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Bi t p l n ha cng II

TNH TON THI T B TRAO I NHI T GIN TI P

A/ C U T O V NGUYN L LM VI C

Thi t b ng chm c dng ph bi n nh t trong cng nghi p ha ch t, n c nh ng u i m l k t c u g n, ch c ch n, b m t truy n nhi t l n. Thi t b truy n nhi t ng chm lo i ng g m: 1- V hnh tr . 2- L i ng. 3- ng truy n nhi t. 4- M t bch c bu lng ghp.Trn v , n p, v y c c a ( ng n i) d n ch t t i nhi t. Thi t b truy n nhi t c t trn chn ho c tai treo hn vo v 1. Ch t t i nhi t I (dung d ch Metylic- n c ) i vo t d i y qua cc ng ln trn v ra kh i thi t b , cn ch t t i nhi t II ( h i n c bo ha ) i t c a trn c a v vo kho ng tr ng gi a cc ng v v r i i ra pha d i. Trong thi t b di n ra qu trnh truy n nhi t c a h i bo ha t i h n h p. Cc ng trong thi t b c b tr theo hnh su c nh, cch l p ng ki u hn. Ch n thng s k thu t:

Ch n v t li u thp CT 3. Chi u cao gi a hai m t bch: H= 2 m. Chu n s Reynolds: Re = 10500. ng knh ng: d = 30x2 mm. B dy ng truy n nhi t: = 2 mm = 0,002 m. Nhi t c a h i n c bo ha c p su t 4 at l: 142,9 oC

Nguy n Vn Tnh MSSV: 20072918L p: CN V C _K52

1


B/ TNH TON

I.Hi u s nhi t trung bnh gi a hai l u th :

Hi u s nhi t l n: Ta ch n t h = 142,9 oC, t2= 20 oC, t2c= ts dung dich= 84 oC

t = 142,9 20 = 122,9 (oC).

Hi u s nhi t b: tc = 142,9 84 = 58,9 (oC).

Vi c

tt

=

122,958,9

= 2,1 > 2

Nhi t trung bnh c a hai l u th c xc nh :

ttb = ln

c

c

t ttt

=122, 9 58, 9

122, 9ln58, 9

= 87,012 (oC).

Nhi t trung bnh c a t ng l u th l: H i t: t 1tb = 142,9 oC

Phia hn hp: t 2tb = 142,9 87,012 = 55,888 (oC)


t1 t1c

t2

t2c

to oC

2


T i t 2tb = 55,888 (oC) n i suy theo [3.171]

Cua Metylic: CM = 2741,496 J/kg.oC

Cua n c: C N = 4186,916 J/kg.oC

Cp = aMCM + (1 aM)CN

Cp =0,25.2741,496 + 0,75.4186,916 = 3825,561 (J/kg.oC)

II. Tnh nhi t l ng trao i Q. Q = G.Cp(tF tf)

Trong : G - l u l ng h n h p ban u, G = 0,944 kg/s; Cp - nhi t dung ring c a h n h p t i t 2tb = 55,888 oC tF = 84 oC nhit si cua hn hp

tf = 20 oC nhit ban u cua hn h p

V i C p =3825,561 J/kg.oC

V y : Q = 0,944.3825,561.(84- 20) = 231125,0934 (W)

III. Tnh h s c p nhi t cho t ng l u th . Tnh h s c p nhi t cho pha h i n c ng ng t theo cng th c:

411

2,04. ..

rAt H

=

, W/m2.oC

Trong : r - Nhi t ng ng t c a h i l y theo nhi t h i bo ha, J/kg; t1 - Chnh l ch nhi t gi a nhi t h i t v nhi t thnh

ng truy n nhi t, oC; H - chi u cao ng truy n nhi t (m) ; ch n H = 2 m;

A - h ng s tra theo nhi t mng n c ng ng.

