Performance evaluation of environmentally benign refrigerants in heat pumps 2: An experimental study with HFC134a

The performance characteristics of HFC134a in an industrial (water to water) heat-pump test facility at Electricité de France with a twin-screw compressor are presented. The performance of HFC134a has been studied in terms of performance parameters of the compressor (e.g. its volumetric and isentropic efficiencies) and of the heat-pump system (e.g. coefficient of performance and volumetric heating capacity) with a view to using it in new installations for low to medium temperature (< 70°C) applications as well as to replacing CFC12 in existing installations. The influence of degree of superheat on the miscibility of HFC134a with ester oil and on the viscosity of the oil-refrigerant mixture has also been studied for various discharge pressures.     

Performance of unitary heat pumps: review of ASHRAE Symposium session

1. 1. System design optimization and validation for single-speed heat pump by S.K. Fischer and C.K. Rice, Oak Ridge National Laboratories.

2. 2. Analysis of on/off cycling for an air-to-air heat pump operating in the heating mode by W.A. Miller, Oak Ridge National Laboratories.

3. 3. Field measured cycling, frosting and defrosting losses for a high efficiency air source heat pump by V.D. Baxter and J.C. Moyers, Oak Ridge National Laboratories.

4. 4. Design and available energy analysis of a heating-only residential heat pump for the Western Pacific Northwest by D.E. Elger, C.M. Reistad and S. Lang, Oregon State University.

5. 5. A study of heat pump service life by Nance C. Lovvorn, Alabama Power Company and Carl C. Hiller, Electric Power Research.

Fundamentals of lubrication in refrigerating systems and heat pumps

The fundamentals concerning the influence of oil on the various components of a refrigeration plant are discussed; especially the favourable and unfavourable influences of the lubricant. The requirements for lubricating oils are also described, as they can be deduced from fundamentals. Requirements for oil are considered with respect to new developments, for example, large heat pumps, high temperature heat pumps, and plants with refrigerant mixtures for cooling systems as well as for heat pumps.     

Carbon dioxide as refrigerant for tap water heat pumps: A comparison with the traditional solution

Increased concern about the environmental impact of the refrigeration technology is leading toward design solutions aimed at improving the energy efficiency of the related applications, using eco-friendly refrigerants, i.e. ozone-friendly and with the least possible global warming potential (GWP). In this respect, carbon dioxide (ASHRAE R744) is seen today as one of the most promising refrigerants and is raising great interest in industrial and scientific fields. In the present work, the plant options are investigated, which are related to the design of air/water heat pumps for tap water using CO₂. A comparison is made, in terms of energy efficiency, between a system working with CO₂ and a similar one working with HFC R134a; such a comparison is carried out by means of a simulation model of a refrigerating machine/heat pump, characterized by a detailed representation of the heat exchangers, based on their subdivision into elementary volumes. Results show that carbon dioxide is an interesting substitute for synthetic fluids, if the design of the system is focused to take advantage of its properties.     

An experimental study of an ejector cycle refrigeration machine operating on R113 Etude expérimentale d'une machine frigorifique à éjecteur au R113

This paper discusses an ejector cycle refrigeration machine that can use a wide range of refrigerants including halocarbons. The feature of such a system is the possibility of using a low grade heat source such as solar energy and waste heat to operate the system. A theoretical analysis was carried out to select a suitable refrigerant for the system. The influence of boiler, condenser and evaporator temperatures on system heat transfer is investigated experimentally under different operating conditions. The experimental machine uses R113 as a working refrigerant.     

Influence of ultrasound on pool boiling heat transfer to mixtures of the refrigerants R23 and R134A

The effect of ultrasound on pool boiling heat transfer to mixtures of the refrigerants R23 and R134a has been investigated in a wide range of heat flux and saturation pressure. The enhancement of the heat transfer coefficient, which can be achieved by ultrasound, is much more pronounced for mixtures than for pure substances. It is, however, limited to rather small heat fluxes ( ). Especially remarkable is the fact, that the maximum influence of ultrasound on the heat transfer coefficient of the mixtures occurs at medium saturation pressures (p/p_c ≈ 0.2); the effect is markedly less for higher and for lower saturation pressures. Obviously, the improvement of the heat transfer to mixtures is mainly caused by a decrease of the local saturation temperature near the heating wall, due to a better mixing in the liquid boundary. This explanation is supported by evaluating important parameters of bubble formation from high-speed photographs of the heating surface. It is further noticeable, that the well known hysteresis effect at the beginning of pool boiling is reduced to a great extent by exposure to ultrasound.     

Feasibility study of a two-stage vapour compression heat pump with ammonia-water solution circuits: experimental results

A breadboard heat pump was designed and built to test the performance of a vapour compression heat pump with two-stage ammonia-water solution circuits. A major improvement in performance was obtained by incorporating a bleed line to attain water balance between the high and the low temperature solution circuits. The new scheme was first investigated by computer simulation and then incorporated in the experimental setup. The advantages of this scheme are reduced first cost (by eliminating the need for a rectifier), a simplified system and its control mechanisms, a 20–30% improvement in cooling coefficient of performance and a 10–15% increase in cooling capacity as compared to the system with a rectifier. Coefficients of performance in the range of 0.69 to 1.04 were obtained experimentally for a temperature lift of 100 K and cooling capacities in the range of 2.02 to 4.22 kW. The pressure ratios encountered were in the range of 6.9 to 11, which are 35 to 50% of the pressure ratio expected for a conventional single-stage heat pump.     

