HEAT E XCHANGE XCHANGERS RS
INTRODUCTION:
Shell and tube heat exchangers are one of the most common equipment found in all plants
How it works?
INTRODUCTION:
Shell and tube heat exchangers are one of the most common equipment found in all plants
How it works?
WHAT ARE THEY USED FOR?
Classification according to service .
Heat Exchanger Both sides single phase and process stream
Cooler One stream process fluid and the other cooling water or air
Heater One stream process fluid and heating utility as steam
Condenser One stream condensing vapor and the other cooling water or air
Reboiler One stream bottom stream from a distillation column and the other a hot utility or process stream
DESIGN CODES: Code
Is recommended method of doing something
ASME BPV – TEMA
Standard
is the degree of excellence required API 660-ASME B16.5–ASME B36.10M–ASME B36.19-ASME B16.9–ASME B16.11
Specifications
Is a detailed description of construction, materials,… etc Contractor or Owner specifications
M AIN COMPONENTS
1- Channel Cover 2- Channel 3- Channel Flange 4- Pass Partition 5- Stationary Tubesheet 6- Shell Flange 7- Tube
8- Shell 9- Baffles 10- Floating Head backing Device 11- Floating Tubesheet 12- Floating Head 13- Floating Head Flange 14 –Shell Cover
TEMA HEAT EXCHANGER
TEMA HEAT EXCHANGER
Front Head Type
A - Type
B - Type
C - Type
TEMA HEAT EXCHANGER
Shell Type
E - Type
J - Type
F - Type
K - Type
TEMA HEAT EXCHANGER
Rear End Head Types
M - Type Fixed Tubesheet
S - Type Floating Head
T - Type Pull-Through Floating Head
CLASSIFICATION:
U-Tube Heat Exchanger
Fixed Tubesheet Heat Exchanger
Floating Tubesheet Heat exchanger
EXAMPLE
AES
EXAMPLE
AKT
HEAT EXCHANGERS MECHANICAL DESIGN
Terminology
Design data
Material selection
Codes overview
Sample calculations
Hydrostatic test
Sample drawing
DESIGN D ATA
Heat Exchanger Data Sheet :
TEMA type
Design pressure
Design temperature
Dimensions / passes
Tubes ( dimensions, pattern)
Nozzles & Connections
Baffles (No. & Type)
M ATERIAL SELECTION Strength
Cost &
Material
Availabilit y
Selection
Fabricability
Corrosion Resistance
HEAT EXCHANGERS: DESIGN CONSIDERATIONS
TYPES
Heat Exchanger Types Heat exchangers are used to energy conversion and utilization. They involve heat exchange between two fluids separated by a solid and encompass a wide range of flow configurations.
• Concentric-Tube Heat Exchangers
Parallel Flow
Counterflow
Simplest configuration.
Superior performance associated with counter flow.
TYPES (CONT.)
• Cross-flow Heat Exchangers
Finned-Both Fluids Unmixed
Unfinned-One Fluid Mixed the Other Unmixed
For cross-flow over the tubes, fluid motion, and hence mixing, in the transverse direction ( y ) is prevented for the finned tubes, but occurs for the un-finned condition.
Heat exchanger performance is influenced by mixing.
TYPES (CONT.)
• Shell-and-Tube Heat Exchangers
One Shell Pass and One Tube Pass
Baffles are used to establish a cross-flow and to induce turbulent mixing of the shell-side fluid, both of which enhance convection.
The number of tube and shell passes may be varied, e.g.:
One Shell Pass, Two Tube Passes
Two Shell Passes, Four Tube Passes
TYPES (CONT.)
• Compact Heat Exchangers Widely used to achieve large heat rates per unit volume, particularly when one or both fluids is a gas.
Characterized by large heat transfer surface areas per unit volume, small flow passages, and laminar flow.
(a) (b) (c) (d) (e)
Fin-tube (flat tubes, continuous plate fins) Fin-tube (circular tubes, continuous plate fins) Fin-tube (circular tubes, circular fins) Plate-fin (single pass) Plate-fin (multipass)
OVERALL COEFFICIENT
Overall Heat Transfer Coefficient
• An essential requirement for heat exchanger design or performance calculations.
• Contributing factors include convection and conduction associated with the two fluids and the intermediate solid, as well as the potential use of fins on both sides and the effects of time-dependent surface fouling.
• With subscripts c and h used to designate the hot and cold fluids, respectively, the most general expression for the overall coefficient is:
1 UA
1 UA
1 UA
c
1 o hA
h
R f , c c
o
A
Rw c
R f ,h o
A
h
1 o hA
h
OVERALL COEFFICIENT
R f
Fouling factor for a unit surf ace area (m2 K/ W)
Table 11.1
Rw
o
Wall conduction resistance (K/W)
Overall surface efficiency of fin array (Section 3.6.5) A f 1 1 o,c or h f A c or h total surface area (fins and exposed base) A At A f surface area of fins only
Assuming an adiabatic tip, the fin efficiency is tanh mL f , c or h
mL
c or h
mc or h
2U p / k wt
U p , c or h
h 1 hR f
c or h
partial overall coefficient c or h
LMTD METHOD
A Methodology for Heat Exchanger Design Calculations
- The Log Mean Temperature Difference (LMTD) Method • A form of Newton’s Law of Cooling may be applied to heat exchangers by using a log-mean value of the temperature difference between the two fluids: q
UA
T 1m
T1m
T1
1n
T 2
T1 / T 2
Evaluation of T1 and
T 2
depends on the heat exchanger type.
• Counter-Flow Heat Exchanger : T1
Th ,1 T c ,1 Th ,i T c ,o
T2
Th ,2 T c ,2 Th, o T c ,i
LMTD METHOD (CONT.)
• Parallel-Flow Heat Exchanger : T1
Th ,1 T c ,1 Th,i T c ,i
T2
Th ,2 Th , o
T c ,2 T c , o
Note that T c,o can not exceed T h,o for a PF HX, but can do so for a CF HX.
For equivalent values of UA and inlet temperatures, T1m,CF
T 1m, PF
• Shell-and-Tube and Cross-Flow Heat Exchangers: T1m
F
T 1m ,CF
F
Figures 11.10 - 11.13
ENERGY BALANCE
Overall Energy Balance
• Application to the hot (h) and cold (c) fluids:
• Assume negligible heat transfer between the exchanger and its surroundings and negligible potential and kinetic energy changes for each fluid. q m h ih,i ih ,o q i
m c ic , o
ic ,i
fluid enthalpy
• Assuming no l/v phase change and constant specific heats, q
m h c p , h Th ,i
Th ,o
Ch Th,i
T h ,o
q
m c c p , c Tc , o
Tc ,i
Cc Tc , o
T c ,i
Ch,C c
Heat capacity rates
SPECIAL CONDITIONS
Special Operating Conditions
Case (a): C h >>C c or h is a condensing vapor C h – Negligible or no change in Th Th , o
T h ,i .
Case (b): C c >>C h or c is an evaporating liquid C c – Negligible or no change in Case (c): C h =C c . –
T1
T2
T 1m
Tc Tc , o
.
T c ,i .
.
PROBLEM : OVERALL HEAT TRANSFER COEFFICIENT
Problem 11.5: Determination of heat transfer per unit length for heat recovery device involving hot flue gases and water.
KNOWN:
Geometry of finned, annular heat exchanger. Gas-side temperature and convection coefficient. Water-side flowrate and temperature. FIND:
Heat rate per unit length.
SCHEMATIC:
Do = 60 mm Di,1 = 24 mm Di,2 = 30 mm t = 3 mm = 0.003m L = (60-30)/2 mm = 0.015m
PROBLEM : OVERALL HEAT TRANSFER COEFFICIENT (CONT.)
ASSUMPTIONS:
(1) Steady-state conditions, (2) Constant properties, (3) One-dimensional conduction in strut, (4) Adiabatic outer surface conditions, (5) Negligible gas-side radiation, (6) Fully-developed internal flow, (7) Negligible fouling.
PROPERTIES:
Table A-6 , Water (300 K): k = 0.613 W/m K, Pr = 5.83,
2
N s/m .
ANALYSIS:
The heat rate is
q
UA
c
Tm,h
Tm,c
where 1/ UA
Rw
c
1/ hA
Rw
c
ln Di,2 / Di,1 2 kL
1/
o hA h
ln 30 / 24 2
50 W / m K lm
7.10 10 4 K / W.
= 855
-6
10
PROBLEM : OVERALL HEAT TRANSFER COEFFICIENT (CONT.)
With 4m
Re D
4 0.161 kg / s
Di,1
0.024m 855 10
6
N s/m
9990
2
the internal flow is turbulent and the Dittus-Boelter correlation gives hc
4/5
k / Di,1 0.023 ReD
hA
1 c
Pr
A
8 2 L w
Af
Di,2
From Eq. 11.4, tanh mL f
0.024m
1883 W / m2 K
The overall fin efficiency is o 1 Af / A 1 A f
0.613 W / m K
0.4
mL
0.024m
0.023 9990
1m
1
4/5
5.83
0.4
1883 W / m
7.043 10 3 K / W.
f
8 2 0.015m 1m
8t w
0.24m2
0.24m 2
0.03m 8 0.003m
0.31m 2.
2
K
PROBLEM : OVERALL HEAT TRANSFER COEFFICIENT (CONT.)
where
m
2h / kt
1/ 2
mL
2h / kt
tanh
2h / kt
1/ 2
1/ 2
2 100 W / m 2 K / 50 W / m K 0.003m L
36.5 m 1 0.015m
L
1/ 2
36.5m 1
0.55
0.499.
Hence f
0.800/1.10
o
1
0.907
Af / A 1
o hA
1 h
f
1
0.24 / 0.31 1 0.907
0.928 100 W / m 2 K 0.31m 2
1
0.928
0.0347 K / W.
It follows that UA UA
1 c c
7.043 10 3
7.1 10 4
0.0347 K / W
23.6 W / K
and
q
23.6 W / K 800 300 K 11,800 W
for a 1m long section.
<
PROBLEM : OVERALL HEAT TRANSFER COEFFICIENT (CONT.)
COMMENTS:
A f
(1) The gas-side resistance is substantially decreased by using the fins
Di,2 and q is increased.
(2) Heat transfer enhancement by the fins could be increased further by using a material of larger k, but material selection would be limited by the large value of T m,h.
ROBLEM: CEAN HERMAL NERGY CONVERSION
Problem 11.47:Design
of a two-pass, shell-and-tube heat exchanger to supply vapor for the turbine of an ocean thermal energy conversion system based on a standard (Rankine) power cycle. The power cycle is to generate 2 MWe at an efficiency of 3%. Ocean water enters the tubes of the exchanger at 300K, and its desired outlet temperature is 292K. The working fluid of the power cycle is evaporated in the tubes of the exchanger at its phase change temperature of 290K, and the overall heat transfer coefficient is known.
FIND:
(a) Evaporator area, (b) Water flow rate.
SCHEMATIC:
ROBLEM: CEAN HERMAL ENERGY CONVERSION (CONT)
ASSUMPTIONS:
(1) Negligible heat loss to surroundings, (2) Negligible kinetic and potential energy changes, (3) Constant properties. PROPERTIES:
ANALYSIS:
Table A-6 , Water ( Tm = 296 K): cp = 4181 J/kg K.
(a) The efficiency is
W
2 MW
q
q
0.03.
Hence the required heat transfer rate is 2 MW
q
0.03
66.7 MW.
Also T m,CF
300 290 n
and, with P = 0 and R = A
q U F T m,CF
A
11,100 m 2 .
292 290 C 300 290
5 C
292 290
, from Fig. 11.10 it follows that F = 1. Hence 6.67 10 7 W 1200 W / m 2 K 1 5 C