ng v i t h = 142,9 oC n i suy, ta c: r = 2135,5.103 J/kg


3


Tnh l n 1 : gi s chnh l ch nhi t gi a mng v h i bo ha l 2 oC.a) Tnh h s c p nhi t cho pha h i n c ng ng t : Gi s chnh l ch nhi t t 1 = 2 oC

tt1= t1tb- t1= 142,9- 2= 140,9 (oC)

Khi ta c nhi t mng n c ng ng l:

1 1142,9 140,9 141,9

2 2tb t

mt t

t+ +

= = = (oC)

T t m = 141,9 oC tra b ng ta c: A = 194,285

V y : 3

41

2135,5.102,04.194,285.2.2

=

1 =10713,45 W/m2.oC

b) Tnh h s c p nhi t pha h n h p ch y xoy 2Ch n Re = 10500H s c p nhi t c tnh theo cng th c:

0,25

0,8 0,43 Pr0,021. .Re .PrPrk t

Nu

=

Suy ra:

0,25

0,8 0,43 Pr0,021. . .Re .PrPrk td

=

Trong :

Prt- chu n s Prandtl tnh theo nhi t trung bnh c a t ng, cn cc thng s khc tnh theo nhi t trung bnh c a dng; k- h s hi u ch nh tnh n nh h ng c a t s gi a chi u di l v ng knh d c a ng.


4


Ta ch n d = 30 x2 mm. H = 2 m . D a vo b ng 1.3 ta c :

2 58,823 50

0,026Ld

= = > k = 1

Tnh chu n s Pr theo cng th c :

.

Pr = pC

Trong : Cp - nhi t ring c a h n h p t 2tb; - nh t c a dung d ch t 2tb; - h s d n nhi t t 2tb tnh theo cng th c:

3. . .pC M =

Trong : - kh i l ng ring c a h n h p, kg/m 3; M - kh i l ng phn t c a h n h p, kg/kmol; Ta c : Cp = 3825,561 J/kg oC

- h s ph thu c m c lin k t c a ch t l ng, v i ch t l ng k t h p ( dung d ch Metylic- n c ) th kt h p = 3,58.10-8.

T i t 2tb = 55,888 oC n i suy ta c : M = 759,7008 kg/m3

N = 984,8504 kg/m3

( )1

30, 25 0,75 916,915 /759,7008 984,8504

kg m

= + = H s d n nhi t c a h n h p l:

8 3916,9153,58.10 .3825,561.916,915.20,212

=

= 0,448 W/m.oC


5


T i t 2tb = 55,888 oC ta co: M = 0,35.10-3 Ns/m2

N = 0,5.10-3 Ns/m2

lg(hh) = xF.lg(M) + ( 1 xF ).lg(N)

lg(hh) = 0,158lg(0,35.10-3) + (1 0,158).lg(0,5.10-3)

hh = 0,4726.10-3 (Ns/m2)

Do : 3. 3825,561.0,4726.10Pr = 3,974

0,455pC

= =

Tnh chu n s Pr t :

.

Pr = pt hhtt

C

Nhi t t i ring v pha h i ng ng t : q1 = 1.t1 = 10713,45.2

q1 = 21426,9 (W/m2)

Hi u s nhi t hai pha thnh ng: tt = tt1 tt2 = q1.rt

Trong : tt2 - nhi t thnh ng pha h n h p, oC; rt - nhi t tr hai bn ng truy n nhi t, m 2.oC /W

1 2t t tr r r= + +

Trong : rt1, rt2 - nhi t tr c a c n b n hai pha c a t ng, m 2. /W; - b dy c a ng truy n nhi t, (m); ch n = 2 mm = 0,002 m; - h s d n nhi t c a v t li u lm ng, W/m. oC; v i thp CT 3 ta c = 46,5 W/m. .D a vo b ng [ 3.4 ] ta ch n: rt1 = 1,16.10-3 , m2. /W rt2 = 0,464.10-3 , m2. /W


6


3 30,0021,16.10 0, 464.1046,5t

r = + + rt = 1,667.10-3 (m2. /W)Do : tt = q1.rt= 21426,9.1,667.10-3 = 35,719 (oC)

tt2 = tt1 tt = (142,9 - 2) 35,719 = 105,181 (oC)

t2 = tt2 t2tb = 105,181- 55,888 = 49,293 (oC)

T i t t2 = 105,181 oC n i suy ta c: CM = 2990,905 J/kg. CN = 4241,657 J/kg. Cpt = 0,25.2990,905 + 0,75.4241,657

Cpt = 3928,969 (J/kg. )T i t t2 = 105,181 oC n i suy ta c: M = 707,783 kg/m3

N = 954,114 kg/m3

( )1

30,25 0,75 877,743 /707,783 954,114t

kg m

= + = T i t t2 = 105,181 oC n i suy ta c:

M = 0,2.10-3 Ns/m2

N = 0,27.10-3 Ns/m2


lg(hh) = 0,158.lg(0,2.10-3) + (1 0,158).lg(0,27.10-3)

hh = 0,257.10-3 (Ns/m2)

Ta c: 3. . . tt k pt tC M =

8 3877,7433,58.10 .3928,969.877,743.20,212t

=

t = 0,434 (W/m2.oC)


7


.

Pr = pt hhtt

C

33928,969.0,257.10Pr 2,327

0, 434t

= =

V y: 0,25

0,8 0,432

Pr0,021. . .Re .PrPrk td

=

0,25

0,8 0,432

0,434 3,9740,021. .10500 .3,974 .0,026 2,327

= 2 = 1191,955 (W/m2. ) q2 = 2.t2 = 1191,955. 49,293

q2 = 58755,04 (W/m2)

y ta thy rng nhit tai ring v phia h i ng ng tu q 1= 21426,9 W/m2 khac rt xa v i nhit tai ring v phia hn h p chay xoay q 2= 58755,04 W/m2. Ma bai toan ta ang xet la truyn nhit n inh nn q1= q2= qtb, do vy, tim gia tri qtb

ta phai tinh lp.

Tnh l n 2 : gi s chnh l ch nhi t gi a mng v h i bo ha l 0,8 oC.a) Tnh h s c p nhi t cho pha h i n c ng ng t theo cng th c:

411

2,04. ..

rAt H

=

, W/m2oC

Gi s chnh l ch nhi t t 1 = 0,8 oC tt1= t1tb- t1= 142,9- 0,8= 142,1 (oC)

Khi ta c nhi t mng n c ng ng l:

1 1142,9 142,1 142,5

2 2tb t

mt t

t+ +

= = = (oC)


8



V y : 3

41

2135,5.102,04.194,375. 13477,690,8.2

= = (W/m2. )

b) Tnh h s c p nhi t pha h n h p ch y xoy 2Ch n Re = 10500

Nhi t t i ring v pha h i ng ng t : q1 = 1.t1 = 13477,69. 0,8 = 10782,152 (W/m2)

Hi u s nhi t hai pha thnh ng: tt = tt1 tt2 = q1.rt

Ta c: rt = 1,667.10-3 , m2. /W. tt= 10782,152.1,667.10-3= 17,974 (oC)

tt2 = tt1 tt = 142,1 17,974 = 124,126 (oC)

t2 = tt2 t2tb= 124,126 55,888 = 68,238 (oC)

T i t t2 = 124,126 oC ta c: CM = 3085,359 J/kg. CN = 4273,158 J/kg. Cpt = 0,25.3085,359 + 0,76.4273,158

Cpt = 3976,208 (J/kg. )T i t t2 = 124,126 oC ta c: M = 792,544 kg/m3

N = 787,582 kg/m3

( )1

30, 25 0,75 861,442 /686,160 941,623t

kg m

= + = T i t t2 = 124,126 oC n i suy ta c:

M = 0,15.10-3 Ns/m2

N = 0,15.10-3 Ns/m2



9


lg(hh) = 0,158.lg(0, 15.10-3) + (1 0,158).lg(0,15.10-3)

hh = 0,15.10-3 (Ns/m2)

8 3861, 4423,58.10 .3976, 208.861, 442.20, 212t

=

t = 0,428 (W/m. )

.

