MODULE 6 HUMIDIFICATION AND AIR CONDITIONING

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1 MODULE 6 UMIDIFICATION AND AIR CONDITIONING LECTURE NO Evaporaton loss of water n coolng tower: Blowdown: Durng the coolng process of hot water n coolng tower, around % water evaporates [-3]. In the long run, t ncreases the sold content n the crculatng water. Some dust partcles also come from the envronment and mx wth crculatng water. But the sold content of the cooled water must be kept under a certan lmt to avod scalng or foulng on the heat exchange equpment. A part of the crculatng water s draned from the bottom of the coolng tower to dscard the deposted solds from the coolng tower. Ths s called blowdown. The losses due to blowdown, evaporaton, drft and leakage are compensated by addng make-up water. Water balance n coolng tower M=B+D+E (6.38) where, M s make-up water rate; B s blowdown rate; D s drft leakage loss rate; E s evaporaton loss. Sold balance M C =(B+D) C +E 0 (6.39) E D( r ) B (6.40) ( r ) where, r=c C ; C s dssolved sold concentraton n the make-up water; C s dssolved sold concentraton n the crculatng water. Jont ntatve of IITs and IISc Funded by MRD Page of 7

2 Evaporaton loss s estmated by a thumb rule as: E=water flow rate (L) range(ºf) gallonmn The other desgn characterstcs are pump horsepower, fan horsepower, source of make-up water and drft elmnators. Nomenclature a contact areatower volume, m m 3 L Water flow rate, kgm s c wl eat capacty of lqud (water), kjkg.k V actve coolng volumeplan area, m 3 m G s Ar rate, kg dry arm s umdty, kg mosturekg h G h L k G eat transfer coeffcent of ar flm, kjm.s.k eat transfer coeffcent of lqud (water), kjm.s.k Mass transfer co-effcent of mosture transport, kgm.s (p A ) K Overall mass transfer co-effcent, kgm 3.s w z λ w dry ar Saturaton humdty, kg mosturekg dry ar Coolng tower heght, m Latent heat of vaporzaton of water, kjkg umd volume, m 3 kg dry ar References. Treybal, R. E., Mass-Transfer Operatons, 3 rd Eddton, McGraw-ll, 98. Geankopls, C.J., Transport Processes and Separaton Process Prncples. 4 th Edton, Prentce-all of Inda, New Delh, Dutta, B.K., Prncples of Mass transfer and Separaton Processes. Prentce-all of Inda, New Delh, 007. Jont ntatve of IITs and IISc Funded by MRD Page of 7

3 UMIDIFICATION PROBLEM Example Problem 6.: A coolng tower s to be desgned to cool water from 45 0 C to 30 0 C by countercurrent contact wth ar of dry bulb temperature 30 0 C and wet bulb temperature of 5 0 C. The water rate s 5500 kgm.h and the ar rate s.5 tmes the mnmum. Determne the tower heght f the ndvdual gas-phase mass transfer coeffcent (k ā) s kgm 3 h (Δ ). The volumetrc water sde heat transfer coeffcent s gven by h L ā=0.059l 0.5 G s, n Kcalm 3 hk, where L and G s are mass flow rates of water and ar (dry bass). Antone Equaton: ln P (bar)= (t-39.74). V A Soluton 6.: T G =30ºC T w =T as =5ºC 0.09 [From psychrometrc chart] ( ) kjkg =78.7 kjkg T L =30ºC Locate pont Q(T L, plane. ) (Lower termnal of operatng lne) at Q(30, 78.7) on T L - Generaton of Equlbrum curve () Calculate p v from Antone Equaton: ln P V A (bar)= (t- () 39.74). v p ( P p v 8.0 ) 8.97 () ( ) ( T T ) 500 G T 0 s 0ºC. 0 ; ere, reference temperature, Jont ntatve of IITs and IISc Funded by MRD Page 3 of 7

4 T (ºC) lnpv (Antone Equaton) p v (bar) ' (kg mostkg dry ar) ' kjkg dry ar Draw a tangent to the equlbrum lne through Q. Slope of the tangent s G s,mn Lc slope WL =63 kgh.m Actual ar rate =G s,mn.5 =63.5 kgh.m = 379 kgh.m Slope of the operatng lne Slope Lc WL ( TL TL ) Gs( (45 30) 379( ) 78.7) Jont ntatve of IITs and IISc Funded by MRD Page 4 of 7

5 84 kjkg Now, locate pont P (T L, ) (Upper termnal of the operatng lne) at P (45, 84) on T L - plane. We have, h L ā=0.059l 0.5 G s =0.059(5500) 0.5 (379) kcalm 3 h.k = kcalm 3 hk= kjm 3 h.k =65,475.9 kjm 3 hk Slope of te lne= hl a k a Jont ntatve of IITs and IISc Funded by MRD Page 5 of 7

6 ' (KJkg) NPTEL Chemcal Mass Transfer Operaton Approach Equlbrum lne Operatng lne Q (TL, ' ) Range P(TL, ' ) T as T L T L T G, T L ( 0 C) A set of te lnes of ths slope s drawn from several ponts on the operatng lne. These te lnes meet the equlbrum lne at (T L, are obtaned. The values of ( ) ). ence, the ponts (, ) are plotted aganst T L and the ntegral N tg T L0 T L dtl s evaluated graphcally. ( ) T L T L Jont ntatve of IITs and IISc Funded by MRD Page 6 of 7

7 ( ) ( ) T L ( 0 C) N tg =Area under the curve= ( ) 0.088=9.7 Gs =( )m=0.56 m k a tg Tower heght= m=5.9 m (Ans.) Jont ntatve of IITs and IISc Funded by MRD Page 7 of 7