M. Jelinek

An experimental investigation of bubble pump performance for diffusion absorption refrigeration system with organic working fluids

An experimental investigation was undertaken to study the performance of the bubble pump for diffusion absorption refrigeration units. The bubble pump is the motive force of the diffusion absorption cycle and is a critical component of the absorption diffusion refrigeration unit. The purpose of the bubble pump (besides the circulation of the working fluid) is to desorb the solute refrigerant from the solution. Therefore the efficiency of the bubble pump will be set by the amount of the refrigerant desorbed from the solution. The performance of the diffusion absorption cycle depends primarily on the efficiency of the bubble pump. A continuous experimental system was designed, built and successfully operated. The experiments were performed in which some of the parameters affecting the bubble pump performance were changed. During the experimental investigation, photographs were taken showing that the bubble pump operates at slug flow regime with a churn flow regime at the entrance of the bubble pump tube. It was obtained that the performance of the bubble pump depends mainly on the motive head and on the heat input to the bubble pump.

Absorption system based on the refrigerant R134a

The possibility of using 1,1,1,2-tetrafluoroethane (R134a) as a refrigerant in combination with different organic absorbents, such as dimethylether tetraethyleneglycol (DMETEG), N-methyl ϵ-caprolactam (MCL) or dimethyl-ethyleneurea (DMEU), in absorption units was investigated. The procedures in this paper are presented for the model working pair R134a-DMETEG. Temperature-pressure-concentration curves were constructed on the basis of vapour-liquid equilibrium measurements. Excess thermodynamic properties of the mixture were calculated, and enthalpy-concentration diagrams were constructed. The density and viscosity of the working fluids were measured. Thermostability testing of R134a-DMETEG showed that no changes were found in either of the phases over the course of the test. The performances of the investigated working fluids were compared in terms of the coefficient of performance (COP) and circulation ratio (f), which were evaluated by a computerized simulation program based on a single-stage absorption cycle containing a jet ejector as a mixer and preabsorber. The values of the COP for the three combinations were similar (R134a-DMEU reached 0.49, R134a-MCL 0.47 and R134a-DMETEG 0.46), but the value of f for R134a-DMTEG was lower than for the other two pairs. R134a-DMETEG was therefore the best of the three candidate working pairs.

Optimum Arrays of Longitudinal, Rectangular Fins in Corrective Heat Transfer

The theoretical basis for the least material optimization of convectively cooled arrays of longitudinal, rectangular fins is presented. The aspect ratio (fin thickness divided by fin width J of the array-optimum fin is found to be only marginally thinner than implied by the. conventional single fin optimization. Using this result, it is possible to define the geometry and thermal performance of least material, air-cooled natural convection arrays and to identify the array geometry yielding the maximum heat dissipation capability. Alternately, the added mass of a forced convection-cooled fin array is shown to be inversely related to the available pumping power and, as a consequence, no single optimum geometry could be defined. This paper includes several illustrative examples that serve to demonstrate the optimization pro cedure and to quantify the theoretical thermal performance of specific natural and forced convection air-cooled fin arrays.

Working fluids for an absorption system based on R124 (2-chloro-1,1,1,2,-tetrafluoroethane) and organic absorbents

Abstract The possibility of using R124 (2-chloro-1,1,1,2,-tetrafluoroethane, CHClFCF3) and organic absorbents as working fluids in absorption heat pumps was investigated. Various classes of organic compounds, all commercially available, were tested as absorbents for possible combination with R124; the absorbents included DMAC (N′, N′-dimethylacetamide, C4H9NO), NMP (N-methyl-2-pyrrolidone, C5H9NO), MCL (N-methyl ϵ-caprolactam, C7H13NO), DMEU (dimethylethylene urea, C5H10N2O), and DMETEG (dimethylether tetraethyleneglycol, C10H22O5). To evaluate the performance of a candidate refrigerant-absorbent pair in a refrigeration or heat pump cycle, the thermophysical properties of the pure components and the mixture and the equilibrium and transport properties have to be determined, either from experimental data or by prediction methods. The thermal stability of the refrigerant-absorbent must also be tested. A method for the calculation of the concentration in the liquid and gas phases and the excess thermodynamic properties of the mixture as a function of the system temperature and pressure based on our experimental setup is described. On the basis of vapor-liquid equilibrium measurements, density and viscosity measurements and thermostability testing, enthalpy-concentration diagrams were constructed. The performance characteristics of the investigated working fluids in terms of the coefficient of performance (COP) and the circulation ratio (f) were calculated for a single-stage absorption cycle. In terms of overall performance (COP, f and stability) R124-DMAC was found to be the superior combination, followed by R124-NMP, R124-DMEU and R124-MCL (the three pairs for which stability problems were found at high temperatures), and finally by R124-DMETEG.

Numerical study on the design parameters of a jet ejector for absorption systems

The purpose of incorporating a jet ejector into an absorption system is to improve the preabsorption of the refrigerant coming from the evaporator by the weak solution, i.e., to improve the overall absorption process. The mixing process in the jet ejector is very intensive as a result of spray generation of the liquid phase and of extensive subcooling of the weak solution in the solution heat exchanger. To facilitate the design of a jet ejector for absorption machines, a numerical model of simultaneous mass and heat transfers between the liquid and gas phases in the jet ejector was developed. The steady-state model was based on unidimensional balance equations of mass, energy and momentum for the liquid and gas phases. Polynomial correlations were employed to calculate the thermodynamic properties. On the basis of the developed model, we studied the following design parameters of the jet ejector as functions of the length and the cone angle of the diffuser: pressure recovery, temperature and concentration of the refrigerant in the solution, and velocities of the gas and liquid drops. The parametric study also involved examination and ways of augmentation of the mass transfer process in the diffuser with the ultimate aim of designing a compact and efficient unit. The calculations were performed for a mixture of the refrigerant R125 (pentafluoroethane) with organic absorbents.

