Application of absorption heat transformer to recover waste heat from a synthetic rubber plant

This paper reports the test results of the first industrial-scale absorption heat transformer (AHT) equipment in China, to recover the waste heat released from mixture of steam and organic vapor at 98 °C in coacervation section, synthetic rubber plant of Yanshan Petrochemical Corporation, Beijing, China. The recovered heat is used to heat hot water from 95 to 110 °C, feeding back to the coagulator as the supplementary heating source. The AHT system is operating with H₂O/LiBr solution with heat flow of 5000 kW. The coefficient of performance (COP), the thermal efficiency and the temperature lift of the AHT system are presented. The heat transfer characteristics of the AHT components, i.e. absorber, generator, evaporator, condenser and the liquid–liquid solution heat exchanger, are also illustrated by comparison of experimental data and the model calculated results. The results show that the mean COP is 0.47, the gross temperature lift of 25 °C can be realized and the payback period is ≈2 years.     

HCFC22-based vapour absorption refrigeration systems. Part II: Influence of component effectiveness

A thermodynamic analysis was performed on a single-stage vapour absorption refrigeration system (VARS) operating with monochlorodifluouromethane (HCFC22) as a refrigerant and dimethylether of tetraethyleneglycol (DMETEG) or dimethyl acitamide (DMA) as a working fluid. Influence of solution heat exchanger effectiveness (E_h) and mass transfer effectivenesses of absorber (E_a) and generator (E_g) on the performance of VARS were studied. The variations in heat quantities at solution heat exchanger, generator and absorber as well as performance characteristics, namely CR, COP_th and second law efficiency (ϵ) at various operating temperatures are reported. As expected, low CR and high COP_th and ϵ can be obtained at high values of E_h, E_a and E_g. The effects of E_h on the performance is more pronounced when compared to that of E_a and E_g. Also, the change in the circulation ratio (CR) due to a given change in E_a is higher than that due to the same change in E_g. CR is not a function of E_h. A comparison of the influence of E_h, E_a, and E_g on the VARS performance for HCFC22-DMA and HCFC22-DMETEG pairs was made.     

Heat and mass transfer characteristics of a helical absorber using LiBr and LiBr + LiI + LiNO3 + LiCl solutions

An experimental study has been performed to investigate the heat and mass transfer performance in a falling film absorber of a small-sized absorption chiller/heater. The components of the chiller/heater were concentrically arranged in a cylindrical form with a low temperature generator, an absorber and an evaporator from the center. The arrangement of such a helical-type heat exchanger makes the system more compact compared to a conventional one. As a working fluid, LiBr + LiI + LiNO₃ + LiCl solution is used to get improved heat transfer effect. The heat and mass transfer coefficients of the helical absorber provide similar values compared with the data obtained for horizontal absorbers at similar solution flow rates. The heat and mass transfer coefficients of LiBr + LiI + LiNO₃ + LiCl solution increase as the solution flow rate per unit length increases. However, if the solution flow rate is larger than 0.03 kg/m s, the heat and mass transfer increase is minimal. Thus, 0.03 kg/m s is recommended as an optimal solution flow rate. The heat and mass flux performance of LiBr + LiI + LiNO₃ + LiCl solution shows the tendency of 2-5% increase compared with that of LiBr solution.     

平板型吸收器的实验研究

由于其工作介质不含氟利昂，以及可利用较低品质废热和太阳能等从而节约能源减少二氧化碳的排放量等优点，近年来吸收式制冷机／热泵越来越受到关注。但是为了与压缩式制冷机在所有市场范围内进行竞争，还需要进一步提高性能，降低成本和减小尺寸。而其中吸收器是吸收式机组中的关键设备，无论对整机性能还是外形尺寸都有很大影响。与传统的溴化锂水平管式吸收器不同，本文从易于布置从而减小机组尺寸的角度出发，开展了平板型吸收器的研究并给出相关实验结果，以期为该类型吸收器的设计提供参考。     

HCFC22-based vapour absorption refrigeration system: Part I: Parametric studies

Influence of operating temperatures at generator, absorber, condenser and evaporator on the performance of a single-stage vapour absorption refrigeration system (VARS) working with HCFC22-DMA, HCFC22-DMF and HCFC22-DMETEG pairs as working fluids is studied by performing thermodynamic analysis. Variations of circulation ratio (CR), heat load at each component, Carnot and thermodynamic coefficients of performance (COP_cr and COP_th) and second law efficiency (ϵ) at various operating parameters are reported. It is deduced that the CR, COP_th and ϵ decrease with T_g while COP_th decreases and CR and ϵ increases with T_a and T_c. Also, CR values are lower whereas COP_th and ϵ values are higher at high T_e. HCFC22-DMETEG yields the lowest value of CR at low heat source temperature while the change in CR between the considered pairs is negligible at high heat source temperature. HCFC22-DMA stands out when both COP_th and ϵ are included.     

