Thiet Bi Truyen Nhiet

7/15/2019 Thiet Bi Truyen Nhiet

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baoduongcokhi.com

THIT B TRUYN NHIT.


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THIT B TRUYN NHIT.1. Gii thiu chung:

Truyn nhit l qu trnh vn chuyn nhit t mt lu th ny sang lu th khc.Qu trnh truyn nhit l qu trnh mt chiu, dng nhit ch truyn t vng c nhit cao sang vng c nhit thp. V vy, s truyn nhit ch xy ra khi c s chnhlch v nhit trong h hay ni cch khc l h tn ti mt gradient nhit . Nnglng c truyn i di dng dng nhit nn khng tho trc tip gi tr nhitlng m xc nh thng qua i lng vt l c tho c l nhit .

S phn b nhit v gi tr ca dng nhit(lng nhit truyn qua mt n vdin tch b mt trong mt n v thi gian) l nhng vn quan tm hng u trongnghin cu v thit k cc thit b trao i nhit. Cc phn tch v qu trnh truynnhit l iu ct li quyt nh kch thc v kt cu ca thit b t a ra nhnggii php k thut v kinh t cho mt thit b trao i nhit.

Ngi ta phn bit qu trnh truyn nhit n nh v qu trnh truyn nhitkhng n nh, trong qu trnh truyn nhit n nh, nhit c th thay i theo

khng gian nhng khng thay i theo thi gian, cn qu trnh truyn nht khng nnh th nhit thay i theo c khng gian v thi gian. Qu trnh truyn nhit nnh ch xy ra trong cc thit b lm vic lin tc, qu trnh truyn nhit khng nnh xy ra trong cc thit b lm vic gin on hoc trong giai on u v cui caqu trnh lin tc.

Nhit c truyn t vt ny sang vt khc theo cc phng thc sau y:

Dn nhit: Dn nhit l qu trnh truyn nhit t phn t ny n phn tkhc trong h khi chng tip xc trc tip vi nhau v c nhit khc nhau. Qutrnh ny thng xy ra trong cc vt th rn. Cc phn t c nhit cao hn th dao

ng mnh, va chm vi cc phn t ln cn v truyn ng nng cho chng. C nhvy , nhit nng c truyn theo mi hng ca vt th. Dn nhit cng xy ra trongmi trng kh v lng nu cht kh v lng trng thi ng yn hay chuyn ngdng.

i lu nhit: i lu nhit l hin tng truyn nhit do cc phn t khhoc lng i ch cho nhau. Si ch l do c s chnh lch v khi lng ring dos khc nhau v nhit gia cc phn t hoc do cc tc ng chc nh bm,qut, khuy...

Bc x nhit: Bc x nhit l qu trnh truyn nht bng sng in t, ngha

l nhit nng bin thnh cc tia bc x ri truyn i trong mi trng, khi gp vt thcn, mt phn nng lng t cc tia bc xai c vt hp th chuyn thnh nhitnng, mt phn b phn x trli v mt phn i xuyn qua vt th.

Trong thc t, truyn nhit l nt qu trnh phc tp, khng xy ra vi mtphng thc duy nht no m lun xy ra ng thi 2 hoc 3 phng thc trn. Khixem xt, nghin cu mt qu trnh truyn nhit, tu tng trng hp c th, nu nhhng ca mt phng thc no khng ng k so vi ton b qu trnh th ta cth b qua.


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2. Cc nh lut v truyn nhit.

2.1 Truyn nhit theo phng thc dn nhit qua b mt vch :

2.1.1 nh lut Fourier v dn nhit :

Qu trnh dn nhit tun theo nh lut Fourier: Mt nguyn t nhit lng dQtruyn qua mt nguyn t b mt dA, trong khong thi gian d t l vi gradient

nhit , vi i lng b mt v thi gian, ngha l:

.... ddAdn

dtdQ = [J].

Vi : dn nhit (W/m.0K).

Q: lng nhit trao i (J).

A: din tch b mt vung gc vi dng nhit truyn (m2).

n: khong cch (m).

Du () v phi ca phng trnh chng t l dng nhit bin i theo chiugim nhit .Gi s lng nhit truyn khng ph thuc vo thi gian ta c:

Lng nhit truyn i:

dAn

dQ ..

=

.

Dng nhit truyn qua mt n v din tch b mt :

..ndA

dQ

==

2.1.2 Dn nhit qua vch phng:

Dn nhit qua vch phng mt lp:

Xt qu trnh dn nhit qua vch phng, ng nht. Theo nh lut Fourier ta c:

Lu lng nhit truyn qua vch :

eAQ 21..

=


m

edAdQ

== . Vi R = e/m.

Vi e: chiu dy ca vch (m).

R: nhit trca vch (m2.0K/W).


