CONFIGURATION OF A 2 Kw CAPACITY ABSORPTION REFRIGERATION SYSTEM DRIVEN BY LOW GRADE ENERGY SOURCE

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1 International Journal of Metallurgical & Materials Science and Engineering (IJMMSE) ISSN Vol. 2 Issue 4 Dec TJPRC Pvt. Ltd., CONFIGURATION OF A 2 Kw CAPACITY ABSORPTION REFRIGERATION SYSTEM DRIVEN BY LOW GRADE ENERGY SOURCE 1 ANIL SHARMA, 2 BIMAL KUMAR MISHRA, 3 ABHINAV DINESH & 4 ASHOK MISRA 1 Department of Production Engineering, Birla Institute of Technology- Extension Centre, Deoghar, India 2 Department of Applied Mathematics Birla Institute of Technology- Mesra, Ranchi, India 3 Department of Electronics & Communication Engineering, Birla Institute of Technology- Extension Centre, Jaipur, India 4 Department of Mechanical Engineering, Birla Institute of Technology- Mesra, Ranchi, India ABSTRACT Consistently increasing CO 2 emission and ozone depletion from CFC s are serious environmental issues challenging scientific community. The dependence on fossil fuels has to be reduced and alternative environmental friendly options need to be explored. In this aspect, vapor absorption system gives scope of utilizing low grade energy source i.e. solar panel for generating cooling effect which is dominated by high grade energy driven compression technology. The LiBr aqueous solution based absorption cycle consists of four stages: generation, condensation, evaporation and absorption with ideally no moving part. In this paper, a configuration of 2kW capacity LiBr-Water based absorption refrigeration system is presented. The selection of evaporator capacity i.e. 2kW guides the operating conditions of other component in the cycle. Empirical correlations had been used to determine heat transfer rate. Based on these results specification for heat exchangers are established. This configuration serves as a platform to design for manufacturing of such systems. KEY WORDS: Absorption Cycle, Low Grade Energy, Overall Heat Transfer Coefficient, System Configuration INTRODUCTION In a basic absorption cycle, low pressure refrigerant vapor is converted to a liquid phase (solution) at same pressure. This conversion is made possible by the vapor being absorbed by a secondary fluid called absorbent. Absorbent absorbs refrigerant because of the mixing tendency of miscible substances, and an affinity between absorbent and refrigerant molecules. Thermal energy is released during this absorption process and should be transferred to a sink. Among various absorbent and refrigerant pair, LiBr- Water is most promising in chiller application due to high safety, high volatility ratio, high affinity, high stability and high latent heat. The working temperature for this pair should be above 0 0 C, because the refrigerant turns to ice at this temperature. The properties of this pair are vastly documented in literature, in this paper, the properties of LiBr aqueous solutions are referred from the reference [3]. In the cycle shown in figure 1, water vapor evaporate and separates from aqueous LiBr solution in generator, and increases the concentration of LiBr in solution. For this process heat is supplied from external source. At the same pressure, the water vapors from generator are condensed in condenser and this condensed water is throttled to the evaporator at low pressure. Due to reduced pressure, the water change phase and evaporate by taking latent heat of vaporization in the evaporator at chiller temperature and generates the cooling effect. At same pressure, the vapors from evaporator are absorbed by LiBr-aqueous solution supplied form generator having higher concentration of LiBr. The absorption of water vapor reduces the concentration of LiBr in aqueous solution, this solution is then passed to generator

2 2 Anil Sharma, Bimal Kumar Mishra, Abhinav Dinesh & Ashok Misra through pump at higher pressure. A solution heat exchanger is used between absorber and generator to increase the efficiency of system. In this paper, the source of heat in generator is low grade energy i.e. hot water at 90 0 C. This low grade energy can be obtained from solar panel, cooling of automobile engine, hot water release in several industries and other source of waste heat. The cooling water is circulated in absorber to release the heat of absorption at 28 0 C and after exit from absorber at 33 0 C, it is passed in the condenser to change the phase of water from vapor to liquid. NOMENCLATURE A area (m 2 ) σ surface tension C p specific heat (J/kg-K) D diameter (m) g gravitational acceleration(m/s 2 ) Nu Nusselt Number h heat transfer coefficient (W/m 2 -K) Pr Prandtl Number k thermal conductivity (W/m-K) Gr Grasfhoff Number m mass flow rate (kg/sec) Subscript: P pressure (Pa) i inside Q Heat transfer rate (W) o outside r radius (m) s Solution side Re Reynolds number h hot T Temperature (K) a absorber U overall heat transfer coefficient (W/m 2 -K) g generator X LiBr mass fraction (%) e evaporator α thermal diffusivity (m 2 /s) c condensor specific film flow rate (kg/m-s) C cold dynamic viscosity (kg/m-s) kinematic viscosity (m 2 /s) density (kg/m 3 ) l liquid f film D Diameter 1-10 Position as mentioned in figure. 1

