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TRNG O TO NHN LC DU KHKHOA O LNG - T NG HA

GIO TRNH:

C S VO LNG QU TRNH

VNG TU 10/05(TI LIU LU HNH NI B)

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Phn I: Cc phn t ca h thng iu khin

MC LC

C s v o lng qu trnh Trang 2/155

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PHN I CC PHN T CA HTHNG IU KHIN


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1. TI SAO CC THIT B O LNG L QUAN TRNG?1.1. An tonCc dng c o lng cn thit cho cc nh my cng nghip hin i hot ng anton v chnh xc. Chng c s dng iu khin qu trnh nhm t c cc ch

tiu k thut ca sn phm cng nh ngng hot ng ca nh my trc khi cc sc xy ra. duy tr hot ng an ton i hi cc dng c o phi:

Gi cho cc bin qu trnh nm trong gii hn vn hnh an ton.Pht hin cc nguy c xy ra nguy him khi chng xut hin.Cung cp cc cnh bo hoc dng h thng khi cn.

Vic s dng cc thit b o lng l quan trng i vi cng ty v bn thn chng tacng ging nh cc cng nhn, bi v:

Chng gip chng ta vn hnh nh my an ton.Chng gip chng ta bo v mi trng v cng ng ni chng ta lm vic.

Cc dng c o lng cn thit gip bo v sc khe v hnh phc ca con ngilm vic ti hin trng cng nh nhng ngi sng v lm vic trong cng ngquanh nh my. Vic theo di v iu khin s hot ng ca nh my cng mang tnhquyt nh n vic bo v mi trng v t c cc quy nh ca php lut.1.2. c tnh k thut ca sn phm t c cc tiu chun k thut ca sn phm, cc dng c o lng phi:

Duy tr cht lngm bo tin cy v tnh lp li ca qu trnhm bo rng cc nguyn vt liu th c s dng cng hiu qu cng tt.Gi cho gi thnh sn phm nm trong gii hn cho php.

duy tr vic kinh doanh c li, h thng phi to ra cc sn phm cht lng nnh theo thit k trong cc iu kin c kim sot. Mt s h thng sn xut hini th qu phc tp v c cn nhc k lng n ni m nhiu sn phm ngy nay

khng th c sn xut m khng c s iu khin chnh xc ca cc h thng olng.2. CC CHC NNG C BN CA CC H THNG IU KHIN .2.1. H thng h H thng iu khin n gin nht l h thng h. tng c bn ca iu khin nyl thit lp mt h thng hot ng t n mc chnh xc cn thit bng cch iuchnh trc tip hot ng ca ng ra h thng (hnh 1.1). Khng c thng tin phn hin b iu khin xc nh hoc hiu chnh tn hiu ra, v vy h thng iu khindng ny c th cho tn hiu ra vi sai s ln.

V d trn my tin NC, ngi vn hnh c th xc nh trc tc ct, lng chydao v chiu su ct sau thc hin qu trnh ct. Nu nh khng c nhng yu tnhiu th cht lng v kch thc b mt gia cng s t yu cu d kin . Tuy nhin,


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nhng yu t nhiu nh mn ca dng c, cng thay i trn b mt chi tit cngv.vs lm kt qu gia cng khng t kt qu nh mong mun. Nh vy, hthng iu khin vng h khng c kh nng b tr cc tc ng do nhiu gy ra

Hnh 1.1: S khi h thng h

2.2. H thng knPhng php iu khin ny lm mt tc hi ca nhiu bng cch o nh hng cnhiu trn tn hiu ra hay trn sn phm ng ra ca h thng (thng tin phn hi), t tnh ton cc tc ng hiu chnh cn thit nhm lm mt tc dng ca nhiu dutr tn hiu ra hay sn phm ra n nh nh mong mun.

Tn hiu sai lch = Gi tr mong mun Gi tr thc t.Phng php ny c dng ch yu trong iu khin lin tc hay iu khin qutrnh

Hnh 1.2: S khi h thng kn

2.3. Cc bc iu khin

Mt qu trnh c th phc tp nh vic tch mt cht ha hc no y t cc nguyliu c bn hoc n gin nh vic vn chuyn vt liu t mt bn ny n bn khCc loi cm bin v thit b c lin quan iu khin mt qu trnh s thay i tC s v o lng qu trnh Trang 5/155

Thit biu khin

i tngiu khin

Nhiu

Yu cu Tc ng Kt qu

Thit biu khin

i tngiu khin

Nhiu

Yu cu Tc ng

Thit bo lng

Kt qu

So snh

Thng tin phn hi

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theo phc tp ca mi qu trnh. C s ca iu khin qu trnh th ging nhau,khng mn n qu trnh phc tp hay n gin nh th no. iu khin qu trnh, chng ta phi:

Thu thp thng tin v qu trnh.

So snh thng tin vi kt qu mong mun hay im thit lp (set point).Hiu chnh qu trnh mt lng cn thit t n v duy tr set point.

Hnh sau y m t mt qu trnh c iu khin bi cc thit b o lng.

Hnh 1.3

Trong qu trnh (Process) ny, thng tin c thu thp l: Vic vn chuyn vt liuvo bn DBinc theo di hoc o lng l: p sut bn trong bn D c to ra bi vicchuyn vt liu vo bn DKt qu mong munl: Duy tr mt p sut c th (set point) trong bn D trong khivt liu c ly ra khi bn D t ng E.Cc hiu chnh cn thitl: iu khin dng vt liu (bin thao tc) t bm A vo bn bng cch thay i mc ng m ca van C.2.4. Cc vng iu khinCc bc c bn cn thit iu khin mt bin thu thp thng tin, so snh vi set point,v hiu chnh to thnh mt vng iu khin.

Hnh 1.4: Vng iu khin


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Trong mt vng iu khin:Phn t cm binthu thp thng tin v bin cn theo di v gi n b iukhinB iu khinso snh thng tin nhn c t cm bin vi set point v gitn hiu n phn t iu khin.Phn t iu khinthc hin s hiu chnh cn thit iu chnh bin thaotc v t n im set point.

2.5. Tn hiu vo v tn hiu raTn hiu vo v ra c s dng m t cc tn hiu c gi t mt thit b o nn mt thit b o khc trong mt vng iu khin. Tn hiu ra ca mt thit b o nc th l tn hiu vo ca thit b o khc. V d, bn cha trong hnh 1.5, tn hiu t b iu khin n van l tn hiu ra ca b iu khin. Tn hiu y li l tn hiu vca van c nhim v iu khin lu lng vo bn.

Chng ta c th s dng m hnh vng iu khin hnh 1.2 ch ra nhng g xy ri vi p sut trong bn hnh 1.1.

Hnh 1.52.6. Truyn tn hiuCc tn hiu vo v ra c truyn nh th no? Cc tn hiu trong vng iu khin thhin thng tin v ln v cht lng ca mt bin. chuyn thng tin ny t m phn t ny n phn t khc i hi thng tin phi c chuyn i thnh dn ph hp cc phn t c th c c. Cc transducer v/hoc transmitter lm nhimv chuyn i tn hiu v gi chng n phn t k tip trong vng iu khin.2.7. Mi trng truyn

Cc tn hiu hu ht thng c truyn gia cc phn t trong vng iu khin btn hiu in hay kh nn. Trn s P&ID, bn cng c th thy cc phng thchoc mi trng khc c s dng truyn thng tin nh:


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ng mao gia cc cm bin v transmitter.Sng in t nh hng gia cc cm bin v transmitter.Tn hiu thy lc.Tn hiu m thanh.

Trong tng lai, cng c nhiu thng tin c th c truyn trong vng iu khin bng nh sng qua ng truyn cp quang.Hnh di y cho thy thng tin t mt cm bin c truyn gia cc thit b otheo hai dng khc nhau, in v kh nn.

Hnh 1.6

2.8. Cc loi tn hiuCc tn hiu c th l mt trong 3 dng khc nhau sau y:

1. Dng nh phn (Binary) - Ch c hai gi tr, on v off.2. Dng tng t (Analog) - Cho di cc gi tr lin tc khng b gin on.3. S (Digital) - Mt chui cc xung.

Nu mt dng c o khng th nhn tn hiu vo c gi t transmitter, tn hiu cth c chuyn i thnh dng ph hp bng mt transducer. V d, khi b iukhin trong mt vng iu khin l mt my tnh, d liu vo i vi my tnh phi dng s. Tn hiu ra t b iu khin my tnh cng l dng s, nhng cc kt qu ct cm bin thng l dng tng t hoc tn hiu lin tc. Nhiu thit b iu khincui cng i hi dng tn hiu vo l lin tc (thng l kh nn). Cc transducer

chuyn i hoc dch cc tn hiu t mt dng ny sang dng khc m cc dngc o c th truyn nhn thng tin vi nhau c.


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Nu b iu khin trong hnh 1.6 l my tnh, cc transducer hoc converter trong hnh1.7 sau y phi c s dng:

P/I Transducer: Chuyn i tn hiu kh nn (P) thnh dngin tng t (I)A/D Converter:Chuyn i dng in tng t (I) thnh tnhiu s (D).D/A Converter:Chuyn i tn hiu s (D) thnh dng intng t (I).I/P Transducer: Chuyn i dng in tng t (I) thnh tnhiu kh nn (P).

Hnh 1.7Hnh 1.8 cho thy:

Cc kiu ng tn hiu c s dng kt ni cc phn ca mt vng ikhin.Di tn hiu chung cho mi loi.Cc trnh by cc dng tn hiu khc nhau trn mt s P&ID tiu biu

Hnh 1.8

3. CC PHN T C BN CA CC H THNG IU KHIN3.1. Cm binCc cm bin thu thp thng tin ca mt bin. Chng theo di hoc o mt thng sno ca mt h thng qu trnh. S hot ng thnh cng ca h thng ph thuvo cc php o chnh xc v tin cy. Vi mc ch ca kha hc ny, chng ta chia cc cc bin thnh hai loi o trc tip v o gin tip.Vi thit b o trc tip, thng thng l chng hot ng c lp v khng phi lmt thnh phn ca vng iu khin. V d:

ng h p sut s dng o p sut.ng h o mc dng chia vchRotameter dng o lu lng


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H thng o nhit kiu in kno khi lng dng chia vch

Vi thit b o gin tip, chng hot ng ging nh cm bin trong vng iu khin.V d:

Cp nhit in v RTD dng o nhit .B chuyn i p sut kiu in dung v kiu lc cng.Thit b o kiu Orifice, kiu Vortex, kiu t trng, kiu tuabin, .o mc kiu chnh p, kiu bt kh, kiu phao, kiu sng siu m,

Trong phn sau y, chng ta s tm hiu cc thit b o c trng dng o cc binqu trnh: nhit , p sut, lu lng, v khi lng. Loi dng c o cn thit tythuc vo cc yu t c trng ca qu trnh nh:

tin cy ca thng tin cung cp t cm bin.Bin qu trnh thay i mt lng bao nhiu th cn phi c pht hin vp ng ca cm bin i vi s thay i ny nh th no. chnh xc ca qu trnh cn phi c tha mn cc yu cu k thut.Thi gian p ng ca cm bin.

Hnh 1.93.2. Thit b ch th v thit b ghi (Indicator v Recorder)Cc thng tin cn c hin th v ghi li cng nh c ti b iu khin. Cc phnt cm bin c th c kt ni ti cc thit b ch th v thit b ghi. Cc thit b chth v thit b ghi trong thc t khng c chc nng iu khin bin qu trnh.


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Thit b ch th dng hin th thng tin di dng tc thi. My o tc trong xehi l mt thit b ch th dng hin th tc . N c th hin th tc di dntng t hoc dng s. Trong c hai trng hp, ng h tc ch th tc tc thca xe.Thit b ghi (Recorder) l mt thit b dng ghi li gi tr ca bin qu trnh theo

thi gian. Chng cho php chng ta gi li cc thng tin trn giy c th xem litrong tng lai.

Hnh 1.10

3.3. Transmitter Transmitter chuyn cc thng tin thu thp c t cm bin ti b iu khin tronvng iu khin. Trong hot ng ca cc nh my thc t, cc phn t cm bin v

cc b iu khin thng nm cch xa nhau. i khi, phn t cm bin c tch hchung trong transmitter.Hai loi transmitter ph bin l in t v kh nn. Cc tn hiu trong vng iu khinthng c truyn i di dng tn hiu in tng t hay tn hiu kh nn tng tCc transmitter kh nn chuyn i thng tin t cm bin thnh tn hiu p sut truyn i trong ng ng n b iu khin. Cc tn hiu kh nn v bn cht t nhis mt nhiu thi gian truyn hn cc tn hiu in.Cc transmitter c th gi thng tin ti cc phn t khc trong h thng nh h thngcnh bo v an ton, cc thit b ghi v thit b ch th c gn trn bng, v my tnh

cng nh cc thit b iu khin.3.4. Cc b iu khinChc nng u tin ca b iu khin l so snh gi tr ca bin o c vi set poinmong mun.Bt k s khc bit no u c coi l lch. Set point c th l gi tr n homt di gi tr chp nhn c. Nu lch l zero, bin ang gi tr set point (gitr mong mun), v khng cn c s hiu chnh no i vi qu trnh ti thi im . Nu lch khc zero, chc nng th hai ca b iu khin l gi tn hiu n phniu khin cui ra lnh cho n hiu chnh bin thao tc theo hng sao cho gim

lch gia set point v gi tr hin ti ca bin qu trnh cn iu khin.


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Hnh sau y trnh by mt b trao i nhit. Vng iu khin o v iu khin nhit ca dng hi qu trnh bng cch iu chnh lu lng dng nc mui lnh qua b trao i nhit.