A mathematical model with experiments of single effect absorption heat pump using LiBr–H2O

A mathematical model of a single effect, LiBr–H₂O absorption heat pump operated at steady conditions is presented. This model took into consideration of crosscurrent flow of fluids for heat and mass exchangers, two-dimensional distribution of temperature and concentration fields, local values of heat and mass transfer coefficients, thermal parameter dependent physical properties of working fluids and operation limits due to the danger of the LiBr aqueous solution hydrates and crystallization. Improvements of the calculation method make this simulation much more convenient and efficient. An improved absorber experiment set-up and a complete absorption heat pump were built and tested for further study. It was found that the mass flux of vapor increased with the increase of absorber pressure, coolant flow rate, spray density of LiBr solution and decrease of coolant and input temperature of solution. And the vapor mass flux increased almost linearly with the increase of absorber pressure. Results derived from this model show agreement within 7% with experimental values.     

Numerical investigation of hydrodynamic and heat transfer performances of nanofluids in a fractal microchannel heat sink

Heat transfer and hydrodynamic performances for nanofluids, Al₂O₃‐water and SiO₂‐water, are numerically investigated with different nanoparticles’ volume fractions and the initial velocities in a fractal microchannel heat sink. The fractal microchannel is 100 μm × 100 μm in the inlet cross‐section, and the length at the 0th level is 2000 μm. A constant heat flux of 500 kW/m² was applied to the bottom wall of the fractal microchannel heat sink. The heat transfer and hydrodynamic performances of different cases are discussed in terms of the mean heat transfer coefficient, mean base temperature, pressure loss, thermal resistance, friction factor f/f₀, and COP/COP₀. Results indicate that increasing the initial velocity and nanoparticles’ volume fraction lead to an enhanced heat transfer at the expense of pressure loss. Al₂O₃‐water has a higher mean heat transfer coefficient and pressure drop than that of SiO₂‐water, a lower f/f₀, mean base temperature, thermal resistance, and COP/COP₀. Ultimately, as compared to pure water, the heat transfer coefficients of 4% Al₂O₃‐water increased by 7.53%, 7.80%, 8.00%, 8.14%, 8.16%, and 8.30%, and the pressure drops increased by 32.09%, 31.41%, 30.81%, 30.05%, 29.21%, and 28.58%, respectively, corresponding to the initial velocities by 4, 5, 6, 7, 8 and 9 m/s.     

Energy and exergy analysis of an experimental single‐stage heat transformer operating with the water/lithium bromide mixture

The first and second law of thermodynamics have been used to analyze the performance of an experimental single‐stage heat transformer operating with the water/lithium bromide mixture. Enthalpy coefficients of performance (COP), external coefficients of performance (COP_EXT), exergy coefficient of performance (ECOP), exergy destruction or irreversibility in the system and components (I) and the improvement potential (Pot) have been calculated against the gross temperature lift and the main operating temperatures of the system. The results showed that the highest COP, COP_EXT and ECOP values are obtained at the highest solution concentrations meanwhile the Pot and the I of the cycle remain almost constant against these parameters. Also it was shown that the COP, COP_EXT and ECOP decrease with an increase with the absorber temperature, meanwhile the Pot and the I increase. Moreover, it was observed that in all the cases independently of the operating temperatures of the system, the absorber accounts with most of the half of the total irreversibility in the system. Finally, it was shown that the improvement potential is considerable for the system. Copyright © 2009 John Wiley & Sons, Ltd.     

Error propagation on COP prediction by artificial neural network in a water purification system integrated to an absorption heat transformer

Numerous authors have reported heat transfer prediction using artificial neural network (ANN). However, the precision or accuracy of the calculation is generally unknown. Error propagation from Monte Carlo method is applied to the coefficient of performance (COP) predicted by ANN. This COP permitted us to evaluate a water purification process integrated into a heat transformer. A feedforward network with a hidden layer was used in order to obtain error propagation in COP prediction. This model used the input and output-temperatures for each component (absorber, generator, evaporator, and condenser), as well as two pressure parameters from the absorption heat transformer and LiBr + H₂O mixture with different LiBr concentrations. The hyperbolic tangent sigmoid transfer-function and the linear transfer-function were used for the network. A new correlation for calculating relative standard deviation (%RSD_COP) of COP as a function of COP_EXP and %RSD_instrument was obtained. This study shows that %RSD_COP of ANN prediction decreased when the experimental COP is increased. The range of COP operations was from 0.21 to 0.39.     

