Carbon Dioxide based Heat Pump Dryer in Food Industry

Transcription

1 1 Carbon Dioxide based Heat Pump Dryer in Food Industry J. Sarkar 1, S. Bhattacharyya, M. Ram Gopal Department of Mechanical Engineering Indian Institute of Technology, Kharagpur West Bengal, India Abstract Drying is one of the common phenomena in food industry. Heat pump assisted mechanical dryer is used in lot of processes such as fruit drying, milk powder production. In food industry, due to the problem of ozone layer depletion and global worming, most of the conventional refrigerants are required to be replaced by some eco-friendly refrigerants. Carbon dioxide is one of the natural refrigerant which can be used in heat pump dryers in food processing applications due to its eco-friendliness, higher volumetric capacity, good heat transfer properties, high temperature heating capability, etc. Modeling of heat pump dryer is complex due to both heat and mass transfer. Ambient conditions (temperature and humidity) influence the performance of heat pump dryer. In the present investigation, the simulation models of open and closed heat pump dryers have been developed to investigate the performance with carbon dioxide as a refrigerant. Finally performance based comparisons have been done with two other conventional refrigerants R22 and R134a, which are generally used in heat pump dryers. Comparison results show that the CO 2 based heat pumps can give comparable performance (slightly better) with respect to R22 and R134a based heat pumps (irreversibility in the expansion valve is higher whereas that in the gas cooler is lower). CO 2 can become a best alternative refrigerant for drying applications. Keywords: CO 2 heat pump, heat pump dryer, food processing, simulation, performance. 1. Introduction Large quantities of agricultural products are dried to extend shelf-life, improve taste, enhance appearance and reduce transportation cost. Drying of food is complex phenomenon, behavior of drying rate with time is depending on the drying materials, although in general the constant period followed by falling rate period is exhibit within total time in the heat drying. In the junction of two-period, the moisture content in the material is called critical moisture content 1 Corresponding author; jahar@mech.iitkgp.ernet.in (J. Sarkar); Tel

2 2 [1]. Better understanding of the effect of drying conditions on the product quality and energy consumption of the equipment has promoted the use of heat pump cycle for drying applications. A heat pump dryer (HPD) is simply an integration of a basic refrigeration cycle with a thermally designed drying chamber. HPD offers several key advantages over other drying systems. It enables close control of the drying conditions, namely, drying air temperature, humidity and flow rate, resulting in better product quality. It reduces energy consumption through efficient regeneration of the drying energy by recovering the latent heat from the drying air. Further, with HPD operations, stage-dryings comprising combinations of hot and cold drying conditions can be carried out. This mode of drying operation is extremely important for delicate products that are sensitive to thermal conditions. Also, HPD ensures a consistent output of hygienically dried products. Depending on the operating temperature, heat pump drying is two types: I) Low temperature drying: Medicinal herbs and spices (specialty crops) such as ginseng, echinacea, feverfew, mother-wort, etc. are heat sensitive and required to be dried at low temperatures (30-40oC) to reduce the risk of loss in nutrient content and damage to physical properties which are important aspects considering their high commercial value. II) High temperature drying required in forage processing for dehydrated alfalfa products (pellets and cubes). The optimum moisture content of forages for cubing and pelleting is less than 10%. High-temperature drying is typically done in rotary drum dryers, where air temperatures are maintained in the range from 200 o C to 800 o C at the inlet to 60oC to 95 o C at the outlet. Pneumatic drying/conveying in a rotary drum dryer does not closely control heat and residence time resulting in under-dried stems and overdried leaves. This results in loss of nutritious leaf fraction and increases the susceptibility of highmoisture cubes and pellets to spoilage during overseas transport. Lot of research is going on drying of agricultural material, product and fruits to optimize the systems and process improvements [2-4]. Choosing of refrigerant for HPD is became a very critical due to the duel prospects of performance and environmental effect. Due to high ODP, R12, which was using for HPD in the earlier days, was replaced by R22. But now-a-day, most of the HPDs are using R134a instead of R22 due lower zero ODP and lower GWP (Table 1), although the value is very higher. So, due to the problem of ozone layer depletion and global worming, most of the conventional refrigerants are required to be replaced by some eco-friendly refrigerants. Carbon dioxide is one of the natural refrigerant which have been gained very importance recently [5], can be used in heat pump dryers in food processing applications due to its eco-friendliness, higher volumetric capacity, good heat transfer properties, high temperature heating capability, etc. Schmidt and coworkers have done Thermodynamic and experimental analyses of CO2 heat pump dryer in laundry recently [6-8]. In the present study, performance comparisons of R22, R134a and R744 heat pump dryers based on both energy and exergy analyses. The design issues of the heat transfer

