72

Chng 7. cc chu trnh nhit ng 7.1. chu trnh ng c t trong 7.4.1. Chu trnh Carno hi nc 7.1.1. Khi nim ng c t trong l ng c nhit m qu trnh chy c tin hnh bn trong xi lanh v sn phm chy c thi ra mi trng. y l chu trnh bin i nhit thnh cng. Hin nay ng c t trong wocj s dng nhiu trong sn xut v sinh hot nh dng lm ng c cho t, my ko, xe la, my pht in . . . Mi cht lm vic trong ng c t trong lc u l khng kh v nhin liu, sau l sn phm chy ca hn hp khng kh v nhin liu. C nhiu cch phn loi ng c t trong, c th phn loi theo nhin liu s dng, theo hnh trnh piston, theo qu trnh cp nhit . . . y, theo quan im nhit ng, da vo chu trnh cp nhit ta phn ng c t trong thnh 3 loi: chu trnh cp nhit ng p, chu trnh cp nhit ng tch, chu trnh cp nhit hn hp. nghin cu cc qu trnh ca ng c t trong, ta gi thit:

- Mi cht l kh l tng v ng nht, - Cc qu trnh xy ra u l thun nghch, - Qu trnh chy l qu trnh cp nhit, qu trnh thi sn phm chy l qu

trnh nh nht. - Cng trong qu trnh np mi cht v qu trnh thi sn phm chy trit tiu ln nhau v bin h y thnh h kn. 7.1.2. Chu trnh cp nhit hn hp 7.1.2.1. M t chu trnh Trong chu trnh cp nhit hn hp, nhin liu s c bm cao p nn n p sut cao, phun vo xi lanh dng sng m. Trong xi lanh khng kh s c nn n p sut v nhit cao, vo xi lanh gp khng kh nhin liu s t bc chy ngay. Qu trnh chy gm hai giai on: giai on u chy ng tch, giai on sau chy ng p. Chu trnh chy l tng ca ng c t trong cp nhit hn hp c trnh by trn hnh 7.1. Chu trnh gm: 1-2 l qu trnh nn oan nhit, 2-2 l qu trnh cp nhit ng tch, mi cht nhn nhit lng q1, 2-3 l qu trnh cp nhit ng p, mi cht nhn nhit lng q1 3-4 l qu trnh dn n on nhit, 4-1 l qu trnh nh nhit ng tch, nh nhit lng q2,

73

7.1.2.2. Hiu sut chu trnh cp nhit hn hp

Hnh 7.1 Chu trnh cp nhit hn hp

* Cc i lng c trng cho chu trnh:

- Thng s trng thi u: p1, T1, - T s nn:

2

1

vv

= (7-1)

- T s tng p:

2

3

pp

= (7-2)

- H s dn n sm:

2

3

'vv

= (7-3)

* Hiu sut ca chu trnh:

1

21ct q

qq = (7-4)

Trong : q1 l nhit lng chu trnh nhn c t qu trnh chy nhin liu, gm q1 l nhit lng nhn c t qu trnh chy ng tch 2-2, q1 l nhit lng nhn c t qu trnh chy ng p 2-3, vy: q1 = q1+ q1, q2 l nhit lng cho ngun lnh trong qu trnh nh nhit ng tch 4-1, T ta c hiu sut cha chu trnh l:

"''

11

2ct qq

q1

+= (7-5)

v 2-2 l qu trnh cp nhit ng tch, nn q1 = Cv(T2 - T2), v 2-3 l qu trnh cp nhit ng p, q1 = Cp(T3 - T2), v 4-1 l qu trnh nh nhit ng tch, nn q2 = Cv(T4 - T1), Thay cc gi tr ca q1, q1 v q2 vo (7-5) ta c:

74

( )

( ) ( )'' 23p22v14v

ct TTCTTCTTC

1+

= (7-6a)

( )

( ) ( )'' 232214

ct TTkTTTT

1+

= (7-6b)

Da vo c im qu trnh ca cc chu trnh, ta tip tc bin i c th tnh hiu sut ca chu trnh theonhit u T1 v cc i lng c trng cho chu trnh nh sau:

- V 1-2 l qu trnh nn oan nhit nn ta c

1k1k

2

1

1

2

vv

TT

=

= , suy ra: 1k12 TT

= ,

2-2 l qu trnh cp nhit ng tch nn:

==2

'2

2

'2

pp

TT

, suy ra: 1k12'2 TTT== ,

2-3 l qu trnh cp nhit ng p nn:

=='2

3

'2

3

vv

TT

, suy ra: 1k1'23 TTT== ,

3-4 l qu trnh dn n on nhit nn:

