GT Ky Thuat Say Nong San

8/14/2019 GT Ky Thuat Say Nong San

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Trng i hc Nng nghip 1 Gio trnh Kthut synng sn -------- ----------------------------------------- 1

b gio dc v o totrng i hc nng nghip i - h ni

GS.TS. Phm xun vng (Ch bin)TS. Trn nh khuyn

Gio trnhK thut sy nng sn

H ni 2006


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li ni u

Gio trnh K thut sy nng sn cp ti nguyn l lm vic, l thuyt tnh ton

cc qu trnh k thut sy, lm c s cho vic thit k cc thit b sy. Mt khc sy l mt qutrnh cng ngh c s dng rng ri trong cc ngnh ca sn xut nng nghip. Sy l cngon quan trng ca cng ngh sau thu hoch trc khi thc hin bo qun sn phm. itng s dng l sinh vin nm cui ca ngnh c kh bo qun ch bin ca trng i hcNng Nghip. ng thi cng c th s dng cho sinh vin c kh nng nghip, cng thn,.v.v. v cc k s lm vic lin quan ti lnh vc ny.

Sinh vin trong qu trnh hc cn nm vng qu trnh sy l mt qu trnh cng ngh,khng n thun l tch nc ra khi vt liu. Yu cu sau khi sy vt liu phi t cht lngcao, chi ph nng lng sy thp.

Gio trnh c trnh by trong 6 chng.

- Chng 1 C s l thuyt qu trnh sy. Trong chng ny trnh by cc khi nim vl thuyt chung ca qu trnh sy.

- Chng 2: Thit b sy i lu.- Chng 3: Thit b sy tip xc.- Chng 4: Thit b sy bc x.- Chng 5: Thit b sy thng hoa.- Chng 6: Cc thit b ph tr h thng sy.Trong tng chng cp ti mt s vn mi cp nht trong thi gian gn y,

h thng ho cc kin thc c bn gip sinh vin nm bt d dng. Sau mi chng c cu hin tp v mt s bi tp ng dng.

Trong qu trnh bin son tc gi c ng nghip gp . Tc gi chn thnh cmn B mn My nng nghip v khoa C in trng i hc Nng Nghip I - H Ni.Cun sch chc chn khng trnh khi nhng thiu xt, rt mong nhn c kin ng

gp ca bn c. Cc kin xin gi v Phng o to, khoa C in trng i hc Nng Nghip I.

Ch bin

GS.TS. Phm Xun Vng


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Chng 1

C s l thuyt ca qu trnh sy

Sy l qu trnh loi b bt k cht lng no ra khi vt liu, kt qu l tng t l hmlng cht kh. Thc t sy i vi lng thc v thc phm l loi b nc ra khi sn phm.Sy vt liu nhm mc ch: gim khi lng, tng bn (g) tng nhit chy (nhin liu),tng n nh khi bo qun (nng sn).

Sy l qu trnh cng ngh phc tp. V nguyn tc, c nhiu phng php sy vt liukhc nhau. Theo du hiu v nng lng ta c hai nguyn tc chnh:

- Loi b m (nc) ra khi vt liu, khng lm thay i trng thi lin kt: Vn dng lng.- Loi b m khi thay i trng thi lin kt: Lng bin thnh hi.

Loi u c th thc hin bng phng php c hc (p, ly tm, lc).Loi th hai lin quan ti chi ph nhit hm nng vt liu m, bc hi nc b mt vt liuv lm si lng bn trong vt liu v dn thot ra ngoi.

1.1. Vt liu ma s nng sn l vt liu m, cha mt lng nc ng k. Nc l thnh phn ca

t chc ng vt v thc vt. ng vt li l ngun thc n cho con ngi. Tuy nhin thanc c th dn n h hi sn phm do tc ng ca vi sinh vt trong mi trng nc. Do phn ln cc sn phm cn phi gi kh.

Trng thi vt liu m c xc nh bi m v nhit ca n. Tnh cht ca vtliu m c c trng bi tnh cht nhit vt l v cc thng s vt l: nhit dung ring, hs dn nhit...vv.

Trong l thuyt sy, m ca vt liu c phn chia thnh m tng i v m tuyt i.1.1.1. m tng i.

m tng i cn gi l m ton phn, l s phn trm khi lng nc chatrong 1kg vt liu m. Khi lng chung ca vt liu m.

g = ga + gk

y: ga Khi lng nc (kg).gk Khi lng cht kh tuyt i (kg).

m tng i s l: %100=g

gW a

Trng hp W = 0 ta c vt liu kh tuyt i.

1.1.2. m tuyt i. m tuyt i l s phn trm khi lng nc chia trong 1kg vt liu kh.

%100=K

a

Kg

gW

Cng thc chuyn i

%100100

+

=K

K

W

WW


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%100100

=W

WWK

T bng ph lc cho ta cc gi tr khi chuyn i t W ra WK.

Do khi lng ca m ga cha trong vt liu c th ln hn khi lng vt liu kh gK,nn khc vi m tng i, m tuyt i WK c th ln hn 100%.

Trng hp t s gia khi lng m ca vt liu m vi khi lng cht kh tnh bngkg/kg vt liu kh, ta c khi lng m ring ( cha m).

K

a

g

gu =

Khi lng m ring c th c trng m khng ch cho ton th tch ca vt lium cn l m lp hoc im no . Nu m phn b u theo tit din th.

WK = 100u hoc

100

WK=u

Mt m trong vt liu C (kg/m3) quan h vi khi lng m ring u theo quan h.

0C u =

y: 0 Khi lng tuyt i cht kh trong mt n v th tchvt liu m (kg cht kh/m3)

Nu gim th tch (gim th tch trong qu trnh sy) nho th 0K (K mt cht kh ca vt liu, hoc khi lng ring ca vt liu kh)

1.1.1. m trong vt liu.a) Tnh cht l ho ca nc: Nc trong vt liu m c th tn ti di 3 dng: rn, lng, kh p sut kh quyn nc chuyn t pha nng sang pha lng, ngc li 0 0C nc chuyn t

pha rn sang pha lng. i vi nc thng thng, nhit 3,980C c mt ln nht 1g/cm3. Khi tng nhit ln mt ca nc bt u gim v chuyn ng ca cc phn tmnh ln. Phn t cht lng cng nh vt rn lin tc chuyn ng dao ng phc tp. Ngoira i vi vt th nhiu nguyn t, cn c chuyn ng dao ng ca nguyn t ny i vinguyn t khc.

Trong bt k thi im no, nng lng chuynng dao ng c xc nh bng tng th nng vng nng ca phn t. Khi phn t v tr trunggian, th nng ca n nh nht cn ng nng t gitr cc i. Ngc li cc v tr tn cng ca phn

t, ng nng bng khng, cn th nng t gi trcc i.

Phn t nc c 3 nguyn t to thnh cu trctam gic, 2prtn y v nguyn t xy nh vtr khng kch ng, nhn nguyn t hi nc cc tnh sau:

Hnh 1.1. S xy dng phn t nc


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Khong cch OH 0,95840

A , HH 1,51500

A gc HOH 104027. Tng ng vi

mc dao ng di; khong cch OH 0,45680

A HH 1,54000

A v gc HOA 105003.

Nc l dch th dnh t. Sc cng mt ngoi 20

0

C bng 0,0727 Pa. Cc c trngvt l trn y, cng nh cc c trng khc ca nc , nc v hi nc u ph thuc vonhit v p sut.b) Hin tng hp ph.

Nghin cu hin tng pht trin trn b mt phn chia pha (lng rn) i vi vt lium c mt ngha quan trng. Qu trnh sy cung cp nng lng cho nc thng cc lc linkt gia nc v vt liu sy, dch chuyn nc t trong lng vt sy ra b mt v i vo tcnhn sy thi vo mi trng. Bn cht cc lin kt gia nc v vt liu l hin tng hpph.

Thng ngi ta kho st hp ph vt l v hp ph ho hc. Hp ph ho hc l s

lin kt gia phn t nc v cc phn t ca vt hp ph thng qua trao i in t vngngoi (tnh ngm nc). V th hp ph ho hc rt bn vng, rt kh dng nhit tch nckhi hp ph ho hc. Hp ph vt l l lin kt gia cc phn t nc vi cc phn t vt hpph khng c trao i in m ch do sc cng mt ngoi. hp ph vt l xy ra khng ng utheo chiu dy lp nc v rt d tch khi vt liu khi sy. Nc c trong vt liu sy ch yul nc hp ph vt l.

Nh bit, ng vi nh lut th 2 ca nhit ng hc, khi h tin dn n trng thicn bng th nng ca h tin dn ti cc tiu, c ngha l b mt cht lng cn tc ng lc

tin ti gim din tch ca n, l lc cng b mt (sc cng b mt) i vi mt n v ditip tuyn vi b mt (in/cm hoc N/m).

Sc cng b mt tnh bng nng lng t do ca lp b mt trn mt n v din tch(in cm/cm2 ngha l erg/cm2 hoc Nm/m2). Ni cch khc, bng mt nng lng t doca mang mng b mt.

i vi nc 00C, 75,7 in/cm 75,7erg/cm20,0757N/m. Kho st s tc ngtng h ca cht lng vi b mt vt cng. Trong trng hp ny phn t b mt vt cng stc dng vi lc xc nh Fc ln phn t lp bin cht lng v pha cht cng tc dng lc Fe

C th trnh by s tc ng tng hcht lng vi vt cng, thng trong c phath 3 l pha kh.

Kh khng c tc ng ca ngoi lc, git

cht lng c dng hnh cu. Trn b mt vt cngtrong mi trng khng kh c dng phn chia1,2,3 nh (hnh 1.2). Dng git nc ph thuc

vo s tng ng gia v gc . Gc cn l so t b mt cng; c th c 3 trng hp sau.

Trng hp 1: 1 -3 > 2 3 + 1 2 cos

Hnh 1.2. Dng git cht lng giihn phn chia 3 pha


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1, 2, 3 pha kh, lng v rn.

y 1 -3,2 -3 v 1 -2 l sc cng b mt gii hn gia pha kh vt cng, Lng vtcng v kh lng.

- Gc to thnh t tip tuyn vi ng vin git nc.Trong trng hp ny git cht lng b dt trn mt vt cng.Trng hp 2:

1 -3 < 2 3 + 1 2 cosGit cht lng b p v c dng elipx - it.Trng hp 3:

1 -3 = 2 3 + 1 2 cosKhi :

1-3 2 3

1 2

cos

= (1-1)

Ro rng trng hp th nht 0 < cos < 1, gc 0 < < 900 (Hnh 1.2) b mt vt

cngb t. Dch th dng ny gi l dnh th dnh t (1 -3 > 2 3).

Nu hiu s1 3 - 2 3 = 1- 2 th ph hp vi cng th (1 1),

cos = 1 v = 0, ngha l b mt vt rn b dnh t hon ton, gc khng to thnh, sccng b mt c trng bi s tha nng lng t do ca lp b mt. Ph thuc vo nhit t,sc cng b mt ca nc c th xc nh theo cng thc (erg/cm2).

t = 75,7(1 0,002t)

Trong trng hp th hai (- 1) < cos < 0, 900 <


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Khi t = 200C, 73 erg/cm2 v0

0,15h

r= (cm)

Trong ng mao bn knh r0 = 10-5 cm, chiu cao h = 0,15.105 cm = 150m v p lc ct

nc bng 1,5 MPa.Nu chiu cao h nh hn v ng vi r0, th lm ca b mt ct dch th s gim, b mt

to thnh p sut dng. i vi vt th dnh t chiu cao ct dch th trong ng mao h > 0 so vichiu cao mt thong, ngc li vi dch th khng dnh t chiu cao ct dch th trong ngmao h < 0 so vi chiu cao mt thong.

p lc ca ng mao gy ra gim p sut

hi bo ho pha trn b mt cong, nng cao p

sut ca n trn mt li so vi p sut pha trn

b mt cht lng.

Ph hp vi nh lut Ken-Vin, n

hi tng i ca hi:

0 0

2exp

.

hr

l

p

p p r

= =

(1.2)

y: pr v p0 - p sut hi bo ho trn mt

cong v mt phng.

Bng 1.1. S ph thuc gia co dn tng i ca hi nc khi t hon ton

ca v r0 ca ng mao. r0.10

7 cm r0.107 cm r0.10

7 cm

0,05 0,36 0,5 1,56 0,95 21,90,1 0,46 0,55 1,8 0,96 26,30,15 0,57 0,6 2,11 0,97 25,30,20 0,67 0,65 2,50 0,98 53,30,25 0,78 0,7 3,01 0,99 107,50,30 0,89 0,75 3,73 0,999 1077,00,35 1,02 0,8 4,83 1,000 10770,00,40 1,17 0,85 6,610,45 1,34 0,9 10,25

T bng trn cho thy khi r0 = 107,510-7cm = 1,0710-5cm, th 1 v p sut hi bo

ho trn mt dnh t thc t bng p sut hi bo ho trn mt phng cht lng. Hin tngny l c s chia ng mao thnh loi cc nh (r0 < 10

-5cm) v ng mao ln (r0 > 10-5cm).Nh thy phn trn, hin tng b mt c pht trin khi tc ng tng h ca

cht lng (nc) vi vt cng, v c bn lin kt khng ch vi tng tc vt l m c tng tc

Hnh 1.4. Chiu cao dch th dnh ttrong ng mao dn


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ho hc. Kho st hin tng phc tp trn l mt kh khn rt ln; m phi p dng phngphp hin i ca c hc lng t.1.1.2. Phn loi dng lin kt m trong vt liu.