3 Configuration of a 2 Kw Capacity Absorption Refrigeration System Driven by Low Grade Energy Source 3 Figure 1: Flow Diagram MASS FLOW RATE AND HEAT TRANSFER Assumptions Following assumption had been made to model the system. 1. Generator and condenser as well as evaporator and absorber are under same pressure. 2. There are no pressure changes except through the flow restrictors and the pump. 3. Refrigerant vapor leaving the evaporator is saturated pure water. 4. Liquid refrigerant leaving the condenser is saturated. 5. Strong solution leaving the generator is boiling. 6. Weak solution leaving the absorber is saturated. 7. No liquid carryover from evaporator. 8. Flow restrictors are adiabatic. 9. Pump is isentropic. 10. No jacket heat losses.

4 4 Anil Sharma, Bimal Kumar Mishra, Abhinav Dinesh & Ashok Misra Selection of Operating Parameters Table 1: Operating Parameters for Cycle Pressure in generator and condenser 7 kpa Pressure in absorber and evaporator 1 kpa Saturation temperature in generator c Saturation temperature in condensor c Saturation temperature in evaporator c Solution Temperature in absorber 36 0 c Solution Concentration at 3 55 % Solution Concentration at 5 60% Mass Flow Rate Calculations for 2kw Capacity Mass flow rate in evaporator (m 9 ) = Load / (Change in enthalpy) = 2 kw / ( ) = kg/sec Mass flow rate for weak and strong solution m 4 X 0.60 = m 3 X 0.55 also, m 3 = m 4 + m 7 m 4 = kg/sec m 3 = kg/sec m 1 = m 2 = m 3 ; m 4 = m 5 = m 6 ; and m 7 = m 8 = m 9 = m 10 HEAT TRANSFER RATE Heat transfer rate in evaporator is 2 kw, base on this selected parameter heat transfer rate at other components are calculated as follows. At Condenser (Qc) = m 7 (h 7 h 8 ) h 7 = Enthalpy of super heated steam at 70 0 C and 7 kpa = kj/kg h 8 = Enthalpy of water (saturated liquid) at 7 kpa and C = kj/kg Qc = KW. At Generator (Qg) = m 4 h 4 + m 7 h 7 m 3 h 3 h 4 = Enthalpy of solution at 82 0 C and 60 % Concentration = kj/kg h 3 = Enthalpy of solution at 62 0 C and 55 % concentration.

5 Configuration of a 2 Kw Capacity Absorption Refrigeration System Driven by Low Grade Energy Source 5 = k J/kg h 7 = Enthalpy of super heated steam at 70 0 C and 7 kpa. = k J/kg Qg = kw At Absorber (Qa) = m6h6 + m 10 h 10 m 1 h 1 h 6 = Enthalpy of solution at 50 0 C and 60 % concentration = k J/kg h 10 = Enthalpy of saturated water vapor at 1 kpa and C. = kj/kg h 1 = Enthalpy of solution at 35 0 C and 55 % concentration. = kj/kg Qa = kW EMPIRICAL CORRELATIONS Inside Tube Heat Transfer Coefficient (h i ) Petukhov-Popov Equation for condenser, generator, evaporator and absorber. In all these heat exchangers water flow inside tube without phase change. Here, Outside tube heat transfer coefficient (h o ) For Condensor For Generator Roshenow Correlation