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Dn nhit qua vch phng nhiu lp:

=i

ieR

Vi e , : b dy v dn nhit vch th i.

Nh vy: Lu lng nhit trao i cng ln nu t c mt hoc tt c cc yu tsau y:

- B mt truyn nhit ln.

- Chnh lch nhit giu cc lu th ln.

- Trnhit nh.

2.1.3 Dn nhit qua vch ng:

Xt qu trnh dn nhit qua mt ng kim loi c ng knh ngoi de, ngknh trong d, dn nhit .

Vi mt nguyn t chiu di l ca ng, lu lng nhit trao i c tnh nh sau:

i

e

ei

d

dLn

Q)(..2

= .


i

ee

ei

e

d

dLn

dld

Q

..2..

=

=

2

e

l 1

e2

21 n

e1 en

21 3 n n+1l

de

i edi

l


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Nhit tr:

i

ee

d

dLn

dR .

.2 =

Trng hp nhiu vch tr kp :

+= m

e

ei

m

e d

dLn

d

dLndR

..2

1

..2

1

. .

2.2 Truyn nhit theo phng thc i lu nhit :

Cc nh lut v truyn nhit i lu rt phc tp do chng lin quan n chchuyn ng ca dng chy. Ch thung ng vai tr cho trong ng lcca qu trnh trao i nhit, hn na cn lin quan n cc nh lut v dn nhit. Vthc cht qu trnh trao i nhit i lu n thun khng tn ti m lun i km viqu trnh dn nhit, qu trnh ny cn c gi l to nhit i lu. To nhit i luxy ra khi c s trao i nhit gia b mt vt rn vi cht lng chuyn ng, qutrnh ny rt ph bin trong truyn nhit.

Trao i nhit i lu ph thuc vo rt nhiu yu t nh : nguyn nhn gy rachuyn ng ca cc lu th, s thay i trng thi ca lu th trong qu trnh truynnhit(truyn nhit c chuyn pha hay khng chuyn pha) ch chuyn ng, tnhcht vt l ca cc lu th v hnh dng, kch thc ca b mt trao i nhit.

Mt cch tng qut, dng nhit trong trng hp trao i nhit i lu c vitnh sau:

..

=

= hR

h=R1 : h s cp nhit hay h s mng.

2.2.1 H s cp nhit trong trng hp trao i nhit khng c sthay i pha:

H s cp nhit l c trng ca qu trnh trao i nhit i lu nn cng phthuc vo cc yu t lin quan n qu trnh: thng s hnh hc ca thit b, iu kinlm vic v tnh cht dng chy. Do s ph thuc phc tp , khng th thit lpc mt cng thc tng qut xc nh h s cp nhit h m ty tng trng hpc th ngi ta a ra nhng cng thc thc nghim tnh. Cc cng thc ny cxy dng trn cscc chun s sau:

Trng hp i lu cng bc, h s cp nhit h c suy ra t cng thc sau:Nu = a.Ren.Prm.

Hoc: St = a.Ren1.Prm1.

Cc h s: a, n, m c xc nh theo tng trng hp c th.

Cc chun s: Nu, Re, St c thit lp tng dng nhit.

Chun s Nusselt: ..

lhNu =

h: H s cp nhit.

l: Kch thc hnh hc c trng ca thit b(m).


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: h s dn nhit(W/m2.K),

Chun s Reynolds: ...

Re

l=

l: chiu di hnh hc c trng ca thit b.

: vn tc trung bnh ca dng chy.

: khi lng ring ca lu th.: nht ng lc.

Chun s Re c trng cho ch chuyn ng ca dng chy.

Re


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lp m lm tng nhit trcp nhit ng thi ngn cn hi tin n b mt truynnhit, lm gim h s cp nhit ca hi.

H s cp nhit khi qu trnh ngng t ch b khng ch bi trng lc:

Nu=1.51Re-1/3

H s cp nhit khi qu trnh ngng t b khng ch bi ma st gia 2 pha lng-hi: Nu=0.22Re0.6Pr0.4

i vi qu trnh ngng t hon ton: =0.5(1+V

L

)

v, l: Khi lng ring ca hi v lng ngng t.

H s cp nhit ca hi:

Nu=0.22Re0.6Pr1/3

2.3 Truyn nhit theo phng thc bc x nhit:

Tt c cc vt th c nhit cao hn 0K u c th pht ra nhng tia nng

lng di dng tia bc x v lan truyn trong khng gian xung quanh vt th. Nichung tia bc x l do kt qu ca s bin i cc dng nng lng khc m thnh,trong ch yu l do s bin i nng lng. Mc bin i ny c xc nhbng trng thi nhit ca vt th, th hin bng nhit ca n.

Ngi ta chia ra bn loi vt th sau:

Vt en tuyt i: Tt c cc tia bc xu c vt th hp th hon ton.