Pr = pt hhtt

C

33976, 208.0,15.10Pr 1,394

0,428t

= =

V y: 0,25

0,8 0,432

0, 428 3,9740,021. .10500 .3,974 .0,026 1,394

= 2 = 1339,425 (W/m2. ) q2 = 2.t2 = 1339,425.68,238= 91399,683( W/m2)

D a vao 2 ln tinh ta co thi:


q2

q1

0

q

t

10

0,8 2 3,5


Tnh l n 3 : Chnh l ch nhi t gi a mng v h i bo ha d a theo thi trn l 3,5 oC.

a) Tnh h s c p nhi t cho pha h i n c ng ng t : D a vo th ta suy ra c t 1 = 3,5 oC v y ta c: tt1 = t1tb- t1= 142,9 3,5 = 139,4 (oC)

1 1142,9 139,4 141,2

2 2tb t

mt t

t+ +

= = = (oC)


V y : 3

41

2135,5.102,04.194,18.3,5.2

=

1 =9309,686 (W/m2. )b) Tnh h s c p nhi t pha h n h p ch y xoy 2Nhi t t i ring v pha h i ng ng t : q1 = 1.t1 = 9309,686.3,5= 32583,901 (W/m2)

Hi u s nhi t hai pha thnh ng: tt = tt1 tt2 = q1.rt= 32583,901.1,667.10-3

tt = 54,317 (oC)

tt2 = tt1 tt = 139,4 54,317 = 85,083 (oC)

t2 = tt2 t2tb = 85,083 55,888 = 29,195 (oC)

T i t t2 =85,083 oC n i suy ta c: CM = 2886,686 J/kg. CN = 4200,166 J/kg. Cpt = 0,25.2886,686 + 0,75.4200,166

Cpt = 3871,796 (J/kg. )T i t t2 = 85,083 oC n i suy ta c: M = 730,4087 kg/m3

N = 968,4419 kg/m3


11


( )1

30,25 0,75 895,484 /730,4087 968, 4419t

kg m

= + = T i t t2 = 85,083 oC n i suy ta c:

M = 0,25.10-3 Ns/m2

N = 0,338.10-3 Ns/m2


lg(hh) = 0,158.lg(0, 25.10-3) + (1 0,158).lg(0,338.10-3)

hh = 0,313.10-3 (Ns/m2)

Ta c:

8 3895, 4843,58.10 .3871,796.895,484.20, 212t

=

t = 0,439 (W/m. )

Ta co .

Pr = pt hhtt

C

33871,796.0,313.10Pr 2,761

0,439t

= =

V y: 0,25

0,8 0,432

0, 439 3,9740,021. .10500 .3,974 .0,026 2,761

= 2 = 1158,095 (W/m2. ) q2 = 2.t2 = 1158,095.29,195

q2 = 33810,584 (W/m2)


12


D a trn s li u tnh ton ta c b ng s li u: S l n tnh

Pha h i n c ng ng t Nhi t tr v hi u s nhi t

t1tb tt1 t1 tm 1 q1 rt tt

1142,

9140,9 2 141,9 10713,45 21426,9 1,667.10-3 35,719

2142,

9142,1 0,8 142,5 13477,690 10782,152 1,667.10-3 17,974

3142,

9139,4 3,5 142,2 9309,686 32583,901 1,667.10-3 54,317

S l n tnh

Pha h n h p ch y xoy

tt2 t2tb t2 Prt (Pr/Prt)0,25 2 q2

1 105,181 55,888 49,293 2,327 1,141191,95

558755,04

2 124,126 55,888 68,238 1,394 1,2991339,42

591399,683

3 85,083 55,888 29, 195 2,761 1,0951158,09

533810,584

T trn ta c:

1 232583,901 33810,584

2 2tbq qq + += =

qtb = 33197,243 (W/m2)

Ki m tra sai s


13


2 1 0 00 02

33810,584 32583,901.100 .10033810,584

q qq

= =

0 00 03,63 10 = < ( ch p nh n)

