heat exchangers design considerations. heat exchangers key concepts heat transfer coefficients...
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Heat Exchangers
Design Considerations
Heat Exchangers Key Concepts Heat Transfer Coefficients Naming Shell and Tube Exchangers Safety In Design of Exchangers Controls for Exchangers
Heat Exchangers
Key Concepts"Allow me to summarise: Hot stuff this side, cold stuff that side. Make the
cold stuff hotter, but use inbetween stuff to not let the cold stuff actually touch the hot stuff. Cold stuff and hot stuff not allowed to destroy inbetween stuff and vice versa. Some kinds of inbetween stuff works better than others. Might need pumps or fans to make the whole shebang work a little better, too.”
- Topher Gayle
4
General Sizing Method Pick an exchanger type (S&T, Plate & Frame
etc.) Choose counter or co-current flow Choose number of tube passes (for S&T) draw Temp diag, Calculate the LMTD and Q Calculate the LMTD Correction Factor (F) if
more than two tube passes Choose a U value based on tables Calculate the Area , A = Q/ U LMTD F Perform rigorous rating as required (not 470)
Key Concepts
Heat Lost Heat Transfer Heat Absorbed
Q = U A Tln
Either
Q = m Cp T, or
Q = m Hevap
=
Either
Q = m Cp T, or
Q = m Hevap
m Cp hot (T1-T2) = U A Tln = m Cp cold ( t1 - t2)
8-9
Combined Equations
m Cp hot (T1-T2) = U A Tln = m Cp cold ( t1 - t2)
• Calculate the unknowns
• Determine the overall heat transfer coefficient (U) value in order to calculate the Area (A) to size the exchanger
9
Duty Considerations - Q Distillation Columns
Start-up and Shut-down usually require the column to operate at “full reflux”
Feed and Outlets are shut down 100% of Overhead vapour being condensed 100% of reflux being boiled Compositions can be completely different (reactor not
online), therefore diff. temps Is the duty in the simulation truly the worst-case
duty? For our purposes assume yes
Mean Temp Difference
T1
T2t2
t1
Counter Current Exchanger Temp Profile
])(
)(ln[
)()(
12
21
1221
tT
tTtTtT
AUQ
26-27
T1
T2
t1
t2 AlwaysDraw This
Graph !!
Mean Temp Difference Correction for not strictly counter or co-
current flow
T2
t1
T1
t2
Hot Side temp & flow direction
Cold Side temp & flow direction
Exchanger Temperature Profile
26-27
200
38 3045
Temperature Correction Factor Form of the heat transfer equations is:
The factor F is usually determined Graphically
lmtdm TFAUTUAQ
27-28
T1 = 200T2 = 38t1 = 30t2 = 45
T1 = 200
T2 = 38t1 = 30
t2 = 45
As A Single PassCounter Current- temp cross?
Temperature Correction Factor Form of the heat transfer equations is:
The factor F is usually determined Graphically
lmtdm TFAUTUAQ
27-28
T1 = 200T2 = 38t1 = 30t2 = 45
T1 = 200
T2 = 38 t1 = 30
t2 = 45
As a 2 Pass exch- temp cross- low F factor
Temperature Correction Factor
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.5
0.6
0.7
0.8
0.9
1
R = 20R = 1.2R=0.6R=0.4
MTD Correction Factor
P = Temperature Efficiency
F =
MTD
Cor
rect
ion
Fac
tor
12
21
11
12
tt
TTR
tT
ttP
28-39
R = 11.2, P = 0.0882 , F = 0.471
Condensing LMTD Divide the Exchanger into segments Evaluate U and LMTD for each segment
Multicomponent, noncondensables? Arghh!
Distance
T1
T2 T2
T3
t1
t2
30
Counter to the Co-current Use Counter Current
maximize LMTD (minimize Area, cost etc.)
minimize utility reqt’s Use Co-current
minimize outlet utility temperatures during turn down - see later
reduced fouling
Determining U Tables for U values Determine U via fundamental equations
note that fouling factors often overshadow much of the accuracy that the fundamental equations provide
Details of exchanger configuration required Computer Programs / Vendors
vendors can and will provide exch sizing Be knowledgeable enough to critique their
design
9-11
Determining U Physical configuration affects U values
Tables assume certain things about the exchanger. If through poor configuration, (ie..inappropriate tube length, or number of tubes) the assumptions are invalidated, then the tables will mislead.
