Kuppan T. Heat Exchanger Design Handbook
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294
Chapter
5
2.
Flow area in the ring opening,
S2:
Step
2.
Calculate the effective crossflow area,
Sq:
sq
=
L,
cx
(
107)
where
Ex
is the sum
of
the clear distances between the neighboring tubes closest to the mean
diameter,
D,
=
0.5
(0,
+
D2).
Determine the sum
of
clear distances between neighboring tubes
closest to
D,,
either from a drawing (as shown in Fig.
46)
or by computational methods.
The number of tubes in the flow areas
S,,
S?,
and crossflow area
S,
(to be defined later)
are
N,,, N,.,
and
N,,,
respectively. They are shown in Fig.
45.
Step
3.
Calculate the hydraulic diameter for the longitudinal flow in the annulus,
&I,
and
in
the ring opening,
Dh?:
(
108)
Step
4.
Calculate the heat-transfer coefficients
h
I
and
hl,
for the longitudinal
flow
through
the
C
:Toss section
S,
and
S2
from the equation of Hausen
[58]:
Figure
46
Mean diameter to calculate effective crossflow area of a disk
and
doughnut heat ex-
changer.
295
Shell
and
Tube Heat Exchanger Design
Nu,
=
0.024Rey8Pr”
”
Nul
=
0.024RePxPro
’’
where
(1
13a)
Re2
=
ul
Dh!
(I
13b)
~
CL\
and
U,
and
U.
are based on
S,
and
S2,
respectively.
Step
5.
Calculate the heat-transfer coefficient
hl
for crossflow across the bundle from
McAdams [47] for the turbulent range:
where
K
=
0.33
for the staggered tube arrangement and
K
=
0.26
for the in-line tube arrange-
ment.
To
calculate Re3, use
U3
calculated using the crossflow area
Sq:
Alternately, Slipcevic recommends calculating the heat-transfer coefficient
of
the individ-
ual tube row with the corresponding flow cross section. From these heat-transfer coefficients,
calculate the weighted mean average
of
heat-transfer coefficients.
Step
6.
Calculate the heat-transfer area in the longitudinal flow section
A
I,
A?,
crossflow
section
A?,
and total heat-transfer area,
A:
(1
16a)
(1
16b)
where
(1
17)
(1
18a)
(118b)
Step 7.
Calculate the heat-transfer coefficient
h
referred to the entire heat-transfer surface
A:
296
Chapter
5
Shell-Side Pressure Drop.
Refer to Slipcevic
[55]
for pressure drop calculation.
Shortcomings
of
Disk and Doughnut Heat Exchanger.
The
disadvantages of this design in-
clude the following
[2]:
(I)
All
the tie rods to hold baffles are within the tube bundle, and
(2)
the central tubes are supported by the disk baffles, which in
turn
are supported only by tubes
in the overlap of the larger diameter disk over the doughnut hole.
Nomenclature
A
total heat-transfer area on the shell side
AI
heat-transfer area for longitudinal flow in the annulus between the shell and the disk
A?
heat-transfer area for longitudinal flow through the ring opening
A3
heat-transfer area for crossflow
D,
disc diameter
D,
diameter of ring opening
D,
mean diameter for calculating the crossflow area
=
0.5
(Dl
+
D?)
D,
shell inside diameter
d
tube outer diameter
gL
acceleration due to gravity
h
I
heat-transfer coefficient for the longitudinal flow corresponding to Nu,
h2
heat-transfer coefficient for the longitudinal flow corresponding to NU?
h3
heat-transfer coefficient for the crossflow corresponding to Nu3
K
heat-transfer correlation coefficient
L
tube length
M
multiplication factor in
Eq.
86
M,
mass flow rate on the shell side
NI
total number of tubes
NI,
number of tubes in annulus between the shell and disk
NI,
number of tubes in the ring opening
NI,
number of tubes in crossflow
Nu, Nusselt number to calculate
h,
Nu?
Nusselt number to calculate
h2
Nu3 Nusselt number to calculate
h3
Pr Prandtl number
Re, Reynolds number in the longitudinal flow area
SI
Re, Reynolds number in the longitudinal flow area
S?
