CHNG 2

VN CHUYN CHT LNG

Phn loi bm

Bm th tch

Bm ng lc

Bm kh ng

Phn loi bm

Bm th tch: Vic ht v y cht lng ra khi bm nh s

thay i th tch ca khng gian lm vic trong bm.

Bm piston

Clip minh ha 2

Clip minh ha 1

Bm roto

Bm cnh trt Bm bnh rng

Clip

Bm ng lc: Vic ht v y cht lng ra khi bm nh s

chuyn ng quay trn ca cc bm, khi ng nng ca cnh qut s

truyn vo cht lng to nng lng cho dng lng.

Bm hng trc

Bm ly tm

Bm xoy lc

(Vortex pump)

Bm kh ng: Vic ht v y cht lng ra khi bm nh s

thay i p sut ca dng kh chuyn ng trong bm v to nng lng

cho dng chy.

Bm tia (ejector)

CC THNG S C TRNG CA BM

Nng sut ca bm: L th tch cht lng c bm cung cp

trong mt n v thi gian. K hiu Q, n v: m3/s

Hiu sut ca bm: L i lng c trng cho s dng hu

ch ca nng lng c truyn t ng c n bm. K hiu

Cng sut ca bm: c tnh bng nng lng tiu tn bm lm

vic. Ni cch khc l nng lng tiu hao to ra lu lng Q

v chiu cao ct p H.

N = gQH / 1000, kW

Trong :

- Khi lng ring ca lu cht, kg/m3

Q Lu lng ca bm, m3/s

H Ct p ca bm (chiu cao ct p ton phn hay p

sut ton phn ca bm), m

- hiu sut ca bm

p sut ton phn ca bm:

=2 1. + 0 +

Trong :

H : Tng p sut khi bm chy tnh theo mt ct cht lng

P1, P2 : p sut trn b mt cht lng khong ht v khong y

(N/m2), Pa

: Khi lng ring ca cht lng c y i, kg/m3

g : Gia tc ri t do, m/s2

H0 : Chiu cao hnh hc a cht lng ln, m

P1 - p sut mt thong b cha s 1.

P2 - p sut mt thong b cha s 2.

Hh chiu cao ht ca bm

H chiu cao y ca bm

H0 = Hh + H khong cch 2 mt thong

Z1 khong cch t mt ct 1-1 n mt chun

Z2 khong cch t mt ct 2-2 n mt chun

Z = Z2 Z1 khong cch 2 mt thong

h khong cch gia p k v chn khng k

Ph, P p sut trong ng ng ht v ng y

Trng hp 1: i vi bi ton thit k hoc chn bm thch hp

Phng trnh Bernulli cho 2 mt ct 1-1 v 2-2:

1 +1. +12

2+ = 2 +

2. +22

2+

Trong :

hf = hms + hcb tng tr lc trn ng ng ht v y, m

Suy ra :

= 2 1 +2 1.

+22 1

2

2+

(Z2 Z1) = Z

Nng lng (ct p) dng khc phc chiu cao nng hnh hc

(P2 P1)/g

Nng lng dng thng li s chnh lch p sut 2 mt

thong, m

(22 1

2)/2g

Nng lng dng khc phc ng nng gia ng y v ng

ht, m

hf

Nng lng do bm to ra thng li tng tr lc trn ng

ng, m

Trng hp 2: i vi bi ton th li bm ( c bm)

Phng trnh Bernoulli cho 2 mt ct 1-1 v 2-2

+. +1 2

2+ = +

. +2 2

2

(Z Zh) = h

Nng lng (ct p) dng khc phc chiu cao gia 2 p k

(P Ph)/g

Nng lng dng thng li s chnh lch p sut ng ht v y

(

)/2g

Nng lng dng khc phc ng nng gia ng y v ng ht, m

Lu : trong trng hp ny i lng hf = 0 v s tn tht nng lng

trn ng ng c o hiu 2 p sut trn hai p k.

1. BM TH TCH

Bm piston tc dng n

Cu to v nguyn tc hot ng

Bm pittng tc dng n gm cc b phn chnh sau: Xi lanh

hnh tr, trong c pittng chuyn ng tnh tin qua li nh c cu

truyn ng tay quay thanh truyn. Pha u xi lanh c 2 xupp ht v

y.

Khi pittng chuyn ng t tri qua phi, p sut trong xi lanh s gim

xung nh hn p sut kh quyn. Di tc dng ca p sut kh

quyn, xupp ht s m ra nc trn vo xi lanh v ng thi

xupp y b ng li. Khi pittng chuyn ng ngc li t phi sang

tri, p sut trong xi lanh s tng ln, khi xupp ht s ng li v

xupp y s m ra v nc c y ra ngoi.

Nh vy trong mt chu k chuyn ng ca pittng qu trnh ht v

y cht lng c thc hin mt ln.

