THERMODYNAMIC SIMULATION OF AMMONIA-WATER ABSORPTION REFRIGERATION SYSTEM. A. SATHYABHAMA and T. P. ASHOK BABU

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1 THERMAL SCIENCE: Vol. 12 (2008), No. 3, pp THERMODYNAMIC SIMULATION OF AMMONIA-WATER ABSORPTION REFRIGERATION SYSTEM by A. SATHYABHAMA and T. P. ASHOK BABU Orig i nal ci en tific pa per UDC: : : BIBLID: , 12 (2008), 3, DOI: /TSCI S The am mo nia-a ter ab orp tion refrigeration ytem i attracting increaing reearch in ter et, ince the y tem can be po ered by ate ther mal en ergy, thu re - duc ing de mand on elec tric ity up ply. The de vel op ment of thi tech nol ogy de mand re li able and ef fec tive y tem im u la tion. In thi ork, a ther mo dy namic im u la tion of the cy cle i car ried out to in ve ti gate the ef fect of dif fer ent op er at ing vari able on the per for mance of the cy cle. A com puter pro gram in C lan guage i rit ten for the per for mance anal y i of the cy cle. Key ord: ab orp tion re frig er a tion, am mo nia-a ter, com puter im u la tion, op - er at ing vari able, per for mance, ther mo dy namic anal y i Introduction In re cent year, the o ret i cal and ex per i men tal re earche on the ab orp tion re frig er a tion y tem (ARS) have in creaed, be caue thee y tem har ne in ex pen ive en ergy ource (like ate heat from ga and team tur bine, o lar, geo ther mal, bio ma) in com par i on to vapour compreion ytem. Beide, ARS caue no eco log i cal dan ger, uch a de ple tion of ozone layer and global arm ing, and hence they are en vi ron ment-friendly. Ex ten ive tud ie have been re ported in lit er a ture on the e lec tion of re frig er ant ab or - bent com bi na tion. The com bi na tion of am mo nia-a ter ha been at trac tive. It per for mance i better than that of flu o ro car bon re frig er ant ued in ab orp tion y tem and it i free from the lim i ta tion im poed by the high freez ing tem per a ture of the re frig er ant and lo cry tal li za tion tem per a ture of the o lu tion, a in a ter-lith ium bro mide y tem, or ex treme cor ro ive ne a in am mo nia-o dium thiocyanate y tem. The only di ad van tage of the am mo nia-a ter y tem i the vol a tile na ture of a ter ued a the ab or bent. Ho ever thi di ad van tage can be over come by in cor po rat ing a prop erly de igned rec ti fi ca tion column. Fig ure 1 ketche the re frig er a tion y tem [1]. Evap o ra tor, ex pan ion valve TV1 and con dener op er ate a in a me chan i cal com pre ion re frig er a tion y tem, ith the re main ing com po nent play ing the role of a com pre or. The va por leav ing the evap o ra tor i ab orbed by the eak o lu tion in ab orber form ing the trong o lu tion, hich i driven by o lu tion pump to the gen er a tor. Thi i com poed of deorber, here am mo nia va por i lib er ated ith a a ter con tent till too high for the evap o ra tor, and the com po nent that re move mot of the a ter: di - til la tion/rec ti fi ca tion col umn and dephlegmator. The part of the va por hich i con dened in the dephlegmator re turn a re flux (R) to di til la tion col umn here it ex change heat and ma ith the a cend ing va por. Af ter the dephlegmator, the am mo nia va por i ad mit ted to the con dener.