Vt trng tuyt i: Tt c cc tia bc xu c phn x hon ton.

Vt trong sut: Tt c cc tia bc xu xuyn qua vt.

Vt th xm: Vt th ch hp th mt phn tia bc x vi mi bc sng.Trong thc t ch c mt vt th duy nht l vt xm, 3 vt th cn li khng tn ti.

Cc nh lut cbn ca bcx:

a. nh lut Stphan-Bnzman.

E0=C0(100

T)4

Trong : E0: kh nng bc x ca vt en tuyt i(W/m2).

C0: H s bc x ca vt en tuyt i, gi tr ca n l 5.7W/m2(0K)4

Ni mt cch cht ch th nh lut Stphan-Bnzman chng i vi vt entuyt i. Tuy nhin, bng thc nghim, Stphan v cc nh nghin cu khc in kt lun l c thng dung nh lut cho vt xm v phng trnh c dng:

E=C(100

T)4

H s bc x C ph thuc vo loi vt th, trng thi b mt v nhit ca vtth. Gi tr ca n c xc nh bng thc nghim.

b. nh lut Kichp:

nh lut kichp thit lp quan h gia kh nng bc x v hng s bc x cavt th.

A

E=E0


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Vi A: Kh nng hp th ca vt th.

Ni dung nh lut: T s gia kh nng bc x v kh nng hp th ca mi vtth l ging nhau v bng kh nng bc x ca vt en tuyt i cuing nhit vch ph thuc vo nhit .

2.4 H s truyn nhit chung :

2.4.1 Trng hp truyn nhit qua vch phng:

Xt qu trnh truyn nhit t lu th nng(lu th 1) c nhit cao, sang luth lnh (lu th 2) c nhit thp.

Dng nhit:

=

=

++

= .

11

21

21P

P

P

UR

h

e

h

.

Vi R l tng nhit tr.- R1: Nhit trca qu trnh cp nhit t lu th 1 n tng.

- Rp: Nhit trca tng.

- R2: Nhit trca qu trnh cp nhit t tng n lu th 2.

- Up: L h s truyn nhit chung.

Ta c:21

111

h

e

hUR

P

P

P

++==

.=R1+Rp+R2

Trong qu trnh truyn nhit, cc cn bn bm trn b ca hai mt vch gy ramt nhit trRS.

RS = RS1 + RS2.

Khi h s truyn nhit trong trng hp vch c bm cn c tnh nh sau:

S

PS

RUU

+=11

=R1+Rp+R2+Rs

21 P1P2

Lu th 1

eP

P

1/h1

1/h2

Lu th 2


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2.4.2 Trng hp truyn nhit qua tng ng:

Dng nhit qua thnh ng, c ng knh ngoi de, ng knh trong d.

).(1

..2

.

1.. eiPei

e

ePe

i

e

P

e

PePi

e

i

i

Pii

e

UR

hd

dLn

d

d

d

h

ld

Q

=

=

=

=

=

= .

Vi he : h s mng ngoi ng.

hi : h s mng trong ng.

H s truyn nhit chung c tnh nh sau :

ei

e

P

e

ieP hd

dLn

d

hU

1.

.2

11++=

.

Nhit trca lp cn RS:

Se

i

e

SiS Rd

dRR += . .

RSi, RSe : nhit trdo lp cn ng bn trong v ngoi ng.

H s truyn nhit khi kn trng hp bm cn US:

.11

S

PS

RUU

+= .

2.5 Cc phng trnh cbn tnh ton lng nhit trao i gia hai lu th quathit b trao i nhit:

Phng trnh cn bng nhit: lng nhit ca lu th nng truyn i bng lngnhit ca lu th lnh nhn c (b qua tn tht nhit ra mi trng).

Q = M.CP.(T1 T2) = m.cP.(t2 t1).

Vi M, m: lu lng khi lng ca lu th nng v lnh.

CP,cP : nhit dung ring ng p ca lu th nng v lnh.

T1,T2: nhit vo v ra khi thit b trao i nhit ca lu th nng.

t1,t2: nhit vo v ra khi thit b trao i nhit ca lu th lnh.Hoc:

e

de

Pi Pedi

i

l


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.)(

)(

21

12

2

1

T

t

TT

tt

C

C

=

=

Vi: C = M.CP gi l nhit dung lu lng khi lng.

Phng trnh truyn nhit: nhit lng trao i gia hai lu th thng qua mtnguyn t b mt dA c tnh nh sau:

dQ = U.(t1 t2).dA.Vi U: h s truyn nhit (W/m2.0K).

(t1 t2) = t: chnh lch nhit gia hai lu th trn b mt nguyn t dA.

