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Thermodynamic Study of Diffusion Absorption Refrigeration System with Organic Fluid

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Thermodynamic Study of Diffusion Absorption Refrigeration System with Organic Fluid Int. J. Mech. Eng. & Rob. Res. 2015 Mukul Kumar and R K Das, 2015 ISSN 2278 – 0149 www.ijmerr.com Vol. 4, No. 1, January 2015 © 2015 IJMERR. All Rights Reserved Research Paper THERMODYNAMIC STUDY OF DIFFUSION ABSORPTION REFRIGERATION SYSTEM WITH ORGANIC FLUID Mukul Kumar1* and R K Das1 *Corresponding Author: Mukul Kumar, [email protected] In the recent years, the research interest on Diffusion Absorption Refrigeration (DAR) technology has increased significantly due to its ability to utilize exclusively low-grade heat to produce cooling effect. The operation of diffusion absorption refrigeration system is quiet and reliable therefore often used in hotel rooms and offices. The diffusion-absorption cycle utilizes ammonia-water- hydrogen as working fluid. In this paper, a mathematical model has been established and solved numerically. The model is based on the mass and energy conservation principles applied for every components of the DAR system, through which the working fluids flow. Equations have been developed to estimate mass flow rate, mass concentration and enthalpy of different fluids at various state points of the cycle by considering the mass balance and heat balance equations. Suitable thermodynamic relations have been used for estimating enthalpies at various points corresponding to their state properties of pressure, temperature and dryness fraction. The study showed that the COP of this refrigeration system, although not comparable with the COP of a vapour compression cycle, is encouraging, considering the fact that waste heat can be utilized for running the cycle and COP of the DAR system depends on different parameters like generator temperature, concentration of ammonia in rich solution, evaporator temperature and condenser temperature. Keywords: Ammonia-water, Bubble pump, COP, Circulation ratio, Diffusion absorption refrigeration INTRODUCTION There are no moving parts in DAR System, Diffusion Absorption Refrigeration (DAR) therefore this refrigeration system has quiet, system is one of the most fascinating field of reliable and maintenance free operation. DAR research interest during the last few decades, system was pioneer in 1920 by Von Platen and especially in developing countries like India. Munters (Pongsid Srikhirin et al., 2001). The 1 Department of Mechanical Engineering, Indian School of Mines, Dhanbad 826004, India. 473 Int. J. Mech. Eng. & Rob. Res. 2015 Mukul Kumar and R K Das, 2015 huge advantage of DAR system is that it can of water. This is rarely true, due to the fact that operate without electricity and thus reduces the rectifler is not 100% efficient in separating demand of electricity. Due to lack of moving water from ammonia. parts DAR system has low maintenance cost In this paper, we investigate, by numerical and because of environment-friendly simulation, the performance of a single stage refrigerant, it does not contribute to ozone diffusion-absorption cycle operating with NH / depletion or global-warming. 3 H2O mixture as working fluids and hydrogen DAR cycle utilizes triple fluids namely as auxiliary inert gas. A parametric study of ammonia, water and an auxiliary inert gas the effect of different system parameters on usually hydrogen, where ammonia is used as the COP is also presented by using the refrigerant and water as absorbent. The unique developed model. The performance of this feature of this cycle, as compared to a mixture is simulated with various generator and conventional ammonia-water absorption evaporator temperatures. cycle, is that introduction of the auxiliary inert gas plays its role to reduce the partial pressure SYSTEM DESCRIPTIONS of the refrigerant in the evaporator and allows A simplified schematic diagram of the DAR the refrigerant to evaporate at low temperatures cycle is depicted in Figure 1. The DAR system producing the cooling effect (Zohar and Jelinek, is similar to conventional ammonia water 2007). This refrigeration system is totally heat absorption refrigeration system with an inert operated and there is no need of electrical or gas usually hydrogen, diffused through the mechanical energy. Generally ammonia/water system to maintain a uniform system pressure mixture provides cooling at quite low throughout the cycle. The DAR system temperatures, –10 to –30 °C, depending on includes: (i) a generator which is basically a the diffusion absorption cycle conûguration but combination of a boiler and a bubble pump, requires moderately high driving temperatures (ii) a rectifier, (iii) a condenser, (iv) an which is higher than 150 °C, Although ammonia evaporator containing gas heat exchanger, (v) has excellent thermo-physical properties, it is an absorber, and (vi) a solution heat toxic, explosive, and corrosive to copper and exchanger. Geometry of the bubble pump is other non-ferrous metals. very simple and it contains a cylindrical hollow tube. Application of bubble pump is to raise