11-13
Film Coef from Velocity (water)
0
500
1000
1500
2000
2500
3000
3500
0 5 10 15
Tube Velocity (ft/sec)
Film
Co
ef
(BT
U/h
r s
q f
t °F
)
40 °F
100 °F
180 °F
U Values & Velocity11-13
Physical Config & U ValuesThe following factors all affect the velocities
of the fluids in the exchanger Tube Length Tube Dia Number of Tube Passes Number of Tubes / Bundle dia Baffle SpacingNote: This does not apply to condensers or
boiling
11-13
U Values & Velocity Adding a Tube Pass doubles the velocity of the
liquid on the tube side
Decreasing Baffle Spacing Increases Velocity Shellside
11-13
0Bulk FlowPath Of Fluid
Baffle "Window"or Opening toFlow Parallelto Tubes, asShell Side Passesfrom One BaffleArea to the Next
1.5 in1.5 inB Baffle Pitch
or Spacing
Physical Configuration & U values Tube Layout
Square Rotated SquarePreferred for cleaning
Triangular(high heat x-fer)
26
Velocity Limitations Maximum Velocity is Dictated by:
Vibration Erosion Hydraulic Exchanger Physical Size
13
Velocity Limitations - Vibration Usually a Shell Side Issue Vibration Can Cause
Collision Damage, Baffle Damage, Fatigue & Tubejoint Failure
Causes Turbulent buffeting Fluidelastic whirling Vibration induced by flow parallel to the
tubes
13-15
Velocity Limitations - Vibration
Analysis determine the natural frequency of the tubes
vibration of tubes between baffles vibration on U bends account for damping (fluid properties, tube
stresses etc.) determine critical flow velocity
minimum cross flow velocity that the span may vibrate unacceptably large amplitudes.
Analysis by Programs or TEMA Standards
13-15
Tube Side Velocity Limitations - Erosion High Velocity causes thinning of the metal walls (erosion). It can be avoided by maintaining velocities (ft/sec) below those given by this
equation. (about 12 ft/sec for water)
TEMA say 2 < 6000 to eliminate tube end erosion
15
Velocity Limitations - Hydraulics Available pressure drop will limit velocity The P rises to the square of the velocity
60 psig
EXCH
CV P
0 psig
15-16
Velocity Limitations - Physical Limitations on Shipping, Floor space etc. all make a difference
(don’t forget about pulling the tube bundle)
16-17
U values of interest
Condensing U values are very high (500 to 800)
Reboiler U values are very high ( 700) liq / liq U values in middle (100 - 300) Cooling / heating gases
(desuperheating) have very low U values (<30)
TEMA
Easier to Clean
Less Costly
Shell Side Fluid Leaks
to Atmosphere
Expensive
Large Annular Space = Low
U Value
Cheap,Hard to clean
Exchanger Selection Require a U-tube or Floating head, instead of
fixed tube sheet, when thermal expansion between shell and tubes is an issue i.e. shell side fluid and tube side fluid
temperatures differ by more than 200 °F Require a Floating Head, instead of U-tube
When cleaning tubes mechanically is important (dirty fluids on tube side)
When errosion may occur on tube side
Reboilers Boiling Phenomena
Nucleate boiling at shell/tube T = 20 to 50 °F
Boiling Regimes
1.00E+03
1.00E+04
1.00E+05
1.00E+06
1.00E+07
1 10 100 1000 10000
Temp Differential (°C)
Q
(W
/sq
m)
Nucleate Boiling
Film Boiling
52
Reboilers Sizing
Common to use “maximum heat flux” 15,000 BTU/hr sq ft
Fundamental Equations can be used to determine the best T
Max Flux is a function of the Number of active nucleation which is in turn affected by the materials of construction, the fluid properties and the temperature difference
53-57
Reboilers Heat Flux can be increased with special
systems (i.e. sintering, brazing, flame spraying, electrolytic deposition). Sand blasting , scoring tends not to provide stable long term enhancement.
Nucleation SitesTrapped
Vapour
55
Heat Exchanger Safety What Can Fail?
Control System Failure Shell & Tube
Tube can rupture Tubes separate from Tube Sheet Blocked in exchanger causes cool fluid to
experience temperatures of hot fluid Plate & Frame
Gaskets can leak mixing hot and cold sides , or releasing either fluid to surroundings
Heat Exchanger Safety Implications
Fires, Explosions, Toxic Releases
Controlling Exchangers
Q = U A Tln
A is fixed U varies slightly with velocity Tln is the controlling variable
Hot In
Hot OutCold Out
Cold In
Design DutyHot In
Hot Out
Cold Out
Cold In
Reduced Duty
Q = m c T
Controls Liquid / Liquid - control on cooling
media
C/w
58
Controls Liquid / Liquid - control on process
C/w
59
Controls - Steam Heating Steam Pressure Control
T
60
Steam Trap
Controls - Steam Heating Condensate Level Control
61
Workshop - Size “Condenser” Duty: 153 x 106 KJ/hr T1 = 213.3 °C T2 = 35 °C t1 = 30 °C dew point: 150 °C (to be confirmed in
PRO 2) U gas/water - 0.51 kW/ m2 °C U condensing / water - 0.85 kW/ m2 °C
END
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