8
HEAT TRANSFER AND HEAT EXCHANGERS
asic concepts of heat transfer are reviewed in this
chapter and applied primarily to heat exchangers,
which are equipment for the transfer of heat
between two fluids through a separating wall.
Heat transfer also is a key process in other specialized
equipment, some of which are treated in the next and other
chapters. The three recognized modes of heat transfer are by
conduction, convection, and radiation, and may occur
simultaneously in some equipment.
8.1. CONDUCTION OF HEAT
which demonstrates that use of a value at the average temperature
gives an exact result. Thermal conductivity data at several
temperatures of some metals used in heat exchangers are in Table
8.1. The order of magnitude of the temperature effect on k is
illustrated in Example 8.1.
B
In a solid wall such as Figure 8.l(a), the variation of temperature
with time and position is represented by the one-dimensional
Fourier equation
For the most part, only the steady state condition will be of concern
here, in which the case the partial integral of Eq. (8.1) becomes
dT
Q=-kA--,
dx
assuming the thermal conductivity k to be independent of
temperature. Furthermore, when both k and A are independent of
position,
AT kA
Q=-kA-=-(To-TL),
A r L
(8.3)
in the notation of Figure 8.l(a).
Equation (8.3) is the basic form into which more complex
situations often are cast. For example,
when the area is variable and
Q = WW,,,
(8.5)
(C)
in certain kinds of heat exchangers with variable temperature
difference.
THERMAL CONDUCTIVITY
Thermal conductivity is a fundamental property of substances that
basically is obtained experimentally although some estimation
methods also are available. It varies somewhat with temperature. In
many heat transfer situations an average value over the prevailing
temperature range often is adequate. When the variation is linear
with
k = ko(l + a T ) ,
(8'6)
the integral of Eq. (8.2) becomes
Q ( L / A ) = k O [ T , -T2+0.5&(T:- T:)]
=ko(Tl- TZ)[1+0.5a(T,+ T+)],
(8.7)
9
(e)
9
(0
8.1. Temperature profiles in one-dimensional conduction of
heat. (a) Constant cross section. (b) Hollow cylinder. (c) Composite
flat wall. (d) Composite hollow cylindrical wall. (e) From fluid A to
fluid F through a wall and fouhng resistance in the presence of
eddies. (f) Through equivalent fluid films, fouling resistances, and
metal wall.
Figure
169
170 HEAT TRANSFER AND HEAT EXCHANGERS
TABLE 8.1. Thermal Conductivities of Some Metals
Commonly Used in Heat Exchangers
[kBtu/(hr)(sgft)W/ft)l
is the logarithmic mean radius of the hollow cylinder. This concept
is not particularly useful here, but logarithmic means also occur in
other more important heat transfer situations.
Temperature ("F)
Metal or Alloy
Steels
Carbon
t
I Cr MO
41 0
304
316
Monel400
Nickel 200
lnconel 600
Hastelloy C
Aluminum
Titanium
Tantalum
Copper
Yellow brass
Admiralty
-100
-
70
225
56
55
64
8.1
11.6
-
-
11.8
-
COMPOSITE WALLS
1000
The flow rate of heat is the same through each wall of Figure 8.l(c).
In terms of the overall temperature difference,
30.0
19.2
13.0
9.4
9.4
12.6
32.5
8.6
7.3
131
11.5
31.8
225
69
-
200
27.6
19.1
14.4
10.0
-
13.8
31.9
9.1
5.6
133
10.9
222
-
22.2
18.0
(8.13)
-
13.7
13.0
22.0
30.6
14.3
10.2
-
12.1
209
-
where U is the overall heat transfer coefficient and is given by
(8.14)
The reciprocals in Eq. (8.14) may be interpreted as resistances to
heat transfer, and so it appears that thermal resistances in series are
additive.
For the composite hollow cylinder of Figure 8.l(d), with length
N,
(8.15)
HOLLOW CYLINDER
As it appears on Figure 8.l(b), as the heat flows from the inside to
the outside the area changes constantly. Accordingly the equivalent
of Eq. (8.2) becomes, for a cylinder of length N,
dT
Q = -kN(2zr)-,
dr
With an overall coefficient U, based on the inside area, for example,
Q =2nNr,U,(T1 - T4)=
2nN( Tl - T4)
l/U,ri
'
(8.16)
On comparison of Eqs. (8.15) and (8.16), an expression for the
inside overall coefficient appears to be
of which the integral is
(8.17)
(8.9)
This may be written in the standard form of Eq. (8.4) by taking
Am = 2xLNr,,
(8.10)
and
L=r2-rl,
(8.11)
where
EXAMPLE
8.1
Conduction through a Furnace Wall
A furnace wall made of fire clay has an inside temperature of
1500°F and an outside one of 300°F. The equation of the thermal
conductivity is k = 0.48[1 + 5.15(E - 4)Tl Btu/(hr)(sqft)("F/ft).
Accordingly,
Q(L/A) = 0.48(1500 - 300)[1+ 5.15(E - 4)(900)] ~ 0 . 7 0 3 .
If the conductivity at 300°F had been used, Q(L/A) = 0.554.
In terms of the logarithmic mean radii of the individual cylinders,
-1= r i [
U,
l
+
l
+
kormo/(r2 - rl) kbrmb/(r3 - r2)
which is similar to Eq. (8.14) for flat walls, but includes a ratio of
radii as a correction for each cylinder.
FLUID FILMS
Heat transfer between a fluid and a solid wall can be represented
by conduction equations. It is assumed that the difference in
temperature between fluid and wall is due entirely to a stagnant film
of liquid adhering to the wall and in which the temperature profile is
linear. Figure 8.l(e) is a somewhat realistic representation of a
temperature profile in the transfer of heat from one fluid to another
through a wall and fouling scale, whereas the more nearly ideal
Figure 8.l(f) concentrates the temperature drops in stagnant fluid
and fouling films.
Since the film thicknesses are not definite quantities, they are
best combined with the conductivities into single coefficients
h = k/L
(8.18)
so that the rate of heat transfer through the film becomes
Q = hAAT.
(8.19)
Through the five resistances of Figure 8.l(f), the overall heat
8.1. CONDUCTION OF HEAT
EXAMPLE
8.2
Effect of Ignoring the Radius Correction of the Overall Heat
Transfer Coefficient
The two film coefficients are 100 each, the two fouling coefficients
are 2000 each, the tube outside diameter is 0.1 ft, wall thickness is
0.01 ft, and thermal conductivity of the metal is 30:
ri/ro= 0.04/0.05 = 0.8,
r, = (0.05 - 0.04)/ln 1.25 = 0.0448,
rm/ro= 0.8963,
Uo= [1/100(0.8) + 1/2000(0.8) + 1/(30/0.01)(0.8963)
171
Basing on the inside area,
U, = [1/100 + l/2000 + [(30/0.01)(0.0448/0.04)1-]
+ 0.8/100 + 0.8/2000]-'
= 52.0898.
