Heat Exchanger design/ Shell and Tube Heat Exchangers Shell-and

Shell-and-Tube Heat Exchangers
Baffles are used to establish a
cross-flow and to induce
turbulent mixing of the shellside fluid, both of which
enhance convection.
 The number of tube and shell
passes may be varied.
 This configuration increases
substantially the heat transfer
Heat Exchanger design/
Shell and Tube Heat Exchangers
One Shell Pass and One Tube Pass
area.
One Shell Pass,
Two Tube Passes
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Two Shell Passes,
Four Tube Passes
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Baffles
Shell-and-Tube Heat Exchangers
Tube
fluid
in
Shell
fluid
in
Shell
fluid
out
Tubes
Baffle
Outlet
header
Inlet header
Tube
fluid
out
Tube sheet
Drain
Baffles help support the
tubes and mix the shell-side
fluid along the length of the
heat exchanger
One-shell and four-tube pass heat exchanger
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Tube considerations
Tubes up to 120” length
Heat exchangers may
have ~10  1000+ tubes (!)
Information about standard tube
dimensions, including areas can be
found in Tables (for example p. 14,
Heaslip)
 Tubes generally handle
the least viscous, most
corrosive & fouling fluid
Shell characteristics – pipe or rolled sheet
Tube characteristics – B.W.G. (Birmingham Wire Gauge) ¾” – 1 ½”
Shell and Tube Heat Exchanger – Temperature Profile
Log-Mean temperature difference
To account for complex flow conditions in multi-pass, shell and tube
and cross-flow heat exchangers, the log-mean temperature difference
must modified:
Tlm  FTlm,CF
where F=correction factor
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Correction Factor
Example
A shell-and-tube heat exchanger must be designed to heat 2.5 kg/s of water from 15
to 85°C. The heating is to be accomplished by passing hot engine oil, which is
available at 160°C, through the shell side of the exchanger. Ten tubes pass the water
through the shell. Each tube is thin walled, of diameter D=25 mm, and makes eight
passes through the shell. If the oil leaves the exchanger at 100°C, what is the
required flow rate? If the overall heat transfer coefficient is estimated at 354 W/m2.K,
what is the total area of heat transfer?
where t is the tubeside fluid
temperature
Fluid properties: Engine Oil at T=130°C: cp=2350 J/kg.K, Water at T=50°C:
cp=4181J/kg.K, =548x10-6 N.s/m2, k=0.643 W/m.K, Pr=3.56
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Correction Factor
where t is the tubeside fluid
temperature
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Overall Heat Transfer Coefficient
Recall:
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Uo 
1
Ao
1
 Ao Rcond 
hi Ai
ho
ho
But: Estimation of U
poses some
challenges for shell
and tube heat
exchangers!
Heat Exchangers
hi
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Determination of tube side film coefficient, hi
•
Approach 1: Using convection correlations and charts (for example Sieder-Tate,
see also Welty et al, Ch. 20, equations 4.1, 2.37, 2.36) :
Nu D 
•

hD
 0.023 Re 4D 5 Pr1 3  b
k
 w



Determination of tube side film coefficient, hi
For these calculations the mass flow rate per tube must be used:
Shell
Inlet
0.14
Tube
Outlet
per tube
Approach 2: Sieder and Tate relationship plotted in graph below (p. 53 Heaslip)
 t : total mass flow rate in tubes
m
n p : number of passes
n t : number of tubes
Shell
Outlet
(Ut is the “local” velocity in each tube)
Re D 
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 np  m
t
U t   

