Experime nt3
1
INME 4032
Table of Contents
Pr incip le
3
Object ive
3
G oals
3
Background
4
a) Experiment al approach
4
§ Ove rall ef f icienc y
4
§ Temperat ure ef f iciencies
5
§ Ove rall heat t ransf er coef f icient U
6
b) Analyt ica l approach
7
Experiment al Set up
9
a) Tubular Heat Exchanger
10
§ Descript ion of t he Tubular Heat Exchanger
10
§ Technica l Dat a
11
b) Plat e Heat Exchanger
11
§ Descript ion of t he Plat e Heat Exchanger
11
§ Technica l Dat a
12
c) Shell & t ube heat exchanger
12
§ Descript ion of t he Shell & Tube Heat Exchanger
12
§ Technica l Dat a
13
Procedure
14
Discussion
14
Experime nt3
2
INME 4032
Uni versi ty o f Puer to Rico
Mayagüez Campus
Depar tmen t of Mechanical Engineering
INME 4032 - LABORATORY II
Spring 2004
I nst ructor: G uillermo Ara ya
Experiment 3: Heat Exchanger Analysis
Principle
This e xper iment is des igned t o acquire expe rience on heat exchangers (being t he most
usually f ound in indust ria l applicat ions: Tubula r, Plat e and Shell & Tube heat
exchangers) and t o underst and t he f act ors and paramet ers af f ect ing t he heat t ransf er
rat es.
Objecti ve
To acquire e xper ience on t hree basic heat exchangers (Tubular , Plat e and Shell &
Tube) and t o underst and t he f act ors and paramet ers af f ect ing t he rat es of heat t ransf er.
Goals
For co-current and counter-current operat ion of t he equipment and f low rat es (hot and
cold f luids ) specif ied b y t he inst ruct or, det ermine:
a) The heat lost t o t he surroundings.
b) The o vera ll ef f ic iency.
c) The t emperat ure ef f iciency f or t he hot and cold f luids.
d) The o vera ll heat t ransf er coef f icient U det ermined exper iment ally.
e) The overa ll heat t ransf er coeff icient U determined t heoret ically. Compa re wit h t he
exper iment al one.
Experime nt3
3
INME 4032
Background
The process of heat exchange bet ween t wo f luids t hat are at diff erent t emperat ures and
separat ed by a solid wall occurs in man y engineering applicat ions. The de vice used t o
imp lement t his exchange is called a heat exchanger, and specif ic applicat ions ma y be
f ound in space heat ing and air-condit ion ing, power product ion, wast e heat recover y and
chemical process ing. Heat exchangers are t ypically c lassif ied according t o f low
arrangement and t ype of const ruct ion. I n t he f irst classif icat ion, f lo w can be
count ercurrent or cocurrent (also called paralle l) . On t he other hand, according to their
conf igurat ion, heat exchangers can be labe led as t ubular, plat e and shell & t ube heat
exchangers.
a) Experimen tal approach
Overall e fficiency
To design o r pred ict t he perf ormance of a heat exchanger, it is essent ia l t o
det ermine t he heat lost to t he surrounding for t he analyzed conf igurat ion. We can
def ine a paramet er t o quant if y t he percent age of losses o r ga ins. Such para met er
may readily be obt ained by app lying o vera ll energ y balances f or hot and cold
f luids. I f Qe is t he heat power em it t ed f rom hot f luid, meanwh ile Qa t he heat power
absorbed by co ld f lu id (neglect ing pot ent ial and k inet ic energ y changes) ;
•
•
Qe = mh (hh,i − hh,o ) = mh Cph (Th,i − Th,o )
•
•
Qa = mc (hc,i − hc,o ) = mc Cpc (Tc,i − Tc,o )
Where,
•
•
m h , m c : mass f low rat e of hot and cold f luid, respect ive ly.
h h ,i , h h, o : inlet and out let ent halpies of hot f luid, respect ive ly.
Experime nt3
4
INME 4032
h c ,i , h c ,o : inlet and out let ent halpies of cold f lu id, respect ively.
