PBOJECT CONCEPTION AND D€FINITION

In the incineration-heat recovery scheme explored in step 2, capacity was fi1edeither by the availability of refuse or by heating requirernents of the university. Thisis typical ol commerciai processes, as weli, which are iimiie<i in capacity erther byraw material availability or by product demand. If limits are broari, anaiysis ofseveral cases, using different capacities, may be necessary. In an approximateestimate typical ofihe predesign stage, it is rather easy to extrapolate costs from onecapacity to another as discussed in Chapter Fi';e.

I#ith feed or effluent flows lixed, many of the remaining entering or leavingstreams can be established by sirnple material balances.'Next, temperatures,pressures, and other process propenies must be defined. For entering sirearns, ihisoftcn requires little more than common sense. For instaace, nonreactive liquids andsolids are normally stored at ambient temperature and pressure. For those that mustbe protected frorn exposure to the atmosphere, storage at I to 5 bara under an inertgas is common. Unless heated or cooiec to retai* eertain pr+perties" stri'aF,*t.6ha?tlrrrp€ nnraqllw lia hetween fl ond ?{a{- lAn pvnpn*ic,h amnna mrnrr icv, \, r.r

number 6 fuel oil, which has the consistency of tar, and must be stored in heatedtanks to be pumped.) Even if feedstock comes from another section of the process, itoften is stored "in process" long enough to reach ambient temperature. Wtiereambient changes are significant, it is custornary to emploJi the'worst case" in lheprocess design. For example, in the northern United States,one might assume a feedtemperature of -20"C if there is to be preheating. If cooling is required. 35" f wouldbe appropriate. If energy costs for preheating or precooling are significanl.

"

consideratiorr of seasoi-ral varialions may 'ue necessary. if in doubt, mosi er-rgineers

make eonservative assumptions. that is- those tlrat assure a safe design.Use of common sense and personal judgment to detine temperat-&res,

pressures, and other canditions is somewhat discorrceriing at first, especialiy forstudents who are accustomed to ready-made homework problems, Students areeven more uncomfortable if their assumptions differ substantially from those of theteacher. rrly'hen such discrepancies are brought up in discussion, I find it upsetsthestudents er.fen more if t re.,'rse rny figures to agree more closely wlth theigs. {Suchapparent abandon and imprecision are acceptable 1o experienced designers, whoreeognize that some assumptions can vary within broad limirs wiiitc,ut h"ving a

significant impact on the final solution",) On the other l:and, same studtnts, caughtup in the spirit, carry t!:is to extremes, arbitrarily assigning parameters that are

uniquely determined h-v nature. A eommon error, f,*r exarnple, is to assr:me ther.rr+!a+ +.**a=^c',-- ,-i -- -l:-L^a:- '."-^- :i a:rls i!+ .i-i.*i-=-' :-*;^-: '..,UULiCt iciiiPciaiijiL- i..ri iiii aiiiauaiii; ji,4U!Ui wIlCIl rt iltw;l w tlE.ttlilttllur lte).seu' u-y

e:{sciiiing an ener5y balance. \,}'ith a iitile thougi;i. it siie uiij !}!..L;rEe eas}- foi :iiaiiiiestudents to identify situations that are arbitrary and th+se tlni are not.

DeI:lir-ag s.:ndiiions and capaciiius coriiain: ixi eicii,;ilii r; ;. i. Thui. i.:r;ltl;,i:sdeveiop:-- '+ith experience. in ;iour i-trst aitenipts to Dieta. E a;: <;;.r:rnai [:o'*' 1:':'-:i.

some boidness wii! 'ue required. As wisdcm a*d judgrr*nt devei*F. this stes "r^-:^-...:!t L", !I.. :!,.-.. .'--.-t-^---.,.. - r:-r -f Fpfoce . ij€iigii siii 3".1 . -- .1 liaiijl;- i;; r.'l:.ii;vur aJ':-iut',': - i rlsi ui

iheet b.=,=:nda-ry conciii+-s, ihe r,:!es +i ri.;s:+ i+ -+;ltnd!:- E :h+';id i-=r usrf' '

Beginning the flowchart, at lhis point, :.:a;r seem prr:eipritcus. 1.. .ireless,

based on a iitr.aeurc suruey, cci'lsiiliatiot=r, anti ai'i';,ts to rjei-, : :iie F-i. . ,i.ou wiiibepreparedtobegin.?heexpell.,nceofprepari?";':rL',;.:rer,'s[6rl'.:.ii.ir".r:.identrn

--eas in which more informatiur, is Eieeded. lt , . n reqt: : tiplt;'--L--- r.r-4-: :- '.:---, '!.L-= f. '-- I r-- - t^;i.iiee..ijeii-iii^iiiji:'.sieF:rltlullU3l1.sltrurij.ri...i*ijiil;aii.;''.

23

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