:::~:":.}: . ~. ~: '~p'; '1" r..:) I J • _ _ • " _ •• ~ _ _ 4 ' • ~__ . ._ ' '
-- 0
. "~:. "', \
. ". r:-. ~- :~ ... ': ." .
t, ,t • • ~: . : ..
,
.
I' I~
.
. ,
t
Jl7{
EF
A
C
E
TO
.
TI
m.
T
HIR.D
,
,
.
E
DITI01\I
... .. : .. I ~."" -, . , " " , ' J ' " , 'The field of thermal ,system design and
analysis
continues to develop. Thenumber of workers is gro\\"ing, technical
papers
appearin
greater numbers,and new textbooks are _ being written. . .
The major objective of this
third
,editionis
tQ organize so.me of the new approaches that are now ,avaHabJe and to provide more flexibibty to- instructors who use
Design
of Thermal Systems as a texL The changes to thetwelve chapters of the second edition are modest and mainly constitute the inclusion of
some
additional end-of-chapter problems.Chapters
13 tPJ"Dugh \ 19, ho\v~yer, are all new_ One possible use of th~ text is to cover thefirst twelve
chapters
in an advanced-levelundergraduate
courseand
theremaining seven chapters as a graduate COjJfse. In some engineering schools students already have some J.01d of
optimiiatlOn
course prior to' taking the' thermal design course. For those classes certai)1 chapters of the fIrst hvelve(usually the ones on search methods, dynamic prograrrul:ring, and linear proirarruning) can be omi.tted and material can be supplemented from the,
new
sevenchapters.
Several of the new
chapters
are exten~ions of the introductions offeredin the lust twelve chapters, especially
mathematical
moqeling, steady~statesystem
simulation',
and search methods.Chapter
14addresses
$.Orne of the challengesthat
arise whensimulating Jarge,
thennalsystems.
Newmaterial
appears in
Chapter
15 on dynamic be'h~yior, in Chapter 18 which introducescalculus of
variationsas a
companion todynamic programming, and in
Chapter 19 on probabilisticapproaches
to design, which is exploratory.'
.,
The author thanks colleagues both at the
University
of
nlinoisat
Urbana-Champaign and at other engineering schools for continued input and
suggestions during [he past several
years
on how to keep the
,
book fresh.
~qraw2Hjl1 would
rus,9. like to thank the
foHowing'reviewers
for
theirmany
useful
comments:John R.
Biddle,California
StatePolytechnic
.
X PREFACE TO TI-IE THIRD EDmON
University, Po'mona; Theodore F.
'
Smith,.
:
1be University
0'- 0a; Edward
Q~ Stoffel" CalifonUa State Polyt~chnic
.
Uriivers~ty; San L
uis O
bis
p
o; John
A. Tichy, ,Rensselaer Polytechnic Institute;, Daniel T.
Va1~I!. e,
~larkson,
College;
?lld
'
William
'
J.
'
Wepfet"
qe
'
<?rgia,1r:lstitute
-of 'fechno
l
o
.
g
y
:
.~_~"
"
. . . . ' , '
--.c,(-' .'- '. ~ " I
"
,
'
-,
w;
,F.-S~o.t:cket--
I, ----~-"'.--'--'~----~-.. - -.. ---....;-... -~---... -~-, • .;:. ... ,.:r : ,l·· . , : -::. . ... :. , / ,The title, Design of Thermal Systems, refie,cts the three concepts embodied
in
this book: design, thennal~ and "systems. 'DESIGN
A frequ~nt product of the erig?neer's efforts is a drawing, a set' of calcu~a t!ons, or a report that is an abstraction an? .descriptio~ of hardware.' Within '
. ,epgineering education, thl? cookbook approach to design, often practiced during the 1940s, discredited tpe' design effort so that many engineering schools dropped design courses'from their curricula in the 1950s. But now design has returned. This reemergence is Dot a re~apse to the earlier proce-dures; design is reappearing as a
cre"ative
and highly technica1 ,~ctivity.THERMAL
Within many mechanical engineering curricula the tenn design J,s limited to machine design. In order to compensate for this frequent lack of recognition
of thermal design I some special emphasis on this subject for the next few years is warranted. The designation thermal
implies
calculations and activities 'based on principles of thermodynamics, heat transfer, and fluid mechanics., ,The, hard,ware associated
'1'ith
therm~. systemsiqcludes
fans) pumps,compressors,
engines,expanders,
,'turbines,
heat
and mass
exchangers, and reactors .. all interconnectedwith
some form of conduits. ' generally, the.,working substances are fluids.
These
types 9fsystems
appearin
such indus-tries as power generatio'n,electric'and
gas utilities,refrigeration,
aircondi-tioning
and heating. andin
the food, chemical,and
process industries ..
'xiv
PREFACE TO nIE FIRST EDITIONSYSTEMS
Engineering e¢lucation
is
predominantlypro,cess )orielZted,
while engineering practice is predornioantlysystem
'orien(eil
.
.
Most
cO,urses
of
study ine;ngi-
" "
_' neering .proyide the student wi~
an
effec~ve exposure to such processes as theflow
of a compressible fluid through' anozzle
and the.bel;l3rytor
of" ,~hydrodynamic and theI"ID:al bou,ndary l~y~rs ,at solid, $q.rfaGes,._lhe ptacti~ing
" . en'gineer, ,however) is lik~Iy
to
be confronted with, a task ~ucp'as.
desigmng,-'
.
:~an: _~eG,o,n.~c:-:.sy.st~,Itl __ !p~t ,}"~ceiv~~ p,~t~~ g~ _from
a. pipeline-aijc;l
.
,
stores
,
it underground '.fo~ )~fef~'~sag~'~·-'Th~r~,~~~~:' a~- big~/iap.:l>.e~,een:Jmoyw~~~qg.~:,;:,~(,.j
-
,:
:
,:;;"
.~'S'~': , inqi~id~hl proces~es4tld
rl1e
,~,~tegraticm-~f these prQc~s .. s~s in ~n.·;engiriee~g.. ,
-:
'~:~.. ::"
,
..
~~',enterprise. -
-Clos~g the
gap should
~ot b~ accomplished by diminishing the empha- . si~'on
pro~esses. A faulty .~no\vledge,'of
fi?1dainentalsmay
result in subs~:' quent failure of the ~ystem. But -within a university enviroJ!illent, it isben-eficial for future engineers to begin thinking in te~s of. systems, Another re~sonfor more emphasis on systems in the llnjversity envirQnment, in
addi-tio,q to, influencing the (hought patterns of students, is that there are sqme technjques-such as simulation and optinrizatjo~-which only re~ently have
been applied to thennal systems '. 'These are usefui t601s and the graduate
should have s6me facility with them. ' ,
While the availability
of
procedures ofsimulation
and optimization jsnot
a new situation, the practical application of these procedures has only recently become widespread because. of ,the- availability of. the: djgital computer. Heretofore, the lirriltation of time, did not permit handcalcula-'tiqns: for example, of an pptirnization of a function that was dependent upon
dO,zens or
hundreds of independent variables. This meant that I in designing systems consisting of dozens or hundreds of comp<?ne,nts, the goal ofachiev-ing
a 11,10rkable system was a significant accomplishmentand
the objective of designing an optinll1J71system
wasusually
abandoned. Thepossibility
ofoprin1iz~tion represents one of the few facets of design,
OD
,
TLINE OF
THIS BOOK'
The goal of this book' is the design of optimum thennal systems. Chapters
6 through 11 cover topics and specific procedures in optimization. After Chap. 6 explains the
typical
statement of the optinlization problem and illustrates howthis staten1enl derives
from thephysical situation,
thechap-ters chat follow explore optimization procedure,s such as calculus
m~thods,search methods, geometric programming, dynamic programming, and linear
programming. All thesemerhods have
applicabiH ty'10 nlany
other
types'of
problems besides thennal ones
and,·in this sense,
aregeneral.