IV. Tnh b m t truy n nhi t.

( )2231125,0934 6,96233197, 243tbQF mq

= = =

V. S ng truy n nhi t.

( )d 0,026 0,03 0,0282 2t n

td dd m+ += = =

S ng truy n nhi t:

6,962 39,59. . 3,14.0,028.2Fnd Hpi

= = =

n: s ng truy n nhi t. D a b ng quy chu n v ch n t ng s ng v i cch s p x p theo hnh l c gic l : n = 61 ng S ng trn m t c nh c a hnh 6 c nh l: 4 ng. S ng trn ng xuyn tm c a hnh 6 c nh l: 9 ng. T ng s ng khng k cc ng trong cc hnh vin phn l: 61 ng. S ng trong cc hnh vin phn l: 18 ng.

VI. ng knh trong thi t b un nng.

D = t.( b 1 ) + 4.dn, m

Trong : t b c ng, th ng l y t = 1,2 1,5 d n ;Nguy n Vn Tnh MSSV: 20072918L p: CN V C _K52

14


dn ng knh ngoi c a ng truy n nhi t, m; b s ng trn ng xuyn tm su c nh. V y: D = 1,5.0,03.(9 1 ) + 4.0,03 D = 0,48 (m)

Lm trn: D = 0,5 m = 500 mm.

VII. Tnh chi u cao c a thi t b .

V i D = 500 mm chon np thit bi hinh elip co g, tra b ng s li u ta ch n: Chi u cao c a n p thi t b : h l = ht + hTrong : ht = 125 mm.

h = 25 mm

V y chi u cao c a thi t b : L = H + 2.hl = 2.1000 + 2.( 125 + 25 ) = 2300 (mm)

VIII. Tnh l i v n t c va chia ngn: Xc nh v n t c th c:

24.

. . .tG

d n

pi =

G = 0,944 (kg/s)

n = 61 ng d = 0,026 m

= 916,915 kg/m3

24.0,944

3,14.0,026 .61.916,915t =

t = 0,0318 (m/s)

Xc nh v n t c gi thi t:

3Re. 10500.0, 4726.10

. 0,026.916,915gt d

= =


15


gt = 0,2082 (m/s)

V: 0 0 00 0 00,2082 0,0318.100 84,73 10

0,2082gt t

gt

= = >

Nn ta c n ph i chia ngn qu trnh c p nhi t ch xoy. S ngn:

S ngn c n thi t: 0,20820,0318

gt

t

m

= =

m = 6,55 ( ngn )

Quy chu n m = 8 ( ngn ).

Tnh l i chu n s Reynolds:

V y cc kch th c c a thi t b un nng h n h p u l: F = 7 m2 b m t truy n nhi t; n = 61 ng s ng truy n nhi t; D = 500 mm ng knh trong c a thi t b ; H = 2 m chi u cao gi a hai m t bch.

L = 2,3 m - chi u cao cua thit bi.


16


A/ C U T O V NGUYN L LM VI C 1

B/ TNH TON 2

I.Hi u s nhi t trung bnh gi a hai l u th 2

II. Tnh nhi t l ng trao i Q 3

III. Tnh h s c p nhi t cho t ng l u th 3

Tnh l n 1 4

a) Tnh h s c p nhi t cho pha h i n c ng ng t 4b) Tnh h s c p nhi t pha h n h p ch y xoy 2 4Tnh l n 2 8a) Tnh h s c p nhi t cho pha h i n c ng ng t 8b) Tnh h s c p nhi t pha h n h p ch y xoy 2 9Tnh l n 3 11a) Tnh h s c p nhi t cho pha h i n c ng ng t 11b) Tnh h s c p nhi t pha h n h p ch y xoy 2 11IV. Tnh b m t truy n nhi t 14V. S ng truy n nhi t 14VI. ng knh trong thi t b un nng 14VII. Tnh chi u cao c a thi t b 15VIII. Tnh l i v n t c va chia ngn 15Nguy n Vn Tnh MSSV: 20072918L p: CN V C _K52

17


Muc Luc 17


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