Journal of Advanced & Applied Sciences Volume 03, Issue 06, Pages , Simulation of an Optimized Vapour Compression Refrigeration System

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1 Journal of Advanced & Applied Sciences Volume 03, Issue 06, Pages , 2015 ISSN: Simulation of an Optimized Vapour Compression Refrigeration System T. S. Mogaji* * Department of Mechanical Engineering, Federal University Of Technology Akure, School of Engineering and Engineering Technology, P M B 704, Ondo State, Nigeria. * Corresponding author. Tel.: ; address: mogajits@gmail.com A b s t r a c t Keywords: Simulation data, Vapor compression Refrigeration system, Sub-cooling, condenser, Coefficient of performance, Performance improvement. This paper presents reports on simulation of an optimized vapor compression refrigeration system using a dedicated mechanical sub-cooling cycle in the system. The study aims at validating a developed double stage vapour compression refrigeration system using numerical investigation approach. A mathematical model was devised by applying the concept of energy balance in the thermodynamic cycle to the components of the vapour compression refrigeration system. The developed model implemented in MATLAB software was used to perform the numerical analysis using Hydrocarbons, R134a as working fluid. Simulation data was generated to observe the performance of the double stage vapor compression refrigeration system for an important parameters such as condensation (35 to 55 C) and evaporation temperatures (-5 to 15 C). Performance evaluation of the system was characterized in terms of cooling capacity and coefficient of performance (COP). The model results compared with the experimental result from literature revealing good agreement with the later. For the vapour compression refrigeration system being validated, simulated data showed that the performance of the VCR system with subcooling cycle improves from 3.3% to 11.4% and 3.1% to 12.2%, as the evaporation and condensation temperature increases and decreases respectively. Statistical analysis of the comparison results revealed that 80% of the experimental data obtained were successfully predicted within an error band of ±10% and the absolute mean error of 25.2%. This shows that the model predicts the system performance to a reasonable accuracy. Accepted:30 December 2015 Academic Research Online Publisher. All rights reserved. I. Introduction Performance of a vapor compression refrigeration system can be improved in a number of ways, other than by testing the system in a controlled environment experimentally; one of those ways is simulation of the system component analysis to achieve rapid and accurate result. Simulation has been widely used for performance prediction and optimum design of refrigeration systems [1]. Simulation techniques have also been used by researchers for design of vapor compression refrigeration system under steady state conditions [2]. [3], gathered empirical information directly by testing the vapor compression refrigeration system in a controlled humidity and temperature chamber

2 , and in other to achieve this, the system was properly instrumented. In the system simulation they employed the conservation laws to establish the governing equations that describe the system behavior, with each component modeled using a lumped approach, based on physical principles and employing empirical parameters such as friction factor and heat transfer coefficient. They concluded that the model showed good agreement with experimental data. Also the results from the study of [4] which is on validation of vapour compression refrigeration system design model showed that the model results were comparable to the actual system data from both quantitative and qualitative points of view under the same operational conditions. Typical vapour compression refrigeration cycle uses capillary tube, thermostatic expansion valve and other throttling devices to reduce refrigerant pressure from condenser to evaporator. Theoretically, the pressure drop is considered as an isenthalpic process (constant enthalpy). However, isenthalpic process causes a decrease in the evaporator cooling capacity due to energy loss in the throttling process. To recover this energy subcooled liquid prior to expansion process can be used by adding extra components such as internal heat exchangers in single-stage cycles and in two-stage cycles of the VCRS as pointed out in the study [5] Moreso, [6], reported in their study that liquid cooling below saturation in the conventional vapour compression cycles reduces the throttling losses and potentially increases COP of the system. In the study of [7], development of sub-cool system was carried out. The authors in their study presented a concept of a sub-cool system in which the liquid receiver is installed before the last pass to a parallel flow micro channel condenser rather than at the exit of the condenser. They observed COP improvement benefitted from subcooling due to an increase in enthalpy difference across evaporator. [8] investigated the performance of a VCRS with R134a, R152a and R12 employed as working fluid and observed that the R134a refrigerant made the VCRS to be highly efficient when compared to other refrigerants, provided there is proper addition of subcooling. The results from the study of [9] which is on experimental analysis of vapor compression refrigeration system with diffuser at condenser inlet revealed approximately 4% and 16% increase in the rate of heat rejection and COP respectively. Additionally, nearly 14% reduction in compressor work of the system was also observed by the authors. [10], worked on the effect of condenser liquid subcooling on system performance for refrigerants CFC-12, HFC-134a and HFC-152a. Their result revealed that the refrigeration cooling capacity of refrigerants; R134a (12.5%), R12 (10.5%) and R152a (10%), benefited from subcooling increase from 6 C to 18 C, while condensing temperature was kept artificially constant. [11] investigated the effects of condensation and evaporation temperatures, motive, suction and diffuser efficiencies as well as subcooling on the performance of the vapour compression refrigeration system. The result from his study revealed that condensation temperature has the highest effect on the performance improvement ratio of the system such that, the performance improvement ratio of the system is found to be doubled as the condensation temperature increases from 30 to 50 C. The objectives of the present paper are to obtain support data for the use of dedicated mechanical sub cooling cycle in VCR system in improving performance of the system using numerical investigation approach. Simulation and comparative 217 P a g e