2 46 Sathyabhama, A., Ahok Babu, T. P.: Thermodynamic Sumulation of Ammonia/Water... Fig ure 1. Am mo nia-a ter ab orp tion re frig er a tion ytem A aborber, R rectifying column, RHX refrigerant heat exchanger C condener, D dephlegmator, SHX olution heat exchanger, E evaporator, TV throttle valve, G generator, P pump Figure 2. h-x diagram The re frig er ant heat exchanger (RHX) ubcool the liq uid re frig er ant from the con dener. The o lu tion heat exchanger (SHX) pre-heat the trong o lu tion hile pre-cool ing the eak o lu tion that leave the deorber, thu re duc ing the amount of ex ter nal heat needed, and alo de crea ing the re quired ize for both the deorber and the ab orber. The var i ou tate point are hon in h-x di a gram in fig. 2. The development of ammonia aborption refrigeration technology demand reliable and effective ytem imulation; everal com puter mod el have been de vel oped, and have proved to be valu able tool for ther mal deign optimization. Whitlo [2] ha ana - lyed both a ter cooled and air cooled cy - cle. Stoecker et al. [3] have com puted the effect of different operating temperature on COP of the cy cle. Since no con id er ation a given to component effectivene, the calculated COP repreent the maximum or ideal cycle performance. Shart et al. [4] analyzed thermodynamically the poibility to operate the olar aborption refrigeration y tem for air con di tion ing. Their re ult hoed that the y tem a uit able for do - me tic ue. Sun [5] an a lyzed and per formed an optimization of the ammonia-ater cycle. A a re ult, he ob tained a math e mat i cal model that al loed the im u la tion of the pro - ce. Sun [6] pre ented a ther mo dy namic de - ign and per formed an op ti mi za tion of the aborption refrigeration proce in order to map the mot com mon cy cle for am mo - nia-ater, and ater-lithium bromide. The re ult can be ued to e lect the op er a tion con di tion in or der to ob tain a max i mum per for mance from the y tem. Sun [7] per - formed a thermodynamic analyi of different bi nary mix ture con id ered in the ab - orption refrigeration cycle. He aumed that the refrigerant vapour con tain 100% am mo nia. Engler et al. [8] per formed a com pre hen ive invetigation of variou ammonia-ater cycle, ith operating condition and deign parameter var ied over a ide range to com pare their per for mance. They em ployed ABSIM code [9], hich ere developed pecifically for flexible imulation of aborption cycle. Large number of governing equa tion and the non-lin ear be hav ior of the ork ing fluid ere olved i mul ta neouly to give the de ign point per for mance. ABSIM provide realitic reult for the tationary behavior under

3 THERMAL SCIENCE: Vol. 12 (2008), No. 3, pp ell-defined boundary condition. Kandlikar [10] ha per formed a de tailed anal y i of of con ven - tional and modified cycle ith aborber heat recovery for ammonia aborption refrigeration ytem. Scope of the pre ent ork Analyi of the aborption cycle i made by everal invetigator for particular operating condition and auming 100% ammonia in the vapour leav ing the gen er a tor. In thi pa per com - plete invetigation of the cycle i made by conidering the individual component efficiencie. Anal y i of the cy cle The anal y i of the cy cle i made un der the fol lo ing a ump tion: the vapour leaving the dephlegmator i aturated at the average temperature of generator and condener temperature, the liquid leaving the condener i aturated at the condener temperature, the trong olution leaving the aborber i aturated at the aborber temperature, the eak olution leaving the generator i aturated at the generator temperature, the trong olution i heated only up to aturation temperature, heat exchanger effectivene i belo 1.0, the refrigerant vapour concentration x R at the dephlegmator exit i equal to 0.999, and con dener and evap o ra tor pre ure are con den a tion and evap o ra tion pre ure, re pec - tively; the ab orber pre ure i equal to evap o ra tor pre ure and the gen er a tor pre ure i im i larly equal to the con dener pre ure; pre ure drop in the y tem are not taken into conideration. In thi tudy, the cy cle pro cee ere mod eled u ing ma and en ergy con er va tion. Equi lib rium tate ther mo dy namic prop er tie of am mo nia-a ter o lu tion re quired for the anal y - i are taken from equa tion de vel oped by Patek et al. [11]. Thee equa tion ere con tructed by fit ting crit i cally a eed ex per i men tal data u ing im ple func tional form. They cover the re - gion ithin hich ab orp tion cy cle com monly ued op er ate mot of ten. Given the gen er a tor, con dener, ab orber, and evap o ra tor tem per a ture, and alo the cool ing load, the tate point prop er tie and the quan ti tie of heat up plied and re jected in the dif - fer ent com po nent are cal cu lated a de cribed be lo: the high ide and lo ide preure are determined from equilibrium correlation: the trong and the eak olution concentration are given by: P h = P(T c, x R ) (1) P l = P(T e, x R ) (2) x = x(t a, P l ) (3) x = x(t g, P h ) (4) Condener, refrigerant heat exchanger and evaporator Due to im per fect rec ti fi ca tion, a mall amount of vapour i alo car ried along ith am - mo nia vapour to the evap o ra tor. The con cen tra tion of a ter in the liq uid tate in creae along

