E.D. Rogdakis

Design and parametric investigation of an ejector in an air-conditioning system

Abstract This paper discusses the behavior of ammonia (R-717) through an ejector, operating in an air-conditioning system with a low temperature thermal source. For the detailed calculation of the proposed system a method has been developed, which employs analytical functions describing the thermodynamic properties of the ammonia. The proposed cycle has been compared with the Carnot cycle working at the same temperature levels. The influence of three major parameters: generator, condenser and evaporator temperature, on ejector efficiency and coefficient of performance is discussed. Also the maximum value of COP was estimated by correlation of the three temperatures for constant superheated temperature (100°C). The design conditions were generator temperature (76.11–79.57°C), condenser temperature (34–42°C) and evaporator temperature (4–12°C).

Performance of solar-driven ammonia-lithium nitrate and ammonia—sodium thiocyanate absorption systems operating as coolers or heat pumps in Athens

Abstract The hour-by-hour performance of solar-driven NH3=LiNO3 and NH3-NaSCN absorption systems operating as coolers or heat pumps in the Athens area is predicted, using 20 yr of local climatological data. The exact thermodynamic cycles are represented by substitution of the composite thermodynamic processes (absorption, generation, heat exchange), which involve interactions of two or three streams by thermodynamically equivalent one-stream changes. Under the assumptions made, it becomes possible to develop two groups of correlations. The first expresses the characteristics and the performance of the absorption systems in terms of the ambient temperature only, while in the second group the behaviour of the systems is expressed in terms of the hour of the day for each day of a typical year in Athens. The main conclusions for operation in the Athens area are: (a) For cooling purposes (summer) the maximum theoretical values of the coefficient of performance and of the cooling power are 90% and 355 W/m2, respectively, while for heating purposes (winter) the maximum theoretical values of the heat gain factor and of the useful thermal power are 210% and 344 W/m2, respectively. (b) For heating purposes (winter), the NH3-LiNO3 system is superior to the NH3-NaSCN one, because it provides a higher heat-gain factor and useful thermal power. (c) For cooling during summer, the choice depends on the special requirements of each application, because the NH3-LiNO3 system provides higher cooling power, while the NH3-NaSCN system achieves a higher coefficient of performance.

A verification study of steam-ejector refrigeration model

In this paper a verification study of steam ejector refrigeration model is conducted. A mathematical model for a steam ejector refrigeration cycle is described and the produced results are compared with experimental ones found in the literature. The present work uses the theory which was developed by Munday and Bagster and takes into account the shock phenomena and isentropic efficiency in the ejector.

Investigation of ejector design at optimum operating condition

Abstract In the present work, an improved ejector theory, which was developed by Munday and Bagster is used, in order to study the thermodynamic behavior of the mixture NH3–H2O through an ejector. This ejector can be operated in combined ejector–absorption cycle using the binary mixture of ammonia–water. Kouremenos et al. showed that the heat gain factor (HGF) of a combined ejector–absorption system is 0.8–37.7% greater than those of the conventional absorption system. The design of ejector is based on Keenan et al.’s theory. Taking into account that the primary, secondary and back pressures are constant, there is a pressure before the shock where the flow entrainment ratio and the performance of the cycle (refrigeration or heat pump cycle) take maximum values. For this optimum value of flow entrainment ratio w, the value of area ratio At/Ad (cross section of minimum area of primary nozzle/cross section of constant area duct) can be estimated for an ejector’s optimum design. The operation conditions were : primary pressure, 30–50 bar; secondary pressure, 3–5 bar; back pressure 10–14 bar; and mass fraction of ammonia vapor 97%.

Thermodynamic analysis, parametric study and optimum operation of the Kalina cycle

The work described here has as major objectives the complete thermodynamic analysis and the parametric study of the Kalina Power Unit. The device layout optimization is based on the presentation of the unit on the T-h and h/T-s thermodynamic charts. The operation of the power unit is simulated by the use of equations describing the thermodynamic behaviour of the NH 3 /H 2 O mixture. The important parameters of the unit, i.e. high, medium and low pressures/rich, weak, working solution and boiler vapour mass fraction are discussed and related. Correlations are developed which describe the optimum operation of the Kalina cycle. The maximum thermal efficiency, the heat required to drive the unit and the work produced may be directly calculated from analytical functions in terms of the ambient temperature and the low pressure of the units. The maximum theoretical efficiency ranges from 42.7% to 46.6%.

A high efficiency NH3/H2O absorption power cycle

Abstract A work producing cycle has been developed showing a thermodynamic efficiency considerably higher than that of the Rankine cycle. The new cycle employs a mixture of H 2 O and NH 3 as the working fluid and uses an absorption process similar to that of absorption refrigerators. Its advantage over existing power cycles working with the same mixture (i.e. the Kalina cycle) is simplicity as far as devices, construction, operation and maintenance are concerned. For the detailed calculation of the proposed cycle a method has been developed, which employs analytical functions describing the thermodynamic properties of the NH 3 /H 2 O mixture. The proposed cycle has been compared with Rankine cycles working at the same temperature levels. For fixed upper (i.e. superheating) and lower (i.e. condensation) temperatures, the new cycle shows an efficiency 20% higher than that of the Rankine cycle if the boiling temperature is high, while for low boiling temperatures the superiority of the proposed cycle is much more pronounced. A parametric study has also been conducted for the new cycle, wwhich showed, inter alia , that the optimum difference between the mass fractions of the rich and weak solution is about 0.1 kg NH 3 /kg mixture.