Th, i , Th , o : inlet and out let t emperat ures of hot f luid, respect ively.
Tc ,i , Tc ,o : inlet and out let t emperat ures of cold f luid, respect ively.
Cp h , Cp c : specif ic heat s of hot and cold f luid, respect ive ly.
Heat power lost (or gained) : Q e − Q a
Percent age of losses or gains P =
Qa
Qe
× 100
I f t he heat exchanger is we ll insu lat ed, Qe and Qa shou ld be equa l. I n pract ice
t hese dif f er due t o heat losses or gains t o/f rom t he en viron ment .
The above f ormulas were deduct ed taking int o account t hat hot f luid is rounded
by co ld f lu id. I f t he average cold f luid t emperat ure is abo ve t he amb ient air
t emperat ure t hen heat will be lost t o t he surroundings resu lt ing in P < 100% . I f t he
average cold f luid t emperat ure is be low t he ambient t emperat ure, heat will be
gained result ing P> 100%.
Temperature e fficiencies
A usef ul measure of t he heat exchanger perf ormance is t he t emperat ure
ef f iciency of each f luid st ream . The t emperat ure change in each f luid st ream is
compared wit h t he ma ximum t emperat ure dif f erence bet ween t he t wo f luid
st reams giving a compar ison wit h an exchanger of inf init e s ize.
Experime nt3
5
INME 4032
Fig 1: Co unte rc u rre n t a nd Co c urre n t o p e ra tio n fo r a s he ll a nd tub e he a t e xc ha ng e r
Temperat ure ef f icienc y f or hot f luid η h =
Th, inlet − Th, outlet
Th ,inlet − Tc, inlet
Temperat ure ef f icienc y f or co ld f luid ηc =
Mean t emperat ure ef f iciency η m =
× 100
Tc , outlet − Tc , inlet
Th ,inlet − Tc, inlet
× 100
η h + ηc
2
Subscript s h and c st and f or hot and cold, respect ively.
Overall heat tr ansfer coefficien t U
Because t he t emperat ure dif ference bet ween t he hot and cold f luid st reams
var ies a long t he lengt h of t he heat exchanger it is necessar y t o der ive an a ve rage
t emperat ure dif f erence (driving f orce) f rom wh ich heat t ransf er calculat ions can
Experime nt3
6
INME 4032
be perf ormed. This average t emperat ure dif f erence is called t he Logarit hmic
Mean Temperat ure Dif f erence (LM TD) ∆t l m.
LMTD ∆t lm =
∆t 1 − ∆t 2
ln( ∆t 1 / ∆t 2 )
Where,
∆t 1 = T1-T4
∆t 2 = T2-T3
Not e: See FI G 1. t o ident if y t emperat ures in cocurrent and count erf low operat ion.
We can def ine an overa ll heat t ransf er coef f icient U as:
U=
Qe
A∆t lm
 W 
m 2 K 


Where,
Qe = Heat power em it t ed f rom hot f luid
A = Heat t ransmission area
b) Analytical approach
Up t o now, a met hodolog y t o e valuat e t he perf ormance of a det ermined heat
exchanger has been deve loped. Here, an analyt ica l st udy will be e xpla ined in
order t o underst and t he init ial st eps of t hermal and s izing des ign.
Analyt ica l met hods are only appro ximat e in order t o get an idea of t he heat
exchanger size. The overa ll heat t ransf er coef f icient is calculat ed assuming t hat
is const ant along all t he heat exchanger and can be predict ed wit h convect ion
correlat ions. Ne vert heless , t here are man y f act ors t hat af f ect t his value, f or
inst ance, t he inf luence of bubbles, corros ion, et c. Manuf act urers pro vide manuals
t hat cont ain inf ormat ion more precise regarding t he heat exchangers t hey t rade.
