Design and Development of a Heat Pump Rough Rice Dryer: A Study of Energy and Quality

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1 Design and Development of a Heat Pump Roug Rie Dryer: A Study of Energy and Quality M. TORKİ, M. SADEGHİ, M. DAVAZDAH EMAMİ 2, A. MOHEB 3 Farm Mainery Department, Isfaan University of Tenology, Isfaan , Iran 2 Meanial Engineering Department 3 Cemial Engineering Department sadegimor@.iut.a.ir Abstrat: Te drying step is one of te most ritial and energy onsuming unit operations in te grain proessing. Heat pumps ave been known to be energy effiient wen assisted wit drying operations. Te prinipal advantages of eat pump dryers emerge from te ability of tem to reover energy from te exaust as well as teir ability to ontrol te drying gas temperature and umidity. Te grain quality in te world marketplae is an important fator. In tis resear, a losed-loop eat pump roug rie drying system was designed, built and evaluated for energy performane, produt quality and oeffiient of performane (COP) of te eat pump. Te eat pump system design and sizing were performed using matematial modeling and omputer simulation for roug rie drying in deep bed bat dryer. Modeling of te eat pump system was arried out in HYSYS software. Te losed-loop eat pump grain dryer system onsisted of a grain drying bin and a eat pump system inluding an evaporator and two ondensers (an internal and an external ondenser). Refrigerant R34a was used as te working fluid in te ooling/eating yle. To primary evaluation of te system, drying experiments were onduted at temperature of 35 C and veloity of m/s. Te initial and final moisture ontents of rie were 8 and 2% (w.b.), respetively. Te results indiated tat te system ould not afford te energy saving in tis stage. So its effiieny is being improved in te resear. Key words: Heat pump, deep bed, matematial modeling, energy, roug rie drying INTRODUCTION Heat pumps ave been extensively used by industry for many years, but less for drying proess. Te modeling of eat pump dryers (HPDs) for design purpose as attrated resear interest more tan 20 years. Yet te literature remains divided on wat design features are appropriate for different appliations of HPDs (Oktay and Hepbasli, 2003). About 6.2 GJ energy is used to produe one ton of rop wit urrent tenology wi more tan 24% of tat is needed for te drying proess (Wang and Cang, 200). An improved dryer wit energy reovery sould redue te total energy requirement. A eat pump is attrative, beause it an deliver more energy as eat tan it onsumes in eletrial energy. Heat pump dryers offer several advantages over traditional oil-fired dryers or onventional gas-fired dryers for te drying of food produts and agriultural materials su as grains. Tese advantages inlude iger energy effiieny (redue energy onsumption by 60-80%), better quality, and te ability to operate independently of outside ambient weater onditions.in te literature te studies arried out on HPDs ave been lassified into tree groups. Te first group inludes studies in wi te performane analysis of tis system as been investigated. Te seond group overs studies on developing simulation models. Te investigations on te appliation of HPDs to drying system for industrial use belong to te last group. In most of tese studies, food produts and agriultural materials ave been seleted for drying in various types of HPDs (Oktay and Hepbasli, 2003). Several researers ave studied eat pump grain drying metods. Davis (949) determined te ability of te eat pump for ontinuous drying of selled orn. Sove (953) made a similar study on a laboratory-sale bat orn dryer. Fritz et al. (990) developed an experimental seed dryer based on te eat pump priniple to dry sweet orn seed on usked ears at a temperature of about 30oC, but energy effiieny of te drying system was not mentioned. Burris (993) used a eat pump deumidifiation system to dry tin layers of orn seed at drying temperatures of 35 to 40 C. He reported tat te eat pump drying system produed ig quality (germination rate) seed and 842

