A. Stegou-Sagia

Use of helium instead of hydrogen in inert gas absorption refrigeration

Abstract The behaviour of helium in place of hydrogen as a pressure equalizing gas in triple-fluid absorption refrigeration units is examined. The main thermodynamic and transport properties of the real NH3/He binary gas mixture are presented in the form of diagrams and analytical expressions, with the temperature and the mass fraction of the mixture as input variables. Among others, the mass or mole fraction of the NH3/He gas mixture as a function of the dry and wet bulb temperatures has been plotted. A computer simulation has been developed to calculate the evaporation of NH3 in a NH3/He atmosphere within an insulated tube, allowing measurement of the composition of the mixture via temperature measurement. The results are compared with the corresponding values of evaporation of NH3 in a NH3/H2 atmosphere.

Measuring the evaporation of NH3 in triple-fluid gas absorption units

Abstract A computer simulation is presented which, among other functions, enables the gas concentrations in NH 3 /H 2 O/H 2 absorption refrigeration units to be determined by suitable temperature measurements. The gas concentration is determined by insulating the evaporator and measuring the inlet and outlet gas temperature, and the wall temperature at the outlet. Because of the pressure influence on the evaporation process, the operating pressure of the unit must also be measured. Numerical results are presented for pressures of 17.5, 20 and 25 bar.

Three-dimensional evaporation process in aqua-ammonia absorption refrigerators using helium as inert gas

Abstract Helium is proposed as a hydrogen substitute in inert gas absorption refrigerators, in order to eliminate the danger of a possible explosion. Attention is focused on the evaporator of such units and a solution is obtained for the three-dimensional evaporation process of a liquid ammonia stream of circular meniscus cross-section flowing parallel to an ammonia-helium gas mixture. The analysis is based on a finite- difference solution of the three-dimensional momentum, heat and mass transfer differential equations within a nearly horizontal evaporation tube. Calculations are also performed using hydrogen instead of helium in order to compare their behaviour as inert gases in aqua-ammonia absorption refrigerators. Under corresponding conditions, higher entropy increase and cooling capacity are observed in the case of helium. Generally, the behaviour of the latter is deemed satisfactory and therefore helium may replace hydrogen.

Binary and ternary blends of R-134a as alternative refrigerants to R-22

Abstract Refrigerant mixtures may be used to solve the ozone layer depletion problem as they offer the optimum combination of favorable technical performance with environmentally acceptable behavior. This work reports the thermodynamic properties and characteristics of the binary mixtures R-32/R-134a in compositions 20–80%, 30–70%, 40–60% by mass and of the ternary mixture R-407c: 23 wt% R-32 + 25 wt% R-125 + 52 wt% R-134a. A computer code has been developed for the phase equilibrium relationship of the binary mixtures using related activity and fugacity coefficients. Our results are in good agreement with existing experimental data. For the ternary blend, we present enthalpy and entropy correlations, diagrams illustrating the real gas behavior by means of k -type isentropic change exponents ( k = c p / c V , k p, V , k T, V , k p, T ) and figures for the refrigerating systems efficiency expressed by coefficient of performance values.

Thermodynamic property formulations and heat transfer aspects for replacement refrigerants: R‐123 and R‐134a

This paper reports analytical relations for the thermodynamic properties enthalpy, entropy, heat capacities at constant pressure and temperature of the replacement refrigerants R-123 and R-134a. These refrigerants are considered promising as substitutes for the fluids R-11 and R-12, respectively, which are two of the most widely used CFC refrigerants. In addition to the properties, the three real gas isentropic exponents kp,v,kv,T, kp,T are calculated, which may be used instead of the classical exponent k=cp/cv in the ideal gas isentropic change equations to describe with good accuracy the real gas behaviour. A systematic study to research the influence of various parameters on heat transfer during condensation of R-123 and R-134a on horizontal integral-fin tubes is also carried out. The results are useful in refrigeration applications to improve the basic design, as a significant concern about new refrigerants to replace the CFCs has increased very rapidly due to the destruction of stratospheric ozone and global warming.

Properties of new refrigerants and predictions for condensation heat transfer enhancement with low-finned tubes

The present article provides a review of thermophysical properties research of environmentally acceptable refrigerants. In recent years, it has become evident that common CFC type refrigerants can cause serious environmental damage when released into the atmosphere. The group of HFCs (HFC-32, HFC-125, HFC-134a, HFC-152a) with no chlorine atom in their chemical formulae and quite short atmospheric lifetime may be promising alternatives to CFCs. A compilation of properties is given which may be used to obtain arithmetic expressions for the vapour pressure, saturated liquid density, viscosity, latent heat of vaporization, saturated liquid thermal conductivity, and surface tension of these hydrofluorocarbons. For easily available comparisons, the results are presented in diagrams illustrating the dependence of thermophysical data on temperature at the saturated state. Heat-transfer predictions are specified for condensation of the HFCs on horizontal integral-fin tubes with rectangular fins. Parameters investigated include fin and tube geometry, etc. This study should be a useful input in developing new condensers with new refrigerants.

Evaporation of NH3 in NH3/H2 atmosphere by 25 bar for neutral gas absorption refrigeration units

The present work considers the evaporation process ofNH3 inNH3/H2 atmosphere, that takes place in neutral gas absorption refrigeration units. The data obtained are used to calculate the “psychrometric problem” and the evaporation process within an insulated tube. The “psychrometric problem” deals with the determination of the mass fraction of theNH3/H2 gas mixture from known “dry” and “wet bulb” temperatures for Lewis numbers not equal to one. The results are in good agreement with some existing experimental data.

An experimental study and a computer simulation of heat and mass transfer for three-dimensional humidification processes

The paper deals with a numerical and an experimental analysis of the humidification process of air in a nearly horizontal tube. The numerical analysis is based on a finite-difference solution of the three-dimensional momentum, heat and mass transfer differential equations within the tube for the evaporation process of a water stream of circular meniscus cross-section counterflowing to an air stream. In order to provide reliable, high-accuracy data to assist in the development of the theoretical model, tests have been conducted using specially constructed apparatus to measure temperature and velocity profiles within the tube. The proposed computer algorithm was validated by evaluating its ability to predict the measurements of temperatures and velocities along the streams flow. The theoretical predictions agree well with the experimental data. The model may be used to give performance characteristics for a wide range of humidification processes with different working fluids and state conditions.

ON ISENTROPIC CHANGES OF ALTERNATIVE REFRIGERANTS (R-123, R-134a, R-500, R-503)

This work deals with the real-gas isentropic exponents kp,v,kv,T,kp,T. These have been calculated for the alternative refrigerants R-123 and R-134a (i.e. the leading proposed replacements for CFC-11 and CFC-12, respectively), as well as for the azeotropic blends R-500 (73.8 wt% R-12 + 26.2 wt% R-152a) and R-503 (40.1 wt% R-23 + 59.9 wt% R-13). Analytical relations, arithmetic values and comparisons are given for a wide range of state conditions. The results should be useful in refrigeration applications.

A basic-2 program for the “psychrometric problem” of ammonia, in neutral gas absorption refrigeration units

Abstract The ‘psychrometric problem’ for small neutral gas absorption refrigeration units concerns among others the determination of the mass fraction of the NH3/H2 gas mixture, when the ‘dry’ and ‘wet bulb’ temperatures are known. By the present work, the values of the mass fraction for various combinations of ‘dry’ and ‘wet bulb’ temperatures are computed and the relevant program codes are given. The pressure range compiles the steps of 17.5 bar, 20 bar, 22.5 bar, 25 bar, and 27.5 bar, i.e. the usual operating conditions of these units.