Heat exchanger is th e device In which the heat exchange between media used in various kinds of systems or machines is executed. As a result ot the heat exchanger operation. the temperature or aggregation state of media flowing through the heat exchanger changes. Marine heat exchanger Is the exchanger used in ship piping and machinery. CLASSIFICATION OF MARINE HEAT EXCHANGERS Marine heat exchangers are divided In the following manner: 1) according to the working principle - into direct-contact (mixing or surtaceless). Indirect (recuperative) and regenerative heat exchangers. 2) according to th eir appli cation In ship systems - into coolers. heaters. condensers and evaporators. The heat exchangers used on board belong to two-media type exchangers. It means th at two medi a of different temperature participate in the heat exchange process . Th ey can possess the same physical and chemi cal properti es (e.g. wa ter- water) or different properti es (e.g. oil - wa ter) and different sta te of aggregation (e.g. steam - oil).
Transcript
Heat exchanger is the device In which the heat exchange between media used in various kinds of systems or machines is executed. As a result ot the heat exchanger operation. the temperature or aggregation state of media flowing through the heat exchanger changes.
Marine heat exchanger Is the exchanger used in ship piping and machinery.
CLASSIFICATION OF MARINE HEAT EXCHANGERS
Marine heat exchangers are divided In the following manner:
1) according to the working principle - into direct-contact (mixing or surtaceless). Indirect (recuperative) and regenerative heat exchangers.
2) according to their application In ship systems - into coolers. heaters. condensers and evaporators.
The heat exchangers used on board belong to two-media type exchangers. It means that two media of different temperature participate in the heat exchange process. They can possess the same physical and chemical properties (e.g. water- water) or different properties (e.g. oil - water) and different state of aggregation (e.g. steam - oil).
The working schemes of the two-media heat exchangers are shown on f1gure 1 .1 In direct-contact heat exchangers (fig. 1.1.a) the working medium and the treated medium are mixed. During this process the warmer medium is cooled and the colder medium is warmed The final temperature of mixture results from the heat balance of the process. For example, if the temperature of warmer medium (mass mt) is lt. and the temperature of colder medium (mass mz) is lz. then the mixture has the mass mt+mz and Intermediate temperature is 13. The relationships between the temperatures are lt>l3>tz.
In case one medium Is condensed under constant pressure its temperature does not change. In this case for 11>12 l3=lr
The working scheme of the recuperative heat exchanger is shown on figure 1.1 . b . The heat exchange is performed though the membrane (wall) thus the media are not mixed. The heat flows from the medium having higher temperature and mass m 1 (which Is cooled). to the medium having lower temperature and mass m 2 (which In turn is heated). The
principles of heat balance are vaiiCI during the process. ana temperature relations are as follows: - in case of heaters and coolers If 11>1z than l4> l z and
13<11, - in case of condensers when the medium of mass m 1 is
condensed l1>tz. f3=11 and l4>lz.
Fig.1.1. Working schemes of two-media heat exchangers
a) direct-contact heat exchanger, b) recuperatiw heat exchanger, c) regeneratiw heat exchanger q . the heat transferred in the heat exchanger
a)
...:.m::z::t::z:::: L > ...
b)
ffl t t1 ~ >
m z tz
c)
> ANIMATION
q m 1 t3 I
I I I I I I I
• • • • • • • m z t4 I
Figure 1.1.c shows the working scheme of regenerative heat exchanger The walls and a special padding of heat exchanger chamber serve as a heat accumulator. The media now through the heat exchanger chamber alternatively. The warmer medium (mass m1) nows during first phase of the
process transfemng the heat to the chamber walls and padding. Its temperature drops from 11 to lz and the chamber is warmed up to temperature 13. The colder medium (mass mz)
flows through the chamber during second phase of the process getting bacK the heat from walls and padding (see animation on fig . 1.1.c ). Its temperature increases from l4 to 15. The temperature relationships are as follows:
in first phase t 1>f:J>Iz,
in second phase I;J>I5>f4 .
Fig. 1.1. Working schemes ottwo.media heat exchangers
a) direct-contact heat exchanger, b) recuperatiw heat exchanger, c) regeneratiw heat exchanger q - the heat transrerred in the heat exchanger
a) > ANIMATION
b)
q
c)
The recuperatiVe heat exchangers are mostly used on board. Separation of media by means of the wall allows the heat exchange between the media of different physical and chemical propertieS. tor example between oil and cooling water. After the heat exchange process is completed bott media keep their chemical and operational properties.
The direct-contact (mbdng) heat exchangers are used in limited number of cases. when the mixing of the media does not change their properties and chemical composition. Thus, these are often the same media but possessing different temperature or state ot aggregation, for example steam and fresh water.
The regenerative heat exchangers allow heat exchange between media or different physical and chemical properties. They are used mostly tor heat exchange between gases. Thus, they have a limited application on board. Air and exhaust gases flowing through the regenerative heat exchanger alternatively can be one example.