Analysis of the process characteristics of an absorption heat transformer with compact heat exchangers and the mixture TFE–E181

In the project described in this paper an experimental rig for a one-stage absorption heat transformer was designed and constructed. One aim of the project was to reduce the investment costs for the apparatus. This incorporates new and less expensive compact brazed plate heat exchangers for generator, evaporator, condenser and solution heat exchanger. The absorber was designed as a helical coil pipe absorber, where the weak solution trickles down as a falling film outside of the coil. The tests of the equipment involved measurements using a mixture of trifluorethanol (TFE) and tetraethyleneglycoldimethylether (E181). The process characteristics were investigated for different temperatures of the rich solution leaving the absorber. Experimental results are presented and compared with the results of a computer simulation model. Additionally the model was used to compare the COP of the heat transformation process with the mixtures lithium bromide–water (LiBr–H₂O) and ammonia–water (NH₃–H₂O). Furthermore, the overall heat and mass transfer coefficients for the plate heat exchangers and the falling film absorber were evaluated and compared with those of shell and tube heat exchangers.     

Increase of COP for heat transformer in water purification systems. Part II – Without increasing heat source temperature

The integration of a water purification system allows a heat transformer to increase the actual coefficient of performance, by the reduction of the amount of heat supplied by unit of heat. A new defined COP called COP_WP is proposed for the present system, which considers the fraction of heat recycled. Simulation with proven software compares the performance of the modeling of an absorption heat transformer for water purification (AHTWP) operating with water/lithium bromide, as working fluid–absorbent pair. Plots of enthalpy-based coefficients of performance (COP_ET) and water purification coefficient of performance (COP_WP) are shown against absorber temperature for several thermodynamic operating conditions. The results showed that the proposed (AHTWP) system is capable of increasing the original value of COP_ET up to 1.6 times its original value by recycling energy from a water purification system. The proposed COP_WP allows increments for COP values from any experimental data for water purification or for any other distillation system integrated to a heat transformer, regardless of actual COP_A value or working fluid–absorbent pair.     

Absorption heat transformers and an industrial application

Absorption Heat Transformer (AHT) systems are devices with the unique capability of raising the temperature of low or moderately warm waste heat sources to more useful levels. The study includes an investigation to analyze the AHT systems using water-lithium bromide solutions with water as the refrigerant. First, a basic AHT system was described, the operating sequence was explained and thermodynamic system analysis was presented. Next, an application of the AHT system to an industrial company was analyzed. A computer code was prepared to determine the effect of different parameters on the AHT system performance and the results were presented in graphical form. Additionally, it was shown that how the basic AHT system could be modified to increase the COP and the heat transfer at the absorber, in other words, the hot process water produced. The system performance data were presented in a tabular form for different system modifications from the base system for comparison. It was proven that, by applying different modifications, the COP could be increased by 14.1%, the heat transfer at the absorber by 158.5% and the hot process water produced by 3.59% compared to the basic AHT system.     

Absorption of water vapour in the falling film of water–lithium bromide inside a vertical tube at air-cooling thermal conditions

In the absorbers of air-cooled water–lithium bromide absorption chillers, the absorption process usually takes place inside vertical tubes with external fins. In this paper we have carried out an experimental study of the absorption of water vapour over a wavy laminar falling film of water–lithium bromide on the inner wall of a smooth vertical tube. The control variables for the experimental study were; absorber pressure, solution mass flow rate, solution concentration and cooling water temperature. Relatively high cooling water temperatures were selected to simulate air-cooling thermal conditions. The parameters considered to assess the performance of the absorber were; the mass absorption flux, the outlet solution degree of subcooling and the falling film heat transfer coefficient. The results indicate that in water cooling thermal conditions the mass absorption fluxes are in the range 0.001–0.0015 kg·m⁻²·s⁻¹, whereas in air-cooling thermal conditions the range of mass absorption values decreases to 0.00030–0.00075 kg·m⁻²·s⁻¹.     

Heat and mass transfer from a single horizontal tube of an evaporative tubular heat exchanger

In this investigation, water side heat transfer coefficients without air flow from a single horizontal tubes are determined. Mass transfer coefficients are determined with water and air flow from the same tube. The total energy dissipated by inside hot fluid when only water is falling is compared with that when both the air and water flow past the tube. The water side heat transfer coefficient and mass transfer coefficient are given by empirical relations h_w = 6.0(Re_p)^0.18(Re_w)^0.87 and K = 3.5(Re_p)^0.18(Re_a)^0.28 (Re_w)^0.54, respectively. The ratio of energies dissipated with water and air flow and with only water flow increases with Re_w and Re_a and its maximum value is 1.72 in the range of variables used.     

