Design of Thermal Systems_stoecker 3rd Edition

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The field of thermal ,system design and

analysis

continues to develop. The

number of workers is gro\\"ing, technical

papers

appear

in

greater numbers,

and new textbooks are _ being written. . .

The major objective of this

third

,edition

is

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 the

twelve 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 the

first twelve

chapters

in an advanced-level

undergraduate

course

and

the

remaining 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

seven

chapters.

Several of the new

chapters

are exten~ions of the introductions offered

in the lust twelve chapters, especially

mathematical

moqeling, steady~state

system

simulation',

and search methods.

Chapter

14

addresses

$.Orne of the challenges

that

arise when

simulating Jarge,

thennal

systems.

New

material

appears in

Chapter

15 on dynamic be'h~yior, in Chapter 18 which introduces

calculus of

variations

as a

companion to

dynamic programming, and in

Chapter 19 on probabilistic

approaches

to design, which is exploratory.

'

.,

The author thanks colleagues both at the

University

of

nlinois

at

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

their

many

useful

comments:

John R.

Biddle,

California

State

Polytechnic

.

X PREFACE TO TI-IE THIRD EDmON

University, Po'mona; Theodore F.

'

Smith,.

:

1be University

0'- 0

a; 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

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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~. systems

iqcludes

fans) pumps,

compressors,

engines,

expanders,

,'turbines,

heat

and mass

exchangers, and reactors .. all interconnected

with

some form of conduits. ' generally, the

.,working substances are fluids.

These

types 9f

systems

appear

in

such indus-tries as power generatio'n,

electric'and

gas utilities,

refrigeration,

air

condi-tioning

and heating. and

in

the food, chemical,

and

process industries .

.

'

xiv

PREFACE TO nIE FIRST EDITION

SYSTEMS

Engineering e¢lucation

is

predominantly

pro,cess )orielZted,

while engineering practice is predornioantly

system

'orien(eil

.

.

Most

cO,urses

of

study in

e;ngi-

" "

_' neering .proyide the student wi~

an

effec~ve exposure to such processes as the

flow

of a compressible fluid through' a

nozzle

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~es

4tld

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?1dainentals

may

result in subs~:' quent failure of the ~ystem. But -within a university enviroJ!illent, it is

ben-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 of

simulation

and optimization js

not

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 hand

calcula-'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 of

achiev-ing

a 11,10rkable system was a significant accomplishment

and

the objective of designing an optinll1J71

system

was

usually

abandoned. The

possibility

of

oprin1iz~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 how

this staten1enl derives

from the

physical situation,

the

chap-ters chat follow explore optimization procedure,s such as calculus

m~thods,

search methods, geometric programming, dynamic programming, and linear

programming. All these

merhods have

applicabiH ty'

10 nlany

other

types'

of

problems besides thennal ones

and,

·in this sense,

are

general.

On

'

the

other

hand, the applications are chosen

fr~m

the therma1 field

to

emphasize

the

opportunity

for

optimization 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 the

performance 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

be

a,

step

in

the optirmzation'

process

-but,

·may

hav'e

a

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

that

m~y

pinpoint

pos_s5hle

operatiI;tg and

c~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~en1

must

be

performed 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 graphical

representations. 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 an

optimum.

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. The

background

assumed is a

knowledge

of

thermodynamics 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 the

University 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 ~xperiences

in

the theilllal design· section' <?fMechanical" .' ~ . ... .

.

....

.

~

...

~~·::~··~~·~~:--~:··Ellgineering~,:N~Ws.

__

and ..

h.ave,

thus, pirectly

and

-indirectly c'oritributed

to

,

,

iaeas

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m anus'cn

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,is'-

the'"

':

:.

third

~

: id{tio~·~

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(jt.~

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t~:it~ ~in~ft te~a)·

::

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t:~ed~

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'.Design of Thermal Systems c611rse"at,th~ ,University 'ofTIlinois"at'Urbana---'-' . --Champaigll7.-I thank the students-Vl·ho have.worked with

me

in

this :course

for

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 and

a~

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 valuable

sugges-. 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 as

in

the technical feattlfeS of

thennal 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. and

Anita

have made the work on this book, as well as anyt .. hing else 1 dO'1 seem worthwhile.

W. F. Stoecker

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"

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

are

abstractions

'of

hardware.

-The

sub-ject

of the design may be a process,' an elem,ent

or

component of a

larger

assembly, or an entire system.

OUf emphasis will be

on

system design, .where a

system

is defined as a collection of

components

with interrelated

performance.