The media in heat exchanger can be divided according to their function, into worKing media I.e. executing a specific task (for example heating or cooling) and treated media, whose temperature or state of aggregation are changed in accordance with the requirements of the ship system or machinery.
The following media participate In heat exchange in the different ship systems: sea water. fresh water. steam. lubricating oil, ruel, air. exhaust gases and technical gases.ln many cases these media can serve as working medium in one heat exchanger and can be used as treated medium in another heat exchanger
The working and treated media in different kinds of ship heat exchangers can be classified as follows:
in the coolers- as cooling medium (e.g. sea water, fresh water, air) and cooled medium (e.g. fresh water, lubricating oil, air),
in the heaters - as heating medium (e.g. steam, fresh water, exhaust gases, oil) and heated medium (e.g. fresh water, fuel, oil, air),
in the condensers - as cooling medium (e.g. sea water, fresh water, air) and condensed medium (e.g. steam, refrigerant vapour),
in the evaporators- as heating medium (e.g. fresh water, steam) and evaporated medium (e.g. sea water, fresh water).
The cooling media mostly used in marine heat exchangers are sea water, fresh water and air while steam and fresh water are mostly used as heat1ng media
Apart from the worl<lng media mentioned above, electric power is very often used as heating medium, for example in the electric heaters or water, oil or fuel.
Taking into account the classification of marine hea exchangers according to their Intended use, the heat exchangers that are mostly usea on board are:
coolers of fresh water, oil ana air,
heaters of water, oil, fuel ana air,
condensers of steam ana refrigerant vapour,
evaporators of sea water.
Basic processes of heat exchange are conduction, convection and radiation.
Conduction of neat takes place Inside a single object, in which the difference of temperature exists. The neat now goes from the area of higher temperature t 1 to tne area of lower temperature t z (fig. 2.1 ) The process essentially consists of
the transfer of energy trom molecules having higher energetic level to the ones having lower energetic level. The amount of neat transferred wtthin the conduction process is characterised by the following equation:
(2.1)
where:
)!. jV\IIm·KJ • coefficient of thermal conduclivily - characteristic
feature of given object describing its ability for heal
transfer I.e. thermal conducllvlly,
O[mJ
F[m2J
Llt [KJ
• distance of heat now Inside the object which is equal to
the thicKness of the object conducting the heat,
• area of object secllon through which the heat is
conducted,
• temperature di1Terence between initial and final area of
heat conducllon process,
t1 [KJ • innlal temperature In Initial area ofthe process. '
> ANIMATION
- -8 A. F ~14--=-~--~
-Fig. 2.1. Pattern or heat conduction
••••• distribution of temperature Inside the object
Convection or neat exists in liquids and gases only. It consists of the transfer or energy by molecules or streams of molecules within the fluia (fig. 2.2). The molecules· motion may be self-acting due to hydrostatic lift effect (molecules of higher energy have lower density than molecules of lower energy), or the motion can be forced by the transferring machine. Convection of heat runs from the space of higher temperature 11 to the space or lower temperature 12. The process of
convection is accompanied by heat conduction in a small degree.
}2-1 l l
l
' l
Fig. 2.2. Pattern of heat corNectlon
I
l t
> ANIMATION
Radiation consists ot heat transmission tJy means of electromagnetic waves trom the object of higher temperature It to the ObJect ot lower temperature tz (fig. 2.3 ). The heat
exchange tJy radiation does not require any conducting or convecting matter and can occur across a vacuum. The electromagnetic waves trom Infrared range of emission spectrum are mainly involVed In the radiation of heat.
Each object emits and absorbs energy at the same time. The intensity of these processes depends on temperature, colour and smoothness both emitting and absorbing energy objects. Objects or higher temperature, darKer tint and more matt surface emit more heat than objects of lower temperature, brighter colours and smooth glossy surface. Matt, darKer and lower temperature objects also absorb heat more easily than bright, light colour and higher temperature objects. In case of equal temperature or both objects, they emit and absorb the same amount of energy, and as a result, the heat transmission by radiation between them disappears. Additionally, the intensity of radiation depends or the slope and shape of the surface of both objects.
I > I ANIMATION
A ... B ~ ...
t1 ~ =-t2
~
'---
Fig. 2.3. Pattern of heat transmission by radiation
In marine heat exchangers, the basic processes of heat exchange do not occur In an Individual pattern. Penetration of heat and transfer of heat are combined processes of heat exchange.