2 required about alf of te input energy of a gasfired drying system. Te objetive of tis resear was design, development and primary evaluation of a losedloop eat pump system for grain drying tat ould be more effiient tan onventional eletrial dryers. Figure illustrates a semati diagram of te setup. () (2) (3) d e ψ ε st ( a. b. )3 (. b. ) a 3 a ρ p ρb ρ p Figure. A semati diagram of te setup: ) drying amber, 2) digital sale, 3) ondenser, 4) evaporaotr, 5) external ondenser, 6) flexible tube, 7) PC, 8) eletrial eater, 9) digital multimeter, 0) frequeny nverter, ) entrifugal fan and 2) ompressor MATERIAL and METHODS Determination of rie pysial properties In order to simulate te rie drying in a bat dryer and design te dryer, determination of rie pysial properties are neessary. To design te dryer te most ommon roug rie variety in Isfaan provine (Central Iran) alled Sazendegi was seleted. Te samples were prepared from Agriultural and Natural Resoures Resear Center of Isfaan in September Table sows te measured dimensions and densities of tis variety at different moistures ontents. Equivalent diameter, speriity and porosity of Sazandegi roug rie were alulated by equations to 3, respetively. Table 2 indiates te values of tese properties at different moistures ontents. Carateristis of te fabriated dryer Te basi operating arateristis of te designed and fabriated dryer are summarized as follow: a) Te dryer was designed and onstruted for grain drying only. b) Te dryer was used for experimental studies. For aieving tis goal, te system was designed to operate at various onditions. ) Air irulation was aieved by means of a entrifugal fan wit a 2 p tree pase motor and maximum speed of 2800 rpm. d) Te dryer amber was a Plexiglas ylinder wit irular ross setion of 4 m internal diameter and 5 mm tikness and was onneted to te pats wit flexible PVC tubes and was loated on a digital laboratory sale. e) Te eletrial eater of te dryer was onstruted of 6 elements wit total eat apaity of 4.2 kw. f) A frequeny inverter was used to adjust te drying veloity. g) A digital multimeter was used to measure te eletrial power onsumption of te eater. ) A portable digital wattmeter was used to measure te eletrial power onsumption of te entrifugal fan. Table. Dimensions and densities of te Sazandegi roug rie ultivar. Moisture ontent (%, w.b.) Lengt Widt Tikness Bulk density (kg/m 3 ) True density (kg/m 3 )

3 Table 2. Equivalent diameter, speriity and porosity of Sazandegi roug rie ultivar. Moisture ontent Equivalent diameter Speriity Porosity (%, w.b.) Simulation of te eat pump dryer Heat pump dryer simulation was arried out in HYSYS software. In a parallel resear, simulation of Isfaan roug rie drying proess and its numerial solution was arried out in MATLAB software (Nagavi, 2008). Te results were used for eat pump dryer simulation. Closed loop onfiguration was seleted for eat pump dryer system. Working fluid of eat pump was R34a, beause of its plus points. Hig and low pressures of te working fluid in eat pump were onsidered 000 and 00 kpa, respetively. Aording to te minimum fluidization veloity and safe temperature for drying of te Sazandegi roug rie variety, veloity and temperature were seleted m/s and 35 C. For te first idea, an evaporator and a ondenser as is sown in Figure 2 was onsidered for te eat pump system. Te system was not stable wit tis arrangement of eat exanger. So in order to stabilize te system (by sedding of te energy from system), as is sown in Figure 3, an external -ooled ondenser was added to te system. After running te program, it was revealed tat te onsidered arrangement was appropriately stable. Te results of te system simulation are sown in Table 3. Carateristis of te fabriated eat pump Te basi operating arateristis of te designed and fabriated eat pump are summarized as follow: a) A ompressor wit maximum power of 84 W (/4 p). b) Te working fluid of te eat pump was R34a. ) Te evaporator and ondensers were aluminum finned opper tubed eat exanger wit dimensions of 0.26 m ig 0.24 m wide 2 rows deep. Tey ad two iruits of 0 tubes ea. d) An expansion valve was used to expand te refrigerant. e) Tree pressure gages were used in te ompressor inlet and outlet and in evaporator inlet to measure te refrigerant pressure. f) A ontrol devie was onsidered to prepare te ompressor safe operation ondition. Performane evaluation of te eat pump Te overall performane of a HPD may be araterized by several riteria. Among tem te oeffiient of performane (COP) and te speifi oeffiient of performane (COP) and speifi moisture extration rate (SMER) are te most important ones and ave been studied by many researers (Oktay and Hepbasli, 2003). Te oeffiient of performane (COP) Te energy effiieny of a eat pump is defined as COP and is defined as useful output eat to te input power ratio. For an ideal refrigeration operating between ondenser temperature (T ) and evaporator temperature (T e ), te maximum eating oeffiient of performane (COP, ) is obtained from Carnot yle as (Oktay and Hepbasli, 2003): COP, T T T e (4) For normal appliation, te power onsumption omes from te ompressor of te eat pump. In tis study te ompressor power onsumption ( ) was used to represent te input power (Oktay and Hepbasli, 2003). 844

4 Figure 2. Heat pump dryer arrangement witout external ondenser: ) Dryer, 2) evaporator, 3) ompressor, 4) ondenser, 5) expansion valve, 6) entrifugal fan and 7) eletrial eater Figure 3. Heat pump dryer arrangement witout external ondenser: ) dryer, 2) evaporator, 3) ompressor, 4) division valve, 5) ondenser, 6) external ondenser 7) refrigerant reservoir, 8) entrifugal fan, 9) expansion valve and 0) eletrial eater. Evaporator eat exange apaity (kj/) Table 3. Te result of te system simulation Condenser eat External ondenser eat exange apaity exange apaity (kj/) (kj/) Mass flow of refrigerant (kg/) Compressor power (W) # # Te ompressor adiabati effiieny was onsidered