Hnh 1.11Hnh 1.12 trnh by mt v d v mt bn cha c lp t mt cm bin mc. Trong vd ny, cc thng tin v mc cht lng trong bn c gi ti mt thit b ghi v biu khin cng c lp t trn bng iu khin. Gi tr set point c a vo biu khin t nt chnh set point trn b iu khin.

Hnh 1.12

Hnh 1.13Trong cc nh my hini, chc nng ca b

iu khin thng cthc hin bi my tnhm n c kt ni ti


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cm bin v phn t iu khin trong vng iu khin. Cc tn hiu c x l ddng s. Mt my tin n c th phc v nh b iu khin cho nhiu vng iu khi(hnh 1.13).3.5. Phn t iu khin cui Phn t cui trong vng iu khin l phn t iu khin m thc t n to ra mt thay i trn qu trnh v ko bin qu trnh v gn hn vi set point nhm gim lch v zero. Mi qu trnh bao gm mt s loi dch chuyn hoc dng chy - hoc sdch chuyn vt liu hoc dng chy nng lng.Cc phn t iu khin iu ha lu lng theo mt s phng php bng ccchuyn tn hiu ca b iu khin thnh mt hnh ng. Phn t iu khin c trcn hai phn:

Mt c cu chp hnh s nhn tn hiu t b iu khin v chuyn i thnchuyn ng.

Mt thit b thao tc bin qu trnh.Hu ht cc phn t cui ph bin trong vng iu khin l mt s loi van c gtrn ng ng qu trnh. Cc v d v cc phn t iu khin khc l bm v cc thi b iu khin ng c khc.Thng thng, vic thao tc mt bin bao gm vic thay i:

V tr ca van iu tit dng cht lng.Tc ca mt thit b nh thit vn chuyn hoc bm iu chnh llng.

Lng nng lng cung cp cho thit b in nh phn t to nhit.V tr ca mng ngn iu chnh dng kh.

Hnh 1.14: Phn t iu khin cui (van)

Hnh 1.15 sau y trnh by mt h thng cung cp nc c trng cho mt cn hHot ng ca h thng nh sau:1. Vi nc nh bp hot ng


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2. Mc nc trong bn gim3. Vng khng kh trn bn tng ln lm gim p sut trong bn.4. Cng tc p sut ng p sut 20 psi.5. Cng tc p sut ng ni kn mch cung cp in cho bm hot ng.


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Hnh 1.15


6. Nc c bm t ging vo bn7. p sut tng ln khi mc nc trong bn tng;

vng khng kh trn nh bn b nn li.8. Khi p sut t gi tr 40 psi, cng tc p sut

m, ngt bm.

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Phn II: Cm bin

PHN II CM BIN


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Phn II: Cm bin

Trang trng

4. CM BIN NHIT 4.1. Cp nhit in (TC)

Nguyn l hot ng nh hng ca nhit n mi ni gia hai kim loi khc nhau to ra mt in nh. Mt trong nhng loi cm bin nhit n gin nht l cp nhit in, chng hng da vo mt nguyn l c gi l hiu ng Seebeck. Seebeck khm ph rahin tng ny vo nm 1821, v trong nhng nm sau cp nhit in tr thnloi cm bin nhit c s dng rng ri nht. T cp nhit in (thermocouple) ghp t hai t: thermo c ngha l nhit v couple biu th hai mi ni.Mt cp nhit in bnh thng gm hai dy kim loi khc nhau, mi dy c ch t mt kim loi n cht hay hp kim. Hai dy ny c ni li vi nhau ti mt

to thnh im o, thng thng c gi l im nng, bi v phn ln nhit o cao hn nhit mi trng. Hai u cn li ca hai dy c ni ti dng c o to thnh mch kn cho dng in chy qua, dng c o ny s o mc in p c tra ti im ni v chuyn i n thnh gi tr nhit tng ng.Khng may, cc im ni gia cp nhit in vi thit b ch th hoc vng iu khili to ra cc mi ni khc gia hai kim loi khc nhau m chng cng to ra nhnin p ph thuc vo nhit . loi b vn ny, cc mch cp nhit in s mt im ni chun kt hp vi mch in t o v hiu chnh nh hng ny. ni chun thng thng v mt in ging nh mt cp nhit in 0oC. im chunthng c t gn hoc thm ch bn trong thit b ch th.in p do cp nhit in to ra rt thp v c o bng mV. in p ny tng knhit tng. Gi tr in p ny cng ty thuc vo t hp hai kim loi c s dn ch to cp nhit in.

Hnh 2.1: Cng ngh cp nhit inChuyn i in p sinh ra Nhng g cp nhit in cho bit l s khc nhau v nhit gia im o v ichun. (Thc t, chng ta khng th o c gi tr tuyt i; vic chng ta c th ll so snh mt i lng bit vi mt i lng cha bit). Nu chng ta bit nh chun, chng ta c th tnh ra c nhit qu trnh bng cch o in p c bi cp nhit in:

nhit cha bit = (in p/h s Seebeck) + nhit chun Nhit qu trnh c th c suy ra t gi tr in p o c bng cch da vo th (hnh 2.2) hoc chnh xc hn, bng cch da vo bng tham chiu cp nhit im trong lit k cc in p tng ng vi nhit ca mi loi cp nhit i


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Phn II: Cm bin

Khng may, quan h in p theo nhit khng phi l ng thng, v h sSeeebeck khng phi l mt hng s (hnh 2.3). Vi mt s cp nhit in trn ton dio ca n, v d nh loi K trn ton di o t 0 n 1000oC (32 n 1832oF), h sSeebeck tng i l hng s (khong 40V/oC), nhng thng thng n thay i theonhit . iu ny trong qu kh dn n mi loi cp nhit in c mt thang oduy nht hoc cn thit phi s dng bng v c tuyn chuyn i in p thnhnhit . Ngy nay, kh nng b nh ca vi x l gii quyt tt c cc vn ny, vnhng cng vic nht nho tn thi gian trc y by gi c gii quyt rt nhanhchng v d dng. Ni tm li, tnh cht khng tuyn tnh t nhin ca cp nhit inkhng cn l vn .

Hnh 2.2: Quan h in p - nhit ca TC

Mt vn na l tn hiu ca cp nhit in rt nh. Nh trnh by trong hnh 2.3, mtcp nhit in platinum s to ra in p khong 10V/oC. Hay ni cch khc, thm chvi mt transmitter cng nghip tt nht c di o nh nht l 1mV v sai s tuyt inh nht khong 0.01mV, ngha l 10V. V th rt kh khn thc hin mt php os dng transmitter cng nghip v cp nhit in platinum m s thay i in p raca n trn mi C nh hn sai s ca transmitter. iu ny c th chp nhn c di nhit cao, nhng khng th chp nhn khi di nhit o hp. V vy, cp nhitin khng c khuyn co s dng trong trng hp di o hp hoc nhng php osai lch nhit nh.

Hnh 2.3: H s Seebeck


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Phn II: Cm bin

Cc bng tham chiu cp nhit inTt c cc bng tham chiu cp nhit in trong ti liu ny c da vo nhit ni chun 0oC (32oF); v vy, vic chuyn i trc tip t cc bng ny c th thchin c khi mi ni chun c t vo chu nc . Nu khng th duy tr nhit mi ni chun 0oC, phi s dng mt h s hiu chnhi vi cc in p cho trong bng. Ch rng, in p c to ra bi mt cp nhin cho trc s gim xung khi s sai lch nhit gia mi ni o v mi ni chgim. Vic hiu chnh khi nhit mi ni chun ln hn 0oC c m t sau y.Chuyn i mV ra nhit p dng h s hiu chnh mi ni chun i vi mV o c t ng h o inta thc hin nh sau:

1. T bng tham chiu cp nhit thch hp, ly gi tr mV (mi ni chun 0oC)

tng tng vi nhit thc ca mi ni chun.2. Cng i s gi tr va ly c bc 1. vi gi tr mV c c trn vn k3. in p c hiu chnh c th c chuyn i trc tip sang nhit

bng tham chiu cho.V d 1Mt vn k ch th in p l 13,033mV khi c ni vi mt cp nhit in loi T, ngi ta mong mun chuyn i in p ny thnh nhit tng ng. Nhit thcca mi ni chun lc ny c xc nh bng nhit k thy ngn chnh xc l 20oC(68oF). Ni suy t bng ca cp nhit in loi T, 68oF = 0.790mV (da vo nhit mi chun 32oF). Cng gi tr ny vi gi tr ch th trn vn k, 13.033 + 0.790 =13.823 mV, y l gi tr in p c hiu chnh da vo nhit mi ni chun32oF. Ni suy t bng tham chiu i vi loi T, 13.823 mV = 539oF (282oC).V d 2Mt cp nhit in loi T trong iu kin hot ng n nh cho ra mt in p trn vk l 3.369 mV. Nhit thc ca mi ni chun l 70oF (21oC). T bng tra loi T,70oF = 0.830 mV da vo nhit mi ni chun 32oF. Cng i s hai gi tr ny,-3.369 + 0.830 = -2.539 mV. Ni suy t bng, -2.539 mV = -98oF (-72oC).

Chuyn i nhit ra mV xc nh chnh xc in p ng vo cn thit cho vic hiu chun dng c

x l nh sau:1. T bng tra thch hp, ly gi tr mV (da vo mi ni chun 32oF) tnng vi nhit thc ti ng vo ca dng c o cn kim tra.

2. Cng t bng tra va ri, ly gi tr mV (da vo mi ni chun 32oF) tnng vi nhit cn kim tra

3. Gi tr in p bc 2 c tr i s i in p bc 1.


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Phn II: Cm bin

V d 1 Ngi ta mun kim tra hiu chun mt dng c o lng nhit 300oF (149oC).Dng c c thang o c chia theooF v s dng cp nhit in loi T. Nhit thcti ng vo ca dng c o ny c xc nh bng nhit k thy ngn chnh xc l70oF (21oC). T bng tra loi T, 70oF = 0.830 mV v 300oF (1490C) = 6.654 mV da

vo nhit mi ni chun 32o

F. Bng cch tr i, in p ng vo mong mun c hiu chnh da trn c s nhit mi ni chun 70oF l 6.654 0.830 = 5.824mV.V d 2 Ngi ta cn xc nh in p chnh xc ng vo kim tra vic hiu chun mtdng c o lng -200oF (-129oC). Thang o ca dng c o c chia theooF v sdng cp nhit in loi T. Nhit thc ti ng vo ca dng c o l 68oF (20oC). T bng tra loi T, 68oF = 0.790 mV v 200oF = -4.152 mV da vo nhit mi nichun 32oF. Tr i s, in p ng vo c hiu chnh trn c s nhit mi

ni chun 68o

F l 4.152 0.790 = -4.942 mV.


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B im lnhKhi s dng thit b o, n chuyn i in p c to ra do s chnh lch nhit gia im nng v im lnh ghi nhn hoc hin th nhit ca im nng. ngn nga sai s do in p c to ra bi s thay i nhit ca im lnh v bntrong thit b o, nhng in p ny phi c b. Mt phng php l gi nhit

im lnh mt nhit c nh, v d c th thc hin c trong phng th nghimvi chu nc (hnh 2.4). C th s dng mt ci l, mc d vic gi nhit ca mt gi tr hng s l c mt vn .

Hnh 2.4Trong mi trng cng nghip th khng s dng chu nc cng khng dng l.Trong cc transmitter nhit s dng trong cng nghip qu trnh, chu nc chun phi c thay th bng mt mi ni chun theo s thay i ca mi trng.iu ny c th t c bng cch thc hin hai thay i t hnh 2.4. Thay i utin l chn thm vo mt on dy ng ngn gia cc u ni ca vn k v cp nhin v t cc mi ni mi ny trn mt khi cch nhit (hnh 2.5). S thay i nyloi b cc mi ni J3 v J4 c trnh by trong hnh 2.4 bi v trong hnh 2.5, ti hai vtr ny l cc mi ni gia ng vi ng. Bng vic thay th cc mi ni J3 v J4 mitrn khi cch nhit, nh c trnh by trong hnh 2.5, cc nh hng ca chng bloi b bi v chng ngc chiu nhau v cng nhit . S thay i th hai l khnt im chun trong chu nc m t trn khi cch nhit. Lc ny vn k o ip ca cp nhit in khi mi ni chun nhit TREF, v s dng mt thit b onhit khc o nhit im chun. Khi nhit TREF c o chnh xc, phn mmlin quan s xc nh c lng mV m mt cp nhit in s to ra nu im nngnhit TREF v im lnh c t trong chu nc . Cng hai gi tr in p ny vni suy t bng tra thch hp ta s c gi tr nhit tng ng. Nhit ny chnhl nhit qu trnh cn o.

