IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

The double-effect parallel flow absorption refrigeration cycle with water-lithium bromide as working fluid is analysed based on the concept of equilibrium temperature at the low pressure generator. Coefficient of performance (COP) and its sensitivity to operating conditions are compared with those for series flow cycle. Maximum attainable COP for parallel flow cycle is greater than that for series flow cycle throughout the range of operating conditions considered here. Performance of parallel flow system is more sensitive to the effectiveness of low pressure heat exchanger than that of series flow system. © 2001 Elsevier Science Ltd.

GENERAL

Department of Mechanical Engineering

Applied Thermal Engineering

Performance comparison of double-effect parallel-flow and series flow water-lithium bromide absorption systems

The equilibrium temperatures at the low pressure (LP) generator for double-effect series flow lithium bromide–water vapour absorption chiller are evaluated and the system performance is estimated at these temperatures. Influence of temperatures at high pressure (HP) generator, evaporator, condenser and absorber, and the effectiveness of heat exchangers on equilibrium temperature and internal heat transfer at LP generator, circulation ratio, and coefficient of performance are studied. Dual-heat mode of operation of the system is also investigated utilising low grade waste heat at the LP generator. Correlations are presented for equilibrium LP generator temperature, internal heat transfer at the low pressure generator, circulation ratio, coefficient of performance, optimum HP generator temperature and optimum circulation ratio for maximum coefficient of performance in terms of operating temperatures, which are useful in the design and control of absorption system even at the off-design conditions.

World University root node

Equilibrium low pressure generator temperatures for double-effect series flow absorption refrigeration systems

Miniaturization of the absorption refrigerator requires an air cooled absorber and condenser and replacement of the customary solution pump by a bubble pump. The operating pressures in the bubble pump operated two fluid vapour absorption refrigerator have to be very low (vacuum) so as to make use of the hydrostatic principle to maintain the required pressure difference between the condenser and evaporator. Hence, only water and alcohol based working fluid combinations are found suitable. Thermodynamic properties, such as specific heat and heat of mixing, and thermophysical properties, such as vapour pressure, density, viscosity, surface tension and solubility, are necessary for the overall performance evaluation of such a refrigerator. In this paper, sixteen water based working combinations have been analyzed and compared with reference to the solution density governing the hydrostatic height, viscosity and specific heat affecting the heat and mass transfer and solubility to avoid crystallization.

Energy Conversion and Management

Influence of thermodynamic and thermophysical properties of water-based working fluids for bubble pump operated vapour absorption refrigerator

The hydrostatic principle and bubble pump technique are used in the two-fluid pumpless continuous vapour absorption refrigerator to create the circulations of the solution and refrigerant. In this paper, the bubble pump, which is the 'heart' of this natural circulation type of refrigerator, has been investigated both analytically and experimentally. The bubble pump is modelled for intermittent slug flow of solution and vapour mixture. A test rig is built in glass to evaluate the performance of the bubble pump, to visualize the flow behavior and to validate the analytical model. Bubble pump tube diameter, pump lift, driving head and heat input are varied to analyze the pump performance. Results indicate that pumping ratio is nearly independent of the heat input, but increases with decrease in pump tube diameter, decrease in pump lift and increase in driving head. © 1998 Elsevier Science Ltd and IIR. All rights reserved.

International Journal of Refrigeration

Studies on bubble pump for a water-lithium bromide vapour absorption refrigerator

Miniaturization of an alcohol-based absorption refrigerator requires an air-cooled absorber and condenser and the replacement of customary solution pumps by the bubble pump. Evaluation of such a refrigerator requires thermodynamic (specific heat and heat of mixing) and thermophysical (vapour pressure, density, viscosity, surface tension and solubility) properties of refrigerant-absorbent solution. These property correlations for five alcohol-based working combinations, majority of them obtained by curve fitting, have been complied and presented in this paper along with their validity ranges and percentage of error. The working fluids have been analyzed and compared with reference to the solution density governing the hydrostatic height, viscosity and specific heat affecting the heat and mass transfer and solubility to avoid crystallization. Further the variations of performance parameters like cut-off temperature, circulation ratio, coefficient of performance and efficiency ratio of these working fluids with respect to various operating conditions are discussed.

International Journal of Energy Research

Comparison of methanol-based working fluid combinations for a bubble pump-operated vapour absorption refrigerator

In a double-effect series flow vapour absorption refrigeration system (VARS), an optimum value of the low-pressure generator temperature exists at which all the vapour generated at the high-pressure generator is condensed. At these conditions, a comparative study of the performance of VARS using environment friendly refrigerants such as, R32, R134a, and R124 with N,N′-dimethyl acetamide (DMAC) as the absorbent is made. It is found that the system with R32-DMAC gives the best performance at high evaporator temperatures. R124-DMAC may be preferred at extreme operating conditions like low evaporator and high heat rejection temperatures. Influence of operating temperatures (high-pressure generator, evaporator, condenser and absorber) and the effectiveness of heat exchangers on the optimum low-pressure generator temperature, cut-off temperature, circulation ratio and coefficient of performance are studied. © 1998 John Wiley & Sons, Ltd.