On
'
the
otherhand, the applications are chosen
fr~mthe therma1 field
to
emphasize
the
opportunity
foroptimization in this cJass of
problems .-
,PREFACE TO THE F1RST EDITION XV
If the .engineer
inLmediately
sets out to try to optimize a rnoderately,c.omplex thermal system9 he 'is soon struckl
lDy
the need for predicting theperformance of that sy~tem. gi veil CeI1aiIl inpu t conditions 3J."1d performance
charac'telistics of' components. This is the process of systein sirn.ulatiolL
System
simulation' fl9t.only rp.ay
bea,
stepin
the optirmzation'process
-but,·may
hav'ea
usefuJne~s
iI!its'
O'1}JI;1 'right.',A-
,
sysren1
may.J~l~,
.
.
.
gesigrted
on.
the '.
--,
'
··bq.sis of
some ..IDaXlrllUm;:-1oad"
,
conditiori\'-but'-maybper~t~' g5·'·:perd~rit'dttl1e··~f~tiille
'
9-t
1es.s'::t)i~in-maxjmiiiQ.load. System'simulitt-ioll.periuirs
an
exar.nination
of the.
ope~a"tini' condi'ti~'n's
thatm~y
pinpointpos_s5hle
operatiI;tg andc~ntrol
problems at non-design c.onditions. ' . .
Since. system simulation and pptimjzation on . any but· the simplest
problems
.4?Ie
complex. operations, the executio~~of
the ptob~en1must
beperformed on ·a computer. When using .~. computer, the equation forin. of
representation. of the performance of corpp~nents and expression of .prop- .
erties of, su-bstances is much more' convenie.nt
than
--
tabular or graphicalrepresentations. Chapter 4 on mathe~atical mo~eling: presents sOTI?-e
tech-niques for equation development for the case where there is and also where
'. the;re is not some insight into the relationships based in thennallaws.
Chapter J~
.
on econom.ics~ is appropriate because engineering design ,and economics are inseparable. and because, a frequent cnterion for
opti-mization is [he economic one. Chapter 2, on workable systems, attempts
to convey one simple but imponant dis(inc~ion =-the difference be~een the
. design process that resui
ts.
in a workable system in contrast to anoptimum.
system. The first chapter on engineering design emphasizes ·the importance
. of design in an engineering under1aking.
The appendix includes
some
problem statements of sevemJ.compre-hensive projects which may run as part-time assignments during an entire
tenn. These tenn projects are industrially oriented b~[ require application
of some of the topics explained in the text. . .
Th~ ~udience for which this book \vas written includes senior or
fiisr-year graduate ·students in mechanical or chemical engineering, or practicing
engineers in
the
thermal field. Thebackground
assumed is aknowledge
ofthermodynamics I heat transfer, fluid mechanics. and an awareness of the
performance charactens[ics of such thermal'equipment as heat exchangers,
pumps, and compressors. The now generally accepted facility of engineers'
to do basic digi tal computer programming is also a requiremen .. t.
ACKNOWLEDGI\1ENTS (,
Thennal system design is gradually
emerging
as an identifiable discipljne.· .. Special recognition should' be
given
to the program coordinated by theUniversity of Michigan
on
Computers
in
Engineering Design Education,which in 1966
.clearly
delineated topics and defined directions that have.since..P'oYed to be
productive.
Acknowledgment should be given to activities"A.-vi" PREFACE TO THE FIRST EDITION
- ,
,.
within the chemical engineering field for developments.that are clos.ely relat-ed, and -in some cases identical, to those in thy theIDlal stem of ll1echanical
. I
.. engmeenng.
_ .
.
M~y faculty members during the past five
years
have arrived, often '.
ind~pendently,
at the same 'conclusion as the author: the rime .is -oppprtune' .'. for .dever~pments in th~rmal-.. q.~s.ign. .
:Mjap.y
,of
th.ese .. fa<:;]Jlty memb~rs have .. -:).,. . -. ':share<;i
some
of.th~ir ~xperiencesin
the theilllal design· section' <?fMechanical" .' ~ . ... ..
....
.
~...
~~·::~··~~·~~:--~:··Ellgineering~,:N~Ws.__
and ..h.ave,
thus, pirectlyand
-indirectly c'oritributedto
,
,
iaeas
'
e*p~ss'~d
'
:jn
'
,
thisjJd6k;~:,=~~
'."'
:
:
'-,;
~
,~,.~~
'
~,"
.
~
,
~,~
,,
\"
I ",,
.
~:
:.<:
.
',',: .:;'~
". " '.: .. .- .' .-',~
, .. ' :._:. - ..This
,
.
m anus'cn
p~
,is'-the'"
':
:.
third
~
: id{tio~·~
·
(jt.~
·
t~:it~ ~in~ft te~a)·
::
:
:~
t:~ed~
~;:
iD:rtht:
·:.
~
'
.~~
:.'~'.
',.
'
:::
'.Design of Thermal Systems c611rse"at,th~ ,University 'ofTIlinois"at'Urbana---'-' . --Champaigll7.-I thank the students-Vl·ho have.worked with
me
in
this :coursefor
their
suggestions for improvement .of
the manuscript. ,The
. second edition. . I ·was an at!:factively printed ~ook1et prepared by my D~partment Publication
Office,
George Morris,Pirector;
June J(empka and Dianne Merridith,typ-ists; and Don
Anderson, Bruce Br~ckenfeld, 'and Paul Stoecker, draftsmen.Special thanks are due to the Engine~Png Department of
Alpoca
Chemicals.Corporation, Chicago, for their intert:st jn ,engineering·,.education and for
. their concrete evidence of this interest shown
by
printing the second edition. Competent colleagues are invaluable as sounding boards for ideas anda~
contributors~f
ideas of their own. Professor L. E. Doyle offered sug-gestions on the econoDljcs chapter and Prof C. O. Pedersen, a coworker in ·the development of the ·thermal systems prqgrarn "at the Unive'rsity of illiriois at ·Urbana-Champaign, prov]d.ed .advice at many stages ..Mr.
Donald -R. \'\tin and a class of architectural engineering students at Pennsylvania State University class-tested '~he manuscript and pf9vided valuablesugges-. tions from the point of view of a user of the book. Beneficial comment~ and criticisms aJso came from the Newark College of Engineering, where Prof. 'Eugene Stamper and a group of students tested the manuscript in one of their classes. Professor Jack P. Holman of Southern Methodist
Universi-ty, -consulting editor of ~cGraw-Hil1 Book Company, supplied perceptive .con1ments both
in.
terms of pedagogy as well asin
the technical feattlfeS ofthennal systems.
The "illustrations in this book 'Were prepared by George Morris of .Champaign~ IlJinojs.
By being the people that they are\ my wife
Pat
and children Paul, Janet. andAnita
have made the work on this book, as well as anyt .. hing else 1 dO'1 seem worthwhile.W. F. Stoecker
---~---- ---.... --.; .,... •• r
'.
," :l ... ' _._" .,J ... ·A ..... _ ' • I .~ 'J.~ • J' . " -." • • ".J- " .1".Il
.
