Lecture 12. Heat Exchangers. Heat Exchangers Chee 318 1

Transcription

1 Lecture 2 Heat Exchangers Heat Exchangers Chee 38

2 Heat Exchangers A heat exchanger s used t exchange heat between tw fluds f dfferent temperatures whch are separated by a sld wall. Heat exchangers are ubqutus t energy cnversn and utlzatn. hey encmpass a wde range f flw cnfguratns. Applcatns n heatng and ar cndtnng pwer prductn waste heat recvery chemcal prcessng fd prcessng sterlzatn n b-prcesses. Heat exchangers are classfed accrdng t flw arrangement and type f cnstructn. All prncples that we have learned prevusly apply. In ths chapter we wll learn hw ur prevus knwledge can be appled t d heat exchanger calculatns dscuss methdlges fr desgn and ntrduce perfrmance parameters. Heat Exchangers Chee 38 2

3 Desgn Prcess Ratng f a heat exchanger: Fr a specfc duty we need t specfy the necessary area (r length) and t decde what type f heat exchanger we need. Desgn f a heat exchanger s smetmes referred t as an art (Perry) MUS READ: heat exchanger lecture ntes by Bb Heaslp abut desgn prcess steps. Heat Exchangers Chee 38 3

4 Cncentrc ube Cnstructn Parallel Flw Cunterflw - : Heat Exchangers Chee 38 4

5 Heat Exchanger Analyss Recall frm Chapter 8 Expressn fr cnvectn heat transfer fr flw f a flud nsde a tube: q cnv mc & p ( m m Fr case 3 nvlvng cnstant surrundng flud temperature: q UA s lm lm ln( / ) ) Heat Exchangers Chee 38 5

6 Heat Exchangers Chee 38 6 Heat Exchanger Analyss In a tw-flud heat exchanger cnsder the ht and cld fluds separately: ) ( ) ( c c c p c c h h h p h h c m q c m q & & lm UA q and he usual desgn gal s t determne the requred area A fr a heatng duty q Cmbne eqs. (.) and (.2) and slve fr A Need t determne U and lm (.) (.2)

7 Heat Exchangers Chee 38 7 lm :. Parallel-Flw Heat Exchangers where lm UA q ) / ln( 2 2 lm c h c h 2 2

8 Heat Exchangers Chee 38 8 lm : 2. Cunter-Flw Heat Exchangers where lm UA q ) / ln( 2 2 lm c h c h 2 2

9 Overall Heat ransfer Ceffcent Fr tubular heat exchangers we must take nt accunt the cnductn resstance n the wall and cnvectn resstances f the fluds at the nner and uter tube surfaces. UA U A UA R U A cnd h A + R ln( D / D 2πkL Heat Exchangers Chee 38 9 cnd ) + h A where nner tube surface uter tube surface A A πdl πd (.3) L

10 Fulng Heat exchanger surfaces are subject t fulng by flud mpurtes rust frmatn r ther reactns between the flud and the wall materal. he subsequent depstn f a flm r scale n the surface can greatly ncrease the resstance t heat transfer between the fluds. An addtnal thermal resstance can be ntrduced: he Fulng factr R f. Depends n peratng temperature flud velcty and length f servce f heat exchanger. It s varable durng heat exchanger peratn. Fulng factrs can be fund n able. textbk (SI unts) r p. 5 heat exchanger lecture ntes (EE unts) Heat Exchangers Chee 38 0

11 Heat Exchangers Chee 38 Overall Heat ransfer Ceffcent he verall heat transfer ceffcent can be wrtten: f cnd f A h A R R A R h A A U U A UA " " (.4a) f cnd f h R A R A A R h A A U " " (.4b)

12 Determnatn f h Apprach : Usng crrelatns frm Chapter 7 Apprach 2: Usng chart by Kern p. 56 heat exchanger lecture ntes Heat Exchangers Chee 38 2

13 Determnatn f tube sde flm ceffcent h Apprach : Usng crrelatns frm Chapter 8 Apprach 2: Seder and ate relatnshp p. 53 heat exchanger lecture ntes Heat Exchangers Chee 38 3

14 Determnatn f Cnductn Resstance Recall that R cnd A R cnd ln( D / D ) 2πkL D ln( D 2k / D ) In EE unts r w A R cnd D 24k w ln( D / D ) Heat Exchangers Chee 38 4

15 Example. A cunterflw cncentrc tube heat exchanger s used t cl the lubrcatng l fr a large ndustral gas turbne engne. he flw rate f clng water thrugh the nner tube (D 25 mm) s 0.2 kg/s whle the flw rate f l thrugh the uter annulus (D 45 mm) s 0. kg/s. he l and water enter at temperatures f 00 and 30 C respectvely. Hw lng must the tube be made f the utlet temperature f the l s t be 60 C? Heat Exchangers Chee 38 5

16 able 8.2 Heat Exchangers Chee 38 6

17 Shell-and-ube Heat Exchangers One Shell Pass and One ube Pass Baffles are used t establsh a crss-flw and t nduce turbulent mxng f the shell-sde flud bth f whch enhance cnvectn. he number f tube and shell passes may be vared One Shell Pass w ube Passes w Shell Passes Fur ube Passes Heat Exchangers Chee 38 7

18 Sme desgn tps A lstng f cmmn heat exchanger tube dmensns s ncluded n page 4 heat exchanger lecture ntes. See p fr useful nfrmatn n baffle desgn Heat Exchangers Chee 38 8

19 Multpass and Crss-Flw Heat Exchangers accunt fr cmplex flw cndtns n multpass shell and tube and crss-flw heat exchangers the lg-mean temperature dfference can be mdfed: lm F lm CF where Fcrrectn factr Sectn th edtn Fr 6 th edtn see supplemental materal (avalable n the curse webste) Heat Exchangers Chee 38 9

20 Crrectn Factr where t s the tubesde flud temperature Heat Exchangers Chee 38 20

21 Example A shell-and-tube heat exchanger must be desgned t heat 2.5 kg/s f water frm 5 t 85 C. he heatng s t be accmplshed by passng ht engne l whch s avalable at 60 C thrugh the shell sde f the exchanger. he l s knwn t prvde an average cnvectn ceffcent f h 400 W/m 2.K n the utsde f the tubes. en tubes pass the water thrugh the shell. Each tube s thn walled f dameter D25 mm and makes eght passes thrugh the shell. If the l leaves the exchanger at 00 C what s the flw rate? Hw lng must the tubes be t accmplsh the desred heatng? Heat Exchangers Chee 38 2

22 Pressure Drp In practce there can be a sgnfcant pressure drp alng the ppes f a multpass heat exchanger. Results n prperty changes Pressure drp must be accunted fr n real desgn stuatns. See pages n heat exchanger lecture ntes. Heat Exchangers Chee 38 22