Penetration of heat consists In the process of heat transmission tram the stream ot liquid or gas into the wall of heat exchanger {fig. 2.4) or trom the wall into the stream of medium. Penetration Is a combination of convection and conduction in the stream of medium. The amount of heat transferred in penetration process Is characterised by the following equation:
where:
a [Wtm2KJ
F[m2[
.dt [KJ
(2.2)
Q= a ·F·L1t [W]
coellicienl of heal penelralion from the stream of medium into wall, or from wall into the stream of mediumcharacterlslic feature of given process of heal penetralion,
-area of wall,
• temperature dlfrerence between the stream of liquid and
wall,
t 1 IKI. lemperature of medium stream core, f2IKI -lemperalure of wall.
Fig.2.4.
> ANIMATION
•
•
Pattern of heat penetration . . • . . distribution of temperature Inside the stream of medium
Transfer of heat Is the combined process of heat transmission consisting of heat penetration from the stream A of liquid or gas into heat exchanger wall, heat conduction through the wall and as a next step, heat penetration from the wall into the stream of another medium B nowing along opposite side of wall (fig 2.5). The amount of heat transferred in the process of heat transfer is described by the following formula:
Q=k ·F ·Llt [W] (2.3) where: k twlm2·KJ. coefficient of heat transfer from the stream of medium A to
the stream of medium 9 through the wall separating them.
characteristic feature of given process of heat transfer
equal to:
or
(2.4)
while: ~ • coefficient of heat penetration from medium A tc
the wall, • coefficient of heat penetration from the wall into
medium 9, • thickness ofwall,
·coefficient of wall thermal conductivity,
F [m2] . area of wall, Llf [I<J · temperature difference between streams of media,
fA [I<J • temperature of medium A core,
fa [KJ • temperature of medium B core.
> ANIMATION
• •
B
F. - - '-... _____ ta
• •
Fig. 2.5. Pattern of heat tranSfer ... .. distribution of temperature during the process
2. BASICS F.IEAli GE
~ ;-- ~ 2.3 DIRECTIONS OF. MEDIA f'LOW DURIN
The issue of mutual flows ot media participating in heat exchange process concerns recuperative heat exchangers. The following mutual flows can occur: parallel-flows . counter-flows, cross-flows and combined flows.
Parallel-flow (fig 2.6) consists In parallel motion ot both fluids A and 8 in the same direction on each side of wall 2 separating them. In the example shown on figure 2.6.a the heat is transferred from medium A Into medium 8 . During the now through the heat exchanger on a distance L the temperature of medium A decreases and the temperature of medium 8
increases (fig . 2.6.b ). and a difference of temperatures between them decreases from .()f1 to .()f2. The amount of heat
transferred in the process equals to:
where:
Q=k· F ·Lltm [W] (2.5J
k [VV!m2·KJ - coelliclent or heat transfer, F tm2J -area of wall, Lltm JKJ - logarithmic mean temperature difference
between media,
.1 t1- .1 t2
In .:1 t1 .:1 t2
(2.6)
• temperature diiTerence in the beginning of heat
exchanger. • temperature diiTerence In the end of heat
a) I > IAMMATION
8 1 2
II t;L I A - - - - A
I (] ll ~
8 b) I
' A
~ I~
B
K
Fig. 2.6. Pattern of parallel-flow In the heat exchanger a) pattern of media now, b) temperature distribution of media along the heat exchanger
Counter-flow (hg 2.7}- media A and B now on each side of wall parallelly but In oppos1te directions. The temperature distribution (fig. 2.7.b) shows. that in case of counter-now heat exchanger it is possible to warm up medium B until its temperature exceeds the final temperature of medium A. This was not possible in me parallel-flow heat exchanger. In practice. this leads to higher work effectiveness of counter-flow heat exchangers and this results in their more common application. The amount or heat transferred from medium A to medium 8 is described by the same formula as in case of parallel flow (formula 2.5). Also the logarithmic mean temperature difference is calculated similarly (formula 2 .6).
a} I > I ANIMATION
B 1 2
~ I 5_1 I
A- - - -A
I
::J
I ! ~
B b) t ,
<i J A
B
.. ~
<I
K
Fig. 2.7. Pattern of counter-now In the heat exchanger a) pattern of media now, b) temperature distribution of media along the heat exchanger
Cross-flow (fig. 2.8) consists in mutual perpendicular motion of media on both sides of separating wall. The distribution of media temperatures is presented on figure 2.8.b. During cross-flow it is impossible to show the direction of media flow on the temperature distribution diagram. The amount of heat transferred between media during the crossflow is calculated in the same way, according to the same formula (2.5).
a) B I >I ANIMATION
1 2
I I 19l
A --+ --+ - A
I I I L I
b) B I
B ~
~ <l
A l r-'-,
~
~ X
L
fig. 2.8. Pattern of cross.flow in the heat exchanger a) 11attern of media flow, b) temperature distribution of media along the heat excham er
The logarithmic mean temperature difference obtained according to the same formula (2 .6) is to be corrected by using the adjustement given in appropriated tables.