Nng sn l mt h trong m c dng lin kt khc nhau vi vt rn. Nhiu cngtrnh nghin cu cho php n gin ho vic phn loi cc dng lin kt nc, nhng hin nayta chia lm hai loi: nc nc t do v nc lin kt.

Cc dng lin kt m c chia thnh ba nhm chnh: lin kt ho hc, lin kt vt lv lin kt c vt l.

Trong m lin kt ho - l khng c th kh c bng qu trnh sy. Mt khc linkt ho hc ca nc vi vt liu ch c loi b nh tc ng ho hc hoc gia cng nhitc bit mnh. Khi sy loi lin kt ny kh loi b.a) Nng lng lin kt m vi vt liu.

. A. Rbiner s dng quan h nhit ng hc c bn, cho c tnh s lng nng

lng lin kt m vi vt liu tip nhn nng lng t do ng nhit. V lin kt nc vi vtliu lm gim p sut hi nc trn b mt ca n, chnh v th nng lng t do tng ng s gim.

Gim nng lng t do F nhit T khng i hoc nng lng lin kt biu dinbi cng L cn chi ph tch ri 1 mol nc khi vt liu (khng thay i thanh phn), c thxc nh theo cng thc (erg/mol).

ln lnh

u

PF L R T R T

P = = =

y: Ph - p sut hi bo ho ca nc t do.Pu - p sut hi nc cn bng trn vt liu vi hm lng m u trong mi

trng khng kh bt k.R rng: u

h

P

P=

Lin kt gia nc vi vt liu cng chc, cng lm gim ln Pu v ngc li, i vi

nc t do Pu t c gi tr Ph, = 1 v nng lng lin kt L = 0.

L = RTln1 = 0Ch lp nc mt phn t, ngha l lp nc c chiu dy mt phn t (ng knh

phn t nc bng 310-8cm) l lin kt hp th vi nng lng ca lin kt phn t thy r.Lin kt ca lp nc xy ra gim t ngt vi vic tng khong cch i vi b mt hp thtng ng vi quy lut ca lc phn t. V vy lng nc lin kt hp th trong vt liu ph

thuc vo tng b mt bn trong ca phn t v c th thng (theo th tch) chim 1 5% ivi th tch chung ca vt. Khi ng knh phn t 10-6cm nc hp th c th chim 18% vln hn nu ng knh phn t cng nh.

Hiu qu nhit lin kt ca nc:

u hQ Q Q= hoc ur r r =

y: Qu - nhit bc hi ca nc lin kt vi vt liu ta xc nh m (ru).


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Qh - Nhit bc hi ca nc t do.Theo phng trnhGips-Hemlhols, s thay i nng lng bn trong hoc entanpi trong

qu trnh ng nhit, ng tch v:F

I F T T

=

T (1.3) suy ra: 2 2ln ln

;u hh h

P PQ R T Q R T

T T

= =

Khi : 2 2ln ln lnu hP PQ R T R T

T T T

= =

(1.4)

Bi v Qu > Qh v Pu < Ph thln ln

u hP P

T T

>

Bt ng thc ny minh ho bi ng cong (Hnh 1.5) biu din s ph thuc gia Pv T. T hnh ta thy, ng cong lnPu = f(T), tang gc nghing ca tip tuyn ln hn so vi

lnPh = f(T). i vi khong nhit khng ln:2 lnQ R T const

T

=

v 0

Q

T

=

Ta c biu thc i vi p sut hi trn nc lin kt:

expu hQ

P PR T

=

(1.5)

T nhng gi thit nhit ng c trng cho ln nng lng lin kt. .A Rbine

tnh nhng dng lin kt nc vi vt liu khuch tn (UCEPCH) khi sy.+ Nc lin kt ho hc.i vi nc ny, nng lng lin kt L t gi tr ln nht. Th d i vi s kt tinh

ca sunpht ng CuSO4 250C Pu = 0,11, Ph = 3,2 KPa v L = 8,4.10

-3J/mol.+ Nc lin kt hp ph.i vi nc lin kt hp ph p lc hi phthuc vo m ca vt liu c gi l hpph ng nhit. Dng lin kt m ny c trngch i vi vt liu c m nh, ngha l gi

tr u

h

P

P= khng ng k; tng ng vi nng

lng lin kt ln.

Th d khi = 0,25, L = 3,47.103

J/mol.+ Lin kt mao dn.Nng lng lin kt i vi nc loi ny gyra lin kt hp ph ca lp a phn t trn thnhng mao v gim p sut hi trn mt congtrong ng mao so vi p sut hi trn b mt

phng ca nc t do. Nc cn li bn trong ng mao khng khc vi nc t do.

Hnh 1.5. S s ph thuc gia P v Ttrong bng bn logarit


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Nng lng lin kt m ca ng mao to khng ln v gn vi nng lng ca m t do.Nu p sut hi trn mt cong ca nc c th xc nh gn ng theo nh lut ca Klu-vin:

02expr h

VP P

r R T

=

(1.6)

Nng lng lin kt L c th xc nh theo cng thc (12) khi thay Pu v Ph.

0

2r

L Vr

=

Vi: V0 - l th tch ring ca hi nc trn mt thong.Khi ng knh ng mao khng ln, L t gi tr ln nht. Th d khi 2r = 10-6cm, L = 5,3.102J/mol.

b/ Lin kt thm thu.Lin kt thm thu in hnh l lin kt ca nc trong cc dung dch, y khi lng

chnh lin kt ca n yu - thm thu, ngha l ngtrpi. Bn cht ca lin kt ny c biudin trong p sut ca hi Pu trn dung dch nh hn p sut hi Ph trn nc. T phng

trnh Gips-Hemlhols.F I T S = + (1.7)

y: I - gim nng lng bn trong (hoc entalpi trong qu trnh ng p vng tch). Xem lin kt nc khi c tng tc phn t.

S - tng ngtrpi trong qu trnh ng nhit.Lin kt thm thu tr ln r rt khi khng c nhng lin kt vng chc hn ngha l

khi I 0.

Khi 0lnF T S RT n = =

y: n0

- l phn mol ca nc trong dung dch,0

u

h

Pn

P= =

1.1.3. Cc c trng nhit ng ca vt liu m.a/ Th dn m.

Khi nim v th dn m cng tng t nh th dn nhit. m ch c th truyn t vthay mt phn ca n c m cao sang vt hay mt phn ca vt khc c m thp hn.

Gia truyn nhit v truyn cht c nhng tnh cht tng t. Th d (hnh 1.6), cho

thy s phn b th chuyn nhit, ngha l nhit v entanpi ca ch v st. trng thicn bng nhit ca hai vt th nh nhau (50 0C) v trn gii hn tip xc c bin chuyn t

ngt ca entanpi (T I = C.t = 0,12750 = 6,3 KJ/Kg i vi ch, n I = 0,4650 = 23,0KJ/Kg ivi st). R rng, s chuyn bin ny l nguyn nhn s khc nhau ca nhit khi ring gia haivt th. V th phn tch hin tng chuyn cht hp l, tng t nhit khi ring ca chtlng nng (nc). a vo khi nim nhit khi ring ng nhit Cm i vi vt tiu chun(giy lc) bng 1/100 UCmax ngha l:

/

1

100mt c Cmax

C u= Kg/Kg0M

Nh vy, tng t th chuyn nhit-nhit , hm lng m ring cc i ca vt tiu

chun, tng ng vi th chuyn m trong 100 (0M) trao i khi.


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Kcal/Kg

Kcal/Kg

Kg/Kgk

k

Kg/Kg

( ) ( )/ /

/

/

100t c t cmt c

mt c CmaxT T

u u

C u

= =

Trong : ut/c - hm lng m tiu chun, khi ut/c = uCmax, mt/nghim = 1000M.

Trong th d tip xc gia than bn v giy lc, hm lng m cc vt liu trngthi cn bng nhit ng ut/c = 0,5Kg/Kg, th chuyn khi (uCmax)T = 0,277Kg/Kg.

0

/

0,5100 180

0,277mt ngh

M = = .

chuyn vt cht (m) v nhit t vt ny n vt khc cn to nn s sai khc thtng ng. c bit ta thy s hin din sai khc th m v nhit truyn t vt c hm lngm nh ti vt c hm lng m ln.

a/ c/

b/ d/

Hnh 1.6. Phn b m, entanpi v th chuyn trong h vt.Vt tiu chun v than bn (a), ch v st (b), chuyn m v nhit khi hin din th (c, d).

Hmlngmu(Kg/Kg)

Khong cch t b mt tip xc (cm)

Th

m

m

thanbnct

than bngiy lc

Khong cch t mt tip xc (cm)

EntanpiI(KCal/Kg)

ch

ch

nhm

st

Nhit

nhit

Kc

k

K

k


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n v o c gi l th dn m v k hiu l . Trong dn m ng nhit, ta c th

xem th dn m l mt hm ca cha m u hay = f(u). Nu trong dn nhit, t khi

nim nhit v nhit lng, ngi ta a ra khi nim nhit dung ring: pdq

C dt Vi:

dq - nhit lng cn hm nng hoc lm ngui 1Kg vt cht nng ln hoc lnh i.dt - bin thin nhit trong qu trnh ng p.

Trong dn m ta cng a ra khi nim m dung ring trung bnh v cc b. m dungring trung bnh c trng cho s thay i hm lng khi ca vt U tng ng vi s thayi th chuyn khi trn mt n v o ca th ny, ngha l:

m

m

uC

=

i vi nhiu vt s ph thuc m = f(u) l phi tuyn, ln Cm thay i vi s thay ihm lng m. Chnh v th a vo khi nim m dung ring thc ng nhit (khi nhit

ca vt l ):

m

m

uC

=

m dung ring thc (cc b) ch ng nhit Cm = f(u) xc nh bng th ngcong vi phn m = f(u) khi = const.

b/m .o nhit theo thang bch phn, ngi ta chn nc lm vt mu v nhit nc

ang tan l 00C v nhit nc ang si l 1000C cng p sut 760mmHg. Nu nhit lngo bng Kcal th nhit dung ring ca nc bng 1Kcal/Kg.K.

Tng t th dn nhit - l nhit ; hm lng m ring cc i ca vt tiu chun

tng ng vi th chuyn m trong 100 trao i khi (0M).

( ) ( )/ /

/

100t c t cmt c

mtc CmaxT T

u u

C u

= = (1.8)

y:ut/c - hm lng m tiu chun, khi ut/c = uCmax, mt/c = 100

0M.Khi kho st v d tip xc ca than bn v giy lc, hm lng m trong trng thi cn

bng nhit ng ut/c = 0,5Kg/Kg, th chuyn khi khi (uCmax) = 0,277Kg/Kg.0

/

0,5100 180

0,277mt c

M = =

chuyn cht (m) v nhit t vt ny n vt khc cn to hiu ca th tng ng(Hnh 1.6c,d).Ta thy rng khi tn ti hiu th m v nhit chuyn t vt c hm m nh vo vt c hm m ln.


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1.1.4. Cc thng s nhit - vt l ca vt liu m.a/ Nhit dung ring ca vt liu m.

Nhit dung ring ca vt liu m (J/Kg.0K) thng c tnh nh l trung bnh cng ln

nhit dung ring ca cht kh ca vt liu CK v nhit dung ring ca nc Cn bng 4,19 (KJ/Kg.0

K).( )100

100

K a

M

C W C W C

+ = (1.9)

Nhit dung ring ca cht kh ca cc sn phm thc phm khc nhau t

1257 1676 J/Kg.0K hoc 0,3 0,4 Kcal/Kg0C.Cng thc trn c th trnh by di dng:

100

a KM K

C CC C W

= +

Nhit dung ring ca vt liu m i vi s kilogam cht kh, th nhit dung quidn J/Kg cht kh.0K.

( )100

100 100 100

cK aqd a

M K K a K a

C W C W C W WC C C C C C u

W W

+ = = + = + = +

(1.10)

Hnh 1.7. S ph thuc ca nhit dung ring ht la tiu mch vo m ca n.1 - Theo BH; 2 - Theo cng thc hn hp; 3 - Theo s liu ca isnha; 4 - Theo s liu

ca .A gRVa; 5 - Theo s liu ca B.C. Klva.

Ph hp vi cng thc (1.9, 1.10) thng dng tnh ton thit b sy, s ph thucgia nhit dung ring ca vt liu v m c c tnh tuyn tnh. Qua thc nghim cho thy

0.

J

Kg K 0Kcal

Kg C


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i vi nhiu loi vt liu v sn phm, hm CM = f(W) khng phi l n iu v th ca nth c nhng im gy khc.

Trn hnh 1.7 cho thy s ph thuc nhit dung ring ca ht la mch vo m ca

n. S ph thuc ny nhn c theo s liu ca nhiu tc gi. Theo cng thc (1.9) thCK = 1548J/Kg

0K = 0,37Kcal/Kg0C.

b/ H s dn nhit ca vt liu m.

Truyn nhit trong vt liu m khc vi truyn nhit trong vt liu kh. H s dn nhitca vt liu m ph thuc vo bn cht vt kh, m ca n, cu trc cc hang xp, ngknh cc mao qun, ...

H s dn nhit (W/m.0K) l h s t l trong phng trnh Fua-ri.

q =

y: q - mt dng nhit trong vt (W/m2) - Gradien nhit 0K/m.

Nu h s dn nhit ca vt liu kh K v ca nc hoc hi nc trong cc hang xp n bit th h s dn nhit ca vt liu m c th tnh:

( )

21

1

2

1 1

1 1K

V

V

=

+

Trong :

1, V1; 2, V2 tng ng l h s dn nhit v th tch ca vt cng v pha hi.

- h s hnh dng ca phn t to nn vt liu m.

- Phn t hnh cu:

1

1 2

3

2

=

+

- Phn t dng hnh tr:

( )1 2

1 2

5

3

+=

+

- i vi ht dng tm mng:

1 2

2

2

3

+=

Thc t i vi vt liu m c th th h s dn nhit ch yu ph thuc vo m vnhit . Trong k thut, tnh ti nh hng ny ta dng cc cng thc thc nghim. ivi la m v ht ng cc:

= 0,070 + 0,0233.K (W/m0K)

y:K - m tuyt i.


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c/ H s dn nhit .Nh ta bit, h s dn nhit a l c tnh quan trng ca vt liu, xc nh qun

tnh nhit ca n. a cng cao, cng xy ra hm nng hoc lm lnh vt liu cng nhanh, ngha

l h s a cn phi tnh khi nghin cu v tnh ton qu trnh khng n nh - hm nng, lmlnh, sy, lm m, ...

aC

=

(1.11)

y: - h s dn nhit ca vt liuC - nhit dung ring

- mt (hoc khi lng th tch).

Hnh 1.8. S ph thuc h s dn nhit a vo hm lng m uI - i vi ct; II - i vi t st; III - i vi than bn.

Tch C. - l nhit dung n v ca th tch vt liu, n c trng cho kh nng tr

nhit: C. cng ln, cng nh gi tr ca h s a s cng nh, ngha l vt liu c kh nngtr nhit cng ln s b hm nng v lm lnh chm.

Phng trnh vi phn truyn nhit bn trong c dng:

2a

=

(1.12)

Trn hnh 1.8 theo s liu ca A.B.Lkp trnh by ng cong c trng ph thucca h s a vo hm lng m u i vi ct, t st v than bn. Cc ng cong c im cc tr.

1.2. Tc nhn syQu trnh sy nhit bao gm chuyn m c trong lng vt liu di dng hi v loi b

hi m vo mi trng bn ngoi. Qu trnh phn t m nhn c nhit lng t hn hpkhng kh m trn ln vi sn phm chy ca nhin liu (tc nhn sy) dch chuyn t trong


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vt th ra b mt v t b mt vo khng gian tc nhn sy bao quanh thi vo mi trng.Nh vy khng kh cng mt lc lm nhim v truyn nhit v truyn m.

Thit b sy gm hai phn t chnh: calorife 1 v bung sy 2. Trong calorife khng kh

c hm nng khi khng thay i hm lng m. Trong bung vt liu nhn nhit, khng khb ngui i, ng thi tip nhn m thot ra t vt liu m. Khi hm lng m ca khng khtng ln, kh nng sy s gim. Do tc nhn sy cn c ci thin, a vo kh chm nng mi, kh hn. lm nhim v ny ta dng qut 3.

Trong qu trnh sy, trng thi ca tc nhn sy lin tc thay i. Khng kh m l hnhp ca khng kh kh vi hi nc. Gn ng c th coi khng kh kh nh l kh thun nht

vi khi lng phn t hiu dng K 29.Khng kh kh l hn hp ca xy v nit v mt s cht kh khc nh CO2, SO2 , ...

cc kh ny c thnh phn khng ng k c th b qua. Thnh phn xy v nit trong khngkh kh tng ng bng 21% v 79% theo th tch hoc 23,3% v 76,7% theo khi lng.

Bng 1.2. Hng s kh v khi lng phn t ca kh.Kh Ri (Kg.m/Kg

0C) Ri (J/Kg0K) K Kh Ri (Kg.m/Kg

0C) Ri (J/Kg0K) K

CO2 19,30 190 44 CO 30,30 298 28N2 30,30 298 28 Kh kh 29,30 287 29O2 26,50 260 32 Hi nc 47,10 462 18SO2 13,25 130 641.2.1. Thng s c bn ca khng kh m.

Ph hp vi nh lut an-Tng, p sut ca hn hp kh, vi th tch xc nh bngtng p sut hi ca tt c cc thnh phn.

1 2 3 ... nB p p p p= + + + +

Trong : B - p sut chung ca hn hp kh.p1, p2, ...pn - p sut hi ca cc kh thnh phn.

Tng ng vi p sut phong v biu ca khng kh m: KK hB p p= +

y: pKK - p sut ring phn ca khng kh kh.ph - p sut ring phn ca hi nc cha trong khng kh.

Bng 1.3. S ph thuc ca p sut hi bo ho ph v nhit ca n.

t,0C

PH,mmHg

pH,Pa

t,0C

PH,mmHg

pH,Pa

t,0C

PH,mmHg

pH,Pa

t,0C

PH,mmHg

pH,Pa

t,0C

PH,mmHg

pH,Pa

-20 0,722 96,5 32 35,66 4,86 58 136,1 18,2 84 416,8 55,7-19 0,850 113,0 7 7,51 1,02 33 37,73 5,1 59 142,6 19,05 85 433,6 57,6-18 0,935 124,0 8 8,05 1,07 34 39,90 5,33 60 149,4 19,92 86 450,9 60,2-17 1,027 137,0 9 8,61 1,15 35 42,18 5,63 61 156,4 20,84 87 468,7 62,4-16 1,128 151,0 10 9,21 1,23 36 44,56 5,95 62 163,8 21,81 88 487,1 65,0-15 1,238 164,1 11 9,84 1,31 37 47,07 6,27 63 171,4 22,81 89 506,1 67,5-14 1,357 181,0 12 10,52 1,40 38 49,69 6,63 64 179,3 23,86 90 525,8 71,0-13 1,486 199,2 13 11,23 1,5 39 52,44 6,99 65 187,5 25,01 91 546,1 72,7-12 1,627 218,3 14 11,99 1,59 40 55,32 7,36 66 196,1 26,2 92 567,0 75,7-11 1,780 238,1 15 12,79 1,71 41 58,34 7,76 67 205,0 27,38 93 588,6 78,4


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-10 1,946 260,0 16 13,63 1,82 42 61,50 8,2 68 214,2 28,31 94 610,9 82,5-9 2,125 285,1 17 14,53 1,94 43 64,80 8,64 69 223,7 29,8 95 633,9 84,5-8 2,321 311,2 18 15,48 2,06 44 68,26 9,1 70 233,7 31,1 96 657,6 87,7-7 2,532 338,0 19 16,48 2,19 45 71,88 9,57 71 243,9 32,45 97 682,1 91,0

-6 2,761 369,0 20 17,54 2,34 46 75,65 10,1 72 254,6 34,1 98 707,3 94,3-5 3,008 400,0 21 18,65 2,49 47 79,60 10,62 73 265,7 35,45 99 733,2 97,7-4 3,276 437,0 22 19,33 2,64 48 83,71 11,15 74 277,2 37,0 100 760,0 101,3-3 3,566 476,2 23 21,07 2,82 49 88,02 11,75 75 289,1 38,45-2 3,879 509,0 24 22,38 2,98 50 92,51 12,35 76 301,4 40,17-1 4,216 560,0 25 23,76 3,17 51 97,20 12,99 77 314,1 41,90 4,579 612,0 26 25,21 3,36 52 102,1 13,61 78 327,3 43,61 4,93 660,2 27 26,74 3,56 53 107,2 14,4 79 341,0 45,52 5,29 706,2 28 28,35 3,78 54 112,5 15,01 80 355,1 47,43 5,69 760,2 29 30,04 4,00 55 118,0 15,75 81 369,7 49,44 6,10 813,4 30 31,82 4,24 56 123,8 16,38 82 384,9 51,495 6,54 875,4 31 33,70 4,46 57 129,8 17,3 83 400,6 53,46 7,01 936,5

Bng trn cho ta p sut hi bo ho ph thuc vo nhit .Theo nh lut an-Tn thy rng p sut ring phn ca hi trong khng kh m phnh hn p sut chung ca hn hp B. Tuy nhin v nhit hi bng nhit hn hp, rrng hi trong khng kh m khng bo ho, c nhit vt qu nhit si ca nctrong khi p sut ring phn ca n cho, ngha l n trng thi qu mc.

p sut phong v biu v p sut ring phn ca hi l nhng thng s c trng trngthi khng kh m. Ngoi nhng thng s ny, trng thi ca khng kh m c c trng bihng lot cc thng s khc: m tuyt i, m tng i, hm lng m, mt , th tchring, nhit v entanpi. Khi bit ba thng s ca khng kh m, c th tm c cc gi trthng s cn li ca n khi s dng cc quan h sau:

- m tuyt i ca khng kha l khi lng ca hi nc cha trong 1m3

khngkh m (g/m3), ng thi a cng l mt ca hi nc cha trong hn hp.

- m tng i ca khng kh l t s gia m tuyt i vi khi lng hi nc

cc i c th (max) cha trong 1m3 khng kh m c cng mt iu kin (nhit v p sut).

a

max

= hoc 100%a

max

=

m tng i l thng s quan trng ca khng kh m. ln ca n c trng

cho kh nng khng kh c bo ho bi m. Gi tr cng nh trong iu kin n ginbng nhau, kh nng sy ca khng kh cng ln.

c tnh ph thuc ca vo nhit , khc nhau: Nu nhit ca khng kh m t, thp hn nhit si ca nc t K p sut phong

v biu cho, ngha l khi t < tK (khi B = 760mmHg, tK = 1000C, khi B = 745mmHg,

tK = 99,40C), ma x l mt hi bo ho h nhit cho, ngha l ma x = a

khi :

a

h

=


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Gi tr ca a trnh by trong bng hi nc. i vi tnh ton k thut c th dng

cng thc thc nghim ca .K.Philnhenc:

6,9

lg 0, 686 230ht

t = + + (1.13)

y: a tnh bng (g/m3).Tnh gn ng c th dng phng trnh c tnh ca kh l tng, i vi hi nc c th vit:

;a a h h

a h

RT RT p p

= = (1.14)

y: a - phn t lng ca hi nc, a = 18.pa - p sut hi bo ho nhit cho (Bng 1.5)

R - hng s ca cht kh, R = 8314,3J/Kmol.0K.

Gi tr bng s ca ln R bng cng dn 1Kg - phn t (Kmol) kh p sut khng

i v nhit tng ln 1 .

T phng trnh (1.14) c:

a a

h h

p

p

= = (1.15)

ln ph ph thuc vo nhit t, nng cao nhit , p sut hi bo ho tng

(Bng1.5), chnh v th khi hm lng m khng i ca kh tng ng vi gim gi tr

ca . Khi hm nng khng kh trong calorife kh nng sy ca n tng ln, ngc li khi lm

lnh khng kh ph s gim xung; m tng i khi hm lng m khng i s nng cao,

t c ph = pa (100%). Khng kh tr thnh bo ho v bt u c ngng t hi nc.

Trong khong nhit t = 00C 1000C, ph (mmHg) c th tnh theo cng thc thcnghim ca Philnhenc:

7,5lg 0,622

238h

tp

t= +

+(1.16)

Nu nhit ca khng kh m cao hn tK p sut cho th bo ho hon ton c

th ch khi khng kh kh khng tuyt i v p sut hi bo ho bng p sut phong v biu

ph = B, ta c:

ap

B=

1.2.2. Lng cha m.Lng cha m (hm lng m), l khi lng hi nc cha trong 1kg khng kh kh.

Thng s ny c dng ph bin tnh thit b sy.

a

K

Gd

G= Kg m/Kg kk.


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hoc 1000 a

K

Gd

G= g m/Kg kk.

S dng phng trnh trng thi i vi hi nc v khng kh kh ta c:

aa a

a

KK K

K

R Tp V G

R Tp V G

=

=

(1.17)

y:Ra = 462J/Kg.

