ABSTRACT
Heat exchanger exchanger is a device used for transfer transfer of thermal thermal energy energy between two or more fluids that are at different temperatures. Heat exchangers work because heat naturally flows from high temperature to lower temperature. Therefore if a hot fluid and a cold fluid are separated by heat conducting surface heat can c an be transferred from the hot fluid to cold fluid. Heat exchangers may be classified according to transfer process, constructio construction, n, flow arrangement, arrangement, surface compactness, compactness, number number of fluids fluids and heat transfer mechanisms or according to process functions. Heat exchangers are useful in many engineering processes like those those in refriger refrigeratin ating g and airair- condit condition ioning ing system systems, s, power power system systems, s, food processing systems, chemical reactors and space or aeronautical applications. Doub Double le pipe pipe heat heat exch exchan ange gerr is a simp simple lest st form form of a heat heat exchanger, for particular advantages for small thermal loads and high pressure applications. It consists of a tube or pipe fixed concentrically inside a larger pipe or tube. They are used when the flow rates of the fluids and the heat duty are small less than !"" k#$.These are simple to construction, but may re%uire a lot of physical space to achieve the desired heat transfer area.
LIST OF FIGURES
Figure
Title
Page No.
&.
'lassification of heat
)
*.
(xchangers +arallel-flow heat
)
).
exchanger 'ounter-flow heat
.
exchangers. 'ross-flow heat
!
!.
exchangers shell-and-tube heat
.
exchanger Double pipe heat
/
0.
exchanger 1traight Tube Double pipe
&"
.
heat exchanger Hairpin or 2-Tube Double
&"
/.
pipe heat exchanger Double pipe heat
&&
exchanger- Two Hairpins &".
in series Double pipe heat
&&
exchanger with &&.
longitudinal fins Double pipe heat
&*
exchanger with longitudinal fins- 3ctual &*.
Image 4low sheet of +arameters
&
LIST OF FIGURES
Figure
Title
Page No.
&.
'lassification of heat
)
*.
(xchangers +arallel-flow heat
)
).
exchanger 'ounter-flow heat
.
exchangers. 'ross-flow heat
!
!.
exchangers shell-and-tube heat
.
exchanger Double pipe heat
/
0.
exchanger 1traight Tube Double pipe
&"
.
heat exchanger Hairpin or 2-Tube Double
&"
/.
pipe heat exchanger Double pipe heat
&&
exchanger- Two Hairpins &".
in series Double pipe heat
&&
exchanger with &&.
longitudinal fins Double pipe heat
&*
exchanger with longitudinal fins- 3ctual &*.
Image 4low sheet of +arameters
&
LIST OF TABLES TABLES
Table Table
Title
Page No.
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Table 4or +iping
&!
*.
'onfigurations (ffectiveness relations of
&0
Heat (xchangers
INDEX
Sr. No. &.
Content Introduction5
Page No. &
Heat (xchanger
*.
'lassification of Heat
*
). .
(xchanger 1hell and Tube (xchangers Double +ipe Heat
/
!.
(xchanger Types of Double +ipe Heat
&"
.
(xchangers 3dvantages of Double
&*
0.
+ipe Heat (xchangers 'omponents of Double
&)
. /. &". &&.
+ipe Heat (xchangers Design constraints 4low sheet of parameters Design +rocedure +ressure Drop
&) & & &/
&*. &).
'alculations 'onclusion 6eferences
*& **
HEAT EXCHANGERS Introu!tion "
Heat (xchanger is any device used for effecting the process of heat exchange between two or more fluids that are at different temperatures. The fluids may be single compounds or mixtures. In most heat exchangers, the fluids are separated by a heat transfer surface, and ideally they do not mix. 1uch exchangers are referred to as the direct transfer type, 7r simply recuperators. In contrast, exchangers in which there is an intermittent heat exchange between the hot and cold fluids via thermal energy storage and re8ection through the exchanger surface or matrix9are referred to as the Indirect transfer type or 1torage type, or simply regenerators. 1uch exchangers usually have leakage and fluid carryover from one stream to the other. Double pipe Heat (xchanger consists of a tube or pipe fixed concentrically inside a larger pipe or tube. They are used when the flow rates of the fluids and the heat duty are small less than !"" k#$.These are simple to construction, but may re%uire a lot of physical space to achieve the desired heat transfer area.
