BASIC SEPARATION THEORYDOE-HDBK-1015/1-93Appendix ACH-01Rev. 0Page A-12The remainder of the converter equipment, with the exception of the gas cooler, is designed todirect the process gas flow inside the converter.TheGasCoolerThe gas cooler's purpose is to remove the heat of compression from the process gas which hasjust been discharged from a compressor. Certain type of stages use a gas cooler containedwithin the converter as shown on Figure A-4. The gas coolers may be separate units placedbetween the compressors and the converters.Barrier TubingIn the early stages of converter design, it was recognized that barriers made in the form of flatsheets and stacked in a container would offer the most barrier area per unit volume. Forpractical reasons, the barrier had to be made in the form of tubes. More engineering knowledgewas available for tubular construction and the urgency of the problem did not allow anyunnecessary development time. The use of tubes also allowed more satisfactory replacementof damaged barrier material. As a result, the barrier is made in the form of thin-walled tubes.ProcessGasFlowWe can now trace the process gas flow within the converter. In the AB cooled converter ofFigure A-4, the process gas enters one end of the converter at the center and is directed to theoutside of the shell by a series of baffles. It flows through the gas cooler and is directed to theouter section, or pass, of the barrier tubes. All of the flow enters the outer, or first pass, andflows through the tubes. Part of the flow diffuses through the barrier tube walls and theremainder of the flow passes through the tubes and is directed by a crossover to the second passof tubes. This crossover is sometimes called a doughnut because of its shape. The process gasflow which does not diffuse through the barrier tube walls of the second pass is directed by asecond crossover to the third pass. The first crossover is sometimes refereed to as the 1-2crossover because it directs gas flow from the first pass to the second pass. Similarly, thesecond crossover is called the 2-3 crossover. The undiffused process gas which passes throughthe third pass is directed to the "B" outlet of the converter and flows to the stage below. Theprocess gas which was diffused through the barrier tubes in all three passes is collected andflows to the "A" outlet and on to the stage above.Since some of the process gas flowing into the first pass will diffuse through the tube walls, areduced amount of process gas flow will enter the second pass. In order to maintain the sameflow velocity, the second pass contains fewer tubes than the first pass. Similarly, the third passwill contain still fewer tubes. The velocity of process gas through the tubes affects the flowthrough the tube walls and thus affects the separation efficiency. The efficiency is highest whenthe flow velocity is the same in all of the tubes.
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