Hnh 2.5


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Hnh 2.6: S kt ni TC cng nghipCc loi cp nhit inC nhiu loi cp nhit in, v mi loi c ch to t nhng kim loi khc nhau.Vic la chn mt loi cp nhit in thng thng da vo:

Cc iu kin ca qu trnh iu khinDi nhit cn oCp chnh xc yu cu

Mi loi cp nhit in c k hiu bng mt ch ci v c th nhn din c chng bng mu dy. Bng sau y trnh by mt s loi cp nhit in thng dng, di nhit v k hiu mu dy ca chng.Kiu Kim loi s dng M mu Di nhit o

E Chromel (+)Constantan (-)(+) ta(-) -200

oC n 900oC

J Iron (+)Constantan (-)(+) Trng(-) 0

oC n 760oC

K Chromel (+)Alumel (-)(+) Vng(-) -200

oC n 1250oC

R Platinum-13%Rhodium(+)Platinum (-) Khng c 0oC n 1450oC

S Platinum-10%Rhodium (+)Platinum (-) Khng c 0oC n 1450oC

T Copper (+)Constantan (-)(+) Xanh da tri(-) -200

oC n 350oC

Vi mi loi cp nhit in, in p do im o sinh ra ng vi mi nhit c ghili thnh bng chuyn i. Bng chuyn i cho bit s chnh lch in p gia mi nio v mi ni chun khi m mi ni chun c duy tr hoc c b v mt in 0oC.Tt c cc mi ni gia hai kim loi khc nhau trong mch s dng cp nhit in phic tnh ti bit chnh xc in p o mi ni o lng to ra.Xem xt k cc bng chuyn i ca cc loi cp nhit in ta thy mi quan h giain p theo s thay i ca nhit l khng tuyn tnh. bng chuyn i ca cpnhit in loi K, in p ra l 0.397mV khi nhit ti mi ni o lng l 10oC. Nuquan h gia nhit v in p ny tht s tuyn tnh th 40oC, in p ra phi l4x0.397=1.588mV. Tuy nhin gi tr thc tra c bng l 1.612mV. Trong vng

iu khin, mch in t s t ng b li tnh cht khng tuyn tnh ny.Khi nhit ti mi ni o lng trn 0oC, in p ti dy dng s cao in p ti dym. Khi nhit mi ni o lng thp hn 0oC, dy dng s tr thnh m v in pC s v o lng qu trnh Trang 22/155

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hin th trn my o s m. Khi nhit ca mi ni o lng bng 0oC, bng vi nhit ca mi ni chun, in p o c s bng 0. Hnh 2.7 sau y cho thy in pca mt cp nhit in trong hai trng hp: khi nhit im o cao hn nhit im chun v ngc li

Hnh 2.7 Kt ni nhiu cp nhit inThnh thong nhiu cp nhit in c ni vi nhau ly gi tr nhit trung bca vi im o. Hnh 2.8 sau y l mt v d v 3 cp nhit in loi J c ni sosong vi nhau ly nhit trung bnh ca mt chic ging si khi hi kh nngqua n. ng h o in p trong hnh v ch gi tr 11.889mV, y l gi tr trung bnhca 3 in p ra trn 3 cp nhit in. in p c th ca cc im o nh sau:

MJ1 = 200oC = 10.779mVMJ2 = 220oC = 11.889mVMJ3 = 240oC = 13.000mV

Hnh 2.8: o nhit bng nhiu TC


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Cu trc cp nhit inThng cc dy ca cp nhit in c t trong mt ng thp khng r mng hoctrong mt lp v bo v chng khi b n mn ha hc hoc b ph hy vt l. Chiudi ca ng hoc v c th thay i t vi in n 30 feet hoc hn. ng knh thngthng l in, tuy nhin, nu ng cha nhiu cp nhit in, kch thc ca n c th

ln ti 1 in. ng bo v thng c lt mt lp gm s gi cho dy cp nhit inkhi chm vi cc phn t khc. iu quan trng phi nh l cp nhit in ch o nhit ti im ni ca hai dy kim loi. Vi cp nhit in n, im o nm u mt.Khi s dng nhiu cp nhit in, chng c th c lp t ti u mt ca ng bo v thc hin nhiu php o cng mt gi tr; hoc chng c th c t dc trong ng bo v. Hnh 2.9 sau trnh by cu trc ca mt cp nhit in v v bc bo v ca n.

Hnh 2.9

Nhng u im v gii hn ca cp nhit inCp nhit in r tin, kch thc nh, chc chn, tin li v linh hot, di o rng, khnng n nh c th chp nhn, c th ti sn xut, chnh xc v nhanh. in p dochng to ra c lp vi chiu di v ng knh dy. Trong khi RTD th chnh xc vn nh hn, nhit tr th nhy hn, cp nhit in l cm bin kinh t nht v chng cth o c nhit cao nht. Nhc im chnh ca cp nhit in l tn hiu ra nh. iu ny lm cho n nhy vi

nhiu in v b gii hn i vi nhng ng dng di o tng i rng. N khngtuyn tnh, v vic chuyn i in p ng thnh gi tr nhit khng n gin nhnhng thit b o trc tip. Cc cp nhit in lun lun cn n b khuch i, v vichiu chun chng c th thay i bi v s nhim bn, s thay i thnh phn do qutrnh xi ha bn trong. Cp nhit in khng th s dng trng thi dy trn trongcht lng dn in, v nu dy ca n khng ng nht, iu ny c th to ra nhngin p m rt kh pht hin. Nhn chung, nn s dng cp nhit in c kch thc dy ln nht c th, trnh sccng v s rung ng, s dng cc transmitter tch hp nu c th (v mt khc s dngdy bc v xon vi v bc c ni b phn bo v ca b chuyn i tng t-stch hp), la chn cn thn ng bao v vt liu ca n.


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4.2. Thit b o nhit kiu in tr (RTD)Gii thiuRTD (Resistance Temperature Detectors) l mt loi thit b c s dng o nhi. RTD thng thng bao gm mt ming kim loi rt nh m in tr ca n thay

theo mt quy lut c bit trc khi nhit thay i. RTD t hn nhiu v hchnh xc hn cp nhit in. Chng c th c s dng hu ht cc v tr m cnhit in c s dng.Platinum l kim loi ph bin nht c dng ch to RTD bi v in tr ca thay i theo nhit rt tuyn tnh. in tr l i lng c trng cho kh nng din ca vt liu. Vi in p bit trc, nu in tr cao th dng in qua vt dnh. in tr c tnh bng m v c k hiu bng mt biu tng hnh mng ng().Vi mt dy kim loi, in tr ca n thay i t l thun vi chiu di. Chiu di dtng gp i th in tr ca n cng tng gp i. Tuy nhin, in tr s gim nu tdin tch tit din ngang ca dy dn. Nu din tch tit din ngang ca dy tng gpth in tr gim mt na. Hnh 2.10 sau y a ra mt s v d v nh hng cchiu di v din tch tit din ngang ln in tr ca ba dy kim loi Platinum khnhau.

Hnh 2.10

o in tr RTDBng cch o in tr ca phn t RTD, ta c th xc nh c nhit qu trnh ns thay i in tr ch do nh hng ca nhit qu trnh. Trong thc t lp t, pht RTD c ni n my o hoc thit b chuyn i tn hiu qua cc dy o. Mchth s dng mt cu o cn bng nh trong hnh 2.11. y h s nhit ca cc intr R1, R2 v R3 l gn bng 0 v gi tr ca R3 c iu chnh cho n khi dng iqua G hoc in p ch th trn vn k s (DVM) bng 0. Trong iu kin ny, gi trca R3 bng vi gi tr in tr cha bit gia hai im o bao gm (A + B + RTD).Gi s rng in tr ca RTD (ti 0oC) l 100 v RTD c ch to bng platinum,khi in tr ca phn t RTD 100 ny s thay i mt lng 0.385/oC. Nu mtdy dn bng ng di 500ft (152m), ng knh 20 gauge c dng ni RTD cu o, th in tr gia hai im o lc ny c cng thm vo 10 (5 cho mC s v o lng qu trnh Trang 25/155

100 feet

Dy A

Dy B

Dy C100 feet

Dy Platinum di 50 feet, din tch tit din ngang 0.1x0.1 c in tr = 0.235Tit din ngang 0.01 in2

Tit din ngang 0.01 in2Dy Platinum di 100 feet, din tch tit din ngang 0.1x0.1 c in tr = 0.47

Tit din ngang 0.0000031416 in2

Dy Platinum di 100 feet, ng knh 0.002 c in tr = 1500

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dy). Lc cu cn bng R3 = A + B + RTD = 10 + RTD. Vi h s nhit 0.385/oC, in tr thm vo ny s gp phn gy nn sai s php o mt lng bng10/0.385 = 26oC.V d trn y cho thy sai s do dy ni tng i ln trong cu hnh RTD 2 dy, vth cu hnh ny khng c s dng trong nhng php o cn chnh xc v chiu

di dy ni ln hn vi in. Khi transmitter c lp t trc tip trn ng bao (hnh2.12) chiu di dy ni s rt nh dn n sai s khng ng k. khc phc trnhtrng ny, cc nh cung cp a ra cu hnh RTD 3 dy m sai s do dy ni cgim n mc nh nht.

Hnh 2.11

Hnh 2.12

RTD 3 dyTrong hnh 2.13 dy C hot ng nh mt cm bin v n l mt b phn ca c haina ca cu o, v th n b loi b khi cu cn bng. Dy A v dy B thuc hai nakhc nhau ca cu v v th lc cu cn bng R3 = B A + RTD. Lc ny sai s do dyni khng cn tn ti thnh phn tng A + B m thay vo l sai lch B A. iu nylm gim ng k sai s dy o v p ng c cc yu cu ca hu ht cc ng dngcng nghip m chiu di dy o ngn. Tuy nhin, y l mt gii php khng trit bi v in tr dy ch c bo m trong gii hn sai s 10% ; v vy, nu A v Bl hai dy ging ht nhau v bng nhau v chiu di th in tr ca chng vn c thkhc nhau trong vng 10%. V th nu c hai cng c in tr 5 trn danh ngha, thtrong thc t mt dy c th l 4.5 v dy cn li l 5.5. Nu iu ny xy ra, sailch 1 vn s gy nn sai s. Nu mt RTD platinum 100 s gy nn sai s tngng 1/0.385 = 2.6oC.


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Trong cc phng th nghim hoc trong cc ng dng i hi chnh xc cao th RTD3 dy khng p ng c, lc ny ngi ta xem xt n RTD 4 dy. Vi RTD 4 dy,nh hng ca dy ni hon ton c loi b.

Hnh 2.13 RTD 4 dy

RTD 4 dy c th c ni vi mt cu o cn bng hoc mt ngun dng hng. C trng hp c m t sau y. Hnh 2.14 minh ha mt cu o cn bng 4 dy. Nhot ng bng cch chuyn mch thng qua mt chuyn mch 3 cc kp v thc hio cn bng lun phin gia hai cu hnh. Mt cu hnh th u A c o cng vin tr RTD trong khi cu hnh cn li th u B, v th chng loi b hon ton v gitr thc t ca in tr RTD c tnh bng (R 3a + R 3b)/2. Vi x l v cc mch in tcao cp s thc hin c gii php tinh vi ny, nhng gi c vn cao, v th cc thk loi ny tng i t tin. Thm vo , chng vn b gii hn bi in tr tip xThm ch vi tip im chuyn mch tt nht (b mt m vng) vn to nn intip xc, v s khc nhau gia cc in tr ny gy ra vi sai lch nh mi khi s dncu hnh ny o in tr.

Hnh 2.14Mt cch khc loi b sai s dy ni l s dng mt ngun dng hng (CCS) troncu hnh RTD 4 dy. Nhng ngun dng hng c sn xut nh gn vi gi thntng i thp v cung cp dng hng kh chnh xc khong 2mA hoc nh hn trnh sai s do nhit t pht. Nh trnh by trong hnh 2.15, trong cu hnh ny cu oc thay th bng mt vn k s dng o in tr ca ring RTD v n khng nhvi nh hng ca dy o bi v khng c dng in chy qua cc dy ni. in tr cdy ni (A v B) khng gp phn to nn sai s bi v in p ri trn chng khngc bao hm trong mch o. t c chnh xc cao nht, phi chc chn rng dng in (Ic) qua RTD phi lhng s v DVM khng tiu th bt k dng in no (i=0), v cng loi b lun ip to ra do mi ni cp nhit in ti im #1 v #2. iu ny l cn thit bi v k


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hai dy (RTD platinum v dy ng) ti im #1 v im #2 to nn nhng mi ni cpnhit in, in p do chng to ra s c ghi nhn bi DVM. nh hng ny b loi b bng cch b. in p lch c to ra do cc mi ni cp nhit in khng mongmun c o bng DVM khi mch CCS h v v th Ic = 0. My o RTD thng minhs ghi nhn in p o c khi khng c dng in chy qua v hiu chnh kt qucui cng bng mt lng khi CCS c kt ni vo mch v Ic khong 2mA.Thng thng, RTD hai dy c s dng trong cc ng dng th cp kiu HVAC,RTD 3 dy c dng trong cng nghip x l, v RTD 4 dy c dng trong cc ngdng c chnh xc cao v trong cc phng th nghim.

Hnh 2.15

Hnh 2.16: RTD cng nghip

RTD kiu dy qunRTD ph bin c ch to t mt si dy Platinum c ng knh rt nh qun quanhmt li bng gm. Li gm sau khi c qun dy s c ph bn ngoi mt lp thytinh nng chy nhm bo v n khi b nh hng bi mi trng. RTD ny cng cth c t thm vo mt lp v bo v cho php lp t n vo cc ng nghoc thit b o qu trnh. Hnh di y trnh by mt RTD tiu biu kiu dy qun.ng knh dy 0.001 in, ng knh li gm khong 0.1 n 0.2 in. in tr ca huht cc RTD thng dng ti 0oC l 100.Mt gii hn ca RTD (so vi cp nhit in) vn kch thc. RTD tng i ln,

bi v t c in tr yu cu (thng l 100) chiu di ca cm bin phi tngi di, thng l vi feet. Gii hn c khc phc bng cch thit k kiu phimmng, rt ph hp vi yu cu kch thc nh gn.


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Hnh 2.17: RTD kiu dy qun

RTD kiu phim mng Cc u o nhit kiu dy qun l ph bin, nhng cc RTD kiu phim mng cnthng c s dng bi v vi cu trc phng v mng, chng p ng nhanh theo thay i ca nhit v c bit chng rt ph hp o nhit b mt v nhica kh. Cu trc ny cng dng t kim loi hn (platinum t hn vng) v t nhncng hn trong qu trnh ch to. Hnh sau so snh kch thc ca mt RTD tiu bivi mt cy bt ch.RTD kiu phim mng c ch to bng cch cho hi platinum ngng t thnh mt lrt mng trn mt tm gm (ceramic) mng v phng. Din tch tit din ngang ca t phim mng ny cc k nh c ngha l ch cn mt on platinum ngn cng to rin tr ln c th o c khi nhit thay i. Cng nh cc RTD kiu dy quRTD kiu phim mng cng cn c bo v bng lp thy tinh hay gm v c vo mt lp v bo v c th lp t chng vo mt h thng o lng qu trnh.in tr ca cc RTD lm bng platinum thay i theo nhit tiu biu nh sau:

0.00385/oCV d: Nu mt RTD lm bng platinum c in tr 0oC l 100 c dng onhit ca nc si (100oC), in tr ca n s tng 38.5 v s thnh 138.5. Gi trny c tnh nh sau:

Lng in tr thay i = 100oC x 100 x 0.00385/oC = 38.5 Nu RTD bng platinum c in tr bng 200 0oC c t vo nc si 100oC thin tr ca n s thay i mt lng bng:

Lng in tr thay i = 100oC x 200 x 0.00385/oC = 77RTD c sn xut o cc di nhit khc nhau. Chng c th c dng nhit t -200oC n 750oC


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Hnh 2.18: RTD kiu phim mng

u im v khuyt im ca RTDRTD l mt trong nhng phn t cm bin nhit chnh xc nht, c th ti sn xut, nnh v nhy. Mt s RTD platinum chnh xc c th o c vi phn ngnoC, v yl l do m trong cc dng c o nh ngha mt phn thang o nhit quc t(ITS-90). Nhng u im khc l nhy tng i tt (0.1 n 10/oF) v s dngdy ni vi mch o bng ng truyn thng (thay v dng dy cp nhit in t tin).Mt u im khc na ca RTD bng ng l c RTD v dy mi vi mch o u bng ng, v th nh hng ca mi ni cp nhit in l nh nht. Mt u im khcca RTD l s dng cu o n truyn thng o s chnh lch nhit gia haiRTD.Khuyt im ca RTD l gi thnh cao, cu trc d v, kch thc ln so vi cp nhitin. Bi v kch thc ln nn thi gian p ng theo s thay i ca nhit tngi chm (hnh 2.19). RTD cng b nh hng bi nhit t pht sinh. Sai s tng theokch thc v in tr v c gim i bng cch gii nhit v bng cch gim n nmc ti thiu (dng hng khong 2mA) hoc loi b hon ton (cu khng cn bng)dng in qua RTD. Sai s c th sinh ra nu in tr cch ly ca RTD b nh hng bi m bm vo v bc hoc do tip xc gia phn t RTD vi v bc. Mt s RTD c nhy dao ng so vi nhng ci khc. RTD cng c chnh xc ph thuc vo n nh ca chng (khng nhy vi s thay i ca nhit ), in tr hng v nguncung cp trong cc cu o c th.


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Hnh 2.19: p ng thi gian cacm bin nhit RTD tiu biu

Trong khi lp t, RTD i hi cng cn thn nh cp nhit in. Thc t lp t nht l t trc tip tt c cc mch in t trn nh ca ng bao (hnh 2.12) v v tloi b c nh hng ca dy ni v nhiu. Nu khng thc hin c cc iu ncc dy ni nn c xon li v bc v; cng nh th, cc dy khng nn b ko cnv i qua vng c sn dc. Dy ni ko di phi c in tr thp (ng knh ln) vdng c o phi m bo.4.3. H thng o nhit kiu bu chaTnh cht gin n v nhit ca cht lng c ng dng ch to ra cc cm bincc h thng o nhit . Di tc ng ca nhit , th tch ca cht lng thay biu th s thay i ca nhit . Trong mt vng iu khin, cc h thng hoc c bin o nhit kiu ny bao gm 3 thnh phn chnh sau:

1. Mt bu nhit l mt bnh cng bn trong cha cht lng, kh hoc kt hcht lng v hi.

2. Mt ng mao l mt ng kim loi c ng knh nh (thng thng khon0,02 in hoc nh hn) kt ni bu cm bin n b chuyn i.

3. Mt b chuyn i v truyn tn hiu c chc nng chuyn i s thay i sut hay th tch thnh tn hiu c, in hoc kh nn a n b phn khtrong vng iu khin.

Cc h thng o nhit kiu bu cha c chia thnh 4 loi:Loi 1: Cc thit b c bu nhit cha cht lng da vo s thay i th tch.Loi 2: Cc thit b c bu nhit cha cht lng/kh da vo s thay i p suLoi 3: Cc thit b c bu nhit cha kh da vo s thay i p sut.Loi 4: Cc thit b c bu nhit cha thy ngn da vo s thay i th tch.

Bu nhit cha cht lng i khi c gi l thit b th tch bi v ng ra ca n l thay i th tch. Bu nhit cha cht kh hoc hi c gi l thit b p sut bing ra ca n l s thay i p sut.


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Hnh 2.20

Bu nhit lun c gn ti v tr o. Tuy nhin, ng mao v b chuyn i nm bnngoi h thng (c th cch mt khong ng k) v tip xc trc tip vi nhit mi trng. Nhit mi trng c th thay i ng k - t im rt lnh nnng. S thay i nhit ny nh hng n th tch ca cht lng hoc p sut cacht kh trong h thng o nhit kiu bu cha.Vic b nh hng ca nhit mi trng ln ng mao thng c thit k sn bntrong b chuyn i v truyn tn hiu.Hnh 2.20 trnh by mt h thng o nhit kiu bu cha c treo bn trong mt bnx l cn o nhit.4.4. Thermowells o nhit qu trnh, cm bin nhit (cp nhit in, RTD, cm bin nhit kiu bucha) phi c tip xc vi qu trnh. Nhiu qu trnh c th lm hng cm bin doha cht hoc cc h hng v mt vt l do tc hoc p sut cao. trnh vn ny v cho php c th thay th cm bin m khng phi gin on qu trnh, cc cm bin nhit thng c t trong mt thermowell. Thermowell l mt ng bo v c

thit k chu ng c cc iu kin ca qu trnh. Thermowell va bo v cc cm bin nhit va c chc nng truyn nhit t qu trnh ti cm bin. Nhit c truynhon ton l quan trng i vi chnh xc ca php o. Cm bin phi c lp tsao cho n tip xc tt vi thermowell ti im o mong mun. Hnh 2.21 sau y trnh by mt thermowell.


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Hnh 2.21: Cu trc mt thermowell

5. CM BIN P SUT5.1. Transducer p sut Cc dng c o p sut l cc b chuyn i l cc thit b c kh nng chuyn

t mt dng nng lng ny sang dng nng lng khc. o p sut, mt dng c phi bao gm mt b phn cm nhn s thay i ca lc tc dng ln n v mt phn khc lm nhim v chuyn i thng tin ny thnh tn hiu ng ra c th s dnc di dng tn hiu c , hoc in, hoc kh nn.Khi o p sut, nh rng tt c cc php o p sut l php o tng i so vi gi t bit trc:

p sut ng h (psig hoc psi) l p sut tng i so vi p sut kh quyti mc nc bin.p sut tuyt i (psia) l p sut tng i so vi p sut chn khng tuyt v phi bao gm p sut kh quyn.p sut chnh lch l p sut khc bit gia hai im o khc nhau.

Cc thit b o p sut cho kt qu trc tip hot ng theo nguyn l l cm nhn thay i p sut v chuyn i s thay i ny thnh s dch chuyn c hc trn mthang o c hiu chun. Cc thit b kiu hp xp v mng chn n gin c c s dng cm bin p sut. S dch chuyn ca cnh tay n gn trn hp xhoc mng chn s ch th gi tr p sut o c theo thang o c hiu chun.


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Hnh 2.22

5.2. Cm bin kiu lc cng ng h o sc cng l mt trong nhng dng c o p sut ph bin nht. Chng lmt v d v thit b cm bin p sut, n hot ng theo nguyn tc chuyn i cc sthay i ca lc thnh tn hiu in. Vng iu khin s s dng cc tn hiu in ny iu khin, qun l cc bin trong h thng. Cc cm bin p sut lc cng tiu biu bao gm:

Mt mng ngn s b bin dng khi c lc tc dng v n s phnh ra theohng tc dng.Mt in tr mu lm theo dng cun dy, l mng hoc mt lp tinh th dnin m in tr ca n s thay i t l thun vi lc tc dng ln mng ngn.

Ng ra ca cm bin p sut lc cng l tn hiu in o c v n thay i theo psut tc dng vo mng ngn theo quy lut bit trc.

Mt s u im ca vic s dng cm bin p sut kiu lc cng so vi cc kiu cm bin p sut khc l:Gi thnh hKch thc nhThi gian p ng nhanh


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Phn II: Cm bin

Hnh: 2.23Hnh 2.23 cho thy mt mng kim loi c lp t sao cho pha trng ca mng ngc tip xc vi p sut qu trnh cn pha gn cm bin lc cng th c txc vi p sut kh quyn. Khi p sut qu trnh ln hn p sut kh quyn, mng ngs phnh ra theo hng tc dng ca p sut qu trnh, lm cng mu dy in tr. D

ny c lm di ra, gim tit din ngang. S thay i nh ny v mt kch thc lm tng in tr ca dy mt lng nh c th o c.Mt mch in c ch to sn bn trong cc cm bin p sut lc cng pht hmt s thay i in tr rt nh c th xy ra khi mng ngn b cong do tc dng ca sut qu trnh. Cm bin p sut thng c 4 dy, 2 dy dng cp in p cho min, 2 dy cn li cp tn hiu in p ra t l thun vi p sut. in p ra thc a vo b chuyn i v truyn tn hiu (transmitter) bin i thnh dng hiu m cc thnh phn khc trong vng iu khin nhn ra c.Trong phn RTD trc y, ta thy in tr thay i theo nhit . Mch in c c

to sn bn trong cm bin p sut lc cng c thit k sao cho loi b c s ti ca in tr gy ra do nhit . V th, cc thay i ng ra ch do s thay i sut gy ra.Hnh 2.24 trnh by mt cm bin p sut lc cng tiu biu. Trong hnh ny, p sutqu trnh t vo b mt mng ngn s lm cho cm bin lc cng c gn mt bn ca mng ngn b bin dng. in tr ca cm bin lc cng thay i t l thun v bin dng. Cc thnh phn in bn trong lp v bo v c cp ngun t in p v to ra in p ng ra. in p ny thay i khi in tr ca cm bin thay i.

Hnh 2.245.3. Cm bin p sut kiu in dung

Hot ng


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Phn II: Cm bin

Mt loi cm bin p sut ph bin khc m n cng cm nhn v bin i p sutthnh tn hiu in l b chuyn i p sut kiu in dung. Mt t in c chto gm:

Hai b mt dn in c ngn cch biMt lp in mi gia, trong trng hp ny l khng kh, vi hng s inmi c bit trc

Hnh 2.25 in dung l lng in tch m cc bn cc ca t in c th tch cin dung ca t in tng khi

Din tch bn cc tngKhong cch gia hai bn cc gimHng s in mi tng


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Phn II: Cm bin

Hnh 2.26Hnh 2.26 cho thy cc thnh phn ca mt cm bin p sut kiu in dung n gip sut o tc ng ln mng kim loi dn in lm cho n gn hn vi b mt din cn li dn n tng in dung gia dy A v dy B. p sut cng ln, in duncng ln khi m khe hn gia hai b mt vt dn gim xung. Loi cm bin thngren hoc mt bch ni vi ng ng hoc bn cha cn o p sut. Cc mch c tch hp vi cm bin p sut kiu in dung to in p hay dng in ngt l vi p sut hay p sut sai lch ng vo. Cc cm bin p sut loi ny cng hi c cp in cho cc mch in bn trong hot ng. o p sut sai lchMt loi cm bin p sut kiu in dung khc s dng mt mng ngn (mt dn i gia hai vt dn kim loi to ra hai t in. Thit k ny ph bin i vi cc bin p sut sai lch. v d hnh 2.27, mt pha ca cm bin c ni vi p sut qutrnh #1 v pha kia c ni vi p sut qu trnh #2.

Hnh 2.27


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Phn II: Cm bin

Bi v cht lng ca mt cm bin p sut kiu in dung ty thuc vo vt liu inmi vi hng s in mi bit trc nm gia hai bn cc ca t in, cn thit phicch ly vng in mi trch n b trn ln vi cht lng qu trnh. Hnh 2.27, ccmng cch ly cng vi khoang rng pha sau c ni ti mng ngn ca t in thngqua mt ng mao. Cc khoang rng ny v cc ng mao c y in mi cngloi vi in mi s dng gia cc bn cc ca t in.Khi p sut qu trnh #1 v #2 khng bng nhau, mng ngn kim loi CD di chuyn v pha p sut thp. Nu p sut #1 ln hn p sut #2, mng ngn kim loi s cong li vhng mng cch ly #2; khong cch gia mng ngn kim loi CD v b mt dn inC2 tr nn nh hn v khong cch gia CD v C1 tng ln.Bi v khong cch gia mt dn in CD v C2 gim, gi tr in dung ca t in nytng ln. Trong khi , khong cch gia mt dn in CD v C1 tng lm cho gi trin dung ca t in ny gim xung. Nu p sut t vo c hai pha ca cm bin bng nhau, p sut sai lch l zero bi v

khong cch gia cc b mt dn in v mng ngn kim loi vn bng nhau.Cc mch in t c gn vi t in c thit k sao cho khi gi tr cc t in ny bng nhau th n s ch th p sut sai lch l zero. Ch khi no gi tr ca hai t inny khc nhau th mi c p sut sai lch.Mt s cm bin p sut kiu sai lch in dung s dng cht lng nh du lm inmi. in mi c vo thng qua mt ng v ng ny c bc kn. Nu in mi l cht lng, p sut P1 v P2 tng ln bng nhau, cc mng s khng dichuyn bn trong bi v cht lng khng chu nn. Mng ch di chuyn khi P1 v P2khng bng nhau.