Single- and double-stage absorption cycles based on fluorocarbon refrigerants and organic absorbents

Abstract A comparison between different fluorocarbon refrigerants and organic absorbents as working fluids in single- and double-stage absorption heat pumps was undertaken. The refrigerants included R22 (monochlorodifluoromethane), R134a (1,1,1,2-tetrafluoroethane), R124 (2-chloro-1,1,1,2,-tetrafluoroethane) and R32 (difluoromethane), and various organic compounds such as DMAC ( N , N′ -dimethylacetamide), DMEU (dimethylethyleneurea), DMETEG (dimethylether tetraethyleneglycol), NMP ( N -methyl-2-pyrrolidone), and MCL ( N -methyl e -caprolactam) constituted the absorbents. The single- and double-stage cycles were calculated for the following working conditions: cooling capacity of 100 kW, generator temperature of 100°C, evaporator temperature of −10°C, water-cooled components outlet temperature of 35°C, and the air-cooled components outlet temperature of 50°C. The performance characteristics of the working fluids in both cycles in terms of the coefficient of performance (COP) and the circulation ratio ( f ), the heat transfer in the main components, the refrigerant and solution mass flow-rates, and the water flow-rates were calculated and compared. Under these working conditions, the performance of the working fluids based on R22 are superior to those based on R124 in both cycles. The working fluids based on R134a cannot be used in a single-stage absorption cycle while in a double-stage cycle the performances were the worst; solutions based on R32 cannot be used in either cycles.

Development of absorption refrigeration units for cold storage of agricultural products

Abstract To meet the requirements of continuous refrigeration installations, powered by low-grade heat sources, e.g. waste heat, flat-plate solar collectors or solar ponds, and providing cooling or refrigeration at subzero (°C) temperatures, rational selection of suitable refrigerant-absorbent pairs is necessary. It is demonstrated that the use of R22 and suitable organic solvent systems is desirable in absorption refrigeration installations. Since no data were available on the vapour-liquid equilibria of these combinations, they were measured over a broad range of pressures, temperatures and concentrations. A rational selection of the optimal combinations was made using results obtained from a computerized simulation model giving the performance characteristics of a specific refrigeration system operating with selected refrigerant-absorbent combinations. An efficient thermodynamic cycle has been constructed for the chosen refrigerant-absorbent pairs. Within the region of feasible operation of the cycle, optimal operating conditions are determined. In selecting these conditions it is also necessary to take into account the heat transferred in the heat exchangers of the unit and the operating stability. On the basis of the research performed a 15 ton refrigeration prototype unit for the storage of agricultural products has been constructed and is in the final stages of being run in. It operates at a coil temperature of −3°C and maintains the storage chamber at 5°C. Hot water is supplied at 95–98°C, and cooling water is provided at 26°C.

Development of Advanced Absorption Systems Driven by Low Temperature Heat Sources

Two refrigerants 1,1,1,2-tetrafluoroethane (R134a) and 2-chloro-1,1,1,2, tetrafluoroethane (R124) that are alternatives to chlorofluorocarbons, in combination with two absorbents N,N’-dimethylacetamide (DMAC) and dimethyl ether tetraethylene glycol (DMETEG) were evaluated for possible utilization in absorption machines powered by low temperature heat sources. The thermodynamic properties of the pure refrigerants R134a and R124 were calculated by a predictive method based on the molecular structure of the substances, the boiling temperature, and the critical temperature and pressure. Enthalpy-concentration diagrams of the refrigerant-absorbent pairs were constructed on the basis of temperature-pressure-concentration relationships. A computerized simulation program was used to compare the different refrigerant-absorbent pairs. The program was based on an advanced complex single-stage cycle containing two generators and an ejector-type mixer and preabsorber for the refrigerant-absorbent pair. Equilibrium pressureconcentration relationships, activity, excess enthalpy, and enthalpy-concentration diagrams are presented for the pair R124-DMAC. Preliminary stability tests of the four investigated working fluids showed that stability of the components was not impared with time. In terms of coefficient of performance and circulation ratio the best refrigerant absorbent pair was found to be R124-DMAC, followed by R134a-DMAC.

FEASIBILITY OF SOLAR-POWERED REFRIGERATION

ABSTRACT The present investigation was directed towards a concerted search for refrigerant-absorbent solutions, which would ensure stable and optimal performance characteristics of solar-powered refrigeration installations over a broad range of operating conditions. The search centered around the fluorochloroalkene family of refrigerants. An extensive experimental study was undertaken of the equilibrium vapor-liquid properties of refrigerant-absorbent systems, and the enthalpy-concentration diagrams of the investigated systems were calculated. To evaluate the feasibility of these in solar-powered refrigeration machines, improved thermodynamic absorption refrigeration cycles were constructed, simulation computerized programs were prepared, and the enthalpy-concentration diagrams were used to evaluate the performance characteristics of the refrigeration installation. Results of the feasibility study indicate that i t is possible to operate absorption-refrigeration installations both with conventional flat-plate collectors and with solar ponds for both domestic and industrial refrigeration purposes.