Ly tch phn theo din tch b mt A phng trnh trn. Ta c:

=A

dAtUQ .. .

S bin thin ca U theo nhit l khng ng k (xem U = Const). Khi :

AtUQ ..= .

:t gi l chnh lch nhit trung bnh gia hai lu th (LMTD). Gi trt xc nh tu thuc vo chiu chuyn ng cc lu th v nhit dung lu lng

khi lng ca cht lng.

Trng hp hai lu th chuyn ng cng chiu:

)(

)()()(

22

11

2211

tT

tTLn

tTtTtcc

=

Trng hp hai lu th chuyn ng ngc chiu:

A

C < c

t2

t1

T1

T2

C > c

A

T1

t2

t1

T2

A

MC< mcT1

t2

t1

T2

T1

t2

t1

T2

MC> mc

A


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)(

)()()(

12

21

1221

tT

tTLn

tTtTtnc

= .

* Xt quan h nhit ra ca cc lu th v b mt truyn nhit:

Kho st phng trnh cn bng nhit v phng trnh truyn nhit, ngi ta

v ra ng c tnh mi quan h gia cc nhit v b mt truyn nhit T-A nhhnh trn. T th trn ta thy rng, trong chuyn ng xui chiu, nhit ra calu th ngui lun b gii hn v lun nh hn nhit u ra ca lu cht nng,trong khi i vi trng hp chuyn ng ngc chiu th c th ln hn.

Mi quan h gia LMTDCC v LMTDNC:

LMTDNC = F. LMTDCC.

i vi cc thit b trao i nhit kiu ng chm, gi tr F xc nh bng cngthc (thit b 1 passe trong ng, 2 passe ngoi ng) hay th thc nghim ph thucvo gi tr R v E nh sau:

12

21

tt

TTR

= .

11

12

tT

ttE

= .

Cng vi mt thit b v cc dng lu th nh nhau, nu cc lu th chuyn ngngc chiu c gi tr DTLM ln hn chuyn ng cng chiu.

1


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- H s cp nhit tng 1.74 ln.

- Tn tht p sut tng ln 8 ln.

Nh vy, vic thay i s passe trong ng cho php iu chnh h s cp nhittrong ng, tuy nhin vn cn hn ch khi tn tht p sut ca lu th trong ng khkhng ch.

Tng t, h s cp nhit bn ngoi ng c iu chnh bng cch thay i tit

din dng chy ca lu thi trong v bc(shell side) khi thay i ng knh vbc, bc ng v lp t cc tm chn dc theo chm ng. Lu thi ngoi ng khith chuyn ng song song vi cc tm chn, khi th chuyn ng dc theo chm ngqua khong trng gia tm chn v than thit b. Vic thay i lin tc hng cadng chy nh vy cho php tng cng ng k hiu qu truyn nhit v tit dinca dng chy thay i, vn tc lu th tng ln, ch chy ri dt c. Tntht p sut tng khi tng s tm chn trong thit b cng ln v khong trng giathn thit b v tm chn cng nh.

Vn tc cc lu th trong thit b cng b khng ch do cc qui tc v an ton

cho thit b nh xi mn chc v cc chn ng trong thit b. Lu lng c khilng ring cng nh, vn tc cho php i trong thit b trao i nhit cng ln

- Cht lng:

i trong ng: 1-2.5m/s

i ngoi ng: 0.3-1m/s

- Cht kh:

p sut chn khng: 45-80m/s

p sut kh quyn 12-30m/s.

p sut cao 6-12m/s.Khi cc thng s hnh hc ca thit b c cnh, h s cp nhit cn ph

thuc vo lu lng ca dng. Khi lu thang chy ri, nu tng lu lng ln10%, h s cp nhit trong ng tng khong 8%, trong khi tn tht p sut tngkhong 20%. Nu gim ng k lu lng c th ko theo s thay i ch chuynng ca dng chy v gim mnh h s cp nhit.

Mt yu t na cng nh hng ln n h s cp nhit l tnh cht vt l calu th: nht, dn nhit v nhit dung ring. Trong , nht c tm quantrng ln nht trong bi ton trao i nhit i lu. i vi nhng cht lng c

nht ln, ch chy ri kh t c v v vy h s cp nhit trong trng hp nythng thp. Ngc li, h s cp nhit t l thun vi dn nhit ca cht lng.

Kh nng ng cn ca lu th cng l mt nhn t lm gim h s cp nhit.Tu thuc vo tng lu th v iu kin lm vic( thi gian, vn tc chuyn ng,nhit ...) m lp cn bm ln b mt truyn nhit nhiu hay t. Lp cn ny ngthi lm tng tn tht p sut khi cc lu th chuyn ng trong thit b. Qu trnh tocn khng khng chc, v vy khi thit k lun phi tnh sao cho h s truynnhit cao hn so vi yu cu b vo phn nhit trgy ra cho cc lp cn ny.Ngoi ra, khi la chn cng ngh cng cn phi tnh n kh nng ng cn ca cclu thm bo kh nng tho d, lm sch.