Cycle analysis of air-cooled absorption chiller using a new working solution

A cycle analysis was achieved to predict the characteristics by comprehensive modelling and simulation of an air-cooled, double-effect absorption system using the new H₂O/LiBr+HO(CH₂)₃OH solution. The simulation results showed that the new working fluid may provide the crystallisation limit 8% higher than the conventional H₂O/LiBr solution. With a crystallisation margin of 3 wt%, the optimal solution distribution ratio was found to be in the range of 37–39%. Variation of cooling air inlet temperature had a sensitive effect on the cooling coefficient of performance (COP) and corrosion problem. The simulation of heat exchangers with UA value revealed that the absorber and evaporator were relatively important for an air-cooled system compared with the condenser and the low temperature generator. The effects of cooling air flow rate, circulation weak solution flow rate and chilled water inlet temperature were also examined. The new working fluid may provide a COP approximately 3% higher than the conventional H₂O/LiBr solution in normal conditions of circulation weak solution.     

Modeling and performance analysis of an absorption chiller with a microchannel membrane‐based absorber using LiBr‐H2O,LiCl‐H2O,and LiNO3‐NH3

In order to develop compact absorption refrigeration cycles driven by low heat sources, the simulated performance of a microchannel absorber of 5‐cm length and 9.5 cm³ in volume provided with a porous membrane is presented for 3 different solution‐refrigerant pairs: LiBr‐H₂O, LiCl‐H₂O, and LiNO₃‐NH₃. The high absorption rates calculated for the 3 solutions lead to large cooling effect to absorber volume ratios: 625 kW/m³ for the LiNO₃‐NH₃, 552 kW/m³ for the LiBr‐H₂O, and 318 kW/m³ for the LiCl‐H₂O solutions given the studied geometry. The performance of a complete absorption system is also analyzed varying the solution concentration, condensation temperature, and desorption temperature. The LiNO₃‐NH₃ and the LiBr‐H₂O solutions provide the largest cooling effects. The LiNO₃‐NH₃ can work at a lower temperature of the heating source, in comparison with the one needed in a LiBr‐H₂O system. The lowest cooling effect and coefficient of performance are found for the LiCl‐H₂O solution, but this mixture allows the use of lower temperature heating sources (below 70°C). These results can be used for the selection of the most suitable solution for a given cooling duty, depending on the available heat source and condensation temperature.     

HCFC22-based absorption cooling systems,part III: Effects of different absorbers and condenser temperatures

Generally in a vapour absorption refrigeration system (VARS) heat rejection temperatures at absorber (T_a) and condenser (T_c) are taken to be equal. However, different temperatures can exist when the cooling water flows in series through the two components. Under such situations, it is essential to know which of T_a and T_c has greater influence on the performance of the VARS. Here the influence of different T_a and T_c on the performance of a single-stage VARS working with HCFC22 as a refrigerant and three organic solvents, namely DMA, DMF and DEMTEG, as absorbents is studied. Results are obtained over a wide range of operating temperatures. To improve the performance of HCFC22-based VARS, results reveal that (i) the cooling water in parallel pipe connections should be used at low values of temperatures at evaporator, cooling water and heat source, and (ii) cooling water should first flow through condenser and then through the absorber when evaporator and heat source temperatures are high over the complete range of cooling water temperatures. COP_th is more sensitive to T_c than to T_a.     

Performance characteristics of an ammonia–water absorption heat pump system

The influences of the performance parameters and the heat transfer characteristics of the absorption heat pump using ammonia–water mixture are theoretically carried out. There is a pronounced effect of the ammonia concentration ξ after rectifier on the temperature glides that has been investigated. At ξ = 0.9000 and saturation pressures of 75 and 0.5 bar, the temperature glides are 64.4°C and 81.21°C, respectively, whereas these glides are 0°C and 16.1°C at ξ = 0.9999 and at the same pressures. This mixture property considerably affects the absorption system performance and the design of the rectifier as well as other absorption components. A correlation of the Nusselt number, Nu, is developed and compared with some published work in the literature for plate type heat exchanger. The effects of ammonia concentration ξ, mass fraction spread Δξ, specific solution circulation ratio f, and pressure ratio R_p on the refrigerant mass flow rate, the pressure drop, and the heat transfer coefficients during the condensation, the evaporation, and the absorption processes are investigated. It was found that increasing ammonia mass fraction spread Δξ results in both specific circulation ratio f and R_p that have insignificant effects on the refrigerant mass flow rate. Mounting Δξ at constant f reduces the pressure drop gradually and subsequently starts to increase as Δξ escalates. The ammonia concentration ξ has insignificant effect on the evaporation heat transfer coefficient but has a little effect on the condensation and the absorber heat transfer coefficients. The ammonia mass fraction spread Δξ and f have considerable effects on the heat transfer coefficient for different absorption heat pump components. R_p has a pronounced effect on the evaporation heat transfer coefficient, although it has a slight effect on the condensation and the absorber heat transfer coefficients. The effect of R_p on the heat transfer coefficient may be eliminated in the absorber for Δξ > 0.18. Copyright © 2013 John Wiley & Sons, Ltd.