DESIGN OF VAPOUR ABSORPTION REFRIGERATION SYSTEM OF 1.5TR CAPACITY BY WASTE HEAT RECOVERY PROCESS

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1 Volume 115 No , ISSN: (printed version); ISSN: (on-line version) url: ijpam.eu DESIGN OF VAPOUR ABSORPTION REFRIGERATION SYSTEM OF 1.5TR CAPACITY BY WASTE HEAT RECOVERY PROCESS Prakash.Matta 1,SaiKiran 2,Khaja SK 3,Manoj A.V.K 4. 1 k.l.university, Green Fields, Vaddeswaram, Guntur Dist. (pin ) Andhra Pradesh, India. 2 k.l.university, Green Fields, Vaddeswaram, Guntur Dist. (pin ) Andhra Pradesh, India 3 khaja754@gmail.com mobile Abstract: this project deals with the design of a vapor absorption refrigeration system. The initial conditions are considered and accordingly the design modules are calculated. The effective area of an evaporator is calculated for fixed cooling rate and the effective area of heat transfer of condenser is calculated. The capacity of a generator and absorber are calculated. The above obtained values are then verified to collaborate this design to an automobile industry where waste heat is recovered and provided as input to the designed absorption system. Keywords:.refrigeration system, evaporator, absorber, condenser, generator. 1. Introduction Refrigeration has become an essential part of the way we live our life. Almost everyone has a household refrigerator, but not many know of the process required to produce the drop in temperature that we know as refrigeration. Nature works much like a heat engine, heat flows from high-temperature elements to low-temperature elements. As it does this, work is also done to its environment. Refrigeration is a process to keep a cool element or to reduce the temperature of one element below that of the other. The refrigeration process is, in essence then, a reverse heat engine, where heat is taken from a cold element to be transferred to a warmer element, generally by adding work to the system. Figure1.1.basic absorption refrigeration cycle Absorption systems have been extensively paid attention in recent years due to the potential for CFC and HCFC replacements in refrigeration, heating and cooling applications. Furthermore, thanks to the progressive reduction of both installation and maintenance cost and energy consumption, their employment may become more and more diffuse. Most of industrial process uses lots of thermal energy by burning fossil fuel to produce steam or heat for the purpose. After the process, heat is rejected to the surrounding as a waste. This exhaust waste heat can be used as refrigeration by using a heat based refrigeration system, like vapor absorption refrigeration cycle. Despite a lower coefficient of performance (COP) as compared to the vapor compression cycle, absorption refrigeration systems are promising for using inexpensive waste energy from industrial processes, geothermal energy, solar energy etc. 613

2 2. Cycle description In such systems, low pressure refrigerant vapor leaves the evaporator and enters the absorber. Here the formation of solution pair takes place i.e. combination of refrigerant and absorbent and the formed solution is strong in nature. Now this strong solution is then pumped to the generator and here the pressure increases. In generator this strong solution is heated by some external source which in this case study is wasteheat. Figure.1.2.basic absorption refrigeration cycle After the heating process is accomplished strong solution at high pressure moves to the condenser leaving back the weak solution which is send back to the absorber using an expansion valve. Now the high pressure refrigerant moves from generator to the condenser where the refrigerant vapor is condensed to high pressure liquid refrigerant. This liquid refrigerant is passed to the expansion valve and where it is forwarded to the evaporator, where the refrigeration effect is achieved and thus completes the complete vapor absorption 3.Methodology The design of a vapour absorption system consists of the following modules. Design of an evaporator. Design of condenser. Design of generator. Design of absorber. The pressure of evaporator and absorber are maintained at 4.7 bar and the pressure of generator and condenser are maintained at 10.7 bar. Temperature of evaporator, condenser, generator and absorber are maintained at 2ºC, 54ºC, 52ºC &120ºC. By constraining the cooling rate at 1.5TR. Specific enthalpies of ammonia water absorbent solution are obtained from the charts. For an overall heat transfer coefficient of 1000w/m 2 using the LMTD procedure effective area of an evaporator is obtained or calculated and the same procedure is repeated for the further modules of condenser, absorber and generator. 4. Calculations The literature values for the design of the Aqua Ammonia vapor absorption system Capacity of system = 1.5 TR= = 5.25 kw Concentration of NH3 in refrigerant, X r = 0.98 Concentration of NH3 in Solution, X s = 0.42 Concentration of NH3 in absorbent, X w = 0.38 Temperature of the evaporator, T E = 2ºC Generator or condenser pressure, P H = 10.7 bar Evaporator pressure, P L = 4.7 bar Temperature of the Condenser, T C = 54ºC Temperature of the Absorber, T A = 52ºC Temperature of the Generator, T G = 120ºC Using the enthalpy-concentration diagram for aqua ammonia, at various concentrations of ammonia/water and corresponding saturation pressure, the corresponding enthalpy (KJ/Kg) and temperature (OC) can be calculated and on the basis of this enthalpy and various mass flow rates calculated, the heat transfer in the various components of the vapors absorption system are calculated. On the basis of this heat transfer the various components are designed. Table4.1: Values of mixture at various state point State Points Pressure in bars Temperature in ºC Specific Enthalpy h in KJ/Kg The enthalpy-concentration diagram has liquid saturation region. If the liquid is saturated at a given pressure and temperature. The enthalpy can be calculated by plotting a point corresponding to 614