3 analysis between the model results and experimental results obtained by [12] over a wide range of operating conditions was carried out. The effects of 2. Vapour compression refrigeration system with subcooling cycle description Fig. 1a shows schematic diagram of a vapour compression refrigeration system with subcooling cycle and the corresponding P-h diagram of the system is presented in Fig. 1b, where the basic vapour compression refrigeration system and the dedicated mechanical subcooled cycle system are represented by processes and processes respectively. The arabic numbers in Fig. 1a and b from 1 to 9 show the different state of the vapour compression refrigeration system with subcooling cycle and the number sequence indicates the flow direction of refrigerant in the system. The refrigerant enters the compressor at state 1, as saturated vapor and also with respect to the evaporation temperature. It follows the irreversible compression process 1-2. At state 2 the refrigerant is with extremely high pressure and superheated. The compressed refrigerant vapor runs from state 2 to state 3 where condensation process occurred. The process 3-4 represents the subcooling of the liquid refrigerant at the condenser condensation and evaporation temperatures on refrigerating effect and coefficient of performance (COP) of the system were also reported. outlet before passing through the expansion valve, hence arriving in state 3 as saturated liquid, the liquid refrigerant undergo further subcooling process at process 3-4. In this study, subcooling process is carried out using a dedicated mechanical subcooling cycle ( ). With the dedicated subcooling modification, liquid refrigerant leaving the condenser is further cooled at constant pressure to an intermediate temperature, T4, as shown in Fig. 1a. The process 4-5 represents the expansion of the subcooled liquid refrigerant by throttling from the condenser pressure to the evaporator pressure. Finally, the vaporized refrigerant is circulated through the compressor (1-2) and then condensate in the condenser (2-3). Shown in Fig. 2. is the system refrigerant thermodynamic flow process. In this way, less work is used to operate the compressor of the the vapour compression refrigeration system with dedicated mechanical subcooling cycle and, consequently, enhance the performance of the system. 218 P a g e

4 Fig. 1a: schematic diagram of the vapor compression refrigeration system with subcooling cycle Fig. 1b: pressure-enthalpy diagram of the model 219 P a g e

5 3. Mathematical model Fig. 2: System refrigerant thermodynamic flow process In this section the system components level mathematical models were developed based on the 3.1 Refrigeration system cooling load model In the present study, the system cooling load ( which comprises of both the product load ( and the infiltration load ( Q CL where Q PT Q PT Q l Q l Q CL Q PT ) is modelled as follows: (1) is the total product cooling load estimated such that for n product stored in the system, the total product cooling load is calculated as: n Q PT Q P 1 (2) ) ) mass and energy conservation principles described as follows. Q P Q AF Q where the terms F Q Q AF BF, Q F and Q BF (3) are defined as sensible heat load above freezing, latent heat of freezing, and sensible heat load below freezing respectively for the selected products and they are given as follow: Q AF mc a T 1 T 2 (4) QF mh fg Q BF mc b T 2 T 3 (5) (6) The term Q in Eq. (2) is the total load required to P cool a product from storage temperature t 1 to final temperature t 3 and is determined using Eq. (3): 220 P a g e

