CC PHNG PHP THU HI NHIT THI L HI

CC PHNG PHP THU HI NHIT THI L HII. Nguyn l lm vic ca l hi

L hi dng nhit lng sinh ra ca nhin liu, bin thnh nhit nng ca hi nc. Hi ny c cung cp cho cc qu trnh cng nghip nh gia nhit cho khng kh sy, ra thit b, cung cp nhit trong cc nh my dt, ng, ha cht, ru bia, nc gii kht trong trng hp ny hi s dng l hi bo ha. Ngoi ra, cc nh my nhit in dng tuabin hi chy my pht in th hi c s dng l hi qu nhit.II. Cc loi nhit thiTrong qu trnh hot ng, l hi c cc ngun nhit lng nh sau: nhit lng ca ngn la khi t chy nhin liu, nhit lng ca khi, nhit lng ca nc v hi nc v ngoi ra cn c nhit lng do x nng chy.Ngn la ca nhin liu nm hon ton trong l hi, hi nc s c a n cc qu trnh cng ngh hoc tuabin; nh vy nhn chung ngn la v hi nc khng mang nhit lng ra mi trng bn ngoi (d nhin l vn c mt phn tn tht ra bn ngoi, nhng l ch l tn thy ch khng phi mt ngun nhit thi). Do , l hi c cc ngun nhit thi sau:

Khi

Nc tuy nm trong bao hi khng thi nhit ra ngoi nhng do trong qu trnh hot ng, x b cu cn tch t li ngi ta vn phi x mt lng nc nht nh ra ngoi

nhng l thi x lng th cng c nhit x thi ra ngoi.

Cc ngun nhit thi ny u c nhit cao, nu khng c tn dng li s gy s lng ph rt ln. Sau y ta s i vo phn tch nhit thi ca tng loi.1- Khi thi:

T nguyn l lm vic, ta thy nhim v ch yu ca l hi l sinh hi (c th l hi bo ha hoc hi qu nhit). Nh vy, sau khi chy, nhit lng ca nhin liu c dng sinh hi, nhit khi ti trung tm bung la c th t 1600-22000C (ty thuc vo tng loi l), khi sau khi i qua vi pass gia nhit cho nc sinh hi th nhit sau khi ra khi bung la t khong 900-13000C. Theo l thuyt, nhim v ca khi n y l ht v c th thi ra ngoi.

Tuy nhin, do nhit ca khi ang cn rt cao, nu thi i m k tn dng th s rt lng ph. Mt khc, hi nc sau khi i qua cc thit b cng ngh s c ngng t thnh lng v c bm tr li l hi tip tc nhn nhit sinh hi. Nc c cp tr li l hi li c nhit kh thp.

V d: hi sau sinh ra qua thit b trao i nhit (LOAD) a li b cha ( y nc b sung (make-up water), c nhit thp dn n nc cp (feedwater) cng b h thp nhit .

Hoc cc nh my nhit in hi nc sau khi gin n tuabin ri c ngng t bnh ngng c p sut khong 0,05 bar vi nhit khong 320C), r rng l ta s phi tn thm mt nhit lng gia nhit cho nc cp t nhit thp n nhit si ng vi p sut tng ng.

V d sau cho thy hiu qu ca vic gia nhit nc cp trc khi a vo l hi:

Cho l hi hot ng vi hi bo ha p sut p = 10 bar, sn lng hi D = 10 tn/gi. Nhit nc cp l 300C. Dng du mazt c thnh phn lm vic nh sau:

Nhit lng cn thit gia nhit nc thnh hi bo ha p sut 10 bar:

Vi i1: entanpi ca hi bo ha p sut 10 bar

i2: entanpi ca nc p sut 10 bar ng vi nhit 300C

Nhit tr thp ca nhin liu l:

Gi s hiu sut nhit l 100%, ta thy lng tiu hao nhin liu l:

Nu nh nc cp c gia nhit n nhit si, th nhit lng sinh hi bo ha l:

Vi : entanpi ca nc p sut 10 bar ng vi nhit si

Lng tiu hao nhin liu trng hp ny l:

Nh vy ta tit kim c mt lng nhin liu

T th ta thy, khi nc c hm nng, th nhit lng cn thit cung cp cho qu trnh sinh hi s gim v lng tiu hao nhin liu cng gim, trong trng hp ny ta tit kim c 24,34% nhin liu.