Cng nh phn tch phn trc, chiu chuyn ng ca lu th cng nhhng rt ln n hiu qu trao i nhit. Vi cng h s truyn nhit v nhit ca


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cc lu th c xc nh, nu cc lu th chuyn ng ngc chiu th gi trLMTD(Log mean temperature difference) tnh c ln hn, v vy b mt truyn

nhit nh hncc

nc

LMTD

LMTD=

nc

cc

UA

UA=

nc

cc

A

A>1. Do vy cch b tr lu th chuyn ng

ngc chiu c li hn.

III. PHN LOI THIT B TRAO I NHIT.

1 Yu cu i vi mt thit b trao i nhit:Mt thit b trao i nhit tt v mt thit k cng nh vn hnh phi cn p

ng nhng nhu cu sau:

H s trao i nhit cng ln cng tt. Cng mt lng nhit cn trao i, nuh s truyn nhit cng ln th thit b s gim din tch b mt trao i nhit.

Tn tht p sut i vi cc dng lu cht cng nh cng tt. Khi s gimcng sut ca bm v qut...

B mt trao i nhit ca thit b phi thit k sao cho t b bm cn bn, d

lm sch v sa cha. B mt ngn cch gia cc lu cht nng v lnh phi tt, trnh hin tng hai

lu th ha trn vo nhau.

Thit b phi bo m an ton khi vn hnh, tui th cao, kt cu n gin dlp rp, gi thnh thp.

2 .Phn loi v cu to thit b trao i nhit:

A. Cu to:

Mt thit b trao i nhit ( heat exchanger) gm c cc b phn chnh sau:

- V bc (shell).- u chp v bc ( shell cover).

- Cc ng (tubes).

- B phn phn phi dng chy ( channel, channel cover, channel nozzle,channel passrib, channel tubesheet).

- Chm ng (tube sheet).

- Cc vch ngn ( baffles).

- Cc hng vo v ra ( nozzles).

Mt thit b trao i nhit ( heat exchanger) c chia lm 3 phn chnh sau:Phn u, phn thn v phn ui. Hnh v di y s ch cho ta, hnh dng v cuto ca 3 vng chnh ca 1 heat exchanger.


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B. Phn Loi:

a. Phn loi theo nguyn l lm vic:

- Trao i nhit trc tip: Hai lu th tip xc trc tip nhau hoc trn ln vonhau.

- Trao i nhit gin tip: Truyn nhit gia 2 lu th qua mt b mt phncch.

- Trao i nhit trn cng mt b mt rn: un nng b mt rn bng mt dng

ti nhit, sau cho dng ngui i qua b mt vt rn ly nhit. Loi trao inhit ny rt t dng trong cng nghip.


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b. Phn loi theo chiu chuyn ng ca lu th:

Chuyn ng cng chiu: cc lu cht i vo v i ra khi thit bcng mt pha,trong thit b cc lu cht chuyn ng cng chiu.

Chuyn ng ngc chiu: cc lu cht i vo v ra khi thit bkhc pha, trongthit b cc lu cht chuyn ng ngc chiu.

Chuyn ng cho dng: cc lu cht chuyn ng ct ngang nhau gin tip quavch ngn.

Chuyn ng hn hp: cc dng lu th chuyn ng hn hp ca hai hay ba kiuchuyn ng trn.

c. Phn loi theo chc nng lm vic ca thit b:

Trong nh my lc du, cc thit b trao i nhit vi cc chc nng ch yusau:

Chc nng lm lnh: Gm cc thit b sau:

+ Thit b lm lnh: Lm lnh bng mt dng kh hoc lng bng mt dng ph

tr( cc dng nguyn liu hoc sn phm trong qu trnh) hoc bng nc.+Thit b lm ngui bng khng kh

.

CoolerCoolerFinned fanFinned fan

+ Thit b Trim cooler. Lm ngui sn phm n nhit yu cu sau khi

lm ngui sb bng cc dng khc m bo nhit lu tr sn phm.

+ Thit b chiller: Lm lnh lu th bng cch bc hi mt tc nhn lnh.

Chc nng ngng t: Dng ngng t mt phn hoc ton b lng hi bngcch s dung nc hoc mt dng c nhit thp, khng kh...

Thit b bc hi v un si li:m bo bc hi mt phn hay ton b cht lng,qu trnh ny c thc hin bng hi nc qu nhit hay mt dng mang nhit khc.


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Kettlereboiler:


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u im: +: Thch hp vi dng cng ngh c phm vi thay i rng.

+: B mt trao i nhit ln.