6 6 Anil Sharma, Bimal Kumar Mishra, Abhinav Dinesh & Ashok Misra For Absorber Wilke s Correlation Film thickness For Evaporator h o = / 3 ρ Re 2 l g k l f µ l Log Mean Temperature Difference (LMTD) Over all Heat Transfer Coefficient (U) Heat Transfer Rate (Q) CONFIGURATION OF HEAT EXCHANGERS Standard dimensions of copper tubes had been selected from manufacturer s guide. Shell and tube type heat exchangers are been analysed due to their efficiency and ease in manufacturing. CONDENSOR Table 2: Specifications for Condenser Inner diameter of tube 0.43 inch Outer diameter of tube 0.5 inch Number of tubes 1 (6-tube pass) Tube orientation Horizontal Length of tubes 2m Overall heat transfer coefficient W/m 2 o C Cooling water in temperature 33 0 C Cooling water out temperature 37 0 C Mass flow rate of refrigerant - water kg/sec Log mean temperature difference (LMTD) 14.89

7 Configuration of a 2 Kw Capacity Absorption Refrigeration System Driven by Low Grade Energy Source 7 GENERATOR Table 3: Specifications for Generator EVAPORATOR Inner diameter of tube inch Outer diameter of tube inch Number of tubes 1 (twelve tube pass) Tube Orientation Horizontal Length of tubes 4.8 m Overall heat transfer coefficient W/m 2 o C Heating water in temperature 90 0 C Heating water out temperature 84 0 C Log mean temperature difference (LMTD) Table 4: Specifications for Evaporator ABSORBER Inner diameter of tube inch Outer diameter of tube inch Number of tubes 1 (8-tube pass) Tube orientation Horizontal Length of tubes 3m Overall heat transfer coefficient W/m 2 o C Chilling water in temperature 12 0 C Chilling water out temperature 8 0 C Mass flow rate of refrigerant - water kg/sec Log mean temperature difference (LMTD) 11 Table 5: Specifications for Absorber Inner diameter of tube inch Outer diameter of tube inch Number of tubes 1 (12-tube pass) Tube orientation Horizontal Length of tubes 5m Overall heat transfer coefficient W/m 2 o C Cooling water in temperature 28 0 C Cooling water out temperature 33 0 C Log mean temperature difference (LMTD) CONCLUSIONS Above calculated specification gives a platform to design for manufacturing of a 2 kw absorption system, driven by low grade energy source i.e. solar panel. Shell and tube type heat exchangers are analyzed due to their efficiency and ease in manufacturing. Among the components, absorber dimensions are highest and required further design modifications to make the system more compact. The design procedure used can be applied to scale up the capacity. This paper assists the economic analysis for manufacturing of absorption chillers. The current calculations has used some assumptions which need more analysis i.e. no jacket heat loss. In the analysis of absorber, empirical correlation for outside heat transfer coefficient ignores the fact of simultaneous mass transfer. Other fields of investigation are creating and sustaining vacuum, throttling process and optimum size of pump.