Mt phn t khc cn c gia nhit trong l hi l khng kh. Nh ta bit, khng kh a vo bung la cung cp oxi cho qu trnh chy. Khng kh thng c ly mi trng bn ngoi c nhit khong 300C, nu a khng kh ny vo bung la th s hp thu mt phn nhit lng do nhin liu chy sinh ra. Do vic sy khng kh tng thm nhit lng a vo bung la s lm tng thm nhit lng hu ch sinh hi. Gi s ta dng du gia nhit khng kh: l phun, nhit khng kh a vo bung la khong 2500C, nhit lng cn thit gia nhit cho khng kh l:

Vi : th tch khng kh l thuyt cn thit t chy ht nhin liu

i1: entanpi ca khng kh 2500C

i2: entanpi ca khng kh 300C

Nu dng du gia nhit th ta phi tn thm mt lng nhin liu:

Ngoi ra, i vi cc l t bng nhin liu lng m ch yu l du nng (FO) th vic hm nng du trc khi a vo bung la l cn thit v nht ca du nhit thng rt cao gy kh khn cho vic dn du trong cc ng cng nh vic phun du vo bung la. Thng thng ngi ta thng gia nhit du ln n khong 900C. Trc y ngi ta thng dng b gia nhit du bng in kt hp vi hi nc. V d sau cho thy nhit lng cn thit gia nhit cho du

Nhit lng cn thit gia nhit du:

ViB: lng tiu hao nhin liu (ly v d trn B = 0,189 kg/s)

Cnl: t nhit ca nhin liu

Trong :

T , ta tnh c

T cc v d trn ta thy nhit lng ca khi thi nu c tn dng hm nc cp, sy khng kh, hm du th s tit kim c mt chi ph rt ln. V l do m ngy nay ngi ta thng lp t cc b trao i nhit pha sau l, ng khi thi gia nhit cho nc, khng kh cng nh du (thng gi l b hm nc, b sy khng kh v b hm du). Ty thuc vo tng loi l m c th c hoc khng c 1 trong 3 phn t k trn. Ngoi ra, i vi nhng l c cng sut ln th nhit thi khi c th ln n rt cao, tuy tn dng nhng vn cn mt ngun nhit lng kh ln, do ngi ta s tm cch tn dng ngun nhit lng ny un nc sinh hot cho cc x nghip hoc h tiu dng hay trong vn v tit kim nng lng th dng nhit do khi thi ny d chy cc my lnh hp th.

Theo s ta thy, khi cp nhit cho bnh pht sinh th NH3 trong hn hp dung dch gm NH3 H2O s bc hi (do cng iu kin p sut th NH3 c nhit bay hi thp hn H2O). NH3 bay ln cn nc s i xung di. Hi NH3 tip tc i vo bnh ngng v c ngng t thnh lng nh nc lm mt, sau NH3 lng i qua van tit lu gim p sut v i vo bnh bc hi. y n s nhn nhit t cht cn lm lnh ( y v d l nc) v bc hi ri i qua bnh hp th. Ti bnh hp th, NH3 s c H2O t bnh pht sinh hp th tr li v hn hp ny c bm v bnh pht sinh tip tc chu trnh ca n. Ta thy v mt nng lng (nu khng k ngun nhit t khi c sn) th my lnh hp th tiu tn t nng lng hn nhiu so vi my lnh c my nn, v cng cn thit cho my nn nn 1kg hi t p sut po ln p sut pk s cao hn nhiu cng bm hn hp lng t p sut po n p sut pk. Do , tn dng nhit thi ca khi cho my lnh hp th khng nhng gip gim tn tht do khi thi ra ngoi m cn gip ta tit kim mt phn nng lng m l ra phi dng khi s dng my lnh c my nn hi.