The temperature distribution during special cases of heat transfer in which one medium is subject to a phase change (condensation or evaporation) are shown on figure 2.9. Figure 2.9.a concerns a typical condenser, in which cooled medium B condenses and its temperature does not change. At the same time, the temperature of cooling medium A increases. In case of evaporator (fig. 2.9.b) the heated medium A evaporates, its temperature is constant and the temperature of heating
medium B decreases. Condensers and evaporators usually worK with cross-flow of worKing media.
a) 1 b) I B
']~ .:: A
~ B
<I ' , A []~ X X
fig. 2.9. Distribution of media tem11eratures in condensers and evaporators a) distribution of temperatures in condenser, b) distribution of temperatures in evaporator
Construction or marine heat exchangers usually is complicated to such degree. that working media do not flow in a simple parallel-. counter- or cross-now pattern. There are various combinations or simple flow patterns of media and such pattern is descnbed as combined flow.
The manufacturers determine the heat transfer coefficient of combined now heat exchangers In an experimental way for each given type or heat exchanger. The logarithmic mear temperature difference Is calculated according to the formula (2.6).
The combined parallel and counter flow in heat exchanger with telescopic pipe (Field's heat exchanger) is shown on figure 2.10. In this type or heat exchanger one medium flows twice inside and outside of inner pipe, and the second medium flows on the outside of the external pipe. The heat exchange in it is effectuated according to the counter-flow pattern of the same medium B on both sides or Inner pipe and according to parallel-flow pattern between medium A and medium B on the sides of external pipe.
a) > ANlMATION
BA
-B A - -
A
b)
A
B
L
Fig. 2.10. Pattern of combined flow In telescopic heat exchanger a) pattern of media flow, b) temperature distribution of media Inside heat exchanger
1 - tube plate; 2 - telescopic pipe
Anotller example of combined flow in shell and tube heat exchanger is shown on figure 2.11 . Medium A flows through different parts of pipes twice, In opposite directions. On the contrary, in different parts of heat exchanger, medium B flows in parallel-, counter- or cross-flow pattern in relation to medium A. This is due to the application of baffle plates 1 mounted outside heat exchanger pipes In the space where medium B flows .
B
A +- ~-
= =
> ANIMATION
B
Fig. 2.11. Pattern or combined flow in shell and tube heat exchanger 1 • transverse baffle plate
Direct-contact heat exchangers are constructed in such a W<l'{, that mixing ot media Is effective. thus ensuring higt efficiency of the heat exchange An example of direct-contad steam heater ot marine bolter teed water is shown on figure 3.1 . The teed water Is supplied Into heater through nange 4 and spraying valve 3 . Next. the water tans in the form of shower into a cylindrical mixing Insert 2 . The healing steam enters the heater via connector 5. anel flows inside the mixing insert 2 through holes on its surface and mixes with the stream of water. The steam condenses and heat of condensation heats the water. A hot mixture Is drawn by boiler feed pum~ from connector 15. The Increase of feed water temperature improves the steam propulsion plant heat balance, decreases thermal stresses of boiler and performs feed water degassing (solubility of gases in water decreases In hlgl1er temperature). The air dissolved In water consists of oxygen, which causes boiler corrosion. Due to degassing (deaeration) the risk of corrosion inside the boiler Is minimised. The feed water heater described above also serves as a compensating reservoir of water in plant steam-condensate system.
> ANIMATION
Fig. 3.1. Oirect.contact reed water heater 1 . heater body, 2 • mixing Insert; 3 • spraying vaiW; 4 . feed water inlet; 5 • heating steam Inlet; 6 • hot water outlet
An example of shell & tube heat exchange is presented on figure 3.3. It can be used as marine engine lubricating oil cooler or fresh water cooler. Sea water (medium A) nows inside tubes and cools down lubricating on or fresh water. which now outside tubes (medium 8).
On figure 3.2 medium A flows through tube stack in one direction, while on figure 3.3 flows twice In opposite directions in separate tube stacks. The heat exchanger with multiple flow of medium inside pipe stacks Is named multi-pass flow heat exchanger.
Fig. 3.3. Shell & tube cooler 1 - body (shell); 2 - cover; 3 . Inlet/outlet chamber; 4 -tube stack; 5 . baftle plate; 6 . shell flange; 7 - bon; 8 - foundation leg; 9 • thermometer; 10 • vent cock; 11 - medium A pipe connections; 12 . medium B pipe connections (photo from brochure F.U.O. "'Rumla··. Centromor)
10
Recuperative heat exchangers possess different constructions of walls. which separate the media participating in the heat exchange process. In most cases various shapes of tubes or plates are used. Thus. depending on the type of separating surfaces the heat exchangers are usually divided into tubular or plate heat exchangers.