. ..::.-) ' . : \ J . ':Jt, : ; . . " ;.. -c.-.---~ • ,..'
I ) ... .~ ... ;" . ... ~. :.~ "!"' .. :. - : ~. ,-. ~: ,::. .. - -!- .' ;. "
"
ENGIl\TEE
RJ]\T
G
DESIGN
-1.1 lNTRODUCTION
\Typical ,professional activities of engineers include sales, construction, . research, development, and design. Design will be our special concern in this book .. The immedia,te
1>roduct
of the design process ·is a report, a set of calculations,andJor a drawing that
areabstractions
'ofhardware.
-Thesub-ject
of the design may be a process,' an elem,entor
component of alarger
assembly, or an entire system.
OUf emphasis will be
on
system design, .where asystem
is defined as a collection ofcomponents
with interrelatedperformance.
Even this definition oftenneeds
interpretation, becau-se a large systemsometimes
includes s·ubsystems. Furthe nn ore , we shall progressively
focus
onther-mal
systems,
where
fluids
and
energy in the form of heat anel work are
conveyed
andconverted. Before adjusting
this
focus,
howe'ver,this
chapter
will examine the
larger picture
fn~o
which thetechnical
engineering activ-ityblends. We shaH call this
larger operation an
engineering-undertaking,
··implying
that
engineering
plays a aecisive role but also
dovetails with other
considerations.
Engineering undertakings include a wide variety of
commer-cial
and industrial
enterprises
as well as
municipally
Istate-. and federally
sponsored projects.
-~ ,..
,', ,.-~' '~ .~' ... ~~.\ ':: ,~:. .'
-
I-,
;~<:r
~
·
'1<'
.
:~
,
:
'
:~ ~
-'
atteritioIi
has been devoted to the.8..-.... ,rlIr . . ', " " .
e~gj~~erhi
:
g ~.dertakings.
Studies onthese
.-;
~'
;"/
'
;'~DJ.
a.,
,
'~IJ)t!o--'.Q
i:;':,l.l~~'~
:
I~~k~i~Rr,t~1~~
,
~.
,~
..
~
,(,~
,
;~
"
steps
:
'
iih~
:
:procedUres
used'
m
.reaching decisions.":'2:::t~f!::'·~,(',';n':;;;:;;jr;",,:":r:.~'n~ ... 'V"UL~.L~~~.~~.4 'Qttii~~e
'
studies'
has been tostimllIate .enginee.rs.
to-reflect'"
.
,,
«
,.>,:
<
,.
:
':
:
~~
p
,
#
·:
'the
,,,
.
'
proces~
,
es
oftheillselvfs
andoth~rs
·
o~ the pro
j
ect'
,'
teani.
"
,:.',-' ,,:,::,.t..':;', ,:~ ::';'~:'i~:~:~;,::,·':'CenaiPIYJh.~~pI:gGes,~,..
~q.I.~f?q~.ep'~e,9f.',s!.~p'?,,~~llowedin
each undertakingis
''''''ll ,"," . - ,. ,~ " ." .... ~. " , ,0 . ') '. • .J~ ' ~ , . ,
',:' -~~.~':~:.''''':' ',;:. <:" diffeient~ 'an:d':no,:oDel's~queDce~' incli.Idii:tgJ~·e·,on,~~·d,~$c;riped,~t.N.s. chapter, ,
is univers,aUy applicabl~.
Since
the ~starting,.Ppjrit, the-:goar~~'::aiitf;:¢8:':sidg :,\r~<'~,.: 'r;:' " c;:onditioris differ from ~ne' ~;ndertaking' to thenext,
the" pIoc~dllre,s,'·':Pillst ' "',v~.
,
The advantage of analyzing the de<;:ision process, especjal~y
in
com~plex undertakings, is that
it
,leads ,toa
more logi~a1 coordinati0D of the ~aJ;ly individual efforts consti~ting the entire venture. The flow diagramin
Fig. 1-1 shows ·typical steps followed in the conception, evaluation, .and execu-,tion of ,the plan. The rectangular boxes, which indicate actions,
may'repre-sent consid,erable effort and e,xpenditures on large projects. The
diamond
boxe~ represent decisions, e.g., whether to c;oIitinue the project or
to
mopit. , J
" The technical engineerlng occurs mostly
in
activities 5 and 7, product or system design and research an~ development. Ljttle will be saidin
this chapter about product or system design because it will be 'studied in the chapters to follow. The'tlow diagra!ll shows only how this design procedure fits into the larger pattern of the '·undertaking. The individual nondesign acrivirie.s will be
discussed next.1.3 NEED OR
OPPORTUNITY
(STEP 1)
Step 1 in the flow diagram of Fig. 1-1 is to define the n~ed or, .opportunity. It
may
seem easy to state the need or opportunity, butit
isDot always
a simple task. Forexample,
theofficials
of a city.may suppose that their need is to enlarge the reservoir so thatit'
can store a largerquantity
of water for municipal purposes. The officialsmay
not have specified the actual need but instead may haveleaped to
one possible solution. Perhaps the needwould
better have
been stated as a
low water reserved'uring certain times of the
year. Enlargem'ent of the reservoir might be one possible solution, but other sol utions might b~ to
restrict
the consumption of water and to se6-k other soutCessuch
as
wells.Sometime
'
s
·
possible-solutions are
precluded bynot
stating the need
properly
atthe begmning.
,The
word "opportunity" has po"sitjve connotations, whereas uneed"suggests a defensi ve action. Sometimos the two cannot be
distinguished.
For-example,
an industrial firmmay
recognize
a new product as an opportunity,but
if
the company
does
not then expand its line of prpducts, business is
ENGINEERING DESIGN .-. .,.J..:R: ~ " ! a' i , , g' f '.' I ,£1
,
' I . f\ 'v' .I r , Define need 1 " , ' , __ .' _,J: ~', . or-, opportunity ':_ , • • • : . I 2: 3 Specify criteria " ' for success Dere~ine probability, of success -.t!-; ',Research, _'c:, , and, 7 Low probabilit~'-drop
development
4
Product or system
5 design and
Refine and revise
Implement-construct or manuf:lc(ure cost estimares NOt feasible-drop -
,
.. i~ " " ~ FIGURE 1·1 , tI:~;~Po~sibre flow diagram in evaluating and planning an engineering und~, 'f
... ~:I .
-
.
~~"In
commercial enterprises
ttypical
needs oroppo~~J.t~rgiie
inthe
ren-ovation or
expansion
of
facilities
to
manufactur~?,f;f-
distribute a current
product. Opportunity
also
arises
when
the sale of
a-.product
not manufactured
by the-fh.ul is rising and the ro.arket potential stems
favorable.
Still a third
form
in
which
nn
opportunity arj'ses is through
research
and developmen[
4 DESIGN OF THERMAL SYSTEMS
, ~
within the
organization.
A new product may bedeveloped
intentionally Qf .,accjperitally. Sometimes.
a new
use.of
alleXlsting proQ.uct·
can befound
by making a slight modification ofit.
An orgahization.maY
know how to"manufacture
,
a
g~JJ?my, stickysup.stance and a.,ssign
'
to the research
,
and
developmefl;t department the
task
offiIid,
ing
som~. useJar it. . .
''':'~J.' Of
interest
to us· a~ the ni~IPent is ~e nee~ or()PP9rWni..ty
th.a~ r~q~~es: .. " 'engineering
designat a
subsequentstage:"
' .
, .
.
'
.
,
'" >" ".;;
;;,~
"
: ~:'
:
~:"'~'
','
~:
,-..