0K, RK = 287J/Kg.0K. Thay p sut ca hn hp khng kh kh v hi

nc p bng k hiu p sut kh tri (p sut phong v biu) B ta c:

622 a

a

pd

B p

=

(g m/Kg kk) hoc 0,622 a

a

pd

B p

=

(Kg m/Kg kk).

Nh vy, bit gi tr p sut ring phn trong khng kh, c th xc nh c lng

cha m ca n, ngc li theo lng cha m c th xc nh c p sut ring phn ca hi.

1.2.3. Mt khng kh m (hn hp khng kh kh v hi nc)

hh K a = +

y: a, K l mt hi nc v khng kh kh.Trong iu kin bnh thng (p sut pK = B), t = 273

0K) K = 1,293Kg/m3. S dng

phng trnh trng thi ca kh, c th vit:

1,293 273

K

K

p B

T

=

Suy ra:( )

1,293 273

273K

t B

=

+

Bit K ap B p= v 622a

a

pd

B p=

Ta c:1, 293 273

1 0,622273

a ahh

a

B p p

t B B p

= +

+

Hoc3

1,293 2730,622

273

1,293 2731 0,378 ( / )

273

a ahh

a

a

B p p

t B B p

pKg m

t B

= + +

=

+

(1.18)

Cng thc trn cho thy, mt khng kh m ph thuc vo hai thng s thay i

trong qu trnh sy: nhit v p sut ring phn ca hi nc p a. Khi tng pa trong qu trnh


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sy, hh s gim bi v hi b thay th bi khng kh kh. Tuy nhin s gim nhit ca

khng kh ko theo qu trnh sy i lu, dn ti tng hh.

1.2.4. Nhit dung ring trung bnh ca khng kh m.

Nhit dung ring trung bnh ca khng kh m khi p sut khng i ng vi 1Kg khm, ngha l 1kg hn hp, coi nh ln trung bnh gia nhit dung kh kh v hi (KJ/Kg m 0K)

1

K K a a K ahh

K a

G C G C C x C C

G G x

+ + = =

+ +(1.19).

y:100

a

K

Gdx

G= =

CK - nhit dung ring trung bnh ca khng kh kh; khi t 2000C

ta c CK 0,24 Kcal/Kg0C 1,004KJ/Kg.0K. Khi t > 2000C cn tnh CK ph thuc vo nhit .

Ca - Nhit dung ring ca hi nc. Ca 0,44 Kcal/Kg

0

C 1,842 KJ/Kg

0

K.c tnh nhit - vt l ca trng thi kh m trong k thut sy thng s dng entanpi.

Thng thng entanpi ring ca khng kh m i vi 1Kg khng kh kh c xc nh nh

l tng entanpi ring 1kg khng kh kh (Ik) v d gam hi m. Do entanpi ca khng kh m l:

1000 1000hh K a K a

d dI I I C t I = + = + (1.20)

y: t - nhit ca hn hpIa - Entanpi ring ca hi m trong hn hp nhit v p sut ring phn cho.

Trong bung sy khng kh khng trng thi bo ho. Hi nc c hm nng trong

hn hp cng vi khng kh. Entanpi ca n ph thuc vo entanpi ca nc lng, nhit ho

hi v nhit hm nng.Entanpi ca nc nhit t

l lI C t =

y: Cl - nhit dung ring ca nc lng (KJ/Kg.0K)Nu entanpi ca nc lng t = 00C bng khng, th entanpi ca hi qu nhit xc nh

theo cng thc:

( )a l K a K I C t r C t t = + + (1.21)

y: tK - Nhit si ca nc lng ph thuc vo p sut.

r - nhit ring ho hi cng ph thuc vo p sutCa - nhit dung ca hi qu nhit p sut khng i.

Bi v trong khng gian thit b sy p sut ring phn ca hi t thay i, do entanpi

ca hi thc t ch ph thuc vo nhit t, khng ph thuc vo p sut. R rng iu ny c

th khi Cl = const, Ca = const.

Khi Cl.tK + r - Ca.tK = const =r0 (r0 - l nhit ho hi tK = 00C)

Nhit ring ho hi:


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( )0 l a Kr r C C t = (1.22)

Nu r0 = 597, Cl 1, Ca 0,44, ta nhn c cng thc gn ng s ph thuc ca r vo t:

597 0,56r t= Kcal/Kg = 4,186(597 - 0,56t) KJ/Kg. ln r ph thuc vo t cho trong bng.

Thay r vo biu thc tnh Ia, ta c cng thc gn ng tnh entanpi ca hi qu nhit:

0a aI r C t = +

r0 = 597Kcal/kg = 2500KJ/Kg; Ca = 0,44Kcal/Kg0C = 1,842KJ/Kg0K.

Tng t c th nhn c cng thc gn ng xc nh entanpi ca hi bo ho:

h l KI C t r = +

Thay gi tr ca r vo cng thc trn ta c entanpi ca hi bo ho (Kcal/kg).

597 0,56 597 0,44h lI C t t t = + = +

T phng trnh (103 - 105) thy rng, gi tr gn ng entanpi ca hi khng bo hoqu nhit bng entanpi ca hi bo ho cng nhit (nu p sut ring phn ca hi khng

qu 0,1Mpa), do tnh ton entanpi ca hi c th ly trc tip t bng (I25) khi nhit

ti 1000C. Nh vy thay vo cng thc (99) gi tr ca IK v Ia, ta thu c entanpi ca khng

kh m.

( )0, 24 597 0,44 / 1000

hh

dI t t Kcal Kgkk = + +

hoc ( )1,004 2500 1,842 / 1000

hh

dI t t KJ Kgkk = + +

Nu ln Ih tnh theo (105) so vi gi tr trn bng (25) cho thy, cng t = 100

0

C csai khc khong 1,9Kcal/Kg (7,9KJ/Kg) hoc gn 0,4%. t = 500C gi tr ca entanpi nh nhau.1.2.5. Nhit ca nhit k t.

Trong l thuyt qu trnh sy mt trong nhng thng s c trng cho trng thi ca

khng kh m, l nhit ca nhit k t. S bay hi ca nc vo khng kh trong iu kin

on nhit nn nhit lng cn bay hi ly ngay t khng kh. Lp khng kh st b mt

bay hi mt i mt nhit lng ng bng nhit lng bay hi ca nc, do nhit lp

khng kh st b mt bc hi gim so vi khng kh xa b mt bay hi. Ngi ta gi nhit

lp khng kh st b mt bay hi l nhit nhit k t t v xa b mt bay hi l nhit

nhit k kh. o nhit khng kh ta dng nhit k thng thng (thu ngn hoc ru).

o nhit nhit k t ta vn dng cc loi nhit k trn, nhng bc bu thu ngn hoc rubng bng lun thm nc nh mao dn t mt cc nc.

Nh vy, nhit ca nhit k t l thng s nhit ng, c trng cho kh nng

khng kh nhng nhit bc hi nc ti khi khng kh bo ho hon ton.

S sai khc gia nhit nhit k kh v nhit k t c trng cho kh nng nhn m

ca khng kh, trong k thut sy ta gi l th sy .


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t t =

Trong : t - nhit khng kh, hoc nhit ca nhit k kh thng thng.

t - nhit ca nhit k t (khng kh bo ho on nhit)

Khi khng kh bo ho hon ton ( = 100%), t = t v = 0.

1.2.6. Xc nh m tng i ca khng kh.Khi bit hiu s (t - t) c th xc nh m khng kh. Gi s q1 l nhit lng m

khng kh cung cp cho bu thu ngn ca nhit k t v q 2 l nhit lng m nc quanh

bu thu ngn tiu th bay hi. R rng ta c:

q1 = q2

Theo phng trnh truyn nhit ta c:

q1 = (t - t)

a vo khi nim bay hi qm (lng m tch ra t 1 n v din tch trong 1 n v thi

gian (Kg/m2.s) ta c:q2 = qm.r.

Trong : r - nhit m ho hi (J/Kg).

Thay gi tr q1, q2 vo cng thc ta c:

( ) mmq r

t t q r t t

= =

Cng bay hi gn ng c th xc nh theo cng thc ca an-tng nh i vi

bc hi nc b mt t do.

760m m

q p

B

= (1.23)

Trong :

- h s bc hi (hoc h s trao i m).

B - p sut phong v biu.

p - hiu p sut ring phn ca hi trn b mt nc v khng kh bao quanh.

ap p p =

p - p sut hi bo ho nhit nhit k t.

Phng trnh cn bng nhit c dng:

( ) ( )760

m at t p p r

B

=

Suy ra:

( ) ( )760

a

m

p p B t t r

=

(1.24)

t760m

Ar

=

(h s m k)

Thc nghim cho thy khi tc khng kh v < 0,5 m/s th A = 66.10-4.


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( )ap p A B t t =

Suy ra m tng i ca khng kh:

a

h

p

p=

ph - p sut hi bo ho khi nhit khng kh o bng nhit k kh.

Sau khi bin i ta c:

( )h h

p ABt t

p p=

p - p sut hi bo ho t.

Khi v 0,5m/s h s A xc nh theo cng thc thc nghim:

56,7565 .10A

v

= +

Cc gi tr v A thit lp thnh bng hoc ton dng trong thc t. m

khng kh l hm ca nhit khng kh (nhit nhit k kho t) v hiu (t - t ) vi tc xc

nh ca khng kh.

1.2.7. th I - d ca khng kh m.Trong k thut sy, khng kh m c c trng bi ba thng s sau:

- Nhit ca nhit k kh t.

- m tng i (%).

- p sut kh tri B (p sut phong v biu).

Ngi ta thng a vo th sy . Cc thng s cn li ca khng kh m c th tnhtheo cc cng thc trn.

a/ Nguyn tc xy dng th I - d.

th c xy dng i vi p sut kh tri xc nh (B = 745mmHg = 99,3 KPa 1Bar).

Trn trc tung t entanpi (Kcal/Kgkk hoc KJ/Kgkk) theo t l xc nh.

Trc honh nghing mt gc 1350 so vi trc tung. Nh vy ng I = const song

song vi trc honh v to vi trc tung mt gc 1350. ng d = const song song vi trc

tung. thun tin tnh ton hm lng m, ta xy dng ng nm ngang - trc ph d.

ng c nhit khng i (ng ng nhit) l s ph thuc I vo d khi t = const,

l nhng ng gn song song ( vng c nhit thp) trong h to I - d, nghing cngln khi nhit cng cao.

597 0,441000 1000

K

d dI C t t = + + Kcal/Kg kk (1.25)

1,004 2,5 0,001842I t d d t = + + KJ/Kg kk

T phng trnh ta thy I l tng ca ba i lng, c th trnh by trn th dng ba

on tng ng. d quan st, xy dng hai ng c tnh ng nhit.


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- ng nhit t = 0.

Khi t = 0 v d = 0 th I0 = 0 ( th bt u t im 0)

Khi t = 0 v d = d1, 1

1 1000

r dI

=

tm ng I1, k ng d1 = const xut pht t ng I = 0 (im a). t trn ng

thng gc t a mt on ab tng ng theo t l xch bng 11000

dr . Ni O v b, ta c ng

ng nhit t = 0.

- ng nhit t1 = const.

Khi t = t1 v d = 0, th I0 = CK.t1, trn th l on Oe ng vi ng d = 0 (trc tung).Khi t = t1 v d = d1

1 11 1 1

0,44

1000 1000

K

d dI C t r t = + +

Trn ng d1 = const t on bc bng Oe (CK.t1) v on ef bng1

10,441000

dt .

ng ng nhit t1 = const i qua im e v f.

T phng trnh c bn thy rng, khi tng t, gc nghing ca ng ng nhit tng

ln, do tng on cf 1 10,441000

dt

. Tuy nhin ln on cf tng khng ng k so vi cc

thnh phn khc ca phng trnh. Gi tr ca I (KJ/Kg kk) trn trc tung, theo gi tr bng s

bng on t tng ng; thc t khi d = 0 th I0 = CK.t1 = 1,004t1.

ng c m tng i = const.

EntanpiI,KJ/kgkkhocKcal/Kgk

m lng m d kg m/kgkk (trc ph)


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Hnh 1.9. Xy dng cc ng chnh trn th I-d.ng ny c xy dng theo cng thc:

622 622a h

a h

p pd

B p B p

= =

(1.26)

Khi = const, khi tng d cn tng ph, ngha l t. Do ng = const, l lin kt cc

im giao ct gia d v t lin tip. ng = const tn ti khi khng kh c nhit v p sut

hi bng p sut phong v biu (tK = 99,40

C khi p sut bng 745mmHg hoc 99,3KPa). Khinhit vt qu tK = 99,4

0C, ng = const tr thnh ng thng (song song vi trc tung),ngha l d = const.