1k1k

1

2

2

3

1k

1

3

1k

4

3

3

4

vv

vv

vv

vv

TT

=

=

=

= . , suy ra:

k1

1k1k

1

1k

34 TTTT =

=

=

Thay cc gi tr T2, T2 , T3 v T4 vo (7-6) ta c:

( ) ( )1k11k11k11k11

k1

ct TTkTTTT

1 +

=

Rt gn li ta c hiu sut chu trnh:

( ) ( )[ ]1k111 1k

k

ct +

=

(7-7)

7.1.3. Cc chu trnh khc Ngoi chu trnh cp nhit hn hp, cn c chu trnh cp nhit ng p, chu trnh cp nhit ng tch. 7.1.3.1. Chu trnh cp nhit ng tch chu trnh cp nhit ng tch, nhin liu (xng) v khng kh c hn hp trc ngoi xi lanh. Sau hn nhin liu v khng kh c np vo xi lanh v nn on nhit n p sut v nhit cao (c biu din bng on 1-2) nhng vn thp hn nhit t bc chy ca n nn n khng t bc chy

75

c. Qu trnh chy xy ra nh bugi bt tia la in, qu trnh chy (c biu din bng on 2-3) xy ra rt nhanh lm cho p sut trong xi lanh tng vt ln trong khi xi lanh cha kp dch chuyn, th tch hn hp kh trong xi lanh khng i, v vy qu trnh ny c th coi l qu trnh chy ng tch. Sau sn phm chy dn n , y piston dch chuyn v sinh cng. Qu trnh dn n ny c coi l on nhit, (c biu din bng on 3-4). Cui cng l qu trnh thi sn phm chy ra ngoi (c biu din bng on 4-1), y cng l qu trnh ng tch. Cc qu trnh lp li nh c, thc hin chu trnh mi.

Hnh 7.2 Chu trnh cp nhit ng tch

y chnh l chu trnh ng c t chy xng hay cn gi l ng c chy cng bc nh bugi nh la. th thay i trng thi ca mi cht c biu din trn hnh 7.2. T cng thc tnh hiu sut ca chu trnh cp nhit hn hp (7-7), ta thy: Nu chu trnh cp nhit hn hp c = 1, tc l v2 = v2 = v3, nh vy qu trnh cp nhit ch cn giai on chy ng tch 2-3, khi chu trnh cp nhit hn hp tr thnh chu trnh cp nhit ng tch. Khi thay = 1 vo cng thc (7-7) ta c hiu sut chu trnh cp nhit ng tch:

( ) 1k1kct11

111

=

= (7-8)

Nh vy hiu sut nhit chu trnh cp nhit ng tch ch ph thuc vo t s nn . 7.1.3.2. Chu trnh cp nhit ng p Nu chu trnh cp nhit hn hp c = 1, tc l p2 = p2 = p3, ngha l qu trnh cp nhit ch cn giai on chy ng p 2-3, khi chu trnh cp nhit hn hp tr thnh chu trnh cp nhit ng p. chu trnh ny, khng kh c nn on nhit n p sut v nhit cao, n cui qu trnh nn nhin liu c phun vo xi lanh di dng sng m, pha trn vi khng kh to nn hn hp chy v s t bc chy. Khi thay = 1 vo cng thc (7-7) ta c hiu sut chu trnh cp nhit ng p:

76

( )1k11 1k

k

ct

=

(7-9)

Nh vy hiu sut nhit chu trnh cp nhit ng tch ch ph thuc vo t s nn v t s dn n sm .

Qu trnh thay i trng thi ca mi cht trong chu trnh c biu din trn th p-v v T-s hnh 7.3. Hin nay ngi ta khng ch to ng c theo nguyn l ny na.

Hnh 7.3 Chu trnh cp nhit ng p

7.1.3. Nhn xt

- Hiu sut nhit ca chu trnh ng c cp nhit hn hp ph thuc vo k, - ng c cp nhit ng p v cp nhit hn hp c th lm vic vi t s

nn rt cao. Tuy nhin khi chiu di xi lanh cng s phi tng ln v gp kh khn trong vn ch to, ng thi tn tht ma st ca ng c s tng v lm gim hiu sut ca n.