3.2.1 . Constructi on of tubular heat exchangers
In tubular heat exchangers also known as shell and tube (shell & tube) heat exchangers one medium flows inside the tubes and the second medium flows outside the tubes. J.
scheme of shell & tube heat exchanger is shown on figure 3.2. A shell (body) 1 of heat exchanger mostly cylindrical in shape is ended with flanges. Into which tube plates 4 as well as covers 2 and 3 are fastened. One cover is stationary fixed and sealed. while the second one is mounted by means of a floating sealing 11 (Its task Is described in further part of this chapter) . The tubes 5 are expanded in tube plates (tube sheets) 4 . One medium. lor example A, flows inside cover 2. which at the same time Is a distribution chamber of the flow of medium A into the pipes 5. Cover 3 is a collecting chamber of medium A at the heat exchanger outlet. Medium B in turn passes through the shell Interior outside tubes. The flow of medium B is guided by means of baffle plates 6.
> ANIMATION
B
fig. 3.2. Shell & tube heat exchanger 1 -body (shell); 2 - Inlet cover; 3 - outlet cowr; 4 - tube plate; 5 - tube; 6 - baffle plate; 7 • medium A inlet connection; 8 -medium A outlet connection; 9 • medium B inlet connection; 10 • medium B outlet connection; 11 - floating tube plate sealing
I> I ANIMATION
I
A principle of multi-pass flow heat exchanger construction is explained on figure 3.4. One-pass flow heat exchangers are shown on figures 3.4.a and 3.4.b (tube stacks are shown in a simplified way). The lett cover forms an Inlet chamber. and the right cover forms an ouuet chamber of medium A. Medium nows through the heat exchanger only once.
In two-pass now heat exchanger (fig. 3.4.c) the left cover is split into inlet chamber 1 and outlet chamber 2 by means of a bame plate 4 . The right cover Is a return chamber of medium A. It passes through the heat exchanger two times in opposite directions inside adjoining tube stacks.
Three-pass now heat exchanger (fig. 3.4.<1 ) has baffle plates 4 inside both covers situated In such way that triple flow of medium A inside tube stacks Is achieved.
a) B I >I ANIMATION
A t~r -+ -+ -+ \ A
)J j}J>ltZ B
Fig. 3.4. l)lpes of medium now In tubes of shell & tube heat exchangers a), b) one-pass now heat exchanger; c) two-pass now heat exchanger; d) three-pass now heat exchanger; 1 - inlet chamber; 2 - outlet chamber; 3 - return chamber; 4 - baffle plaCe; 5 - tube stack
b) B > ANIMATION
c) 1 B
~'L_IJI:=====:::::J" A• -+
A -B
d)
B
Application of multt-pass flow heat exchangers makes it possible to shorten the tubes· length. though the diameter of heat exchanger shell does not Increase considerably. Thus. the overall dimensions or heat exchanger are minimised and tube replacement as well as cleaning Is easier.
On board, two-pass now heat exchangers are mostly used (comp. fig. 3.3), while three-pass. tour-pass and five-pass now heat exchangers are more seldom used.
A proper direction of medium now outside tubes makes it possible to incluCle a whole surtace of all tubes in the heat exchange process. This task Is executed by appropriate baffle plates placed in the space among heat exchanger tubes. The flow of medium 8 outside tubes In case no baffle plate used is shown on figure 3.15 . Medium 8 flows In the shortest way 2 between inlet and outlet connector. Thus "dead" spaces 1 are formed. where the flow or medium 8 Is not considerable, and where the heat exchange Is not effective. The application of baffle plates inside or heat exchanger allows to avoid this inconvenience. For example longitudinal baffle plates 3 (fig. 3.5.b) force medium B to flow within the whole space between the pipes. Additionally, the flow distance of medium inside shell increases and the medium speed also increases. It results in high turbulence of flow, which raises the coefficient of heat penetration between tubes and medium. The heat exchange intensifies.
a) B > ANlMATION
J A --+
\ B
b) B
J A --+ - A
\ L'/ B
Fig. 3.5. Application or bailie plates In space outside tubes in shell & tube heat exchanger a) shell space whhout bailie plates, b) application or longhudlnal bailie plates 1 . "dead" space; 2 • medium B now direction; 3 . longhudinal bailie plates
varied types of transverse baffle plates (fig. 3.5.c - 3.5.f) are more frequently used In shell & tube heat exchangers than longitudinal baffle plates The most popular are nat cut segment baffle plates 4 (Ng. 3.5.c). whereas concentric baffle plates 5 (fig. 3.5.d). shape segment baffle plates 6 (fig. 3.5.e) and sieve sheet baffle plates 7 (fig. 3.5.f) are less frequently used. Besides the improvement of heat exchange conditions. the segment baffles 4 and 6 support pipes of heat exchanger. thus guarding these pipes against arching ana deformation . In a similar way, concentric baffles extend the distance of medium flow and increase the turbulence of Its movement. Sieve sheet baffles force a considerable increase of medium flow speed and turbulence within the sheet holes. while the flow direction generally remains unchanged.
