Measurement of Reliability of Evaporator used in Vapour Compression Refrigeration System (VCRS) 1 VivekDwivedi,Dr. A. K.Bajpai 2 1 Research Scholar, College of Engineering,TeerthankarMahaveer University, Moradabad. 2 Principal/Professor, Mechanical Engineering Department, Sri RammurtiSmarak, College of Engineering & Technology,Lucknow. ABSTRACT In VapourCompresion Refrigeration System (VCRS) evaporator is the component where the actual cooling takes place. It is the space from which the actual COP of refrigeration system is calculated. Generally in evaporator heat transfers in the form of latent heat where a refrigerant in liquid state is converted into vapour state by absorbing heat in the form of latent heat[2]. Evaporator is also called chiller. In this research paper the technique to measure the reliability of evaporator for fifteen years if its life span is given on the basis of Modeling and Simulation on VENSIM-PLE Software. KeyWords:-VapourCompresion Refrigeration System, evaporator, Modeling and Simulation, VENSIM-PLE 1.INTRODUCTION The evaporators used in VCRS must fulfill the following requirements (a). The temperature difference between the refrigerant and cooling substance should be very small when refrigerant absorbs heat. For this purpose the surface area of evaporative/cooling coil is kept large in basic design[3]. (b). It should have sufficient space that the liquid refrigerant completely changes into vapour form. (c). The space provided inside the evaporator should be such that the refrigerant an flow easily even for very small pressure difference between evaporator inlet and evaporator outlet[1]. The amount of het transferred by evaporator can be given by the following equation; Q= U A (t 2 - t 1 ) Where; U = Overall Heat Transfer Coefficient A = Surface area of evaporator t 2 = Temperature of the medium to be cooled t 1 = Saturation Temperature of refrigerant at evaporator pressure Some important causes which can affect the performance of evaporator are: The material of High Conductivity Non corrosive with refrigerant Eg. Evaporators made of iron or steel can be used for any type of refrigerant. Evaporators made of brass or copper can be used for any refrigerant except ammonia. Evaporators made of aluminum can be used for any refrigerant except brass and Freon. The temperature difference between refrigerant and the product to be cooled should not increase to 8 0 C. The evaporator s performance is much affected by the velocity of refrigerant flowing inside the evaporator. On increasing the velocity of the refrigerant, the pressure difference between inlet and outlet also increases which may produce adverse effect. Therefore the velocity of different refrigerant should be optimized or according to the recommended in data book[4]. Even it is seen that the wall thickness of evaporative coil does not affect much more to the performance or heat transfer rate of evaporator because the material used for its manufacturing is already of high thermal conductivity. The contact surface area of evaporative coil is another important factor which affects the heat transfer capacity or effectiveness of evaporator. In the design of evaporator the exposure of coil surface area should be more as much as possible according to its physical shape and size[3]. Volume 3, Issue 5, May 2015 Page 71
2.MAJOR PROBLEMS ON EVAPORATOR 1. Blockage in evaporator due to valve damage, introduction of foreign materials. It affects the cooling capacity of the evaporator. 2. Leakage of refrigerant due to holes or damage inside it. 3. Choking of evaporative coil when defrosting switch is damaged or over freezing takes place[5]. 2.1. OBSERVATION TABLE This Observation is based on the complaints in evaporators of hundred vapour compression refrigeration systems that are used simultaneously before fifteen years. Table 1 : Practically observed data recorded for different types of problems in evaporator No. of complaints Where λi = ------------------------------------ (Max. Yr. No. )* 100 λi λ av (C) = ---------------------------- 14 λi λ av (L) = ---------------------------- 9 λ av (B) = ---------------------------- λi 12 Volume 3, Issue 5, May 2015 Page 72
In modeling the reliability of components of evaporator is considered as it decays exponentially with time. 3.SYSTEM DYNAMIC MODELING Figure 1 :System Dynamic Modeling for reliability of evaporator 3.1. PROGRAMING DETAILS (1) BOEC= EXP(-0.003*Time) (2) CIEC= EXP(-0.011*Time) (3) FINAL TIME = 15 Units: Year The final time for the simulation. (4) INITIAL TIME = 0 Units: Year The initial time for the simulation. (5) LOR= EXP(-0.0034*Time) (6) ROE= BOEC*CIEC*LOR (7) SAVEPER = TIME STEP Units: Year [0,?] The frequency with which output is stored. (8) TIME STEP = 1 Units: Year [0,?] The time step for the simulation. Volume 3, Issue 5, May 2015 Page 73
4.GRAPHICAL OUTPUTS (a). Figure 2 : Time dependent reliability for choaking in Evaporative Coil (b). Figure 3 : Time dependent reliability for leakage of refrigerant (C). Figure 4 : Time dependent reliability for blockage of Evaporative Coil Volume 3, Issue 5, May 2015 Page 74
(d). Figure 5 : Time dependent reliability for Evaporator 4.1. OBSERVATION TABLE Table 2 : Reliability of different components with respect to time 5.RESULTS AND DISCUSSION The reliability of evaporator mainly depends on three parameters as choking in evaporative coil, leakage of refrigerant and blockage of evaporative coil. Their reliability degradation rate with respect to time is calculated by experimentally collected data. From the above data it is confirmed that choking of evaporative coil is important parameter that considerably affects the reliability of evaporator. During modification in the functioning of evaporator we must concentrate towards the rectification in the choking of evaporative coil than the other two parameters. From the observation table it is found that at the end of fifteenth year the reliability of choking coil falls to 84.78% regarding to its choking. The reliability of other two parameters like leakage of refrigerant and blockage of evaporative coil falls to 95.02% and 95.59% at the end of fifteenth years which do not much affect the performance of evaporator. Overall reliability of evaporator decreases from 100% to 77.02% at the end of fifteenth year for starting. This percentage is not feasible for confident running of vapor compression refrigeration system for fifteen years. Hence at this stage of design modification is necessary. Volume 3, Issue 5, May 2015 Page 75
6. CONCLUSION It is confirmed from the above data recorded for last fifteen years that major problems arise in evaporator due to the choking of evaporative coil. Therefore the reliability of evaporator mainly depends on the proper and healthy functioning of evaporative coil. If there is a need of increase in the reliability of evaporator, first it is important to modify the evaporative coil. REFERENCES [1]. Cullimore, B; Optimization, Data Correlation, and Parametric Analysis Features in SINDA/FLUINT; SAE- 981574. [2]. Cullimore, B; Reliability Engineering and Robust Design: New Methods for Thermal/Fluid Engineering; C&R White Paper, www.crtech.com, March 2000. [3]. Optimisation of Industrial Refrigeration Plants https://txspace.tamu.edu/bitstream/handle/1969.1/ 5624/ESL-IE- 06-05-09.pdf?sequence=1 [4]. [Pham, 2005] Pham, Q. T. (2005). Modelling heat and mass transfer in frozen foods: a review. Int. J. Refrigeration, 29:876 888. [5]. [Powell, 2002] Powell, P. (2002). In search of energy efficiency. Air Conditioning, Heating and Refrigeration News, 217(10):17. Volume 3, Issue 5, May 2015 Page 76