Liquid dynamic viscosity: A general prediction method with application to refrigerants and refrigerant mixtures

This paper discusses prediction methods which are able to provide the dynamic viscosity, μ, of liquids along the saturation line. The best empirical or semi-empirical correlations existing in the literature are critically presented and checked to outline the usefulness of the new prediction method presented in this paper. Fifty substances (organic compounds, inorganic compounds and pure elements) are examined to show the reliability of the new simple equation which contains three factors (A, B and C) related to the molecular structure and the most important physical properties. The general scheme of prediction is then applied to the particular case of refrigerant fluids belonging to the methane and ethane families and to their binary mixtures. The accuracy of the proposed prediction method is checked using the most recent and reliable experimental dynamic viscosity data available in literature, and the mean and the maximum deviations between predicted and experimental μ values are shown to be less than 3 and 8%, respectively.     

An NTU- model for alternative refrigerants

This paper presents a steady state simulation model to predict the performance of alternative refrigerants in vapour compression refrigeration/heat pump systems. The model is based on the NTU- method in analysing the heat exchangers following an elemental approach. The model extends its applicability to new refrigerants including hydrocarbons and uses a large database of REFPROP package for refrigerant properties. The main inputs to the model include the physical details of the heat exchangers, compressor efficiency, mass flow rates of heat transfer fluids and their inlet temperatures to the evaporator and the condenser, the pressure drops across the heat exchangers and the capacity of either the evaporator or condenser (in kW). The model results are validated with a wide range of experimental data of HCFC22 and propane (HC290) on a heat pump test facility for a number of parameters, e.g. coefficient of performance, condenser capacity, mass flow rate of the refrigerant and compressor discharge temperature. Although the model is currently tested for pure refrigerants using compact brazed plate (counter flow type) heat exchangers, it can also be applied to mixture of refrigerants as well as to other types of heat exchangers.

Résumé

Dans cet article, on présente un modèle de simulation de régime permanent pour prédire la performance des frigorigènes de remplacement dans les systèmes frigorifiques ou les pompes à chaleur à compression de vapeur. Fondé sur la méthode NTU- utilisée pour analyser les échangeurs de chaleur, ce modèle emploie une approche élémentaire. Ce modèle étend la méthode aux nouveaux frigorigènes, y compris deees hydrocarbures, et utilise une base de données étendue, celle de REFPROP, pour les propriétés des frigorigènes. Les principaux paramètres du modèle comprennent des détails physiques sur les échangeurs de chaleur, le rendement des compresseurs, et les débits massiques des fluides de transfert de chaleur et leurs températures à l'entrée de l'évaporateur ou du condenseur, la chute de pression à travers les échangeurs de chaleur et la puissance soit de l'évaporateur, soit du condenseur (exprimés en kW). Les résultats obtensus en utilisant ce modèle sont validés pour une large gamme de données expérimentales obtenus avec le HCFC22 et avec le propane (le HC290) sur un banc d'essai de pompe à chaleur et pour un certain nombre de paramètres, par exemple le coefficient de performance, la puissance du compresseur, le débit massique du frigorigène et la température du frigorigène à la sortie du compresseur. En ce moment, le comportement des frigorigènes purs utilisés dans des échangeurs de chaleur compacts à plaques brasées (de type contre-courant) est en train d'être étudié; le modèle peut également être appliqué aux mélanges de frigorigènes et à d'autres types d'échangeurs de chaleur.     