Experime nt3
7
INME 4032
Then, it is e xpect ed t hat t he t heoret ical values d if f er f rom t he experiment al ones,
f undament ally due t o t he presence of bubbles. Of course, experiment al result s
are mandat ory because t hey ref lect rea l condit ions of operat ion. Howe ver, f or
heat exchanger select ion it is conven ience t o have a met hodolog y in o rder t o
est imat e t he overall heat t ransf er coef f icient or t he size according t o given
t emperat ure range and f low specif icat ions.
Bef ore sett ing t he equat ion t hat det ermines t he Overall Heat Transf er Coeff icient ,
let ’s t ake some assumpt ions. The conduct ion resist ance bet ween hot and cold
f luid could be neg lect ed, also resist ance due t o f ouling.
U=
1
1/ h h + 1/ h c
Where,
hh : Heat t ransf er coeff icient of hot f luid [W /m2 K]
hc : Heat t ransf er coeff icient of cold f luid [W/ m2K]
I n order t o calculat e hh and hc , t he appropriat e correlat ion will be used.
For f low in c ircula r t ubes:
NuD : 4.36 (Lam inar f lo w, ReD < 2300)
Colburn equat ion
NuD : 0.023 ReD4/ 5 Pr1/ 3 (Turbulent f low, ReD > 2300)
Nu D =
hD
k
D: Diamet er of t ube
k: Conduct ivit y of f lu id
I f the t ube is non circula r, h yd raulic dia met er is used , inst ead.
Experime nt3
8
INME 4032
Dh =
4A c
P
Where Ac and P are t he cross-sect ional area and t he wett ed perimet er,
respect ive ly.
Experimen tal setup
There are t hree opt ional small-scale heat exchangers t hat can be inst alled t o illust rat e
t he principles and dif f erent t echniques of heat t ransf er bet ween f luid st reams. The heat
exchangers are ind ividually mount ed on a com mon bench-t op Heat Exchanger Ser vice
Unit . The unit supplies hot and co ld wat er st reams t o t he dif f erent heat exchangers
inst alled on it .
The f ollowing paramet ers can be mod if ied f or each sma ll-scale heat e xchanger:
vo lumet ric f low rat es of hot and cold f luids, hot f luid t emperat ure and f low arrangement s
(count ercurrent or cocurrent ).
Fig. 2: Heat Exchanger Service Unit with the Tubular Heat Exchanger installed.
Experime nt3
9
INME 4032
a) Tubular Heat Exchanger
Fig. 3: Tubular Heat Exchanger
Fig. 4: Diagram of tubular heat exchanger
under countercurrent operation.
Descrip tion of the Tubular Heat Exchanger :
Please ref er t o f igures 3, 4, and 5
The t ubular heat exchanger cons ist s of t wo concent ric (coa xia l) t ubes carr ying t he
hot and cold f luids. The t ubes are separat ed int o t wo sect ions.
Fig. 5: Diagram of tubular heat exchanger
under co-current operation.
Experime nt3
10
INME 4032
The accessor y consist s of t wo concent ric t ube heat exchangers arranged in series
in t he f orm of a U. The hot wat er f lo ws in t he inner t ube and cold f luid in t he out er
annulus. The equipment allo ws t he conversion f rom count ercurrent t o co-current
operat ion.
Six t emperat ure sensors are inst alled in t he hot and cold f luid in let s, out let s and
mid pos it ions.
Technical Data:
• Each inner t ube is const ruct ed f rom st ainless st eel t ube, 9.5 mm OD.
• Each out er annulus is const ruct ed f rom clear acr ylic t ube, 12.0m m I D.
• Each heat t ransfer sect ion is 330mm long giving a combined heat t ransf er
area of approximat ely 20000mm2. Heat t ransf er area is equiva lent t o t hat
of t he HT33 Shell and Tube Heat Exchanger.
b) Plate Heat Exchanger
Descrip tion of the Plate Heat Exchanger
The plat e heat exchanger consist s of a pack of plat es wit h sealing gasket s held
t oget her in a f rame bet ween end plat es. Hot and cold f luids f low bet ween channels
on alt ernat e sides of the plat es t o promot e heat t ransf er. The plat e heat exchanger
supplied is conf igured f or mu lt i-pass operat ion wit h passes in ser ies.