Thc t t phng trnh: a a

h h

p

p

= = cho thy, ti nhit 99,40C, khi tng t vi d = const,

ngha l pa v gim do t ng ph. Khi t > 99,40C, hn hp trng thi hi v a

p

B= . Bi v th I-d

xy dng vi B = const, th khi thay i t (pa = const) khng nh hng ti , do khi xc

nh d (ngha l pa), const.

ng = 100%.ng ny chia din tch th lm hai phn.- Phn trn i vi khng kh m l hn hp khng kh kh v hi nc khng bo ho.- Phn di hi nc bo ho c phn ngng t thnh nc. im sng trn th,

ng t = const i qua im ct, ng tng ng d = const vi ng = 100%. Tip tc lmlnh khng kh bo ho xy ra ngng t tng phn hi nc, kt qu l hm lng m ca

khng kh gim v qu trnh din ra theo ng = 100%.

p,(p

a,mmHg)


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ng p sut ring phn ca hi nc.

ng ny xy dng theo phng trnh:

. 622a

d

p B d= + (Thng d 0 th I2 > I1.

Trong qu trnh bc hi hm lng m ca khng kh d s tng v khi bo ho honton ( = 100%), hm lng m ca khng kh ng vi t l d. Tuy nhin, vic tng thm hm

lng m ca khng kh s tng ng vi lng nc bc hi:

1 21000

n

d dI I C t

= . (1.28)

Theo phng trnh trn, ta xy dng ng t trn th I - d.


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Hnh 1.10. Xy dng ng t= const v = const trn th I - d.

Ti im B bt k trn ng = 100%, xc nh t bng cch v ng I2 = const. Ct

trc tung, ngha l vi ng d = 0 (im C). T C h xung mt on1000

n

dCD C t = .

im D c th xc nh I1. ng ni D v B l ng cn tm t = const. Bi v khi = 100%,

t = t (nhit khng kh mi trng), r rng gi tr ca tng vi gi tr ng nhit t = const i

qua im B.

Trn th ng t = const l ng chm chm. Khi bit t v t c th tm trn th

gi tr tng ng trn ng = const, i qua giao im ca ng t = const, v t = const.

Th d trn hnh khi t = 400C v t = 300C th 50%.

Khi dng ng t v t trn th I - d, n cng a vo ng th sy = t = t =const. Trn th ng = 100C (im A, A1, A2) v = 50

0C (im C1C2). ng = 0 l

ng = 100%. Th sy c trng cho kh nng sy ca khng kh, v n xc nh tc

bc hi ca nc ph thuc nhit v hm lng m ca khng kh.

S dng th I - d khi p sut kh tri khc nhau.


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th I - d xy dng vi p sut kh tri B = 745mmHg = 99,3KPa; ta c th tnh

lng m bo ho trong 1Kg khng kh cng nhit , nhng p sut kh tri khc nhau.

622 622a h

a h

p p

d B p B p

= =

Khi = 1, hm lng m s l:

622 hh

h

pd

B p=

Khi t = const, ph = const th1

hd f

B

=

ngha l vi 1Kg khng kh kh p sut khc nhau,

s c khi lng hi m khc nhau. T cho thy, m tng i ca khng kh m phthuc vo p sut kh tri.

Nh vy khi c sai lch ng k i vi B = 745mmHg, cn phi c hiu chnh ng

= const trn th I - d. Hnh di cho ta ng khng kh bo ho = 100% vi cc p

sut kh tri khc nhau. Xy dng ng = const trn th I - d c th trnh by di dng:

622

1h

dB

p

=

T suy ra, m khng kh c trng bi ng = const v trn th vi p sut

khc nhau, c xc nh bi t sB

(d = const, t = const) cng khng i (

Bconst

= ). V

d trn th I - d, i vi ng khng kh bo ho 745 = 100%.

7457,45

100

B

= =

i vi p sut 700mmHg, s tng ng:

700

70094%

7,45 =

i vi p sut khc nhau, ng c th xc nh theo cng thc:

745

745

745 745B

B B

= =

Hoc 99,399,3

B

B =

Trong : B - p sut kh tri cho (mmHg hoc KPa).

745, 99,3 - gi tr ng trn th tnh ton xy dng vi p sut 745mmHghoc 99,3KPa.


30/145


Hnh 1.11. V tr ng khng kh bo ho trn th I - d vi p sut kh tri khc nhau.1.2.8. Khi l.

Khi l l tc nhn sy ph bin, va trc tip cung cp nhit cho vt liu sy, vamang m thi ra mi trng (sy trc tip) hoc l ngun cung cp nhit gin tip hm nng

tc nhn sy (trong calorifer kh - khi). Nh vy nu bit entanpi, lng cha m d, mtng i v nhit t ca n ta c th coi khi l l mt dng khng kh m tng ngv c th s dng th I - d ni trn.

Sy trc tip, khng kh trn vi khi l, thit b sy n gin, gim c chi ph innng v gim c sc cn ca khng kh. Tuy nhin s dng trc tip sn phm chy canhin liu lm tc nhn sy, phi lc ton b bi, tro, v chng lm gim cht lng sn phm sy.

Khi nhin liu chy, trong khi l cha mt lng ng k lu hunh v hp cht calu hunh c tnh c hi. Do khng c sy trc tip i vi cc sn phm c tnh hpph cao.

Tnh ton nhit qu trnh chy, cn xc nh khi lng sn phm chy, hm lng m

v entanpi. Xc nh cht lng nhin liu chy trong l v iu kin chy ca n. Cht lngnhin liu rn v lng c trng bi thnh phn ca chng trong nhin liu.

C + H + O + N + S + W + A 100%.

y: C (cacbon), H (hydr), O (xy), N (Nit), S (sunfua), W (nc), A (tro).Nhit tr cao ca nhin liu xc nh theo cng thc ca D-. Mendlep.

QC = 81C + 300H - 26 (O - S) Kcal/Kg.hoc QC = 419.10

-5[81C + 300H - 26(O - S)] MJ/Kg.(1.29)

mmHG

mmHG

mmHG


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Khi lng khng kh l thuyt cn chy 1Kg nhin liu, tnh theo:

0

2,67 8

23

C H S Og

+ + = Kg/Kg nhin liu (1.30)

trnh tn tht do chy khng hon ton trong l, cn phi a tha khng kh vol. Do khi lng khng kh thc t a vo l cho 1Kg nhin liu chy l g t, cn ln hn g0

l thuyt. T s gt/g0 = gi l h s tha khng kh. Khi khi lng sn phm kh to thnh,khi t chy 1Kg nhin liu.

01 1100 100

r t

A Ag g g= + = +

Lng nc cha trong khi l: 0 09

100 1000

g dH Wg

+= + Kg/Kg nhin liu

y: d0 - hm lng m khng kh mi trng bn ngoi a vo l t.Hm lng m ca sn phm chy:0 0

0

90 101000

91

100 100

r

K

H W g d gd

A H W gg

+ += =

++

g/Kg kk.

hoc

( ) ( )0 0

0

9000 1000 100

100 1 9r

H W g d d

g A H W

+ +=

+ + +g/Kg kk (1.31)

Khi lng kh l: 09

1100 100

K

A H W g g

+= + (1.32)

c th s dng cng thc ny, ngoi hm lng (%) cc thnh phn ca nhin liu(H,W, A) cn bit h s tha khng kh.

Chng l mt phn ng ho hc, tc v s chy hon thin ph thuc vo nhit .Nhit cng cao, phn ng chy cng nhanh v hon ton. Tuy nhin nhit cao qu cngkhng cho php v c th lm nng chy tro v to x, dn ti ph hoi ch chy. Nhit

chy trong l, cung cp cho thit b sy dng kh nng, l 900 10000C. nhit ny, nhinliu chy hon ton v trit , tro ca nhin liu khng b nng chy; nhng vi kh nhit cao khng th a vo bung sy i vi a s cc sn phm. Ngi ta thng b tr gia l vbung sy thit b lm sch kh khi tro v m hng. Kh l v khng kh s ho trn trongbung hn hp. Nhit ca hn hp cn t gi tr t c xc nh bi ch sy.

tnh ton thit b sy dng kh nng cn phi bit hai h s tha khng kh: mt

trong l l, bo m nhit cho php ca n v mt h s bung sy . Nhng h s nyc gi tr th no nhit hn hp a vo bung sy t c gi tr chnh xc i vi ch nhit yu cu t.

Nhit trong l:( )0 1

h ll

l pc

Qt

g C

=

+ (1.33)


32/145


y: l - hiu sut ca l, tnh ti tn tht c v ho chy khng hon ton, cng nh tntht qua thnh l vo mi trng. l = 0,85 0,95.

Cpc - nhit dung trung bnh ng p ca sn phm chy.

Nhit tr thp ca nhin liu:Qt = Qc - 6(9H + W) Kcal/Kg.

hocQt = Qc - 0,223H - 0,025W MJ/Kg (1.34)

tnh h s tha khng khl, cn bit h s tha khng kh ca hn hp a votrong bung sy.

T iu kin cn bng nhit qu trnh chy trong l v hn hp kh l trong bung hnhp, c th xc nh h s tha khng kh trong hn hp.

( )

( ) ( )00 0 0

9 91

100 100

1000

c T r hr Kr r

hr h Kr h

H W H W AQ C t I C t

dg I I C t t

+ + + +

= +

(1.35)

dy: CT - nhit dung ca nhin liu bng 2,09KJ/Kg 0K (0,5Kcal/Kg 0C).Ihr = 2500 + 1,842tK (KJ/Kg) = 597 + 0,44t (Kcal/Kg). Entanpi ca hi nhit hn

hp a vo bung sy.CKr - nhit dung ca hn hp kh kh, tng t nhit dung ca khng kh: Nu nhit

ca hn hp ny khng qu 2000C, th nhit dung ca n c th ly bng 1,004KJ/Kg 0K(0,24 Kcal/Kg 0C).

Ih0 - entanpi ca hi cha trong khng kh ngoi tri, nhit t 0.

Sau khi xc nh , d dng tnh c hm lng m ca hn hp a vo bung d rtheo cng thc trn.

Entanpi ca hn hp kh, a vo bung I r xc nh theo cng thc (Kcal/Kg kk)hoc KJ/Kg kk).

0 0C l T r r

K

Q C t g I I

g

+ + = (1.36)

Khi bit hm lng m ca hn hp kh dr, entanpi ca hn hp kh Ir c th tnh theocng thc bit:

( )0,24 597 0, 441000 1000

r rr K r hr K r

d dI C t I t t = + + + Kcal/Kh kk.

Hoc

( )1, 004 2500 1,8421000

rr r r

dI t t + + KJ/Kg kk.

Hng s kh ca sn phm chy:

2 2 2 2 2 2 2 2

0

91

100

CO CO SO SO O O N N

r

g R g R g R g RR

A H W g

+ + + =

+ + +

(1.37)


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Khi lng cc thnh phn ring ca sn phm chy 1Kg nhin liu xc nh theo:

2COg

2SOg

2Og

2Ng

3,67100

C

2 100

S

( )

0

0,23 1 g 0

0,77 g

S thay i trng thi ca hn hp kh trong qu trnh sy d dng xc nh trn th I - dtng t nh i vi khng kh.

1.3. Truyn nhit v truyn m trong qu trnh sy1.3.1. Truyn m t b mt vt liu vo mi trng.

Qu trnh sy l qu trnh vt liu nhn nhit nng t mt ngun nhit chuyn m ttrong lng vt liu ra mt vt liu, sau i vo mi trng thng qua tc nhn sy. Nh vyqu trnh truyn nhit v truyn cht xy ra ng thi.

u tin, kho st truyn m bn ngoi, ngha l trao i m gia b mt vt liu v

mi trng ca bung sy. Tc nhn sy nht thit phi chuyn ng tng i so vi b mtca vt liu, v to thnh lp bin, c nh hng ln ti qu trnh sy. Trong trng hp ny ttc cc thng s c bn ca kh chuyn ng trong lp bin, khc vi cc thng s trng thitng ng ca kh trong bung sy. iu ny lm xut hin sc cn ph ca qu trnh truynnhit v m. Ngha l lp gii hn l yu t cn tr qu trnh sy. Nghin cu qu trnh ny cnhiu kh khn, khi cn xc nh nhit , m v tc kh. Nhng nghin cu gn ycho thy, trng nhit ca lp bin cng ging nh trng m v trng p sut ringphn. Hnh di cho s phn b nhit v p sut ring phn trong lp bin khi sy vt

liu m (n t) theo s liu thc nghim ca H..cutrap.

Hnh 1.12. S phn b nhit (ng cong 1) v p sut ring phn (ng cong 2) tronglp bin trn b mt vt liu m qu trnh sy i lu.

Khongcchivibmtvtliu

Nhit p sut ring ca hi

mmHg


34/145


Trn (Hnh 1.13) (S liu ca .M.Smlski) ng cong phn b nhit trong lpbin i vi tm kh (khng bc hi) v tm t (trao i nhit gy kh cho trao i m). Tnhng ng cong ny cho thy trao i khi mnh hn trao i nhit, do nhit tm ttng mnh hn. Tng hm lng m ca kh trong lp bin s nng cao s dn nhit ca n, do lm tng mt dng nhit.