- Trong ng c cp nhit ng tch qu trnhchy l cng bc (nh bugi), nu tng cao qu tr s gii hn (6-9) th hn hp chy s t bc chy khi bugi cha nh la, s nh hng xu n ch lm vic bnh thng ca ng c. Ngoi ra khi t s nn ln th tc chy c th tng ln mt cch t ngt gy ra hin tng kch n (v hn hp nn l hn hp chy) ph hng cc chi tit ng c. V vy t s nn cn c la chn ph hp vi tng loi nhin liu. 7.1.5. So snh hiu sut nhit ca chu trnh ng c t trong (ctp, ct, ctv)

nh gi hiu sut nhit ca ng c t trong lm vic theo cc chu trnh khc nhau, ta so snh cc chu trnh vi cc iu kin sau:

a. Khi c cng t s nn v nhit lng q1 cp vo cho chu trnh: Trn th T-s hnh 7.4 biu din 3 chu trnh: 123v4v1 l chu trnh cp

nhit ng tch, 122341 l chu trnh cp nhit hn hp v 123p4p1 chu trnh cp nhit ng p. 3 chu trnh ny c cng t s nn v nhit lng q1, ngha l cng v1, v2 v cc din tch a23vd, a223c v a23pb bng nhau. T (7-4) ta thy: cc chu trnh c cng q1, chu trnh no c q2 nh hn s c hiu sut nhit cao hn.

q2 ca chu trnh cp nhit ng tch bng din tch a14vb l nh nht,

77

q2 ca chu trnh cp nhit ng p bng din tch a14pd l ln nht, q2 ca chu trnh cp nhit hn hp bng din tch a14c c gi tr trung gian

so vi hai chu trnh kia. Vy hiu sut ca chu trnh cp nhit ng tch l ln nht v hiu sut ca

chu trnh cp nhit ng p l nh nht: ctv > ct > ctp (7-10)

Hnh 7.4. So snh cc chu trnh Hnh 7.5. So snh cc chu trnh b. Kh c cng p sut v nhit ln nht v nh nht: y ta so snh hiu sut nhit ca chu trnh cng nh mt nhit lng q2 ging nhau, cng lm vic vi ng sut nhit nh nhau (cng Tmax v pmax). Vi cng iu kin , cc chu trnh c biu din trn th T-s hnh 7.5. 12p34 l chu trnh cp nhit ng p, 122341 l chu trnh cp nhit hn hp v 12v34 chu trnh cp nhit ng tch. Trn th, 3 chu trnh ny c cng p1, T1 v cng p3, T3 ngha l cng nh ra mt lng nhit q2 (din tch 14ab) trong : nhit lng q1 cp vo cho chu trnh cp nhit ng p bng din tch a2p3b l ln nht, nhit lng q1 cp vo cho chu trnh cp nhit ng tch bng din tch a2v3b l nh nht.

Vy theo (7-4) ta thy hiu sut ca chu trnh cp nhit ng p l ln nht v hiu sut ca chu trnh cp nhit ng tch l nh nht:

ctp > ct > ctv (7-11) Gii hn trn ca p3, T3 ph thuc vo sc bn cc chi tit ca ng c. 7.2. Chu trnh tuc bin kh

u im ca ng c t trong l c hiu sut cao. Tuy nhin, ng c t trong c cu to phc tp v phi c c cu bin chuyn ng thng thnh chuyn ng quay, nn cng sut b hn ch. khc phc cc nhc im trn, ngi ta dng tuc bin kh. Tuc bin kh cho php ch to vi cng sut ln, sinh cng lin tc, thit b gn nh nn c s dng rng ri ko my pht in, s dng trong giao thng vn ti. Da vo qu trnh chy ca nhin liu, c th chia thnh hai loi: tuc bin kh chy ng p v tuc bin kh chy ng tch. 7.2.1. S thit b v nguyn l hot ng ca tuc bin kh

78

S thit b v nguyn l hot ng ca tuc bin kh c biu din trn

hnh 7.6. Khng kh c nn on nhit trong my nn kh I, phn ln c a vo bung t III, mt phn nh c a ra pha sau bung t ho trn vi sn phm chy nhm lm gim nhit sn phm chy trc khi vo tuc bin.

Nhin liu c bm hoc my nn II a vo bung t III. Nhin liu v khng kh c s to thnh hn hp chy v chy trong

bung t III. Sn phm chy c p sut v nhit cao ( khong 1300-15000C) c pha trn vi khng kh trch t my nn, to thnh hn hp c nhit c nhit khong 900-11000C. Sau , sn phm chyc a qua ng tng tc IV, tc s tng ln v i vo tuc bin, bin ng nng thnh c nng trn cnh tuc bin, lm quay tuc bin ko my pht quay theo. Sn phm chy sau khi ra khi tuc bin c thi ra mi trng.

