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<br />~ 2.8.1 Quan~itv and Quality <br /> <br />en A cooling system is both a concent=ating device and a form of treatment, <br />~ <br /> <br />~ as Qiscussed above in Section 2.6. Additional trea~~ents c~' be placed either <br /> <br />on the makeup or in the blowdcwn and the quant~ties and qualities fed eo the <br /> <br />trea~~ents will depend on the location. Cost estimating must begin with a <br /> <br />measure of the flow rates. Fur~hermore, since the cooling tower is a concentrating <br /> <br />device, quality and qu~'tity will alter together and cannot be separated. <br /> <br />The water evaporated for cooling is very much a matter of plant design. <br /> <br />Table 2-11 shows the calculation for a 65\ efficient gasification plant with <br /> <br />25\ of ~~e unrecovered heat dissipated by evaporative cooling. The evaporation <br /> <br />rate is 100 Ib/106 Btu. !n a series of detailed studies5 we have considered <br /> <br />the individual cooling loads in a variety of coal conversion pl~~ts, anc dete~i~ed <br /> <br />the degree co which wet cooling, dry cooling or wet/crj cooling should be usee <br /> <br />as a function of the availability and cost of water in the Wes~e=~ United States. <br />We found that the dissipation of 25\ of the unrecovered heat by evaporati~e <br />cooling is not excessive, but not minimum, in ~~e Colorado River basi~. ~~e <br />balance of ~he unrecovered heat is dissipated directly, as up a stack, or <br />t~raugh air cooled heat excnangers_ This conclusion is borne out by all <br />available commercial plant cesigns. <br />As was discussed in Section 2.6, ~~e cooling system ~ust be bla~ down ~o <br />prevent the formation of scale_ Acidification of sus?ending or ant:-scaling <br />agents, and, occasionally, softening to ~emove calcicm, all help to minimize <br />blowdown. In cur~ent steam-elect:ic power plant practice in the West, a <br />concent=ation in ~~e circulating water of fivefold (called "five c/cles of <br />concent=ation") is low; te~fold is beginning to be found economical and highe~ <br />concentrations are being pl~Lned. In the example on Table 2-11 we ~ound that <br />a thirtsenfold concentration would yield just enough blowdown to dispose of <br />with the coal ash. If this were chosen, ~~en ~~e circulating water would be <br />treated to prevent scaling at thirteenfold concentration. <br />For a typical plant we will take 10\ of the makeup as blowdown and 100 lb <br />evaporated/l06 Btu product giving 11 lb blowdown/l06 Btu and III lb makeuP/I06 <br />Btu. A plant making 250 x 106 scf/day pipeline gas would produce 109 x 103 lb;r.r <br />cooling system blowdown (= 218 gals/min = 351 acre-ft/yrJ. This is similar in <br />quantit1 to the ion exchange waste. <br /> <br />44 <br />