Cu to

a. B hm nc:

B hm nc cng nh mt thit b trao i nhit, dng nhit ca khi nng i bn ngoi gia nhit cho nc lnh i bn trong ng.

tng cng din tch trao i nhit, ngi ta b tr thm nhiu ng ni vi nhau: dng phng hoc khng gian

tng cng h s trao i nhit, ngi ta lm thm cnh v pha khi, do khi c h s ta nhit i lu b hn nc

Tu theo mc gia nhit trong b hm nc m b hm nc c th lm vic trng thi si hoc khng si. cc l ghi do nhit khng kh nng khng cao nn ton b lng nhit cn li thng dng gia nhit cho b hm nc. V vy b hm nc thng lm vic trng thi si.Cu to ca b hm nc thng chia lm 3 loi: ng thp trn, ng thp c cnh v ng gang. Do tnh cht ca gang l chu va p km nn thng dung gang cho nhng b hm nc khng si.

cc b hm nc bng gang, ngi ta thng lm thm cnh bn ngoi (v khi c h s ta nhit i lu b hn nc) tng cng h s trao i nhit. Do gang c tnh c cao hn thp nn vic lm cnh i vi gang d hn thp nhiu. Tuy nhin, ngy nay do cng ngh ch to pht trin nn vic lm cnh i vi thp khng cn kh khn nh trc na.b. B sy khng kh:

Cng l mt thit b trao i nhit. Theo nguyn tc truyn nhit, b sy khng kh c chia lm hai loi: loi thu nhit v loi hi nhit. Thu nhit: nhit truyn trc tip t khi ti khng kh qua vch kim loi ( y l cc ng dn). B sy khng kh kiu thu nhit c dng rng ri nht hin nay. N cng c dng ca mt b trao i nhit loi chm ng trong khi i trong ng cn khng kh i bn ngoi ng. Ngi ch to h thng ng ng t so le v c gi vi nhau bi hai mt sng thnh mt cm. Khi lp l, cc cm ny c lp vi nhau thnh b sy khng kh.Mt cm ca b sy khng kh

C nhiu cch b tr ng i ca khi v khng kh:

B sy khng kh kiu ng c nhng u im sau:

n gin trong ch to, lp rp v lm vic chc chn

Tro bm trong ng khng nhiu, d dng thi sch

Khc phc hin tng lt khng kh vo trong ng khi

Xut tiu hao kim loi tng i b

Khuyt im: ch yu l ng thp khng bn vng di tc dng n mn ca khi nhit cao v mi mn bi tro bay. B sy khng kh bng ng thp c dng gia nhit khng kh ti khong 4000C.

Khi mun nhit khng kh cao hn, ngi ta dng b sy khng kh bng gang. tng h s truyn nhit, ng gang thng c cnh ngoi v c rng trong.

Khuyt im ca b sy khng kh bng gang l kch thc cng knh v nng n.B sy khng kh kiu hi nhit: khi u tin t nng kim loi ri sau nhit tch t kim loi s truyn li cho khng kh. B phn chnh l mt rto qua vi tc 2 5 vng/pht xung quanh trc ng. Trn rto c gn cc l thp, nhng l thp ny trong qu trnh rto quay s ln lt khi th tip xc vi khi nng, khi th tip xc vi khng kh lnh v qua chuyn nhit lng t khi sang khng kh. ng khi v khng kh c b tr hai pha c nh v c ngn bi vch ngn. Hnh nh cc khung v rotor ca b sy khng kh kiu hi nhit, ngi ta s lp vo nhng khung ny nhng l thp tng din tch trao i nhit. Trong qu trnh hot ng, cc l thp b bm tro nn s lm gim h s trao i nhit nn ta dng nc p sut cao ra sch.

Thng thng, b sy khng kh c lp sau b hm nc. Hoc ngi ta chia b hm nc v b sy khng kh lm hai v t chng xen k. Vic chia cp nh vy s gim nhit khi pha cui ng ng v nh th cho php ch to b sy khng kh (hoc b hm nc) cp 2 vi kim loi r tin hn.

V d v vic tn dng nhit do khi thi

Nhin liu: du mazt c cc thnh phn lm vic nh sauC = 83% - H = 10,4% - S = 2,8% - O = 0,7% - A = 0,1 % - W = 3%

Thng s hi:p = 10 bar - D = 10 tn/gi

Cc tn tht khc: q3 = 3% - q4 = 0% - q5 = 1% - q6 = 0%

H s khng kh tha: = 1,2

Khi khng tn dng khi thi:

Gi s nhit khi ra khi bung la l 11000C, nu khng tn dng nhit thi ny thTn tht nhit do khi thi ra ngoi Q2 = (I1 Ikkl)(1-q4/100)