3 the given concentration. The other regions are saturated vapor and superheated vapor region. Calculation Equations Mass Flow Rate: Consider State Point 2:- (saturated liquid) P 2 =10.7bar X 2 =0.98 Using the enthalpy concentration diagram for Ammonia / Water We get: T 2 = 54ºC h 2 = 200KJ/Kg State Point 3: (expansion of refrigerant through expansion valve from high pressure to low pressure at constant enthalpy) h 2 = h 3 = 200 KJ/Kg T 3 = 2ºC P 3 = 4.7 bar State Point 4: (extraction of heat by low pressure ammonia vapors in the evaporator) Saturation Pressure in evaporator; P 4 = 4.7 bar Evaporator temp;t 4 = 2ºC Using Enthalpy concentration diagram; Considering the ammonia vapor as saturated. h 4 = 1220 KJ/Kg Heat Extracted by evaporator Q E = m r (h 4 -h 3 ) m r = Mass flow rate of refrigerant Given that Q E = 5.25 KW=m r ( ) m r = gm/sec Using Mass Balance Equation: Mass Of solution (m s )=Mass of refrigerant(m r )+ Mass of absorbent(m w ) m s = m w + m r Using Mass Balance Equation for NH3; m s X s = m r X r + m w X w (m w + m r ) X s = m r X r + m w X w (m w ) 0.42 = m w (0.38) m w = gm/sec m s = m w + m r = m s = gm/sec Design of Evaporator Evaporator is an equipment in which refrigerant vaporizes to generate the desired refrigeration. It is also known as chiller. Considering the evaporator made of tubes and air cooled. Let air inlet temperature to evaporator th 1 = 30ºC Air outlet temp; th 2 = 5ºC θ 1 = 30-2 = 28ºC θ 2 =5-2 = 3ºC Figure 4.2.Temperature changes in evaporator [LMTD]= (θ m ) =. [ LMTD]= (θ m ) = (28-3) / ln (28/3) = º C Assuming, Overall heat transfer coefficient (U)=1000 W/m 2 Assuming, correction factor (F) = 1 Q E = FUA θ m = A E Area of the evaporator A E = m 2 Considering the number of evaporator tubes (n) = 30 Length of each tube (L) = 70cm Diameter of evaporator tube (D) = to be calculated The effective area of evaporator (A E ) = n π D L = 30 π D 0.7 D = 1.382cm = inches. Design of Condenser The function of the condenser is to remove the heat of the hot vapor refrigerant. State point 1 Ammonia Vapor Entering the condenser shell as a Saturated Vapor P 1 = 10.7 bar X r = 0.98 Using h-x Diagram for Ammonia/Water T 1 = 54ºC h 1 = 1135 KJ/Kg 615