Hnh 1.13.Phn b nhit trong lp bin trn b mt tm kh v tm t, khi tc dng khng kh v = 2m/s.

Ngoi ra, dng nhit ti b mt tng lm xut hin dng khng kh kh cng hng tib mt, kt qu gy ra s sai khc p sut ring phn ca hi b mt vt liu v trong kh,thun li cho tc sy tng. .D.Lbp nghin cu trao i nhit bn ngoi trong qu trnhi lu - hng knh thu c cc t s tnh ton quan trng khi sy cc vt liu khc nhau.

2 0,4

0,5Re

n

p

u

c

TT uN A

T T u

=

(1.38)

Trong :

TGu

T

=

- chun Gc - Man, tnh ti kh nng ng ho ca tc nhn sy.

pT

T

- Chun thng s, t s gia nhit pht Tp vi nhit trung bnh trong bung

sy T, ngha l nh hng ca ngun bc x n trao i nhit i lu.Gi tr h s A, n ph thuc vo loi vt liu.

Nhit

Tm kh

Tm t

Khong cch vi b mt tm

v = 2m/s


35/145


c

u

u- l t s gia hm lng m trung bnh ca vt liu thi im bt k vi hm

lng m chun trung bnh.Trng hp gradien nhit trong lp si khng ln hoc iu kin ng nhit th

truyn hi nhn c p sut ring phn. Khi cng bc hi khi b mt vt liu uthi k sy biu din bi phng trnh tng t phng trnh cng bc hi nc t b mtt do (cng thc an - Tn).

( )760

m m aq p p

B= Kg/m

2.h (1.39)

Trong :qm - cng bc hi (Kg/m

2.h)

m - h s trao i m khi sy vt liu m, dn ti sai khc p sut ring phn,

(Kg/m2

.h.mmHg).B - p sut kh tri.S bc hi m khi b mt, lm di chuyn m lp trong vt liu ra b mt, gy ra

gradien. H s trao i m ph thuc ch yu vo tc khng kh v cc yu t khc m(hnh dng v kch thc b mt bc hi, iu kin chy ca khng kh i vi b mt,nhit ). Dng tng qut trao i m gia vt liu v mi trng, c trng bng chuntrao i m Nu-xen.

m

mu

m

N l

= (1.40)

Trong :

m - h s trao i m (Kg/m2.h.mmHg)m - h s dn hi (tng t h s dn nhit) (Kg/m.mmHg)l - kch thc xc nh b mt bc hi (m).

Thay gi tr ca m vo ta c:

( )760

mu m

m a

Nq p p

l B

=

Chun m Nu xc nh theo:2

0 ,33 0 ,135Re Prm

n

u m

p

TN A Gu

T

=

(1.41)

Trong :Re - h s Rynn.Prm - chun trao i khi Prandt, c trng tnh cht vt l ca mi trng.Gu - chun Gcman.T v Tp - nhit mi trng v nhit b mt bc hi (nhit pht)

Khi Re = 3150 22.000 th A = 0,49 , n = 0,61.


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Khi Re = 22.000 315.000 th A = 0,025 , n = 0,90.1.3.2. Truyn nhit v truyn khi bn trong vt liu m.

Qui lut c bn truyn cht (m) c dng:

m m mq = (1.42)Trong :

qm - mt dng m (lng m G truyn trong 1 n v thi gian, qua mt n v b

mt ng ths.

m

dGq

d s=

(Kg/m2.h)

m - gradien th cht, ngha l th/m, c trng tc khng gian thay i th phptuyn vi b mt ng th.

m - h s t l, gi l h s dn m (Kg/m.h.n v th) theo nh lut Fourrier:

q = Gradien th truyn cht m, tng t gradien nhit , t l vi gradien p sut hi

(truyn khuch tn hi).

m mu

n u n

=

(1.43)

Trong :

m

u

- o hm ring ca th truyn theo hm lng m ring nhit khng i.

Nu s dng dung tch m th:

m

m

uC

=

.

Khi coi gi tr trn khng i ta c:

m mu C A= +

A - l hng s.

T y rt ra: mm m

u A

C C =

Ngha l khi nhit khng i v dung tch m ca vt liu khng i, th truyn ml mt hm tuyn tnh ca hm lng m. Thc t dung tch m l i lng thay i. Nu a

khi nim dung tch m trung bnh ca vt liu mC th:

m mu C =

T y ta c th vit s tng quan gia gradien th truyn v gradien hm lng mc dng:

1m

m

uC

= (1.44)


37/145


Biu thc quy lut truyn c dng:

mq uCm

=

uur

m

Hoc 0 00

mm

m

u a uC

= =

uuurqm

H s am gi l h s khuch tn:

.m

aC

=

H s khuch tn m ph thuc vo m v nhit ca vt liu.i vi ht la m, hm lng m v nhit ca n c dng (theo L-Kp).

Khi 0,283 0,360 / u Kg Kg=

9

21 .10

293

K

h tm Ta

A u B u C

= + +

(m2/s).

Khi 0,360 / u Kg Kg= 430

( ) 90,147 0,055 .10293

K

h t

m

Ta u

= +

(m2/s)

y: A, B, C, K h s ph thuc hm lng m ca vt liu.Khi 0,283 0,324 / u Kg Kg= A = 558, B = -382, C = 76,8

Khi 0,324 0,360 / u Kg Kg= A = 3620, B = -2340, C = 380,4

Tht - nhit ht (0K).Nu hng gradien m v gradien nhit trng nhau, th hng dng m tng ng l

dng m chung:

=+= 00 mmmmum auaqqq (1.45)

Trong :

muq - mt dng khuch tn m (dn m) (Kg/m2.h).

mq - mt dng khuch tn nhit ca m (dn nhit m) (Kg/m2.h).

- gradien nhit (0C/m).

- h s khuch tn nhit tng i vt liu m (Kg m/Kg kk0

C);m

m

a

a

= (am v am h s khuch tn v h s khuch tn nhit)

Biu thc ca dng nhit m qm c dng:

mm m

m

qC

= =


38/145


'mm mm

C

= = l h s dn nhit m.

H s'

mC

= l h s dn nhit m tng i.

trng thi n nh, khi qm = 0, L-Kp kho st h sp (h s gradien nhit) nhl thng s nhit ng hc l t s gia truyn hm lng m u vi truyn nhit .

0m

p

q

u

=

=

Lin quan gia h s tng i khuch tn nhit m , a vo phng trnh truyn m

(qm 0) v thng sp ng vi qm = 0, ln qm c xc nh. Nu qm = 0 th t biu thc (1.45)cho ta:

0 0m

mp

m q

q u u ua

=

= = = =

d dd d

Khi truyn nhit cn tr s truyn khi, nh lut dn nhit c dng:

mq Iq = +

Trong :q - mt tng dng nhit (W/m2).

- h s dn nhit vt liu m (W/m.0K)

- gradien nhit (0C/m)qm - mt tng dng m (Kg/m

2.s)

I - entanpi m dch chuyn bn trong vt liu (J/Kg).Trong phng trnh ny khng tnh ti s thay i entanpi m theo to vt liu.

1.3.3. Phng trnh vi phn truyn nhit.Trn c s nh lut bo ton nng lng, s thay i phn t entanpi th tch vt trong

1 n v thi gian bng s phn k ca vect q mt dng nhit.

( )( )

0qd

l ml

Cdi q I q

= =

r(1.46)

Nhit dung ring qui dn ca vt liu:

100qd K l K

WC C C u C = + = + (J/Kg kk 0K).

y: CK, Cl - nhit dung ring cht kh ca vt liu v lng.

100

Wu = - hm lng m ca vt liu.

Bi v entanpi ca lngl l

I C = nn:

( )0qd ml lC q C

=


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Thnh phn u phn phi ca phng trnh trn tnh truyn nhit bng dn nhit,thnh phn th hai truyn nhit bi dng cht lng.

Nu k hiu mll

l

q

=v (l - mt cht lng; vl - tc di dch chuyn ca cht lng m/h),

th ta nhn c phng trnh tng t Fua-ri-Kic-Xp.

( )0qd l l lC C

=

v (1.47)

Trong trng hp chung tnh ton truyn nhit, truyn hi cng nh ngun nhit. Ktqu l s bin i pha trong lng vt liu (ngng t hi, bc hi ca lng).

Truyn nhit bng dng nhit (khng bin i pha), tnh tng t truyn lng ngha l

thit lp Ca, a, va, . y : a , Ca l mt hi v nhit dung hima

a

q

=av tc di

truyn hi.Ngun nhit khi c bin i pha

0

ur

y: r - nhit chuyn pha, u

- s thay i hm lng m chung ca vt theo thi gian, -

chun bin i pha.Chun ny c trng s thay i dng m hoc hm lng m ca vt liu do bin i

pha (bc hi bn trong hoc ngng t) i vi s thay i dng m chung bn trong vt hocs thay i hm lng m ca vt.

Chun thay i t 0 (khng bin i pha) n 1 (dng m gy bin i pha v truyn hi).

Trng hp chung = f(u,).

Hnh 1.14. S ph thuc ca chun bin i pha vo m ca vt liu.

C

hunbinipha

m %


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Chun ph thuc to vt, ngha l ln ca n khc nhau i vi cc lp vt liu

khc nhau. Theo M..Cnin, khi tng m vt liu, gim t ngt. Khi m ln th 0,

ngha l m c di chuyn trong vt liu dng lng. Nh vy truyn nhit trong vt liu m b cn

tr bi truyn khi bn trong. Phng trnh chung c dng:

( )0 0qd l l l a a au

C r C C

= +

v v (1.48)

Nu = 0 th cui cng, phng trnh c dng:

2

0 0

l l a al a

qd qd qd

C Cr ua

C C C

= +

v v (1.49)

Trng hp sy i lu, trong vt liu gradien nhit khng ng k, tc truynm nh (Re < 10), thng b qua truyn nhit bi dng cht lng v hi. Khi phng trnhvi phn truyn nhit c dng:

2

qd

r uaC

= +

(1.50)

1.3.4. Phng trnh vi phn truyn m. c c phng trnh ng cong sy, ngha l tnh c tc sy v tm trng

khng n nh ca hm lng m trong qu trnh sy, cn gii phng trnh vi phn truyn mbn trong vt liu. Sy l mt qu trnh khng n nh, ngha l m tch khi vt liu l hmca to v thi gian. Ni cch khc, m thay i theo tit din v thi gian.

Tng t phng trnh truyn nhit, phng trnh vi phn truyn m c dng:

( )( )mK mm VmK

udi q q q

= + +

y: qmK - Mt dng m khuch tn.qmm - mt truyn mol.qVmK - mt th tch dng cht gy ra chuyn pha.Nu b qua s truyn mol ca cht lng, cn tnh truyn mol ca hi, ta c phng

trnh truyn m tng t phng trnh vi phn truyn nhit.

2 2

m m

ua u a

= +

(1.51)

Phng trnh ny thit lp quan h gia s thay i m theo thi gian

u

v theo to

(x, y, z) hoc ni theo cch khc, m t tc gim mu

cc im bt k ca vt.

n gin ho, cho php coi am v khng i, khng ph thuc vo m. Do phng trnh trn tr thnh phng trnh vi phn tuyn tnh.

Trng hp sy i lu, khng kh c t nng (nhit khng cao v tc khng ln)trong tnh ton khng cn tnh qu trnh dn nhit - m. Giai on sy u gradien nhit khng


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c hoc nh. Giai on sy th hai, ln ca h s khng ng k, thng nhit sythp. Phng trnh vi phn truyn m khi sy vt liu dng tm, s l:

2

2m

u u

a x

= (1.52) gii bi ton ny cn cho iu kin bin v bit m cc im bt k thi

im ban u (iu kin ban u).

1.4. ng hc qu trnh sy1.4.1. ng cong sy.

ng cong sy c trng cho s thay i m trung bnh ca vt liu W trong thi gian .T th ta thy, giai on u ca qu

trnh, m vt liu gim theo ng cong,vt liu c hm nng nhanh. Thi gian ca

giai on ny ph thuc dy mu thnghim, sau m thay i theo qui lutng thng (on AK1). Giai on sy thnht gi l giai on tc sy khng i.

Ti im K1 (gi l m chun th nhtW K1), tc gim m bt u gim v btu giai on th hai ca qu trnh sy; tc sy gim. Cui qu trnh sy, ng cong sytim cn vi ng cn bng m, Wcb ng vi

ch sy cho. m cn bng, vic syHnh 1.15.ng cong sy

kt thc, tc sy bng khng. i vi vt liu mng, thi k t nng rt ngn, khng c

ng cong sy. i vi vt liu dy, c dn m nh, cng tch m khi b mt vt

liu (v d la m) trong thi k tc sy khng i c th khng c.

1.4.2. ng cong tc sy.