6 The detail of the terms ( shown in Table 1. n,m,t ) for the selected products considered in the present study are,c,c, h 2 a b fg Products Mass,m Kg Highest temperature, Table 1: Properties of the selected products [ o C] freezing T 2 Specific heat above freezing, KJ c a KgK Specific heat below freezing, KJ c b KgK Latent heat of fusion, Apple Fresh meat Water Source: Heat load in refrigeration systems [13] It is interesting to highlights that the average ambient temperature of the environment temperature T 3 T 1 and the final are assumed to be 30 o C and -10 o C respectively. The infiltration load ( Q ) given in Eq. (1) is calculated from: Q l 1.08V T T 1 3 where 1.08 is a multiplying factor and V average velocity of the door and is computed as: V v where, v r r H H r T T a r is the average velocity of the reference door H is the height of the door H r Q VC is the height of the reference door v l de (7) is the (8) 2 2 o VC i m o ho gz o m i hi dt 2 2 v T a KJ h fg is the temperature difference between the refrigerated space and the environment T r is temperature difference of the reference door. Therefore, the total cooling load for the main system is evaluated as follow: Q CL Q Q l PT KgK (9) Thus, the mass flow rate of the systems working fluid is calculated from : QCLEq m h h 1 9 (10) ' 5 In the present study, the concept of energy balance in thermodynamics cycles, applying first Law of thermodynamics for control volumes to obtain performance result that can meet the operating conditions imposed on each component shown in Fig.1a was adopted and is mathematically expressed according to Eq. (11) gz i W VC (11) Where the subscripts i and o in Eq. (11) stands for inlet and outlet states, respectively. It is well known that in vapour compression 2 refrigeration system, changes in kinetic, v 2 and potential energies, gz are negligible. Thus Eq. (11) becomes: Q W m h m h (12) VC VC o o i i 221 P a g e

7 Referring to the P-h diagram shown in Fig. 1b and application of Eq. (12) to each component of the system, the mathematical equations used to obtain the energy balance in each component are presented in the next subsections Heat exchangers model: evaporator, condenser and subcooler The heat rejected by the main VCR and sub-cooling VCR system condenser are calculated as follows: Q Cmain Q Csub m h 2 h 3 (13) m h 8 h 9 (14) The refrigeration capacity of the main and subcooling VCR systems are accounted as follows: Q Emain m h 1 h 5 ' (15) Q Esub m h 7 h 6 (16) 3.3. Compressor model The work done by the main and the sub-cooling VCR systems compressor is calculated as follows: W Cmain W Csub h 2 h 1 h 8 h 7 (17) (18) where, the subscript 2 and 8 refers to the enthalpy at the exit of the main and the sub-cooling VCR system compressor. Thus, the compressor power required by the main and the sub-cooling VCR system is estimated using Eqns. 19 and 20 given below respectively as: P P m Cmain W cmain m Csub W csub (19) (20) 3.4. Capillary tube model For the expansion process, the overall energy balance in the capillary tube for both main and sub-cooling The Mathematical models described above was implemented in MATLAB software to predict the response of principles components for both main and sub-cooling systems i.e compressor, condenser, expansion valve (capillary tube) and evaporator. The simulation model of the vapour compression refrigeration system with subcooling cycle shown in Fig.1a was devised assuming the following conditions (i) the refrigeration system operates at systems is accounted for using Eqns. 21 and 22 respectively: m h m h m 9 h 6 m 4 h 5 (21) (22) steady state regime, (ii) irreversibilities within the evaporator, condenser and compressor are ignored, (iii) no frictional pressure drops, (iv) refrigerant flows at constant pressure through the two heat exchangers (evaporator and condenser), heat loss to the surrounding are ignored and compression process is isentropic. The solution algorithm is illustrated in the information flow diagram depicted in Fig. (3). 222 P a g e