+: Khng gian bay hi ln.


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+: Lng y ra n nh nhc vch chy trn.

Nhc im: Gi thnh cao.

Thermosyphon reboiler:

Thng dc p dng khi lu lng vn hnh gn vi lu lng thit k. Viloi ny th gi thnh thp nht trong cc loi reboiler, kh nng to cn thp.

Forced circulation reboilers:

Fired heater reboiler:


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* Chc nng un nng: un nng mt lu th bng hi nc hoc bng mt dngnhit khc tn dung c t qu trnh, gia nhit trc nguyn liu ca qu trnh bngcc dng nhit do cc sn phm sinh ra.

d. Phn loi thit b trao i nhit theo cu to:

Tu thuc vo dng trao i nhit trc tip hay gin tip, c nhiu loi thit btrao i nhit khc nhau. Sau y, ta ch xt cc kiu thit b trao i nhit gin tip.

Thit b trao i nhit kiu ng trn:

Loi thit b ny c chm ng t so le hoc song song. Trong lu cht cnun nng i ngoi ng, lu cht nng i ngoi ng. Loi ny thng c s dng cc thit bun nng nc, hi nc, khng kh... hay gia nhit trong cc thit bphn ng

Thit b trao i nhit kiu ng lng ng:

ng ngoi c ng knh D bao bc ng bn trong c ng knh d2/d1. Bntrong c th mt hay nhiu ng, c cnh hoc khng c cnh. Cc lu cht chuynng ngc chiu nhau.

Loi thit b ny c u im d lm v sinh v sa cha. Tuy nhin n c din

tch b mt trao i nhit nh so vi cc loi thit b khc cng th tch, cc lucht d b r r.


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ng dng: lm ngng t hi, lm lnh lu cht th lng, un nng nc...

Thit b trao i nhit kiu ng chm:

THIT B TRAO I NHIT DNG NG V V

Parts Identification


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Loi thit b ny c s dng rt rng ri trong nh my lc du do c nhiu uim nh : din tch b mt trao i nhit ln, chc chn, gn, hiu sut v an toncao trong s dng.

Thit b gm v bn ngoi bng kim loi dy, bn trong cha nhiu ng nh.Cc ng ny l ng trn hoc ng c cnh gn theo chu vi bn ngoi ng, ng thnghoc hnh ch U v c gn ln mt sng bng phng php hn hay nc ng.

Mt lu thi trong trong cc ng nh, lu cht cn li i trong khng gian giacc ng. Dc theo chm ng, ngi ta b tr cc tm ngn nhm tng cng qu trnhtrao i nhit v chng rung ng cho cc ng.

hai u thit b c gn cc b phn phn phi, cc b phn phn phi c tcdng nh hng cc dng chy v phn phi mt cch ng u lu lng cht lngi trong ng. Cn i vi lu thi ngoi ng, ca vo v ra c gn ln thn vthit b.

Gi qung ng i ca lu th tu ny n u kia ca thit b l mt Passe,thit b trao i nhit c mt hay nhiu Passe i trong v ngoi ng. Thng th micht i ngoi ng l 1 pass, 2 passe trong ng, hoc s passe 12, 14,16,18... Spasse tng ln s lm tng tc chy ca lu th trong khi lu lng khng i, do s lm tng h s trao i nhit ca thit b.

chng gin ndo nhit ngi ta ch to cc np b v thit b, hoc s dngng hnh ch U, hoc dng loi thit b c phn ui c th di chuyn c.

V trt thit b c th nm ngang hoc thng ng. Khi t nm ngang thit bs vng chc hn nhng chim nhiu din tch, khi cht lng chy trong ng schim y ng. Cn khi t thng ng, thit b chim t din tch nhng t vngchc, cht lng i trong ng khng chim y ng m c phn b to ra lp mngmng xung quanh b mt trong ca ng v chy t trn xung.

C nhiu cch bng trn mt sng, thng thng nht l l b tr theo hnhvung v tam gic u. Ngoi ra, c thc phn b theo theo hnh trn ng tm

hay hnh lc gic u.

Dch

h h

Dch

B


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Khi b tr ng theo hnh tam gic, cho php gim c ng knh tngng ca lu thi ngoi ng l 8,6% so vi cch b trng theo hnh vung, gimc chi phu t. Tuy nhin, vi cch b trng theo tam gic khng ph hp vilu thi ngoi ng c khuynh hng to cn cao.

Ssp xp cc tm ngn trong thit b trao i nhit theo cc cch sau:


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La chn lu thi trong v ngoi ng :

Vic la chn lu thi trong ng hay ngoi ng nh hng n h s mng, hs trao i nhit ca thit b. ng thi, n cng thun tin trong vic bo dng, tusa thit b. Thng thng, lu thc chn i ngoi ng hay trong ng da vocc c im sau:

Tit din dng chy ngoi ng ln hn trong ng. Lu th c lu lng thtch ln hn c chn i ngoi ng.