0;>=;
:,
< , ..~
-
:",
,), L ,;", ,,: 5'" -- <"'" .' ,', .' ",'
~:'
-- '-,", "1.4 CRlTERJA'
'
OF
SUCCES·S~.(STEP~·2J'::-,··,
...
' '
.'
" .
'
:".:.;: ..
'.:----~'~.. ; ..
~,.~';.~:,...
.
~:'::.
:
.
,
..
-In·
c'ommercial ente'rprises the'usual
mterion
ofsllcc,ess
is''
showillg
a ,profit~"i.e.~ providing a certairi" rate of telurtr .on the -investment.
In
public' projects . the criterionof
success is the d~gree tq which Q-le need' is satisfied in relatipn .to tile cos~~ monetary or otherwise: ' .
. In a ,profit-and-loss ecopbmy,
0e
expected earning power of a' pro-posed conimercial project isa
.dqmjn~tiDg influence on the ·decisionto
pro- .ceed with the project. Strict monetary concerns are always tempered, bow~ ever: by. hunlan~ social,
and
political considerations to a. greater or ~esserdegree.
In
other words, a price tag is placed, on the' nonmonetary factors . .. A. factoD' may be located' at- a more remote site at a penalty in !be form of , transportation costs so that'its atmospheric. pollution or noise affects fewerpeople. As an
alternative,
the plant mayspend
a lot onsuperior
pollut~on .c0!ltIo] in order
to
be a good neighbor to the surroundIng community ..
' So:metlmes
'
a finn will desjgn and' manufacture a product· that offers·Jj'ale opportunity for profit
sin1ply
to round out a line of products. The a\'aiI~biliry of this product, product A, peMits the sales force tosay
to aprospective customer, "Yes, we can sell you product A, but'we recommend
product 'B 7~'
which is a more
profitable item inthe
company's line an~may
acrually be
superiorto
product A. ," .Often a decjsion, particularly in an emergency, appears
outside the
realm of economics. If a boiler providing steam for beating a
rental
officebuilding fails, the decjsion
\vhether
to repair orreplace
theboiler inay seem,
to
be
outside therealm of
econQmics.The
question can still be consideredan economic one. however, the penalty for nor executing the project being
nn
overpov';'ering loss.1.5' PROBABILITY QF
,SUCC~~S(STEP 3)
Plans and
designsare
a]ways dirdcted
roward thefuture,
for which
only
probabiI
ity Inot
cenajnty,
is applicable: There is no absolute assurance
Chl;H the
pJanr willn)eet the success. criteria discussed in Sec .. 1.4,
'Only
a
- li.k;]
l
ihood or probability
thai
it
wilJ
po
so.
The
mention ofprobability suggests [he
nonna}distribution
cUrY"e(Fig.
1·2),
an excel~en[starting
pointfor expressing
uncertainty in the
decision-making
l?J:Ocess The significance of rhe distribution
.
curvelies
particolarly
• ; ' .- .4 ,' , -. ~ .. '. -f. , • .I ...... -;,-{# ... ;:; FIGURE 1-2
Probability distribution carve ..
,
!'. }' p:" -/ -, .x=a ENGINEERfNG DESIGN 5 ~ ·1 J ) -." .:':~ " ,- ./-.,.- -\ . , '':' '.:':". ':"' .~-:'.-:
'
\
'"
'- -.•• j •.•• ,. xin the evaluation of the area be~eath the cprve. The area under the curve between x l arid-X2, for example~ represents the probability
P
of the event'soccurring between valli~s x 1 and X2', Thus,
Since the. probability .of the event's occurring somewhere in the range of x
is unity, the integration. over .the entire range of x is
equal
to 1.0:I~o>Ydx
=
1The equation for
the probability distribution
curve is. h _h2(.:r -a)2
. y
=
- - e
'
fo
. (1.1)The maxiffium value of the
ordinate ishlF, which occurs when x
==
a.
This fact suggests that increasing the value ofh
alters the sh4pe of the distributioncurve, as
shown inFig.
1-3.If
hI
is
greaterthan
h2 ,
tbe
peakof the
h
Icurve
rjses higherthan lhat
of theh2
curve: . To extend theprobability
"
idea
to decision
making in an engineering undertaking, suppose that a new product or facilityis proposed and
that the criterion forsuccess
is a 10 percentrate
of return onthe investment
for a
5-yearlife of. the plant. After. a
preliminarydesign, the
probab~itydistribution curve i's shown as indicated
in
Fig
.
1-4. Since
roughfigures
were u~tIoaghout the
evaluation,
thedistribution
curveis
,flat,
indicating
-.;:- J ,
-j ' , ... ..." - "'0'-" ,\i..
x=a x
F:IGliRE 1-3
Several different shapes of the probability distripution curve,
18 perc~nt, The expected rate of return is ,attractive enough, ho~ever~ to proceed to _ a complete design, including cost estimates. If the most probable return on investn1en{ after
till
s complete design were 16 percent, for example, the confidence in this figure would be greater than-the confidence in the 18 percen t figure after thepreliminary
design because costs have nowo
, After construcrion\
FIGURE 1-4 8 16 I After 5 years After 1 yeaI pf operari on Retum on investment. %Disr.tibutieilllAi ~eS at -various stages of.decision making.
ENGINEERING DESIGN '"/
been analyzed more carefuUy C).nd mar.keting studies have been conducted_
more thoroughly. _ . I L
The. probability distribution curves at Hvo other stages~ after construc-tion and after 1 year.of operation, show progressively greater q~grees of .
confidence. in·. the rate of retuill;after a
5-ye~r
life.A
.
fier:.5
.
ye~rs; th~
rate of return- is kllQWnexac
tlY1
and th.e.
:prob)abi1ifY;
,
distiibut~dn
·"
c~rve
-
degeD.e~tes-- .>degeD.e~tes--,"degeD.e~tes--'
into'
'a: curve-...
that:-is
'
infinites tmally'··ttli·riw·and'
:
:illfini-tely ... :;hi gh~,~:;' -;,',;\ .:~:o ;'>:->.:~,-..
:~.,:-:~:.~;;. :'\.~.':.-
~h¢
re¢9ghitionth.~~
·
pr:edj.¢.ti9~
:
pf:fjJ.Jilte ?
-
eli.~vi6r is
"rioi
:deterrmriisii.c!t·"
so that only one ~Set of events or cOD:ditions \vill prevail, has spawned a ne\v probabilistic approach' to design (see the addirional readings p-~ the e~d of the chapter and Chapter 19). One of the activities
of
this new study is . that"-of quantifying the curves sho\vn in Fig_ 1-4. It is valuable for the.decision maker to know not
.only
the most likely v3Jue of the rerum on investrTIent .but also whether there
i$'
a high- or low.probability of achieving this mO${_ likely value~ "
1.6. I\tIARKET .ANALYSIS (STEP 4)
If the undertaking is one in which a product or service' must eventually be sold or leased to customers, there mus! be some indication of favorable reaction by the potential consumer. An ideal fonn of the information pro-vided by a' market ana-lysis would- be a set of cu[\(es like tl:ose in Fig.
l~? With an increase in.price, the potential volume of sales decreases until - such a high price is reached that no sales can be made. The sales-volume _
to
price relationship affects the· size of the plant or process because the unit-price is often lo\ver in a large plant. For this reason, (h.e marketing and. plant capabilities must be evaluated in conjunction with each other.