n
 t   f D 2t
4
 f D t U t 4m pertube

f
Di 
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Determination of tube side film coefficient, hi
Note that in the Sieder-Tate plots, the “mass velocity”, G, is used, where
where At is the cross-sectional area available for flow on the tube-side
and G is in units of lb/(hr)(ft2)
(multiply  in cp by 2.42 for units to
•
Based on this definition,
•
match)
To find G, we need to determine the flow area on the tube side:
.
.
/
(in ft2)
 The diameter D and flow area/tube can be found from tables (p. 14 Heaslip,
see next slide), Table 10 Kern
p. 14 Heaslip (2007)
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Tube
Inlet
Determination of shell side film coefficient, ho
Tube patterns and layout
The problem:
•
•
Tubes create a complex combination of flow patterns.
Variable cross section confronted by the fluid.
The solution:
•
•
•
Square
Proprietary software packages or experimental facilities – e.g. Heat
Transfer Research Inc. (HTRI).
Delaware method – Serth (later chapters)
Tabulated values presented in various heat transfer references.
Square
(rotated)
Triangular
Tube pitch is the centre-to-centre distance between tubes
Square and rotated square patterns permit mechanical cleaning of
the outside of the tubes
Determination of ho
•
•
Approach 1: Using convection correlations (for example Welty et al, Ch. 20)
Approach 2: Using charts by Kern, (p. 56 Heaslip). See also curve fit eq.
3.21 in Serth, (but check out assumptions)
Serth, pg. 3/110 See also curve fit eq. 3.21 in Serth, (but be aware of assumptions!)
Typical values of baffle cuts 20-25% for liquids and 40-45% for vapour
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Determination of ho
•
Calculation of shell-side equivalent diameter
Once again we need the “mass velocity” on the shell side, Gs.
(where As is the cross-sectional area available for flow on
Square pitch
the tube-side).
•
 The equivalent diameter must be calculated based on the pitch (see
next slide), or can be found in the charts (such as p. 56 Heaslip).
 The shell-side cross-flow area must also be calculated based on the
pitch, baffle spacing and internal tube diameter (see next slide)
Triangular pitch
The shell-side crossflow area is:
 The equivalent diameters are
commonly included in the charts for
the shell-side heat transfer
coefficients
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in ft
where B is the baffle spacing
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Fluid velocities and pressure drops in tubes
Overall Heat Transfer Coefficient
Up to now we have determined the “design” (or “clean”) overall heat
transfer coefficient (UD ) .
•
•
 To account for fouling:
Higher fluid velocity gives higher heat transfer coefficient.
Higher fluid velocity helps reduce build-up of scale and
contaminants on pipe/tube walls.
But …
Uo 
•
•
1
Ao Rf",i
Ao
1

 Ao Rcond  Rf",o 
hi Ai
Ai
ho
Target values …
•
•
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Higher fluid velocity gives a higher pressure drop.
Higher fluid velocity requires larger internal fluid pressures (potential
safety issues?).
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For liquids, velocities of 1-3 m/s in pipes or tubes are recommended.
For liquids, pressure drops of 30-60 kPa (or less) are permissible.
Pressure Drop
Tube Side Pressure Drop
In practice there can be a significant pressure drop along the pipes of
a multipass heat exchanger.
 Results in property changes
 Pressure drop must be accounted for in real design situations and
maximum allowable pressure drops must be respected (see
specifications in design assignment)
• See pages 67-69 (Heaslip).
The tubeside pressure drop is the sum of the pressure drop through the tubes plus the pressure
drop through the channels:
Pt =
Where:
f Gt2 L n
+ 4n V2
10
5.22 x 10 De s t
s 2g'
lbs/in2
Pt = Pressure drop across the tubeside, lbs/ft2
f = Friction factor, ft2/in2
Gt = Tube mass velocity, lb/hr-ft2
L = Tube length, ft
n = Number of tube passes
De = Equivalent diameter, ft
s = Specific gravity = density, lbs/ft3 / 62.4
s = The viscosity ratio (/w)0.14
g' = Acceleration due to gravity = 32.2 ft/sec2
•
Useful to obtain an estimate of a suitable combination of L,n and D for
your H.E.(i.e. starting point)
 In calculations of double-pipe heat exchangers, for flow in annulus just
use equivalent diameter
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Shell Side Pressure Drop
The isothermal equation for pressure drop for the shellside flow of a fluid being heated or cooled and
including the entrance and exit losses is:
Ps =
Where:
f Gs Ds (N+1)
5.22 x 1010 De s s
lbs/in2
Ps = Pressure drop across the shell, lbs/ft2
f = Friction factor, ft2/in2
Gs = Shell mass velocity, lb/hr-ft2
Ds = Shell inside diameter, ft
N = Number of baffles
De = Equivalent diameter, ft
s = Specific gravity
s = The viscosity ratio (/w)0.14
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Some design tips
The pressure drop through the shell of an exchanger is proportional to the the number of times the
fluid crosses the bundle between baffles. It is also proportional to the distance across the bundle
each time it is crossed.
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 Step-by-step instructions to size double pipe heat exchangers are
provided in Serth (2007), Chapter 4 (Example 4.1)
 Step-by-step instructions to size shell and tube heat exchangers are
provided in Serth, Chapter 5 (Example 5.1), and multiple examples are
provided by Kern.
 A listing of common heat exchanger tube dimensions is included in page
14 (Heaslip).
 See p. 22-23 (Heaslip) for useful information on baffle design
 Useful shell dimensions are found in pages 16-17 (Heaslip)
 Criteria for the placement of the fluid (tube side or shell side) can be found
in Serth Table 3.4 and section 5.7 and in p. 30 Heaslip.
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