Ignoring the corrections,
U = (2/100 + 2/2000 + 1/30/0.01)-'
+ 1/100 + 1/2000]-' = 41.6721.
= 46.8750.
The last value is very nearly the average of the other two.
transfer coefficient is given by
(8.20)
where L , is the thickness of the metal.
If the wall is that of hollow cylinder with radii r, and r,, the
overall heat transfer coefficient based on the outside surface is
where r,,, is the mean radius of the cylinder, given by Eq. (8.12).
Since wall thicknesses of heat exchangers are relatively small
and the accuracy of heat transfer coefficients may not be great, the
ratio of radii in Eq. (8.21) often is ignored, so that the equation for
the overall coefficient becomes simply
1
1 1
1
1
+-+- 1 +-+hi hz k3lL3 h, hs
(8.22)
_=-
u
The results of the typical case of Example 8.2, however, indicate
that the correction may be significant. A case with two films and
two solid cylindrical walls is examined in Example 8.3.
EXAMPLE
8.3
A Case of a Composite Walk Optimum Insulation Thickness
for a Steam Line
A 3 in. IPS Sched 40 steel line carries steam at 500°F. Ambient air is
at 70°F. Steam side coefficient is 1000 and air side is
3Btu/(hr)(sqft)("F). Conductivity of the metal is 30 and that of
insulation is 0.05 Btu/(hr)(sqft)("F/ft). Value of the steam is
$S.OO/MBtu. cost of the insulation is $1.5/(yr)(cuft). Operation is
8760 hr/yr. The optimum diameter d of insulation thickness will be
found.
Pipe:
do = 0.2917 ft,
d, = 0.2557 ft,
In(do/di) = 0.1317.
Insulation:
In(do/di) = ln(d/0.2917).
(1)
Heat transfer coefficient based on inside area:
1
0.1317+ln(d/0.2917)
U,=di __
0
[lOOOd, 3
0.05
Q/Ai = UiAT = 430U,..
+
1
+3d]
'
3
70 F
18 ! E x a m p l e 8 . 3 . O P t i m u r n insul
ation thickness
2r3 RERD D 1 .*D 2
38 DATR .2317,.2557
48 INPUT D
5 8 Ul=.BBl/DZ+. 1387/38+LOG(D./Di
j/.85+1/3/D
6 8 U= 1 /U 1 l D 2
78 @=4JB*U
89 C1=.8438tG!
98 CZ=1.5*(Dn2-D1*2)/D2rr2
189 C=Cl+C2 ! ( r e q ' d t o be minim
Urn j
118 PRINT USING 128
Steam cost:
I
DD.DD,X,DD.DDDD
C, = 5(10-6)(8760)Q/Ai
= 0.0438Q/Aj,
j
DrU,Cl,C2>
c
1 28 I MRGE . DDD ,X . DUD ,X DD . DD . X ..
$ (yr)(sqft inside).
(3)
172 HEAT TRANSFER AND HEAT EXCHANGERS
EXAMPLE
8.S-(continued)
D
U
c2
- - - ,498
,494
,495
.496
.SO8
6.66
.349 6 . 5 7
,347 6.S4
346
6.52
.?I41 6 . 4 3
.354
Total cost:
c1 + c 2
C = C , + C2+ minimum.
3 . 5 6 1r3.2147
3.65 18.2118
3 . 6 7 18.2117-w
3.659 1 0 . 2 1 1 8
3.78 16.2148
Insulation cost:
C , = 1.5Kn,/A,
- 1.5(dZ- 0.2917’)
, $/(yr)(sqft inside).
(0.2557)’
(4)
Substitute Eqs. (2)-(4) into Eq. (5). The outside diameter is the
key unknown.
The cost curve is fairly flat, with a minimum at d = 0.50ft,
corresponding to 1.25 in. thickness of insulation. Some trials are
shown with the computer program. A more detailed analysis of
insulation optima is made by Happel and Jordan [Chern. Process
Econ., 380 (1975)], although their prices are dated. Section 8.12
also discusses insulation.
+ * < -
Heat transfer coefficients are empirical data and derived
correlations. They are in the form of overall coefficients U for
frequently occurring operations, or as individual film coefficients
and fouling factors.
(5)
,
T
,hot
(AT),
(AT),
cold
II
II
T’
8.2. MEAN TEMPERATURE DIFFERENCE
In a heat exchanger, heat is transferred between hot and cold fluids
through a solid wall. The fluids may be process streams or
independent sources of heat such as the fluids of Table 8.2 or
sources of refrigeration. Figure 8.2 shows such a process with inlet
and outlet streams, but with the internal flow pattern unidentified
because it varies from case to case. At any cross section, the
differential rate of heat transfer is
dQ = U(T - T’) dA = -mc d T = m’c’ dT’.
(8.23)
The overall heat transfer rate is represented formally by
Figure 8.2. Terminal temperatures and temperature differences of a
heat exchanger, with unidentified internal flow pattern.
SINGLE PASS EXCHANGER
The simplest flow patterns are single pass of each fluid, in either the
same or opposite directions. Temperature profiles of the main kinds
of thermal behavior are indicated on Figure 8.3(a). When the
unbroken lines [cases (a)-(e)] are substantially straight, the mean
temperature is expressed in terms of the terminal differences by
Q = UA(AT),.
(8.25)
.-
The mean temperature difference (AT), depends on the terminal
temperatures, the thermal properties of the two fluids and on the
flow pattern through the exchanger.