4 48 Sathyabhama, A., Ahok Babu, T. P.: Thermodynamic Sumulation of Ammonia/Water... the evap o ra tor a am mo nia va por ize fater, due to hich the at u ra tion tem per a ture at the exit of the evap o ra tor i higher than the tem per a ture at the in let, i. e.: Tem per a ture of the ubcooled re frig er ant at tate 2: here E i the ef fec tive ne of RHX. For throt tling pro ce 2-3: T 4 = T 3 + DT c (5) T 2 = T 1 (T 1 T 4 )E (6) h 3 = h 2 (7) The re frig er ant flo rate for the given ca pac ity i cal cu lated by a heat bal ance acro the evap o ra tor: TOR mr (8) h h Heat ab orbed in the evap o ra tor: Heat re jected in the con dener: 4 3 Q e = m r (h 4 h 3 ) (9) Q c = m r (h 12 h 1 ) (10) Aborber, olution heat exchanger, generator Enthalpy at tate point 7: Pump ork: h 7 = h 6 + (P h P l )v 6 (11) W p = m (P h P l )v 6 (12) The ma flo rate of the trong and eak o lu tion are cal cu lated by mak ing a heat bal ance over the ab orber: mr ( xr x ) m (13) x x m = m m r (14) The cir cu la tion ra tio f i de fined here a the ra tio of the ma flo rate of the trong o - lu tion en ter ing the gen er a tor to the ma flo rate of re frig er ant: m f m r x x R x x Enthalpy at tate point 10 i cal cu lated by an en ergy bal ance acro SHX: m h10 h9 ( h8 h7 ) m and T 10 i no cal cu lated at the knon val ue of h 10 and x from: (15) (16)

5 THERMAL SCIENCE: Vol. 12 (2008), No. 3, pp h 10 = h l (T 10, x ) (17) For throt tling pro ce 10-11: h 11 = h 10 (18) Heat re jected in the ab orber i cal cu lated by mak ing an en ergy bal ance acro ab - orber: Q a = m h 11 + m r h 5 m h 6 (19) Dephlegmator The ex ten ion of the line join ing 8 and 8v in ter ect the line x R at h min. The enthalpy h o1 can be cal cu lated by the tri an gle prin ci ple, from graph i cal con truc tion hon in fig. 2 xr x hmin h8 ( h8 v h8 ) (20) x x 8 v But the prin ci pal op er at ing line mut be teeper than thi mix ture re gion io therm. Hence the op er at ing enthalpy i given by: h o =h 12 + [FR min (h min h 12 )] (21) here R min, the min i mum re flux in the dephlegmator i given by: R min h h min h h 12 1 and the re flux ra tio F i de fined a the ra tio of op er at ing re flux to the min i mum re flux. Heat re jected in the dephlegmator fig. 2: 12 (22) Q d = m r (h o h 12 ) (23) Then enthalpy h p i given by the tri an gle prin ci ple, from the graph i cal con truc tion in Heat in put to the gen er a tor: h p h o x x R R x x COP Q e W Qg Fi nally en ergy bal ance yield: ( h h ) (24) o Q g = m (h 9 h p ) (25) p (26) Q g + Q e + W p = Q c + Q a + Q d + error (27) A com puter pro gram in C lan guage i rit ten for the above anal y i. De crip tion of the pro gram The pro gram i con tructed to al lo for a e quen tial com pu ta tion of more than one prob lem. For each prob lem, the uer pro vide the fol lo ing in for ma tion: capacity of the ytem, generator temperature,

6 50 Sathyabhama, A., Ahok Babu, T. P.: Thermodynamic Sumulation of Ammonia/Water... condener temperature, evaporator temperature, and aborber temperature. The main pro gram call on ub rou tine for the ther mo dy namic prop er tie of the am - mo nia-a ter mix ture at dif fer ent tate. For the cal cu la tion of ther mo dy namic prop er tie, both in vapour and liq uid phae a ell a the vapour-liq uid equi lib rium, the im pli fied for mu la tion pro poed by Patek et al. [10] are ued in thi ork. Thee equa tion are in the fol lo ing form: T x = F 1 (p, x) T y = F 1 (p, y) y = F 2 (p, x) h l = F 3 (T, x) h v = F 4 (T, y) For cal cu lat ing any prop erty on right hand ide of the above equa tion, for ex am ple, to cal cu late T hen h l and x are given, an it er a tive cheme u ing Ne ton-raphon method i ued. The main pro gram then cal cu late the heat re jected/ab orbed in var i ou com po nent of the cy cle like the condenor, evap o ra tor, ab orber, dephlegmator, pump, gen er a tor, and the COP of the cy cle. The re ult along ith in put data are then printed in a file named out put. Re ult and di cu ion Out put of the pro gram for one et of op er at ing con di tion are hon be lo. In put data ca pac ity 1.0 TOR gen er a tor tem per a ture K condenor tem per a ture K ab orber tem per a ture K evap o ra tor tem per a ture K vapour con cen tra tion af ter dephlegmator Re ult condenor pre ure MPa evap o ra tor pre ure MPa eak o lu tion con cen tra tion rich o lu tion con cen tra tion vapour con cen tra tion af ter gen er a tor ma flo of re frig er ant kg/ rich o lu tion florate kg/ eak o lu tion florate kg/ cir cu la tion ra tio heat in put to the gen er a tor Q g kj heat re jected in the ab orber Q a kj heat re jected in the con dener Q c kj heat ab orbed in the evap o ra tor Q e kj heat re jected in the dephlegmator Q d kj