CLASSIFICATION"
n
s r
u
c i
n
S u r f a c e Cc o om p na c t tn e s as c t i n g
F l o w
B a s i s o f c l a s s i c a t i o n
#. T$%e& o' HX (.r.t Flo( ") There are two primary classifications of heat exchangers according to their flow arrangement.
•
Parallel"'lo( *eat e+!*anger&) If two fluids enter the
exchanger at the same side and moves in the same direction.
•
Counter"'lo( *eat e+!*anger&) The fluids enter the exchanger
from opposite ends, moves in parallel but in opposite direction. The counter design is most efficient, in that it can transfer the most heat from the heat transfer$ medium.
•
Cro&&"'lo( *eat e+!*anger&) In cross-flow heat exchanger, the
fluids travel at right angle to each other through the heat exchanger.
,. Cla&&i'i!ation a!!oring to tran&'er %ro!e&& ")
#. Dire!t !onta!t t$%e *eat e+!*anger& ") In direct contact type, heat
is
transferred through direct contact between the hot and
cold immiscible fluids. Direct contact heat exchange takes place between two process streams. The streams can include combinations such as gas-solid, gas-li%uid, li%uid-li%uid, li%uid-solid, or solid-solid streams. 4or obvious reasons, gas-gas systems cannot be achieved directly. Direct contact heat exchangers include heat transfer between hot and cold streams of two phases in the absence of a separating wall. Hence such exchangers can be categori:ed as gas li%uid, immiscible li%uid and solid li%uid or solid gas.
-a I//i&!ible 'lui e+!*anger " In this type, two immiscible fluid
streams are
brought into direct contact. These fluids may be single-phase fluids, or they may involve condensation or vapori:ation. 'ondensation of organic vapors and oil vapors with water or air are typical examples. -b Ga&"li0ui e+!*anger - In this type, one fluid is a gas more
commonly, air$ and the other a low-pressure li%uid more commonly, water$ and are readily separable after the energy exchange. In these exchangers, more than /"; of the energy transfer is by virtue of mass transfer due to the evaporation of the li%uid$, and convective heat transfer is a minor mechanism. 'ooling tower with forced- or natural-draft airflow is the most common application. 7ther applications are the air-conditioning spray chamber, spray drier, spray tower, and spray pond. -! Li0ui"1a%or e+!*anger " In this type, typically steam is partially
or fully condensed using cooling water or water is heated with waste steam through direct contact in the exchanger.
,. Inire!t !onta!t t$%e *eat e+!*anger& -5 In this type of heat
exchangers, the fluid streams remain separate, and the heat transfer takes place continuously through a separating wall. There is no direct mixing of the fluids because each fluid flows in separate fluid passages.
This type of heat exchanger also referred to as a surface heat exchanger, can be further classified into a$ Direct-transfer type, b$ 1torage type
a$ Dire!t"tran&'er t$%e - In this type, heat transfers continuously from the hot fluid to the cold fluid through a dividing wall. There is no direct mixing of the two or more$ fluids because each fluid flows in separate fluid passages. 1ome examples of direct transfer type heat exchangers are tubular, plate-type, and extended surface exchangers. -b Storage t$%e " In a storage type exchanger, both fluids flow
alternatively through the same flow passages, and hence heat transfer is intermittent. The heat transfer surface or flow passages$ is generally cellular in structure and is referred to as a matrix. #hen hot gas flows over the heat transfer surface through flow passages$, the thermal energy from the hot gas is stored in the matrix wall, and thus the hot gas is being cooled during the matrix heating period. 3s cold gas flows through the same passages later i.e., during the matrix cooling period$, the matrix wall gives up thermal energy, which is absorbed by the cold fluid. Thus, heat is not transferred continuously through the wall. This storage type heat exchanger is also referred to as a regenerator.
2. Cla&&i'i!ation a!!oring to %a&& arrange/ent& ") These are either
single pass or multipass. In single pass, fluid flows through a section of heat exchanger through it=s full length once. In multipass arrangement, a fluid is reversed and flows through the flow length two or more times.
3. Cla&&i'i!ation a!!oring to %*a&e o' 'lui& ")
-a Ga&"Li0ui ") >as-?i%uid heat exchangers are mostly tube-fin type
compact heat exchangers with the li%uid on the tube side. The radiator is best example of gas-li%uid heat exchanger. some other examples are air coolers for aircraft, intercoolers and aftercoolers in compressors, and condensers and evaporators of room air-conditioners. -b Li0ui"Li0ui ") @ost of the li%uid-li%uid heat exchangers are shell
and tube, +H( to a lesser extent. Aoth fluids are pumped through the exchanger, so the principal mode of heat transfer is forced convection. The relatively high density of li%uids result in very high heat transfer rate. -! Ga&"Ga& ") These type of exchanger found in rotary generators,
intercoolers. 'ompare to li%uid-li%uid exchanger, si:e of gas-gas much larger.
4. Cla&&i'i!ation a!!oring to !on&tru!tion ") 3ccording to
constructional details, heat exchangers are classified as 5 -a Tubular Heat E+!*anger& " 5 1hell-and-Tube (xchangers, Double
+ipe, 'oiled Tube -b Plate"T$%e Heat E+!*anger& ") >asketed +late Heat (xchangers,
#elded, 1piral +late Heat (xchangers, ?amella Heat (xchangers. -! E+tene Sur'a!e Heat E+!*anger& ") +late-4in Heat (xchangers 5
Tube-4in Heat (xchangers. - Regenerator& ") 6otary 6egenerators, 4ixed-@atrix 6egenerator
#. S*ell"an"Tube E+!*anger& ") 1hell-and-tube heat exchangers are
fabricated with round tubes mounted in cylindrical shells with their axes coaxial with the shell axis. The differences between them any variations of this basic type of heat exchanger lie mainly in their construction features and the provisions made for handling differential thermal expansion between tubes and shell There are various design considerations to be taken into account such as routing of fluids shell or tube$, pressure drop especially in the case of increasing number of baffles and tube diameter and ad8usting the area with the suitability of the exchanger to conduct the heat re%uired to heat or cool a fluid with another one.
A%%li!ation& ") They are extensively used as process heat exchangers
in the petroleum-refining and chemical industriesB as steam generators, condensers, boiler feed water heaters and oil coolers in power plantsB as condensers and evaporators in some air-conditioning and refrigeration applicationsB in waste heat recovery applications with heat recovery from li%uids and condensing fluidsB and in environmental control.
,. Double Pi%e Heat E+!*anger ") 3 typical double-pipe heat
exchanger is shown in 4igure below. (ssentially, it consists of one pipe placed concentrically inside another one of larger diameter, with appropriate end fittings on each pipe to guide the fluids from one section to the next. The inner pipe may have external longitudinal fins welded to it either internally or externally to increase the heat transfer area for the fluid with the lower heat transfer coefficient. The double-pipe sections can be connected in various series or parallel arrangements for either fluid to meet pressure-drop limitations and ?@TD re%uirements.
Fig" Double %i%e *eat e+!*anger -*air"%in A%%li!ation& ") The ma8or use of double-pipe exchangers is for sensible
heating or cooling of the process fluid where small heat transfer areas typically up to !" m.$ are re%uired. They may also be used for small amounts of boiling or condensation on the process fluid side. The advantages of the double-pipe exchanger are largely in the flexibility of application and piping arrangement, plus the fact that they can be erected %uickly from standard components by maintenance crews.
T$%e& o' Double Pi%e Heat E+!*anger" # Straig*t tube *eat e+!*anger& - It consists of two coaxial
pipes,is simple to fabricate and relatively easy to clean, maintain or modify.However, it takes up a lot of space and single units have limited thermal capacity.Heat exchangers of this types are sometimes made Cin house for specific small scale applications,but most double pipe heat exchangers are purchased from specialist manufacturers who provide a wide range of designs, including straight tubes,2-tubes and multiple units.
, Hair%in or U"tube *eat e+!*anger&) 2nits of this types are
designed so that the 2-tube can be withdrawn from the shells for cleaning and maintainance. There is a removable shell cover at the 2 bend and a bolted flange.The tube is held by a split ring, that can be extracted after unbolting the flange to allow the tube to slide through the shell in the direction of the 2-bend or tail end.The sealing ring between the tube and the shell is normally made from a compressible metal,however other metals are used when corrosive fluids are involved. >uy,&/)$
2 6ultitube Unit& - In this,the tubes pass through and are sealed
into a perforated plate,called a tube sheet, at the head end.4or low pressure applications the tube sheet is sealed by a single compressible ring that prevents leakage of the tube- amd shell-side fluids, in an arrangement called a unihead.4or high pressures,separated heads are employed.>uy &/)$ #hen the pressure drop available for driving the cold fluid is limited then parallelEseries arrangement may be adopted.The conventional log mean temperature difference used to calculate the performance of one double pipe heat exchanger is not applicable to the parallelEseries arrangement.