Qu trnh lm sch cn bn trong ng d dng hn ngoi ng. Lu th ckh nng n mn v bm cn b mt trao i nhit cao c chn i trong ng.

Lu th c p sut ln c chn i trong ng, gim bt chi phu t do

tng chiu dy v thit b.


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Lu th c nhit thp c chn i ngoi ng, nhm gim bt tn tht nhitra mi trng.

Thit b trao i nhit kiu tm:

Loi thit b trao i nhit kiu tm c s dng hu ht trong cc ngnh cngnghip ha cht, ha du, thc phm...

B mt trao i nhit l cc tm kim loi c lp ghp song song nhau. Trn tmkim loi ny c to nhiu knh dn cho c hai lu thi hai bn v xen k nhau.Cc knh dn thng l cc khe hp song song hoc an li, hnh dng cc khe hpv mt ct ngang cc tm trao i nhit rt phc tp. Cc lu th nng v lnh itrong knh dn hp ca cc tm xen k lin tip nhau. iu , s lm cho thit b htsc gn, nh m tt c cc loi thit b trao i nhit khc khng c c.

Gia cc tm trao i nhit c cc m lm kn, trnh hin tng cc lu cht rr vo nhau. Mt thit bc lp ghp nhiu tm trao i nhit v cnh bng ccbulong.

Vic tnh ton h s truyn nhit v cc thng s thy ng hc ca thit b traoi nhit kiu tm rt phc tp, ty thuc vo kt cu ca tng loi tm khc nhau stng ng vi mi phng php tnh c th.

Loi thit b trao i nhit ny c rt nhiu u im nh sau:

m bo c h s truyn nhit cao vi tn tht thy lc b.

Thit b gn nh, chi ph ch to thp. Din tch b mt trao i nhit ln so vi cc loi thit b khc vi cng mt

th tch.

- S r r do khuyt tt hoc ph hu gasket l bn ngoi v d dng pht hin.

- bm cn thp do ch chy ri cao to ra bi cc tm chn.

Qu trnh lm vic m bo an ton, khng b r r tr phi vt liu b hng.

Thit b d lp t, vn hnh, bo dng v sa cha.

Vic tiu chun ha cc chi tit cho php p dng rng ri loi thit b ny lmvic iu kin cc nhit v p sut khc nhau.

Nhc im:


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- Ngi thc hin cng vic bo dng sa cha phi cn thn trnh s phhy gasket trong sut qu trnh tho d, lm sch v lp ghp li.

- Nhng vt liu lm gasket khng ph hp vi tt c cc dng.

- Nhit v p sut lm vic tng i thp hn so vi chu ng ca vtliu.

Thit b trao i nhit kiu tm c nhiu u im ni bt hn so vi loi thit b

kiu ng chm. Tuy nhin, trong cng ngh lc du vn s dng thit b kiu ngchm ph bin hn, iu c th l do kinh nghim s dng, cc phng php tnhton thy ng hc r rng, qu trnh lm vic ca thit bn nh trong iu kinnhit , p sut v mi trng khc nhau

3. Cc thng s cn gim st i vi thit b trao i nhit loi shell and tubeexchanger, reboiler, plate exchanger:

- Nhit vo v ra ca lu th nng.

- Nhit vo v ra ca lu th ngui.

- Lu lng ca lu th nng i qua thit b.

- Lu lng ca lu th ngui i qua thit b.

- Chnh p ca lu th nng ti u vo v ra ca thit b.

- Chnh p ca lu th ngui ti u vo v ra ca thit b.

- Mc lng phn shell side( i vi mt s loi thit b trao i nhit).

- Nhit dng hi i ra i vi cc thit bi trao i nhit c chc nng bc hihoc un si li.

IV.Sng cn bn trong thit b trao i nhit:Cc nguyn nhn gy ng cu bn:

Sng cu bn ca thit b trao i nhit c th l kt qu t s tch t cacn g hoc cht bn trong thit b trao i nhit.

Cn nc c th lm bn ng trao i nhit ca thit b lm lnh hoc ngngt.


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Thng thng, vt cht bm bn b mt du khng th loi b ngay bngphng php n gin. Khi cn thit, h thng chm ng (tube bundle) phic tho ra lm sch.

Tuy nhin cn nc c th loi b m khng cn ko chm ng ra khi v. Cnnc c th l bn, b, bn ct, v/ hoc cn g.Loi b vt cht gy ng cu:

Nu vt cht gy ng cu l bi: Vic s dng dng nc ra c th ci thin mt cch ng k hiu sut lm

vic Vic o ngc dng chy tc thi, lc ca dng nc snh bt tp cht

bm cht t cui ng trao i nhit v loi bc mt t bn bn trn thnhng.