8 START Select Refrigerant R134a Is Tes & Tcs available in the Cool pack Software saturation table[13]? No Input Condenser Temperature and Evaporator Temperature (Te, Tc) Input Product Properties and Air infiltration parameter Yes Set Values of enthalpy, entropy, Specific volume & Pressure from the Cool pack Software Saturation table [13]. Compute product, Infiltration, Cooling load Compute Refrigeration effect, COP sub Print Product, Infiltration & Cooling load. Print effect, COP sub Refrigeration STOP Is Te & Tc available in the Cool pack Software saturation table[13]? No Yes Set Values of enthalpy, entropy, Specific volume & Pressure from the Cool pack Software Saturation table [13]. Compute Refrigeration effect, COP main Print Refrigeration effect, COP main Input Subcooling System Temperature (Tes, Tcs) and Subcooling degree Fig. 3: Flow chart of the simulation program 223 P a g e

9 The software input data are, product properties, evaporation temperature and condensation temperature. It is interesting to point out that the effect of quantity of products refrigerated on the performance of vapour compression refrigeration system can be evaluated with the developed model taking into consideration Eqns. 1-8 thus, the performance prediction and optimum design of refrigeration systems can be achieved with this model. The simulation of both refrigeration systems are characterized by refrigerating effect in terms of cooling capacity, compress power and coefficient of performance (COP) of the systems. The cooling capacity is calculated as: Q sub m h 1 h 5 (23) The compressor power is obtained as follows: P sub m h 1 h 2 (24) Thus, based on the simulation procedure carried out for both main and sub-cooling systems, the performance of the systems is evaluated as follows: COP sub Q P sub sub (25) 3.5 Simulation model validation The simulation results were compared against the experimental data obtained in the study of [12] for a wide range of condensation (35 to 55 C) and evaporation (-5 to15 C) temperatures in Fig. 4. In this figure, it can be noted the simulation results have a good agreement with the published data from literature. Table 1 presents the results of the statistical analysis of comparisons between the simulation data and the experimental data. The variation between the model and the experimental data is due to heat loss, which at present it is almost impossible to prevent since there is no perfect insulation of heat. Comparisons are based on the following parameters: (i) the percentage of the experimental data predicted by the model results within an error band of ±10%, (ii); and the absolute mean error defined as follows: The COP improvement COP imp expressed in % is calculated as follows; COP imp COPsub COPmain COPmain (26) where, the COP main is the COP at the same evaporator and condenser temperatures of the main VCR system. 224 P a g e

10 number of datapoints COP imp COP estimated imp exp erimental number of COP data points imp exp erimental (27) Table 1: Statistical analysis of comparisons between the simulation data and experimental data obtained in the study of [12]. Methodology Model validation/prediction result [%] ζ [%] Evaporation Condensation Evaporation Condensation Temperature [ o C] Temperature [ o C] Temperature [ o C] Temperature [ o C] 80 % 80% COP improvement ( o C) Present study model Mogaji and Yunisa [12] COP improvement ( o C) Present study model Mogaji and Yunisa [12] Condensation temperature ( o C) Evaporation temperature ( o C) Fig. 4: Comparison between present study model and previous experimental data from [12] It is interesting to point out that the comparisons results presented in Table 1 and Fig. 4 shows that the model predicts the system performance to a reasonable accuracy. 4. Result and discussion In this section, analysis of performance of vapour compression refrigeration system (VCRS) with and without subcooling cycle based on the use of numerical model developed for the systems are comparatively reported. Simulation data was generated over a wide range of evaporator and condenser temperatures of ( 5 to 15 o C) and (35 to 55 o C) respectively. 4.1 Comparative analysis An overview of the performance evaluation of the VCRS with and without subcooling cycle based on the effects of evaporation and condensation temperatures using the simulation model developed are shown in Figures 5-7. It should be noted that when one of them varied, the other parameters remain constant at a practical value. Figure 5 illustrate the effects of the evaporation temperature on COPs of both the VCRS with and without subcooling cycle. According to this figure, as expected the COPs of the systems increase with increasing the evaporation temperature. The VCRS with subcooling cycle is more sensitive to increase in evaporation temperature as higher COP is observed at any operational condition considered in this study. 225 P a g e