3 Double %i%e *eat e+!*anger& (it* longituinal 'in& -4ins are
formed from a strip metal, fabricated in the shape of a 2 and usually attached to the tube by spot welding.'ommonly used fin materials are carbon steel,stainless steel and alloys. 4ins made from brass or similar materials are usually soldered to copper,nickel or aluminium tubes.They have limited temperature range and are not normally used used above *!"Fc.
A1antage& o' Double Pi%e Heat E+!*anger&" # Si/%li!it$ o' !on&tru!tion" In applications that re%uires only a
relatively small heat ratingG&"""k#$ and where heat transfer enhancement is not necessary,a double pipe heat exchanger with plain tubes may be advantageous because of simplicity of construction. , Ea&e o' A!!e&& 'or /aintenan!e" The sealing of double-pipe heat
exchangers is achieved by means of flanged 8oints and sealing rings. This allows the inner pipes to be disconnected from the shells and withdrawn for cleaning, an advantages that applies both to plain and finned tubes. 2 Counter!urrent Flo( 7 It permits pure countercurrent heat exchange
in which the cold fluid can be heated to a temperature above that of the hot fluid at exit.This eliminates the restriction of Ctemperature approach or Ctemperature crossthat applies to concurrent ,or multipass systems. 3 Fea&ibilit$ o' 'inne tube&" The double pipe heat exchanger is
particularly suitable for the application of extended surface heat transfer enhancement in the form of fins.4ins are used when the shell side heat transfer coefficient is poor.This may occur when the fluid on the shellside is a gas or a high-viscosity li%uid. 4 Hig* %re&&ure a%%li!ation&" 4or a given duty, a series of double pipe
heat exchangers will re%uire much smaller shell diameters!"-*""mm$ than the e%uivalent shell and tube exchanger.Aecause of this ,the shell wal thickness is much smaller,and for high pressure applications this may be a significant factor in determining the cost and even feasibility.
Co/%onent& o' ouble %i%e *eat e+!*anger # Pa!8ing 9 glan"
The packing and gland provides sealing to the annulus and support the inner pipe. , Return ben"
The opposite ends are 8oined by a 2-bend through welded 8oints . 2 Su%%ort lug&"
1upport lugs may be fitted at these ends to hold the inner pipe position. 3 Flange"
The outer pipes are 8oined by flanges at the return ends in order that the assembly may be opened or dismantled for cleaning and maintenance. 4 Union :oint"
4or 8oining the inner tube with 2-bend .
De&ign Con&traint&"
&. Co&t to have an exchanger that costs the least. *. E''i!ien!$ to have an exchanger that operates most efficiently, with minimum loss of energy in the transfer, and minimum drop in pressure of the fluids. 2. S%a!e 7t o have an exchanger that is small . 3. 6aterial& 7 an exchanger built from materials that are
compatible with the process streams and don=t cost a lot . 4. 6aintenan!e 7 an exchanger that can be easily cleaned. ;. Ea&e o' Con&tru!tion
Flo( &*eet o' %ara/eter&"
Ste% #) Ba&i!& A1ailable 6et*o&) Logarit*/ 6ean Te/%erature Di''eren!e 6et*o or NTU 6et*o
#L6TD •
Inlet and (xit Temperatures
•
'alculate Tln
•
'alculate 4ouling factor if necessary
•
'alculate Heat transfer coefficient
•
JK 23 Tln
NTU /et*o •
Inlet Temperatures are needed
•
'alculate 'max L 'min
•
Jmax K Tmax Tmin$'min
•
'alculate (ffectiveness using appropriate expression
•
JK MNJmax
Ste% ,) Pi%ing Con'iguration
-Division of H( in six different :ones graphix add$ -'alculation of total no. of pipes in each region - Tube Dimensions - 7ptimally transverse pitch 1t$ to outer diameter 7D$ ratio is &.*! &.! &.! for & st Iteration$ - ?inear 'onfiguration or 1taggered. - "o for maximum density. Total no. of pipes can now be calculated
Table" Pi%ing !on'iguration
Ste% 2) Cal!ulation o' Re$nol& No. Re$nol& No. < Criti!al Lengt* = > -/a+. ? @ine/ati! >i&!o&it$
