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.     

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.     

Generalized correlation for condensation of binary mixtures inside a horizontal tube

The effect of mixture composition on the heat transfer coefficient during forced convection condensation of R22 and R12 mixtures inside a horizontal tube has been investigated. The effect of mixture composition is complex and the heat transfer coefficients have not been found to vary in a simple manner with the composition. The generalized correlation which best fits the R22/R12 mixture heat transfer data is:     

HFC 134a as a substitute refrigerant for CFC 12

In response to international protocol agreements and national regulatory actions promoted by the increasing concern for ozone depletion and the greenhouse effect, HFC 134a has emerged as a leading candidate for CFC 12 substitution in automotive air conditioners, centrifugal chillers and residential refrigerators and freezers. This Paper discusses compressor and refrigeration system requirements and information gaps for HFC 134a application as a CFC 12 substitute.     

Flow visualization applied to a small refrigeration compressor

This paper describes a flow visualization technique that was used to evaluate qualitatively the gas flow pattern inside a small, hermetically sealed, reciprocating refrigeration compressor. The applicable compressor designs are those in which the suction gas from the evaporator is dumped into the compressor shell, and is then drawn through a muffler into the suction plenum of the compressor. The physical separation of the muffler inlet from the suction gas inlet serves to reduce compressor noise and also provides an easy and convenient means of separating any liquid (compressor oil or liquid refrigerant) from the refrigerant gas. For the flow visualization studies the compressor housing was replaced by a clear plastic shell. Atmospheric air seeded with white smoke was the working fluid. The suction inlet and muffler were parts from a commercial compressor. The flow pulsations were modelled by connecting the muffler outlet to the input plenum of an auxiliary compressor. The flow patterns near the muffler inlet were recorded with a video camera. The mixing of the inlet gas with the gas circulating inside the muffler was studied. The effect of alignment and offset of the muffler inlet relative to the suction inlet, the effect of muffler size, and the effect of a shroud around the muffler were studied. The results were used to guide a companion study of detailed temperature and pressure measurements inside a working compressor.     

Impact of CFC regulations on the air conditioning and refrigeration industry

I welcome the opportunity to speak about a subject of great importance to our industry. The biennial conferences at Purdue have acquired a justified reputation for substance and excellence, and I'm glad to see the large attendance. Given the recent significant developments pointing toward possible further CFC restrictions and the continuing need for high efficiency compressors, the conferences take on a greater significance than ever. Everyone in our industry, and indeed all people in the world who depend upon the products we provide, face some difficult choices and possibly some disruptions during the next decade or two as we shift away from refrigerants that have been used for more than 50 years. With modern technology, and with ingenuity and determination, we should be able to find alternative ways of refrigerating. But it will take time. It will not be easy. We have an enormous investment in current technology and a vast worldwide inventory of equipment which will continue to need service for years. Will we be given the time needed to accomplish the transition? How do we do it? How long will it take? What are the implications if our governments, responding to public concerns, insist on rapidly terminating production of CFC refrigerants? I don't anticipate that I can answer those questions completely in this presentation, but I will attempt to identify the main concerns as I see them and suggest some actions which we should all be considering in responding to this developing issue.     

Start-up and shut-down operation in a reciprocating compressor refrigeration system with capillary tubes

Refrigerant migration during start-up and shut-down cycles can affect energy losses. Two reciprocating compressor refrigeration systems were studied: (1) a conventional cycle with refrigerant migration through the capillary tube after compressor shut-down and (2) a system in which a magnetic cut-off valve was closed on compressor shut-down. It was found that by preventing the refrigerant from migrating during shut-down, energy losses during start-up could be reduced, resulting in a 4% decrease in motor power input at this time. A computer program has been developed to calculate energy losses due to refrigerant migration.     

Comparison of the refrigerants HFC 134a and CFC 12

A comparison of the refrigerants HFC 134a and CFC 12 has been carried out and the results from a theoretical analysis and from tests with an open piston compressor are reported in this paper. The results indicate that the tested compressor will give a greater refrigerating capacity with HFC 134a than with CFC 12 for certain operating conditions. However, the results also indicate an increased operating power for the compressor over the entire temperature range. As a result the coefficient of performance is decreased. Another noticeable result is dependency of the compressor's isentropic efficiency on temperature when using HFC 134a. This might be explained by the properties of the polyalkene glycol oil which is used with HFC 134a. The increased cost of using HFC 134a is justified if the environmental aspects are considered and the practical problems, such as the influence on the material in the refrigeration cycle, can be solved.     

Impact of the decrease in CFC emissions on refrigeration: target of the IIR initiative

According to the Montreal Protocol, agreed upon in 1987, a 50% decrease in CFC emissions will become effective in 1999. Optimistic scenarios do not foresee any problems for refrigeration, supposing alternative, environmentally more acceptable, refrigerants become available in a period of five to seven years from now. The newest analysis of ozone measurements, yielding an unexpected seasonal depletion everywhere, might have the consequence that additional reductions will be agreed upon. The refrigeration industry will then have to search more actively for alternatives and various types of other measures. In the following text, developments regarding the CFC—ozone hypothesis are once more considered. What has been the trend so far in searching for alternative substances and alternative measures is presented thereafter. The attitudes and actions of various (inter)national institutes are considered. Substitutes are then listed and scenario investigations are presented. The main part of this contribution deals with the IIR initiative, set up to investigate the effects of measures and the research necessary to decrease CFC emissions. The IIR initiative and its specific set-up, as well as some preliminary results, can be found in the later sections of the paper.     