Tc sy l s thay i m ca vt liu trong 1 n v thi gian (dW/d %/h).

ng cong tc sy c xy dng bng phng php vi phn th theo ng cong sy:

Tc sy thi im cho c xc nh nh l tang gc nghing ca tip tuyn qua im

ng cong sy (Hnh 1.16) tng ng vi m xc nh ca vt liu. Phng php ny cha

t chnh xc y , c bit cui qu trnh sy khi ng cong sy tim cn vi ng m cn bng, vic v chnh xc ng cong sy tr nn kh khn. khc phc nhc

im ny ngi ta dng phng php o hm s v tnh ton tc sy trung bnh sau

khong thi gian trung gian khng ln.

Thi gian (h)

mvtliu

W%

B


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a/ b/Hnh 1.16. ng cong tc sy.

1 - Vt liu m xp mao dn c b mt ring bc hi ln: giy, cctng.

2 - Cng nh trn: vi, v mng; 3 - Vt liu m xp mao dn c b mt bc hi ring nh;

4 - 5 - 6. H phc tp, vt keo xp mao dn: ht lng thc.

hnh trn, a v b cho th d v ng cong sy ca vt liu khc nhau. u tin - giai

on hm nng - tc sy tng t 0 n gi tr cc i N. thi k tc khng i N = const. Sau

d (t im chun K1) bt u gim tc sy. Trong thi k gim tc c tnh ca ng cong

ph thuc vo cu trc ca vt liu, kch thc mu, dng lin kt m vi vt liu v c cu di

chuyn m. V vy ng cong sy ca vt liu khc nhau s khc nhau. Trn ng cong c

im chun th hai K2 tng ng vi m chun th hai.

1.4.3. ng cong nhit sy.

Trn (Hnh 1.17) trnh by ng cong nhit sy vi mu vt liu mng v dy khi

sy i lu. Giai on u hm nng, nhit b mt vt liu tng cao, nhanh chng t nhit

nhit k t. Nhit b mt vt liu khng thay i trong sut giai on u, bt u bay

hi mnh, ton b nhit lng b mt nhn c dng ho hi. Trong thi gian ny vt liukhng c hm nng v nhit ca n bng nhit bc hi ca lng. .

Trong giai on tc sy khng i, nhit tm vt liu m cng khng i. Khi

m khng cn bay hi mnh na, th nhit b mt cng nh ca tm vt bt u tng ln v

giai on tc sy khng i kt thc v bt u tc sy gim dn. Khi m ca vt t

m cn bng, nhit ca n bng nhit khng kh tK (tc nhn sy).

TcsydWc/d

TcsydWc/d

m vt liu W1c % m loi b (W0 - Wcp)


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Hnh 1.17. ng cong nhit sy.Nh vy giai on u hm nng vt liu m t trong bt ng thc sau:

tK > tM > tb > t0Trong :

tK - nhit tc nhn sytb - nhit b mt vt liut0 - nhit tm vt liu.

Giai on cui hm nng, ngha l bt u thi k th nht sy ta c:tK > tM = tb > t0

Cui cng, thi k th hai ca sy, c trng bi:

tK > tb > tM t0.Ph thuc vo su ca vng bc hi nhit ca n c th cao hn hoc thp hn

nhit ca vt liu. Trong thi k th hai tu theo su ca vng bc hi, nhit trungbnh ca vt liu tng v c th cao hn nhit bc hi, cn nhit b mt gn bng nhit mi trng. Nu qu trnh t m cn bng, th cui qu trnh, tc sy s bng khngv nhit ca vt liu tin gn ti nhit tc nhn sy:

tb = t0 tK.

i vi vt liu mng, gradien nhit trong vt liu khng ng k, c th xem thik u t0 = tK. Hnh di cho ng cong phn b nhit trong mu vt liu dy v mng

trong sy i lu (vi khc nhau).

m vt liu W1c %

Nhitvtliu,

0

t =


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Hnh 1.18. Phn b nhit trong mu vt liu khi sy i lu (thi gian khc nhau). Trngnhit i xng vi trc vt liu.

Gi s c mt mu vt liu no , m mi im nh nhau, ngha l bn trongmu khng ch c trng nhit m trng m cng thun nht. Nu t mu vo bung sy,vt liu bt u c t nng. u tin t nng lp ngoi ca mu, m ca lp ny bt ubc hi ho vo khng kh. Bn trong m trng thi khng thun nht, xut hin gradien m c m. Di tc dng ca gradien m ny trong dng lng, bt u dch chuyn ti bmt mu. Dng m cn thng lc cn thu lc ca vt liu, lm gim tc di chuyn ca m.

Theo mc pht trin ca qu trnh sy gradien m c ca m trong vt liu sgim, cng lm gim c lng m di chuyn ti b mt, lm gim hm lng m ca b mtvt liu.1.4.4. Phn tch qu trnh sy.a/ Thi k tc sy khng i.

Ta bit rng, trong cc qu trnh sy nh nhit, m b loi b khi vt liu ch sau khho thnh hi, ngha l cn chi ph nhit nng (nhit ho hi). Trong sy i lu, lng nhitny cn hm nng vt liu m ti nhit bc hi, do trao i nhit gia b mt vt liu vtc nhn sy.

Dng tng qut c m t bng phng trnh cn bng sau:( ) ( ).K K p h a bhdQ C g C g dt r C t t dg = + + + (1.53)

y:CK , C - Nhit dung ring cht kh ca vt liu v m (J/Kg

0K)

gK , g - khi lng cht kh ca vt liu v khi lng m cha trong vt liu ti thiim cho (Kg).

Chiu dy vt liu

Chiu dy vt liu

Trcvt

Trcvt


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dt - s thay i nhit trung bnh (theo th tch) trong khong thi gian v cng

nh (0C).r - nhit ho hi ca m (J/Kg)

Cp.h - nhit dung ring trung bnh ng p ca hi nng ca m bc hi (J/Kg. 0K)ta - Nhit hi nng ca m tch khi b mt vt liu.tbh - nhit bc hi m.

dg - khi lng m bc hi t vt liu trong khong thi gian d.

Lng nhit truyn i lu qua din tch vt liu F sau khong thi gian v cng nh d, xcng theo:

( )K adQ F t t d =

y: - h s trao i nhit i lu (W/m20K)

tK - nhit trong bung sy.

( ) ( )

( ).

K a K K

p h a bh

tF t t C g C g

dg t

d r C t t

+ =

+

d

d (1.54)

Phn tch cng thc trn, ta thy c th c hai trng hp:- Nhit vt liu vo bung sy t1 < tbh, l trng hp ph bin.t1 > tbh thng trong trng hp khi sy ng knh, dung dch nng, ... trng hp u, khi t1 < tbh , vt liu a vo bung sy trong khong thi gian no

, nhit b mt vt sy tng dn cho ti khi t tbh. Khong thi gian xc nh theo:

( ) ( )K a K K F t t d C g C g dt = +

Khi t1 > tbh th qu trnh sy s bt u ngay khi vt liu a vo bung sy. R rng, thi im bt k khi bt u ti khi kt thc sy, biu thc sau lun mang gi tr dng:

( ) ( )K a K K t

F t t C g C gd

> + d

Thi k th nht sy, g v c bit tc hm nngt

d

dnhanh chng gim, sau

giai on t nng ban ut

d

dv tc sy ch ph thuc vo ( )K aF t t .

Nu ch lm vic ca my sy c thit lp th ti mi im ca bung sy trngthi kh v vt liu khng i, cn nhng im khc th s khc nhau.

Lin quan ti h s trao i nhit , phng trnh chun c dng:

Re, ,Pr,K

uNu f Gr

u

=

(1.55)


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Trong :

u - hm lng m trung bnh theo th tch ca vt liu thi im

uK - hm lng m tiu chun.

Thi k u khi u > uK, chun Nu khi ch kh ng khng i, c th xem l hngs. Nh vy thi k u, ln c th chu nh hng ca cc ln trong biu thc ca

Nussen:l

Nu

=

Bi v Nu = const, do kch thc vt liu sy l gim th tng. Thc t, trong thik sy u, kch thc ca nhiu loi sn phm c thay i, ln ca l v din tch trao i

nhit F thng gim. Nh vy trong nhiu trng hp v F thay i theo hng ngc nhauv thng c gi tr thay i khng ln. Do , mc gn ng c th coi thng s tc nhnsy khng i, v tc sy khng i.

Nhiu tc gi a ra cc cng thc tnh tr s Nu. Di y A.V NhesChenko ngh cc cng thc:

Khi sy i lu t nhin Nu1 = 4(Pr1Ar)0,103 khi (Pr1Ar) = 310

6 2108

Nu2 = 0,665(Pr2Ar)0,243 khi (Pr2Ar) = 110

4 3108

Khi sy i lu cng bc.

( )

( )' '

0,33

1 1

0,33'

2 2

2 Pr Re

2 Pr Ren m

n mNu A Gu

Nu A Gu

= +

= +

Trong

- 111

lNu

=

Tiu chun Nussen v trao i nhit

- 222

lNu

=

Tiu chun Nussen v trao i m

- 11

VPr

a= Tiu chun Prng i vi trao i nhit

-2

2

PrV

a

= Tiu chun Prng i vi trao i m

-( )K b

K

t tGu

t

= Tiu chun Gucman

-3

2

g l PAr

V

= Tiu chun Acsimet

Cc h s trong cc phng trnh trn cho trong bng di.


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Bng 1.4Re A n m A n m

1 2102 1,07 0,48 0,175 0,83 0,53 0,135

3,15103

2,2104

0,51 0,61 0,175 0,49 0,61 0,1352,2104 3,15105 0,027 0,90 0,175 0,0248 0,90 0,135

1-5. Phng php xc nh thi gian sy1.5.1. Phng php T.K Philnhenko: nghin cu cc ng cong sy ca cng mt loi vtliu ch sy khc nhau (sy i lu, vt vi si) phi hp trn cng mt ng cong trong

h to : tc sydw

d; tc sy qui dn , t s gia tc sy thi im bt k vi tc

sy thi k u N. Ta c

1 dw

N d

=

i vi vi si:( )28,5 0,73cb

cb

w w

w w

=

+

Trong : W - m ca vt liu thi im sy bt kWcb - m cn bng ca vt liu i vi ch sy cho

Lng nhit di chuyn qua b mt vt liu sau khong thi gian v cung nh.

( )K adQ F t t d r dg = =

Cng thc xc nh thi gian sy.

( )1 1 22

12,3 28,5 lg 0,73cb

cb

w ww w

N w w

= +

(1.56)

Trong : w1, w2 - m ban u v cui cng ca vt liu.Tc sy qui dn vi vt liu khc nhau

( )

( )

m

cb

m

cb

w w

A w w

=

+

Bng 1.5. Gi tr cc hng s trong cng thc ca T.K Phi lon hen koRau c A m

Khoai ty 1200 - 3,0 1,0Ci 1000 - 2,0 1,0C rt 1550 - 2,0 1,0Hnh c 1600 - 3,0 1,0

1.5.2. Phng php A.B Lkp.u tin xc nh thi gian sy i vi thi k tc khng i. Xc nh chun

Nussen trao i khi2

0,33 0,135Re Prn K

m m

bh

TNu A Gu

T

=


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Bit ( )760

;13

mm m m a

m

lNu q p p

= =

Tc sy thi k u:

100m

K

q

N Sg

=

Trong : S - l din tch b mt bc hi.Thi gian giai on u, xc nh theo biu thc

11

Kw w

N

=

xc nh thi gian sy giai on hai vt liu mng, c th li dng vic gii phng

trnh (1.52) khi = 0, u = uK1 = const, c dng

1

; ;p

m m m

K p

u uE f Bi Fo

u u R

= =

x(1.57)

Trong : Bim chun trao i khi Bi.

x

R- to khng kch thc

2

mm

aFo

R

= - chun trao i khi Phua ri

T phng trnh (1.57) xc nh chun F0m, sau thay gi tr am v R vo t1 xc nh c II.

2

1ln

Kd p

II

p

w w

K w w

=

H s sy:Kd p

NK Nw w

= =

Trong : wKd - m dn xut tiu chun

- H s sy tng ng0

1,8

w

m ti tiu chun2

K1 1w 1003

1w w

1

w w

K

m

Kd p

Kd p

N Ru

a

K

N

= +

= +

= =

Thi gian sy chung t m w1 ti m w2 s l.

11

2

w ww w 12,3 lg

N w w

K pKdI II

pK

= = +

(1.58)


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1.5.3. Phn loi v th t tnh ton thit b sy:a) Phn loi thit b sy.

Phng php k thut ca qu trnh sy v cu trc ca thit b sy khc nhau.

Thng ta phn loi h thng sy theo cch sau: h thng sy tip xc, h thng sybc x, h thng sy i lu. Trong cng nghip thc phm ph bin nht l h thng i lu.Bng sau cho ta cc loi sy i lu ph bin nht.