Nu in mi l kh (kh th chu nn) v d nh khng kh ng thi P1 v P2 tng lnmt lng bng nhau, cc mng ngn s:

Mi mng s di chuyn bn trong mt lng bng nhau.Mi t in s tng gi tr in dung mt lng bng nhau.


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Phn II: Cm bin

Hnh 2.28: Transmitter p sut

Hnh 2.29: Transmitter p sut chnh pCc ng dng

Cc cm bin p sut sai lch c s dng khi sai lch p sut gia hai imquan trng trong vic iu khin qu trnh. Hnh 2.30 trnh by mt v d v mt c bin p sut sai lch c lp t qua mt bnh lc theo di khi no bnh lc y ltt ngn dng chy. Nu p sut chnh lch thp, chng ta bit bnh lc l sch. Nusut chnh lch gia pha vo v ra ca bnh lc tng ln ng k, chng ta bit bnh l b nghn cc b v kha dng chy. Nu trng hp ny xy ra, ng ra t cm bin sut sai lch c th c s dng kch hot cnh bo rng bnh lc cn c sch hoc chuyn dng chy qua ng dn khc.

Hnh 2.30


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Phn II: Cm bin

6. CM BIN MC6.1. Cc loi cm bin mc

Cc cm bin mc c th c s dng o mc cht lng hoc cht rn dng bthoc xc nh khi no vt liu t n mc gii hn hoc di o c thit lptrc. V d, mc ch ca mt cm bin mc trong vng iu khin c th l:

m bo mc c duy tr trong gii hn hot ng l tng t hiu sutcao nht.Kch hot bo ng cnh bo mc vn hnh khng an ton v ng van.Khi ng bm v m van khi mc trong bn gim n mc thp c thitlp trc.

Ngi ta p dng cc k thut khc nhau trong vng iu khin o v iu khinmc vt liu trong bn. Kiu cm bin c s dng ty thuc vo cc yu t nh kchthc v hnh dng ca bn, nhit v p sut hot ng, v cc c tnh ca vt liu.Trong mc ny, chng ta s xem xt cch ngi ta s dng cc phng php v kthut sau y trong cc cm bin mc:

1. p sut chnh lch2. Bt kh3. Phao

4. Th ch5. in dung6. Dn in7. Sng siu m v rada8. Nhng thit b mt im khc

6.2. S dng p sut chnh lchHnh 2.31 trnh by mt php o p sut xc nh mc vt liu trong bn.

Chng ta bit rng, c th xc nh c p sut gy ra bi ct cht lng nu chng ta bit c trng lng ring ca vt liu v chiu cao ca ct. Tng t, nu chng ta bit p sut gy ra bi ct cht lng v trng lng ring ca vt liu, chng ta c thtnh c chiu cao ca ct cht lng.

p sut = trng lng ring x chiu cao (p = .h)Hy nh rng, p sut o c trong bn hnh 2.31 bao gm c p sut gy ra bi bukh quyn ln b mt cht lng bi v bn h vi kh quyn. Tuy nhin, ng h och o p sut tng i ch khng phi p sut tuyt i, n ch hin th p sut gy ra bi ct cht lng.


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Phn II: Cm bin

Hnh 2.31ng h o p sut trong hnh 2.31 cho kt qu 4.33 psi (hoc 4.33 lbs/in2). Chng ta bit trng lng ring ca nc l 62.4 lbs/ft3 hoc 0.0361 lbs/in3. Nh vy chiu caoct nc trong bnh s bng 4.33 lbs/in2 0.0361 lbs/in3 = 120 in. Cm bin p sutc gn vo y bnh s gi tn hiu n mt thit b o s c hiu chun thmc ct nc tnh bng in. ng h o s s chuyn i tn hiu p sut thnh mnc l 120 in. Nu tn hiu ny c gi n mt b iu khin, n c th cdng vn hnh van iu khin mc nc trong bn.

V tr ca ng h p sut hoc cm bin l im ti hn xc nh mc. Ch

rng ng h p sut trong hnh 2.31 c gn ti y bn o p sut ct cht lTrong Hnh 2.32 th ng h o c lp t v tr c mc cht lng khc. Nu nc lp t ti v tr cch y l 120 in, ct cht lng ti im l 0 psi. Nu n gn thp hn mt nc 60 in, ct nc ti im ny s l 2.16 psi.


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Phn II: Cm bin

Hnh 2.32Trong hnh 2.33, cng vi chic bn vi mc nc nh trc nhng c thm p

sut Nit t ln b mt ct nc. Trong v d ny, xc nh mc nc thc t phily p sut o c y bnh tr i p sut ca Nit p ln b mt ct nc, vic ny

phi dng n mt thit b o chnh lch p sut.


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Phn II: Cm bin

Hnh 2.33ng h y bn cho gi tr 24.33 psi, v ng h nh bn ch gi tr 20 psi. psut chnh lch gia y v nh bn l 4.33 psi. Cm bin chnh lch p sut gi thiu n my o s v my o ny s cho kt qu l mc nc tnh bng in

6.3. o mc bng bt kh Bt kh c th c s dng o mc lin tc hoc tch cc ng/ngt c

chuyn mch khi t n mc thit lp trc. Chng lm vic theo cng nguyn l nvic o p sut ti y bn xc nh mc.

Mt ng c t trong bn vi u h ca n cch y bn vi in. Kh (thnl khng kh, nit, hoc mt loi kh m khng phn ng vi qu trnh) c a vng p sut cao hn p sut ct p thy tnh cao nht cn o. Khi cc bt kh va bu thot ra khi u h ca ng, p sut kh trong ng bng vi p sut ct p thtnh ca cht lng trong bn (Nh rng chiu cao ct cht lng bng vi p sut th

tnh ca cht lng). Bng cch gi lu lng kh rt thp, p sut trong ng ti v tr t cm bin p sut v c bn bng vi p sut ti y ng. Khi mc cht lng tr bn thay i, p sut ti v tr kh thot ra khi ng cng thay i t l thun vi m


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Phn II: Cm bin

cht lng. Nu bn b nn, ta phi dng t bo cm bin chnh lch p sut xc nh chnh lch gia p sut trong ng v b mt hoc p sut hi bn trn mc cht lng. Mt s lu khi s dng kiu bt kh

chnh xc c gii hn

Tp cht c th ln vo vt liu ca qu trnhCm bin bnh thng khng c t vo trong vt liu qu trnh, nhng nup sut kh gim xung t ngt, vt liu qu trnh c th trn vo ng nhhng n chnh xc v c th ph hy cm bin cng nh to ra mt minguy him.

Hnh 2.34

6.4. o mc bng thit b kiu phaoPhng php dng phao l mt cch khc o mc. Ty thuc vo th tch v trnglng ca phao m chng c th ni trn b mt ca mt cht lng hoc trn lp tip

xc gia hai cht lng c trng lng ring khc nhau. C bn loi cm bin kiu phaoc bn nh sau:


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Phn II: Cm bin

Cc thit b phao v cp Phao ni trn b mt cht lng v c ni vi msi cp dn n mt b ch th hoc mt c cu cm bin v tr.Phao t - Phao trt trn mt thanh dn, v mt thit b in hoc c kh g bn trong thanh dn s cm nhn v tr phaoPhao c ni vi mt n by, v n by ny s tc ng vo mt chuymch.Phao cng vi mt chuyn mch c buc vo mt si cp

Phao v cpHnh 2.35 trnh by mt h thng phao v cp n gin, h thng ny ch th mc chlng trong bn trc quan. Bi v h thng ny khng c tn hiu ra no khc ngoi v trca qu cn trn thc o, n khng th c s dng nh mt cm bin trong viu khin. Phao c gn vi mt qu cn bn ngoi bn bng mt si cp vt qua m b rng rc. Khi phao chuyn ng ln xung theo s thay i mc cht lng, v tr c

qu cn cng thay i theo s ch th mc trn thang o c chia . Ch khi bch th c gn qu cn nm nh ca bn th phao nm y ca bn, cho bit bang rng.

Hnh 2.35

Vi h thng ny, cung cp tn hiu ra n vng iu khin, n phi cn n mmch in t cm nhn s di chuyn ca cp v chuyn i thnh tn hiu mc. H2.36 trnh by mt cm bin nh th. Mch in t v mt ng c gi cho cng cp ging nh qu cn m khng nhc phao ln khi b mt cht lng. Mch in tcho ra mt tn hiu in t l vi v tr ca phao. H thng ny lm vic khng tt kc cht lng hoc vt liu qu trnh bm cht vo cp.


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Phn II: Cm bin

Hnh 2.36 Phao t Mt kiu h thng phao khc l s dng mt phao t trt trong ng cha mt b nhnt, b ny s cung cp tn hiu ra. Xem mt v d minh ha n gin v tng ny(hnh 2.37). t mt thanh nam chm mt pha ca tm ba cng, mt chic inhc t pha cn li ca tm ba. Khi bn di chuyn thanh nam chm, chic inh sdi chuyn theo.

Hai u im ca thit k ny l:Phao di chuyn dc ng t b nh hng bi nhng gn sng nh trn b mtcht lng.Khng gy nhim bn vt liu x l bi v c ch cm nhn v b nhn t ccch ly vi vt liu x l.

Hnh 2.37


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Phn II: Cm bin

Hnh 2.38 trnh by php o mc trong bn s dng tng ny. u cm bin nca bn tng t nh trong hnh 2.36. B nhn t trng bn trong ng di chuyn lxung cng vi phao. V tr cp c cm nhn bi b iu khin cp v cm bin vca b nhn t trng m b ny s cung cp tn hiu ra t l vi mc cht lng tr bn.

Hnh 2.38

Mt s h thng phao t khng s dng b nhn t. Thay vo , v tr ca phao t th c pht hin bng cc phng php in hoc in t khc nhau, bao gm:Tn hiu radio hoc tn hiu sng siu m c dn hng trong ng.Mt chui cc chuyn mch nh hot ng bng t tnh gn dc lin tc btrong ng.

Cng vi nhiu loi cm bin m chng ta bit, cc h thng phao s c cc thnh phc bn ging nhau v nguyn l hot ng tng t nhau, nhng hnh dng s thay ty theo cc yu cu c bit ca qu trnh v cc thit k ca nh sn xut. Hnh 2.trnh by hai cm bin phao t tiu biu m chng khng s dng b nhn t v cp btrong ng.


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Phn II: Cm bin

Hnh 2.39

Phao vi n byCc cm bin phao m trong s dng mt n by gn vi phao thng c lp ttrn thnh bn hoc c t trong hp kn gn trc tip trn thnh bn. Chng c thc kt ni:

V mt c hcti mt thit bch th c thango c hiuchun.V mt in nmt thit b nhthit b ghi, cnh bo v chuynmch cho bm.V mt thy lcti thit b ghihoc van trong vng iu khin.

Hnh 2.40


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Phn II: Cm bin

Cc phao trong cc dng c o loi ny thng l mt qu cu kim loi rng. Kim los dng ty thuc vo p sut, nhit v bn cht t nhin ca vt liu trong bcha. ng, thp khng r, nhm v nickel l mt s vt liu thng c s dng.Phao nm trn b mt cht lng vi mt phn chm. Khi n i ln xung theo mc chlng, n by s di chuyn. Phao cu nn c kim tra u n, t xut hoc t

mi trong trng hp p sut v nhit c th ph hng n.Mt cm bin kiu phao v n by ch c thxc nh mc trong mt di rt hp. V d, phaotrong bn trn hnh 2.41 ch hu dng khi sdng o mc trong gii hn di chuyn vi ca n by.

Hnh 2.41

Hnh 2.42 trnh by mt cp cm bin kiu phao v n by c sdng tc ng cc chuyn mchti im t cao v im t thp.

Hnh 2.42


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Phn II: Cm bin

Phao v chuyn mchPhao cha chuyn mch thy ngn v c gn vo bn bng mt si cp mm c thc s dng xc nh mc khi mc trong bnh cha t n im m ti chuynmch c chuyn sang ng hay ngt. Kiu cm bin ny khng th dng o mclin tc, nhng l mt cch n gin tn hiu in khi t n mt mc cao hay mcthp no y. Hnh 2.43 trnh by mt s chuyn mch thy ngn.