Khi trc quay t B A, Th tch nc ht vo ng bng th

tch ca xilanh (.D/4)S. Khi trc quay na vng cn li (t A B) th

pittng y lng nc trong xi lanh ra ngoi.

Nh vy, khi trc quay 1 vng th lng nc do bm pittng tc

dng n cung cp l (.D/4)S. Khi bm quay n vng/pht th lng

nc do bm cung cp l n.(.D/4)S, m3 / pht.

Vy, nng sut ca bm pittng

Q = .F.S.n = .(.D/4)S.n , m3/ph

D ng knh pittng, m

S khong chy ca pittng, m

- hiu sut th tch, v 1 phn th tch lu cht b r r

Bm pittng tc dng kp c 2

pittng v hai xilanh. Khi pittng

chuyn ng v pha phi, th tch

khong trng trong xi lanh bn tri

tng, p sut gim nn cht lng

c ht vo bung xi lanh bn

tri qua xupp 1, ng thi khi

th tch khong trng trong xilanh

bn phi gim, p sut tng, y

cht lng cha trong xi lanh bn

phi qua xupp 4 vo ng y. Khi

pittng chuyn ng v phi tri,

cht lng c ht vo bung xi

lanh bn phi qua xupp 2 v ng

thi y cht lng cha trong xi

lanh bn tri qua xupp 3 vo ng

y.

Bm piston tc dng kp

Khi trc quay na vng, pittng chuyn ng t tri sang phi:

Bm ht vo 1 lng : F.S = (D/4)S

Bm y ra mt lng : F.S f.S = (D/4 d/4)S

(.S l th tch cn pittng ng knh d chim ch)

Khi trc quay na vng cn li, pittong i t phi sang tri

Bm ht vo 1 lng : F.S f.S = (D/4 d/4)S

Bm y ra mt lng : F.S = (D/4)S

Nh vy, khi trc quay 1 vng, lng cht lng do bm cung cp:

F.S + (F.S f.S) = (2F- f)S .

Khi trc quay n vng/pht lng cht lng bm cung cp:

n.(2F- f)S .

Nng sut ca bm pittong tc dng kp s l:

Q = .n.(2F - f).S, m3/pht

thy r hn s khc nhau lng cht lng c cung cp bi

bm pittng tc dng n v tc dng kp ta xem th sau. Khi trc

quay na vng (180), bm cung cp c cht lng.

Bm piston tc dng ba

Bm pittng tc dng 3 cng tng t nh bm pittng tc dng

kp nhng lng nc cung cp s u hn

a. Bm bnh rng

Thng c nng sut nh, (0,3 2 l/s)

p sut t 100 200 mH2O.

Nng sut ca bm

Q = (.b.n/4.60).(D1 - D2).

b chiu rng bnh rng, m

n s vng quay ca bnh rng, v/ph

D1, D2 ng knh nh v chn rng, m

- hiu sut th tch, = 0,7 0,8

Cc loi bm th tch khc

Vn chuyn cc loi cht lng c nht cao (0,2 100cm2/s)

b. Bm cnh trt (Sliding-vane pump)

Gm v 1, bn trong trc 2 c s rnh theo hng bn knh. Trong rnh

c t cnh trt 3. Khi trc quay, do lc ly tm nn cc cnh trt

vng ra pha ngoi v p st vo v bm, chia thn bm thnh hai vng

ht v y.

Nng sut ca bm cnh trt c xc nh theo cng thc:

=.. 2.

30 m3/s

Trong

b chiu rng rto, m e khong cnh lch tm, m

S chiu dy cnh trt, m z s cnh trt

r bn knh rto, m - hiu sut th tch

R bn knh v my (R = r + e), m

n s vng quay ca rto, vng/pht

Bm cnh trt c th to ra p sut ti 70 at v lu lng ti 3,5 l/s.

2. BM LY TM

1 gung; 2 v bm

3 ng ht; 4 ng y

5 xupp (li lc)

Trc khi hot ng, bm cn c mi y nc trong bnh

gung. Di tc dng ca lc ly tm, cht lng trong cnh gung s chy

theo cnh hng dng t tm cnh gung ra mp cnh gung v i theo

v bm ra ngoi. Khi cht lng trong bnh gung chuyn ng ra ngoi,

di tc dng ca lc ly tm, s tao ra p sut chn khng ti tm bnh

gung. Do s chnh lch p sut bn ngoi v tm bnh gung cht

lng s theo ng ht chuyn ng vo bnh gung.

Hin tng xm thc v cch khc phc

Cht lng chuyn ng vo ming bm ly tm do p sut y

thp hn p sut kh quyn, iu ny to iu kin cho cc kh ha

tan c trong cht lng bc hi to ra cc bt kh ming ht ca bm.