Fig. 3.5. Application of baftle plates In space outside tubes in shell & tube heat exchanger c) - d) application Of dltrerent baftle plates 2 . medium B now direction; 4 - nat cut segment baffle plates; 5 - concentric baftle plates;
c)
d)
B
4
B
A
J J ---+
\
> ANIMATION
4
''L --+ A
r/ B
e) B > ANIMATION
Varied types of transverse baffle plates (fig. 3.5.c - 3.5.f) A J
are more frequently used 1n shell & tube heat exchangers than -
~ longitudinal baffle plates The most popular are nat cut segment bame plates 4 (fig 3.5.c ). whereas concentric bame plates 5 (fig. 3.5.d ). shape segment bame plates 6 (fig. 3.5.e ) B and sieve sheet bame plates 7 (fig. 3.5.f) are less frequently used. Besides the Improvement of heat exchange conditions. the segment bames 4 and 6 support pipes of heat exchanger. thus guarding these pipes against arching and deformation. In a similar way, concentric baffles extend the distance of medium flow and increase the turbulence of Its movement. Sieve sheet •
6 baffles force a considerable Increase of medium flow speed B and turbulence within the sheet holes. while the flow direction f) JJ generally remains unchanged.
A • \c~
A
Fig.3.5. Application of bailie plates In space outside tubes in shell & B lube heat exchanger e) - f) application of different bailie plates 2-medium 8 now direcclon; 6- shape segment bailie plates; 7 - slew sheet bailie plates
The shell & tube heat exchangers used on board usually are multi-pass flow on a side of medium flowing inside pipes, and have baffle plates lor medium flowing outside pipes. An example of four-pass trow shell & tube heat exchanger with segment transverse baffle plates Is shown on figure 3.6 .
I > !ANIMATION
B B
A ...../IJ ._[ -,------.---'1 L = ......
Fig. 3.6. Four .pass now shell & tube heat exchanger wHh segment transverse baftle plates
Performance of recuperative heat exchangers can be increased by application of fined (ribbed) heat exchange surfaces (fig. 3.7.a).
Finned surfaces 2 made of material of good heat conductivity are mounted on exchanger pipes 1. Instead of ribbed surfaces other shape elements are also used, tor example metal stubs (item 3, fig. 3.7.b) padded on pipe surface. The stubs can be made of good thermal conductor covered with a higher strength material (items 4 and 51n fig. 3.7 .c) .
a)
' -+-
Fig. 3.7. Finned heat exchange surfaces In tubular heat exchangers a) mounted finned surface, b) padded stubs, c) padded stubs whh good heat conductor insert 1 . tube; 2 • finned surface; 3 • stub; 4 • good heat conductMty stub core; 5 • stub strength lining
Examples of different shapes of tined surfaces are shown on figure 3 . 8 . usually me tinning Is used on me side of medium. which have a lower coefficient of heat penetration.
Aside from increasing the heat exchange surface. the tinning also increases turbulence and speed of medium flow which additionally benefits the coefficient of heat penetration value. It results in minimising the heat exchanger dimensions and in savings of Ship engine room space.
a) b)
c)
Fig. 3.8. Examples of different finned heat exchange surfaces in tubular heat exchangers
a) disc !inning, b) stub !inning, c) ribbed disc !inning, d) longitudinal nat !inning
A high performance tubular heat exchangers with finned heat exchange surfaces are shown on figure 3.9 . These are telescopic heaters or fuel, oil or liquid cargo (compare fig. 2.10), in which the outside or telescopic pipes is finned. The shape of ribbing is presented in enlargement.
Fig. 3.9. High performance telescopic heat exchangers a) single-pipe telescopic heat exchanger, b) muHi-pipe telescopic heat exchanger 1 . casing; 2 - telescopic pipe; 3 - tinning (photos from brochures STANEX Heat Exchangers, Zander & lngestrOm, Alfa·Laval Group and SUNROD and BENDEK Heat Exchangers for Cargo Heating, Frank Mohn AS, Norway)
a)
B
b)
1 3 2 3 > ANIMATION
> ANIMATION
During tubular heat exchanger operation. thermal stresses can occur due to tne drtterence of temperatures of the construction elements, drtterent coefficients of thermal expansion of materials and variable running temperatures. It can cause heat exchanger failure because of excessive elements deformation. loss of joints llghtness and in extreme circumstances also crackS and damage of tne construction components. That is wrry tne design of tubular heat exchangers should take into consideration the problem of thermal expansion compensation In order to avoid excessive thermal stresses. The means for compensating thermal deformations in shell & tube heat exchangers are presented on figure 3.10.