Vi I1 = IRO2 + IN2 + IH2O + (1 )Ikk

= VRO2(Ct1)RO2 + VN2(Ct1)N2 + VH2O(Ct1)H2O + (1 )Vkk(Ct1)kkIkkl = .Vkk .ckkl.tkkl Vi nhit khi thi l 11000C v nhit khng kh lnh l 300C, ta tnh c:Q2 = (I1 Ikkl)(1-q4/100) = 21664,002 kJ/kgNhit lng a vo bung la Qv = Qt + Qnl + Qkkn + Qp

Do khng c sy khng kh, hm nng nhin liu cng nh phun hi nn

Qv = Qt = 39002,9 kJ/kgPhn trm tn tht do khi thi

Lng tiu hao nhin liu

Vi Q1 = (1 q2 q3 q5)Qvibh: entanpi ca hi nc bo ha p sut 10 bar

inc: entanpi ca nc c nhit 300C p sut 10 bar

T ta tnh c

B = 0,467 kg/s

Tn dng nhit thi hm nc:

Gi s nc cp c gia nhit n trng thi lng si p sut 10 bar

Tn tht nhit do khi thi ra ngoi Q2 = (I2 477,968)

Nhit lng a vo bung laQv = 39002,9 kJ/kg Phng trnh cn bng nhit

Qknr = Qnnv

B(I1 I2) = D(i inc)

(1-q5/100)B(I1 I2) = D(i inc)

0,99B(22141,987 I2) = 1769,03Phng trnh c 2 n, nn khng gii c, ta s dng thm phng trnh cn bng nhit th 2:

Kt hp 2 phng trnh ta tnh c cc gi tr sau:I2 = 14739,28 kJ/kg

B = 0,241 kg/s

Q2 = 14261,3 kJ/kg

q2 = 36,56 %

= 59,44 %

qth = 18,79 %

Tn dng nhit thi sy khng kh

Gi s khng kh c sy n nhit 4000C vi nhit khng kh lnh l 300C

Tn tht nhit do khi thi ra ngoi Q2 = (I3 Ikkl)

Nhit lng a vo bung la Qv = Qt + Qkkn Phng trnh cn bng nhit

Qknr = Qkknv

B(I2 I3) = .B.Vkk(Ikkn Ikkl)

Ta tnh c cc gi tr:I3 = 8645,935 kJ/kg

Q2 = 8167,95 kJ/kg

q2 = 18,14 %

= 77,86 %

Phng trnh cn bng nhit:

Suy ra

B = 0,159 kg/s

qth = 29,67 %

Tn dng khi thi hm du

Gi s du c gia nhit t nhit 300C ln 900C

Tn tht nhit do khi thi ra ngoi Q2 = (I4 Ikkl)

Nhit lng a vo bung la Qv = Qt + Qkkn + Qnl Phng trnh cn bng nhit

Qknr = Qnlnv

B(I3 I4) = B. (Cnl2t2 Cnl1t1) Ta tnh c cc gi tr:

I4 = 8518,397 kJ/kg

Q2 = 8040,412 kJ/kg

q2 = 17,78 %

= 78,22 %

Phng trnh cn bng nhit:

T ta tnh c:

B = 0,158 kg/s

qth = 29,83 %

Tn dng nhit thi qua cc thit b:Gi s tn dng ngun nhit un nc dng trong sinh hot, vi yu cu ca khi thi ra khong 2000C, v nhit khi thp hn nhit ng sng s gy ra hin tng n mn ng ng.

Tn tht nhit do khi thi ra ngoi Q2 = (I5 Ikkl)Vi I5 = 3606,178 kJ/kg

Suy ra Q2 = 3128,193 kJ/kg

q2 = 6,92 %

= 89,08 %

Nhit lng a vo bung la Qv = Qt + Qkkn + Qnl Phng trnh cn bng nhit

Ta tnh c:

B = 0,139 kg/s

qth = 40,58 %

Lng nc tn dng nhit

Qknr = Qnnv

B(I4 I5) = G.cp (tn2tn1) Suy ra:

G = 2,305 kg/s th cho ta thy khi tn dng nhit khi thi th hiu sut nhit ca l hi tng ln, tn tht nhit do khi thi gim xung, ng thi lng tiu hao nhin liu cng gim xung.

th cho bit, phn trm nhit lng thu hi c v lng nhit thu hi c so vi nhit lng a vo.

