Heat exchangers: Heat exchanger types:

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1 Heat exhangers: he proess of heat exhange between two fluids that are at different temperatures and separated by a solid wall ours in many engineering appliations. he devie used to implement this exhange is termed a heat exhanger. Heat exhanger types: Heat exhangers are typially lassified aording to flow arrangement and type of onstrution.

2 Conentri tube heat exhangers: he simplest heat exhanger is one for whih the hot and old fluids move in the same (parallel flow) or opposite diretions (ounter flow) in a onentri tube (or double pipe) onstrution. 2

3 Cross flow heat exhangers: he fluids may move in ross flow (perpendiular to eah other). he finned and un finned tubular heat exhangers are shown in the figure. he two onfiguration are typially differentiated by the fluid motion over the tubes as unmixed or mixed. 3

4 Shell and tube heat exhanger: Speifi forms differ aording to the number of shell and tube passes, and the simplest form whih involves single tube and shell passes is shown in the figure. 4

5 Baffles are usually installed to inrease the onvetion oeffiient of the shell side fluid by inluding turbulene and a ross flow veloity omponent. Baffled heat exhangers with one shell pass and two tube passes and with two shell passes and four tube passes are shown in the figure. 5

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7 Compat heat exhangers: A speial and important lass of heat exhanger is used to ahieve a very large heat transfer surfae area per unit volume. ermed ompat heat exhangers, these devies have dense arrays of finned tubes or plates and are typially used when at least one of the fluids is a gas and is hene haraterized by a small onvetion oeffiient. 7

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9 Fouling Fators: After a period of operation the heat-transfer surfaes for a heat exhanger may beome oated with various deposits present in the flow systems, or the surfaes may beome orroded as a result of the interation between the fluids and the material used for onstrution of the heat exhanger. In either event, this oating represents an additional resistane to the heat flow, and thus results in dereased performane. 9

10 he overall effet is usually represented by a fouling fator, or fouling resistane, Rf, whih must be inluded along with the other thermal resistanes making up the overall heat transfer oeffiient. R f U dirty U lean Fouling fators must be obtained experimentally by determining the values of U for both lean and dirty onditions in the heat exhanger. 0

11 List of reommended values of the fouling fator for various fluids, is given in the table:

12 he log mean temperature differene: If q is the total rate of heat transfer between the hot and old fluids: q m. h ph ( hi ho )... A q m. p ( o i )... B 2

13 Relating q to the temperature differene between the hot and old fluids h... C Sine varies with position in the heat exhanger q UA m... D Where m differene. is an appropriate mean temperature 3

14 For the parallel flow heat exhanger. 4

15 he temperature differene is initially large but deays rapidly with inreasing x, approahing zero asymptotially. he outlet temperature of the old fluid never exeeds that of the hot fluid. he energy balanes are subjet to the following assumptions: 5

16 . he heat exhanger is insulated from its surroundings, in whih ase the only heat exhange is between the hot and old fluids. 2. Axial ondution along the tubes is negligible. 3. Potential and kineti energy hanges are negligible. 4. he fluid speifi heats are onstant. 5. he overall heat transfer oeffiient is onstant. 6

17 Applying the energy balane to the differential element, dq m. h ph d h C h d h... E dq m. p d C d... F Where Ch and C are the hot and old fluid heat apaity rates, respetively. he heat transfer aross the surfae area da may be expressed as: dq UdA... G Where = h - is the loal temperature differene between the hot and old fluids. 7

18 d( ) d h d... H By substituting equations E and F in H: d( ) dq( C h C Substituting for dq from equation G and integrating aross the heat exhanger, 2 ln( d( 2 ) ) U ( C UA( C C C Substituting for Ch and C from equations A and B h h ) ) ) 2 da 8

19 ln( 2 ) UA( hi UA [( q q hi ho i ) o ( q ho i ) o )] From the figure, for the parallel flow heat exhanger q ( hi i 2 UA ln( / 2 ) and ) Comparing the above expression with equation D, we onlude that the appropriate average temperature differene is a log mean temperature differene, m or LMD. 2 ( ho o ) : 9

20 Jb... Ja... : flow exhanger - For theparallel I... ) ln( ) ln( ho 2 h2 2 hi h o i m 20

21 For the ounter flow heat exhanger: 2

22 Equations A, B, D and I apply to any heat exhanger. For the ounter flow exhanger the endpoint temperature differenes must be defined as: 2 h h2 2 hi ho Ka Kb o an exeed ho for ounter flow but not for parallel flow. o i 22

23 Note that, for the same inlet and outlet temperatures, the log mean temperature differene for ounter flow exeeds that for parallel flow, m, CF m, PF Hene the surfae area required to effet a presribed heat transfer rate q is smaller for the ounter flow than for the parallel-flow arrangement, assuming the same value of U. 23

24 If heat exhanger other than double-pipe type is used, the heat transfer is alulated by using a orretion fator (F) applied to the LMD for a ounter flow double-pipe arrangement with the same hot and old fluid temperature: q UAF m... L Value of the orretion fator F are plotted in the figures. For several different types of heat exhangers. 24

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29 When a phase hange is involved, as in ondensation or boiling (evaporation), the fluid normally remains at essentially onstant temperature. For this ondition, P or R beomes zero and we obtain: F =.0 for boiling or ondensation 29

30 Example : Water at the rate of 68 kg/min is heated from 35 to 75 C by an oil having a speifi heat of.9 KJ/kg. K. he fluids are used in a ounter flow double-pipe heat exhanger, and the oil enters the exhanger at 0 C and leaves at 75 C. he overall heat-transfer oeffiient is 320 W/m². K. Calulate the heat exhanger area. Given Cpw = 480 J/kg.k 30

31 Example 2: Instead of the double-pipe heat exhanger of the previous example, it is desired to use a shell and tube exhanger with the water making one shell pass and the oil making two tube passes. Calulate the area required for this exhanger, assuming that the overall heat transfer oeffiient remains at 320 w/m². C. 3

32 Effetiveness NU method: he LMD approah to heat exhanger analysis is useful when the inlet and outlet temperatures are known or are easily determined. he LMD is then easily alulated, and the heat flow, surfae area, or overall heat-transfer oeffiient may be determined. When the inlet or exit temperatures are to be evaluated for a given heat exhanger, in these ases the analysis is performed more easily by utilizing a method based on the effetiveness (Ԑ) of the heat exhanger in transferring a given amount of heat. 32

33 he effetiveness method also offers many advantages for analysis of problems in whih a omparison between various types of heat exhangers must be made for purposes of seleting the type best suited to aomplish a partiular heat transfer objetive. Effetiven ess atual heat transfer maximum possibleheat transfer he atual heat transfer may be omputed by alulating either the energy lost by the hot fluid or the energy gained by the old fluid. 33

34 For the parallel flow exhanger: For the ounter flow exhanger: In a general way the effetiveness is expressed as: 34 ) ( ) ( h h h h at m m q ) ( ) ( ) ( ) ( min. max i hi h h h h at m q m m q Maximum temperature differene in heat exhanger (minimum fluid)

35 f ( NU, C C min max ) he number of transfer units (NU) is a dimensionless parameter that is widely used for heat exhanger analysis and is defined as, NU UA C min NU is indiative of the size of the heat exhanger. 35

36 Kays and London have presented effetiveness ratios for various heat exhanger arrangements in hart form: 36

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42 he effetiveness and the NU in terms of effetiveness and apaity ratio are listed in the tables: 42

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