4or Inside the tubes5 a$ J is known and total no. of tubes is known b$ Inner cross-section area through A#> c$ O max$ is calculated d$ 'ritical length is inner diameter
4or outside the tubes5 a$ JK3 x O b$ 3rea is approx. to be consist of &Eth of total length of Heat (xchanger i.e. ".m 6egion &$. 1o total area is ".m N *.*!m c$ J can be obtained from fan calculations
Ste% 3) Nu&&elt No
?aminar or Turbulent Different for Inside and outside flow$ 4low inside the tube - ?aminar 4low 6e G &","""$ nielinski e%uation,
4 is Darcy=s friction factor given as,
Ste% 4) Cal!ulate Total Re&i&tan!e an *en!e total no. o' tran&'er unit& -NTU
Total resistance 6 x Total no. of Tubes in region$
Ste% ;) E1aluate E''e!ti1ene&&5 -/a+i/u/5 -a!tual an Outlet Te/%erature
JmaxK 'minThin - Tcin$ J K Jmax x M K 'hotThin - Thout$ K 'cold Tcout - Tcin
Ste% ) Fin Sele!tion" •
•
4in 3naly:ed5 'ircular QK4in heightR".!N4in Thickness$N1%rt'onvection Heat Transfer 'oeff.E4in @aterial=s thermal conductivityN4in Thickness$
•
Total heat capacityK(fficiencyN4in 1urface 3reaN
T*e De&ign Pro!eure"
&$ 'alculate the log mean driving force, ?@TD. *$ 1elect the diameters of the inner and outer pipes.If the allowable pressure drops for the individual streams are given,they may provide a basis for selection of the pipe diameters. )$ 'alculate the inner fluid 6eynolds numberB estimate the heat transfer coefficient hi from the Dittus-Aoelter e%uation.
Nu = hidi/k = 0.023(Re)0.8(Pr)0.3 $ 'alculate the 6eynolds number of the outer fluid flowing through the annulus.2se the e%uivalent diameter of the annulus.(stimate the outside heat transfer coefficient ho using the e%uation or the chart mentioned above. !$ 'alculate the clean overall heat transfer coefficientB calculate the design overall coefficient 2d using a suitable value of the dirt factor. $ 'alculate the heat transfer area 3for a counter flow double pipe exchanger ?@TD correction factor, 4K& Determine the length of the pipe that will provide the re%uired heat transfer area.If the length is large use a number of hairpins in series. 0$ 'alculate the pressure drop of the fluids.2se the 6eynolds number calculated above to determine the friction factor.
Pre&&ure ro% !al!ulation&" #Tube"&ie %re&&ure ro%"
∆ P t =
fGt * Ln * g ρ t d i Φ t
where, f K friction factor >t K mass velocity of the fluid ? K length of the tube, m g K/.mEs* pt K density of tube fluid diK inside diameter of tube n Kthe number of tube passes St K dimensionless viscosity ratio ∆Pt = pressure drop StKviscosity at bulk temperatureEviscosity at wall temperature$m where mK".& for 6e P *&""
and mK ".*! for 6e G *&""
In a multi-pass exchanger , in addition to frictional loss the head loss known as return loss has to be taken into account.
∆ P r = ,n V ρ t g * The pressure drop owing to the return loss is *
given by-
#here, nKthe number of tube passes OKlinear velocity of the tube fluid The total tube-side pressure drop is
∆PT = ∆Pt + ∆Pr ,S*ell"&ie %re&&ure ro%"
4or an unbaffled shell the following e%uation may be used
∆ P s =
#here,
fG s* LN * g ρ t d i Φ s
?Kshell length, m sKshell-side mass velocity, kgEm* s DHKhydraulic diameter of the shell, m SsKviscosity correction factor for the shell-side fluid
U
CONCLUSION
3 double pipe heat exchanger is one of the simplest form of Heat (xchangers.
U
The wall of the inner pipe is the heat transfer surface.
U
The ma8or use of these Heat (xchanger is sensible cooling or heating applications.
U
Aut Oery long, even for moderate capacities.
U
2nviable to accommodate in an industrial space.
U
To make a 2nit Isotropically 'ompact, the arrangement is made in @ultiple Times and 'ontinuous 1eries and +aralle l flow. >eneral design considerations are routing of fluids and the
suitability of the calculated area of heat transfer and other important parameters like baffles arrangement to meet with the maximum pressure loss re%uirement in shell-an-tube heat exchanger.