Những Kiến Thức Cơ Bản Về Không Khí Ẩm (Part1)

Nhng kin thc c bn vkhng kh m (part1)

Bi:

V Ch Chnhinh Vn Thun

NHNG KIN THC C BN V KHNG KH M

KHNG KH M

Khi nim v khng kh m

Khng kh xung quanh chng ta l hn hp ca nhiu cht kh, ch yu l N2 v O2ngoi ra cn mt lng nh cc kh tr, CO2, hi nc . . .

- Khng kh kh: Khng kh khng cha hi nc gi l khng kh kh. Trong thc tkhng c khng kh kh hon ton, m khng kh lun lun c cha mt lng hi ncnht nh. i vi khng kh kh khi tnh ton thng ngi ta coi l kh l tng.

Thnh phn ca cc cht kh trong khng kh kh c phn theo t l phn trm sauy:

Bng 1.1. T l cc cht kh trong khng kh kh

Thnh phn

T l phn trm, %

Theo khi lng Theo th tch

- Ni t: N2- xi : O2- Argon -A- Carbon-Dioxide: CO2-Cht kh khc: Nn, Hli,Kripton, Xnon, zn, Radonvv . . .

75,523,11,30,0460,05 78,08420,9480,9340,030,004

Nhng kin thc c bn v khng kh m (part1)

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www.princexml.comPrince - Non-commercial LicenseThis document was created with Prince, a great way of getting web content onto paper.

- Khng kh m: Khng kh c cha hi nc gi l khng kh m. Trong t nhin chc khng kh m v trng thi ca n c chia ra cc dng sau:

a) Khng kh m cha bo ha: L trng thi m hi nc cn c th bay hi thm voc trong khng kh, ngha l khng kh vn cn tip tc c th nhn thm hi nc.

b) Khng kh m bo ha: L trng thi m hi nc trong khng kh t ti a vkhng th bay hi thm vo c. Nu tip tc cho bay hi nc vo khng kh thc bao bao nhiu hi bay vo khng kh s c by nhiu hi m ngng t li.

c) Khng kh m qu bo ha: L khng kh m bo ha v cn cha thm mt lnghi nc nht nh. Tuy nhin trng thi qu bo ho l trng thi khng n nh vc xu hng bin i n trng thi bo ho do lng hi nc d b tch dn ra khikhng kh . V d nh trng thi sng m l khng kh qu bo ha.

Tnh cht vt l v mc nh hng ca khng kh n cm gic ca con ngi phthuc nhiu vo lng hi nc tn ti trong khng kh.

Nh vy, mi trng khng kh c th coi l hn hp ca khng kh kh v hi nc.Chng ta c cc phng trnh c bn ca khng kh m nh sau:

- Phng trnh cn bng khi lng ca hn hp:

G = Gk + Gh (1-1)

G, Gk, Gh - Ln lt l khi lng khng kh m, khng kh kh v hi nc trongkhng kh, kg.

- Phng trnh nh lut Dantn ca hn hp:

B = Pk + Ph (1-2)

B, Pk, Ph - Ap sut khng kh, phn p sut khng kh kh v hi nc trong khng kh,N/m2.

- Phng trnh tnh ton cho phn khng kh kh:

Pk.V = Gk.Rk.T (1-3)

V - Th tch hn hp, m3;

Gk - Khi lng khng kh kh trong V (m3) ca hn hp, kg;


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Rk - Hng s cht kh ca khng kh kh, Rk = 287 J/kg.K

T - Nhit hn hp, T = t + 273,15 , oK

- Phng trnh tnh ton cho phn hi m trong khng kh:

Ph.V = Gh.Rh.T (1-4)

Gh - Khi lng hi m trong V (m3) ca hn hp, kg;

Rh - Hng s cht kh ca hi nc, Rh = 462 J/kg.K

Cc thng s vt l ca khng kh m

p sut khng kh.

Ap sut khng kh thng c gi l kh p, k hiu l B. Ni chung gi tr B thay itheo khng gian v thi gian. c bit kh p ph thuc rt nhiu vo cao, mc mtnc bin, p sut kh quyn khong 1 at, nhng cao trn 8000m ca nh Everestth p sut ch cn 0,32 at v nhit si ca nc ch cn 71oC (xem hnh 1-1). Tuynhin trong k thut iu ha khng kh gi tr chnh lch khng ln c th b qua vngi ta coi B khng i. Trong tnh ton ngi ta ly trng thi tiu chun Bo = 760mmHg.

th I-d ca khng kh m thng c xy dng p sut B = 745mmHg v Bo =760mmHg.


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S thay i kh p theo chiu cao so vi mt nc bin

Nhit .

Nhit l i lng biu th mc nng lnh. y l yu t nh hng ln nht ncm gic ca con ngi. Trong k thut iu ha khng kh ngi ta thng s dng 2thang nhit l C v F. i vi mt trng thi nht nh no ca khng khngoi nhit thc ca n trong k thut cn c 2 gi tr nhit c bit cn lu trong cc tnh ton cng nh c nh hng nhiu n cc h thng v thit b l nhit im sng v nhit nhit k t.

- Nhit im sng: Khi lm lnh khng kh nhng gi nguyn dung m d (hocphn p sut ph) ti nhit ts no hi nc trong khng kh bt u ngng t thnhnc bo ha. Nhit ts gi l nhit im sng (hnh 1-2).

Nh vy nhit im sng ca mt trng thi khng kh bt k no l nhit ng vi trng thi bo ha v c dung m bng dung m ca trng thi cho. Hay ni


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cch khc nhit im sng l nhit bo ha ca hi nc ng vi phn p sutph cho. T y ta thy gia ts v d c mi quan h ph thuc.