3 3 components for all R22, R134a and R744 based HPDs have been discussed. Although different types of configurations can be used for HPD (Fig. 1), the simple configuration of HPD (a) is used for this study. Fig. 1. Heat Pump Dryer Configurations (E: Evaporator, C: Condenser/Gas cooler, D: Dryer) Table 1. Comparison of R22, R134a and R744 as refrigerants Refrigerant NBP C. P. C. T. ODP GWP Flammability Toxicity ( o C) (bar) ( o C) R22 (CHClF 2 ) No Yes R134a (C 2 H 2 F 4 ) No Yes R744 (CO 2 ) No No R22: Chlorodifluoro Methane, R134a: Tetrafluoro Ethane, R744: Carbon dioxide 2. Performance analysis Thermodynamic cycle of air in the closed heat pump dryer is shown in psychometric chart (Fig. 2) contains following processes: (i) a1-a3: cooling and dehumidifying in evaporator, (ii) a3-a4: Sensible heating in condenser/gas cooler, (iii) a4-a1: Cooling & humidifying in dryer. T-s diagrams of heat pump dryer for R22 or R134a and that for R744 are shown in Figs 3 and 4

4 4 respectively. Main difference of R744 cycle with the conventional cycle is the gas cooler instead of condenser, where the refrigerant is in the supercritical state, so instead of condensation the R744 gas is cooled by gas cooler at constant pressure. The following assumptions have been made for the performance analysis: 1. Compression process is adiabatic but non-isentropic. 2. Refrigerant at evaporator outlet is considered as saturated vapor. 3. Refrigerant at condenser outlet is considered as saturated liquid. 4. Evaporation and compression processes are isobaric 5. Heat transfer with the ambient has been neglected 6. Air at the evaporator outlet or the gas cooler inlet is considered as saturated (relative humidity is 100 %). 7. Outlet air condition from the dryer is same as the inlet condition to the evaporator. 8. Temperature approach (AT) for both the evaporator and the condenser/gas cooler is taken as 5 o C Fig. 2. Thermodynamic cycle of air in closed HPD From the energy balance in the evaporator, the cooling load for cooling and dehumidification of air is given by: Qev = m r ( h1 h4) = m a ( ωa 3)( Ta1 Ta 3) ( ωa 1 ω a3) (1) and the heating load in sensible heating of air is given by: Q = m h h = m ω T T (2) ( ) ( )( ) gc r 2 3 a a3 a4 a3 Power input to the compressor:

5 5 cp = gc ev = r ( 2 1 W Q Q m h h ) (3) Cooling coefficient of performance is given by: COP = Q W = h h h h (4) c ev cp ( ) ( ) Fig. 3. T-s diagram R22 & R134a heat pump dryer Fig. 4. T-s diagram R744 heat pump dryer Moisture extraction rate (MER) and specific moisture extraction rate (SMER) are the important performance measures of heat pump dryer, which are given by: MER = m ω ω ) (5) a( a1 a4 SMER = MER W c (6)

6 6 3. Numerical calculation A computer code has been written based on energy and exergy balance equations incorporating the property codes CO2PROP (based on Span and Wagner correlations [9]) for R744 and REFPROP [10] for R22 and R134a. Although the AT is attained at point 4 in the evaporator (T T = ), In the condenser the AT may be attained at saturated vapor point or a3 4 AT point 2, so the temperature approach condition is mathematically written as for R22/R134a HPD: h2 h min ( t 3 2 ta4), t2 t a3+ ( ta4 ta3) = AT (7) h2 h3 For CO2 system, the entire temperature range is divided into elemental sub-ranges to find the minimum temperature between refrigerant (AT) and the gas cooler pressure is set as the optimum compressor discharge pressure (bar), given by [11], P = t 0.17t t (8) dopt, c e c 2 Valid for evaporation temperatures between 20 C and 20 C and cooler exit temperatures: between 30 C and 60 C. The detailed algorithm of computer code is given bellow: 1. Give all the input parameters: t a1, ω a1, t, T, m a4 R a. 2. Assume ω a3 and take corresponding saturated temperature t a3. 3. t 1 = t a3 -AT and ωa4 = ωa3. 4. Find the state points of refrigerant (1, 2,3, 4) by cycle simulation satisfying AT 5. Find cooling load and refrigerant mass flow rate using Eqn. (1): 6. Find heating load and new value of ω a3 using Eqn. (2). 7. If the new value of ω a3 is not matching with old one, find new t a3 corresponding to new value and repeat the steps Find performance parameters COP c, MER, SMER and irreversibilities. 4. Result and discussion Results are presented in Table 2 for unit mass flow rate of air (1 kg/s) and input parameters: ta1 = 30 o C, t 4 80 o a = C, inlet moisture content of evaporator ω a1 = 0.02 kg/kg of dry air and reference temperature T R. Evaporator outlet saturated air temperatures are 11.5 o C, 13.0 o C and 12.4 o C for R22, R134a and R744 respectively. Results clearly show that due to the high system pressure the volumetric capacity of R744 is very lager compared to others. Another advantage for R744 is the smaller pressure compared to others, although the high system pressure can make some difficulty in design of components. COP of R744 based heat pump dryer is 10 % higher than that of R134a, whereas 7 % lower than that of R22. In term of moisture extraction rate also R22 is best compared to others. SMER of R744 system is 11% higher than that of R134a