/ High sales and advertising effort
~ ~---~---~----~~ Prier:
f1GURE;M~_
_ o,! " r_
. --,... . ~":.,
8 DESIGN OF THERMAL SYSTEMS
Because the sales and advertising effort influences the!"Volume of sales "
for a given price, a family of curves -is'expected. Since a cost is associated' :'~,:,
with
the sales and advertising effort,-and
sib~e a continuous increase of ,', this effort resuits ~n dirninishffig improvement in sales I there eXls'ts an " "optimum level of sales and' adv~rtising effort. A marketing plan, should,:' '
emerge 'simultaneously \vith the ,tec~cal plans Jor the undertaking~',
' I, '
" '. ~, -: ' ' " .; • •• ~ • • - fI ', : - ~.: , " • ' .: ' . '
,'~':,1'.7: ,~~FEASIBlLITY,..(STEP
6} , -
,
:--:'" ~ . . '. \' . . . .. ", .... > ... :-~ .-.~:::'~~~~.-
:.! ..
,~~·~:'I&A.,"·-:; 'I.-",,:2:,I:~:-~·;,;,"· ... .;-:-' "" - & . . -• • ~. " ... - '. , ; .. . . . ~'. _ i'"; . a,' • • ' _ - •. i.The' feasibility, sWdy',"
,step
"
6, .;n-d,:'the"
:sllbs~qu~nt'feasi@rty,
:
a~ci#O:ii~
refer
:,:,~:," ~
:,~
~
,:"
t~, w~ether the'
project
'
is
'everi',possibie~'It
"proJecl:may' be-feasible,"
,arpossible, ,but not economi,cal. Infeasibility may result from unavailability of inves~ent capital, land, labor; or favorable zoning regulations. Safety codes or other regulatory Jaws may proJ:ribit.the enterprise. If an undertaking is shown, to be infeasible, either altertlatives
must
be found or the project must be dropped.1.8 RESEARCH AND DEVELOPl\1ENT
(STEP
'.
7)
If the product or process is one new to the organization, the results from research and qevelopment (R&D)
may
be an important input' to the decision'process. Research efforts
may
provIde the originor
improvement of the " basic idea, and development, work may supply .. \vorking mo_dels or ,a pilotpJant, dep~nding upon the natl.l!e of the undertaking.
Placing R&D in a late stage of decision' ~aking, as was done in Fig. 1-1, suggests that an idea originates somewhere else in the organization or in the field and eventually is placed at the doorstep of R&D for transfonnation
into
a workable idea. The possibility of the idea's originating in the researchgroup should also be exploited .and is indicated
by
the dashed line in Fig. 1-},. Rese:arch people often learn of new ideas in other'fields which might be applied to their--Q\vn activity.1.9 ITERATIONS
The loop in
Fig.
1-1 emphasizes that the decision-making process involvesmany
iterations. Each pass through the loop improves the amount andtbe
quality of infQtmation anddata.,
Eventual]y la pointlS
reac~ed" ,wherefinal
decisionsare
made regardi~gthe
de~igntprod\lction,
and marketing of theproduct.
Thesubstance
that circulatesthrough
~sflow
diagramis infonnatio
,
n
twhich
'
may be in. the form of reports and
~onyersatjonsa'nd
f!1aybe both verbal and pictorial. The iterations are
'
accompl ished by
c6mmunicatron between people,
an'd
thiscommunication
isinterspersed
by go-~r-no-go decisions.ENGINEERL"IG DESIGN 9
1.10
OPTI1VIIZA.TIOI\T O~FO
P
ER
A.,.
TJO
l
r
1
: . )
The
1low
diagr:ln1 of Fig. 1-1 termipates with1the constnJction or begin~ingof lnanufacture of a product.
or
service. Actually lli.~other stage takes over . at thi:s point,which
seeks tooptimize
rh~ ope~ation of- a given facil.ity. Thefacility was designed on the ba~is, of.qc~ain,9-~sig.r:t p~ram~Fer5. which almost
jnevit~bly
cha.11ge
by thetirne
thefacility )s
iri-'optration~"rr(e,ile
xt
,challeng'e, :. ,~thell ~ is": to··'op~.rate ihe-1~cility'·
iri
'
:'rJ1e-
'
bes't-~p'6s,s'i5ret'fri'a.n.:ne';:'ip--
'
tii
:
e
~Jigl11:
~6..r".""
'such
facto(s a$-acnt~al costs arid' pric~s.A
'
painful'activity
occurs when ,th~" 'project is not profitable and the ob.jective becomes, that of minimizing the
loss.
1011
'
TECHNICAL DESIGN (STEP
5)
I
. S,t~p 5 in Fig. 1-1, the product or system design, ,has no~,. been discussed~
'The reason
for
this omission is that the system design is 'the subject of thisbook from this point on. This step 'js where the largest portion of eng'ineering
, time, is spent. System design as an activity lies some\vhere between the study
and analysis of individual processes or cOillponents and the larger decisions,
\vhich are heavily economic. Usually one person coordi~ates the planning of
the undertaking. This manager normally ~merges with a background gained
from experience in one of the subactivities. The manager's experience might
be in finane'e, engineering, or marketing, for example. Whatever the original
- discipline7 the manag~r must become conversant with -all the fields that play
a
role in the .decision-making process.. .. The word "desjgn~' encompasses a wide range of activities,. Design
may be applied to (he act of selecting a single member or part, e.g., the
size of a tube
in
a heat exchanger;to
a ]arger co'mponent, e.g., the entireshell-and-tube heat exchanger; or to the design of the system in which the
heat'ex:chang'er is only one component. Design activities can· be, directed
toward mechanical devices which incorporate linkages, gears'~ and orher
moving solid members, electrical or electronic systems, thermal systems,
and a multitude of others. -OUf concenrration
win
be on thennal systemssuch as those in power generation, heating and refrigeration plants, the
food-processing
jndustry, and in [he chemjcal and process industries.1~12
SUMMARY
The flow
diagram
and description<o~
the decision processesdiscussed
in thischapter are
highly
simplified and are nor sacred. Sincemmost
everyunder-t~ing is different. there are
almost
infinitevariations
in startit:lgpoints,
goals,
and
intervening circumstanc~. Thepurpose of
thestudy is to
empha-size the advantage of
sys.tematicplanning. Certain
functionsare
common inthe evaluation
andpJanning of
undertakings,particularly the iterations
and10 DES1GN OF THERMAL SYSTEMS
ADDITIONAL READINGS
Introductory 'bQoks on engineering design I
)
Alger, 1. R. M .. and C. V. Hays: Creative Synthesis in Design, Prentice-Hall. Englewo,od
Cliffs, N. J.~ 1964.
Asimow, M.: Introduction to Design, " 'Prentice-Hall, Englewood Cliffs, N. J., c 1962.,_. . , _ )
Beakley, Q. ~., and H. W. Leach: El1gi~1eering, ,f-n Introduction to a Creative Profession,
, , Macm,!l1an, New York, 1967. " , ' " , ' : , ,',-r'" ' :.', "B'iml; .H .', ,R.:. c..r.e~riv,e. ~ngi.neering Design, Iowa State Un,iversity Press, 'Ames,
1960
-
.
·
-
.'
,~ '. " DJxori.-J:;R::·.bes"gn-'Erfifrreeri1'):g:~il:tVe!llil~eness, Analysis~' ,and Decision Ma~ng, :M:c(h:ay.;-' ,
, Ifill, 'New' Y~~k,_ ~966., "-, ~'~c/:'~' :~~::~.'i:,:,,::'..~:~,-::.,~'~i;-i,_,/.,:,<,~, ,~~/ .~::,;
__
« ,(,,:~,"''':~'< ~:~,,
:
:
:,;
';'~: "_ ',;-,;,~:.~,.
