heat transfer/heat exchanger - جامعة نزوى · a shell and tube heat exchanger is a class of...
TRANSCRIPT
As we discussed early in the first chapter that heat can transfer through materials and the surrounding medium whenever temperature gradient exists until thermal equilibrium is reached.
Heat transfer by: Radiation is often categorized as either
ionizing radiation or non-ionizing radiation depending on the energy of the radiated particles.
Conduction is the transfer of heat through materials by the direct contact of matter.
Convection is the transfer of heat by the motion of the fluid (liquids and gases). Natural convection Forced Convection
Natural and forced Convection
Natural convection occurs whenever heat flows between a
solid and fluid, or between fluid layers.
As a result of heat exchange, change in density of effective
fluid layers taken place, which causes upward flow of heated
fluid.
If this motion is associated with heat transfer mechanism only,
then it is called Natural Convection
If this motion is associated by mechanical means such as
pumps or fans, the movement of the fluid is enforced.
And in this case, we then speak of Forced convection.
Forced Convection
Usually involves convection in each fluid and conduction
through the wall of separating the two fluids.
The overall heat transfer coefficient U contributes to all
these factors.
Usually work with the LMTD.
LMTD = Logarithmic Mean Temperature Difference
Heat Exchanger is a device that provide the flow of thermal energy
between 2 or more fluids at different temperatures..
They are used in a wide variety of applications. These include power
production process, chemical, food and manufacturing industries,
electronics, environmental engineering, waste heat recovery, air
conditioning, reefer and space applications.
Heat Exchangers may be classified according to the following criteria.
Recuperators/ regenerators
Transfer process: direct and indirect contact
Geometry of construction; tubes, plates, and extended surfaces.
Heat transfer mechanism: single phase and two phase
Flow arrangement: Parallel, counter, cross flow current.
Classification of Heat Exchangers
As mentioned in the previous slight, according to transfer process heat
exchangers are classified as direct contact type and indirect contact type.
In direct contact type, heat is transferred between cold and hot fluids
through direct contact of the fluids (e.g. cooling towers, spray and tray
condensers)
In indirect heat exchanger, heat energy is transferred throw a heat transfer
surface.
Heat Exchangers
prevent car engine
overheating and
increase efficiency
Heat exchangers are
used in Industry for
heat transfer
Heat exchangers are
used in AC and
furnaces
Heat Exchanger
Double pipe heat exchanger
Concurrent\Parallel Countercurrent
Advantages of Double Pipe Heat Exchanges:
1. Simplest type of heat exchangers
2. Can be easily assembled
3. Relatively low cost
4. Small sizes
Disadvantages of Double Pipe Heat Exchanger:
1. Leakages are very common
2. Requires a lot of time in dismantling and cleaning
3. Small surface area of heat transfer/pipe
4. Space requirements are large
Double pipe heat exchangers should be considered first in design. The
heat transfer surface should not exceed 200 ft2.
If several double pipes are required, their weight increases and thus the
shell and tube heat exchangers is better.
Shell and Tube Heat Exchangers
A shell and tube heat exchanger is a class of heat exchanger designs. It is
the most common type of heat exchanger in oil refineries and other large
chemical processes.
Shell and tube heat exchangers normally consist of a bundle of tubes
fastened into holes, drilled in metal plates called tube sheets.
The Tubular Exchanger Manufacturers Association (TEMA) provides a
manual of standards for construction of shell and tube heat exchangers,
which contains designations for various types of shell and tube heat
exchanger configurations.
The most common types are summarized below.
Shell and Tube Heat Exchangers
The E-type shell and tube heat exchanger, illustrated in Fig. 2, is the
workhorse of the process industries, providing economical rugged
construction and a wide range of capabilities.
E-Type
The E-type shell is usually the first choice of shell
types because of lowest cost, but sometimes requires
more than the allowable pressure drop, or produces a
temperature, so other, more complicated types are used.
Tubular Exchanger
Manufacturers Association
F-Type
The F-type shell can be effective in some cases if well designed, but has a
number of potential disadvantages, such as :
Thermal and fluid leakage around the longitudinal baffle.
High pressure drop.
Tubular Exchanger
Manufacturers Association
When an F-type shell cannot be used because of high pressure drop, a J-type
or divided flow exchanger, shown in Fig. 4, is considered.
J-Type
Tubular Exchanger
Manufacturers Association
When a J-type shell would still produce too high a pressure drop, an X-type
shell, shown in Fig. 5, may be used.
This type is especially applicable for vacuum condensers, and can be
equipped with integral finned tubes to counteract the effect of low shellside
velocity on heat transfer.
X-Type
Tubular Exchanger
Manufacturers Association
Baffles are used to increase velocity of the fluid
flowing outside the tubes (shellside fluid) and to
support the tubes. Higher velocities have the advantage
of increasing heat transfer and decreasing fouling
(material deposit on the tubes), but have the
disadvantage of more energy consumption.
Baffle-Type
Baffle types commonly used are shown in Fig. 9, with pressure drop
decreasing from Fig. 9a to Fig. 9c.