This is called the logarithmic mean temperature difference. The
temperature profiles are straight when the heat capacities are
TABLE 8.2. Properties of Heat Transfer Media
Medium
Trade Name
-
Electricity
Water
Water
Flue gas
Diphenyl-diphenyl oxide eutectic
Dowtherm A
Di + triaryl cpds
Ethylene glycol, inhibited
Dimethyl silicones
Dowtherm G
DOWSR-1
Dow Syltherm
Mixed silanes
Aromatic mineral oil
Hydrotherm
Mobiltherm,
Mobil
Therminol,
Monsanto
Hi-Tec. DuPont
-
“F
atm,
gage
100-4500
200-1 100
300-400
100-2000
0-300
6-15
0-7
450-750
0-9
nontoxic, carbonizes at high temp
20-700
-40--250
-40-750
0-3
0
0
sensitive to oxygen
acceptable in food industry
low toxicity
liquid
liquid
-50-675
100-600
0
0
react with oxygen and moisture
not used with copper based materials
liquid
50-600
0
toxic decomposition products
liquid
liquid
vapor
300-1100
100-1400
600-1000
0
resistant alloys needed above 850°F
stainless steel needed above 1000°F
low pressure vapor, toxic, and expensive
Phase
vapor
liquid
gas
liquid or
vapor
liquid
liquid
liquid
-
Remarks
-
800
Chlorinated biphenyls
Molten nitrites and nitrates of K and Na
Sodium-potassium eutectic
Mercury
0
0-12
8.2. MEAN TEMPERATURE DIFFERENCE
173
r Shell fluid
1
I
I
I
I
I
'
' e
I
I
I
-Tube
fluid
CDNDENEING
COOLING
~~
01
BOTH FLUIDS
PHASE
01 COUNTERFLOW , NO PHASE
CHANGE
CHANGING
CONDENSING
--
1,
P I
SUBCOOLING
CONDENSING
Shell fluid
HEATING
b l ONE
FLUID
PHASE
CHANGING
I1 ONE
FLUID CHANGING
PHASE
I
COOLING
1
Tube
fluid
\
COOLING
E VAPOR AT ING
SUPERHEATING
t
HE AT ING
~
CI ONE
fLUlD
PHASE
ONE FLUID
PL1ASE
CHANGING
CHANGING
PARTIAL
CONOENSATION
12
HEATING
6) PARALLEL F L O W , NO
PHASE CHANGE
HE4TING
CONOE NSABLE A N 0
NON -CONDENSABLE
COMPONENTS
Figure 8.3. Temperature profiles in heat exchangers. (a) In parallel or countercurrent flow, with one or two phases. (b) One shell pass, two
tube passes. (c) Two shell passes, four tube passes.
substantially independent of temperature over the range of the
process, or when a phase change occurs at constant temperature.
When the profiles consist of linear sections, as in cases ( f ) and
(g), the exchanger can be treated as a three-section assembly, each
characterized by its own log mean temperature difference, for which
intermediate temperatures may be found by direct calculation or by
trial. Heat transfer for a case such as (h) with continuously curved
profile must be evaluated by integration of Eq. (8.23).
each stream flows without lateral mixing, for instance in equipment
like Figure 8.6(h). In Figure 8.6(i) considerable lateral mixing
would occur on the gas side. Lateral mixing could occur on both
sides of the plate exchanger of Figure 8.6(h) if the fins were absent.
Mean temperature differences in such flow patterns are obtained
by solving the differential equation. Analytical solutions have been
found for the simpler cases, and numerical ones for many important complex patterns, whose results sometimes are available in
generalized graphical form.
MULTIPASS EXCHANGERS
For reasons of compactness of equipment, the paths of both fluids
may require several reversals of direction. Two of the simpler cases
of Figure 8.3 are (b) one pass on the shell side and two passes on
the tube side and (c) two passes on the shell side and four on the
tube side. On a baffled shell side, as on Figure 8.4(c), the dominant
flow is in the axial direction, so this pattern still is regarded as single
pass on the shell side. In the cross flow pattern of Figure 8.5(c),
F-METHOD
When all of the terminal temperatures are known, the mean
temperature difference is found directly from
(AT),
= F(AT)logmeanr
(8.26)
where the correction factor F depends on the flow pattern and is
174 HEAT TRANSFER AND HEAT EXCHANGERS
Fluid A inlet
Hot Fluid
Cold Fluid
Outlet
Outlet
(C)
Vopor
Steam
lory
les
t
Condensote Liquid
outlef
feed
t
Y
Tube
Hot liquid
supports
outlet
(4
Figure 8.4. Example of tubular heat exchangers (see also Fig. 8.14). (a) Double-pipe exchanger. (b) Scraped inner surface of a double-pipe
exchanger. (c) Shell-and-tube exchanger with fixed tube sheets. (d) Kettle-type reboiler. (e) Horizontal shell side thermosiphon reboiler. ( f )
Vertical tube side thermosiphon reboiler. (g) Internal reboiler in a tower. (h) Air cooler with induced draft fan above the tube bank. (i) Air
cooler with forced draft fan below the tube bank.
8.2. MEAN TEMPERATURE DIFFERENCE
175
Vapor
control
f- -t u
I
1
Bottoms
(e)
t’
L-.
t
a-x
+
Bottoms
(f)
1
Bottoms
(9)
Section-suwt
t
fluid out-
(h)
F k r e 8.4.-(continued)
Figure 8.5. Correction factor F, effectiveness and number of transfer units in multipass and cross flow heat exchangers (Bowmanet al., Trans
ASME 283, 1940; Kays and London, 1984):
T on the tubeside, T’ on the shellside. i = input, o = output. (a) One pass on shellside, any multiple of two passes on tubeside. (b) Two
passes on shell side, any multiple of four on tubeside. (c) Cross flow, both streams unmixed laterally. (d) Cross flow, one stream mixed
laterally. (e) Cross flow, both streams mixed laterally. ( f ) Effectiveness and number of transfer units in parallel and countercurrent flows. (g)
Three shell passes, multiples of six on tubeside. (h) Four shell passes, multiples of eight on tubeside. (i) Five shell passes, multiples of ten on
tubeside. (j) Six shell passes, multiples of 12 on tubeside.
176 HEAT TRANSFER AND HEAT EXCHANGERS
P
P
P
0
1
2
1
2
4
3
5
P
0
Figure 8.5-(continued)
3
NTU
4
5
8.2. MEAN TEMPERATURE DIFFERENCE
177
cci
,
:,+
'
o!
110
'
'
210 NTU
'
310
'
4;
0
5.;
0
1.0
20
mu
3.0
4.0
5.0
1.0
0.9
2" 0.8
I
c
2 0.7
e
b
u
0.6
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
K
(9)
1.0
1.0
0.9
0.9
rs
t.
5
--
0.8
-;
0.8
u
c
c
0.7
0.7
0.6
0.6
0.5
0.5
2
E
V
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
X
K
(i)
(i)
Figure IS-(continued)
expressed in terms of these functions of the terminal temperatures:
T-Zactual heat transfer
p="---maximum
possible heat transfer '
Ti - 7;
(8.27)
(8.28)
Some analytical expressions for F are shown in Table 8.3, and more
graphical solutions in Figure 8.5.
This method is especially easy to apply when the terminal
temperatures are all known, because then F and (AT)logmeanare
immediately determinable for a particular flow pattern. Then in the
heat transfer equation
Q = UAF(AT),,
(8.29)
any one of the quantities Q, U,or A may be found in terms of the
others. A solution by trial is needed when one of the terminal
temperatures is unknown, as shown in Example 8.4. The next
178 HEAT TRANSFER AND HEAT EXCHANGERS
(e)
(9)
Liquid flow
(i)
Figure 8.6. Examples of extended surfaces on one or both sides. (a) Radial fins. (b) Serrated radial fins. (c) Studded surface. (d) Joint
between tubesheet and low fin tube with three times bare surface. (e) External axial fins. ( f ) Internal axial fins. (9) Finned surface with
internal spiral to promote turbulence. (h) Plate fins on both sides. (i) Tubes and plate fins.
179
8.3. HEAT TRANSFER COEFFICIENTS
TABLE 8.3. Formulasfor Mean Temperature Differenceand
Effectiveness in Heat Exchangers
method to be described, however, may be more convenient in such
a case.
&METHOD
One measure of the size of heat transfer equipment is the number
of transfer units N defined by
(8.30)
N = UA/Cmin,
1.
where Cminis the smaller of the two products of mass flow rate and
heat capacity, mc or m’c‘. N is so named because of a loose analogy
with the corresponding measure of the size of mass transfer
equipment.
A useful combination of P and N is their ratio
Parallel or countercurrent flow,
(AT),
= (AThogmean
= (AT, - AT,)/In(AT,/AT,).
2. In general,
(AT),
= F(AThog,,,,
(AT),
= 8(7;.- 71,
or
r)
where f and 8 depend on the actual flow paths on the shell and tube
sides and are expressed in terms of these quantities:
c = c,in/~max.
P= (To - 7;.)/(7
- 6)=actual heat transfer/
(maximum possible heat transfer),
R = (7;. - To)/(TA- J’)= m’c’/mc.
3. Number of transfer units, N or NTU, is
N = UA/Cmi,= P/8,
where Cmi, is the smaller of t h e two values mc or m’c’ of the
products of mass rate of flow times the heat capacity.
4. In parallel flow, P = N8 = { 1 - exp[-N(l + C)1}/(1+ C).
5. In countercurrent flow, P = N8 = { 1 - expl-N(l - C)]}/{
1 - C exp
l-N(1 -CHI.
6. One shell pass and any multiple of two tube passes,
F=-
m.
In(l-P)/ln[
R-1
1-PR
2 - P(R + 1 - G)
R+1,
2-P(R+l+mTi) ’
]
where (To is the temperature change of the stream with the
smaller value of mc. Thus 0 is a factor for obtaining the mean
temperature difference by the formula:
(AT), = 0(T: -
r)
(8.32)
when the two inlet temperatures are known.
The term P often is called the exchanger effectiveness.
Equations and graphs are in Table 8.3 and Figure 8.4. Many graphs
for 8, like those of Figure 8.7, may be found in the Heat Exchanger
Design Handbook (HEDH, 1983). When sufficient other data are
known about a heat exchange process, an unknown outlet
temperature can be found by this method directly without requiring
trial calculations as with the F-method. Example 8.5 solves such a
problem.
SELECTION OF SHELL-AND-TUBE NUMBERS OF PASSES
A low value of F means, of course, a large surface requirement for
a given heat load. Performance is improved in such cases by using
several shells in series, or by increasing the numbers of passes in the
same shell. Thus, two 1-2 exchangers in series are equivalent to one
large 2-4 exchanger, with two passes on the shell side and four
passes on the tube side. Usually the single shell arrangement is
more economical, even with the more complex internals. For
economy, F usually should be greater than 0.7.
EXAMPLE
7.
A shell side fluid is required to go from 200 to 140°F and the tube
side from 80 to 158°F. The charts of Figure 8.5 will be used:
Two shell passes and any multiple of four tube passes,
m -]/
1-P
F = [-In
2(R-1)
1-PR
2/P-1 - R + ( 2 / P ) ~ ( l - P ) ( l - P R ) + ~
2/P - 1 - R + (2/P)Y(l - P)(1 - P R ) -
rn1
8. Cross flow,
(a) Both streams laterally unmixed, P= 1 -exp{[exp(-NCn)- l]/Cn},
where n = N-o.22.
(b) Both streams mixed, P = (1/11 - exp(-N)I C/[1 - exp(-NC)] -
+
l/N}-’.
mixed, Cmi, unmixed, P=(l/C){l -exp[-C(1 - e-N)]}.
mixed. ,
C
unmixed, P = 1 - exp{-(l/C)[l -exp(-NC)]).
9. For more complicated patterns only numerical solutions have been
made. Graphs of these appear in sources such as Heat Exchanger
Design Handbook (HEOH. 1983) and Kavs and London (1984).
(c) C
,,
(d) Cmi,
P = (200 - 140)/(200 - 80) = 0.5,
R (158 - 80)/(200 - 140) = 1.30.
For a 1-2 exchanger, F = 0.485:
2-4
0.92
4-8
0.98
The 1-2 exchanger is not acceptable, but the 2-4 is acceptable. If
the tube side outlet were at 160 instead of 158, F would be zero for
the 1-2 exchanger but substantially unchanged for the others.
8.3. HEAT TRANSFER COEFFICIENTS
Data are available as overall coefficients, individual film coefficients,
fouling factors, and correlations of film coefficients in terms of
180 HEAT TRANSFER AND HEAT EXCHANGERS
EXAMPLE
8.4
Performance of a Heat Exchanger with the F-Method
Operation of an exchanger is represented by the sketch and the
equation
tabulation:
T
Q / U A = 50 = F(AT),,
80
Ia
7
100
L 120
The outlet temperature of the hot fluid is unknown and designated
by T. These quantities are formulated as follows:
p=- 200- T
200-80'
R = - 200 - T
120 - 80 '
T - 80 - (200 - 120)
(AT)''"=In[(T -80)/(200- 12O)J
F is represented by the equation of Item 6 of Table 8.3, or by
Figure 8.4(a). Values of T are tried until one is found that satisfies
G = 50 - F(AT),,
0. The printout shows that
--
T = 145.197
The sensitivity of the calculation is shown in the following
physical properties and operating conditions. The reliabilities of
these classes of data increase in the order of this listing, but also the
ease of use of the data diminishes in the same sequence.
OVERALL COEFFICIENTS
The range of overall heat transfer coefficients is approximately
10-200 Btu/(hr)(sqft)("F). Several compilations of data are
available, notably in the Chemical Engineers Handbook (McGrawHill, New York, 1984, pp. 10.41-10.46) and in Ludwig (1983, pp.
70-73). Table 8.4 qualifies each listing to some extent, with respect
to the kind of heat transfer, the kind of equipment, kind of process
stream, and temperature range. Even so, the range of values of U
usually is two- to three-fold, and consequently only a rough
measure of equipment size can be obtained in many cases with such
data. Ranges of the coefficients in various kinds of equipment are
compared in Table 8.5.
FOULING FACTORS
Heat transfer may be degraded in time by corrosion, deposits of
reaction products, organic growths, etc. These effects are accounted
for quantitatively by fouling resistances, l/hP They are listed
separately in Tables 8.4 and 8.6, but the listed values of coefficients
include these resistances. For instance, with a clean surface the first
listed value of U in Table 8.4 would correspond to a clean value of
U = 1/(1/12-0.04)=23.1. How long a clean value could be
P
R
(AT),,
F
G
145.0 0.458 1.375 72.24 0.679 0.94
145.197 0.457 1.370 72.35 0.691 0.00061
145.5 0.454 1.363 72.51 0.708 -1.34
1 9 ! E x a m p l e 8 . 4 . T h e F-met hod
28 SHORT P . S R , F , T l
39 INPlJT T
1.' P = ( 2 9 B - T ) / . 1 2 @
50 R = ( Z B # - T j / 4 8
68 Tl=(T-168j/LOG({T-SBj/~8)
78 E = C R * Z + 1 ) * . 5
88 F=E/(R-lltLOG({l-P),(l-P*R)~
38
F=F/LOG((2-Pt{R+l-E)j/~~-P*~
R+ 1+E)
>j
315 G z 5 8 - F t T 1
198
110
128
138
148
158
168
178
T=
PRINT "T=";T
P R I N T "G="jC
PRINT "P=" ;P
PRINT "R="iR
PRINT "F="jF
PRINT " T l = " i T l
GOTO 30
END
145.197
G= .98248286
P= . 4 5 6 6 9
R= 1 . 3 7 8 1
F= , 6 4 1 8 9
Tl= 72.346
maintained in a particular plant is not certain. Sometimes fouling
develops slowly; in other cases it develops quickly as a result of
process upset and may level off. A high coefficient often is
desirable, but sometimes is harmful in that excessive subcooling
may occur or film boiling may develop. The most complete list of
fouling factors with some degree of general acceptance is in the
TEMA (1978) standards. The applicability of these data to any
particular situation, however, is questionable and the values
probably not better than +SO%. Moreover, the magnitudes and
uncertainties of arbitrary fouling factors may take the edge off the
importance of precise calculations of heat transfer coefficients. A
brief discussion of fouling is by Walker (1982). A symposium on this
important topic is edited by Somerscales and Knudsen (1981).
INDIVIDUAL FILM COEFFICIENTS
Combining individual film coefficients into an overall coefficient of
heat transfer allows taking into account a greater variety and range
of conditions, and should provide a better estimate. Such individual
coefficients are listed in Tables 8.6 and 8.7. The first of these is a
very cautious compilation with a value range of 1.5- to 2-fold.
Values of the fouling factors are included in the coefficient listings
of both tables but are not identified in Table 8.7. For clean service,
for example, involving sensible heat transfer from a medium organic
to heating a heavy organic,
U = 10,000/(57 - 16 + 50 - 34) = 175
8.3. HEAT TRANSFER COEFFICIENTS
--
E. 5
Er-
5 I
-8.4
2. 5
3.0
-;8.5
E?
0 A-
t=.
181
+
2. 8
2. 5
a 4
3.0
(b)
Figure 8.7. 0 correction charts for mean temperature difference: (a) One shell pass and any multiple of two tube passes. (b) Two shell passes
and any multiple of four tube passes. [(HEDH, 1983); afier Mueller in Rohsenow and Hartnett, Handbook of Heat Transfer, Section 18,
McGraw-Hill, New York, 1973. Other cases also are covered in these references.]
182 HEAT TRANSFER AND HEAT EXCHANGERS
EXAMPLE
8.5
Application of the Effectiveness and the 8 Method
Operating data of an exchanger are shown on the sketch. These
data include
V A = 2000,
m'c' = 10o0, mc = 800,
C = Cmin/Cmax
= 0.8.
m'c' = 1000
N = UA/Cmin
= 2000/800 = 2.5,
c = C,/C,
= 0.8,
D=
= 1.2806,
2
P=
1 + C + D[1+ exp(-ND)]/l - exp(-ND) = 0.6271,
e = P/N = 0.2508,
AT, = e(200 - 80) = 30.1,
Q = UA(AT), = 2000(30.1) = 60,200,
= 800(200 - &) = 1000(T; - 80),
:. & = 124.75,
Ti = 140.2.
also may be found from the definition of P:
UA = 2000
T;
The equation for effectiveness P is given by item 6 of Table 8.3 or it
can be read off Figure 8.4(a). Both P and 0 also can be read off
Figure 8.4(a) at known N and R = CJC, = 0.8. The number of
compared with a normal value of
V = 10,000/(57 + 50) = 93,
where the averages of the listed numbers in Table 8.6 are taken in
each case.
METAL WALL RESISTANCE
With the usual materials of construction of heat transfer surfaces,
the magnitudes of their thermal resistances may be comparable with
the other prevailing resistances. For example, heat exchanger
tubing of 1/16in. wall thickness has these values of l/h, = L / k for
several common materials:
Carbon steel
Stainless steel
Aluminum
Glass
transfer units is
l / h , = 1.76 x
5.54 x 1 0 - ~
0.40 x 1 0 - ~
79.0 x 1 0 - ~
which are in the range of the given film and fouling resistances, and
should not be neglected in evaluating the overall coefficient. For
example, with the data of this list a coefficient of 93 with carbon
steel tubing is reduced to 88.9 when stainless steel tubing is
substituted.
actual AT =200 - T2 = o.6271,
max possible AT 200 - 80
.: T2 = 124.78.
P=
With this method, unknown terminal temperatures are found
without trial calculations.
The Nusselt number, hL/k = h / ( k / L ) , is the ratio of effective
heat transfer to that which would take place by conduction through
a film of thickness L.
The Peclet number, D G C / k = G C / ( k / D )and its modification,
the Graetz number wC/kL, are ratios of sensible heat change of the
flowing fluid to the rate of heat conduction through a film of
thickness D or L.
The Prandtl number, C p / k = ( p / p ) / ( k / p C ) ,compares the rate
of momentum transfer through friction to the thermal diffusivity or
the transport of heat by conduction.
The Grashof number is interpreted as the ratio of the product
of the buoyancy and inertial forces to the square of the viscous
forces.
The Stanton number is a ratio of the temperature change of a
fluid to the temperature drop between fluid and wall. Also,
St = (Nu)/(Re)(Pr).
An analogy exists between the transfers of heat and mass in
moving fluids, such that correlations of heat transfer involving the
Prandtl number are valid for mass transfer when the Prandtl
number C p / k is replaced by the Schmidt number p / p k , . This is of
particular value in correlating heat transfer from small particles to
fluids where particle temperatures are hard to measure but
measurement of mass transfer may be feasible, for example, in
vaporization of naphthalene.