11 Such behavior is related to the fact that less work is used to operate the compressor of the VCRS with subcooling cycle compared to its counterpart without subcooling cycle. This trend is similar to those observed in the studies of [11] and [14]. Moreover, as can be noticed in Figure 4, the COP improvement ratio of the modified system is observed to increase from 3.3 to 11.4%,as the evaporation temperature increases from -5 to15, these simulated values validate those obtained experimentally in our previous study predicting 80% of the data within an error band of ±10 and the absolute mean error of 25.2% which proves the high energy efficiency of the VCRS with subcooling cycle. Fig 6 shows that the COPs for both VCRS with and without subcooling cycle decrease as the condensation temperature increases. It can also be notice that the COP of the modified system at lower condensation temperature of 35 o C is more sensitive higher than the COP of the VCRS without subcooling cycle. Additionally, as condensation temperature decreases from 55 to 35 C, the improvement ratio in COP increases from 3.1% to 12.2% these simulated values as displayed in Figure 4 predicted 80% of the experimental data obtained in our previous study within an error band of ±10% and the absolute mean error of 25.2%.These trends are similar to those observed in the study of [11] and [15]. This rate of improvement can be attributed to the fact that during a process through the VCRS with dedicated subcooling cycle, temperature of refrigerant increases. Due to this difference between temperature of refrigerant flows through condenser tubes and that of outside air flowing over condenser tubes increases resulting into increase in the rate of heat transfer from the condenser. Illustrated in Fig. 7 is the comparison between COP of the VCRS with and without subcooling cycle for a wide range of subcooling degrees. Obviously as can be notice from this figure, as subcooling degree increases both COPs are increased but with different rates. This behavior is due to reduction in exergy loss of the system under these operating conditions. (i.e decreasing condensation temperature). Thus, dedicated subcooling modification is responsible for the betterment of the system performance. From Figs.5 and 6, it was observed that by using dedicated subcooling cycle, up to 11.4 and 12.2 % performance improvement ratio of VCR system are achieved at evaporation temperature of 15 o C and condensation temperature of 35 o C respectively. Applying first law of thermodynamics to VCRS with subcooling cycle, it was observed that increase in subcooling degree increasing refrigerating effect, due to the reduction of the condenser exit temperature. Hence, net compressor work was reduced which result in betterment performance of the VCR system. 226 P a g e

12 Coefficient of performance COPsub COP main Evaporation temperature ( o C) Fig. 5: Variation of coefficient of performance with evaporation temperature 6 COP sub Coefficient of performance COPmain Condensation temperature ( o C) Fig. 6: Variation of coefficient of performance with condensation temperature 227 P a g e

13 15 COP sub 12 COPmain COP improvement ( o C) Subcooling temperature ( o C) Fig.7: Variation of performance improvement with sub-cooling temperature 5. Conclusion In this study, a simulation model is developed for VCRS with sub-cooling cycle where a main system is mechanically sub-cooled with another complete cycle system using R-134a as the working fluid. Numerical analysis on the performance of the system considering the effect of evaporation temperature, condensation temperature and subcooling degree using the simulation model has been identified. The performance evaluation of the system was characterized by refrigerating effect in terms of cooling capacity, compressor power and coefficient of performance (COP). Subsequently, the obtained simulation data were comparatively analyzed with experimental result from the study according to [12]. The model results compared with the experimental results revealed good agreement with the later. From the present study, the following main conclusions can be drawn: i. The COP of VCR system with dedicated mechanical sub-cooling cycle is higher compares with the main VCR system counterpart. ii. By using dedicated mechanical subcooling cycle, up to 11.4 and 12.2 % performance improvement ratio of vapour compression system are observed at evaporation temperature of 15 o C and condenser temperature of 35 o C respectively iii. As the subcooling degree temperature increases the COP of the VCR system increases. Similarly, Refrigerating effect of the system increases. iv. Evaporation temperature has the highest effect on the system performance improvement ratio. As the evaporation temperature increases from -5 to 15 o C, the performance improvement ratio of the system increases geometrically. v. According to experimental and simulation data the preceding comparative analysis on performance evaluation of both basic VCR system with and without subcooling cycle. It 228 P a g e