Even this definition often

needs

interpretation, becau-se a large system

sometimes

includes s·ubsystems. Furthe nn ore , we shall progressively

focus

on

ther-mal

systems,

where

fluids

and

energy in the form of heat anel work are

conveyed

and

converted. Before adjusting

this

focus,

howe'ver,

this

chapter

will examine the

larger picture

fn~o

which the

technical

engineering

activ-ity

blends. 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

I

state-. and federally

sponsored projects.

-~ ,..

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atteritioIi

has been devoted to the

.8..-.... ,rlIr . . ', " " .

e~gj~~erhi

:

g ~.dertakings.

Studies on

these

.-;

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a.,

,

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:

I~~k~i~Rr,t~1~~

,

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,(,~

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steps

:

'

iih~

:

:procedUres

used

'

m

.reaching decisions.

":'2:::t~f!::'·~,(',';n':;;;:;;jr;",,:":r:.~'n~ ... 'V"UL~.L~~~.~~.4 'Qttii~~e

'

studies'

has been to

stimllIate .enginee.rs.

to-reflect'

"

.

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,.>,:

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,.

:

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p

,

#

·:

'the

,,,

.

'

proces~

,

es

of

theillselvfs

and

oth~rs

·

o~ the pro

j

ect'

,'

teani.

"

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..

~q.I.~f?q~.ep'~e,9f.',s!.~p'?,,~~llowed

in

each undertaking

is

''''''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 the

next,

the" pIoc~dllre,s,'·':Pillst ' "',

v~.

_,

The advantage of analyzing the de<;:ision process, especjal~y

in

com~

plex undertakings, is that

it

,leads ,to

a

more logi~a1 coordinati0D of the ~aJ;ly individual efforts consti~ting the entire venture. The flow diagram

in

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

mop

it. , J

" The technical engineerlng occurs mostly

in

activities 5 and 7, product or system design and research an~ development. Ljttle will be said

in

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 acri

virie.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, but

it

is

Dot always

a simple task. For

example,

the

officials

of a city.may suppose that their need is to enlarge the reservoir so that

it'

can store a larger

quantity

of water for municipal purposes. The officials

may

not have specified the actual need but instead may have

leaped to

one possible solution. Perhaps the need

would

better have

been stated as a

low water reserve

d'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 soutCes

such

as

wells.

Sometime

'

s

·

possible-solutions are

precluded by

not

stating the need

properly

at

the 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 firm

may

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

t

typical

needs or

oppo~~J.t~rgiie

in

the

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 be

developed

intentionally Qf .

,accjperitally. Sometimes.

a new

use

.of

all

eXlsting proQ.uct·

can be

found

by making a slight modification of

it.

An orgahization

.maY

know how to"

manufacture

,

a

g~JJ?my, sticky

sup.stance and a.,ssign

'

to the research

,

and

developmefl;t department the

task

of

fiIid,

ing

som~. use

Jar it. . .

''':'~J

.' Of

interest

to us· a~ the ni~IPent is ~e nee~ or

()PP9rWni..ty

th.a~ r~q~~es: .. " '

engineering

design

at a

subsequent

stage:"

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,

..

-In·

c'ommercial ente'rprises the'

usual

mterion

of

sllcc,ess

is'

'

showillg

a ,profit~"

i.e.~ providing a certairi" rate of telurtr .on the -investment.

In

public' projects . the criterion

of

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 is

a

.dqmjn~tiDg influence on the ·decision

to

pro- .

ceed with the project. Strict monetary concerns are always tempered, bow~ ever: by. hunlan~ social,

and

political considerations to a. greater or ~esser

degree.

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 fewer

people. As an

alternative,

the plant may

spend

a lot on

superior

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 to

say

to a

prospective customer, "Yes, we can sell you product A, but'we recommend

product 'B 7~'

which is a more

profitable item in

the

company's line an~

may

acrually be

superior

to

product A. ," .

Often a decjsion, particularly in an emergency, appears

outside the

realm of economics. If a boiler providing steam for beating a

rental

office

building fails, the decjsion

\vhether

to repair or

replace

the

boiler inay seem,

to

be

outside the

realm of

econQmics.

The

question can still be considered

an 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

designs

are

a]ways dirdcted

roward the

future,

for which

only

probabiI

ity I

not

cenajnty,

is applicable: There is no absolute assurance

Chl;H the

pJanr will

n)eet the success. criteria discussed in Sec .. 1.4,

'Only

a

- li.k;]

l

ihood or probability

thai

it

wilJ

po

so.

The

mention of

probability suggests [he

nonna}

distribution

cUrY"e

(Fig.

1·2),

an excel~en[

starting

point

for expressing

uncertainty in the

decision-making

l?J:Ocess The significance of rhe distribution

_.

curve

lies

particolarly

• ; ' .- .4 ,' , -. ~ .. '. -f. , • .I ...... -;,-{# ... ;:; FIGURE 1-2

Probability distribution carve ..

,

!'. }' p:" -/ -, .x=a ENGINEERfNG DESIGN 5 ~ ·1 J ) -." .:':~ " ,- ./-.,.- _{-\ .} , '':' '.:':". ':"' .~-:'.-

:

'

\

'"

'- -.•• j •.•• ,. x

in 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's

occurring 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

=

1

The equation for

the probability distribution

curve is

. h _h2(.:r -a)2

. y

=

- - e

'

fo

. (1.1)

The maxiffium value of the

ordinate is

hlF, which occurs when x

==

a.

This fact suggests that increasing the value of

h

alters the sh₄pe of the distribution

curve, as

shown in

Fig.

1-3.

If

hI

is

greater

than

h2 ,

tbe

peak

of the

h

I

curve

rjses higher

than lhat

of the

h2

curve: . To extend the

probability

"

idea

to decision

making in an engineering undertaking, suppose that a new product or facility

is proposed and

that the criterion for

success

is a 10 percent

rate

of return on

the investment

for a

5-year

life of. the plant. After. a

preliminary

design, the

probab~ity

distribution curve i's shown as indicated

in

Fig

.

1-4. Since

rough

figures

were u~tIoaghout the

evaluation,

the

distribution

curve

is

,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 the

preliminary

design because costs have now

o

, 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 kllQWn

exac

tlY1

and th.e

.

:prob)abi1ifY;

,

distiibut~dn

·"

c~rve

-

degeD.e~tes-- .>degeD.e~tes--,"degeD.e~tes--'

into'

'a: c

urve-...

that:-

is

'

infinites tmally'

··ttli·riw·and'

:

:illfini-tely ... :;hi gh~,~:;' -;,',;\ .:~:o ;'>:->.:~

,-..

:~.,:-:~:.~;;. :'\.~.':.

-

~h¢

re¢9ghition

th.~~

·

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,"

,ar

possible, ,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 origin

or

improvement of the " basic idea, and development, work may supply .. \vorking mo_dels or ,a pilot

pJant, 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 research

group 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 involves

many

iterations. Each pass through the loop improves the amount and

tbe

quality of infQtmation and

data.,

Eventual]y la point

lS

reac~ed" ,where

final

decisions

are

made regardi~g

the

de~ignt

prod\lction,

and marketing of the

product.

The

substance

that circulates

through

~s

flow

diagram

is infonnatio

,

n

t

which

'

may be in. the form of reports and

~onyersatjons

a'nd

f!1ay

be both verbal and pictorial. The iterations are

'

accompl ished by

c6mmunicatron between people,

an'd

this

communication

is

interspersed

by go-~r-no-go decisions.

ENGINEERL"IG DESIGN 9

1.10

OPTI1VIIZA.TIOI\T O~F

O

P

ER

A.,.

TJO

l

r

1

: . )

The

1low

diagr:ln1 of Fig. 1-1 termipates with1the constnJction or begin~ing

of lnanufacture of a product.

or

service. Actually lli.~other stage takes over . at thi:s point,

which

seeks to

optimize

rh~ ope~ation of- a given facil.ity. The

facility was designed on the ba~is, of.qc~ain,9-~sig.r:t p~ram~Fer5. which almost

jnevit~bly

cha.11ge

by the

tirne

the

facility )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 this

book 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 entire

shell-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 systems

such 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 processes

discussed

in this

chapter are

highly

simplified and are nor sacred. Since

mmost

every

under-t~ing is different. there are

almost

infinite

variations

in startit:lg

points,

goals,

and

intervening circumstanc~. The

purpose of

the

study is to

empha-size the advantage of

sys.tematic

planning. Certain

functions

are

common in

the evaluation

and

pJanning of

undertakings,

particularly the iterations

and

10 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·

I

2.1 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 design

prpblem'" 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 workable

system

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 workable

toward

an optimum

system

may

not be justified because of limitations in

calendar

time,

cost

of

engi-neering time, or

even'

the

r~liability

,of the

fundamental data on which

the

design is·

based. One

point

to

be explored in this chapter is how superior solutions

may be

ruled out in

the

design process by prematurely

eliminating

so~e

system concepts. Superior soiutions may also

be

precluded

by

fixing

interconnecting parameters betweerl components and selecting the

compo-nents based on these parameters instead of letting the parameters float unti1

the optimum total

system

emerges.

~

12 DESIGN OF TIfERMAL SYS1EMS

2~2

A WORKTCAJ3LE SYSTEM

A .workable system is

one that .

1

. , .

, ',:'

I'

. 1 .

.