5 (5) COP p, Q& W& d Instead of measuring te mass flow rate on te refrigerant side, te eat delivered in te ondenser ( Q & d ), was estimated as (Oktay and Hepbasli, 2003): (6) (7) Q& d were m & ρ & C V p, ( To, Ti, Te speifi moisture (SMER) ) extration rate SMER an be defined as te energy required to remove Kg of water and may be related to te input power into te ompressor (SMER p ) or to te total power into te dryer inluding te fan and eater powers (SMER wsl ), as given by (Oktay and Hepbasli, 2003): (8) (9) SMER And SMER p ws W& d SMERp W& f + W& ( + ( W& )) For primary evaluation of te designed and fabriated HPD some experiments were arried out. Sazandegi roug rie was dried in deep bed (20 m dept) in two modes, HPD and eletrial eater dryer system (EHD). As mentioned before te veloity and temperature were seleted m/s and 35 C. RESULTS and DISCUSSION In tis study a HPD was designed, developed and evaluated primarily. Te COP and SMER were investigated for te system. Te quality of te dried roug rie (perentage of fissured kernels) was examined as well. Te results of te COP and SMER examinations are sown in Table 4. As sown, te mean of Q & d obtained in te experiments is 38.3 J/s (or 498 kj/) wile te required Q & d predited by HYSYS software was 820 kj/ (for two ondensers). Tis is beause of low value of COP (0.76). Also te SMER ws of EHD was kj/kw. tat it is more tan SMER ws for te HPD (0.09). Te results of te fissured kernels perentage before drying and after drying by EHD and HPD are sown in Table 5. CONCLUSIONS In tis study a eat pump dryer was designed and developed based on simulation of Isfaan (entral Iran) roug rie drying proess and numerial solution of partial differential equations (PDE) arried out in MATLAB and HYSYS softwares. Ten te system evaluated primarily form energy onsumption and dried produt quality aspets. Te results sowed tat te eat pump dryer ould not afford te energy saving in tis stage. So its effiieny is being improved in te resear. Experimental study Table 4. Te results of COP and SMER examinations for HPD. T i, T o, d ( C) ( o C) Q & COP p, d SMER p f (kg/kw.) SMER ws (kg/kw.) (g/) (J/s) (J/s) (J/s) Table 5. Te results of roug rie quality evaluation. Treatment Fissured kernels(%) Before drying 9.3±0.88 After drying by EHD 9.3±.76 After drying by HPD 5.3±

6 NOMENCLATURE Q & eat delivered in te ondenser (kw) d mass flow rate of (kg/s) f input power to te entrifugal fan (kw) input power to te ompressor (kw) input power to te eletrial eater(kw) V & volumetri flow of (m 3 /s) COP, eating oeffiient of performane of te Carnot yle (-) COP p, eating oeffiient of performane of te eat pump (-) COP ws COP of te wole system (-) C p, speifi eat of (kj/kg K) d drying rate in kg water per our (kg/) SMER p speifi moisture extration rate for te eat pump (kg/kw.) SMER ws speifi moisture extration rate for te wole system (Kg/KW.) T ondenser temperature ( K) T e evaporator temperature ( K) T o, average temperature leaving te ondenser ( o C) T i, average temperature entering to te ondenser ( o C) Greek letter ρ density of (kg/m 3 ) REFERENCES Brooker, D. B., W. Bakker-Arkema and C. W. Hall, 992. Drying and Storage of Grain and Oilseeds. Publised by Van Nostrand Reinold. New York, N.Y.: Van Nostrand Reinold, In. Burris, J. S., 993. Impat of Deumidifiation Drying on Seed Quality and Preonditioning in Maize. Postarvest Biol. Te. 3: Davis, C. P. J., 949. A Study of te Adaptability of te Heat Pump to Drying Selled Corn. MS tesis. Purdue Univ., Lafayette, Ind. Fritz, V. A., H. A. Cloud, R. F. Deef and A. m. Borowski, 990. A Versatile Heat Pump Seed Dryer. Hort. Siene 25: Nagavi, Z., Simulation of Fixed Bed Rie Drying using Non Equilibrium Model Failitated wit Steady State Heat Pump. MS tesis. Isfaan University of Tenology, Isfaan, Iran. Oktay, Z. and A. Hepbasli, Testing of a Heat- Pump-Assisted Meanial Opener Dryer. Appl. Term. Eng. 23: Sove, G. C., 953. A Laboratory Investigation of te Adaptability of te Heat Pump to Bat Drying of Selled Corn. MS tesis. Kansas State Univ., Manattan, Kansas. Wang, D. and C. S. Cang, 200. Energy Effiieny of a New Heat Pump System for Drying Grain. Transations of te ASAE 44: