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Lecture on Heat Exchanger
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bblee@UniMAP
1
Heat ExchangerERT 216 HEAT & MASS TRANSFER
bblee@UniMAP 2
1. Types of Heat Exchangers2. Log Mean Temperature
Difference Correction Factors3. Heat Exchanger Effectiveness4. Fouling Factors and Typical
Overall U values5. Heat Exchanger Design
bblee@UniMAP3
Heat exchangers: Heat transfer between 2 fluids.
Common type: The hot & cold fluids do not come into
direct contact with each other but are separated by a tube wall or a flat or curved surface.
Heat transfer from the hot fluid to the wall or tube surface is accomplished by convection, through the tube wall or plate by conduction, & then by convection to the cold fluid.
bblee@UniMAP4
Double-pipe (concentric-pipe) heat exchanger: The simplest exchanger, where one fluid
flows inside one pipe and the other fluid flows in the annular space between the two pipes.The fluids can be in co-current or counter-current flow.It is useful mainly for small flow rates.
bblee@UniMAP5
Figure 1: Flow in a double-pipe heat exchanger.
bblee@UniMAP6
Shell-and-tube exchanger:In these exchangers the flows are continuous.Many tubes in parallel are used, where
one fluid flows inside these tubes. The tubes, arranged in a bundle, are
enclosed in a single shell & the other fluid flows outside the tubes in the shell side.
bblee@UniMAP7
The cold fluid enters & flows inside through all the tubes in parallel in 1 pass.
Figure 2: Shell-&-tube heat exchangers: (a) 1 shell pass & 1
tube pass (1-1 exchanger) (a)
(b)
(b) 1 shell pass and 2 tube passes (1-2 exchanger).
bblee@UniMAP8
1-2 parallel counter-flow exchanger: The liquid on the tube side flows in two
passes as shown & the shell-side liquid flows in one pass.
In the first pass of the tube side, the cold fluid is flowing counter-flow to the hot shell-side fluid; in the second pass of the tube side.
The cold fluid flows in parallel with the hot fluid.
bblee@UniMAP9
Cross-flow exchanger:It is commonly used to heat or cool a gas.One of the fluids (liquid), flows inside through the tubes, and the exterior gas flows across the tube bundle by forced or sometimes natural convection. The fluid inside the tubes is considered
to be unmixed, since it is confined and cannot mixed with any other stream.
bblee@UniMAP10
Figure 3: Flow patterns of cross-
flow heat exchangers: one fluid mixed (gas) and one
fluid unmixed.
Both fluids unmixed type: It is typically used in air-conditioning
and space-heating applications.
bblee@UniMAP11
In this type the gas flows across a finned-tube bundle and is unmixed, since it is confined in separate flow channels between the fins as it passes over the tubes.
Figure 4: Flow patterns of cross-
flow heat exchangers: both fluids unmixed.
bblee@UniMAP12
When the hot & cold fluids in a heat exchanger are in true counter-current flow or in co-current (parallel) flow, the log mean temperature difference should be:
The equation holds for a double pipe heat exchanger & a 1-1 exchanger with one shell pass & one tube pass in parallel or counter-flow.
12
12
ΔΔ
ΔΔΔ
TTln
TTTlm
Temperature difference at one
end of the exchanger
bblee@UniMAP13
In cases where a multiple pass heat exchanger is involved, it is necessary to obtain a different expression for ∆Tlm.
Depending on the arrangement of the shell & tube passes.∆Tlm which applies either to parallel or to
counterflow but not to a mixture of both types cannot be used without a correction.The usual procedure is to use a correction
factor (FT) which is so defined that when it is multiplied by ∆Tlm, the product is the correct mean temperature drop ∆Tm to use.
bblee@UniMAP14
In using the correction factor (FT), It is immaterial the warmer fluid flows through the
tubes or the shell. For a 1-2 exchanger, two dimensionless
ratios are used:
cico
hohi
TT
TTZ
cihi
cico
TT
TTY
Inlet temperature of hot fluid (K)
Outlet of hot fluid (K)
Inlet of cold fluid (K)
Outlet of cold fluid (K)
bblee@UniMAP15
Fig 5: Correction factor (FT) to ∆Tlm for 1-2 & 1-4 exchanger
Fig 6: Correction factor (FT) to ∆Tlm for 2-4 exchanger
bblee@UniMAP16
It is not recommended to use a heat exchanger for conditions under FT < 0.75.Another shell and tube arrangement
should be used.
Fig 7: Correction factor (FT) to ∆Tlm for cross flow exchangers
(a) Single pass, shell fluid mixed, other fluid unmixed.
(b) Single pass, both fluids unmixed.
bblee@UniMAP17
Then, the equation for an exchanger is:
cohocohi
cihocohi
TTTTln
TTTT
lmTΔ
moomii TAUTAUq ΔΔ
lmTm TFT ΔΔ
bblee@UniMAP18
Example 4.9-1:A 1-2 exchanger containing one shell pass
and two tube passes heats 2.52 kg/s of water from 21.1 to 54.4 0C by using hot water under pressure entering at 115.6 and leaving at 48.90C.
The outside surface area of the tubes in the exchanger is A0 = 9.30 m2.
(a) Calculate the mean temperature difference ∆Tm in the exchanger and the overall heat-transfer coefficient U0.
(b) For the same temperatures but using a 2-4 exchanger, what would be the ∆Tm?
bblee@UniMAP19
Solution [Example 4-9-1]:(a)
bblee@UniMAP20
From Fig. 5:
FT = 0.74
)K(C.).(.TFT o
lmT 331342740ΔΔ
bblee@UniMAP21
From Fig. 6:(b) FT = 0.94
bblee@UniMAP22
In the design of heat exchangers, ∆Tlm was used in the equation:
This form is convenient when the inlet & outlet temperatures of the two fluids are known or can be determined by a heat balance. The surface area can be determined if U
is known. Heat exchanger effectiveness (ε) is used
which does not involve any of the outlet temperatures.
lmTUAq Δ
bblee@UniMAP23
ε is defined as the ratio of the actual rate of heat transfer in a given exchanger to the maximum possible amount of heat transfer if an infinite heat transfer area were available.
The temperature profile for a counter-flow heat exchanger is shown:
Fig 8:Temperature profile for counter-
current heat exchanger
(mcp)H
(mcp)C
bblee@UniMAP24
CH>CC and the cold fluid under-goes a greater temperature change than the hot fluid, Cc is designated as Cmin (minimum heat capacity):
If the hot fluid is the minimum fluid, THO = Tci,
CiHimin
HoHimax
CiHiC
HoHiH
TTC
TTC
TTC
TTCε
CiHimin
CiCOmax
CiHiH
CiCOC
TTC
TTC
TTC
TTCε
bblee@UniMAP25
The denominators of both equations are the same, the numerator gives the actual heat transfer:
For the case of single-pass, counter-flow exchanger,
CiHimin
CiCOC
CiHimin
HOHiH
TTC
TTC
TTC
TTCε
CiHimin TTCεq
Only inlet temperatures
bblee@UniMAP26
We consider the case when the cold fluid is the minimum fluid:
After re-arrangement & solving,
COHiCiHO
COHiCiHOCiCOC
TTTTln
TTTTUATTCq
max
min
minmax
min
max
min
min
C
C
C
UAexp
C
C
C
C
C
UAexp
ε
11
11 Graphical form
(See Fig. 9)
bblee@UniMAP27
NTU could be defined as the number of transfer units:
For parallel flow,
minC
UANTU
Same result, CH = C min
max
min
max
min
min
C
C
C
C
C
UAexp
ε
1
11 Graphical form
(See Fig. 10)
bblee@UniMAP28
Fig 9: Heat-exchanger effectiveness (ε) for
counter-flow exchanger.
Fig 10: Heat-exchanger effectiveness (ε) for
parallel flow exchanger.
bblee@UniMAP29
Example 4.9-2:Water flowing at a rate of 0.667 kg/s
enters a counter-current heat exchanger at 308 K and is heated by an oil stream entering at 383 K at a rate of 2.85 kg/s (cp=1.89 kJ/kg.K).
The overall U = 300 W/m2.K and the area A = 15.0 m2.
Calculate the heat transfer rate and the exit water temperature.
bblee@UniMAP30
bblee@UniMAP31
In actual practice, heat-transfer surfaces do not remain clean.Dirt, soot, scale, & other deposits form
on one or both sides of the tubes of an exchanger and on other heat-transfer surfaces. These deposits offer additional resistance to the flow of heat & reduce the overall heat-transfer coefficient, U. Biological growth (e.g. algae) can occur
with cooling water & in the biological processes.
bblee@UniMAP32
Water velocities above 1 m/s are generally used to help reduce fouling.
Large temperature differences may cause excessive deposition of solids on surfaces and should be avoided if possible.
The effect of such deposits & fouling is usually taken care of in design by adding a term for the resistance of the fouling on the inside & outside of the tube:
doo
i
oo
i
lmAA
iio
dii
i
hA
A
hA
A
Ak
Arr
hh
U11
1
bblee@UniMAP33
hdi: The fouling coefficient for the inside (W/m2.K).
hdo: The fouling coefficient for the outside of the tube (W/m2.K).
Table 1: Typical Fouling Coefficients
bblee@UniMAP34
Table 2: Typical values of overall heat-transfer coefficients in shell-and-tube exchangers
bblee@UniMAP35
A substantial number of parameters is involved in the design of a shell-and-tube heat exchanger for specified thermal and hydraulic conditions and desired economics, including:
tube diameter size of shell length
number of passes
number of shell baffles
pitch
square or triangular
baffle type thickness
baffle windows baffle spacing
bblee@UniMAP36
A logic diagram of a heat exchanger design procedure appears in Fig. 11.