2- Nc x l:

Nc cp cho l hi d c x l k nhng vn c th cn cu cn. Trong qu trnh hot ng, nc trong l bc thnh hi li cu cn, lu ngy bm di y l lm gim h s truyn nhit v gy ra nhng h hng khc do cn x b nhng cu cn ny. Qu trnh x b cu ng thi phi x lun nc ang si p sut cao trong l. Lng nc ny c nhit cao nn khi thi b i s gy ra tn tht.V d: L hi hot ng vi thong s p = 10 bar, D = 10 tn/gi, lng nc x l Dx = 300 kg/gi Nhit nng do nc x = Dx . i = 63,56 kWTrong i l entanpi ca nc trng thi lng si ng vi p sut 10 bar

Tuy nhin, do cha cu nn lng nc ny khng dng li c m phi c cch thu hi nhit ca n. Mt c im na ca lng nc ny l: do ang p sut cao, nn khi x nc vo mi trng p sut thp s lm bc hi mt phn nc, hi ny c nhit cao khi bay ra gy nguy him. Do , cn c bin php cch ly lng hi ny vi mi trng bn ngoi. Bn cnh , lng hi ny cng c nhit lng ng k nn cng cn c bin php thu hi li ngun nhit thi ny.Lng nc x l ha hi (Flash steam) c tnh qua phng trnh cn bng nhit sau (gi s nh nc c x vo mi trng c p sut 1,2 bar):

i10.Dx = x.Dx.i1,2 + (1 x)Dx.i1,2Trong :

i10 l entanpi ca nc trng thi lng si ng vi p sut 10 bar

i1,2 l entanpi ca hi bo ha p sut 1,2 bar

i1,2 l entanpi ca nc trng thi lng si ng vi p sut 1,2 bar

x l t l nc ha hi

T , ta tnh c x = 14,41% Nhit nng ca lng hi ny = x . Dx . i = 32,22 kW

T th ta thy, lng hi tuy chim t l nh nhng nhit lng ca n li chim mt t l kh ln.

c th tch c hi ra khi nc x l, ngi ta dng bnh phn ly

Nc x l (blowdown) sau khi ra khi l hi c a vo bnh phn ly. p sut bnh phn ly vo khong 1,2 bar, ti y xy ra s bc hi mt phn ca nc. Nc x l i xung theo hnh xon c ng thi hi nc bc ln bay ra khi bnh phn ly, nc x ngng i theo ng ng pha di. Hi bc ln l hi sch nn lng hi ny c a tr li bn cha nc cp v gia nhit cho nc cp v ngng t li thnh lng. Qu trnh ny xy ra trong bnh hi nhit.

Nh vy, ta tn dng c ngun nhit ca hi nc phn ly. Cn lng nc x ngng t bnh phn ly s c thi b??? Nh v d trn, lng nhit do nc x ngng d chim khong 50% nhit lng nc x l, v vy nn tm cch tn dng li ngun nhit ny.

Nh ni, lng nc x ngng ny cha cu cn nn khng th s dng li m ta s cho n i qua mt thit b trao i nhit gia nhit cho nc cp. Ngoi ra, vic nh nhit cho nc cp s lm cho nhit ca nc x ngng gim xung v nh th vic x b n ra ngoi s an ton hn

Lng nc x ngng t = (1 x)Dx = 0,0713 kg/s

Gi s sau khi nh nhit cho nc cp, nc ngng c thi ra mi trng vi nhit 300C

Lng nhit thu hi = 0,0713 . (i1,2 i30-1 bar) = 22,35 kW

Tng nhit lng thu hi = 32,22 + 22,35 = 54,57 kW

Lng nhit thu hi c chim khong 86% lng nhit nc x l

S x l trn hot ng hiu qu khi nc cp lun c bm cng thi gian khi c x l. Tuy nhin do x l khng phi lc no cng thc hin, cn nc cp th cn cp lin tc. Do ngi ta s dng s sau:

Lc ny, khi c nc x l th cm bin nhit s a tn hiu m bm tun hon a nc tun hon nhn nhit t nc x ngng v nh nhit cho nc cp trong bn.