Meets the

requi:rem:ents

of

the

.

pllrpose~;,

of

the

system,

e.g.,

pr~viding

the. .,

req~ired

amount

of

power, heating.,

cooling, or

fluid

flow., or

sllITouild

.

ing

''''''.:~! •. ! •.•. . ,1. .

it

spac~'vith

a 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,

·

sllch

as

'size,'

.

we~ght~

.

i~~p~r~trires~

·'jp

r~ss~i'e';<

.'

~

'.' " materia) properti~s,

noise,

pollution,' etc. . .. . .

' . . -

-

-In

summary,' a

workable

system ·.perfortlls

.

the

'as~igned ·task

within

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 the

concept 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 'the

design

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 for

adequate exploration, discouragen1ent by supervision or environment, or the laziness and timjdity of the

engineer.

It

is

particularly

in selecting the concept that creativity· can be exercised. Too often only' one concept is ever considered, the concept

that

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 or

materials

on the market

D1ay make an

app'roach successful today that was not .attractive 1

a.

years

.

ago.

2.5

WOEKABLE VS. OPTIMUM 'SYSTEM

. <

The distinction between the approaches used in aniving

at

a workable system

and an optimum system· can be inustrated

by

a simple example. Suppose

that the pump and piping are to be selected to

convey

3

kgls

from one

location

to another 250 m

away from the original position and 8 m higher... If

the

design is

approached with the

limited objective of

achieving

a workable

system,

the

foJlowing procedure

might

be

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 kPa

Arbitrarily choose. an additional 100 kPa to COlnpensate for, fg~tion in the 250 ill of pipe. , ' . I ... .'.~' :'

.

,~."..

'. ' . '

, '~2o' itccording to'

the

foregoi]J,g

decision

1

select a p

ur

.o.p

,;vh:kI~l'.

delivers

3

legIs

,

'. '. -,--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 in

Ui 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.\1S

a 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~e

for.

the'

. pipe is the pump-pr~ssuIe rise

minus

78.5 .kPa. need~d for the difference in , '.elevation. An appropriate' optimization

technique

~ ~~

used

to deten;n1ne

the 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 is

a

small

one

"

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 further

example

"B(the contrast

between 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 be

constructed. 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 and

found

to be potentially piofita ble. We are likely I then, to

am

ve at a sol

utian

that

is an

improvement

over the arbitrary

selection.

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 located

near

(he producing area, in the market. city, or somewhere between. The size

\vilI be

strongly

influenced

by

the market

expectati'on. The third

decision,

the type of

freezing

plant,

embraces ,the engineering

desjgn~

T

h

ese

three

major ·decisions are interrelated.

For

example,

'

the

location

and' size

of

plant

might reasonably

influence

the type of system

selected.

The seJec£ion of the

freezing-DESfGNING A WORKABLE SYSTE:v{

15

).

plant design \vv.li be based. A.fter the

cO

.

ncept

has been decided:. the inter:nal

design 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 .plant

i~

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·:basis

o.f

the: ..

curre

.

nt

.'~.

-availability

of

the crop, the pqtential sale in the city, ~nd il vaiIahIe

fini!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 .computed

by

d.eciding upon the nUffi-.' ber of shIfts to ·be operated.

Assume

that one shift is s.elected, so that

DQ":' 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'ents

can

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 rate

of

heat transfer. Th~. air~cooling evaporator can be selected from

a

catalog because the airflow rate, air temperatures, 'and r~frigerant evaporating temp~rature fix the choice. The

compress'or must

provide 220 kW

of.r~frigeration with

an

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 commercially

available

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 product

first and

immersing

the p'ackage

in

cold brine until frozen; or

freezing

the

product with liquid nitrogen purchased

in

Jiquid fann in bulk. An example

of 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 ant

may

be

best located

,in .the, cjry .as ~n, ~xtension

of

e):.isting freezing facilities, and

it

may

be' un~jse to locate"

it

close to the produdng

area

because'

of

Jack

of t'rained operators. The liquid-nitrogen freezing p]ant,

.

on the

.

other

hand,

is

simple in

'operation

'

and

could

be

located close to

we field;

furthermore,

it

could

be

shut down

for

the

idle

off-season more

conveniently than

the

compression

plant.

If

the

possibiJiry of

two

or

even

three

shifts were considered.

·

the

processing

,

rate of the plant could be

reduced

py

a factor of·2 or 3

1

respectively. for the same

daily

throughput.

-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 each

component around lhose temperatures and resw.tlng flo'w rates.· 'Vhen one

approaches the design \vith the 'objective of optimization,

aU

those

inter-.' connecting

parameters

_are left .free to Jfloat. and one 'finds .. th~ comhination

..

~.'"

of

v2E~~~· of'tbeSe:"p8Iam~tef,s· :-vvhich

results. in'

"the

opri: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 with

given 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 2-3

SuppJy 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 2

FJow 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 6}

Flow 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. ,ENeglect

the 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)

..

' -( " I " ... .