Cc bt kh ny cng cht lng s chuyn ng trong cnh gung. Khi

p sut li tng ln, kh li ho tan ngc li vo cht lng.

Do qu trnh bay hi - ngng t - ha tan kh xy ra rt nhanh,

th tch bt kh tng ln v gim t ngt dn n p sut trong cc bt

kh tng ln rt ln. Hin tng to ra s va p thy lc, bo mn

cc kt cu kim loi, to ra s rung ng v ting n. Hin tng ny

gi l hin tng xm thc.

trnh hin tng xm thc, ngi ta cn tng p sut

cht lng ca vo ca bm bng cch gim chiu cao ht nh

t bm thp hn mc cht lng trong b ht.

Chiu cao ht ca bm c xc nh theo cng thc:

1 1

.

.+ 1 +

12

2+

P1 : p sut b ht, N/m2

1 : Tng tr lc ng ht , m

Pt : p sut hi bo ha ming ht, N/m2

1 : Vn tc dng ng ht , m/s

h : Tn tht ma st do xm thc

=5,62. .

c : h s xm thc ; c = 500 - 1000

Nhit (oC)

Chiu cao ht, m

10 6

20 5

30 4

40 3

50 2

60 1

65 0

c tuyn ca bm ly tm

Mi quan h l thuyt gia cc gi tr : Lu lng Q, ct p H,

cng sut N khi s vng quay thay i c th hin theo t l

Q1/Q2 = n1/n2 H1/H2 = (n1/n2) N1/N2 = (n1/n2

Tuy nhin trong thc t quan h gia cc i lng khng ng

hon ton theo t l nh trn m n thay i khi mt trong cc thng s

ca bm thay i. V vy i vi mi loi bm mi quan h trn u

xc nh bng thc nghim.

c tuyn ca bm ly tm l mi quan h hm s gia cc thng

s ca bm : Ct p, lu lng, cng sut, hiu sut khi s vng quay

c nh hay thay i

H = f(Q); N = F1(Q); = F2(Q)

Mi quan h H Q ph thuc vo hnh dng cnh gung

Hnh dng cnh gung :

Cong v pha sau ( < 90o)

Cong v pha trc ( > 90o)

Theo hng bn knh ( = 90o)

Q

H

n = const

< 90o

> 90o

= 90o

Q

H > 90o

Q

H

< 90o

Qmax Qmax

Hmax

c tnh l thuyt ca bm

c tnh thc ca bm

Q

Qmax

Hmax

1 2

3

on 1 2 trn ng c tuyn l vng lm vic

khng n nh ca bm.

on 2 3 l vng lm vic n nh ca bm

H

c tuyn mng ng

Trong mt mng ng dn, tn tht ct p khi cht lng chuyn

cng trong ng dn c xc nh bng cng thc

= 2 1 +2 1.

+22 1

2

2+

P1; P2 : p sut u vo v u ra ca ng dn

Z1; Z2 : Chiu cao u vo v u ra ca ng dn

22; 1

2 : Vn tc cht lng ti 2 u ng

h : Tng tn tht ma st trong ng ng

= +

2

2

Q

Qmax

Hmax

1 2

3

H ng c tuyn mng ng

ng c tuyn ca bm

A im lm vic

Khi ghp bm vo mng ng ta s c th phi hp c tuyn

gia bm v mng ng

Giao im A gia ng c tuyn ca bm v mng ng gi l

im lm vic ca bm trong mng ng.

Ghp bm

Bm lm vic song song : khi cn gi nguyn ct p H v tng

lu lng Q

Bm lm vic ni tip : khi cn gi nguyn lu lng Q v tng

ct p

Q

H

Q1 Q2

A1

A2

Bm ghp song song

Q

H

Q1 Q2

A1

A2

Bm ghp ni tip

Ghp song song

Ghp ni tip

III. CC LOI BM KHC

Bm sc kh

Loi bm ny lm vic theo

nguyn tc bnh thng nhau. Kh nn

qua ng 2 thi vo ng 1 lm cho cht

lng trong ng 1 si bt to thnh hn

hp lng kh c hh < l nn hn

hp ny dng ln qua np 4 vo b

cha.

1 ng dn, 2 ng dn kh nn, 3 bnh gim p, 4 b cha

Phng hn hp kh lng 4 phi t cao hn ca ht cht

lng ng 1 khong 1 1,5 m gi cho kh nn khng b pht ra

ngoi.

u im:

n gin, khng c b phn truyn ng, c th lm vic

nhit cao khi bm li tm khng ht c.

Nhc im:

Hiu sut thp (25 35%), nng sut nh. Cn c trm nn

kh v phi duy tr ct cht lng nht nh m bo nhng su ca

ng 1.

Bm mng (Diaphragm pump)