Fig. 7: Schematic diagram of plate heat
exchanger showing countercurrent fluid flow
Fig. 6: Plate heat exchanger
Experime nt3
11
INME 4032
The plat e heat exchanger consist s of a pack of seven plat es and gasket s arranged
f or mult i-pass operat ion wit h passes in se ries (Pat t ern of holes in t he plat es and
shape of t he gasket s det ermine t he direct ion of f low t hrough t he exchanger ). Four
t emperat ure sensors are inst alled at f luid inlet s and out let s.
Hot and cold f luid connect ions allow connect ion and conversion f rom
count ercurrent t o co-current operat ion.
Technical Data:
• Number of act ive p lat es 5
• Plat e o vera ll d imens ions: 75m m x 115m m
• Ef fect ive dia met er: 3.0m m
• Plat e t hickness: 0 .5mm
• Wet t ed perimet er: 153.0mm
• Pro ject ed heat t ransmission area 0.008m2 per plat e
• Correct ion f act or f or LMTD: F = 0.95
• Temperat ures are measured using t ype K t hermocouples wit h min iat ure
plug f or direct connect ion t o t he elect rical console on H T30X.
• Plat es are manuf act ured f rom 316 st ainless st eel.
c) Shell & tube heat exchanger
Descrip tion of the Shell & Tube Heat Exchanger
The shell and t ube heat exchanger consist s of a number of t ubes in parallel
enclosed in a cylindrica l shell. Heat is t ransf erred bet ween one f luid f lowing
t hrough t he t ubes and anot her f luid f lowing t hrough t he cylindr ical she ll around t he
Experime nt3
12
INME 4032
t ubes. Baff les are inc luded ins ide t he shell t o increase t he ve locit y of t he f luid t o
impro ve t he heat t ransf er.
The exchanger is designed t o demonst rat e liquid t o liquid heat t ransf er in a 1-7
shell and t ube heat exchanger (one shell and 7 t ubes wit h t wo t ransverse baf f les in
t he shell).
Technical Data:
• Hot f luid f lows in t he inner t ubes and cold f luid in out er shell. The se ven t ubes
are const ruct ed f rom st ainless st eel t ube, 6.35mm OD. The out er annulus,
end caps and baf f les const ruct ed f rom clear acr ylic. The lengt h of t ube
bundles 144mm (act ual lengt h of heat t ransf er region) giving nominal
combined heat t ransf er area of 20 000mm2.
• Heat t ransf er area is equ ivalent t o t hat of the Concent ric Tube Heat
Exchanger f or direct comparison.
• Cold f luid ent ers one end of t he shell at the bot tom and exit s at t he opposit e
end at t he top having f lowed o ver and under t wo t ransverse baf f les inside t he
shell.
• Temperat ures are measured using t ype K t hermocouples.
• Thermocoup les are inst alled at t he f ollowing 4 locat ions (when operat ed
count ercurrent ):
? Hot f luid inlet ( T1)
? Hot f luid out let (T2)
? Cold f lu id inlet ( T3)
? Cold f lu id out let ( T4)
Experime nt3
13
INME 4032
Fig. 8: Tube and shell heat exchanger.
Fig. 9: Schematic diagram of tube and shell heat
exchanger showing countercurrent fluid flow
Hot and cold f luid connect ions allow connect ion and conversion f rom
count ercurrent t o co-current operat ion.
The st ainless st eel t ubes can be remo ved f rom t he heat exchanger f or c leaning.
Procedure
Allow t he syst em t o reach st eady st at e, and t ake readings and make adjust ment s as
inst ruct ed in t he individual procedures f or each experiment . Record t emperatures, V, I ,
and ot hers if any. Repeat t he lect ures t hree t imes t o assure t hat t he syst em has reached
st eady st at e.
Comput e t he mean value and st andard deviat ion. Report your result s f or a conf idence
le vel of 95%.
Discussion
Experime nt3
14
INME 4032