.
':'J~1:';:,!.,_:v ~,, Harrisberger, L.: Engin{!ersmD;lzshipJ ..
APhilosopny·
'oj Des;in~ Brb6kS1C6re~·Berriiont~'·Caiif.'~'
-., .,
-,
',
'
I 1966. ' ' ,
-KriGk. E. V.,: 'An Introduction to Engineen·ng and Engineering Design, Wjley, New York,_
1965.
Middendorf, W. H.: Engineering Design, Al~yn
and
Bacon, Boston, 1968.·1'4.isc;hke~
c.
R.: ~ lnrroduciion [0 Computer-Aid,ed,Des(gn7 I!entice-Hall, Englewood Cliffs,N. l., 1968. - ,
11orris, G. E.: Engineenng. 'A pecision-Making Process, Houghton -Mifflin Company,
Boston~ 1977.
Woodson, T. T.:-Introduction to Engineering Des,ign, McGra~-Hill, New York, l,~§6 .
. Probabilistic approaches to design
Ang, A. H-S., and \V, H. Tang: Probal;iliT), Concepts in Engineering ,Planning and Design.
Wiley, New York. 1975.
Haugen, E. B.: Probabilistic Approaches to Design, Wiley. New Yqrk, 1968.
Rudd, D. F., and
C.
C. \Vatson: Strategy of Process Engilleen'ng, Wiley, New York, .1968.I - ... '-- .
. ', ... , ! 'r: I I ,.,: .. , " •• 1 . ' r~)
..
.
"
. . [/ . • , '. r ' ""t~:':::,:::).~.,:-;~, '.J~ : '. • .. '"DESIGNING
.
A WORKABJ-.JE
SYSTEIVl·
I2.1 WORKABLE
'
AND OPT1l\1UM SYSTEMS
The simple but important point of this chapter' is the distinction ·betwee:p.'
designing a workable system and an optimum syst~m. This chapter also
continues the progression from the broad concerns of an undertaking~ as d~scribed in Chapter 1, to a concentration on engineering systems and, even
,more specificull
y,
on thermal systems.It is so often said that ~4there are
many
possible ans'wers to a designprpblem'" tba.t the idea is sometimes conveyed that all solutions are equally
desira151e~ Actu~lly only· one solution is the optimum, where the optimum is
base~ Or:l some defined criterion, e.g., cost, size, or weight. The distinction then
win
be made between a workable and an optimum system. It should not be suggested that a workablesystem
is' being sc·omed. Obviously, a work-able syster:IJ is infinitely preferwork-able to a nonworkwork-able system. Fu rthenn ore ,extensive effort in
progressing
from a workabletoward
an optimumsystem
may
not be justified because of limitations incalendar
time,cost
ofengi-neering time, or
even'the
r~liability,of the
fundamental data on whichthe
design is·
based. One
pointto
be explored in this chapter is how superior solutionsmay be
ruled out inthe
design process by prematurelyeliminating
so~e
system concepts. Superior soiutions may also
beprecluded
byfixing
interconnecting parameters betweerl components and selecting the
compo-nents based on these parameters instead of letting the parameters float unti1
the optimum totalsystem
emerges.~
12 DESIGN OF TIfERMAL SYS1EMS
2~2
A WORKTCAJ3LE SYSTEM
A .workable system is
one that .
1. , .
, ',:'
I'
. 1 .
.
Meets therequi:rem:ents
ofthe
.
pllrpose~;,
of
thesystem,
e.g.,pr~viding
the. .,req~ired
amount
ofpower, heating.,
cooling, orfluid
flow., orsllITouild
.
ing
''''''.:~! •. ! •.•. . ,1. .
it
spac~'vitha specified
environm~nt so·that
peopI~ will,be .. coIPf6rtable '., ... ~.
:~~..
~,~Qri_a~.,ch~lnJca4 P.tq_c~s.~_"Yi1Lp!oceed
or not proceed ..
.
2~
'
~ ~iiil·
'
Na~e
··
:~ati~f~-~
'
t6~;/lif~~
:
~~·~fu.
:
~i~ten~ce·£_6s¢
:/
~
:
'
!.
~};,
;
~
~c~::
·
.
:.~'<'
.
:.
:J :.
,
:
.
::'
~~
<
::
.:::/~
: ~
--'
:'
~
'
:"'
__3.
Abid~s
by.ali
constraints,
·
sllchas
'size,'.
we~ght~
.
i~~p~r~trires~
·'jp
r~ss~i'e';<
.'~
'.' " materia) properti~s,noise,
pollution,' etc. . .. . .' . . -
-
-In
summary,' aworkable
system ·.perfortlls.
the
'as~igned ·taskwithin
the. imposed con~traints.
2.3
STEPS
IN
ARRIVING
AT
A WORKABLE
. . : ~ ...:
SYSTEM
.The t\VO rriaj~r'
steps
in. achieving a workable system are (1) to select theconcept to be 'used an~ (2) 't6 fix whatever parameters ar~ necessary to select
the components of the system. These parameters
must
be chosen so that 'thedesign
requirements. and constraints are satisfied.2.4 CREATIVITY
.
'
IN CONCEPT
'
SELECTION
'Engineering~ especially engineering
design.
is a potentially creative activity. In practice creativity may not 'be exercised because of lack of ~ime foradequate exploration, discouragen1ent by supervision or environment, or the laziness and timjdity of the
engineer.
It
isparticularly
in selecting the concept that creativity· can be exercised. Too often only' one concept is ever considered, the conceptthat
was used on the last similar job. As a standard practice, engineers should discipline themselves to review all the alternative concepts in some manner appropriate to.·the scope of the project. .Old ideas that
were
once discarded as impractical or uneconomical should be constant1y reviewed. Costs change;new
devices ormaterials
on the marketD1ay make an
app'roach successful today that was not .attractive 1a.
years.
ago.
2.5
WOEKABLE VS. OPTIMUM 'SYSTEM
. <
The distinction between the approaches used in aniving
ata workable system
and an optimum system· can be inustrated
bya simple example. Suppose
that the pump and piping are to be selected to
convey
3
kgls
from one
locationto another 250 m
away from the original position and 8 m higher... If
thedesign is
approached with thelimited objective of
achievinga workable
system,
thefoJlowing procedure
mightbe
followe'd:
DESIGNlNG A WORKABLE SYSTEM
1
3
2 < • • The ·;::;lcvauoIl of
8
ill imposes a prc$sure difference of. . I
. 3 ) ?
(8 m)(lOOO kg/ill )(90807 mls-'-)
=
7805 kPaArbitrarily choose. an additional 100 kPa to COlnpensate for, fg~tion in the 250 ill of pipe. , ' . I ... .'.~' :'
.
,~."..
'. ' . ', '~2o' itccording to'
the
foregoi]J,gdecision
1select a p
ur
.o.p
,;vh:kI~l'.delivers
3legIs
,
'. '. -,--Y:.agalnst a -pres~llre-~-d.i[ference.
.
.of~\L1,8 .. 5);k:f)~ .. <~jnqllY-'~;iselect~,a~~:g~H~,}si?;~:~~:) r:,:f.~'Y\'"frqni-a h,!IldqQok such "thaf the pressure' drop:in 250
m·
·'
of.Jength
is-)UO··::···· .,. "kPa or less.
