where nitrogen is trapped while oxygen is allowed to flow through. 3.2.2 When the sieve in the first bed becomes full of nitrogen, the air flow is directed into the second bed. 3.2.3 As the second bed separates oxygen from the air now, the first bed vents the adsorbed nitrogen into the atmosphere simultaneously. 3.2.4 On getting the second bed full of nitrogen, compressed air is once again fed into the first bed and the process is repeated continuously, resulting in a constant flow of oxygen enriched air in its downstream buffer tank. 3.3 From the buffer tank, high pressure oxygen enriched air with optimal moisture content enters the Pressure Regulator to reduce the high pressure of oxygen enriched air to the desired level, before it passes to the recuperator. 3.4 In recuperator, the oxygen-enriched air becomes hot on getting requisite heat from the waste flue gas and then is passed to the tuyeres. 4. WORKING PRINCIPLES OF THE COMPONENTS 4.1. Continuous cycle vapor absorption system works on Dalton's law of partial pressures, which states that the total pressure of a closed mixture of gases is the sum of the partial pressures of the gases in the mixture. It also explains that each gas behaves, as if it occupies the space alone. The present model of the absorption refrigerator uses two gases, viz. ammonia and hydrogen, of which ammonia is absorbed by water at room temperature in the closed system (Figure 2). On absorbing the waste heat from the post recuperative flue gas, the ammoniacal water solution drives out the ammonia. Here hydrogen remains as gas, because it is not absorbed by water. Under high uniform pressure within the system, the ammonia condenses into a liquid within the condenser. Total pressure of the system is the sum of the partial pressures, exerted by the vapor ammonia and hydrogen. When the pressure of the ammonia vapor within the system falls below that corresponding to the vapor pressure of the ammonia alone, liquid ammonia starts to evaporate with a view to reach- ing the vapor pressure corresponding to the temperature of the absorber. The unit consists of four main parts: boiler, condenser, evaporator and the absorber. The boiler element is fitted below the central tube (A), which transfers heat to the system from the post recuperative waste flue gas. The unit charge consists of ammonia, water and hydrogen, which are at a sufficient pressure to condense ammonia at room temperature. On getting adequate heat from (A), ammonia bubbles are produced, which rise and carry with them quantities of weak ammonia solution through the siphon pump (C). This weak ammonia passes into tube (D), while the ammonia vapor passes into the vapor pipe (E) and onto the water separator, which condenses water vapor and runs back the boiler system, leaving the dry ammonia vapor to pass to the condenser. Air circulating over the fins of the condenser removes the heat from the ammonia vapor to condense it into liquid ammonia, which flows then into the evaporator. The evaporator is supplied with hydrogen, which passes across the surface of ammonia to lower its vapor pressure enough to allow the liquid ammonia to evaporate. The evaporation of ammonia extracts heat from the evaporator, which, in turn, receives heat from the surface of air chilling unit to help in lowering the temperature of the compressed air to condense its moisture. The moisture of ammonia and hydrogen vapor passes from the evaporator to the absorber. A continuous trickle of weak ammonia solution enters the upper portion of the absorber, which is fed by 28 METAL Casting Technologies March 2005 TECHNICAL FEATURE Figure 2. A schematic diagram of continuous cycle vapour absorption system.