7 7 and 4% lower than that of R22 systems. For optimum evaporator inlet moisture content of kg/kg, system is giving maximum cooling COP as shown in Fig 5. This type of behavior is exhibit due to the nature of saturated line in psychometric chart. Table 2. Performance Comparison for R22, R134a and R744 HPD Refrigerant ω a1 ω a4 r p COP c V c 3 kj m MER kg/min SMER kg/kwh R R134a R Fig. 5. Variation of COP with evaporator inlet moisture content Table 3. Component irreversibilities Refrigerant I CP (%) I EXP (%) I EV (%) I GC (%) I R R134a R The irreversibility analysis of the components show that due to the gliding temperature in the gas cooler, irreversibility of the gas cooler for R744 heat pump dryer is much smaller than that for R22 and R134a heat pump dryers. Due to the huge pressure drop and near critical operation, irreversibility of expansion device for R744 HPD is much larger than that for R22 and R134a HPD. In the actual heat exchanger design temperature approach for R744 system can be lower compared to R22 and R134a due to better heat transfer properties [12].

8 8 5. Conclusion The performance-based comparison of R744 based heat pump dryer has been done with two other conventional refrigerants R22 and R134a based heat pump dryer in terms of both 1 st and 2 nd laws and the following conclusions can be drawn: 1. R744 based heat pump dryer gives better performance than that of R134a whereas poor performance than that of R22 2. Irreversibility of expansion device for R744 is higher whereas that of gas cooler is smaller that of R22 and R134a. 3. There is some optimum moisture content for which the system can give the maximum performance due to behavior of psychometric chart. 4. Use of expansion turbine instead of throttling device can improve the system performance of R744 HPD significantly. 5. Because of the superior environmental properties and favorable heat transfer properties of R744 compared to R22 and R134a, the use of R744 as refrigerant in heat pump dryer in near future is promising. References 1. Arora CP. Refrigeration and air-conditioning, 2 nd Ed. Tata McGraw Hill Prasertsan S, Saen-saby P. Heat pump drying of agricultural materials, Drying Tech 1998; 16(1,2); Oktay Z, Hepbasli A. Performance evaluation of a heat pump assisted mechanical open dryer, Energy Conv Magmt 2003; 44; Teeboonma U, Tiansuwan J, Soponronnarit S. Optimization of heat pump fruit dryers, J Food Engg 2003; 59; Kim MH, Pettersen J, Bullard CW. Fundamental process and system design issues in CO 2 vapor compression systems, Prog Energ Comb sc 2004; 30; Schmidt EL, Klocker K, Flacke N, Steimle L. Applying the transcritical CO2 process to a drying heat pump, Int J Refrig 1998; 21(3); Klocker K, Schmidt EL, Steimle L. Carbon dioxide as a working fluid in drying heat pumps, Int J Refrig 2001; 21(3); Neksa P. CO 2 heat pump systems, Int J Refrig 2002; 25; Span R, Wagner W. A new equations of state for Carbon dioxide covering the fluid region from triple point temperature to 1100 K at pressure up to 800 MPa, J Phys Chem Ref Data 1996; 25; NIST database REFPROP version 6.0, USA. 11. Sarkar J, Bhattacharyya S, Ramgopal M. Optimization of a transcritical CO 2 heat pump cycle for simultaneous cooling and heating applications, Int J Refrig 2004; 27(8); Sarkar J, Bhattacharyya S, Ramgopal M. Transcritical carbon dioxide based heat pumps: Process heat applications. Int Ref A/c conf, Purdue, USA.