A
pipe size of-5~rlli-n'
(2'hi)
satisfies
the requirement . ~ .. 'A~poaching the' same problem with the 9bjective of achieving
an
opti-mum system presupposes agreement on a c~terion to optimize .. A frequeptly chosen criterion. is cOS.t (sometimes fn-st .~ost only
in
speculative projects,.and sometimes the lifetime cost, GOI?-sisting of first plus lifetime pumping and maintenance costs).
~ designing th~ optimum pump
and
piping system for minimum 'life-time cost~ ,the pressure rise to be developed by the -pump is not fixed imme-diately but left free to float.If
[he three major contributors to cost are (1) the rust cost of the .pump, (2) the fIrst cost' of the pipe, and (3) the .life-time pumping costs, these costs will vary as a function. of pump pressure, as shown in Fig. 2-1. As the pump-pressure rise jnc.reases, the cost of the Pl!mp probably Increases for the required flow rate of 3 kg/s because of the - need for higher speed ~dJor larger impeller piameter. W~th the increase in pressure rise) the power required. by' the' pump increases and is reflected inUi o U " 0 FIGURE 2 ... J 50 First cost of pipe 100 150
Pressure developed by pump. kPa
ContribUtions to costs by pump and piping systems.
, '.' ~ .
14
DESIGN OF THE.KMAL SYSffi.\1Sa higb~~ lifetime 'pumping cost.' The first cost of the pipe, the third contrib- .
uto.r to, i:p.e t~t~
co.st,.
"i;>ec~m~s 'enornollsly hi~h as th~ presSUre available-to'
overcome fricTIon ill the pIpe .reduces to ~ero.
The
avaIlable press~efor.
the'. pipe is the pump-pr~ssuIe rise
minus
78.5 .kPa. need~d for the difference in , '.elevation. An appropriate' optimizationtechnique
~ ~~used
to deten;n1nethe optim~ pump-pressure rise,.which
in
Fig. 2-1 is approximately-ISO kPa~'
-,
_
'
, -.: . ",.-, .,. ! " :.: " Fip~lly the pump can '~e selec~~d. to develop l.?O-J~~" p'ressur~ -rise':," ~~,
,
a
,~;,... ~~::.::-; .', "';;~-<., .. ' :.~ ... pipe si.ze ~an be chosen such that the pressure drop
due
to friction is 71.5,
-"
--- , .
-
.
:··J~P~.'or
"
Ie~'s'
:
.
~J - , : •• !",:". :,;\'-,' - .-, '" --.'." •. " ' . ': ,:' ,; " .. :_-~, .. -' .... '~" ; .,'.' .. ' . : . '--I' . . . .
.Th~t~ne'er'ili~'~pr~edi~~-diseri~~i6rr' mal6ires\t~ti()1~~ pref-~ten~~
:
"
.:
-!
-towax:d designing optinium systems.-· To·,temper thi.s bias, several additional:
c~nsiderations
should ,be_ mentiqried. ,If the job isa
smallone
"
the' cost 'of,the increased engineering tim~ required Jor optimization ~a'y devour the
, -savings, if any. Not only the engineer's time but pressure of c!iIendar time,
-nlay
not permit. the desigil to proceed beY0!1d a workable design.t
2 .. 6
'
DESIGN OF A FOOD-FREEZING
_PLANT
'
Large-s'cale engineering projects are extremely complex, ~'d decisions are often intricately interrelated; not onJy do they influence each other in· the
purely technical area but als9 c~oss over into the technoeconomic ) social,
and human fields. To illustrate a few of the decisions.involved in a.;-ealistic
,
~commerclal
undertaking and- to provide a furtherexample
"B(the contrastbetween a workable ~ystem and an optimum system~ consider the following project.
.(\. [ooq company can buy sv/eet com and peas from farmers during the '
season and'sell the vegetables as frozen food throughout the year in a
city
300 Ian away. What are the decisions and procedures involved in designing
the plant to process and freeze the crops? .
The ,statement of the task· actually starts at an advanced stage jn
the decision process, because
it
is already assumed that a plant will beconstructed. This decision cannot realistically be made until some cost data are available to evaluate the attractjveness of the project. Let us aSSUDle,
therefore, that
an
'arbitrarily selected soJuriq,p has been priced out andfound
to be potentially piofita ble. We are likely I then, to
am
ve at a solutian
thatis an
improvement
over the arbitraryselection.
Son1e
major
decisions tqat_rpy..~,~ ,.pe
, ,
m'lc;le
.
are
(1)thew
location,(2)
size,
~nd(3) type of
freezinp pl~t.The
plant
could
be locatednear
(he producing area, in the market. city, or somewhere between. The size
\vilI be
stronglyinfluenced
bythe market
expectati'on. The thirddecision,
the type of
freezing
plant,embraces ,the engineering
desjgn~T
h
ese
threemajor ·decisions are interrelated.
For
example,'
the
location
and' sizeof
plantmight reasonably
influence
the type of systemselected.
The seJec£ion of thefreezing-DESfGNING A WORKABLE SYSTE:v{
15
).
plant design \vv.li be based. A.fter the
cO
.
ncept
has been decided:. the inter:naldesign of the plant can proceed. I ) .
An outline of the sequence of tasks and decisions by vvhich a wQrkable , design could be arrived at 'i~ as follows: , . ..:.-:
' .... ' _ -. --l.·
I?~Gjde-.t.o
locate the .planti~
the ·.tharket~ity adjacenf::t~
a~etpgernted
,
." . . .
:--'·:"~::'w~reflouse~~perated"by:.the-co_inpany.· ."~'" '.0.·,,,,. -.,; . . - . ' ": " . '.' ".- ...2~ Select the, freezing · capac'ity of the plaiil
on
.the·:basiso.f
the: ..curre
.
nt
.'~.-availability
of
the crop, the pqtential sale in the city, ~nd il vaiIahIefini!l~.iQg. ' . .
-'34 Decid~ upon' the co~cept to be used in the freezing plant, e.g. 7 the one
. .. shown in Fig.' 2-2. In this system the food 'particles are frozen in a flu-idized' bed4 in' \vhich low-temperature air blows up through a conveyor
chain, suspending the product being· frozen. This air returns from the'
fluidized-bed conveyor to a. heat exchanger that 'is the evaporator of a
: re~rigerating unit. The refrigerating unit uses a reciprocating cOlnpressor
and water-cooled condenser. A cooling to\ver. in tum, cools the con-denser water. rejecting heat to the atmosphere.
4. The design can be quantified by establishing certain valueS. Since the throughput of the plant has already been detelmined, [he freezing capac~
ity
in kilograms per second can be .computedby
d.eciding upon the nUffi-.' ber of shIfts to ·be operated.Assume
that one shift is s.elected, so thatDQ":' the .refrigeration load can be' calculated at, say, 220 kW. To proceed
with the design, the parameters shown in Table 2.1. can be pinned down.
Air
Water
Pump
f:m Condtnser
FlG~ ~.
16
DESIGN OF ~ SYSTE."1S TABLE 2.1 , , 0 Temperature, .·C . . , ~ " .' ,Air. chilled supply - 30
Retu.Ql . ~ 23
.' , Refrigerant,' .ev~poration - 38' •• __ ~:J
';",. ":-0-" :"'." . . J:ondensation . . '. 45 .: ,','.-, ' . Condenser, cooling :water. inlet,' -I 3D ·?·· ..
:<
'r
:~,~-=,:"'~'':'::~·':::Olltlet::::~~,>.~:.-L.~~.:'»r'''~,i'::'' : ,',:, ~"-'::'" ' ,35 " . ._ . _ . . .. ' .... ....