DIMENSIONLESS GROUPS
The effects of the many variables that bear on the magnitudes of
individual heat transfer coefficients are represented most logically
and compactly in terms of dimensionless groups. The ones most
pertinent to heat transfer are listed in Table 8.8. Some groups have
ready physical interpretations that may assist in selecting the ones
appropriate to particular heat transfer processes. Such interpretations are discussed for example by Grober et al. (1961, pp.
193-198). A few are given here.
The Reynolds number, Dup/p = pu2/(pu/D), is a measure of
the ratio of inertial to viscous forces.
8.4. DATA OF HEAT TRANSFER COEFFICIENTS
Specific correlations of individual film coefficients necessarily are
restricted in scope. Among the distinctions that are made are those
of geometry, whether inside or outside of tubes for instance, or the
shapes of the heat transfer surfaces; free or forced convection;
laminar or turbulent flow; liquids, gases, liquid metals, nonNewtonian fluids; pure substances or mixtures; completely or
partially condensable; air, water, refrigerants, or other specific
substances; fluidized or fixed particles; combined convection and
radiation; and others. In spite of such qualifications, it should be
fl
?
71 I I l l
u!
9
0
J I I I l l
N
?
0
I I Y Y I I Y I Y I I Y I 13x1
0
555
0
u ) u ) u )
c c c
L
m
c
8.4. DATA OF HEAT TRANSFER COEFFICIENTS
I l l 1
> > > I
Y Y Y S
L
L
L
L
.-6 .p .p ,p
?!?!?!?!
r r r r
183
3
s
0
>
f
z
c
8
"
.E
D
m
U
u
8
I
I
3
184 HEAT TRANSFER AND HEAT EXCHANGERS
33
z
u
Y Y I I I Y I I I I X I I
m
C
m
5
&
c
I13
3
Y
C
e
'c
b
P
>
W
3
n
7
II
3
3
3
0
n
8.4. DATA OF HEAT TRANSFER COEFFICIENTS
185
TABLE 8.5. Ranges of Overall Heat Transfer Coefficients in Various Types of
Exchangers [U Btu/(hr)(sqft)("F)]"
Equipment
U
Process
Shell-and-tube exchanger [Fig.
8.4k)l
gas (1 atml-gas (1 atm)
gas (250 atm)-gas (250 atm)
liquid-gas (1 atm)
liquid-gas (250 atm)
liquid-liquid
liquid-condensing vapor
1-6
25-50
2-12
35-70
25-200
50-200
Double-pipe exchanger [Fig. 8.4(a)l
gas (1 atm)-gas (1 atm)
gas (250 atm)-gas (250 atm)
liquid-gas (250 atm)
liquid-liquid
2-6
25-90
35- 100
50-250
Irrigated tube bank
water-gas (1 atm)
water-gas (250 atm)
water-liquid
water-condensing vapor
3-10
25-60
50-1 60
50-200
Plate exchanger [Fig. 8.8(a)l
water-gas (1 atm)
water-liquid
3-10
60-200
Spiral exchanger [Fig. 8.8(c)l
liquid-liquid
liquid-condensing steam
Compact [Fig. 8.6(h)l
gas (1 atm)-gas (1 atm)
gas (1 atm)-liquid
Stirred tank, jacketed
liquid-condensing steam
boiling liquid-condensing steam
water-liquid
90-260
120-300
25-60
Stirred tank, coil inside
liquid-condensing steam
water-liquid
120-440
90-210
~.
120-440
160-600
2-6
3-10
Btu/(hr)(sqft)("F)= 5.6745 W/m2 K.
Data from (HEDH, 1983).
borne in mind that very few proposed correlations are more
accurate than f20% or so.
Along with rate of heat transfer, the economics of practical
exchanger design requires that pumping costs for overcoming
friction be taken into account.
DIRECT CONTACT OF HOT AND COLD STREAMS
Transfer of heat by direct contact is accomplished in spray towers,
in towers with a multiplicity of segmented baffles or plates (called
shower decks), and in a variety of packed towers. In some processes
heat and mass transfer occur simultaneously between phases; for
example, in water cooling towers, in gas quenching with water, and
in spray or rotary dryers. Quenching of pyrolysis gases in transfer
lines or towers and contacting on some trays in fractionators may
involve primarily heat transfer. One or the other, heat or mass
transfer, may be the dominant process in particular cases.
Data of direct contact heat transfer are not abundant. The
literature has been reviewed by Fair (1972) from whom specific data
will be cited.
One rational measure of a heat exchange process is the number
of transfer units. In terms of gas temperatures this is defined by
difference is 234.5 and Ng = 4.48. The height of a contact zone then
is obtained as the product of the number of transfer units and the
height H, of a transfer unit. Several correlations have been made of
the latter quantity, for example, by Cornell, Knapp, and Fair (1960)
and modified in the Chemical Engineers Handbook (1973, pp.
18.33, 18.37). A table by McAdams (1954, p. 361) shows that in
spray towers the range of H, may be 2.5-10 ft and in various kinds
of packed towers, 0.4-4 ft or so.
Heat transfer coefficients also have been measured on a
volumetric or cross section basis. In heavy hydrocarbon fractionators, Neeld and O'Bara (1970) found overall coefficients of 13603480 Btu/(hr)(OF)(sqft of tower cross section). Much higher values
have been found in less viscous systems.
Data on small packed columns were correlated by Fair (1972)
in the form
Ua = CG"L",
Btu/(hr)(cuft)("F),
where the constants depend on the kind of packing and the natures
of the fluids. For example, with air-oil, 1in. Raschig rings, in an
8in. column
Ua = 0.083G0.94L0.2s.
Ng= T,.in - T,.out
( T p - TL)mean
(8.33)
'
The logarithmic mean temperature difference usually is applicable.
For example, if the gas goes from 1200 to 150°F and the liquid
countercurrently from 120 to 4WF, the mean temperature
(8.34)
(8.35)
When G and L are both 5000 lb/(hr)(sqft), for instance, this formula
gives Ua = 2093 Btu/(hr)(cuft)("F).
In spray towers, one correlation by Fair (1972) is
h,a = 0.043G0.8L0.4/Z0.5Btu/(hr)(cuft)("F).