4 Heat rejected by condenser Q C = m r (h 1 -h 2 ) Q C = ( ) Q C = 4.81 KW The cooling medium used is air. Inlet temperature of air is (t c1 ) = 30ºC Exit temperature of cooling air (t c2 ) = 45ºC θ 1 = = 24ºC θ 2 = = 9ºC Figure 4.3Temperature changes in condenser LMTDθ m = (θ 1 - θ 2 )/ ln (θ 1 /θ 2 ) LMTD (θ m ) = (24-9) / ln (24/9) = 15.23ºC Assuming, Overall heat transfer coefficient (U) =1000 W/m2 Assuming, correction factor (F) = 1 Q C = FUA θ m = A C Area of the Condenser A C = m 2 Considering the number of Condenser tubes (n) = 30 Length of each tube (L) = 70 cm Diameter of Condenser tube (D) = to be calculated The effective area of Condenser (A C ) =n π D L = 30 π D 0.70 D = cm = 0.19 inches. Design of Generator State Point 5: strong solution entering the pump as saturated liquid P 5 = 4.7 bar Xs = 0.42 Using enthalpy-concentration diagram; T 5 = 5ºC h 5 = KJ/Kg State Point 6: high pressure saturated strong solution entering the generator P 6 = 10.7 bar X s = 0.42 h 6 = 180 KJ/Kg State point 7: weak solution leaves the generator at saturation temperature of generator P 7 =10.7 bar X w = 0.38 Using h-x diagram h 7 = 255 KJ/Kg T 7 = 120ºC Using energy balance for generator Q G = Heat added to generator Q G = m r h 1 +m w h 7 -m s h 6 = ( ) + ( ) ( ) =12.27 KW Design of Absorber Heat rejected in the absorber Q A = m w h 8 + m r h 4 - m s h 5 Q A = ( ) + ( ) (72x179) = = Kw Considering the absorber to be direct contact heat exchanger in which the weak solution from the generator mixes with the ammonia gas from the evaporator and due to the direct mixing the heat is rejected. Air is used as cooling medium. COP of System Now, COP = Q E /Q G (Neglecting pump work W p ) COP=5.25/12.27= Further scope The final measurements of plant are finalized. It is observed this plant is not much large in size the absorption plant is coupled with an automobile where heat rejection is available of 12KW. The low grade thermal energy is used to run the absorption plant. Heat rejected in an automotive plant can be used todrive the absorption plant. The typical layout of an AC components are mentioned in below diagram for an idea 616

5 Figure 5.1. isometric view of air conditioned automobile Figure 5.4 top view of bus evaporator chamber is placed The AC compartment is modelled in Ansys to check whether the designedd air conditioner is capable of cooling the space. It is simulated in CFD FLOW criteria fixing boundary to the air conditioner and still compartment. The temperature contours of air flowing in compartment is observed to check cooling is attained. The boundary contains are inlet velocity of 3m/s at inlet of evaporator. Convective coefficient of still air 5(W/m 2 ). The evaporator interface at 290k and still air at 300k Figure5.2 front view of bus condenser is placed in radiator compartment Figure5.5 model of compartment with air conditioner at top\ Figure 5.3 backside view of bus generator is kept at heat rejection Chamber 617

6 Figure 5.6 loading model in ANSYS CFD FLUENT 6. Results COP = Q E /Q G (Neglecting pump work W p ) COP=5.25/12.27=0.427 Capacity of generator=12.27 KW Heat rejection at absorber=11.75 Heat exchange at condensor Q C = 4.81 KW Effective areas of heat exchange Evaporator A E = m 2 Condenser A C = m 2 busses. As the plant require the generator as heat input. So heat rejected from the automobile is diverted as input to the generator compartment which is beside it. The rest of cycle continues to heat exchanger from there to evaporator to absorber to generator again. So this type of plant are pollution n free uses low grade thermal energy so they are economical to use. 8. References [1] Absorption Refrigeration System as an Integrated Condenser Cooling Unit in a Geothermal Power Plant(Geothermal Training Programme Report Orkustofnun, Grensásvegur 9, IS-108 Reykjavík, Iceland) [2] Mohamed Arif.N, Y.udhayakumar and Inbarasan.G, Design of high frequency earthingsystemused for gas insulated substation, International Innovative Research Journal of Engineering and Technology, Vol.2, No..1, Sep [3] Design of Solar Powered Vapor Absorption System (Proceedings of the World Congress on Engineering 2012 Vol III WCE 2012, July 4-6, 2012, London, U.K) [4] Wikipedia [5] PREDICTION METHODOLOGY FOR THE HEAT REJECTION FROM TURBOCHARGED OR NATURALLY ASPIRED AUTOMOBILE ENGINES by OVERTON L. PARISH IV. B.S.M.E., M.S.M.E Figure 6.7 temperature contour of compartment 7. Conclusion It is evident that the compartment highest temperature is 297k which is sufficient to keep cool the surrounding. This particular size of plant is compatible to couple with an heavy automobile of (50-100) KW BP capacity as luxury travel 618

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