7 THERMAL SCIENCE: Vol. 12 (2008), No. 3, pp pump ork W p kj COP In or der to find the per for mance of the y tem un der dif fer ent op er at ing con di tion fol - lo ing range of op er at ing con di tion are ued: generator temperature from 60 to 140 C, condener and aborber temperature from 30 to 50 C, evaporator temperature from 5 to +5 C, and ith heat exchanger efficiency of 80%. The variation of COP ith gen er a tor tem per a ture i hon in fig. 3 for three val ue of condener and aborber temperature. Increae in condener and aborber temperature decreae the ytem performance. COP, in gen eral in creae ith gen er a tor tem per a ture reache a max i - mum and then de creae a a re ult of in creae in ir re ver ibil ity at higher gen er a tor tem per a ture. For con dener and ab orber tem per a ture of 25 C, the max i mum COP i 0.75 and oc cur at gen er - a tor tem per a ture of 65 C. For con dener tem per a ture of 30 C, max i mum COP i 0.67 and oc cur at generator temperature of 77 C. Maximum COP i 0.57 at gen er a tor tem per a ture of 97 C, for con dener tem per a ture of 40 C. There i a min i mum gen er a tor tem per a ture for each et of pec i - fied operation condition belo hich the ytem cannot function. Fig ure 3. Ef fect of gen er a tor tem - per a ture on COP Fig ure 4 ho the vari a tion of cir cu la tion ra tio ith gen er a tor tem per a ture. At loer gen er a tor tem per a ture the dif fer ence be teen rich and eak o lu tion con cen tra tion i mall re - ult ing in higher cir cu la tion ra tio a per eq. (15). Thi mean that the o lu tion pump ha to run fater or a big ger pump i re quired. At a given gen er a tor pre ure, eak o lu tion con cen tra tion (x ) i de ter mined by gen er a tor tem per a ture. If the gen er a tor tem per a ture reache it loer limit, the cir cu la tion ra tio in creae dra mat i cally mak ing the y tem in ef fi cient. Alo cir cu la tion ra tio i loer for lo con dener tem per a ture due to higher x val ue. Fig ure 5 ho the vari a tion of COP ith evap o ra tor tem per a ture for dif fer ent val ue of gen er a tor and con dener tem per a ture. Fig ure 4. Ef fect of gen er a tor tem per a ture on cir cu la tion ra tio

8 52 Sathyabhama, A., Ahok Babu, T. P.: Thermodynamic Sumulation of Ammonia/Water... Figure 5. Effect of evaporator temperature on COP COP in creae ith in creae in evap o ra tor tem per a ture and higher COP i ob tained for loer con dener and ab orber tem per a ture at con tant gen er a tor tem per a ture. At con tant con dener and ab orber tem per a ture, COP i higher at loer gen er a tor tem per a ture. Fig ure 6 ho the vari a tion of COP ith con dener and ab orber tem per a ture for gen er a tor tem per a ture 100 C. It i ob erved that vari a tion of COP ith ab orber tem per a ture i more prom i nent than ith con - dener tem per a ture. Same pat tern i ob erved at gen er a tor tem per a ture 150 C (fig. 7). Fig ure 6. Ef fect of con dener and ab orber Tem per a ture on COP at T g = 100 C Fig ure 7. Ef fect of condener and ab orber Tem per a ture on COP at T g = 150 C Con clu ion A com puter im u la tion i done to tudy the ef fect of op er at ing vari able on the per for - mance of am mo nia-a ter ab orp tion re frig er a tion y tem ith their ther mo dy namic prop er tie ex preed in poly no mial equa tion. The im u la tion i car ried out for pe cific op er at ing tem per a - ture. From the re ult it i ob erved that e can not chooe an ar bi trary com bi na tion of op er at - ing tem per a ture. For con dener and ab orber tem per a ture of 25 C, the max i mum COP i 0.75 and oc cur at gen er a tor tem per a ture of 65 C. For con dener tem per a ture of 30 C, max i mum COP i 0.67 and oc cur at gen er a tor tem per a ture of 77 C. Max i mum COP i 0.57 at gen er a tor tem per a ture of 97 C, for con dener tem per a ture of 40 C. One ha to be care ful in e lect ing the op er at ing vari able a lo heat re jec tion or high gen er a tor tem per a ture, do not nec e ar ily give higher COP val ue.