Performance of solar NH3/H2O absorption cycles in the athens area

Abstract The performance of solar driven NH3/H2O absorption units, operating in conjunction with high and intermediate temperature solar collectors in Athens, is predicted along the typical year, in the cases (a) of absorption refrigeration units working as refrigerators, (b) of absorption refrigeration units working as heat pumps and (c) of reversed absorption units working as heat transformers. In all cases, the operation of the units and the related thermodynamics are simulated by suitable computer codes, and the required local climatological data (i.e. the incident solar radiation and the ambient temperature) are determined by statistical processings of related hourly measurements over a considerable number of years. It is found that in the case of the refrigerator, for operation over the whole year, the theoretical coefficient of performance varies in the range from 72 to 75% and a maximum theoretical specific cooling power of 223 W/m2 is observed on July at 13 hrs. In the case of the heat pump, for operation from November to April, a maximum theoretical heat gain factor of about 170% is obtained on December with corresponding specific heat gain power amounting to 213 W/m2 at 14 hrs, while a maximum theoretical specific heat gain power of 344 W/m2 is observed on April at 13 hrs with a corresponding heat gain factor of about 165.5%. Lastly, in the case of the heat transformer, for operation over the whole year, a maximum theoretical heat gain factor of about 48.3% is observed during winter at about 13 hrs but with very small specific heat gain power, while a maximum specific heat gain power of 175 W/m2 is obtainable on July at noon with a corresponding heat gain factor of 44.5%.

Performance characteristics of two combined ejector-absorption cycles

Abstract The paper describes the performance of an ammonia–water combined ejector–absorption cycle as refrigerator using two simple models. In the first an ejector draws vapour from an evaporator and discharges to a condenser. In the second, an ejector draws vapour from an evaporator and discharges to an absorber. The thermodynamics cycles and ejector operation on the temperature–entropy charts are shown. The thermodynamics of the combined ejector–absorption cycle are simulated by a suitable method and a corresponding computer code, based on analytic functions, describes the behaviour of the binary mixture NH 3 –H 2 O. It was found from the first model that the refrigerator (theoretical) coefficient of performance (COP) varied from 1.099 to 1.355 when the operation conditions were: generation temperature (237°C), condenser temperature (25.9–30.6°C), absorber temperature (48.6–59.1°C) and evaporator temperature (−1.1–7.7°C). In the second the theoretical COP vary from 0.274 to 0.382 when the operation conditions were: generation temperature (237°C), condenser temperature (91°C), absorber temperature (76.7–81°C) and evaporator temperature (−1.1–7.7°C).

Predicted performance of solar driven H2OLiBr absorption units in Athens

Abstract The hour by hour performance of solar driven H 2 Oue5f8LiBr absorption units, operating as refrigerators or heat pumps, is predicted during a typical year in the Athens area. The thermodynamic analysis is based on a developed model, which simulates the exact absorption thermodynamic cycle. The hourly values of solar radiation and ambient temperature are estimated from numerical processing of related measurements corresponding to about 20 years. In the case operation as a refrigerator, it has been found that the theoretical coefficient of performance varies during the year from 86 to 96%, and the calculated maximum values of the specific cooling power for January, March, May, July, September and November are 155, 260, 369, 363, 277, and 197 W m −2 per collector, respectively. In the case of operation as a heat pump, the theoretical heat gain factor has been found to be practically constant during the typical year (⋍ 192%) and the calculated maximum values of the specific useful thermal power for the above mentioned months are 264, 504, 825, 890, 658 and 369 W m −2 per collector, respectively.

Performance of solar driven methanol–water combined ejector–absorption cycle in the Athens area

The performance of a solar driven CH4O-H2O combined ejector– absorption unit, operating in conjunction with intermediate temperature solar collectors in Athens, is predicted along the five months (May–September) in case of the unit working as heat pump in an industrial area. The operation of the unit and the related thermodynamics are simulated by suitable computer codes and the required local climatological data are determined by statistical processings over a considerable number of years. It is found that the heat gain factor varies in the range from 2.1330 to 2.4481 for the above period of time. The maximum HGF of about 2.4481 is obtained in July at 14.25 hrs with corresponding specific heat gain power 915 W/m2. The maximum Qgain of about 1086 W/m2 is obtained in June at 12.54 hrs with corresponding HGF 2.3572. Also the maximum value of HGF was estimated by correlation of three temperatures: generator temperature (85.0°C–97.2°C), condenser temperature (43.3°C–47.6°C) and evaporator temperature (12.6°C–25.4°C).