(8.36)
186 HEAT TRANSFER AND HEAT EXCHANGERS
TABLE 8.6. Typical Ranges of Individual Film and Fouling Coefficients [ h Btu/(hr)(sqft)("F)]
Fluid and Process
Sensible
Water
Ammonia
Light organics
Medium organics
Heavy organics
Heavy organics
Very heavy organics
Very heavy organics
Gas
Gas
Gas
Condensing transfer
Steam ammonia
Steam ammonia
Steam ammonia
Steam ammonia
Steam ammonia
Light organics
Light organics
Light organics
Medium organics
Heavy organics
Light condensable mixes
Medium condensable mixes
Heavy condensable mixes
Vaporizing transfer
Water
Water
Ammonia
Light organics
Light organics
Medium organics
Medium organics
Heavy organics
Heavy organics
Verv heavv oraanics
,.,
Conditions
P (atm)
("F)
(AT),,
liquid
liquid
liquid
liquid
liquid heating
liquid cooling
liquid heating
liquid cooling
1-2
10
100
all condensable
1 % noncondensable
4% noncondensable
all condensable
all condensable
pure
4% noncondensable
pure
narrow range
narrow range
narrow range
narrow range
medium range
pure
narrow
pure
narrow
pure
narrow
narrow
range
range
range
ranae
I
0.1
0.1
0.1
1
10
0.1
0.1
10
1
1
1
1
1
<5
<loo
<30
20
20
20
20
20
20
20
45
36
36
36
27
36
27
36
27
27
io4h
io4 I+
7.6-1 1.4
7.1-9.5
28-38
38-76
23-76
142-378
189-568
378-946
450-700
140-230
57-113
6-14
0-6
6-11
9-23
1 1 -57
11-57
23-1 70
23- 1 70
0-6
0-6
0-6
4.7-7.1
9.5-14.2
19-28
3.8-5.7
2.3-3.8
28-38
57-76
8-19
14-38
28-95
23-57
38-95
95-1 90
0-6
0-6
0-6
0-6
0-6
0-6
0-6
0-6
6-30
11-28
0-1 1
6-23
11-45
5.7-19
3.8-14
11-19
14-57
19-76
16-57
23-95
23-95
38-142
57-189
6-12
6-12
6-12
6-12
6-17
6-17
6-17
11-28
11-45
11-57
Light organics have viscosity <1 cP, typically similar to octane and lighter hydrocarbons.
Medium organics have viscosities in the range 1-5cP. like kerosene, hot gas oil, light crudes, etc.
Heavy organics have viscosities in the range 5-100 cP, cold gas oil, lube oils, heavy and reduced crudes, etc.
Very heavy organics have viscosities above 100 cP, asphalts, molten polymers, greases, etc.
Gases are all noncondensables except hydrogen and helium which have higher coefficients.
Conversion factor: 1 Btu/(hr)(sqft)("F) = 5.6745 W/m2 K.
(After HEDH, 1983, 3.1.4-4).
In a tower with height Z=3Oft and with both G and L at
5000 Ib/(hr)(cuft), for example, this formula gives h,a = 215.
In liquid-liquid contacting towers, data cited by Fair (1972)
range from 100-12,000 Btu/(hr)(cuft)("F) and heights of transfer
units in the range of 5 ft or so. In pipeline contactors, transfer rates
of 6OOO-60,OOO Btu/(hr)(cuft)("F) have been found, in some cases
as high as 200,000.
In some kinds of equipment, data only on mass transfer rates
may be known. From these, on the basis of the Chilton-Colburn
analogy, corresponding values of heat transfer rates can be
estimated.
NATURAL CONVECTION
Coefficients of heat transfer by natural convection from bodies of
various shapes, chiefly plates and cylinders, are correlated in terms
of Grashof, Prandtl, and Nusselt numbers. Table 8.9 covers the
most usual situations, of which heat losses to ambient air are the
most common process. Simplified equations are shown for air.
Transfer of heat by radiation is appreciable even at modest
temperatures; such data are presented in combination with
convective coefficients in item 16 of this table.
FORCED CONVECTION
Since the rate of heat transfer is enhanced by rapid movement of
fluid past the surface, heat transfer processes are conducted under
such conditions whenever possible. A selection from the many
available correlations of forced convective heat transfer involving
single phase fluids, including flow inside and outside bare and
extended surfaces, is presented in Table 8.10. Heat transfer
resulting in phase change, as in condensation and vaporization, also
is covered in this table. Some special problems that arise in
interpreting phase change behavior will be mentioned following.
8.4. DATA OF HEAT TRANSFER COEFFICIENTS
187
TABLE 8.7. Individual Film Resistances (1/ h ) Including Fouling Effects, with h in
Btu/(hr)(sqft)(OF)
Kind of Heat Transfer
Fluid
Aromatic liquids
Benzene, toluene, ethylbenzene,
styrene
Dowtherm
Inorganic solutions
CaCI, Brine (25%)
Heavy acids
NaCl Brine (20%)
Misc. dilute solutions
Light hydrocarbon liquids
c., c, c,
Chlorinated hydrocarbons
Miscellaneous organic liquids
Acetone
Amine solutions
Saturated diethanolamine and monoethanolamine (CO, and H,S)
Lean amine solutions
Oils
Crude oil
Diesel oil
Fuel oil (bunker C)
Gas oil
Light
Heavy (typical of cat. cracker feed)
Gasoline (400" EP)
Heating oil (domestic 30"API)
Hydroformate
Kerosine
Lube oil stock
Naphthas
Absorption
Light virgin
Light catalytic
Heavy
Polymer (CLs)
Reduced crude
Slurry oil (fluid cat. cracker)
Steam (no noncondensables)
Water
Boiler water
Cooling tower (untreated)
Condensate (flashed)
River and well
Sea water (clean and below 125°F)
Gases in turbulent flow
Air, CO, CO,, and N,
Hydrocarbons (lighithrough naphthas)
Sensible
Boiling
Condensing
0.007
0.007
0.01 1
-
0.007
0.004
0.013
-
-
0.0035
0.005
-
-
0.004
0.004
0.007
0.004
0.009
0.007
0.007
-
-
-
-
-
-
0.007
0.005
0.015
0.011
0.018
0.0125
0.014
0.008
0.010
0.006
0.009
-
0.010
-
0.013
-
0.008
0.010
0.010
0.010
0.01 1
0.010
0.008
0.008
0.018
0.015
0.015
0.018
0.008
-
0.018
0.007
0.006
-
-
-
-
-
0.006
0.007
0.007
0.0085
0.008
-
-
0.001
0.003
0.007
0.002
0.007
0.004
-
-
0.045
0.035
(Fair and Rase, Pet Refiner 33(7),121, 1854; Rase and Barrow, Project Engineering of
Process Plants, 224, Wiley, 1957.)