14 can be concluded that the use of dedicated mechanical sub cooling cycle in VCR system is most efficient and suitable for any cooling system application (air conditioning refrigeration and freezing). Acknowledgement The authors would like to acknowledge the assistance of the Refrigeration and Air References [1] Guo-liang Ding. Recent developments in simulation techniques for vapour-compression refrige ration systems. International Journal of Refrigeration 2007; 30: [2] Sanaye S, Malekmohammadi H.R. Thermal and economical optimization of air conditioning units with vapour compression refrigeration system. Journal of Applied Thermal Engineering 2004; 24(13): [3] Joaquim, M. G., Claudio, M. Christian J.L. A semi-empirical model for steady-state simulation of household refrigerators. Applied Thermal Engineering 2009; 29(8-9): [4] Akintunde, M. A. Validation of vapour compression refrigeration system design model. American Journal of Scientific and Industrial Research 2011;2(4): [5] Domanski, P.A., Didion, D.A. Evaluation of suction-line/liquid-line heat exchange in the refrigeration cycle. International Journal of Refrigeration 1994; 17: [6] Hrnjak, P S. Gustavo P. Effect of Condenser Subcooling of the performance of vapor compression systems: experimental and numerical investigation. international refrigeration and air conditioning conference at purdue, July 16-19, 2012 Conditioning Unit of the Department of Mechanical Engineering, Federal University of Technology Akure in supplying the equipment used in the present study. The technical support given to this investigation by Mr K. R. Yunisa and Mr. K. A. Adewole are also appreciated and deeplyrecognized. [7] Yamanaka Y, Matsuo H, Tuzuki K, Tsuboko T. Development of sub-cool system. SAE Technical Paper Series 1997; paper [8] Kamei, A; Piao, C.C; Sato, H, Watanabe, K. Thermodynamic Charts and Cycle Performance of FC-134a and FC-152a ASHRAE Transaction. 1990; 96(1): [9] Saudagar, R.T., Wankhede, U.S. Experimental Analysis of Vapour Compression Refrigeration System with Diffuser at Condenser Inlet. International Journal of Engineering and Advanced Technology 2013; 2(4): [10] Linton, J.W., Snelson, W.K., Hearty, P. F. Effect of condenser liquid subcooling on system performance for refrigerants CFC-12, HFC-134a and HFC-152a. ASHRAE Transactions 1992; 98: [11] Elgendy E. Parametric Study of a Vapor Compression Refrigeration Cycle Using a Two- Phase Constant Area Ejector. International Journal of Mechanical, Aerospace, Industrial and Mechatronics Engineering 2103;7(8): [12] Mogaji T.S. and Yinusa R. K. Performance evaluation of vapour compression refrigeration system using double effect condensing unit (Subcooler). International Journal of Engineering and Technology Sciences. 2015; 3(1): P a g e

15 [13] Ebook manual file on Heat load in Refrigeration System's by Roger D. Holder, CM, MSME [14] Mohan, C. Exergy analysis of vapour compression refrigeration system using R12 and R134a as refrigerants. International Journal of Students Research in Technology & Management 2014; 2(04): [15] Mishra, R. S. Methods of improving thermodynamic performance of vapour compression system using twelve eco-friendly refrigerants in primary circuit and nanofluid (water-nano particles based) in secondary circuit. International Journal of Emerging Technology and Advanced Engineering 2014; 4(6): P a g e