Re3 Reynolds number in the longitudinal flow area
S3
cross-sectional area for longitudinal flow through the annulus between the shell and the disk
cross-sectional area for longitudinal flow through the ring opening
annular area between the disk and the shell
area inside the ring
crossflow area through tube bundle
mean longitudinal flow area
=
0.5
(S,+
S,)
bulk mean temperature of the shell-side fluid
tube wall temperature
shell-side velocity through the cross-sectional area
S,
shell-side velocity through the cross-sectional area
Sz
shell-side velocity through the cross-sectional area
S3
viscosity at bulk mean temperature
of
the shell side fluid (kg/m
s)
viscosity at tube wall temperature of the shell side fluid (kg/m
s)
297
Shell and Tube Heat Exchanger Design
Al.3
Phillips
RODbaffle
Heat Exchanger
Phillips RODbaffle Heat Exchanger
The RODbaffle exchanger is a shell and tube heat exchanger that uses an improved support
system for the tubes. It consists of rods located in a predisposed manner such that they confine
tube movement. RODbaffle heat exchangers are used in process industries to enhance thermo-
hydraulic performance, eliminate flow-induced tube vibration occurrence, minimize shell-side
pressure losses, increase shell-side flow-field uniformity, and reduce shell-side fouling [59].
The RODbaffle concept and design came about as a result of tube vibration failures in three
shell and tube exchanger bundles resulting in frequent plant shutdowns in a Phillips Petroleum
plant in 1970. The emergency repairs made included the substitution of rod tube supports for
the plate baffles, resulting in significantly improved operating characteristics. This was the
starting point of the RODbaffle concept. References 59-64 provide general and specific infor-
mation on RODbaffle heat exchanger design.
RODbaffle Exchanger Concepts
The RODbaffle exchangers (Fig. 47) use rods, with a diameter equal to the clearance between
tube rows, inserted between alternate tube rows in the bundle in both the horizontal and vertical
directions. The support rods are laid out on a square pitch with no nominal clearance between
tubes and support rods. Support rods are welded at each end to a fabricated circumferential
baffle ring. Major components of each individual RODbaffle are support rods, baffle ring,
cross-support strips, partition blockage plate, and longitudinal slide bars, as shown in Fig. 47a.
Four different RODbaffle configurations
(W,X,Y,Z)
are welded to longitudinal slide bars to
form the RODbaffle cage assembly as shown in Fig. 47c. Tube and rod layout is shown in
Fig. 47d.
Important Benefit-Elimination of Shell-Side Flow-Induced Vibration
Although RODbaffle heat exchangers are being increasingly used because of thermohydraulic
advantages, flow-induced vibration protection of tubes remains one of the major design consid-
erations. The RODbaffle bundle eliminates harmful flow-induced vibration by using these ma-
jor design innovations:
1.
Each tube is supported in all four directions.
2.
Positive four-point confinement of the tubes.
3.
Minimum convex point contact between rods and tubes.
Since these exchangers have longitudinal flow fields, vortex shedding and fluid elastic
instability mechanisms (which lead to flow-induced
tube
vibration in crossflow exchangers)
are not present
[60].
Similarly, because zero nominal clearance exists between tubes and sup-
port rods, fretting wear and collision damage that may occur in plate baffle units is not evident
in RODbaffle exchangers. Under design conditions where plate baffles are required and posi-
tive tube support is critical, combination RODbaffle-plate-baffle exchangers may be used
[60].
Proven RODbaffle Applications
Typical applications for RODbaffle exchangers include gas-gas exchangers, compressor after-
coolers, gas-oil coolers, reactor feed-effluent exchangers, condensers, kettle reboilers, waste
heat boilers, HF acid coolers, and carbon black air preheaters.
Operational Characteristics
The unobstructed flow path created by the support rod matrix makes the flow field in a ROD-
baffle heat exchanger predominantly longitudinal. Longitudinal flow fields produce the highest
298
Chapter
5
Figure
47
Rod
baffle heat exchanger and
support.
(a) Schematic; (b) details; (c) cage assembly; and
(d)
tube and
support
rod layout. (From
Ref.
59.)
thermohydraulic effectiveness of any commercial heat exchanger geometry. The main charac-
teristics of longitudinal flow on the outside of tube bundles include the following
[65]:
1.
Extremely low pressure drop and a very high effectiveness of pressure drop
to
heat-transfer
conversion, compared to baffled crossflow pattern. In longitudinal flow, the friction occurs
directly at the heat-transfer surface, and there are no parasitic pressure losses due to win-
dow turnarounds.
2.
