SENSITIVITY ANALYSIS FOR COUNTER FLOW COOLING TOWER- PART I, EXIT COLD WATER TEMPERATURE

Transcription

1 SENSITIVITY ANALYSIS FOR COUNTER FLOW COOLING TOWER- PART I, EXIT COLD WATER TEMPERATURE *Citranjan Agaral Department of Mecanical Engineering, College of Tecnology and Engineering, Maarana Pratap Univerity of Agriculture and Tecnology, Udaipur-3300, Rajatan, India *Autor for Correpondence ABSTRACT Water cooling toer are ized and elected on te bai of economic conideration and contraint impoed by ytem component. Te exit cold ater temperature a direct impact on te economic of deign and performance of proce equipment. Terefore te accurate prediction of cooling toer performance and exit cool ater temperature i of great importance. In ti tudy enitivity analyi i performed for te counter flo type cooling toer to identify te critical parameter to ic cooling toer exit cold ater i more enitive. Key Word: Cooling Toer, Exit Cold Water Temperature, Senitive Analyi, Wet Bulb Temperature INTRODUCTION No a day large number of indutrial application are uing cooling toer to remove te proce eat epecially in poer generating, refrigeration and air conditioning, cemical, petrocemical and petroleum indutrie. Cooling toer are extenively being ued, erever ater i ued a a cooling medium or proce fluid. In mot indutrial location, cooled fre ater i canty terefore, continuou reue and re cooling of te limited fre ater it te elp of cooling toer i more common and economical. Anoter trong motivation for te increaed ue of cooling toer i te environmental protection provided troug te reduction of ater itdraal and minimizing of termal dicarge. Figure : Scematic of a et counter flo Cooling toer Kan et al., (2003). Figure 2: Temperature relationip beteen ater and air in a counter flo cooling toer Moiuddin and Kant (996). 5

2 Te mot common type of cooling toer i et cooling toer in ic temperature of te re circulated ater i reduced by bringing it into direct contact it unaturated air. Figure o a cematic of a et cooling toer ic conit of large camber, looely filled it tray and deck of ooden board, abeto eet a late or PVC material film called packing Kan et al., (2003). Tee filling or packing provide ufficient area and time to get contact beteen air and ater for energy tranfer Moiuddin and Kant (996), Moiuddin and Kant (996). Figure 2 o te temperature relationip beteen ater and air a tey pa troug a counter flo-cooling toer. Te difference beteen te ater temperature entering and leaving te toer i defined i te Range. Te difference beteen te leaving ater temperature and entering air et bulb temperature i knon a Approac of te cooling toer. In te poer generation plant, te temperature of cooling ater fixe te ultimate recovery of eat from te turbine and dicarge preure of te eat engine. In addition te cooling ater temperature etablie te operating preure of te condener of ditillation and evaporation operation. For tee vital reaon exit cold ater temperature from te cooling ater play an important role in deigning a cooling toer? Terefore te parameter tat affect te exit cold ater temperature te mot i te paramount critical parameter for te cooling toer deign. Tu a enitive analyi a been performed for te counter flo cooling toer to identify te critical parameter to ic cooling toer exit cold ater i more enitive. MATERIALS AND METHODS Folloing procedure i adopted to determine te exit cold-ater temperature form a counter flo cooling toer baed on te Merkel and accurate model of cooling toer deign Citranjan (2004). Te equation for NTU calculation can be ritten a folloing equation. 2 A V d NTU = d v G ma 2 d AV v dt NTU L C (2) m t Te equation () i knon a number of tranfer unit (NTU) of Air pat i.e. (NTU) G and equation (2) i knon a number of tranfer unit of liquid pat i.e. (NTU) L. Wit elp of equation () e can rite 2 m a d So Z Were V = A*Z (3) d A Here te term (m a / d A) i knon a eigt of tranfer unit Kern and Donald (997) i.e. (HTU) G terefore, Z HTUG (4) NTU G Te integral involved in equation () and (2) could not be olved directly a te ( ) cannot be expreed imply in term of an expreion of temperature. Variou metod being ued to evaluate ti integral viz: Grapical metod Majumdar and Singal (983), ue of Carey and Williamon cart Hill et al., (990), numerical integration uing Simpon rule Arora (997), Tecebyceff metod or by incremental tep metod ASHRAE Sytem and Equipment Hand Book (2000). Te cooling toer ize can be calculated by equation (5) for te required value of exit cold ater and inlet ot ater temperature, given urrounding condition, packing type and material. t t 2 m C dt V (5) d t () 6

3 Initially for required inlet and outlet ater temperature, NTU i calculated ere by uing erie of incremental metod. In ti metod, cooling toer i divided into mall control volume (dv). Te calculation tart at te bottom of toer, ere inlet air and ater condition are knon. Water to airflo ratio (m / m a ) i kept contant. Water temperature i increaed by a mall amount until te inlet ater temperature i reaced; correponding air entalpy i calculated by equation (6). In our calculation, mall incremental value of temperature i taken 0 C. m 2 C( t t2) ma (6) Air propertie at ater temperature t can be calculated by folloing empirical relation eq. (7-0) P exp t (7) P P (8).88t 2500 (9) g. 005t (0) g NTU for a control volume (dv) can be obtained a d dv dt NTU C m () For complete toer volume NTU i te um of NTU for mall control volume. Ti calculated NTU i te required NTU of te cooling toer to cool te ater from temperature (t ) to temperature (t 2 ) for te given condition of urrounding air and packing material. For performing enitivity analyi ti calculated NTU i ued to determine exit cold-ater temperature it uing te ame equation but te calculation require iteration. Te exit cold ater temperature a been evaluated eparately for % cange in different operating parameter. RESULTS AND DISCUSSION Senitivity analyi curve for te exit cold ater temperature from te counter flo cooling toer i on in Figure 3. Te bae value for te operating parameter are taken a NTU =.5, m = 00 kg/ec. m a = 00 kg/ec. m /m a =.00, t bt =25 0 C, t =40 0 C and te atmoperic preure P= 0.32 k Pa. 7

4 Te variation in te bae value i done for -30% to +30%, te correponding value of cold ater temperature are given in Table. Te percentage cange in cold ater temperature by canging bae value of operating parameter are on in Table. It i oberved from Figure 3 and Table tat te cold ater temperature i decreae by increae in m a and NTU and it i increae by increae in t, t bt, m and preure value. From Table it i alo oberved tat te exit cold ater temperature i more enitive to te cange in et bulb temperature (t bt ) and te inlet ot ater temperature (t ). Cange in 60% from te bae value of t bt and t cange te cold ater temperature (t 2 ) by 27.9% and 8.06% repectively. From ti table it i alo oberved tat, by increaing operating parameter value exit cold ater temperature increae it tee operating parameter value except for te increae in Air flo (m a ) and NTU. Table : Senitivity Analyi for Exit Cold Water Temperature % Cange Cange in (t ) Cange in (t bt ) Cange in m in bae Hot ater Cold ater value (t in 0 C) (t 2 in 0 t C) bt in 0 Cold ater m C in (t 2 in 0 C) kg/sec -30% % % % % % Bae value NTU=.5, m =00 kg/ec, m a =00 kg/ec, t bt =25 0 C, t =40 0 C, P=0.32 kpa S. No. Cold ater (t 2 in 0 C) S. No Cange in m % Cange in a Cange in NTU Cange in Preure m bae value a Cold ater Cold ater Preure Cold ater kg/sec (t 2 in 0 NTU C) (t 2 in 0 C) kpa (t 2 in 0 C) -30% % % % % % CONCLUSION Wit te performed enitivity analyi it i found tat Exit cold ater temperature from te cooling toer i more enitive to te et bulb temperature and inlet ot ater temperature but te effect of et bulb temperature on cold ater temperature i more compare to inlet ot ater temperature. Terefore election of location and range for ic cooling toer i deigned are alo important for te performance of a cooling toer. REFERENCES Kan Jr, Yaqub M and Zubair SM (2003). Performance caracteritic of counter flo et cooling toer. Energy Converion and Management Moiuddin AKM and Kant K (996). Knoledge bae for te ytematic deign of et cooling toer Part I Selection and toer caracteritic. International Journal of Refrigeration 9()

5 Moiuddin AKM and Kant K (996). Knoledge bae for te ytematic deign of et cooling toer Part II Fill and oter Deign parameter. International Journal of Refrigeration 9() Citranjan (2004). M.Tec.Tei, Centre for Energy Studie, Indian Intitute of Tecnology Deli. Kern and Donald Q (997). Proce Heat Tranfer, Second edition, Pubied by Mc Gra Hill (Ne Deli) Majumdar AK and Singal AK (983). Numerical modeling of et Cooling Toer Part: Matematical and Pyical Model. ASME Journal of Heat Tranfer Hill GB, Pring EJ and Oborn Peter D (990), Cooling toer Principle and Practice, Tird editon, Publied by Butter Wort, Heinemann Arrora CP (997). Refrigeration and air conditioning, Sevent edition, Publied by Tata Mc Gra Hill, (Ne Deli) , ASHRAE Sytem and Equipment Hand Book (2000). Cooling Toer 36. 9