THE USE OF AIR FLOW THROUGH WATER FOR WATER EVAPORATION

Transcription

1 Prceedings f the Fifth Internatinal Cnference n Energy and Envirnment, Cair, Egypt, 1996 EG THE USE OF AIR FLOW THROUGH WATER FOR WATER EVAPORATION AshnfA Lashin, * Hassan ASiiman** * Assistant Prfessr, ** Asnciate Prfessr, *** Prfessr, Department f Mcebanical Engineering, Shubra Facility f Engineering, Zagizig University, EGYPT. ABSTRACT Iru water desahnatin system the prductivity rate is imprved by increasing the rate f water evapratin either by heating the water r by frcing an* t carry mre vapr befre cndensatin. This paper describe an experimental investigatin int the effect f frcing the air t flw thrugh a ht water cntained in a clsed tank thrugh a perfrated end f inlet tube. When the air bubbles pass thrugh the water, it increases the rate f vaprizatin. The effect f sme perating parameters are investigated and the results are presented and discussed. INTRODUCTION Slar energy utilizatin fr water desalinatin has been under investigatin fr many years. Many types f basin types slar still have been develped, cnstructed and tested fr their prductivity rateref. [1,2]. The results btained frm indr and utdr testing f the classical basin type slar still (cnvectin type) indicated that the maximum prductivity achieved is in the rder f 3.5 t 4 h't/m 2 day. Many mdificatin have been develped -171-

2 t enhance the prductivity f these types f stills bv imprving the methds t slar energy utilizatin and reduces the energy lss within the still cmpnents ref. [3,4]. Anther apprach emplyed t enhance the still prductin MH use frced cnvectin fr increasing water evapratin. Ref. (5,6.71 prpsed a frced cnvectin slar desalinatin plants where air enters the evapratr and carries the water vapr ver water surface t an external heat exchanger In sme f the plants described the prductivity rate achieved was in the rder f 6.5 lit/m 2. day at a slar radiatin f 24 MJ/m-. and system efficiency f 62%. The present wrk suggested a desalinatin plant which can be used fr slar energy applicatin. In this system air is frced t flw thrugh a perfrated end f a tube inside a water cntained in a clsed tank (evapratr). When the air bubbles thrugh the water it will carry mre vapr and increase the rate f prductivity f the still. The effect f water level inside the evapratr and water temperature n the rate f evapratin are experimentally investigated EXPERIMENTAL SET-UP In rder t carry ut this experimental study, a test desalinatin plant is cnstructed as shwn in fig. (1). The plant cnsists f an evaprating tank made f 6mm glass sheet f 80 x 80 x 30 Cm dimensins with metal tp A 7 5 Cm (P.V.C) pipe with 50 Cm length clsed at the end and 156 hles are drilled in 6 rws each has 26 hles. The higher three rws, the diameter f the hle is 6 mm while 8 mm diameter hles are used in the bttm three rws. The pipe is immersed inside the water tank The utlet pipe is als made f -172-

3 PVC f 7.5 evapratr Cm diameter which takes the air water mixture frm the Air flw can be measured by an standard rifice having 3.75 Cm diameter with a water, manmeters. A heat exchanger is added fr vapr cndensatin. An air blwer is used t frce the air t flw thrugh the water in the evapratr. The water in the evapratr is heated by tw electric heaters f, 1 KW heating Capacity. Water temperature and air temperature at the pints indicated in Figured) are measured by thermmeter having! 0.1 C accuracy Details f the evapratr under test are shwn in Figure! 2) TEST RESULTS AND DISCUSSIONS In this paper the results btained frm indr tests f the evaprating tank will be presented and discussed The effect f water level n the evapratin at cnstant air flw rate and cnstant water temperature is shwn in FigureO). The results shw that the rate f evapratin is increased with the increase in the water level as the water cvers all hles in the perfrated pipe. As the water level is increased ver 18.6 Cm fr the prpsed evapratr, the rate f evapratin is reduced again due tp the increase in the pressure head f the water which reduce blwer mass flw rate. The variatin f maximum rate f evapratin achieved frm this system with water temperature at the ptimum perating cnditins is shwn in Figure(4). The results indicate that the maximum rate f evapratin is increased as the temperature f the water is increased up t 80 C which is the maximum water temperature culd be reached

4 Fig (5) shws, the variatin f evapratin rate and water temperature with time. The decrease in the evapratin rate and water temperature are due t the drp in the water level during evapratin. The variatin in dry bulb and wet bulb temperature f the air evapratr utlet are als shwn in fig.(6) CONCLUSIONS 1- Passing air thrugh heated water enhanced the rate f vaprizatin. 2- Fr evapratr dimensins used in this prject the ptimum level f water is fund t be 18.6 Cm. 3- The rate f evapratin is increased with the increase in water temperature. 4- The prcess used in this study can be applied t enhance the prductivity rate f slar stills. REFERENCES 1- Hew, E.D., "Distillatin f Sea Water" In : Slar Energy Technlgy Handbk [W.C. Dickinsn and P.N. Cheremisinff], Dekker, New Yrk, PP (1980). 2- Malik, M.A.S. and Tiwari, G.N., "Slar Distillatin", First Editin. Pergamn Press, Oxfrd (1982). 3- Akinsete, V.A. and Duru, C.U., "A Cheap Methd f Imprving the Perfrmance f Rf Type Slar Still.", Slar Energy, Vl.23 (3) PP 271., (1979). 4- Lawrence, S.A, Gupta, S.P. and Tiwari, G.N., "Experimental Validatin f thermel Analysis pf slar still with Dye". Int. J. Slar Energy, Vl.6, PP ,(1988). 5- Tiwari, G.N and Gupta S., "Analytical Mdel f Inverted wick Slar still", Int, J. Slar Energy, PP , (1988)

5 6- EL- Desuky, M.A.S.,"Optimum Perfrmance f A Slar Energy Desalinatin System", Ph. D. Thesis, Zagazig University., (1991). 7- Ashraf, A. Lashin, "Water Desalinatin Using Slar Energy", Ph. D. thesis, Zagazig University., (1993)

6 A - Air Outlet Cling Water A I Manmeter Air Inlet In Out w 1 \ II 1 Thermmeter Water Level A >' PAir Blwer Water Heaterl Fig. (1) Test-Bed Layut

7 Air Outlet Air Inlet Perfrated End Tube -^ Thermmeter Water Level Make Up e u Water Heater O O O 3 OO O O O O O O 80 Cm Heater (1) Air Outlet i Heater (2) Fig. (2) Evapratr Details -177-

8 s * c 9! O Initial water Temperature (Twi) C ^>60 C»70 C A80 Airlnlct Water Level ( *0.01 m ) Fig.(3) Effect Of The Water Level n The Rate Of Evapratin at Cnstant Water Temperature and Cnstant Air Flw-Rate

9 3 O 3 E * B O O c ON E s Water Temperature ( C) 90 Fig.(4) Effect Of Water Temperature n The Maximum Rate f Evapratin at The Optimum Water Level (18.6 Cm)

10 E <*> Water Level '^t t Cm > n a Evaprat / r~ r -= H Si 3 re ^ater Tt / L- -^Average Water Temperature j ^ Ttal Water Evaprated J C n Operating Time (min) Fig.(5) Variatin f Water Temperature and Ttal Water Evaprated with Time

11 Air Temerature After The Evapratr ( C) ts> p' O 3 2. n t 2 5* as 5' 3. 5' " NEXT PAOE(S) left BLANK