9 THERMAL SCIENCE: Vol. 12 (2008), No. 3, pp Nomenclature COP coefficient of performance, [ ] E effectivene of heat exchanger, [ ] F reflux ratio, [ ] f circulation ratio h enthalpy [kjkg 1 K 1 ] m ma flo rate [kg 1 ] P preure, [bar] Q heat quantity, [kw] R reflux T temperature, [K] TOR ton of refrigeration, [t] n pecific volume, [m 3 kg 1 1 ] W ork quantity, [kw] x ammonia mole fraction in liquid phae y ammonia mole fraction in vapour phae, [ ] Subcript a aborber c condener d dephlegmator e evaporator g generator l liquid phae of ater-ammonia mixture min minimum o operating p pole r refrigerant trong ammonia olution v vapour phae of ater-ammonia mixture eak ammonia olution Ref er ence [1] Threlkeld, J. L., Ther mal En vi ron men tal En gi neer ing, Prentice Hall, Up per Sadle River, N. J., USA, 1962 [2] Whit lo, E. P., Trend of Ef fi cien cie in Ab orp tion Re frig er a tion Ma chine, ASHRAE Jour nal, 8 (1966), 13, pp [3] Stoecker,W. F., Reed, L. D., Ef fect of Op er at ing Tem per a ture on the Co ef fi cient of Per for mance of Aqua-Am mo nia Re frig er at ing Sy tem, ASHRAE Tranaction, 77 (1971), 1, pp [4] Shartz, I., Shitzer, A., So lar Ab orp tion Sy tem for Space Cool ing and Heat ing, ASHRAE Jour nal, 19 (1977), 11, pp [5] Sun, D. W., Com puter Sim u la tion and Optimization of Ammonia-Water Aborption Refrigeration Sy - tem, En ergy Source, 17 (1976), 3, pp [6] Sun, D. W., Ther mo dy namic De ign Data and Optimum Deign Map for Aborption Refrigeration Sy - tem, Ap plied Ther mal En gi neer ing, 17 (1997), 3, pp [7] Sun, D. W., Com par i on of the Per for mance of NH 3 -H 2 O, NH 3 -LiNO 3 and NH 3 -NaSCN Aborption Re - frig er a tion Sy tem, En ergy Con ver ion Management, 39 (1998), 5/6, pp [8] Engler, M., Groman, G., Hellmann, H. M., Comparative Simulation and Invetigation of Ammonia/Wa - ter Ab orp tion Cy cle for Heat Pump Ap pli ca tion, Int. J. Re frig er a tion 20 (1997), 7, pp [9] Groman, G., Wilk, M., Ad vanced Modular Simulation of Aborption Sytem, Int. J. Re frig er a tion, 17 (1994), 4, pp [10] Kandlikar, S. G., A Ne Ab orber Heat Re cov ery Cy cle to Im prove COP of Aqua Am mo nia Ab orp tion Re frig er a tion Sy tem, ASHRAE Tranaction, 88 (1982), 1, pp [11] Patek, J., Klomfar, J., Sim ple Func tion for Fat Calculation of Selected Thermodynamic Propertie of the Am mo nia-wa ter Sy tem, Int. J. Re frig er a tion, 18 (1995), 4, pp Author' addre: A. Sathyabhama T. P. Ahok Babu Dept. of Mech. Engg., National Intitute of Technology Karnataka Surathkal Pot Srinivanagar, Mangalore , Karnataka, India Correponding author A. Sathyabhama athyabhamaa@hotmail.com Paper ubmitted: November 3, 2007 Paper revied: March 25, 2008 Paper accepted: April 10, 2008