Hnh 7.6. S thit b tuc bin kh

Qu trnh chy c th l: - Chy ng p p = const. y mi cht vo v ra khi bung t mt

cch lin tc, cu to bung t n gin. - Chy ng tch v = const. y khi chy, cc van ca bung t phI

ng li th tch hn hp khng i, nhm thc hin qu trnh chy ng tch, do sn phm chy ra khi bung t khng lin tc. Mun sn phm chy vo v ra khi bung t mt cch lin tc th cn c nhiu bung t, do cu to phc tp v tn tht qua cc van cng ln. V vy, trong thc t ngi ta thng ch to tuc bin chy ng p. 7.2.2. Chu trnh tuc bin kh cp nhit ng p

Chu trnh tuc bin kh cp nhit ng p c biu din trn th p-v v T-s hnh 7.7. 1-2 l qu trnh nn oan nhit mi cht trong bung t, 2-3 l qu trnh cp nhit ng p trong bung t, 3-4 l qu trnh dn n on nhit trong ng tng tc v trong tuc bin, 4-1 l qu trnh nh nhit ng p (thi sn phm chy), * Cc i lng c trng ca chu trnh gm:

79

- T s nn:

1

2

pp

= (7-12)

- H s dn n sm trong qu trnh cp nhit:

2

3

vv

= (7-13)

- Hiu sut ca chu trnh:

1

21ct q

qq = (7-14)

Trong : q1 l nhit lng sinh ra trong qu trnh chy ng p, q1 = q23 = Cp(T2 - T2), q2 l nhit lng thi ra mi trng trong qu trnh 41, q2 = Cp(T4 - T1), T ta c hiu sut ca chu trnh l:

( )( )

23

14ct TT

TT1

=

Hnh 7.7. th p-v v T-s ca chu trnh tuc bin kh cp nhit ng p

Tng t nh i vi chu trnh ng c t trong, thay cc gi tr vo ta c:

k

1kct11

= (7-15)

Ta thy hiu sut nhit ca chu trnh tuc bin kh cp nhit ng p ph thuc vo v k. Khi tng v k th hiu sut nhit ca chu trnh s tng v ngc li. 7.3. Chu trnh ng c phn lc i vi ng c t trong, mun c cng sut ln th kch thc v trong lng rt ln, do khng th s dng trong k thut hng khng c. ng c phn lc c th t c cng sut v tc ln m kch thc v trng lng thit b li nh, do c s dng rt nhiu trong k thut hng khng, trong cc tn la v tr.

80

Nguyn l ca ng c phn lc l: nhin liu c t chy, nhit nng bin thnh ng nng ca dng kh, phun qua ng phun ra ngoi vi vn tc ln, to ra phn lc mnh y thit b chuyn ng v pha trc. ng c phn lc c chia thnh hai loi: ng c my bay v ng c tn la. ng c my bay v ng c tn la ch khc nhau ch: Oxy cp cho my bay ly t khng kh xung quanh, cn ng c tn la oxy c cha sn di dng lng ngay trong ng c, v vy tn la c tc ln hn v c th bay trong chn khng. 7.3.1. ng c my bay Vic tng p sut khng kh trong ng c my bay c th nh ng tng p, c th nh my nn. Hin nay my bay c ch to theo kiu tng p mt phn nh ng tng p, nhng phn ch yu l nh my nn, do di y ta ch kho st loi ny.

Hnh 7.8. S cu to ng c Hnh 7.9. th p-v chu trnh

my bay c my nn ng c my bay c my nn

S cu to ca ng c my bay c my nn c biu din trn hnh 7.8. Cu to ca ng c gm cc b phn chnh nh sau: ng tng p 1, my nn 2, vi phun nhin liu 3, tuc bin kh 4, ng tng tc 5 v bung t 6. Chu trnh ca ng c my bay c biu din trn hnh 7.9, gm cc qu trnh: 1-2 l qu trnh nn oan nhit khng kh trong ng tng p, 2-3 l qu trnh nn oan nhit khng kh trong my nn, 3-4 l qu trnh chy ng p hn hp Khng kh-nhin liu trong bung t, cp cho chu trnh mt lng nhit q1, 4-5 l qu trnh sn phm chy dn n on nhit trong tuc bin kh, sinh cng chy my nn, 5-6 l qu trnh dn n on nhit sn phm chy trong ng tng tc, 6-1 l qu trnh thi sn phm chy ng p, nh ra mi trng lng nhit q2.

81

Chu trnh ca ng c my bay c my nn chy ng p hon ton ging nh chu trnh tuc bin kh cp nhit ng p. Hiu sut ca chu trnh c xc nh theo (7-15):

( )( )

23

14ct TT

TT1

= k

1k

11

= (7-16)

Ta thy hiu sut nhit ct tng khi tng ( l t s tng p trong qu trnh nn 1-2 c trong ng tng tc ln trong my nn). R rng l t s y ln hn chu trnh ng c my bay khng c my nn, ng c ny c hiu sut so vi cc ngc khng c my nn. 7.3.2. ng c tn la S cu to ca ng c tn la c biu din trn hnh 7.10. Cu to ca ng c gm cc b phn chnh nh sau: Bnh cha nhin liu A, Bnh cha oxy lng B, bm nhin liu C, bopm oxy lng D, bung t E v ng tng tc F. Chu trnh ca ng c my bay c biu din trn th p-v hnh 7.11, gm cc qu trnh:

Hnh 7.10. S cu to Hnh 7.11. th p-v chu trnh ng c tn la ng c tn la

1-2 l qu trnh nn oan nhit nhin liu v oxy trong bm (v cht lng khng chu nn nn c th coi l qu trnh ng tch), 2-3 l qu trnh chy ng p hn hp Khng kh-nhin liu trong bung t, cp cho chu trnh mt lng nhit q1, 3-4 l qu trnh dn n on nhit sn phm chy trong ng tng tc, 4-1 l qu trnh thi sn phm chy ng p ra mi trng, nh lng nhit q2. Hiu sut ca chu trnh c xc nh:

=ct 1ql

(7-17)

y cng k thut ca qu trnh dn n on nhit 3-4 (b qua cng bm trong qu trnh 1-2).

82

7.4. chu trnh nh my nhit in 7.4.1. Chu trnh Carno hi nc chng 2 ta bit chu trnh Carno thun chiu l chu trnh c hiu sut nhit cao nht. V mt k thut, dng kh thc trong phm vi bo ha c th thc hin c chu trnh Carno v vn t c hiu sut nhit ln nht khi cng phm vi nhit . Chu trnh Carno p dng cho kh thc trong vng hi bo ha c biu din trn hnh 7.12. Tuy nhin, i vi kh thc v hi nc th vic thc hin chu trnh Carno rt kh khn, v nhng l do sau y: - Qu trnh hi nh nhit ng p, ngng t thnh nc (qu trnh 2-3) s thc hin khng hon ton. Mun nn on nhit hi m theo qa trnh 3-4, cn phi c my nn kch thc rt ln v tiu hao cng rt ln. - Nhit ti hn ca nc thp (374,15 0C) nn chnh nhit gia ngun nng v ngun lnh ca chu trnh khng ln lm, do cng ca chu trnh nh.

Hnh 7.12. chu trnh Carno hi nc

- Cc git m ca hi s va p vo cnh tuc bin gy tn tht nng lng v n mn v mimn nhanh cnh Tuc bin. 7.4.2. Chu trnh Renkin (chu trnh nh my in) Nh phn tch trn, tuy c hiu sut nhit cao nhng chu trnh Carno c mt s nhc im khi p dng cho kh thc, nn trong thc t ngi ta khng p dng chu trnh ny m p dng mt chu trnh ci tin gn vi chu trnh ny gi l chu trnh Renkin. Chu trnh Renkin l chu trnh thun chiu, bin nhit thnh cng.

Hnh 7.13. S thit b nh my in Hnh 7.14. th T-s ca chu trnh NMN Chu trnh Renkin l chu trnh nhit c p dng trong tt c cc lai nh my nhit in, mi cht l nc. Tt c cc thit b ca cc nh my nhit in

s

2

T

0

3

2

4 5 1

P1

P2

83

u ging nhau tr thit b sinh hi I. Trong thit b sinh hi, nc nhn nhit bin thnh hi

i vi nh my nhit in thit b sinh hi l l hi, trong nc nhn nhit t qu trnh t chy nhin liu. i vi nh my in mt tri hoc a nhit, nc nhn nhit t nng lng mt tri hoc t nhit nng trong lng t. i vi nh my in nguyn t, thit b sinh hi l thit b trao i nhit, trong nc nhn nhit t cht ti nhit trong l phn ng ht nhn ra. S thit b ca chu trnh Renkin c trnh by trn hnh 7.13. th T-s ca chu trnh c biu din trn hnh 7.14.

Nc ngng trong bnh ngng IV ( trng thi 2 trn th) c thng s p2, t2,, i2, c bm V bm vo thit b sinh hi I vi p sut p1 (qu trnh 2-3). Trong thit b sinh hi, nc trong cc ng sinh hi nhn nhit ng p n si (qu trnh 3-4), ho hi (qu trnh 4-5) v thnh hi qu nhit trong b qu nhit II (qu trnh 5-1). Qu trnh 3-4-5-1 l qu trnh ha hi ng p p sut p1 = const. Hi ra khi b qu nhit II ( trng thi 1) c thng s p1, t1 i vo tuc bin III, y hi dn n on nhit n trng thi 2 (qu trnh 1-2) v sinh cng trong tuc bin. Hi ra khi tuc bin c thng s p2, t2, i vo bnh ngng IV, ngng t thnh nc (qu trnh 2-2), ri li c bm V bm tr v l. Qu trnh nn on nhit trong bm c th xem l qu trnh nn ng tch v nc khng chu nn (th tch t thay i).