Fig 11: A procedure for the design of a heat exchanger, comprising a tentative selection of design parameters
bblee@UniMAP37
The key elements of heat exchanger design are:i. Selection of a tentative set of design
parameters. ii. Rating of the tentative design, which
means evaluating the performance with the best correlations and calculation methods that are feasible.
iii. Modification of some design parameters, then rerating the design to meet thermal and hydraulic specifications and economic requirements.
bblee@UniMAP38
Stainless Steel Tubes:The simplest method to increase heat
transfer is to increase the number of tube-side passes, if the controlling resistance to heat transfer is on the tube side and current tube-side velocities are low (< 3 ft/s).Other than high velocities, the next best
method to control fouling on the both shell and tube sides is to prevent the formation of rough surfaces due to corrosion.
bblee@UniMAP39
A smooth, mirror-finished surface will retard the accumulation of fouling deposits.
Two notes of caution regarding re-tubing with stainless:i.Do not put stainless tubes in direct
physical contact with carbon steel tube support baffles/carbon steel tube sheets. The results will be galvanic corrosion of
the carbon steel components. nine chrome tubes are consistent with
carbon steel components.
bblee@UniMAP40
ii. 304, 316 and 317 stainless should not be used in crude preheat.
At least, not upstream of the desalter. The problem is chloride stress corrosion
cracking.
Sintered Metal Tubes:Rough surfaces are bad for sensible heat
transfer due to fouling.But in clean services, rough surfaces are
critically important to boil water and hydrocarbons.
bblee@UniMAP41
Rough surfaces provide nucleation sites for bubbles to form.
For one butane reboiler, an old reboilerbundle with pitted carbon steel tubes had double the heat transfer capacity of a new bundle.
There are two ways to prevent loss of heat transfer on reboilers when a newly retubedbundle is commissioned. A sintered metal coating can be applied to
the tubes, or the tubes can be lightly sand blasted to
roughen their surface.
bblee@UniMAP42
Tube Inserts:These are springs that are inserted into
the tubes. Some types are fixed and some spin with
the flow. The objective is to create turbulence
that reduces the tube-side film resistance, and the rate of tube-side fouling.
bblee@UniMAP43
Twisted Tubes:It looks like a 1-inch hollow drill bit. The idea is that the twisted surface
generates more turbulence on both the tube side and the shell side than a straight tube.
The twisted tube does not require any tube support baffles.
The tubes touch and thus are self-supporting.
bblee@UniMAP44
However, without any tube support baffles, an external sleeve or shroud is needed to keep the tubes in place.
Care must be taken in handling the shroud so as not to alter the alignment of the twisted tubes.
Fig 12:A twisted tube. Bundle does not have tube support baffles.
bblee@UniMAP45
Helical Tube Support Baffles:The flow through the tube side of this
type of exchanger is conventional. However, the shell-side flow is unique. It is neither perpendicular to the
tubes nor parallel. Rather, the shell-side flow follows a
screw-type pathway across the tubes. It is the angled slope of the baffles that
induce this sort of helical or screw-type flow to the shell-side liquid.
bblee@UniMAP46
Fig 13:Tube bundle with helical tube support baffles. Liquid flows in a screw type path.
bblee@UniMAP47
The advantage of this sort of flow is that dead zones are eliminated in the exchanger areas where ordinarily the flow changes directions along the edge of the tube support baffles.
For this strategy to work, the controlling resistance to heat transfer must be on the shell side.
bblee@UniMAP48
Practical ideas to enhance heat transfer efficiency:
Several more conventional methods to enhance heat transfer are:
i. Minimize clearance between tube support baffles and shell ID, as per TEMA specifications.
ii. Use ½-inch rather than the standard ¼-inch space between tubes. o This will reduce dirt bridging between
tubes on the shell side. o This bridging problem creates dead
zones with no flow and no heat transfer.
bblee@UniMAP49
iii.Specify the highest possible allowable pressure drops on the exchanger data sheets. o This permits vendors to design for a
high velocity, which suppresses fouling rates.
iv. Include block valves and bypasses to stop flow for brief periods. o The result is thermal spalling and/or
melting of deposits from tube surfaces.
bblee@UniMAP50
v. Use floating head, not U tube exchangers. o U tube exchangers cannot be visually
inspected after cleaning the tube side of the bundle.
vi. Do not rerun cracked recovered slops through heat exchangers after the slops have been exposed to air.o Polymerization will result at about
300°F to 350°F. The polymers form gums which promote fouling.
o Virgin materials do not polymerize.
bblee@UniMAP51
vii. Use vertically cut baffles in boiling or condensation services for the shell-side flow.
viii.Charge from tanks with floating suctions. o Dirt will settle out of the bottom of
the tanko Wait until just before a unit
turnaround to start the internal tank mixers.
o Exchangers will then foul rapidly.
bblee@UniMAP52
ix. Watch for hydrocarbon leaks in cooling water system. o Hydrocarbons promote biological fouling inside the tubes.
x. Elevate condensers above reflux drums for drainage to avoid condensate backup.
xi. Place the fluid with the lowest Reynolds number (i.e., the high viscosity fluid) on the shell side. o the most important point
bblee@UniMAP53
o The resulting vortex shedding as the liquid flows perpendicularly across the tubes will avoid laminar flow and the resulting high heat transfer film resistance.
o As long as the tube pitch is rotated square, the shell will still be able to be cleaned even though the shell-side flow is dirty.