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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)