8 8 Anil Sharma, Bimal Kumar Mishra, Abhinav Dinesh & Ashok Misra REFERENCES 1. Arzoz, D., Rodriguez, P., Izquierdo, M. Experimental study on the adiabatic absorption of water vapor in to LiBr- H2O solutions, Applied Thermal Engineering 25 (2005) Asdrubali, F. and Grignaffini, S. Experimental evaluation of the performances of a H 2 O-LiBr absorption refrigerator under different service conditions, International Journal of Refrigeration, 28, (2005), pp ASHRAE Fundamental Handbook (SI): 2001, Atlanta, USA. 4. ASHRAE Handbook of fundamentals, Bourouis, M., Valles, M., Medrano, M., Coronas, A. Absorption of Water vapour in the falling film of water- (LiBr+ LiI+LiNO 3 +LiCl) in a vertical tube at air-cooling thermal conditions, Internatinal Journal of Thermal Scineces 44 (2005) Bredow, D., Jain, P., Wohlfeil, A., Ziegler, F. Heat and mass transfer characteristics of a horizontal tube absorber in a semi-commercial absorption chiller. International Journal of Refrigeration 31 (2008) Castro, J., Oliva, A., Segarra, C.D. and Oliet, C. Modeling of the heat exchangers of a small capacity, hot water driven, air-cooled H 2 O-LiBr absorption cooling machine, International Journal of Refrigeration, 31, (2008), pp Florides, G.A., Kalogirou, S.A., Tassou, S.A., Wrobel, L.C. Design and construction of a LiBr water absorption machine, Energy Conversion and Management 44 (2003) Goodheart, K.A. Low firing temperature absorption chiller system, Thesis submitted for Master of Science, University of Wisconsin-Madison, (2000). 10. Islam, M.R. Absorption process of a falling film on a tubular absorber: An experimental and numerical study, Applied Thermal Engineering, Elsevier, 28 (2008), pp Islam, M.R., Wijeysundera, N.E., Ho, J.C. Evaluation of heat and mass transfer coefficients for falling-films on tubular absorbers, International Journal of Refrigeration 26 (2003) Kaynakli, O. and Horuz, I. Comparison of parallel and counter flow coil absorber performance, International Communications in Heat and Mass Transfer, 33 (2006), pp Killion, J.D., Garimella, S. A critical review of models of coupled heat and mass transfer in falling film absorption, International Journal of Refrigeration, 24 (2001) Kohlenbach, P. and Ziegler, F. A dynamic simulation model for transient absorption chiller performance. Part-I: The Model, Part-II: Numerical results and experimental verification, International Journal of Refrigeration, 31 (2008), pp Kyung, I., Herold, K.E. and Kang, Y.T. Experimental verification of H 2 O/LiBr absorber bundle performance with smooth horizontal tubes, International Journal of Refrigeration, 30, (2007), pp Martineza, P.J., Pinazo, J.M. A method for design analysis of absorption machines, International Journal of Refrigeration 25 (2002)

9 Configuration of a 2 Kw Capacity Absorption Refrigeration System Driven by Low Grade Energy Source Medrano, M., Bourouis, M., Coronas, A. Absorption of water vapour in the falling film of water-lithium bromide inside a vertical tube at air-copoling thermal conditins, Internatinal Journal of Thermal Sciences 41 (2002) Misra, R.D., Sahoo, P.K., Sahoo, S. and Gupta, A. Thermo economic optimization of a single effect water/libr vapour absorption refrigeration system, International Journal of Refrigeration, 26 (2003), pp Nosoko, T., Miyara, A., Nagata, T. Characteristics of falling film flow on completely wetted horizontal tubes and the associated gas absorption, International Journal of Heat and Mass Transfer 45 (2002) Papaefthimiou, V.D., Karampinos, D.C., Rogdakis, E.D. A detailed analysis of water-vapour absorption in LiBr- H 2 O solution on a cooled horizontal tube, Applied Thermal Engineering 26 (2006) Patnaik, V., Perez-Blanco, H. A study of absorption enhancement by wavy film flows, International Journal of Heat and Fluid Flow 17 (1996) Sencan, A., Yakut, K.A. and Kalogirou, S.A. Exergy analysis of lithium bromide/water absorption systems, Renewable Energy, Elsevier, 30 (2005), p.p Sharma, A., Mishra, B.K., Dinesh, A., Misra, A. Theoretical investigation and parametric study of a solar driven absorption refrigeration system, Renewable Energy Asia 2008 An International Conference & 4 th SEE Forum Meeting, Center for Rural Development and Technology, Indian Institute of Technology Delhi, Hauz Khas, New Delhi- India. 24. Srikhirin, P., Aphornratana, S., Chungpaibulpatana, S. A review of absorption refrigeration technologies, Renewable and Sustainable Energy Reviews, 5 (2001) Subramaniam, V., Garimella, S. From measurements of hydrodynamics to computation of species transport in falling films, International Journal of Refrigeration 32 (2009) Wassenaar, R.H. Measured and predicted effect of flowrate and tube spacing on horizontal tube absorber performance, International Journal of Refrigeration 19 (1996) Xie, G., Sheng, G., Bansal, P.K., Li, G. Absorber performance of a water/lithium-bromide absorption chiller, Applied Thermal Engineering 28 (2008) Xu, Z.F., Khoo, B.C., Wijeysundera, N.E. Mass transfer across the falling film: Simulations and experiments, Chemical Engineering Science 63 (2008) Yoon, J., Phan, T., Moon, C., Bansal, P. Numerical study of heat and mass transfer characteristic of plate absorber, Applied Thermal Engineering 25 (2005)

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