C mt vi cch sp xp lm cho n c tht c hiu qu lm vic nhthit k m khng phi dng thit b.

Cu cn nc hnh thnh c thc loi b m khng phi tho ri thit btrao i nhit bng vic s dng ha cht lm sch. Cn nc c loi b bng

bin php chc v c hon thnh bi p lc phun nc ra.Lm sch chc ca cu bn b mt du c th thc hin mt s cch. Khoan,

hi nc, phun ct c thc s dng lm sch b mt ng. B mt v cngthng c lau chi bng tay hay ra bng nc p sut cao. Ha cht cng c thc s dng lm sch cu bn ny

V. Nhng cch gim vic bo dng sa cha ( tips to reduce maintenance)

Vn hnh vin l ngi c nh hng rt ln n trong vic gim qu trnhbo dng v ci thin hiu sut vn hnh. Bng vic tun th cht ch cc qui trnhvn hnh, s nhn thc r tc hi ca vic khng tun th qui trnh vn hnh snhhng ln tui th thit b, vn hnh vin c th ngn nga s h hng v s mt khnng ca thit b

Mt vi v d c th gip chng ta gim bo dng, tng tui th v hiu sutlm vic ca thit b trao i nhit:

Duy tr chnh xc nhit du ra nc lm mt (khng c vt qu 125oF)Nu lm sai khuyn co ny c th dn ti kt qu:

1. ng cn qu nhiu trn thnh ng.2. Tng tc n mn cc phn ca thit b trao i nhit.

3. Bin dng, thay i tnh cht chc ca cc ng i nhit. Trnh dn dng hi c nhit cao vo thit b trao i nhit trc khi dngmi cht lnh c thit lp. N c th ngn chn c ng sut qu mc chokim loi.

Duy tr lu lng dng thch hp qua thit b trao i nhit lm sch cubn trong chm ng v duy tr tc gia nhit trong gii hn thit k trnhqu nhit.

Vn hnh trong gii hn thit k ngn nga s vt qu ng sut kim loi.

S dng cc b by hi ( steam traps) nhng thit b gia nhit bng hi nc duy tr p sut hi ti a trong thit b trao i nhit. S r ca cc steamtraps ny s lm cho s tiu th steam vt qu mc, gim h s truyn nhit


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ton phn v ph hng cc thit b cung cp steam nhng ng, cc gi ( support), cc van v hin tng HAMMERING.

Ngn nga mt cch cn thn vic s dng hi nc qu nhit trong thit btrao i nhit c thit k cho hi bo ha nhit. Hi qu nhit trong nggim tc truyn ton phn khong 10 ln v c th y qu nhit thit b

VI. Vn Hnh:1. m bo ton b h thng c lm sch trc khi bt u vn hnh

ngn nga s bt( plugging) cc ng truyn nhit, phn v thit b(shellside) do s di chuyn cc cn bn. Nn s dng cc b lc ( strainer hocsettling tanks) trn cc ng ng dn n thit b trao i nhit.

2. Mcc van vent trc khi khi ng.3. Khi ng mt cch t t. Tham kho bng sau, qui trnh start up v shut

down cho hu ht s p dng ca thit b trao i nhit.4. Sau khi h thng c in y hon ton bi mi cht vn hnh, khng

kh c vent hon ton, ng tt c cc van vent li.5. Sit cht li tt c cc c, cc im ni, vng m(gasket) khi thit bt

n nhit vn hnh ngn nga r r v h hng cc vng m.

6. Khng c vn hnh cc heat exchanger qu p sut v nhit thit k.7. Ngn nga mt cch cn thn water hammer(hammering), x nc ngng

t trong cc thit b steam heat exchanger cng nh cc thit b tng tkhi thc hin khi ng v dng.

8. X tt c lng khi thc hin dng loi tr sng lnh hoc n mn cth xy ra.

9. Tt c cc thit b lp t khng nn b rung khi c dng chy v s rungng ny s lm gim tui th thit b.

10.Khng c vn hnh vi lu lng ln hn thit k v lu lng ln nyco th gy rung ng v ph vchum ng.

11. Cc heat exchanger khng c a vo vn hnh trong thi gian di nnc bo v trnh sn mn. Nu cc heat exchanger khng c a vovn hnh trong thi gian ngn v s dng nc nh l mi cht th phi xlng v thi bng khng kh nng mt cch cn thn. Nu khng th thc hiniu ny th nn s dng nc tun hon hng ngy qua thit b ngn nganc ng c th gy n mn thit b.


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VII. Sbo dng:

Thc hin bo dng theo cc bc hng dn sau y:


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Thiet Bi Truyen Nhiet