Design of Rankine Cycles for power generation from evaporating LNG

Of the different ways of using the cold available from the large scale evaporation of LNG the generation of electric power, using the Rankine Cycle, appears the most practical. Fourteen such plants have so far been installed in Japan. The present study was undertaken for a plant at Zeebrugge. Six different cycles using nine different refrigerants and mixtures were examined. The results (Tables 2 and 3) show the importance of using a heat source with the highest possible temperature. No one refrigerant is ideal for all configurations Environmental and safety considerations also affect the choice of refrigerant.     

A new three-parameter cubic equation of state for refrigeration engineering calculations

A new three-parameter cubic equation of state is proposed in which the repulsion pressure term of the semiempirical van der Waals equation has been modified based on the hard-sphere potential and a functional form similar to Redlich-Kwong attraction term is adopted for the attraction term. All three parameters are treated as functions of temperature. For the purpose of examining the applicability of the new equation, it was applied to describe the thermodynamic properties of difluoromethane (R-32) and pentafluoroethane (R-125). It has been found that the essential thermodynamic properties are represented by the new equation as accurately as a conventional far-complicated modified Benedict-Webb-Rubin equation in the working range of refrigeration equipment. The new cubic equation was also compared with the Peng-Robinson equation and the Carnahan-Starling-De Santis equation.     

Comparative assessment of HFC134a and some refrigerants as alternatives to CFC12

A composite plot relating evaporating temperature T_EV, condensing temperature T_CO, pressure ratio (PR) and theoretical Rankine coefficient of performance (COP)_RR is presented for HFC134a. The theoretical performance of HFC134a has been comparatively assessed along with HCFC22, HFC134, HFC152a, HCFC124 and HCFC142b as alternatives to CFC12 by using the standard refrigeration parameters including pressure ratio, specific compressor displacement, theoretical Rankine coefficient of performance, shaft power per ton of refrigeration. A discussion of the practical implications of the choice of the alternatives to CFC12 is also presented.     

Performance evaluation of a heat pump cycle using NARMs by a simulation with equations of heat transfer and pressure drop

Simulation analyses for a vapour compression heat pump cycle using nonazeotropic refrigerant mixtures (NARMs) of R22 and R114 are conducted under the condition that the heat pump thermal output and the flow rate and inlet temperatures of the heat sink and source water are given. The heat transfer coefficients of the condensation and evaporation are calculated with empirical correlations proposed by the authors. The validity of the evaluation method and the correlations is demonstrated by comparison with experimental data. The relations between the coefficient of performance (COP) and composition are shown under two conditions: (1) the constant heat transfer length of the condenser and evaporator; and (2) the constant temperature of refrigerant at the heat exchanger inlet. The COP of the NARMs is higher than that of pure refrigerant when the heat transfer lengths of the condenser and evaporator are sufficiently long.     

Commercial refrigeration system using CO2 as the refrigerant

Various field-test systems using carbon dioxide as the only refrigerant have been installed since December 2001. In this paper we will analyse an ‘all-CO₂’ supermarket, which has been operating in the North of Italy since January 2003.The seasonal COP is calculated, based on prior laboratory measurements, and a comparison is made with a conventional direct expansion system using R404A.The total annual energy consumption of the installed CO₂ system is estimated to be about 10% higher than the direct expansion R404A solution. It is still possible to further improve efficiency and approach the efficiency of present R404A systems. These improvements are identified.The cost of the CO₂ installation is compared to the cost of an equivalent direct expansion R404A installation, the most economic among the various present types of commercial refrigeration systems. Because of the lack of suitable mass-produced components, the CO₂ installation is estimated to be, today, about 20% more expensive.     

冰箱的节能与CFCs替代

冰箱耗电量较大，研制节能型冰箱具有重要意义。另外，目前冰箱中使用的制冷剂ＣＦＣ１２不久将被禁用，替代物研究是十分迫切的任务。这也可在研制节能冰箱时与ＣＦＣｓ替代联系起来。本文综述了目前正在研究或已经使用的冰箱节能措施与ＣＦＣ５替代，有利于制订出我国的产品发展方向。     

Property equations for saturated water

The fundamental relations for the thermophysical properties of refrigerant R22 over an extended range of saturation temperatures have been documented in this Short communication. These empirical expressions, which are associated with saturated liquid and saturated vapour states, speed up the computations involved in system modelling and computer simulation of vapour compression units.     

Interaction parameter, k12, for non-azeotropic refrigerant mixtures

The interaction parameter, k₁₂, is determined from the experimental equilibrium data obtained by other authors. Vapour-liquid equilibria for binary mixtures of halocarbon refrigerants are predicted using the Redlich-Kwong-Soave equation of state. The mixtures considered are: R14-R23, R23-R12, R13-R12, R13-R11, R13B1-R22, R13B1-R152a, R22-RC318, R12-RC318, R12-R11.     

Reciprocating compressors for refrigeration and heat pump application

Reciprocating compressors have been used in refrigeration for about 150 years. At first they operated at a maximum of 100 r.p.m. and so were very large. Much of the development has been directed to increasing speed and hence reducing size and cost. Recent improvements have been directed mostly to valve design because valve performance largely controls the indicator diagrams and hence the isentropic efficiency. Present studies are directed to improving the mechanical efficiency. Despite all this progress, further improvement is still possible.     

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.     

Compressor technologies for low temperature applications of R-22

Substitutes for the commonly used refrigerants CFC-12 and CFC-502 are not commercially available. HCFC-22 is not suitable for low temperature applications because of the high discharge temperature caused by high compression ratios. Staged compression and liquid injection are two approaches to prevent compressor overheating in HCFC-22 low temperature applications. This paper describes the above approaches and their efficiency as compared to conventional applications.