7.4.2.2. Hiu sut nhit ca chu trnh Renkin Hiu sut nhit ca chu trnh t c tnh theo cng thc:

11

21ct q

lq

qq=

= (7-18)

Nhit lng nhn c trong trong l hi theo qu trnh ng p 3-1 l: q1 = i1 i3

Nhit lng mi cht nh ra cho nc lm mt bnh ngng trong qu trnh ng p 2-2 l: q2= i2 i2 Thng thng, p sut khng cao lm, cng tiu tn cho bm nc cp rt b so vi cng Tuc bin sinh ra nn ta c th b qua cng bm, ngha l coi i2 i3. Khi cng ca chu trnh s bng: l = q1 - q2 = i1 - i3 - i2- i2 i1 - i2 (7-19)

Hiu sut nhit chu trnh s bng:

31

21

1ct ii

iiql

== (7-20)

7.4.2.3. Cc bin php nng cao hiu sut ca chu trnh Hiu sut nhit ca chu trnh Renkin cng c th biu th bng hiu sut

chu trnh Carno tng ng:

84

1

2tcarnot T

T1max == (7-20)

T (7-20) ta thy: hiu sut nhit ca chu trnh khi gim nhit trung bnh T2tb ca qu trnh nh nhit trong bnh ngng hoc tng nhit trung bnh T1tb ca qu trnh cp nhit trong l hi. * Gim nhit trung bnh ca qu trnh nh nhit T2tb

Hnh 10.7 biu din chu trnh Renkin c p sut cui gim t p2 xung p2o , khi nhit u t1 v p sut u P1 khng thay i.

Khi gim p sut ngng t p2 ca hi trong bnh ngng, th nhit bo ha ts cng gim theo, do nhit trung bnh T2tb ca qu trnh nh nhit gim xung. Theo (10-29) th hiu sut nhit t ca chu trnh tng ln. Tuy nhin, nhit ts b gii hn bi nhit ngun lnh (nhit nc lm mt trong bnh ngng), do p sut cui ca chu trnh cng khng th xung qu thp, thng t 2Kpa n 5Kpa ty theo iu kin kh hu tng vng. Mt khc, khi gim p sut p2 xung th m ca hi cc tng cui tuc bin cng gim xung, s lm gim hiu sut v tui th Tuc bin, do cng lm gim hiu sut chung ca ton nh my.

Hnh 10.15. nh hng ca p sut cui

* Nng cao nhit trung bnh ca qu trnh cp nhit T1tb

nng nhit trung bnh ca qu trnh cp nhit T1tb, c th tng p

sut u p1 hoc nhit u t1.

Hnh 7.16. nh hng ca nhit u Hnh 7.17. nh hng ca p sut u

0

T

50

x = 0 x = 0

3

10

s

2 x= 1

3

4 5

2

1

0 s

10 1

2

T

2 x = 1

4

20

5

40

20

s

20

T

0

2x = 1

x = 0

30 3

4 5

1

2

85

- Hnh 7.16. biu din chu trnh Renkin c nhit hi qu nhit tng t t1 ln t10 khi p sut hi qu nhit p1 v p sut cui p2 khng i. Khi nhit trung bnh T1tb ca qu trnh cp nhit 3451 tng ln, do theo (7-21) th hiu sut nhit t ca chu trnh tng ln.

- Hnh 7.17. biu din chu trnh Renkin c p sut u tng t p1 n p10, khi nhit hi qu nhit t1 v p sut cui p2 khng thay i.

Nu gi nguyn nhit hi qu nhit t1 v p sut cui p2, tng p sut p1 th nhit si ca qu trnh 4-5 tng, do nhit trung bnh T1tb ca qu trnh cp nhit 3451 cng tng ln trong khi T2tb gi nguyn, dn n hiu sut nhit t ca chu trnh tng ln.

Tuy nhin khi tng p sut p1 th kh ca hi cc tng cui tuc bin s gim, l gim hiu sut v tui th tuc bin.

Khi tng nhit u th m gim, nhng tng p sut u th m tng. Do trn thc t ngi ta thng tng ng thi c p sut v nhit u tng hiu sut chu trnh m m khng tng, nn hiu sut ca chu trnh Renkin thc t s tng ln. Chnh v vy, ng vi mt gi tr p sut u ngi ta s chn nhit u tng ng, hai thng s ny gi l thng s kt i.