Zohar et al. (2005) developed a complex the solution from the lower level to the higher thermodynamic model that explored the DAR one. Different operations of a DAR system are cycle performance. They found that the as follows: maximum COP values reachable with a DAR cycle can be obtained when the ammonia • A generator where the rich solution of concentrations in rich and weak solutions are ammonia is heated increasing the 0.25 to 0.30 and 0.10, respectively and that temperature of the solution, the generator temperature is approximately • A bubble pump through which bubbles or 200 °C. They assumed that the vapor exiting vapours formed during heating of solution the rectiûer was pure ammonia with no traces rise to the separator, 474 Int. J. Mech. Eng. & Rob. Res. 2015 Mukul Kumar and R K Das, 2015 • A rectifier for condensing out the water evaporator through a heat exchanger where vapour, if any, in the ammonia vapour, the liquid ammonia is sub cooled at 9 by the leaving the generator. cold ammonia and hydrogen mixture returning • A condenser for de-superheating and from the evaporator, which is mixed with the condensing ammonia vapour, uncondensed ammonia 8 and the mixture flows into the reservoir. • An evaporator where liquid ammonia and hydrogen gas mix and liquid ammonia The ammonia and hydrogen gas mixture evaporates at partial pressure absorbing enters the absorber coil from the bottom and heat from the region to be cooled, flows upward while the weak solution of ammonia enters the absorber coil from the top • An absorber where ammonia vapour is 16 and flows downward in a counter-current absorbed in the weak solution of ammonia arrangement. The ammonia vapor is readily and water, and absorbed in the weak solution and the resulting • A solution heat exchanger for exchanging rich solution flows to the reservoir. The auxiliary heat between the hot weak solution gas hydrogen is not absorbed and continues returning from the generator and the cold to flow to the evaporator with ammonia rich solution going toward the generator. residuals at 10. The strong solution leaves the reservoir at 14 towards the generator. Rich solution from the reservoir at 14 is Hydrogen leaving the absorber is at higher heated in the solution heat exchanger by the temperature since it absorbs some part of heat return weak solution and enters the generator liberated during absorption of ammonia in at 15. When heat is supplied to the generator weak solution. at 1, bubbles of ammonia gas are produced from the ammonia-water mixture. The Hydrogen gas with traces of ammonia after ammonia bubbles rise through the bubble leaving the absorber at 10 enters the pump lift tube to the separator where evaporator and passes through the heat ammonia vapour is separated and the exchanger. When hydrogen is allowed to mix remaining weak ammonia-water solution with the liquid ammonia at 12, the partial (weak in ammonia) is sent to the absorber at pressure of ammonia is reduced and this 16 through the liquid-liquid solution heat allows the liquid ammonia to evaporate at a exchanger. The ammonia vapor along with lower temperature. The evaporation of some amount of water vapour from the ammonia extracts heat from the evaporator, separator flows through the rectifier, where providing refrigeration to the region to be ammonia and water vapour mixture is cooled cooled. From the evaporator, the mixture of and water vapour is condensed to join the ammonia and hydrogen at 13 returns to the weak solution at 2. Almost pure ammonia reservoir. In the passage this mixture absorbs refrigerant produced by rectification at 6 some amount of heat separately from the liquid enters into the condenser and condenses to ammonia coming from the condenser and liquid at 7 the total pressure of the system. hydrogen gas coming from the absorber and The condensed ammonia flows to the moving towards the evaporator. 475 Int. J. Mech. Eng. & Rob. Res. 2015 Mukul Kumar and R K Das, 2015 One of the advantages of DAR system is 3. Supply of cooling water in the absorber that it is self circulating system due to the where ammonia vapour is absorbed in the gravity and density differences of the working weak solution liberating latent heat of fluid. liquefaction which is to be carried away by the coolant water. It is necessary to have the following external services for the functioning of the cycle. 4. Supply of coolant in the rectifier. This coolant may be air or water. In the rectifier 1. Supply of the heat at the generator (NH + H O) vapors are cooled and since temperature from the external sources, 3 2 water has higher saturation temperature which may be any source of waste heat than ammonia at any given pressure, available at generator temperature, like water vapour is condensed out and almost exhaust gas from automobiles or boilers, pure ammonia proceeds to the exhaust gas from gas turbines, solar energy, condenser. etc. 2. Supply of cooling water in the condenser, NUMERICAL MODELING OF in which ammonia vapour is de- DAR SYSTEM superheated and condensed to liquid, Numerical modeling of DAR system is liberating latent heat of liquefaction.
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