Nhng trng thi khng kh c cng dung m th nhit ng sng ca chng nhnhau. Nhit ng sng c ngha rt quan trng khi xem xt kh nng ng sngtrn cc b mt cng nh xc nh trng thi khng kh sau x l. Khi khng kh tipxc vi mt b mt, nu nhit b mt nh hn hay bng nhit ng sng ts thhi m trong khng kh s ngng kt li trn b mt , trng hp ngc li th khngxy ra ng sng.

- Nhit nhit k t: Khi cho hi nc bay hi on nhit vo khng kh cha boha (I=const). Nhit ca khng kh s gim dn trong khi m tng i tng ln.Ti trng thi bo ho = 100% qu trnh bay hi chm dt. Nhit ng vi trngthi bo ho cui cng ny gi l nhit nhit nhit k t v k hiu l t. Ngita gi nhit nhit k t l v n c xc nh bng nhit k c bu thm t nc(hnh 1-2).

Nh vy nhit nhit k t ca mt trng thi l nhit ng vi trng thi bo hav c entanpi I bng entanpi ca trng thi khng kh cho. Gia entanpi I v nhit nhit k t t c mi quan h ph thuc. Trn thc t ta c th o c nhit nhitk t ca trng thi khng kh hin thi l nhit trn b mt thong ca nc.

Nhit ng sng v nhit k t ca khng kh

m

m tuyt i.

L khi lng hi m trong 1m3 khng kh m. Gi s trong V (m3) khng kh m ccha Gh (kg) hi nc th m tuyt i k hiu l h c tnh nh sau:


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V hi nc trong khng kh c th coi l kh l tng nn:

trong :

ph - Phn p sut ca hi nc trong khng kh cha bo ho, N/m2

Rh - Hng s ca hi nc Rh = 462 J/kg.oK

T - Nhit tuyt i ca khng kh m, tc cng l nhit ca hi nc, oK

m tng i.

m tng i ca khng kh m, k hiu l (%) l t s gia m tuyt i h cakhng kh vi m bo ha max cng nhit vi trng thi cho.

hay:

m tng i biu th mc cha hi nc trong khng kh m so vi khng khm bo ha cng nhit .

Khi = 0 l trng thi khng kh kh.

0 < < 100 l trng thi khng kh m cha bo ho.

= 100 l trng thi khng kh m bo ha.

- m l i lng rt quan trng ca khng kh m c nh hng nhiu n cmgic ca con ngi v kh nng s dng khng kh sy cc vt phm.

- m tng i c th xc nh bng cng thc, hoc o bng m k. m k lthit b o gm 2 nhit k: mt nhit k kh v mt nhit k t. Nhit k t c bu


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bc vi thm nc hi nc thm vi bc xung quanh bu nhit k khi bc hivo khng kh s ly nhit ca bu nhit k nn nhit bu gim xung bng nhit nhit k t t ng vi trng thi khng kh bn ngoi. Khi m tng i b, cng bc hi cng mnh, chnh nhit gia 2 nhit k cng cao. Do chnh nhit gia 2 nhit k ph thuc vo m tng i v n c s dng lm c s xcnh m tng i . Khi =100%, qu trnh bc hi ngng v nhit ca 2 nhitk bng nhau.

Khi lng ring v th tch ring.

Khi lng ring ca khng kh l khi lng ca mt n v th tch khng kh. Khiu l , n v kg/m3.

i lng nghch o ca khi lng ring l th tch ring. K hiu l v

Khi lng ring v th tch ring l hai thng s ph thuc.

Trong :

Do :

Mt khc:

Thay vo ta c:


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trong B l p sut khng kh m: B = pk + ph

- Nu l khng kh kh hon ton:

- Nu khng kh c hi m:

Lu trong cc cng thc trn p sut tnh bng mmHg

iu kin: t = 0oC v p = 760mmHg: = o = 1,293 kg/m3. Nh vy c th tnh khilng ring ca khng kh kh mt nhit bt k da vo cng thc:

Khi lng ring thay i theo nhit v kh p. Tuy nhin trong phm vi iu hokhng kh nhit khng kh thay i trong mt phm vi kh hp nn cng nh p suts thay i ca khi lng ring ca khng kh trong thc t k thut khng ln nnngi ta ly khng i iu kin tiu chun: to = 20oC v B = Bo = 760mmHg: =1,2 kg/m3

Dung m ( cha hi).

Dung m hay cn gi l cha hi, c k hiu l d l lng hi m cha trong 1 kgkhng kh kh.

- Gh: Khi lng hi nc cha trong khng kh, kg

- Gk: Khi lng khng kh kh, kg

Ta c quan h:


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Sau khi thay R = 8314/ ta c

Entanpi

Entanpi ca khng kh m bng entanpi ca khng kh kh v ca hi nc cha trongn.

Entanpi ca khng kh m c tnh cho 1 kg khng kh kh. Ta c cng thc:

Trong :

Cpk - Nhit dung ring ng p ca khng kh kh Cpk = 1,005 kJ/kg.oK

Cph - Nhit dung ring ng p ca hi nc 0oC: Cph = 1,84 kJ/kg.oK

ro - Nhit n ha hi ca nc 0oC: ro = 2500 kJ/kg

Nh vy:

I = 1,005.t + d (2500 + 1,84.t) kJ/kg kkk (1-21)


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Nhng kin thc c bn v khng kh m (part1)NHNG KIN THC C BN V KHNG KH MKHNG KH MKhi nim v khng kh mCc thng s vt l ca khng kh mp sut khng kh.Nhit . m m tuyt i. m tng i.

Khi lng ring v th tch ring.Dung m ( cha hi).Entanpi

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Những Kiến Thức Cơ Bản Về Không Khí Ẩm (Part1)