Optimal performance of double-effect series flow vapour absorption refrigeration systems with new working fluids

Thermodynamic analysis of a water-based vapour absorption refrigeration system with four binary mixtures [H2O-LiBr, H2O-NaOH, H2O-LiI and H2O-LiCl], five ternary mixtures [H2O-LiBr + -Lil (salt mole ratio 4:1), H2O-LiCl + LiNO, (2.8:1), H2O-LiBr + LiNO3 (4:1), H2O-LiBr + ZnBr2 (2:1) and H2O-LiBr + LiSCN (1:1)] and seven quaternary mixtures [H2O-LiBr + LiCl + ZnCl2 (3:1:4), H2O-LiBr + ZnCl2 + CaBr2 (1:1:0.13), H2O-LiBr + ZnBr2 + LiCl (1:1.8:0.26), H2O-LiBr + LiI + -C2H6O2 (3:1:1), H2O-NaOH + KOH + CsOH (4.3:3.6:2.4). H2O-LiNO3 + KNO3 + NaNO3 (5.3:2.8:1.9) and H2O-LiCl + CaO2 + Zn(NO3)2 (4.2:2.7:1)] has been carried out by computer simulation. Variations of performance parameters (cut-off temperature, circulation ratio, coefficient of performance and efficiency ratio) of these aqueous solutions are compared with operating temperatures (generator, evaporator, condenser and absorber temperatures) and heat exchanger effectiveness as parameters. Correlations for the performance parameters are obtained in terms of operating temperatures and heat exchanger effectiveness from the regression analysis. It has been concluded that the H2O-LiCl combination is better from the cut-off temperature and circulation ratio points of view and H2O-LiBr + -LiCl + ZnCl2 combination is better from the coefficient of performance and efficiency ratio points of view. © 1998 Elsevier Science Ltd. All rights reserved.

Thermodynamic comparison of water-based working fluid combinations for a vapour absorption refrigeration system

Miniaturisation of the vapour absorption refrigerator requires replacement of the solution pump by a heat operated bubble pump and air cooled condenser and absorber. The replacement necessitates the selection of working media restricted to vacuum operation, and the air cooling poses the problem of high pressure drops in the refrigerator. Thermodynamic analysis of the absorption refrigerator with such a suitable working medium is performed considering the pressure drops in the system as parameters. The analysis shows that the effect of the evaporator to absorber pressure drop on the system performance is more significant than that of the generator to condenser pressure drop, and it becomes more predominant at the low generator temperature normally encountered in solar operated systems. © 1997 Elsevier Science Ltd.

Effect of component pressure drops in two-fluid pumpless continuous vapour absorption refrigerator

A single-storey office building in a hot and dry climate is modelled for evaporative cooling. The counterflow cooling tower is modified to precool the tower inlet air by the tower exit air in a heat exchanger. Centralized evaporative air cooling, using the modified cooling tower, and roof evaporative cooling are considered to provide comfortable living conditions in the space. The thermal performance of such a building is analysed for various operating parameters. The study indicates that centralized evaporative air cooling is feasible, to maintain near-perfect comfort conditions in hot and dry climates. Modified cooling tower and roof evaporative cooling further enhance the scope of evaporative cooling for comfort applications.

Thermal performance of an evaporatively cooled building: Parametric studies

Evaporative air cooling for human comfort and other applications, whenever possible and feasible, is less expensive than conventional air conditioning. A cooling tower is used in the centralised evaporative air cooling system. In the present study, the counter-flow cooling tower is modified to pre-cool the air at the tower inlet, either by the water after the cooling load, or the tower exit air. These two modified cooling towers are modelled and their thermal performances are compared with the conventional counter-flow cooling tower. For generality, the results are presented in terms of number of transfer units (NTU), a dimensionless number for a cooling tower indicating its size. The modified cooling towers are found to be better than the conventional one in lowering the tower approaches below about 2°C. These cooling towers seem to be distinctly advantageous when operating at low approaches, small ranges and large wet-bulb depressions, thus making them quite useful in evaporative air cooling applications. © 1995.

Heat Recovery Systems and CHP

Analysis of modified counter-flow cooling towers

This Paper deals with theoretical and experimental investigations on the triple fluid vapour absorption refrigerator (TFVAR) in relation to the gas circuit, thermo-siphon and inert gas charge pressure. The charges of the various fluids were estimated. Helium seems to be a better inert gas than hydrogen. A compact absorber design is proposed.