Uniformity of flow distribution, without any stagnant areas characteristic of segmental
baffle exchangers, results in uniform local heat-transfer rates and less fouling.
299
Shell
and
Tube Heat Exchanger Design
Architectural
44
Data
-
Base/Rules
Su
b-System
Figure
48
Schematic
of
integrated expert system
for
heat exchangers
[66].
3.
The grid baffles increase pressure drop through the flow contraction process, but also
enhance heat transfer.
As
reported in Refs.
60
and
61,
RODbaffle exchanger heat-transfer rates compare favor-
ably with double segmental plate baffle exchangers
and
are generally higher than in comparable
triple segmental plate baffle and “no-tube-in window” designs. Shell-side pressure losses
in
RODbaffle exchangers are lower because neither bundle crossflow form drag nor repeated
flow
reversal effects are present.
Thermal Performance
For shell side, the expressions for Nusselt number, Nu, for laminar flow and turbulent flow
are given by
[59]:
300
Chapter
5
10
00
Figure
49
Coefficient of heat transfer, fluids inside tubes
[44].
Nu
=
CLRei6
Pro4(44)”
for laminar flow
(
120a)
Nu
=
CTRe;’
ProJ(@Jn
for turbulent flow
(
120b)
where
CL
and
CT
are RODbaffle exchanger geometric coefficient functions for laminar flow
and turbulent flow, respectively.
Pressure Drop.
Shell-side pressure loss for flow through a RODbaffle heat exchanger bundle,
excluding inlet and exit nozzles, is defined as the sum of an unbaffled frictional component,
ApL,
and a baffle flow contribution
Apb
as
[59]:
AP
=
APL
+
APb
(121)
According to Gentry
[59],
shell-side pressure losses in RODbaffle exchangers are gener-
ally less than
25%
of shell-side losses produced in comparable double segmental plate baffle
exchangers.
Design and Rating Program Available
Phillips Petroleum engineers have an ongoing development and testing program on RODbaffle
exchangers from which they have developed basic correlations to predict heat transfer and
pressure drop. The correlations are based on experimental data obtained from different bare
and low-finned tube test bundles using water, light oil, and air as test fluids. The program is
available for use
on
an IBM mainframe or IBM
PC
or equivalent.
Nomenclature
CL
laminar heat-transfer bundle geometry function
CT
turbulent heat-transfer bundle geometry function
Nu Nusselt number
301
Shell and Tube Heat Exchanger Design
Pr Prandtl number
Re,, Reynolds number
ApL
pressure drop due
to
unbaffled frictional component
Apb
pressure drop due to baffle flow contribution
pw
viscosity
of
shell side fluid at wall temperature (kg/m
s)
($J"
bulk-to-wall viscosity correction,
(~~/uy)~.'~
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6
Regenerators
1
INTRODUCTION
Regeneration is an old technology dating back to the first open hearths and blast furnace stoves.
The drastic escalation of energy prices has made waste heat recovery more attractive over the
past two decades. Manufacturing and process industries such as glass, cement, primary and
secondary metals, etc. account for a significant fraction of all energy consumed. Much of this
energy
is
discarded in the form of high-temperature flue gas exhaust streams. Recovery of
waste heat from flue gas by means of heat exchangers can improve the overall plant efficiency
and serves to reduce the national energy needs and conserve fossil fuels. Figure
1
shows the
cross section of a side-port regenerative furnance used in the container and flat glass industry
[
13. Currently, interest in storage-type regenerators has been renewed due to their applications
of
heat recovery, heat storage, and general energy related problems
[2].
The objective of this
chapter is to acquaint the readers with various types
of
regenerators, construction details, and
thermal and mechanical design. Additionally, some industrial heat recovery devices for waste
heat recovery are discussed.
1
.I
Regeneration Principle
For many years the regeneration principle has been applied to waste heat recovery by preheat-
ing the air for blast furnaces and steam generation. In the former the regeneration is achieved
with periodic and alternate blowing of the hot and the cold stream through a fixed matrix of
checkered brick. During the hot flow period, the matrix receives thermal energy from the hot
gas and transfers it to the cold stream during the cold stream flow. In the latter, the metal
matrix revolves slowly with respect to two fluid streams. The two gas streams may flow either
in parallel directions or in opposite directions. However, countefflow is mostly preferred be-
cause of high thermal effectiveness. The inclusion of an air preheater in a steam power plant
improves the boiler efficiency and overall performance of the power plant.
303