Hnh 2.43

Hnh 2.44Hnh 2.44 cho thy cch thc thy ngn lm ng hoc ngt chuyn mch khi phao dichuyn ln v xung theo mc cht lng trong bn. Khi thy ngn lp y khe h giahai vt dn, chuyn mch ng v c th dn in. Chuyn mch ng hoc m khi phao xoay ln hoc xung t v tr ngang. Khi phao nm ngang, v tr ca chuyn mchkhng thay i so vi trng thi ca n c thit lp khi n v tr nm ngang bnhthng. Ti mt mc bit (cao hoc thp), cp bt u ko phao ra khi v tr nmngang. Cp cng di, di thay i ca mc cng ln, trong di ny cp vn nm ngang.Hnh 2.45 trnh by hot ng ca mt chuyn mch kiu phao


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Phn II: Cm bin

Hnh 2.456.5. Cm bin kiu th ch

Cm bin mc kiu th ch th tng t vi cm bin kiu phao trong mqu cn c ni vi mt n by. Nhng cm bin ny cng c th c gn btrong mt bnh cha hoc bn trong mt hp gn trong mt bnh cha (hnh 2.46). B phn th ch di hnh tr (khng ging nh phao) v c t chm trong cht lng qtrnh. B phn th ch c gn vi h thng n by m n cho php o trng l biu kin ca b phn th ch khi mc cht lng thay i.

Hnh 2.46Di o ca cm bin s c xc nh bi chiu di ca qu cn th ch. Khi mc clng thp hn b th ch, n by chu ton b trng lng, v ch th mc zero. Kmc cht lng tng ln quanh b th ch, do lc y ca cht lng, n by s chu nh hn lm cho b ch th ca n by di chuyn trn thang o c hiu chu

hoc tn hiu ng ra thay i t l vi s thay i ca mc cht lng. Khi mc cht ltng ti nh ca b th ch, b phn ch th s ch kt qu l 100% thang o.


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Phn II: Cm bin

Hnh 2.47 Hnh 2.48Hnh 2.47 v 2.48 trnh by cch xc nh khi lng biu kin ca ng th ch ti btk v tr no trong bn c th c s dng xc nh mc cht lng trong bn .ng trong v d ny c lm kn c hai u v nng 25 pounds

Hnh 2.49 ch ra cch thc s dng mt cm bin mc kiu th ch xc nh mcca mt tip xc gia hai cht lng c trng lng ring khc nhau.

Hnh 2.49


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Phn II: Cm bin

50 15.4 9.660 15.9 9.163 16.0 9.0

Ngp vt th ch 16.0 9.0Hnh 2.50

Cc bng trong hnh 2.50 so snh mc tnh bng in tng ng vi lc y ln b thch v trng lng b th ch c dng o. Cng ng th ch nh th c sdng o mc trong bn cha ton nc, ton du v pht hin b mt tip xc cahn hp du v nc. o b mt tip xc, b th ch phi lun lun ngp trong chtlng cho kt qu ng.6.6. Cm bin kiu in dung in dung c th c s dng o mc cng nh p sut. in dung l mt php olng in tch ly trn bn cc ca mt t in. Mt t in c ch to gm hai bncc dn in c cch ly bng cht in mi (khng dn in). Cm bin kiu indung c th c s dng cm nhn mc ca c cht lng v cht rn. Hnh 2.51 trnh by phng php sdng u d kiu in dung o mctrong bnh. u d hoc in cc hotng nh mt bn cc ca t in, vthnh bn kim loi l mt bn cc cnli ca t. C mt s khc bit ng kgia hng s in mi ca kh hoc hiv hng s in mi ca cht lng vcht rn qu trnh. Trong php o mclin tc, gi tr in dung s tng khimc ca vt liu cn o tng. Cm binnhn ra s thay i v in dung khimc thay i v chuyn i n thnh tnhiu in tng ng kch hot cnh bo hoc chuyn mch hoc a nvng iu khin.

Hnh 2.51Ch : u d in dung s c di o bng vi chiu di ca n.Mt iu quan trng cn xem xt khi s dng u d in dung l kh nng dn inca cht lng qu trnh. u d in dung s lm vic vi c cht lng dn in vkhng dn in. Tuy nhin, khi o cht lng dn in, i hi u d phi c mt lpcch ly vt liu qu trnh khng lm mt i hiu lc ca in dung. Cc u dthng c bc mt lp vt liu cch in nh Teflon tng chnh xc. Hai yut nh hng n kt qu o ca u d in dung l:

Nhit tng lm gim hng s in mi ca vt liu.

Nu cu to hoc kch thc cc ht ca vt liu qu trnh thay i, hng sin mi cng b nh hng.


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Phn II: Cm bin

Nhng vt liu khc nhau s c hng s in mi khc nhau. Nu thnh phn cacht cn o mc thay i th dng c o phi c hiu chun li Hnh 2.52 trnh by mt trng hp m i hi cm bin c hai u d. Bi v khoncch t thnh bn n u d s khc nhau theo cao, ng ra ca cm bin s khtuyn tnh theo s thay i ca mc trong bn nu thnh bn c s dng nh m

bn cc ca t in. o chnh xc, khong cch gia hai bn cc (hai vt dn) cin phi lun ging nhau trn ton di o. C th s dng mt cm bin mc vi u d nu nh thnh bnh khng phi l vt liu dn in.

Hnh 2.52Hnh 2.53 ngi ta s dng mt ng kim loi ng bao quanh u d v hot ng nhmt in cc th hai.

Hnh 2.536.7. u d kiu vt dn

u d dn in o mc da trn c s tnh dn in tng i tt ca nhiu cht lChng c s dng tiu biu iu khin ng/ngt chuyn mch ca bm, vsolenoid, v trang thit b. Mt in cc kim loi c lp t trong bn ko di n


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Phn II: Cm bin

tr cn thit lp mc cht lng. Khi mc cht lng tng v chm vo u d, n hnhthnh mt mch in gia u d v bn kim loi, bn kim loi lc ny ng vai tr lt ca mch in. Khi iu ny xy ra, dng in s ng hoc ngt chuyn mch iukhin. Cc bin o mc loi dn in ch lm vic vi vt liu qu trnh c tnh dnin.

Mc cn o hoc iu khin bng u dkiu vt dn ty thuc vo v tr ca u d. Nu bn khng phi bng kim loi, ngi ta phi thm vo mt in cc lm im tca mch in. Cm bin kiu vt dn c thc thit k vi nhiu u d c nhiu dikhc nhau gi tn hiu khi mc trong bnt n im nh ngha trc bng chiudi ca mi u d. Hnh 2.54 trnh by mtthit b o mc kiu vt dn c 3 u d cthit k gn thng ng.

Hnh 2.54

6.8. Cc cm bin kiu sng siu m v RadaCm bin o mc bng sng siu m v rada pht ra sng m thanh hoc sng in ttrc tip ti b mt cht lng hoc cht rn cn o mc. Sng m i vi vn tc xp x1054 feet/second trong khng kh v sng rada xp x 186000 miles/second. Cc hthng rada kiu xung v siu m, cm bin o thi gian t lc pht tn hiu n khinhn sng phn x t b mt cht lng. Nu bit c tc v thi gian truyn casng, khong cch t cm bin n b mt c th tnh ton c.Cc cm bin siu m v rada khng tip xc trc tip vi vt liu qu trnh v rt ca thch trong cc ng dng o mc lin tc m :

Vt liu c axit v kim cao c th lm hng thit b oVt liu m b mt cn o mc sn siVt liu nht hoc c bm v n c th bm vo dng c o

Cm bin mc bng sng siu m bao gm mt b pht v b nhn sng hoc xung sium c pht ra t mt b dao ng thch anh n b mt v c phn x ngc li.

(Thch anh dao ng tn s in p xoay chiu tn s cao s to ra sng siu m). Vd, mt 0.0189 second sng siu m i t ni pht (u cm bin) n b mt cch 10 feet v tr v li u cm bin.. chnh xc ca b o mc bng sng siu m


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Phn II: Cm bin

b nh hng bi bt, bi bn, v hi ngng t. Chng cng khng ph hp lm dng trong cc bn nh bi v sng di lm sai lch kt qu o.

Hnh 2.55

Cm bin o mc bng sng rada cng c hai dng: kiu xung v kiu iu tn lin t(FMCW). Khi s dng mt cm bin rada, sng rada c th c to ra bi mt m pht v c gi ra ngoi bng anten. Thi gian sng rada phn x tr v xp bng mt phn triu so vi sng siu m. Hnh 2.56 l mt lp t cm bin o m bng sng rada tiu biu. Cc cm bin o mc bng sng rada khng b nh hng bs thay i trng lng ring, hi trn b mt, bt, hoc nhit . Chng c chxc cao v gi c vn hnh bo dng thp. Cc h thng xung hot ng ging nhcc thit b siu m kiu xung v o thi gian xung i t ni pht n b mt v phx ngc li. Vi cc cm bin kiu xung, sng di do vt cn trong bn nh nc thathanh chn, hoc thnh bn c th nh hng n chnh xc.


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Phn II: Cm bin

Hnh 2.56Cc cm bin FMCW khng tnh ton mc bng cch o thi gian tn hiu di chuyn vt b nh hng do sng di. Cc thit b FMCW so snh tn hiu truyn i v tn hiu

nhn v v phn tch bng mch in t nhng thay i trong dng sng b gy ra bis phn x trch hng t b mt n mc cn xc nh. chnh xc ca c hai loicm bin rada c th c ci thin bng cch thit lp cc thit b to ra chm tiahp. Chm tia cng hp th cng hi t khi tn s ca sng tng ln.Hnh 2.57 trnh by cch thcm mt chuyn mch mc kiusiu m lm vic. Cm binny pht hin mt im mcn bng cch tip xc vi vtliu qu trnh. Hai thch anh pin c gn i din nhau.Khi khng c cht lng invo khe h gia chng, tn hiusng m m c gi bngmt thch anh tch cc skhng th c nhn bi thchanh th hai. Khi mc cht lngtng in vo khe h, tnhiu c truyn qua cht lngn thch anh i din, tchcc n.Hnh 2.57


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Phn II: Cm bin

6.9. Cc loi cm bin mc mt im khcHnh 2.58 trnh by mt kiu cng tc mc dng tip xc khc s dng dao

ng. Thit b ny hot ng nh mt m thoa. p in thch anh c iu khin dng tn s t nhin ca m thoa trong khng kh s lm cho m thoa dao ng. Khmc cht lng tng n gia hai nhnh ca m thoa, s dao ng ca n chm li ho

dng hn. Mch in t bn trong cm bin mc s nhn bit iu ny do m thkhng cn dao ng tn s ban u do thch anh to ra, v cho bit rng mc chlng t n im thit lp.

Hnh 2.58Chuyn mch cnh qut c trnh by trong hnh 2.59 l mt v d v cm bi

o mc mt im c s dng cng vi bt. Mt cnh qut quay lin tc cho n kvt liu qu trnh tng n mc lm cn tr cnh qut quay. Khi cnh qut dng quang c iu khin cng dng theo. Trong khi dng, ng c quay thm c vi vtc ng vo mt cng tc. Nu mc ca vt liu gim xung v cnh qut c g phng, n bt u quay tr li, gii phng cng tc.


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Phn II: Cm bin

1. Lu lng th tch l th tch trn mt n v thi gian nh gallons trn ph(gal/min, gpm) hoc feet khi trn pht (ft3/min)

2. Lu lng khi lng - l khi lng trn mt n v thi gian nh poundtrn giy (lb/sec) hoc pounds trn pht (lb/min)

3. Vn tc dng chy l khong cch trn mt n v thi gian nh feet trn pht (ft/min)

Mi quan h c bn gia lu lng v vn tc l:Lu lng th tch = Vn tc x din tch tit din ngang ca ng ng

Trong hnh 2.61, chng ta thy ng knh ca hai ng c kch thc khc nhau nhhng nh th no n vn tc dng chy khi xt cng lu lng.

Hnh 2.61

7.2. Cc yu t xem xt Mi quan h gia p sut v vn tcTng nng lng ca bt c dng cht lng no cng c to nn t 3 thnh phn: sut, vn tc v cao. S lng ca mi thnh phn c th thay i, nhng tng nlng vn khng i tr khi nng lng b ly i hay thm vo cht lng. V d, mtch nc khi nm trn nh ca ta nh vn khng c p sut hoc vn tc, nhng nc cao ln. Nu n b ra ngoi, cht lng s trao i nng lng ca cao chvn tc. Khi n chm vo t, n khng cn vn tc hoc cao nhng n tc nmt lc ln vo mt t do nng lng ca vn tc chuyn i thnh p sut. Khi lva chm c truyn ti t, nc thc cht khng cn nng lng na.Dng cht lng chy qua ng ng tc ng cng ging nh th. Nu mt lu lth tch cho trc c cho qua mt ng ng nh hn, vn tc s tng ln v p sgim v cht lng trao i nng lng ca p sut cho nng lng ca vn tc. Nu lu lng th tch duy tr khng i, p sut s thp hn ni cht lng di chuvi tc nhanh nht.