Shell and Tube Heat Exchangers
w,cp,t1
w,cp,t2
• Non-baffled Heat Exchangers W,Cp,T1
W,Cp,T2
IDs
do
di
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
Heat Exchangers 24
One Shell Pass and One Tube Pass
One Shell Pass,
Two Tube Passes Two Shell Passes,
Four Tube Passes
Shell-side flow
TEMA Designations
Tubular Exchanger
Manufacturers Association
The heat transfer surface consists of a number of thin
corrugated plates pressed out of a high grade metal.
The pressed pattern on each plate surface induces
turbulence and minimizes stagnant areas and fouling.
Unlike shell and tube heat exchangers, which can be
custom-built to meet almost any capacity and
operating conditions, the plates for plate and frame
heat exchangers are mass-produced using expensive
dies and presses.
Plate heat exchangers
Superior thermal performance is the hallmark of plate heat
exchangers.
Compared to shell-and-tube units, plate heat exchangers offer
overall heat transfer coefficients 3 to 4 times higher.
These values, typically 4000 to 7000 W/m2 ºC (clean), result in very
compact equipment.
This high performance also allows the specification of very small
approach temperature (as low as 2 to 3 ºC) which is sometimes useful
in geothermal applications.
Selection of a plate heat exchanger is a trade-off between U-value
(which influences surface area and hence, capital cost) and pressure
drop (which influences pump head and hence, operating cost).
Casketed plate heat exchangers (plate and
frame heat exchangers)
Brazed plate heat exchangers
Welded plate heat exchangers
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Finned - Both Fluids Unmixed
Finned - Both Fluids Unmixed
Unfinned - One Fluid Mixed the Other Unmixed
Unfinned - One Fluid Mixed the Other Unmixed
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 (>700 m2/m3), small flow
passages, and laminar flow.
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37
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Tubes
Standard tube lengths are 8, 12,
16 and 20 ft.
Tubes are drawn to definite wall
thickness in terms of BWG
(Birmingham Wire Gauge) and
true outside diameter (OD), and
they are available in all common
metals.
The spacing between the
tubes (center to center) is
referred to as the tube
pitch (PT). Triangular or
square pitch arrangements
are
used. Unless the shell side
tends to foul badly,
triangular pitch is Used.
Tube Pitch
•Heat Exchanger (HEX) Rating
Checking the existing design for compatibility with the user requirements (outlet temperature, heat load etc.)
given: flow rates, inlet temperatures, allowable pressure drop; thus HT area and passage dimensions.
find: heat transfer rate, fluid outlet temperatures, actual pressure drop.
•HEX Sizing
Thermal and pressure drop considerations, maintenance
scheduling with fouling consideration.
given: inlet and outlet temperatures, flow rates, pressure
drop
find: dimensions -type and size of HEX.
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Assumptions for Basic Design Equations for Sizing
◦ steady-state, steady flow
◦ no heat generation in the HEX
◦ negligible ΔPE, ΔKE
◦ adiabatic processes
◦ no phase change (later)
◦ constant specific heats and other physical
properties.
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Because the temperature difference between
the hot and cold fluid streams varies along
the length of the heat exchanger, it is
necessary to derive an average temperature
difference from which heat transfer
calculations can be performed.
This average temperature difference is called
the Logarithmic Mean Temperature Difference
(LMTD) ΔTlm.
)/ln( io
iolm
TT
TTT
Where, ΔTo = T1 – T4
ΔTi = T2 – T3
Log Mean Temperature Difference (LMTD) is the heat flows between the hot and cold
streams due to the temperature difference across the tube acting as a driving force. As seen
in the Figure below, the difference will vary with axial position within the HX.
2
1
21
ln
LMTD
Where, θ1 = T1-t2
θ2 = T2-t1
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Parallel Flow CounterflowParallel Flow Counterflow
• : • :
Parallel Flow CounterflowParallel Flow Counterflow
LMTD Method
Expression for convection heat transfer for flow of a fluid inside a tube:
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)( ,, imompconv TTcmq
lms TAUq )/ln( io
iolm
TT
TTT
U = Overall heat exchanger coefficient, q = heat transfer rate
In a two-fluid heat exchanger, consider the hot and cold fluids separately:
)(
)(
,,,
,,,
icoccpcc
ohihhphh
TTcmq
TTcmq
lmTUAq and
Need to define U and Tlm
Where:
qh : the heat power emitted from hot fluid.
qc : the heat power absorbed by cold fluid.
ṁh , ṁc : mass flow rate of hot and cold fluid, respectively.
hh,i , hh,o : inlet and outlet enthalpies of hot fluid, respectively
hc,i , hc,o : inlet and outlet enthalpies of cold fluid,
respectively.
Th,i , Th,o : inlet and outlet temperatures of hot fluid, respectively.
Tc,i , Tc,o : inlet and outlet temperatures of cold fluid, respectively.
Cph , Cp
c : specific heats of hot and cold fluid, respectively
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)(
)(
,,,
,,,
icoccpcc
ohihhphh
TTcmq
TTcmq
With the LMTD method, the task is to select a heat exchanger that will meet
the prescribed heat transfer requirements. The procedure to be followed by the
selection process is:
Example 1