;7'~.
:"
~
~::.;
!;p
:
;''';,_<~;o:\~;:':
':. ;;-i.
;.c:(
;j:
r~;_
;:.~.
...,
:.
/,
;
5 .. :
After,
thes~ valu~;
:
DaVe·been-
·
fi,,,ed,.
.
·
th
·
e-..
:indivrdual'·comp6n'entscan
be
selected. The flow of chilled , - - '
,arr
can
be . C'alculated to remove 220 kW with a temperature"rise"
of 7°C. The c'onveyor length' and speed mllst now be chosen to achieve the required rateof
heat transfer. Th~. air~cooling evaporator can be selected froma
catalog because the airflow rate, air temperatures, 'and r~frigerant evaporating temp~rature fix the choice. Thecompress'or must
provide 220 kW
of.r~frigeration withan
evaporating temperature of -38°C and a condensing ·temperature 'of 45°C, which'is adequate information for selecting the compressor or perhaps a two-stage. compression system. The heat-rejection rate at the condenser exceeds the .220-kW refrigeration capacity by the amouD:t of work added in the compresso~ and may be in the neighborhoo.d 'of 300 kW_ The condenser and cooling to\ver can be sized on
the
basis of the ra~e of he'at flow and' the water temperature of 30' and 35°C. Thus.,' a workable' system can be- d e s i g n e d . ' .
Unlike the above procedure, an attempt to achieve on optimum system returns to the pojnt \vhere the first decisjons are made. Such decisions, as the locat~on, size, and freezing concept should be considered in connection with ea'ch other instead of independently. The choice of fluidized-bed freezing
\vith a conventi.onaI refrigeration plant
is only
one of the commerciallyavailable
concepts, to say nothing of the possibility (admittedly remote) of devising an entirely new concept. Other concepts are a freezing tunnel, \\'here the air blows over the toP. of the product; packaging the productfirst and
immersing
the p'ackagein
cold brine until frozen; orfreezing
theproduct with liquid nitrogen purchased
in
Jiquid fann in bulk. An exampleof the interconnection of decisions is that the location of the plant that
is
best for one concept may not be best
for
another concept.A compression
,
refrigeration
pI antmay
bebest located
,in .the, cjry .as ~n, ~xtensionof
e):.isting freezing facilities, andit
may
be' un~jse to locate"it
close to the produdngarea
because'
of
Jack
of t'rained operators. The liquid-nitrogen freezing p]ant,
.
on the
.
other
hand,is
simple in
'operation'
and
couldbe
located close towe field;
furthermore,
it
could
be
shut downfor
theidle
off-season more
conveniently than
thecompression
plant.
If
thepossibiJiry of
twoor
eventhree
shifts were considered.
·
theprocessing
,
rate of the plant could bereduced
py
a factor of·2 or 3
1respectively. for the same
dailythroughput.
-i .
DESIGNJrjG A \VORKAELE SYSTElvI 17
vVf;~h~j] rh,e .internal
design
of the compression re£-:-igerfl.tioD pla.nt?the procedure was' to select r~asonable
'
tempenatures
and then design eachcomponent around lhose temperatures and resw.tlng flo'w rates.· 'Vhen one
approaches the design \vith the 'objective of optimization,
aU
thoseinter-.' connecting
parameters
_are left .free to Jfloat. and one 'finds .. th~ comhination..
~.'"of
v2E~~~· of'tbeSe:"p8Iam~tef,s· :-vvhichresults. in'
"theopri:tnu11r
'
Cprdbably
the ·-_/~.co~omic optiIn~rh).J"·:~<··..,
..
:--- ...
:
...
,.
,.'
( . . ,,- - .' .... ~
~ • . : '\ ... ... • ... :-~ 'I. ~ -" .. ' . ; ' - : ' . ,: .: • -' . ' , . ~ .'
2~7
PRELnVllf\JARIES
TO
.
THE
STrFDY
OF OPTIMIZATION
.
Any attempt to apply optimization theory to .thennal systems at. this st.age
'is destined for frustration. There are a variety of optimizarion techniq'ues
available, some of which are introduced ill Chapters 8 to 12. The first
. attempts to
optimize
a thermal system\virh
a· dozen· components, however~\vill be detoured by the need to predict the performance
:
6f
the system withgiven input: conditions. This assignment. called system si}nuiatiolZ, must'be
studied first' and will be considered in Chapters 6 and '14 .. For complex
systemS, system simulation must be performed with a computer. For this
purpose the performance characteristics.~aking up the system could possibly
,be stored as tables~ but a far more
efficient
and llsefu1 form is equ'ation-t;ipe. formulation;', .. ·Translating catalog tables into· equatioos, called: . . . . , cOJnponent .
. silnulation or mathernatical In ode lillg , is a routine preliminary step to system·
simulation and \vill' be treated in Chapters 4, 5, and 13.
Finally. since optimization presupposes a cliterion, 'which in'
engineer-ing practice is often an economic one. a review of investment econ'omics in·
Chapter 3 will be appropriate.
The sequence to be followed in the ensuing srudies; then, will be
. (1) economics, (2) mathematical
modeling,
(3) system simulation, and(4) optimization.
PROBLEMS
2.1. Location S in Fig .. 2-3 is an -adequate source of \l./at~r. and location A) B ,
and C are points at which water' must be provided at
the
following rates of. flow:
Us . 2.5 3.5 1.5
Points S .A, B. and C are all at the same elevation. The demands for water
at A and C occur inrerminen.tly and only during the working day, nnd they
may ·coinCide. The demand for wUler o.t B occurs only during nonworking
hours nnd is also intermittent. .GrQund-level ·access exists in II 3-m border
~g the building. ~.ccess is not permined over, through, or under the
'18 pESIGN OF THERMAL SYSTEMS ''':;''J ., .... :~ ....
:
. .-.-... . ~._ ; ! _ l
/
.\
.
/
\
/~~~---~---~m---.~~ fiGURE 2-3SuppJy and consumption points in water-distribution system.
1.0 . 0.8 0.6. 0.4
c..
-c 0.2 L-"C 2 ;; v. :...0 !1: O. J 0 0.08 0.06 0 .. 04 0.1 0.2 0.4 0.6 2FJow of wat~r. LIs .
10 FIGURE 2-4
-,
DESIGNlNG A WORKABLE SYSTEM -
19
120 --~~~"'-"Q~-
.
;
I I ~IOO "1 ~T,
~
80 --Z
~
~
~~~~
,
g-
,
,'
0 ~ , S ' -~ ,-:- J , ~-~ , ,~ I, ' 2 3 "4 6Flow rare, LIs ..
FIGURE 2-5
Pump perfonnance curves.
(a) Describe all the concepts ot" workable methods you can devise to fulfill the assignment .
.(b) The influence of such factors as the expected life of the system has
resulted in the decision to use a' system in which a pump delivers water into an elevated storag~ tank, which supplies me piping system. A
water-, level switch starts and stops the pump. Design .. the system; this includes sketchjng the pipe network chosen, ,listing all the pipe sizes,"-select-ing the pump, and speci~ying the elevation of the
'
storage
tank. Use pr~ssure drop data from' Fig. 2-4 and pump performance from F~g_ 2-5. ,ENeglectthe pressure drop in, the pipe fittings and pressure conversions due to,
kinetic energy.) Fill out Table 2.2.
TABLE 2.2
Design data for Prob. 2.1
Pipe Design flow, Us Apt kPa
size,
Pipe section rom Day NIght Day Night
(S to A. for example)