CON DEN SAT1ON
Depending largely on the nature of the surface, condensate may
form either a continuous film or droplets. Since a fluid film is a
partial insulator, dropwise condensation results in higher rates of
condensation. Promoters are substances that make surfaces
nonwetting, and may be effective as additives in trace amounts to
the vapor. Special shapes of condensing surfaces also are effective in
developing dropwise condensation. None of these effects has been
generally correlated, but many examples are cited in HEDH and
elsewhere. Condensation rates of mixtures are influenced by
both heat and mass transfer rates; techniques for making such
calculations have been developed and are a favorite problem for
implementation on computers. Condensation rates of mixtures that
form immiscible liquids also are reported on in HEDH. Generally,
mixtures have lower heat transfer coefficients in condensation than
do pure substances.
BOILING
This process can be nuclear or film type. In nuclear boiling, bubbles
detach themselves quickly from the heat transfer surface. In film
boiling the rate of heat transfer is retarded by an adherent vapor
film through which heat supply must be by conduction. Either mode
188 HEAT TRANSFER A N D HEAT EXCHANGERS
TABLE 8.8. DimensionlessGroups and Units of Quantities
Pertaining to Heat Transfer
Symbol
Bi
Fo
Gz
Gr
Nu
Pe
Number
Biot
Fourier
Graetz
Grashof
Nusselt
Peclet
Pr
Prandtl
Re
sc
St
Reynolds
Schmidt
Stanton
Notation
k
kd
L
T, AT
0
U
W
B
e
P
P
Group
realized heat transfer to the heat transfer that would be obtained if
the fin were at the bare tube temperature throughout. The total
heat transfer is the sum of the heat transfers through the bare and
the extended surfaces:
Ab is the tube surface that is not occupied by fins. Example 8.6
performs an analysis of this kind of problem.
8.5. PRESSURE DROP IN HEAT EXCHANGERS
Name and Typical Units
heat capacity [Btu/(lb)(“F),
cal/(g)(’C)]
diameter (ti, m)
acceleration of gravity [ft/lhr)’, m/sec’]
mass velocity [Ib/(hr)(ft)’,kg/sec)(m)’]
heat transfer coefficient [Btu/(hr)(sqft)(”F),
~/(m)’(sec)l
thermal conductivity [Btu/(hr)(sqft)(“F/ft).
ca1/(sec)(cm’~(~/cm)1
diffusivity (volumetric)[ft’/hr, cm2/secl
length (ft, cm)
temperature,temperature difference (“F or “C or K)
linear velocity (ft/hr, cm/sec)
overall heat coefficient (same as units of h )
mass rate of flow (Ib/hr, g/sec)
Thermal expansion coefficient (l/”F, l/”C)
time (hr, sec)
viscosity [Ib/(R)(hr), g/(cm)(secll
density ~ b / ( f t g) ~/ (, ~ r n ) ~ ]
OR,
can exist in any particular case. Transition between modes
corresponds to a maximum heat flux and the associated critical
temperature difference. A table of such data by McAdams (Heat
Transmission, McGraw-Hill, New York, 1954, p. 386) shows the
critical temperature differences to range from 42-90°F and the
maximum fluxes from 42-126 KBtu/(hr)(sqft) for organic substances and up to 410 KBtu/(hr)(sqft) for water; the nature of the
surface and any promoters are identified. Equations (40) and (41) of
Table 8.10 are for critical heat fluxes in kettle and thermosyphon
reboilers. Beyond the maximum rate, film boiling develops and the
rate of heat transfer drops off very sharply.
Evaluation of the boiling heat transfer coefficient in vertical
tubes, as in thermosyphon reboilers, is based on a group of
equations, (42)-(48), of Table 8.10. A suitable procedure is listed
following these equations in that table.
EXTENDED SURFACES
When a film coefficient is low as in the cases of low pressure gases
and viscous liquids, heat transfer can be improved economically by
employing extended surfaces. Figure 8.6 illustrates a variety of
extended surfaces. Since the temperature of a fin necessarily
averages less than that of the bare surface, the effectiveness likewise
is less than that of bare surface. For many designs, the extended
surface may be taken to be 60% as effective as bare surface, but this
factor depends on the heat transfer coefficient and thermal
conductivity of the fin as well as its geometry. Equations and
corresponding charts have been developed for the common
geometries and are shown, for example, in HEDH (1983, Sec.
2.5.3) and elsewhere. One chart is given with Example 8.6. The
efficiency 1 of the extended surface is defined as the ratio of a
Although the rate of heat transfer to or from fluids is improved by
increase of linear velocity, such improvements are limited by the
economic balance between value of equipment saving and cost of
pumping. A practical rule is that pressure drop in vacuum
condensers be limited to 0.5-1.0 psi (25-50 Torr) or less, depending
on the required upstream process pressure. In liquid service,
pressure drops of 5-1Opsi are employed as a minimum, and up to
15% or so of the upstream pressure.
Calculation of tube-side pressure drop is straightforward, even
of vapor-liquid mixtures when their proportions can be estimated.
Example 8.7 employs the methods of Chapter 6 for pressure drop in
a thermosiphon reboiler.
The shell side with a number of segmental baffles presents more
of a problem. It may be treated as a series of ideal tube banks
connected by window zones, but also accompanied by some
bypassing of the tube bundles and leakage through the baffles. A
hand calculation based on this mechanism (ascribed to K.J. Bell) is
illustrated by Ganapathy (1982, pp. 292-302), but the calculation
usually is made with proprietary computer programs, that of HTRI
for instance.
A simpler method due to Kern (1950, pp. 147-152) nominally
considers only the drop across the tube banks, but actually takes
account of the added pressure drop through baffle windows by
employing a higher than normal friction factor to evaluate pressure
drop across the tube banks. Example 8.8 employs this procedure.
According to Taborek (HEDH, 1983, 3.3.2), the Kern predictions
usually are high, and therefore considered safe, by a factor as high
as 2, except in laminar flow where the results are uncertain. In the
case worked out by Ganapathy (1982, pp. 292-302), however, the
Bell and Kern results are essentially the same.
8.6. TYPES OF HEAT EXCHANGERS
Heat exchangers are equipment primarily for transferring heat
between hot and cold streams. They have separate passages for the
two streams and operate continuously. They also are called
recuperators to distinguish them from regenerators, in which hot
and cold streams pass alternately through the same passages and
exchange heat with the mass of the equipment, which is intentionally made with large heat capacity. Recuperators are used
mostly in cryogenic services, and at the other extreme of temperature, as high temperature air preheaters. They will not be
discussed here; a detailed treatment of their theory is by Hausen
(1983).
Being the most widely used kind of process equipment is a
claim that is made easily for heat exchangers. A classified directory
of manufacturers of heat exchangers by Walker (1982) has several
hundred items, including about 200 manufacturers of shell-and-tube
equipment. The most versatile and widely used exchangers are the
shell-and-tube types, but various plate and other types are valuable
and economically competitive or superior in some applications.
These other types will be discussed briefly, but most of the space
following will be devoted to the shell-and-tube types, primarily