3- Nc ngng:Nc ngng l hi nc ngng t sau khi qua cc thit b cng nghip hoc bnh ngng trong nh my nhit in. Hi nc ngng t t cc thit b cng nghip thng c nhit cao (ng vi nhit si p sut tng ng)

Lng nc ngng ny hoc c a tr li l hi tip tc sinh hi, hoc tip tc qua cc b trao i nhit tn dng nhit lng ca n. iu ny s lm gim nhit nc cp cho l hi nhng nh trnh by trn, ta hon ton c th dng khi gia nhit li cho nc cp. Do , ty thuc vo iu kin c th m nn c nhng phng php thch hp.Ngc li, nc ngng t bnh ngng nh my nhit in c nhit kh thp (320C ng vi p sut ngng t 0,05 bar). Do , lng nc ngng ny kh c th tn dng c nhng nhit lng ca n l ng k. V vy hin nay ngi ta kt hp c chu trnh cp in v cp nhit, khi hi nc s gin n xung p sut cao hn p sut kh quyn, do nhit nc ngng cng s cao hn v c dng cung cp cho h sinh hot hoc thit b cng nghip.

Mt loi nc ngng khc l nc ngng t cc ng dn hi. Cc ng tuy c bc cch nhit cn thn nhng cng s c mt lng tn tht nht nh ra mi trng xung quanh. Chnh lng tn tht ny lm ngng t mt phn hi nc v lng nc ngng ny cn c ly ra v nhng l do an ton. V mt c bn, vic ly nc ngng t ng dn hi cng nh vic x l trnh by trn do nc ngng trong ng cng c nhit v p sut cao. Nh vy, nc ngng t cc cc ngng s c a vo bnh phn ly v tn dng nhit.

4- X cc l t than bt cn c mt ngun nhit thi ng k l x. Theo nghin cu, x l l mt ngun nguyn liu rt tt c th thay th cho mt phn xi mng, do hin nay ngi ta thng c xu hng tn thu x l dng trong xy dng.

X bao gm hai loi: x kh v x lng.Vi x kh, nhit x thi ra khng cao nn tn tht nhit hu nh khng ng k, ngi ta c th vn chuyn trc tip chng n bi cha ri n cc cng trnh xy dng.

i vi x lng, ngoi nhit kh cao nn tn tht nhit do x lng thi ra l ng k, vn quan trng l kt ht x c th tn thu n. Trc y ngi ta thng s dng phng php kt ht t: x thi ra c cho vo mng dn nc, khi vo mi trng c nhit thp lp tc x s b rn v v kt thnh cc ht nh. Sau ngi ta a c nc v x n bi cha, cho nc bc hi v thu c x ht.

Cch lm ny gp phi nhng khuyt im sau: Tiu hao lng nc ln nhim khng kh do thnh phn lu hunh trong nhin liu s bc hi vo kh quyn

nhim nc Tn tht nhit khng thu hi c do nc v x ht trn ln vi nhau tuy c nhit cao nhng li c nn vic lp thm cc b trao i trn ng i ca n s gy ra tr lc ln.V vy, ngi ta nghin cu c cch kt ht mi kt ht kh. Nguyn l ca phng php ny c Sumitomo Metals Industries tm ra vo thp k 80 nhng do nhng kh khn v vt liu m n gn y mi p dng c. X lng sau khi thi ra c a vo mt mng dn rt vo mt a ang quay vi tc cao, nh lc ly tm x lng s bn ra chung quanh di dng cc ht nh ng thi n trao i nhit vi mi trng xung quanh ( y c th khng kh). Kt qu l cc ht x s ngui dn ng thi khng kh s nng ln, nh vy ta va tn thu c x ht v ng thi gia nhit cho khng kh.

Cch kt ht x kh hon ton khc phc c nhng nhc im ca kt ht t k trn. Tuy nguyn l ca n sm c tm ra nhng mi cho n thi gian gn y mi p dng c. Do trong qu kh ngi ta vn cha tm c kim loi c th hot ng bn di tc cao v nhit cao ca x.

C 2 cch kt ht x theo phng php ny: theo a quay hoc theo trng quay

Di y l hnh nh thc t ca mt a kim loi vi cc ht x bn ra nh lc ly tm.

Kt ht bng trng quay

Nc ra

Nc vo

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