7.4.3. Chu trnh trch hi gia nhit nc cp

Mt bin php khc nng cao hiu sut chu trnh Renkin l trch mt phn hi t tuc bin gia nhit hm nc cp. S thit b chu trnh gia nhit hm nc cp c biu din trn hnh 7.18. Chu trnh ny khc chu trnh Renkin ch: Cho 1kg hi i vo tuc bin, sau khi dn n trong phn u ca Tuc bin t p sut p1 n p sut pt, ngi ta trch mt lng hi g1 v g2 gia nhit nc cp, do lng hi i qua phn sau ca tuc bin vo bnh ngng s gim xung ch cn l gk: gk = 1 - g1 - g2 (7-22)

Lng nhit nh ra trong bnh ngng cng gim xung ch cn:

Hnh 7.18. chu trnh gia nhit hm nc cp

q2hn = (i2 - i2)(1 - g1 - g2) < (i2 - i2) (7-23)

Hiu sut chu trnh c trch hi hm nng nc cp l:

11

hn21tr

ct ql

qqq

=

= (7-24)

1kg

IV

g2

VII

VII

VI

II

V

I

III

g1g

86

1

hn2tr

ct qq

1= > 1

2ct q

q1= (7-25)

Lng hi vo bnh ngng gim, ngha l lng nhit q2 m hi nh ra cho nc lm mt trong bnh ngng cng gim. T (7-25) r rng ta thy hiu sut nhit chu trnh c trch hi gia nhit hm nc cp tng ln. 7.4.4. Nh my in dng chu trnh hn hp Tuc bin kh - hi Chu trnh hn hp l mt chu trnh ghp, gm chu trnh Renkin hi nc v chu trnh Tuc bin kh. S thit b v th T-s ca chu trnh c th hin trn hnh 7.19. H thng thit b bao gm: thit b sinh hi 1 (bung t); tuc bin hi nc 2; bnh ngng hi 3; bm nc cp 4; b hm nc 5; tuc bin kh 6; my nn khng kh 7.

Hnh 7.20. S thit b v th T-s ca chu trnh hn hp Nguyn l lm vic ca chu trnh thit b nh sau: Khng kh c nn on nhit trong my nn 7 n p sut v nhit cao, c a vo bung t 1 cng vi nhin liu v chy trong bung t di p sut cao, khng i. Sau khi nh mt phn nhit cho nc trong dn ng ca bung t 1, sn phm chy i vo tuc bin kh 6, dn n sinh cng. Ra khi tuc bin kh, sn phm chy c nhit cn cao, tip tc i qua b hm nc 5, gia nhit cho nc ri thi ra ngoi. Nc c bm 4 bm qua b hm nc 5, vo dn ng ca bung t 1. y nc nhn nhit v bin thnh hi qu nhit. Hi qu nhit i vo tuc bin hi 2, dn n on nhit v sinh cng. Ra khi tuc bin, hi i vo bnh ngng 3 nh nhit ng p, ngng t thnh nc ri c bm 4 bm tr v l, lp li chu trnh c. th T-s ca chu trnh nhit c biu din trn hnh 7.20. Nhit lng do nhin liu chy ta ra trong qu trnh b-e chia thnh hai phn: mt phn dng sn xut hi nc trong thit b sinh hi 1, mt phn cp cho tuc bin kh 6. - a-b: qu trnh nn on nhit khng kh trong my nn kh 7;

87

- b-c: qu trnh cp nhit (chy) ng p trong bung t 1; - c-d: qu trnh dn n on nhit sinh cng trong tuc bin kh 6; - d-a: qu trnh nh nhit ng p trong b hm nc 5; - 3-1-1-1: qu trnh nc nhn nhit ng p trong b hm 5 v bung t 1;

- 1-2; 2-2; 2-3 l cc qu trnh dn n on nhit trong tuc bin, ngng ng p trong bnh ngng, nn on nhit trong bm nh chu trnh Renkin.

Hiu sut chu trnh l:

ct = 1ql

(7-28)

Trong : l: Cng ca tuc bin hi v tuc bin kh, l = lh + lk

q1: nhit lng nhin liu ta ra khi chy trong bung t 1. 7.5. chu trnh thit b lnh chy bng Amoniac, Fron

Chu trnh thit b lnh chy l chu trnh ngc chiu, nhn nhit t ngun c nhit thp, nh nhit chongun c nhit cao. Mi cht s dng trong cc lm thit b lnh thc t thng l hi ca mt s cht lng c nhit si thp p sut bnh thng, h s to nhit ln, r tin, khng c hi. Tu theo phng php tng p sut ca mi cht ta chia ra hai loi: chu trnh thit b lnh c my nn v chu trnh thit b lnh hp th (khng c my nn). 7.5.1. Chu trnh thit b lnh c my nn Mi cht thng dng trong my lnh c my nn l Amoniac (NH3) hay Fron F12, F22 (c cng thc: CmHxFyClz). Amnian thng dng trong my lnh cng nghip sn xut nc hoc lm lnh thc phm, v nhit n ho hi ln nn c th ch to vi cng sut ln. Fron thng dng trong my lnh gia nh nh t kem, t lnh gia nh v khng i hi cng sut ln, khng mi v khng c hi. S nguyn l ca my lnh c my nn c th hin trn hnh 7-20. Hi mi cht trng thi bo ho kh t bung lnh IV c p sut p1 c my nn ht vo v nn on nhit n p sut p2, nhit t2. Sau i vo bnh ngng II ngng t ng p p sut p2, nh lng nhit q1 cho khng kh hay nc lm mt. Lng ngng t t dn ngng II i qua van tit lu III, gim p sut t p2 xung p1 v chuyn t dng lng sang dng hi m. Hi m tip tc i vo bung lnh IV nhn nhit lng q2 ca vt cn lm lnh p sut p1 = const bin thnh hibo ho kh v chu trnh lp li nh c. Cc qu trnh ca my lnh dng hi c my nn c biu th trn th hnh 7-21. 1-2 l qu trnh nn on nhit trong my nn, p sut tng t p1 n p2,