Fig. 3.10. Means for compensation of thermal deformations in shell & tube heat exchangers
a) noating tube plate, a1) fixed tube plate detail, a2) noatlng tube plate detail, b) telescopic pipes,
On figure 3.10.a one tube sheet 31s fixed (detail at). while the second tube sheet Is floating (detail a2). The floating tube
sheet is sealed by means or conical sealing rings 6 placed between flanges of snell 1 and cover 2. and separated by leakage nng 5 . A tigntening of flanges presses sealing rings 6 to tne side of tube plate 3. tnus sealing tne joint and simultaneously keeping tne floating feature of tube plate. Tubes of neat excnanger whlcn are manufactured witll good neat conducting material nave higher tnermal expansion tnan tne shell. When working. tne tube expands more tnan tne casing. The floating joint or tube plate allows to avoia possible thermal stresses. In case one of sealing rings 6 loses its tightness. it is indicated by leak of medium through holes in leakage ring 5. The leak through holes of ring 5 also protects against the mixing of media.
In the telescopic heat exchanger (fig. 3.10.b) telescopic pipes 9 are fixed one-side to the tube plates only, which allows their free expansion Independently of the expansion of casing 1. Fig. 3.10. Means for compensation of thermal deformations in shell &
tube heat exchangers
a) noating tube plate, a1) fixed tube plate detail, a2) noatino tube plate detail, b) telescopic pipes,
In turn, bent tubes 1Z are used In the heat exchanger shown on figure 3.10.e. A change of the tubes' length in ditferen working temperatures Is compensated by the change of tube bending radius For example, spiral tubes possess a similar
feature. In the heat exchanger shown on figure 3.10.f tubes are
permanenUy fiXed to one side of tube plate (detail '') and in movable w;ry by means of stuffing-boxes in the second tube
plate (detail fz). Each pipe 4 has Its own stuffing-box with packing rings 13 pressed by packing ring holder 14.
Fig. 3.10. Means for compensation of thermal deformations in shell & tube heat exchangers
e) expansion shape tubes, f) stuffing-box pipe nxlng; r1) tube rolling end nxlng detail, r2) tube packing end nxlng detail,
Lamella heat exchangers (tig. 3.11 ) are a modification of tubular heat exchangers. In which ltle heat exchange surface is formed by lamella walls made of tubes connected by nat metal strips. Tubes 3 have a hexagonal (fig. 3.11 .a) or oval section (fig. 3.11.b). A whole Internal space of the heat exchanger is divided by lamella walls into flat channels. One medium flows inside pipes and the second medium flows through the flat channels. The pipes are fixed to tube plates 2. Compensation or tubes thermal elongation is executed by floating sealing 5 attached to one connector of medium floating through the pipes.
Plate heat exchangers (fig. 3.12) have a heat exchange membrane shaped as a pack of corrugated metal plates 1.
Plates in the pack are separated by gaskets 2. They form flat corrugated channels between plates. which at the same time constitute ways of flow for media A and B participating in heat exchange. The flow of media between plates is guided by proper shape of gaskets. The plate pack is closed from both sides by frame plates 4 and tightened by horizontal tightening
bolts 6. Inlet and outlet pipes of media A and Bare fastened to the frame plates
a)
B
Fig. 3.12. Construction of lllate heat exchanger a) general view, b) construction of plates 1 . plate; 2 . nasket; 3 . frame plate; 4 - pressure plate; 5 .
> ANIMATION b)
There are two types ot plates In each heat exchanger. The gaskets shape Is mirror like in relation to each other (fig. 3.12.b. 3.13.3 , 3.14.a . 3.14.b). The gaskets are placed in groves extruded on plates Ports at the plate comers are alternatively surrounded completely or partly With gaskets. Thus, media A and B can flow separately through adjoining channels of plate pack. Plate heat exchangers used in marine
applications execute counter-flow Of media (fig. 3.12 . 3.13 , 3.14).
The corrugation of plates results from the groove shape of their surface. Grooves can be chevron type with different angle of inclination (fig. 3.12.b , 3 .13.3) or transverse type with different shape of grooves (fig. 3.14.3, 3.14.b). The grooves at adjacent plates are placed and directed in such way that channels between plates consist or a number of chambers and narrowings (fig. 3.13.b). The distance between plates depends on gasKets' thicKness and amounts to a few up to a dozen of miliimetres.
Fig. 3.13. Construction or chevron type plates a) construction or adjacent plates, h) section detail or plate pack
a)
b)
> ANIMATION
l
8 A 8 A. 8 A 8
There are two types ot plates In each heat exchanger. The gaskets shape Is mirror like In relation to each other (fig. 3.12.b , 3.13.a , 3.14.a . 3.14.b ). The gaskets are placed in groves extruded on plates Ports at the plate comers are alternatiVely surrounded completely or partly with gaskets. Thus. media A and B can now separately through adjoining channels of plate pack. Plate heat exchangers used in marine applications execute counter-flow of media (fig. 3.12 . 3.13 , 3.14).
