HEAT EXCHANGER
Introduction
Heat transfer is perhaps the most important as well
as most applied process in chemical and petrochemical plants. The economics of
plant operation often are controlled by the effectiveness of the utilization
and recovery of heat .The word exchanger applied to all type of equipment in
which heat is exchange but is often specifically to denote equipment in which
heat is exchange between two process streams. A heat exchanger is a piece of
equipment that continually transfers heat from one medium to another, without
mixing the process fluids. Heat exchangers are used to pre heat the feed for
the Stripper.
Significance in Process
Basic purpose of heat
exchanger is to heat the waste oil to its bubble point temperature (134oC)
from 25oC, so it can be used in flash distillation column
CLASSIFICATION OF HEAT EXCHANGER
In general
industrial heat exchangers are classified according to there: Construction
1.
Transfer
processes
2.
Degrees of
surface compactness
3.
Flow
arrangements
4.
Pass arrangements
5.
Phase of the
process fluid
6. Heat transfer mechanism
Classification according to Construction
According
to construction heat exchangers are:
1)
Tubular heat
exchanger (double pipe, shall and tube, coiled tube)
2)
Plate heat
exchanger (gas kited, spiral, plate coil, lamella)
3)
Extended surface exchangers (tube fin, plate fin)
4)
Regenerators
(fixed matrix, rotary)
Classification according to
Transfer Process
These
classifications are:
1.
Indirect Contact
(double pipe, shall and tube, coiled tube)
2.
Direct contact
(cooling towers)
Classification according to
Surface Compactness
A compact heat
exchanger incorporates a heat transfer surface having a high area density,
which is the ratio of heat transfer area (A) to its volume (V) it is somewhat 700
m2/m3. They can often achieve higher thermal
effectiveness than shall and tube exchangers. (95% vs. 60-80% for STHE) which
makes them particularly useful in energy intensive industries.
Classification according to Flow Arrangement
The basic flow
arrangements in a heat exchanger are
1. Parallel flow
2. Counter flow
3.
Cross flow
The choice of a particular flow arrangement is
dependent upon the required exchanger effectiveness, fluid flow paths,
packaging envelope, allow able thermal stress, temperature levels etc.
Classification according to Pass Arrangement
A fluid is
considered to have made one pass if it flows through a section of heat
exchanger through its full length once. There are either single pass or multi
pass , in a multi pass arrangement the fluid is reversed and flows through the
flow length two or more times. The multi pass arrangements are possible with
compact shall and tube and plate exchangers.
Classification according to Phase of Fluid
These
classifications is made according to the phase of the fluid I-e gas-gas,
1. liquid-liquid
2. Gas-liquid
Heat Exchanger Selection
Criteria
When selecting a heat exchanger for a given
duty the following points must be considered
1.
Material of
construction
2.
Operating
pressure and temperature
3.
Flow rates
4.
Flow
arrangements
5.
Performance parameters--thermal
effectiveness and pressure drops
6.
Fouling
tendencies
7.
Types and phases
of fluids
8.
Maintenance,
inspection, cleaning, extension, and repair possibilities
9.
Overall economy
10. Fabrication technique
11. Intended applications
Types of Heat
Exchanger
1.
Double pipe
2.
Shell and tube
3.
Spiral type
4.
Plate and frame
5.
Compact heat exchanger
Types of shell & tube heat exchanger
1.
Fixed tube
2.
U tube
3.
floating head
Advantages of Shell and Tube Exchanger
1.
It is used for
high heat transfer duties.
2.
It occupies less
space.
3.
Its compactness
is more.
4.
Its maintenance
is easy.
5.
It can be
fabricated with any type of material depend up fluid properties.
Typical Shell-and-Tube Heat Exchanger
FLUID
ALLOCATION
1-Shell
side fluid selection (Reactor Effluent):
Ø High viscosity fluid.
Ø Fluid which exhibit low heat transfer coefficient.
2-Tube
side fluid selection (steam):
Ø Fluid at high pressure.
Ø Corrosive fluid.
Ø Fluid having high heat transfer coefficient.
GEOMETRICAL
DESIGN
1. SHELL:
i.
Shell type
ii.
No. of shell
iii. Shell id.
2. TUBES:
i.
Outer dia.
ii.
Wall thickness
iii.
Tube pitch
iv. No. of tubes
3. BAFFLES
i.
Baffle type
ii.
Baffle cut
iii.
Orientation
iv.
No. of cross
passes
SHELL SELECTION
Shell
diameter
Range
is 200mm to 2500mm
Shell orientation
i.
Horizontal
ii.
Vertical
I have selected horizontal because,
Ø Frequently used
Ø Easy maintenance
Ø Economical
TUBE SELECTION
Type:
Plane
tube
Ø Commonly used
Ø Readily available
Ø Wide range of wall thickness
Tube
OD and wall thickness:
0.0195m, 10 BWG
Tube length:
Standard length are used
(2.44,3.66,4.88 m)
Tube
effective length:
For floating head
Le=L-2Ts
L= length
Ts= tube sheet
thickness
for
Ds<500mm ; Ts=50mm
Tube pattern:
i.
Triangular pitch
ii.
Rotated
triangular
iii.
Square
iv.
Rotated square
I have selected triangular pitch because
Ø Large
no of tubes can be accommodated
Ø It
provides large surface area
Ø High
heat transfer
Tube pitch:
Standard
way is 1.25times or more than this outside diameter
BAFFLES
i.
Segmental baffles
ii.
Disc and doughnut
iii.
Orifice baffles
Segmental baffles:
Cross
baffles are provided in shell and tube heat exchanger in order to force the
shell side fluid to flow across the tube bundles and to support tube bundles
Types:
i.
Single segmental baffle
ii.
Double segmental
iii.
Triple segmental
Single
segmental baffles are used unless there is no
pressure drop limitation
Baffle cut
For
segmental and single phase service 25 percnt recommended.
Orientation:
i.
Vertical
cut
ii.
Horizontal cut
iii.
Single phase with solids
Baffle Spacing:
Normally
0.2-1 shell ID
Minimum baffle spacing should not be less than the
1/5 ID of shell or 50mm
Design
procedure for shell-and-tube heat exchangers:
|
Reactor Effluent |
Steam |
||
|
Flow rate |
3116 (Kg/hr) |
Flow rate |
3086(Kg/hr) |
|
Inlet Temperature |
220 oC |
Inlet Temperature |
370oC |
|
Outlet Temperature |
335oC |
Outlet Temperature |
370oC |
|
Pressure |
40 atm |
Pressure |
21 atm. |
|
CP |
3.99 (Kj/Kg.oC) |
Latent heat |
452.6 (Kj/Kg. ) |
Design of heat exchanger
DESIGN
STEPS
1-HEAT LOAD:
used oil:
Q = m cp (tc,out
– tc,in)
=3116*3.99*(335-220)
=388
kj/sec
steam:
M steam=q/λ
AT 370 oC
;
λ=452.6Kj/kg
M steam
=406.7e3/2041
=0.85kg/sec
m=mass flow rate of oil
m=mass flow rate of steam
λ=latent
heat
Calculation of
LMTD:
=79 oC
ASSUME:
Ud=220 W/m2 c
A=Q/Ud.LMTD
=70.3 m2
OD=19.5mm
BWG=10
0.024 m triangular pitch
L=length=4.88 m
Le=L-2Ts (Ts=50mm for ID<500mm)
=4.78 m
Area of 1
tube, At,=πDoL =0.31m
No of tubes,Nt=Ao/At
=150
For 2 tube passes,no of tubes=75
|
HOT
FLUID(tube side) 4-FLOW AREA: at=Nt*π/4*Di2
=0.012m2 5 vol. flow rate=W/ρ 7-MASS VELOCITY: Gt=W/at
=71.58Kg/m2.sec 8-at Tavg=370 oC μ=9.8e-5 pa.sec 9-Di=0.01483 m 10-Re=D*Gt/ μ
=10832 11- hio=15000W/m2.k
|
COLD
FLUID(Reactor Effluent) 4-FLOW AREA: as=(Pt-Do) *Ds*Lb/Pt =0.07m2 5- vol flow rate=w/ρ =3.3e-03 6-shell side vel=vol flow/as =0.5m/sec 7-MASS VELOCITY: Gs=W/as
=123.7kg/m2.sec 8- Tavg
=277.5oC μ=3.89e-4 pa.sec De=4(Pt2
–(π/4) do2)/πdo
=0.014m 10- Re=De*Gs/
μ =4452 9- Jh=4.5e-03 10- Ta=277.5oC k=0.142W/m.
oC (Cp
μ/k)⅓ = 10.7
11- ho =Jh.k/DePr(Cp
μ/k)⅓. ho=466
W/m2.k |
1/Uo=1/ho+1/hod+(do*ln(do/di))/(2Kw)+(do/di)*hid+(do/di*hi
U0=247 W/m2 oC
PRESSURE DROP
|
TUBE: 1- Re =
10832 2- Jf =2.3e-03 ΔPt=Np[8Jf(L/di))*(µ/µw)0.14+2.5]*ρ*ut2/2 =13
KPa |
SHELL: 1- Re= 4452 Jf
=6e-02 ΔPs=8*Jf(Ds/de)*(L/Lb)(ρ*us2
/2) =42.2 KPa |
SPECIFICATION
SHEET
|
Equipment |
1-2 pass shell
& tube heat exchanger |
|
Shell passes |
1 |
|
Tube passes |
2 |
|
Heat transfer area |
70.3m2 |
|
Shell diameter |
400 mm |
|
Tube pitch |
0.024 m |
|
No of tubes |
150 |
|
Type of tube uses |
10 BWG |
|
NO 0f baffles |
3 |
|
Tube outer
diameter |
0.0195 m |
|
Tube inner
diameter |
0.01485 m |
|
Pressure drop
on tube side |
13 KPa |
|
Pressure drop
on shell side |
42 KPa |
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