Beyond CFCs: Extending the search for new refrigerants

Previous work in developing environmentally acceptable alternatives to fully halogenated chlorofluorocarbons (CFCs) has concentrated almost exclusively on methane and ethane based compounds. A review of toxicity and boiling point data for a large variety of fluorine compounds reveals additional classes of compounds which may be suitable as refrigerants. Fluorinated derivatives of dimethyl ether and cyclopropane appear to have both low toxicity and suitable boiling points. They also have a relatively simple structure which means that they should have a reasonably good cycle efficiency. Propane based CFCs may also be useful if simpler compounds prove to be unacceptable. Specific compounds that warrant further investigation include bis-difluoromethyl ether (for R114), difluoromethyl dichlofluoromethyl ether (for R113), difluoromethyl fluoromethyl ether (for R11) and hexafluorocyclopropane (for R12). In addition, the compound trifluoroiodomethane may be a useful alternative to R13B1 in fire extinguishers. A cooperative programme of synthesizing and evaluating fluorinated derivatives of dimethyl ether and cyclopropane is recommended.     

Theoretical and experimental freezing times of strawberries

Strawberries were frozen at different air velocities in an air blast freezer at −30°C. The freezing time was taken as the time required to lower the temperature at the geometric centre of the samples to −10°C. The freezing times measured in the experiments were compared with the values calculated using Plank's equation. The freezing times calculated by Plank's equation were fround to be higher than those found experimentally at any given air velocity. Freezing time decreased with increasing air velocity. This is attributed to the increase in heat transfer coefficient at increased air velocities.     

Comparative efficiencies and implications for greenhouse gas emissions of chiller refrigerants

This paper summarizes analyses of refrigerant options for chillers. It presents tabulated global warming potentials and other environmental data for candidate refrigerants. The paper discusses briefly the historic progression in refrigerant release reductions and presents analyses to compare the efficiencies of historic and current refrigerant options. The 28 refrigerants addressed include chlorofluorocarbon (CFC), hydrochlorofluorocarbon (HCFC), hydrofluorocarbon (HFC), hydrocarbon (HC), and inorganic (such as ammonia) fluids. The paper discusses the relative importance of the refrigerant-related and energy-related components of chiller emissions.     

Prediction of evaporation heat transfer coefficient and pressure drop of refrigerant mixtures in horizontal tubes

A study on the prediction of heat transfer coefficient and pressure drop of refrigerant mixtures is reported. Heat transfer coefficients and pressure drops of prospective mixtures to replace R12 and R22 are predicted on the same cooling capacity basis assuming evaporation in horizontal tubes. Results indicate that nucleate boiling is suppressed at qualities greater than 20% for all mixtures, and evaporation becomes the main heat transfer mechanism. For the same capacity, some mixtures containing R32 and R152a show 8–10% increase in heat transfer coefficients. Some mixtures with large volatility difference exhibit as much as 55% reduction compared to R12 and R22, caused by mass transfer resistance and property degradation due to mixing (32%) and reduced mass flow rates (23%). Other mixtures with moderate volatility difference exhibit 20–30% degradation due mainly to reduced mass flow rates. The overall impact of heat transfer degradation, however, is insignificant if major heat transfer resistance exists in the heat transfer fluid side (air system). If the resistance in the heat transfer fluid side is of the same order of magnitude as that on the refrigerant side (water system), considerable reduction in overall heat transfer coefficient of up to 20% is expected. A study of the effect of uncertainties in transport properties on heat transfer shows that transport properties of liquid affect heat transfer more than other properties. Uncertainty of 10% in transport properties causes a change of less than 6% in heat transfer prediction.     

A simplified cycle simulation model for the performance rating of refrigerants and refrigerant mixtures

A simulation program, CYCLE11, which is useful for the preliminary evaluation of the performance of refrigerants and refrigerant mixtures in the vapour compression cycle is described. The program simulates a theoretical vapour-compression cycle and departures from the theoretical cycle that occur in a heat pump and in a refrigerator. The cycles are prescribed in terms of the temperatures of the external heat-transfer fluids with the heat exchangers generalized by an average effective temperature difference. The isethalpic expansion process is assumed. The program includes a rudimentary model of a compressor and a representation of the suction line and liquid line heat exchange. Refrigerant thermodynamic properties are calculated by using the Carnahan-Starling-DeSantis equation of state. Refrigerant transport properties are not included in the simulations. The program can generate merit ratings of refrigerants for which limited measured data are available. An example of simulation results stresses the need for careful application of simplified models and consideration for the assumptions involved.     

Prediction of freezing and thawing times for multi-dimensional shapes by simple formulae part 2: irregular shapes

Calculated and experimental data for multi-dimensional irregular shapes wer used to assess various methodologies to include the effect of shape in empirical freezing and thawing time prediction methods. The principles underlying two existing geometric factors, EHTD and MCP, were found to be valid; so there seems to be no need for other approaches. Used in conjunction with accurate slab freezing and thawing time prediction methods, the proposed empirical formulae for EHTD and MCP gave accurate predictions for all of the two-dimensional shapes and most of the three-dimensional shapes tested, except those with oval cross-sections in the third dimension. This was attributed to the lack of data for this group of shapes.     

Prediction of freezing and thawing times for multi-dimensional shapes by simple formulae Part 1: regular shapes

Numerical prediction methods were used to generate data to assess and develop geometric factors taking account of the effect of product geometry on freezing and thawing time. Improved empirical formulae for two existing geometric factors were developed; these depend only on the Biot number and parameters that describe object shape. The new formulae are accurate for both freezing and thawing of an extended range of regular multi-dimensional shapes and for a wider range of conditions than the original formulae. Used in conjunction with accurate slab freezing and thawing time prediction formulae, the improved geometric factors accurately predicted a large set of experimental freezing and thawing times for various shapes. As the improved geometric factors are both accurate and generally applicable there is no need for shape-specific freezing and thawing time prediction formulae to be developed.     

An implicit curve-fitting method for fast calculation of thermal properties of pure and mixed refrigerants

Calculations of refrigerant thermal properties are desired to be very fast and stable in cases of simulation of refrigeration system, etc. The traditional method based on equation of state cannot meet such requirement because of unavoidable iterations in calculation. In this paper, a new calculation method for refrigerant thermal properties is presented. Low order implicit polynomial equations are got by using curve-fitting method at first, and then explicit formulae for calculating refrigerant thermal properties quickly are obtained by getting the analytical solution of these implicit equations. Explicit fast calculation formulae for thermal properties of R22 and R407C, covering the saturated temperature of −6080 °C and superheat of 0–65 °C, are presented as examples. The calculation speeds of the formulae of R22 are about 140 times faster than those of REFPROP 6.01 while the formulae of R407C are about 1000 times faster. The total mean relative deviations of the fast calculation formulae for R22 and R407C are less than 0.02%.     

Nucleate boiling heat transfer coefficients of pure halogenated refrigerants

Nuclate pool boiling heat transfer coefficients (HTCs) of HCFC123, CFC11, HCFC142b, HFC134a, CFC12, HCFC22, HFC125 and HFC32 on a horizontal smooth tube of 19.0 mm outside diameter have been measured. The experimental apparatus was specially designed to accomodate high vapor pressure refrigerants such as HFC32 and HFC125 with a sight glass. A cartridge heater was used to generate uniform heat flux on the tube. Data were taken in the order of decreasing heat flux from 80 to 10 kW m⁻² with an interval of 10 kW m⁻² in the pool of 7 °C. Test results showed that HTCs of HFC125 and HFC32 were 50–70% higher than those of HCFC22 while HTCs of HCFC123 and HFC134a were similar to those of CFC11 and CFC12 respectively. It was also found that nucleate boiling heat transfer correlations available in the literature were not good for certain alternative refrigerants such as HFC32 and HCFC142b. Hence, a new correlation was developed by a regression analysis taking into account the variation of the exponent to the heat flux term as a function of reduced pressure and some other properties. The new correlation showed a good agreement with all measured data including those of new refrigerants of significantly varying vapor pressures with a mean deviation of less than 7%.     

Options and outlook for chiller refrigerants

This paper reviews the progression in refrigerants for chillers, from historical selections through current options and projections for the future. It examines the global environmental issues that catalyzed recent changes. It then discusses candidate refrigerants in the context of future availability (or phaseout) based on controls for environmental protection, efficiency, toxicity, flammability, and escalating future costs. It notes that negative marketing and conflicting claims, intended to discredit competitor's approaches, create confusion and retard replacement of older, less-efficient equipment. The result hurts the environment, increases costs, and stifles the chiller market. The paper concludes that most of the current anxiety with refrigerant selections is unwarranted. Engineers, building owners, and others involved in chiller decisions should revert to traditional chiller specifications based on cost, performance, local manufacturer support, service options, and reliability. Anticipating more stringent environmental regulations, they also should take all practical steps to reduce refrigerant releases and increase efficiency. The paper examines future refrigerant options for chillers. Noting that there are no ideal refrigerants and that none are likely to be found, it recommends scientific determination of acceptability rather than market manipulation.     

Rheological study of TBAB hydrate slurries as secondary two-phase refrigerants

In the refrigeration field, the use of ice slurries (suspensions of ice) as secondary refrigerant fluids is spreading. This technology could be extended to air-conditioning systems provided that one can propose a phase-change material (PCM) with melting temperature belonging to the range [+6 °C; +12 °C]. Aqueous solutions of tetra butylammonium bromide, which crystallise under atmospheric pressure and temperatures between 0 and +12 °C, into ice-like compounds (hydrates), seem to be good candidates for air-conditioning applications. We focus here on the rheological properties of TBAB hydrates suspensions and propose an experimental determination of their flow behaviour through flow rate and pressure drop measurements. In laminar regime, we observe Bingham behaviour. Apparent viscosity and yield shear stress are determined for different hydrate contents.

Résumé

Dans le domaine de la réfrigération, l'utilisation de coulis de glace en tant que fluides frigoporteurs connaît un vif intérêt. Cette technologie pourrait être étendue aux systèmes de climatisation dans la mesure où il existe un matériau dont la température de fusion est comprise dans le domaine de fonctionnement usuel des climatisations [6 °C; 12 °C]. D'un point de vue thermodynamique, l'hydrate de bromure de tetra-bultylammonium (TBAB) apparaît bien approprié pour une telle application puisqu'il est stable à pression atmosphérique et pour des températures comprises entre 0 et 12 °C. Nous nous intéressons ici aux propriétés rhéologiques de coulis d'hydrates de TBAB. Nous proposons une méthode expérimentale de l'étude de leur comportement en écoulement à travers des mesures de débits et de pressions différentielles. En écoulement laminaire, les coulis d'hydrates de TBAB présentent un comportement du type fluide de Bingham. Les dépendances de la viscosité et de la contrainte seuil avec la température, et par conséquent la teneur en cristaux d'hydrates, est mise en évidence.     

Screening of pure fluids as alternative refrigerants

Hydrofluorocarbons are now well established as refrigerants because of their zero ozone depletion potential. Since they have a high global warming potential, other alternatives as, e.g. fluorinated ethers or cyclic hydrocarbons are considered as next-generation refrigerants. Screening of alternative refrigerants is difficult because mostly no or only few data are available. To evaluate, e.g. the cycle performance, the thermodynamic properties of the refrigerants must be known and described accurately by an equation of state. Here, the physically based BACKONE equations are used to describe alternative refrigerants, such as natural refrigerants, hydrofluorocarbons, fluorinated cyclic hydrocarbons, and fluorinated ethers. BACKONE needs only a few substance specific parameters to describe thermodynamic properties with high accuracy. Thus, even alternative refrigerants, with very few available experimental data can be described. Calculations with BACKONE of the performance of many refrigerants show that some hydrocarbons and fluorinated ethers are a good alternative.     

An excess function method to model the thermophysical properties of one-phase secondary refrigerants

This paper describes an easy-to-use and accurate method to calculate some of the thermophysical properties of aqueous solutions which are used as secondary refrigerants. This method is based on the correction of the ideal behaviour of aqueous solutions by excess functions. This method allows to determine the following properties: freezing points, densities, heat capacities, thermal conductivities and dynamic viscosities. As an illustration, it is applied to aqueous solutions of (i) ethyl alcohol, (ii) ammonia, (iii) sodium chloride, (iv) ethylene glycol and (iii) propylene glycol.     