The corrugation of plates results from the groove shape of their surface. Grooves can be chevron type with different angle of inclination (fig. 3.12.b, 3.13.a) or transverse type with different shape of grooves (fig. 3.14.a, 3.14.b ). The grooves at adjacent plates are placed and directed in such way that channels between plates consist of a number of chambers and narrowings (fig. 3.13.b). The distance between plates depends on gaskets' thickness and amounts to a few up to a dozen of millimetres.
a) b)
Fig. 3.14. Construction oftran!Mirse corrugated plates a) washboard corrugated plates, b) arborescently corrugated plates
The shape of channels between plates and small thickness of channels cause the characteristic high turbulence of media now. Grooving also Increases the area of heat transfer Thickness of plates Is small- 0.4 - 1,5 mm. These features, in particular turbulence of now considerably inftuence the increase of heat transfer coetticient. In plate heat exchangers it amounts 3500 • 7500 W/m2K. and it is about three times higher than the same In tubular heat exchangers. Thanks to this feature. the plate heat excnangers are lighter and smaller than the same heat transfer capability tubular heat
exchangers. Typical plate heat exchanger and comparison of dimensions of plate and tubular heat exchangers are shown on figure 3.15 . Plate heat exchanger is approximately three times smaller and has six times less weight than the same thermal capacity tubular heat exchanger. Besides the tubular heat exchanger requires additional space 9 (fig. 3.15) in engine room for pipes replacement and cleaning.
Fig. 3.15. Plate heat exchanger a)lliew, b) overall dimensions and comparison of tubular and plate heat exchangers 1 . plate pack; 2 • frame plate; 3 • tightening bolt; 4 • plate pack upper carl)llng bar; 5 • plate pack lower guiding bar; 6 . support column; 7 • tubular heat exchanger; 8 . plate heat exchanger; 9 • tube stack serllice space of tubular heat exchanger (photo from brochure Plate Heat Exchangers, ALFA LAVAL)
a)
• . . ~ .
.A~
b)
Ui!------llk - - - - - - - ,
Different kind of plate heat exchangers are spiral heat exchangers (fig. 3.16) Its heat transfer surface is shaped by helical reeled metal sheets 2 sealed In end covers 4 . Spiral channels 3 are formed between the metal sheets. in wtlich a counter-flow of media A and B Is executed.
Fig. 3.16. Spiral heat exchanger a) view, b) pattern of construction 1 . casing; 2 • spiral plate; 3 • spiral channel; 4 . end cover; 5 -foundation leg (photo from brochure ·wymlennlkl Clepla", Alfa Laval Polska Sp. z o.o.)
a) > ANIMATION
b)
-c> A
The working space ot regenerative heat exchangers is built as a chamber through which media participating in heat exchange process flow alternatively . First. the hot medium nows through and warms the walls and padding which accumulate the heat In turn the cold medium nows through the chamber and Is warmed t:ty hot walls and padding. The working process Is repeated.
In marine applications the regenerative heat exchangers are usually used tor heat exchange between gas media. Typical example or marine regenerative heat exchanger is thE rotary air heater used in main steam boilers shown on figure 3.17 . The air delivered into boiler furnace is heated by hot exhaust gases. The heat exchanger has rotary working chamber built in shape of drum 2 filled with padding of corrugated metal sheet padding. Thickness of s11eets 0,5 mm and distance between them 3-5 mm. The drum is rotated by electric motor 3 about 2-3 rpm. Exhaust gases (stream A) flow through a part of drum warming the metal padding. A hot part of padding due to the rotating motion of drum moves into the space of air flow (stream B) and transfers the heat into air, thus warming it up.
Fig. 3.17. Rotary regenerlltlve air heater 1 • casing; 2 • drum padding; 3 • electric motor; 4 . reduction gear; 5 • air and exhaust gases now channels
> ANIMATION
A B
The part of padding cooled down by air returns into the space of exhaust gases flow. Thus, the rotating drum padding transfers the heat trom exhaust gases Into air in a permanent manner. During the process the temperature of metal sheet padding achieves 280-300°C and the temperature of air increases up to 250-260oc To improve the heat exchange process. the elements of drurr padding are lined with special ceramic coating or made as whole ceramic pads. It allows to Increase the amount of heat transferred from exhaust gases Into air ana increases boiler efficiency. Besides the ceramic coating or pads are more resistant to corrosion caused by sulphur contained in the fuel. A fast flow of gases and air streams prevents also deposits forming on the heat exchange surfaces.
The presented regenerative rotary heat exchanger is known on board as LjungstrOm heat exchanger.
Fig. 3.17. Rotary regeneratiw air heater 1 • casing; 2 • drum padding; 3 • electric motor; 4 . reduction gear; 5 . air and exhaust gases now channels