2-3 l qu trnh ngng t ng p p sut p2 = const, nh lng nhit q1 cho khng kh hay nc lm mt,

3-4 l qu trnh tit lu trong van tit lu, p sut gim t p2 xung p1,

88

4-1 l qu trnh bc hi dn bc hi trong bung lnh, mi cht nhit lng q2 p sut p1 = const.

H s lm lnh:

( ) ( )514241

21

22

iiiiii

qqq

lq

=

== ,

v trong qu trnhtit lu i4 = i3, do :

( )12

41

iiii

= (7-26)

Nng sut ca my lnh: Q0 = G.q2, Cng sut ca my nn: N = G.l, y: G l lu lng mi cht trong chu trnh, kg/s. 7.5.2. Bm nhit

Bm nhit cn c gi l my iu ho hai chiu. Bm nhit c th lm lnh, ht m v cng c th si m, hin c dng kh ph bin min Bc nc ta. Khi dng vi chc nng si m, bm nhit s tit kim c in nng rt nhiu so vi dng l si in tr.

Nguyn l lm vic ca bm nhit nh sau: Mi cht trng thi bo ho kh t bung lnh IV c my nn ht vo v nn on nhit t p sut p1 n p sut p2, nhit t2. Sau i vo dn ngng II ngng t ng p p sut p2, nh lng nhit q1 bin thng lng. Lng t dn ngng II i qua van tit lu III, gim p sut t p2 xung p1 v chuyn t dng lng sang dng hi m, ri vo dn bay hi nhn nhit lng q2 . Nu s dng nng lng hu ch t dn bay hi (dn lnh, c b tr trong phng) th my lm vic theo ch lm lnh; Nu s dng nng lng hu ch t dn ngng (dn nng, c b tr trong phng) th my lm vic theo ch si m (bm nhit). Trong thc t cc dn c b tr c nh, ch cn i chiu chuyn ng cu dng mi cht nh van i chiu.

S nguyn l ca bm nhit c th hin trn hnh 7-22. Ch cn thay i vai tr ng, m ca cc van, thit b c th lm lnh hoc si m. Thit b

89

chnh gm my nn C, hai dn trao i nhit A v B, hai dn ny thay nhau lm dn lnh (dn bc hi) hoc dn nng (dn ngng t); van tit lu D v cc van ng m t 1-8 thay i chc nng lm vic ca my. Mi cht c th l Fron hoc Amniac. xt nguyn l vn hnh ca thit b, ta coi dn A t trong phng. * My lm vic vi chc nng si m: M cc van 2, 4, 6, 8 v ng cc van 1, 3, 5, 7, mi cht t my nn C i theo chiu C4A6D8B2C. Mi cht c my nn ht vo v nn n p sut v nhit cao, qua van 4 vo dn ngng A, nh lng nhit cho khng kh trong phng. Bn thn mi cht mt nhit, s ngng t, i qua van 6 v van tit lu D, bin thnh hi bo ho m nhit v p sut thp, qua van 8 vo dn bay hi B nhn nhit t mi trng xung quanh, bc hi v c hut v my nn, hon chnh mt chu trnh ngc chiu.

* My lm vic vi chc nng lm mt: ng cc van 2, 4, 6, 8 v m cc van 1, 3, 5, 7, mi cht t my nn C i theo chiu C1B7D5A3C. Mi cht c my nn ht vo v nn n p sut v nhit cao, qua van 1 vo dn ngng B, nh lng nhit cho mi trng xung quanh. Bn thn mi cht mt nhit, s ngng t, i qua van 7 v van tit lu D, bin thnh hi bo ho m nhit v p sut thp, qua van 5 vo dn bay hi A nhn nhit t khng kh trong phng, lm cho nhit trong phng gim xung, mi cht bc hi v c hut v my nn, hon chnh mt chu trnh ngc chiu lm mt phng.