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Phn II: Cm bin

Hnh 2.62 nht, khi lng ring v ma st l nhng yu t quan trng phi xem xt khi chnloi cm bin lu lng sao cho n tht ph hp vi qu trnh xc nh. nht lma st ni trong bn thn vt cht, n c nh hng n s chuyn ng ca vt chtkhi chy trong ng ng. nht cng cao th lc cn dng chy cng ln. Mt ong lmt v d v cht lng c nht cao. N khng chy d dng nh sa khi m nhtca sa thp.Khi lng ring l khi lng trn mt n v th tch vt liu. V d v n v khilng ring l pounds trn foot khi (lb/ft3) hoc pounds trn gallon (lb/gallon) hocgam trn centimet khi (g/cm3). Khi lng ring ca kh ty thuc vo thnh phn cuto, nhit v p sut ca n. Bi v cht lng th khng chu nn nh cht kh, khilng ring ca cht lng l mt hm s ca thnh phn cu to v nhit ca n.Cht lng nng hn s kh y hn v s chy chm hn khi cng chu mt lc tcdng (p sut t ln tit din ngang ca ng ng) so vi cht lng nh hn.Bit c lu lng chnh xc ch khi loi cht lng, nhit , p sut v lu lng choqu trnh ph hp vi c tnh k thut c thit k ca cm bin. Khi vt liu qutrnh, lu lng hoc cc iu kin nm ngoi di o xc nh, cn thit phi hiuchun li cm bin hoc thay i cm bin bng mt loi khc ph hp vi iu kinhin ti.Khi lng ring c s dng chuyn i gia lu lng th tch v lu lng khilng. V d, i vi du:

Lu lng th tch x khi lng ring = Lu lng khi lngV d: 5 gpm x 7.2lb/gal = 36 lb/min

Ma st Cng vi ma st ni lin quan n nht, ma st cng l s cn tr i vi dng chyca vt cht do s tip xc ca dng chy vi thnh ng hoc do s tc nghn. ngcng nhm hoc s lng cc on khc khuu, cc van, cc u ni cng nhiu th mast cng ln. Ma st xy ra khi vt liu chy qua ng ng dn n s mt mt p suth thng vnh vin v to thnh nhit. Trong khi s tn tht p sut thng l hinnhin, bt k s thay i nhit no u khng ng k so vi cc ngun nhit khc.Cc dng dng chyKhi lng ring, ma st v nht c th nh hng n dng dng chy trong ng.Dng dng chy c th on trc bng ton hc t ng knh ng, vn tc dngchy, khi lng ring v nht. Hnh 2.63 minh ha cc v d v 3 loi dng chy c

bn. Cc v d ny l cng mt lu lng, ch khc nhau v cc yu t khc ca dngchy. Hu ht cc thit k cm bin lu lng da vo dng chy hn lon ng nhtcho cc php o chnh xc.


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Phn II: Cm bin

Hnh 2.63Bn nhn thy rng, dng chy qua ng ng b nh hng bi nhiu yu t kngoi phm vi ca ti liu ny, nh l:

Ch un cong ca ngGc to bi ng ng so vi phng ngangKhong cch di chuynKh nng chu nn ca kh

Hu ht cc thit k cm bin lu lng u gi s l dng chy hn lon ng nho lu lng th tch. trnh dng chy hn lon khng ng nht, thng phi t mt meter run ca ng ng thng khng gy tc nghn dng chy pha trc sau cm bin to mt dng chy tng i ng nht. Thng s dng ng thng c ng knh 20-25 pha trc cm bin v ng knh 5-10 pha sau cm biTrong mt s trng hp, nhng mnh kim loi mng c lp t bn trong ng sosong vi dng m bo c dng chy ng nht. Nu khng c c dng chng nht, chnh xc ca cm bin lu lng s b nh hng.7.3. p sut sai lchKiu cm bin lu lng c s dng ph bin nht l da vo php o p sut trn mt on ng thu hp. Bi v lng ln thng tin v cc th nghim sn c, ccm bin kiu da vo s thu hp ng ng ny cho kt qu c chnh xc cao kc s dng chnh xc.Cc ng venturi, cc tm orifice v cc nozzle l nhng v d v cc thit b gimp thng c t trong ng ng qu trnh o lu lng. C ba u to nn mchnh lch p sut m c th d dng o c v t tnh c lu lng th tcMt s u im ca phng php ny l:

Gi thnh tng i thpD lp t v thay thKhng c b phn chuyn ng


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Phn II: Cm bin

Thch hp vi nhiu loi vt liu, di nhit v p sut hot ng rng.Hnh 2.64a l mt ng venturi. Hnh 2.64b cho thy lm th no m mt cht lng dichuyn qua mt ng venturi.

Hnh 2.64a

Hnh 2.64bCc cm bin hot ng da vo s gii hn s dng mt yu t l khi cht lng clu lng th tch khng i b buc i qua mt vng gii hn, vn tc dng chy khi s tng ln m bo cng mt lng cht lng i qua trong cng mt khong thigian. S thay i v vn tc lm gim p sut tnh ni m dng chy tng tc. p sut

trn mi pha ca vng gii hn c th o c nh s dng ng h o p sut hocthit b cm bin p sut chnh lch. Mt ng h p sut c t pha trc vng giihn, v ci cn li c t ni dng chy hp nht v c vn tc cao nht. Vn tcdng chy qua vng gii hn c th c tnh ton t vic o p sut chnh lch nuc tnh ca vt cht chy qua ng l bit trc. Mi khi dng cht lng tng gp i, chnh lch p sut tng 4 ln. Quan h p sut lu lng ny l mt hm cn bc hai.Cc tm orifice l thit b gii hn c s dng ph bin nht. Tm orificie ct trong dng chy qu trnh gia hai mt bch nm trn cc ng nm ngang hay thngng. Hnh 2.65 l mt v d v mt tm orifice tiu biu. Dng chy b gii hn khii qua tm orifice c l h 1,345 in.


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Phn II: Cm bin

Hnh 2.70

Hnh 2.71: Cm bin kiu Vortex c trng

7.5. Cm bin lu lng kiu t trng Cc my o lu lng kiu t trng c s dng o lu lng th tch ca chtlng hoc cht dng va dn in. Chng s dng nguyn l tng t vi my phtin ; khi vt dn in i qua t trng, mt in p s c to ra. Khi s dng myo lu lng kiu t trng, vt liu dn in l cht lng hay cht dng va. Dngchy cng nhanh, in p to ra cng ln.


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Phn II: Cm bin

mi bnh cng tc c dng tnh lu lng th tch. S vng quay c nhn vth tch c dng tnh tng th tch o c.

Hnh 2.74Hnh 2.74 trnh by mt kiu cm bin th ch tch cc khc, mt cm bin kiu cqut rotor. Khi vt liu qu trnh chy qua my o, cnh qut s lp tc cch ly ch

lng trong ngn c to bi cc cnh qut v v ca my o. Cc cnh qut, c gvi rotor quay nh cc l xo, trt vo hoc l ra s bt kn thnh cho n khi chng dchuyn n cnh ca v m o v cho php cht lng tip tc chy dc ng ng. Mtn hiu c gi ra t my o mi khi trc rotor hon thnh mt vng quay. S lnvng quay trn pht c th c s dng xc nh lu lng th tch bi v th tca cc ngn my o l bit trc.Cc my o kiu th ch tch cc rt chnh xc. Chng c th c dng tnh tnth tch hoc mt phn cng nh o lu lng th tch ca cht lng v kh. Cng h o nc v kh gia nh s dng loi th ch tch cc ny. Hnh 2.75 cho thmt ng h kiu th ch tch cc tiu biu cng vi thit b c gi tr tng.

Hnh 2.75Do cc b phn chuyn ng ca ng h o kiu th ch tch cc cn phi c bng my tinh vi, n t c la chn s dng trong trng hp cc cht n mn hocc cht lng c c tnh ph ln hay bo mn thit b n tip xc.

7.7. Cm bin lu lng kiu tuabinC s v o lng qu trnh Trang 71/155

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Phn II: Cm bin

Hnh 2.79Phng php th hai, trnh by trong hnh 2.80, l thay i lng nhit u vo gicho sai lch nhit gia hai cm bin l khng i. Lu lng khi lng c tc tnh bng cch o lng nhit cn thit duy tr sai lch nhit l khi.

Hnh 2.80

Trong cc cm bin lu lng khi lng da vo nhit thc t, cc phn t nhitcc cm bin khng lun lun nm trong dng qu trnh m c lp t trn thnh con ng nm trong cm bin. Hnh 2.81 trnh by mt cm bin lu lng khi ltiu biu. Trong kiu dng c o ny, ch mt phn trm nh dng chy qu trnh gia nhit, trong khi phn cn li th chy thng qua m khng c gia nhit.

Nu thnh phn ca dng qu trnh thay i, nh vy lng nhit c trng s khcvi nhng g m cm bin lu lng khi lng c hiu chun, cm bin s kt qu sai. Nu dng kh b ngng t v ng li trn cc cm bin nhit hoc p


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Phn II: Cm bin

Hnh 2.82

8. CM BIN KHONG CCH

8.1. Gii thiuTrong cc loi cng tc hnh trnh lun c mt s tip xc, l chng phi chm vmt i tng mi cm nhn c s tn ti ca i tng. Cc cm bin tip thng trnh s dng trong cc h thng t ng bi v mi khi cc phn chm vnhau s lm mn v sinh ra in th dn n li chc nng cc cm bin. Cc h tht ng v ang c thit k vi s gia tng cc cm bin khng tip xc. Ba locm bin khng tip xc ph bin nht c s dng ngy nay l cm bin khong ckiu cm ng (inductive proximity sensor), cm bin khong cch kiu in dun(capacitive proximity sensor), v cm bin khong cch quang hc (optical proximity

sensor). Tt c cc loi cm bin ny thc s l cc b chuyn i (transducers), nhchng bao hm lun c mch iu khin cho php chng c chc nng nh l cchuyn mch. Mch in thay i bn trong khi ng ra ca transducer t n gi tno .


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Phn II: Cm bin

Hnh 2.83 Hall effect limit switches

Hnh 2.84: Hnh dng thc t mt s cm bin khong cch

8.2. Cm bin khong cch kiu cm ng

Hnh 2.85: Cm bin khong cch kiu cm ng


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Phn III: Vng iu khin v cc phn t ca n

bao gm lun cc mch in t cn thit thc hin cng vic ca mt transmittchuyn i v hiu chun tn hiu ra t cm bin thnh dng tn hiu chun.9.2. Transmitter kh nn Di tn hiu chun

Mt transmitter kh nn to ra cc tn hiu p sut ng ra t l vi bin qu trnh cno. Kh c s dng cho tn hiu ng ra transmitter thng thng c cung cp bh thng cung cp kh. Ng vo t phn t cm bin n mt transmitter kh nnthng l mt tn hiu c hc.Di p sut ra tiu chun t transmitter kh nn l 3-15 psi, thng n gin ngta gi di p sut ny l 3-15 pounds. Kh t h thng cung cp kh n transmitterthng c duy tr mc 20 psi.

Ngun kh 20 psi

Hnh 3.4

Hot ng Hot ng ca mt transmitter kh nn tiu biu c trnh by hnh 3.5 v hnh 3.6Tn hiu ra (3-15 psi) t transmitter trong v d ny c iu khin bi khong cchgia vt cn v ming vi h. Vt cn chuyn ng nh lin kt c kh ni ti phncm bin. Bnh thng, cm bin v transmitter c cng n v o trong v d ny.Hnh 3.5 v 3.6 l v d v transmitter p sut kh nn c s dng o p sut qutrnh. Cm bin p sut, l mt hp l xo xp, gin ra hoc co li khi p sut tng hagim v lm chuyn ng vt cn. Mt vt cn ging nh th cng c th c ikhin nh lin kt c kh ti nhiu loi cm bin khc. V d, nu y l transmittemc, vt cn c th c ni trc tip n cm bin kiu th ch.Transmitter p sut kh nn trong hnh 3.5 gi mt tn hiu ra 15 psi, gi tr o trn cadi o (200 psi i vi qu trnh ny). Hp l xo xp gin ra v y vt cn ra.Khi vt cn chn b y ra xa khi ming ng do hp l xo xp gin ra:

1. Kh thot ra khi ming ng. p sut kh trong ming ng gim xung. (Kh iqua b phn gii hn cung cp kh nh khng th duy tr p sut. V th,

2. p sut b h thp trong ng ni t ming ng n vt cn.3. Mng chn vng xung.4. Qu cu di ng c ni vi mng chn ri xung, m cc b ng dn k

gia ngun kh v ng kh ra ca transmitter.


Transmitter kh nn

Tn hiu vo t cm bin(thng tin bin qu trnh) Tn hiu ra 3-15 psi

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5. Ngun kh i qua ng kh ra ca transmitter, lm cho tn hiu p sut ng ra catransmitter tng ln.

Hnh 3.5Trong hnh 3.5, p sut ra tng ln khi khong cch gia vt cn v ming ng tngtheo p sut qu trnh.

Hnh 3.6Hnh 3.6 cho thy cch thc m p sut ng ra gim khi khong cch gia vt cn vming ng gim.


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Transmitter kh nn trong hnh 3.6 gi mt tn hiu p sut thp (3 psi) ng ra. Khivt cn b ko gn li ming ng do hp l xo xp co li:

1. Kh thot ra t ming vi t hn. V th,2. Ngun kh c th duy tr p sut cao hn trong ng ng ni n mng ngn.

3. p sut tng lm cho mng ngn gin ra.4. Qu cu c nng ln s hn ch bt ngun kh cp n ng ra ca transmittv m khe h thng kh.

5. ng p sut ng ra by gi thng vi khe h thng kh, v p sut ra catransmitter b gim xung.

Quan h t l thunTransmitter c trnh by trong hnh 3.5 v 3.6 l transmitter hot ng trc tip; tnhiu c gi tr cao th tn hiu ra khi transmitter cng cao. (i vi transmitter hotng ngc, gi tr ca bin vo cao th tn hiu ng ra ca transmitter thp).Transmitter trong v d ny p ng t l thun vi p sut qu trnh (bin qu trnhc o) trong di o, 0 n 200 psi trong hnh 3.5 v 3.6

Ti mc cao ca di o, p sut qu trnh 200 psi, vt cn nm v tr xa nht svi ming ng. Ng ra ca transmitter tng ng l 15 psi.Ti mc thp ca di o, p sut qu trnh l 0 psi, vt cn nm v tr gn nhso vi ming ng. Ng ra ca transmitter tng ng l 3 psi. Nu p sut qu trnh nm u gia 0 v 200 psi, vt cn s nm v tr giim xa nht v im gn nht; ng ra ca transmitter s l mt tn hiu t lthun gia 3 v 15 psi.