Bng 1.6. Xp loi thit b syDu hiu phn loi Kiu thit b sy

a Tc ng chu kb Tc ng lin tc

Ch lm vic (phng php tc ng)

c Tc ng tun hona) Cc mnh (ming)b) Ht

c) Dng bid) Bt nho

Dng vt liu lm kh

e) Dung dcha) Bung kn p sut khng kh cao

b) p sut khng kh bnh thngp sut trong bung sy

d) chn khng trung bnh v chn khng su(p sut thp hn im bc hi nc)a) Chu trnh t nhinb) Khc) Hi

Dng tc nhn sy

d) Lnga) Chu trnh t nhinPhng to p di chuyn tc nhn syb) Chu trnh cng bca) Khngb) Ngc

Hng chuyn ng ca vt liu v tcnhn sy

c) Dng ct nhaua) Chtb) Tic) Sid) Treo

Trng thi lp vt liu (vt liu dng ht)

e) Phuna) Clorifer hib) Clorifer ncc) t nng bng in

Phng php hm nng tc nhn sy

d) t nng b xung tc nhn sya) X kh lm vic ra ngoiCc loi qu trnh syb) Tc nhn sy hi lu


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c) Phn phi tc nhn syd) t nng b xung tc nhn sya) Sy bung, tuy nen, trc cun, sy trngCu trc bung sy

b) Sy hia) X kh lm vic ra mi trngb) Trn vi tc nhn mi v t nng (hi lu)c) Dng xilicagend) Ngng t nc trong bnh ngng

Phng php loi b hi m kh lmvic

e) Ngng t nc nh lm lnh

b) Th t tnh ton thit b sy:Mi lin hp my sy bao gm cc b phn chnh sau: Bung sy, calorifer, h thng

thng gi. Phn ny ta chi quan tm ti tnh ton nhit thit b sy (ch yu l bung sy).Qu trnh tnh ton, ta cc tm cc s liu sau:

-Theo mc ch: Nng sut, dng tc nhn sy, phng php truyn nhit ti vt liusy, phng php hm nng tc nhn sy (kiu calorife), c im chuyn ng ca tc nhnsy (t nhin, cng bc), phng php lp nguyn liu v chuyn ng.

- Theo vt liu: m u v cui, nhit cho php ban u v cc i, nhit dungcht kh ca vt liu, kch thc v khi lng sn phm, khi lng th tch...vv.

- Theo ch sy: thng s khng kh ngoi v ca tc nhn lm vic, nhit chophp cc i ca tc nhn sy, tc ca tc nhn sy, thi gian sy.

Cc s liu c th khng cho, ta c th chn khi thit k. V d chn kiu calorife,phng php vn chuyn vt liu... tnh ton cn phi c ng cong sy, bit phng trnhca n. Trn c s ng cong sy, bung sy phn chia thnh vng, v tnh ton thc hin

theo vng. Di y s tnh ton theo v d sau:+ Phn chung: chn kiu my sy, chn loi qu trnh sy.+ Tnh ton bung sy: Tnh kch thc bung sy, tnh khi lng m bc hi, chn

cu trc bung sy v tnh tn tht nhit vo mi trng, tnh tn tht cn li ca bung sy v

xc nh ln , xy dng th I d, tnh chi ph khng kh, tnh chi ph nhit.- Tnh thit b hm nng(calorife) chn cu trc ca calorife v cht mang nhit, tnh

hiu nhit trung bnh, tnh h s truyn nhit, tnh din tch t nng, tnh l t (calorifet bng ngn la).

- Tnh thit b thng gi: thit k qu o ca khng kh, tnh lc cn trong thit b sy,chn qut v tnh cng sut ng c.

- thit k v tnh ton c kh ca thit b.- Xy dng c tnh kinh t k thut ca qu trnh sy.Lp bng ch tiu kinh t k thut chnh so vi thit b thc t lm vic.


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cu hi n tp chng I1. Phn bit m tng i v tuyt i? ngha s dng thc t?2. Trnh by cc dng lin kt m? ngha ca tng loi lin kt?3. Trnh by cc thng s c bn ca khng kh m? Cch s dng th I - d ca khng khm?4. Trnh by phng trnh vi phn truyn nhit v truyn m trong qu trnh sy?5. Phng php xc nh thi gian sy?6. Trnh by th t tnh ton thit b sy?7. Hy xc nh m tng i ca khng kh, khi bit p sut kh tri bng 1 bar, tc khng kh nh hn 0,5 m/s, nhit nhit k kh t = 400C v nhit k t t = 30

0C.

8. Cho t0 = 250C, d0 = 0,017 kg m/kg kk v t1 = 90

0C. Hy xc nh 1, I1 v nhit lng mcalorifer tng ng cn cung cp q.

9. Gi s t = t0 = 250

C, = 0 = 85% xc nh d0 v I0.10. Xc nh lng nhin liu tiu hao lm bc hi 250 kg m/h trong thit b sy. Hiu

sut bung t b = 0,90, nhin liu l than c cc thnh phn (%).C =36,70 H = 2,70 S = 3,20 N = 0,70O = 11,10 A = 20,60 W = 25Nhit dung ring ca nhin liu Cnl = 0,5 kcal/kg

0KNhit tc nhn khi vo ra khi bung sy t1 = 150

0C v t2 = 600C. Trng thi khng

kh ngoi tri t0 = 200C v d0 = 9,80 g/kg kk, tng tn tht = - 200 kcal/kg m.


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Chng 2.thit b sy i lu

2.1 Khi nimPhng php sy i lu l phng php c dng kh ph bin trong sn xut, s

dng tc nhn sy l kh nng va lm nhim v truyn nhit v ly m ra khi vt liu sy.Nh phn phn loi thit b sy chng trc, tc nhn sy c th c nhiu dng khc nhauph thuc vo kiu my sy.

Nguyn tc cu to:Thit b sy i lu c cu to khc nhau theo tnh cht, kt cu, hnh dng v kch

thc ca vt liu m khi a vo sy. Nhng nm gn y s dng rng ri sy i lu nhsn phm chy ca nhin liu hoc hn hp vi khng kh nhit cao. Qu trnh sy ny ccng kh mnh. S nguyn tc cu to cc loi sy i lu cho bng di.

I - Thit b sy vt ri, ht lp cht v lp ri.

A - Sn phm ri (mnh), sy theo lp.

Tunen

Bng chuyn

Gong di chuyn

Bng ti

Bung

T sy

Gong c nh


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B - Ht ri.a/ Sy theo lp.

Qun c

Khe chp

Qun cc khechp quay

b/ Sy lp ti

Trng

Trc cun

Thanh

Rung, gia tc nh

a chng

Vng

Bung

Rto

Tuc bin


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II - Thit b sy vt liu ri.1 - Sy tng si. 2 - Sy kh ng

Mt bung

hiu bung

Khi

Lp rung

Khi rung

n hp - lytm c ng hi

n hp

Khi kh ng

Xycln

ng khng


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III - Thit b sy vt liu nho. IV - Thit b sy vt liu lng

Lp si

Lp rung

Lp rung kh ng

Trc

Sy phun

hun a ly tm

Vi phun hi

Lp rung

Lp si


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Cc k hiu:Vt liu:Tc nhn sy:

Hng quay caphn t c cu

Sng phn phi kh:

Qun trnh vt liu:

2.2. L thuyt tnh tonTnh ton thit b sy, thng bt u tnh ton bung sy, gm cc giai on chnh

sau: Tnh lng m bc hi, thit lp phng trnh cn bng v xc nh chi ph khng kh, xcnh kch thc chnh ca bung sy, tnh ton nhit bung sy.

(Hnh 2.1) y l s tnh ton sy bng khng kh kiu tunen, c calorife ring vqut t trc n.

Hnh 2.1. S sy bng khng kh kiu tu nen

Kcal/Kg

g/Kg

Vt liu

Kcal/Kg

g/Kg

g/Kg

Khngkh

I

Khng kh

Vt liu

Khng kh

Kcal/Kg


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Thng s ca khng kh bn ngoi: nhit t0, m 0, hm lng m , entanpy I0

c hm nng khi qut thi vo calorifer v kh ra vi cc thng s t 1, 1, d1, I1 c a trctip vo bung sy. Trong qu trnh sy, nhng thng s ca khng kh s thay i v c gi tr

t2, 2, d2, I2 v thot ra khi bung sy.

Trng thi vt liu trc khi sy c trng bi w1 ( m ban u theo % i vi khilng chung) v t1 (nhit ), sau khi sy l w2 v t2.

S thay i thng s chnh ca khng kh v tnh cht ca vt liu trc v sau khi sych trn s dng th khi sy la m.

2.2.1. Tnh lng m bc hi.

Ph hp vi nh lut bo ton cht, khi lng vt liu a vo my sy phi bngkhi lng vt liu ra khi my cng vi khi lng m bc hi.

g1 = g2 + U (2.1)

Trong : g1, g2 - khi lng vt liu m a vo bung sy v a ra khi bung sy (kg/h).

u khi lng m bc hi (kg/h).Khi lng vt liu gm khi lng cht kh tuyt i gK v khi lng m (nc)

cha trong vt liu.

Trong qu trnh sy gK = const, ta c th vit

11 1

1

1

100

1 100

K

K

wg g g

gg

w

= +

=

(2.2)

22 2

22

100

1100

K

K

wg g g

gg

w

= +

=

(2.3)

Ta c t s

1 2

2 1

100

100

g w

g w

=

(2.4)

Suy ra

22 1

1

100

100

wg g

w

=

(2.5)

T phng trnh c bn (2.1) v tng quan (2.4), c th nhn c tng quan thngdng trong tnh ton thit b sy.


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( )

( )

2 1 1 2

1 1 2 2

1 2 1 2

2 2 1 1

1001 1 2.6

100 100

100

1 1 2.7100 100

g w w wU

g g w w

g w w wU

g g w w

= = =

= = =

Trong :

1

U

g- khi lng m bc hi qua 1 kg vt liu kh (kg/kg).

2

U

g- khi lng m bc hi qua 1 kg vt liu sau sy (kg/kg).

Ta c: ( )1 2 1 21 22 1

2.8100 100

w w w wU g g

w w

= =

Nh cng thc (2.5) v (2.8) khi bit gi tr u v cui ca vt liu m v khi lngvt liu kh, c th xc nh c khi lng vt liu sau khi sy (hoc ngc li) v lng mbc hi.2.2.2. Cn bng m v chi ph khng kh trong bung sy.

Trong qu trnh sy khi lng m trong hn hp khng kh tng, khi lng kh cakhng kh khng i. (Thc t c thay i mt cht, do ro r...). Trong qu trnh sy khilng khng kh kh L (kg/kg) c th coi l khng i.

Phng trnh cn bng m i vi bung sy, biu din s cn bng gia m a voca vt liu v trong tc nhn sy trc khi sy v m a ra khi bung sy (trong tc nhn vm cn li trong vt liu sy)

1 1 1 2 2 2

100 100 100 100

g w d g w d L L

+ = + (2.9)

Trong :

1 1

100

g w- khi lng m a vo bung sy cng vi vt liu (kg/h).

2 2

100

g w- khi lng m tch khi vt liu (kg/h).

1

100

L d- Khi lng m trong khng kh vo bung sy (kg/h) (dg/kgkk,

100

dkg/kgkk).

2

100

L d- Khi lng m tch khng kh (kg/h)

T phng trnh trn ta vit.

1 1 2 2 2 1

100 100 1000

g w g w d d L

=


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Hiu s: 1 1 2 2100 100

g w g w - tn tht m ca vt liu sau khi sy, hoc lng m bc hi

trong 1 gi u (kg/h).

T ta c:2 1

1000

d du L

=

Khi lng khng kh kh.

2 1

1000L u

d d=

Ll

u= Chi ph khng kh cho 1 kg nc bc hi (kg/kg) trong m bc hi.

2 1

1000l

d d

=

(2.11)

Khng kh i qua calorife, d = const, ngha l d1 = d0, ta vit.

2 0

1000l

d d=

(2.11)

y l phng trnh c bn xc nh chi ph khng kh trong thit b sy. T (2.11)cho thy chi ph khng kh tng khi tng d0. Hm lng m bn ngoi vo ma h cao hn mang, do thng tnh qut theo iu kin ma h.

Tnh ton qut theo th tch khng kh di chuyn. bit chi ph khng kh theo khilng khng kh kh L, c th xc nh chi phi khng khi theo th tch V (m3 m/h).

V = LV0 y: V0 - Th tch khng kh m, xc nh theo phng trnh trng thi V 0 = Vc (thtch ring ca khng kh kh).

Thay gi tr 2

2

2

2

2

622 h

h

Pd

B P

=

vo phng trnh (2.11); ch rng d0 0 v gi tr ca

n bng 5 10 g/kg qu nh so vi d2; ta c

( )2

2 2

2

2 0 2 2

100010001,6 1

622

h

h h

B P Bl

d d P P

= = =

Qua phng trnh ny cho thy; chi ph khng kh l gim: kh

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