Solar absorption system for air conditioning

This Paper presents a new continous operating solar desiccant absorption system in which CaCl₂-H₂O is used as the absorbent. The flat-plate solar collector is utilized as the desorber where water from the solution is evaporated to ambient air in passing over the collector above the solution film. The plant is equipped with a latent heat accumulation system which is extremely compact in size and very efficient. The coefficient of performance—solar collector efficiency product, indicating the grade of solar energy utilized, is estimated as being the highest of all presently known systems.     

Propriétés thermodynamiques et physiques des mélanges de fluides frigorigènes et d'huilesThermodynamic and physical properties of mixtures of refrigerants and oils

The influence of lubricants circulating within refrigerating plant on boiling and convective condensation mechanisms, and the lack of data supplied by manufacturers mean that predictive models have to be used in order to determine the thermodynamic and transport properties of lubricating oils and of mixtures of refrigerants and oils. This study provides a series of correlations making it possible to calculate the properties of oils. This article also compares literature references to a few methods used to determine the properties of such mixtures.     

A general dynamic simulation model for evaporators and condensers in refrigeration. Part II: simulation and control of an evaporator

A mathematical model of an evaporator based on one-dimensional partial differential equations representing mass conservation, and tube wall energy has been formulated. These equations are then restructured and linked to a program data base of all major refrigerants and refrigerant mixtures. The result is a simulation model of an evaporator that is general and flexible. The model is tested over a wide range of operating conditions and a simple controller is implemented to demonstrate the effectiveness of the model for controller and systems design.

Résumé

On a établi un modèle mathématique d'un évaporateur basé sur des équations aux dérivés partielles unidimensionnelles qui représentent la conservation de masse et l'énergie de la paroi du tube. Ces équations ont été restructurées ensuite, puis reliées à une base de données sur les principaux frigorigènes purs et en mélanges. De cette manière, on obtient un modèle d'évaporateur d'application générale et souple. Ce modèle a été éprouvé dans des conditions de fonctionnement très variées et on a employé un système de régulation simple pour montrer l'efficacité du modèle pour la conception et la régulation des systèmes.     

Liquid side heat transfer and pressure drop in finned-tube cooling-coils operated with secondary refrigerants

In this study full-scale experiments with two different conventional cooling-coils aimed for display cabinets were performed. Heat transfer and pressure drop on the liquid side for three different single phase secondary refrigerants were studied and compared to predictions by existing correlations. Predominantly, the laminar flow regime was studied. The results show that when predicting the heat transfer performance on the liquid side of a cooling-coil the Gnielinski correlation for thermally developing flow and uniform wall temperature boundary conditions (T) leads to good agreement for 0.0014 < x^* < 0.017 if 50 < Re < 1700, assuming a new entrance length is formed after each U-bend. In addition, these entrance lengths must also be accounted for, when predicting the pressure drop on the liquid side of the cooling-coil. The uncertainty of measurement can be a problem in this type of investigations and this has been taken into consideration when analysing the results.     

Choice of sink for air conditioning systems. Part 2

The technical superiority of water-cooled air conditioning systems for different applications in terms of overall power rating have been established by the authors in an earlier publication. This paper deals with the generalized approach for evaluating the techno-economic choice of sink for an air conditioning system. A single parameter of unit cooling cost in fils/kWh_c (100 fils = 1 Kuwaiti dinar = US$3.25), combining the system's technical performance and economic aspects, is an effective cost-benefit criterion for making the choice for the type of sink. The results of an analysis, applied to example case studies in Kuwait, favour the water-cooled system for both the direct expansion (mostly used in the residential sector) and the chilled water (commonly used for non-residential applications) air conditioning systems.