V d, nu p sut qu trnh ng vo l 100 psi, ng ra transmitter s l 9 psi - mt nagia 3 v 15 psi, bi v 100 l mt na gia 0 v 200 psi p sut qu trnh .

Hnh 3.7


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Hnh 3.12 Hnh 3.13Vi mc nc trong bn l 72 in tng ng vi 50% thang o, ng ra ca transmitter kh nn s l 9 psi. (Transmitter kh nn c tn hiu ra nh nht l 3 psi v ln nht l15 psi, tng ng vi di o l 12 psi. 50% ca 12 psi l 6 psi v 6 psi + 3 psi l 9 psi).Vi v d ny, tn hiu ra 9 psi tng ng vi mc nc trong bn l 50% hoc 5 feet.9.4. Transmitter nhit Transmitter nhit l mt loi transmitter in t, do chc nng tng qut ca nhon ton ging nh chc nng ca mt transmitter in t m t trn. Khi nhit ng vo thay i t gi tr thp nht n gi tr cao nht ca di o th tn hiu ngra cng thay i tng ng t 4 n 20 mA. Quan h gia nhit v dng in ngra l quan h ng thng (tuyn tnh).Phn t s cp ca transmitter nhit l cm bin nhit . Chc nng ca cm binnhit l chuyn i nhit cn o thnh tn hiu in hoc in tr. Hai loi cm bin nhit c s dng rng ri trong cng nghip hin nay l cp nhit in vRTD. Tn hiu t cm bin nhit c a n transmitter, mch in t bn trongc nhim v chuyn i n thnh tn hiu chun ng ra, y l dng in DC 4-20mA, v gi n n cc phn t khc trong vng iu khin.Cm bin nhit v transmitter nhit l hai phn t ri nhau v c sn xut clp nhau. Khi lp t vo vng iu khin, chng c th ng gn nhau hay xa nhau vc kt ni vi nhau bng dy dn.Ty theo nh sn xut v ty tng ng dng m ng vo ca transmitter nhit cth thch hp vi cp nhit in hoc RTD hoc c th c hai. Vi mt s loitransmitter nhit thng minh, ta c th dng thit b chuyn dng cu hnh chotransmitter, vic cu hnh c th bao gm:

La chn loi cm bin ng voThit lp di nhit cn o


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Mt s ty chn khc ty theo nh sn xut

Hnh 3.14: Tit b giao tip dng cu hnh cho transmitter

9.5. Transducers v Converters

Trong mt vng iu khin, thng c s kt hp ca cc phn t m yu cu ng vhoc l in hoc l kh nn. V d, b iu khin nhn ng vo t transmitter p suin t hnh 3.11 cn ng vo l tn hiu in, nhng ngc li, vi recorder trong phng iu khin dng hin th mc nc trong bn c l li cn ng vo kh nCc b chuyn i (converters) c s dng cho php cc phn t trong mt vniu khin giao tip c vi nhau khi chng s dng cc dng tn hiu khc nhau.Converters v transmitters c hai u l transducers bi v chng chuyn mt tn hiut mt dng ny sang dng khc. Cc converters m chuyn i cc tn hiu dng i(c th hin bng ch I) thnh cc tn hiu kh nn (th hin bng ch P) c tl I/P converters. Nhng converters m chuyn i cc tn hiu kh nn thnh cc tnhiu in c tn l P/I converters. Ch u tin trong tn gi trn m ch dng tnhiu vo v ch th hai m ch dng tn hiu ra.Chc nng ca 3 loi converters hoc transducers c trnh by hnh 3.15:

1. Transducer in sang kh nn (I/P)2. Transducer kh nn sang in (P/I)3. Converter cch ly (I/I)

Hnh 3.15


Dng DC 4-20mA Kh nn 3-15 psi

Converter

hoctranducer Tn hiu Tn hiu

IP

PI

II

Kh nn 3-15 psi Dng DC 4-20mA

Dng DC 4-20mA Dng DC 4-20mA

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I/P v P/I converter Converter I/P c s dng chuyn t l mt tn hiu in tiu chun 4-20mA sangmt tn hiu kh nn 3-15 psi. Converter P/I c chc nng chuyn i t l mt tn hiukh nn chun 3-15 psi sang mt tn hiu in 4-20mA. Converter cch ly c mt ngdng c bit s c bn sau trong mc ny.Khi dng tn hiu c chuyn i bng mt converter, thng tin gc v gi tr ca bin o lng phi c lu gi. Nh th trong hnh 3.16, nu ng vo converter l20mA, ngha l 100% thang o, p sut ra tng ng l 15 psi, cng l 100% thang o. Nh th, c ng vo 12mA v ng ra 9 psi u tng ng vi bin o ti 50% thango.

Hnh 3.16 I/I converter Mt converter cch ly c trnh by trong hnh 3.17. Thit b ny c t gia mt

transmitter 4-dy v thit b khc c lp v ngun in trong vng iu khin. Ncch ly transmitter ngn nga s nhiu lon do cc tn hiu c to ra t cc phnt khc s dng ngun cung cp khc.


Dng in vo4-20 mA

20 ------- 100% ------- 15

16 ------- 75% ------- 12

12 ------- 50% ------- 98 ------- 25% ------- 6

4 ------- 0% ------- 3

Kh nn ra3-15 psi

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10.2. Cc loi Recorder Recorder v theo chiu ngang Hnh 3.21 trnh by mt thit b v th dng cun ngang. Giy c cun vo thit b ghi ny t bn phi; giy di chuyn sang tri trong qu trnh ghi d liu. Thit b ghiny c hai cy vit v ghi li gi tr ca hai bin qu trnh khc nhau. i vi v dny:

Cy vit A biu th cho ng vo t transmitter c hiu chun ng ra4mA (0%) tng ng vi ng vo l 0 gallons trn pht, v 20mA (100%)tng ng vi 1000 gallons trn pht.Cy vit B c hiu chun sao cho ng ra ca n 4mA tng ng vi ngvo l 0 gallons trn pht, v 20mA tng ng vi ng vo l 200 gallons trn pht.

Ch rng, giy trong hnh 3.21 khng c nh du trn trc thi gian. tm tr s

lu lng ca dng qu trnh A cch mt gi, bn phi bit tc v tnh tonxem cy vit nm v tr no cch mt gi. Trong trng hp ny, cn thit phighi li thi gian v tc giy trn th khi thit b ghi bt u ghi. Tc giy tiu biu cho thit lp ny l 2 inchs trn gi.

Hnh 3.21

Recorder v theo chiu dcHnh 3.22 trnh by mt thit b v th cun theo chiu dc. Giy c di chuyntheo chiu dc t cun giy c gn nh ca thit b ghi v cun xung y can.


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Hnh 3.24


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Nhiu khi, my tnh thc t mm sau bng iu khin. Nhng th nhn thy c mn hnh hin th v bn phm. Vic s dng thit b ghi bng my tnh thng cho php kt hp cc chc nng ca cc thit b ghi khc, gim bt thit b v gim chi ph u t v thi gian bo dng. Bi v c nhiu cch hin th v thao tc ccthng tin bin qu trnh trn my tnh, chc chn rng bn quen vi cc c tnh ca phn mm h thng.11. CONTROLLER 11.1. Chc nng ca b iu khinTrong phn trc, tc gi gii thiu cc khi nim c s dng iu khin mqu trnh:thu thp, so snh v iu khin.Chc nng ca b iu khin trong vngiu khin l nhn bit d liu bin qu trnh c bng gi tr mong mun hay khn bng cch so snh ng vo t transmitter (thng thng tn hiu 4-20mA hoc 3-15psi)vi gi tr set point c lu trong b iu khin. Bt k s khc nhau no so vi gitr set point c gi l lch. Nu lch l zero, bin qu trnh ang bng gi tr

set point, v khng cn s hiu chnh no i vi qu trnh. Nu lch khng phi lzero, b iu khin gi mt tn hiu hiu chnh ti ng ra n phn t iu khin cu a bin qu trnh gn hn vi set point.

Hnh 3.29Trong tho lun v iu khin qu trnh, c 3 thut ng c lin quan ti t bin(variable): bin o lng, bin iu khin v bin thao tc.Bin iu khin(controlled variable): l bin qu trnh cn phi c duy tr tronggii hn cho php i vi mt qu trnh.Bin o lng(measured variable): l iu kin qu trnh c theo di bi cm bintrong vng iu khin xc nh gi tr cabin iu khin.Bin thao tc(manipulated variable): l iu kin c iu chnh bi phn t iu

khin cui lm cho bin iu khin t n v duy tr gi tr set point mong munTrong mt s trng hp, bin o lng, bin thao tc, v bin iu khin l ginnhau. Lu lng l mt v d, n l mt bin m c th c o v iu khin tr


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tip, ty thuc vo thit k ca qu trnh. Nu mt cm bin o lu lng trc tip, lulng lc ny va l bin o va l bin iu khin. Nu mt van iu khin lulng, lu lng cng l bin thao tc, v khng c bin no khc hn lu lng.Tuy nhin, vic duy tr mc cht lng trong bn gi tr mong mun c th phc tphn nhiu. V d, nu mt bin iu khin l mc, chng ta c th o p sut theo

di mc. B iu khin c th gi tn hiu n van iu khin, phn t iu khin cuitrong vng iu khin, iu chnh lu lng (bin thao tc) i vo bn. Van iukhin c gn trn ng ng dn cht lng vo bn c m v ng duy trmc cht lng trong bn gi tr set point khi vt liu c ly ra.

Hnh 3.30

Trong cc nh my cng nghip, ngi ta s dng nhiu loi b iu khin khc nhau.Chng c th l loi kh nn, loi in t tn hiu tng t hoc loi in t tn hiu s.Cc b iu khin s c th ng mt mnh hoc kt hp vi my tnh qua cc mchvo v phn mm khc nhau. Khi kt hp vi my tnh, my tnh thng thc hin ccchc nng ca mt s b iu khin trong cc vng iu khin khc nhau. Cc mytnh iu khin nhiu vng iu khin khc nhau c gi l DCS (Distributed ControlSystems)Hnh 3.31 trnh by cc v d tiu biu v: cc b iu khin kh nn, b iu tng t, b iu khin s v my tnh c s dng lm DCS.


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Bin iukhin Bin thao tc

Quan hvo/ra

Kiu b iukhin

Tng/tngTc ng thun

Gim/gimTng/gim

Tc ng nghchGim/tng

Hnh 3.32

Yu t thi gianThi gian l mt nhn t quan trng phi xem xt trong cc h thng iu khin qutrnh. Tt c cc h thng iu khin u c mt khong thi gian cht (dead time),thi gian tr (lag time) hay thi gian p ng l thi gian tr gia lc gi tn hiut b iu khin n phn t iu khin cui n khi bin iu khin thay i m biu khin c th cm nhn c. Khong thi gian cn thit thc hin s thay imong mun trong h thng c th ngn v khng quan trng i vi hot ng ca qutrnh hoc c th l mt yu t quan trng. V d, khi s dng mt cng tc nh sng bnh thng, p ng (lng nh sng tng thm) thay i trng thi cng tc hunh l thc thi. Tuy nhin, khi iu chnh nhit ca mt l si, c th mt hng pht trc khi t c nhit mong mun k t khi l si c bt hoc tt. Cmt khong thi gian tr khi thay i lu lng nc ca vi tm hoa sen, nhng cth mt hng gi trc khi bn c th thy s thay i ca mc nc trong h bi nu bn iu chnh van bm nc vo h.Thng thng, thi gian tr cng ln th cng kh duy tr bin iu khin ti gi trset point. p ng nhanh tng ng vi thi gian tr ngn v thi gian p ng ding vi thi gian tr di.Cc kiu ch iu khinTn hiu ra ca b iu khin c th c iu chnh tha mn cc yu cu ring bit ca h thng qu trnh bng cch thm vo mt hoc nhiu cc ch iu khinsau y:

On-off hoc binary: phng thc iu khin n gin nhtT l (Proportional)Reset hoc tch phn (Integral): phi c s dng kt hp vi ch t lVi phn hoc tc (Derivative): phi c s dng kt hp vi ch t lv hu nh lun lun c s dng kt hp vi ch reset

Cc b iu khin c chc nng t l, tch phn v vi phn c gi l b iu khinPID. P l vit tt ca Proportional, I l vit tt ca Integral hoc Reset v D lvit tt ca Derivative. Cho d ch iu khin On-off hoc s kt hp ca cc ch


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PB = 100/K pH s khuch i (gain) l mt cch biu th khc ca quan h bng t l. H skhuch i l t s ca s dch chuyn ca phn t iu khin cui vi s thay i ca bin iu khin. Trong hnh 3.36, h s khuch i l 1, s thay i ng ra (v trvan) chia cho s thay i ng vo (v tr phao). hp v rng cng c s dng

m t bng t l. Bng hp l mt bng t l phn trm thp hoc h s khuch icao. Bng rng l mt bng t l phn trm cao hoc h s khuch i thp. Bng t lcng hp th nhy iu khin cng cao.

Hnh 3.37. Quan h gia bng t l v h s khuch iS khc nhau gia gi tr bin iu khin thc t v set point c gi l offset. Gi tr bng t l nh hng nh th no n offset khi ti thay i; phn trm cng cao (bngcng rng), offset cng ln. Xem hnh 3.36, nu ti (lu lng ra) tng n im m van iu khin cn c m hon ton, mc s gim xung n 0% van m 100%. Phao lc ny v tr thp hn set point l 5 feet, vy offset l 5 feet.Hnh 3.38 trnh by mt h thng ging nh v d trc nhng y lin kt phao/vanm khi van d

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