Ammonia high pressure heat pumps in food refrigeration applications

The use of the environment-friendly and natural refrigerant ammonia (R717) for industrial heat pumps is described. Compressors for higher operating pressures equivalent to 40 bar have been developed and characteristic data such as COP, performance and operating limits are specified. The operating data from two application examples are 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.     

Experimental investigation of a direct driven radial compressor for domestic heat pumps

The presence of oil in domestic heat pumps is an obstacle toward higher efficiency, particularly for enhanced surface evaporators and for advanced concepts based on two-stage cycles. Very compact direct driven radial compressors supported on oil-free bearings represent a promising alternative. This paper presents the derivation of the specifications, the choice for an appropriate refrigerant fluid and the design of a proof of concept prototype with the various tradeoffs between the impeller characteristics to follow the seasonal heat demand, the bearing and rotordynamics for a stable operation. Heat pump simulation results, the design of the impeller as well as the layout of the experimental facility and first experimental results are presented. An impeller with a tip diameter of 20 mm has been tested at rotational speeds of up to 210 krpm reaching pressure ratios in excess of 3.3 and efficiencies above 78%. The refrigerant chosen for this first experimental approach is HFC 134a.     

HFC32, a low GWP substitute for HFC410A in medium size chillers and heat pumps

This paper presents the experimental heat transfer coefficients and pressure drops measured during refrigerant HFC32 condensation inside a commercial Brazed Plate Heat Exchanger (BPHE) and compares this data with similar measurements previously obtained for refrigerant HFC410A to assess its capability as low GWP substitute for HFC410A in medium size chillers and heat pumps. The effects of saturation temperature, refrigerant mass flux, and vapour super-heating are investigated. HFC32 exhibits heat transfer coefficients much higher and frictional pressure drop slightly higher than those of HFC410A. Therefore, considering that HFC32 exhibits a GWP just one-third that of HFC410A, taking into account also its good thermodynamic properties, it seems to be a very promising low GWP substitute for HFC410A in medium size chillers and heat pumps.     

Methods for comparing the performance of pure and mixed refrigerants in the vapour compression cycle

Methods of comparing pure and mixed refrigerants are considered by computing the coefficient of performance and the heating capacity for an ideal vapour compression cycle for R22/R11 aand R22/R11 mixtures. For comparisons based only on one characteristic condensation temperature and one evaporation temperature, the results depend entirely on how the characteristic temperatures are defined. A method specifying the heat transfer fluid temperatures and a total heat exchanger area per unit capacity is thought to offer a comparison applicable to both pure and mixed refrigerants. Using this method, the effects of compressor superheat, heat exchanger approach temperatures, and the match of refrigerant and heat transfer fluid temperatures are discussed.     

Condensation of refrigerants in horizontal microfin tubes: comparison of prediction methods for heat transfer

A comparison was made between the predictions of previously proposed empirical correlations and theoretical model and available experimental data for the heat transfer coefficient during condensation of refrigerants in horizontal microfin tubes. The refrigerants tested were R11, R123, R134a, R22 and R410A. Experimental data for six tubes with the tube inside diameter at fin root of 6.49–8.88 mm, the fin height of 0.16–0.24 mm, fin pitch of 0.34–0.53 mm and helix angle of groove of 12–20° were adopted. The r.m.s. error of the predictions for all tubes and all refrigerants decreased in the order of the correlations proposed by Luu and Bergles [ASHRAE Trans. 86 (1980) 293], Cavallini et al. [Cavallini A, Doretti L, Klammsteiner N, Longo L G, Rossetto L. Condensation of new refrigerants inside smooth and enhanced tubes. In: Proc. 19th Int. Cong. Refrigeration, vol. IV, Hague, The Netherlands, 1995. p. 105–14], Shikazono et al. [Trans. Jap. Sco. Mech. Engrs. 64 (1995) 196], Kedzierski and Goncalves [J. Enhanced Heat Transfer 6 (1999) 16], Yu and Koyama [Yu J, Koyama S. Condensation heat transfer of pure refrigerants in microfin tubes. In: Proc. Int. Refrigeration Conference at Purdue Univ., West Lafayette, USA, 1998. p. 325–30], and the theoretical model proposed by Wang et al. [Int. J. Heat Mass Transfer 45 (2002) 1513].     

Comparison of the working domains of some compression heat pumps and a compression-absorption heat pump

The working domains of a model of a compression heat pump using different fluids and a model of a compression-absorption heat pump using water-ammonia mixtures are defined, plotted and discussed. These domains are defined by means of limiting values for their electrical coefficient of performance, volumetric heating capacity, and low and high pressure. In the case studied in the present paper, the disappearance from use of CFC and HCFC fluids leaves only one alternative for the implementation of high temperature electric heat pumps: hydrocarbons in compression devices or water-ammonia mixtures in compression-absorption devices. Problems relating to the implementation of these systems are also mentioned.     

Flow condensation heat transfer coefficients of pure refrigerants

Flow condensation heat transfer coefficients (HTCs) of R12, R22, R32, R123, R125, R134a, and R142b were measured experimentally on a horizontal plain tube. The experimental apparatus was composed of three main parts; a refrigerant loop, a water loop and a water-glycol loop. The test section in the refrigerant loop was made of a copper tube with an outside diameter of 9.52 mm and 1 m length. The refrigerant was cooled by cold water passing through an annulus surrounding the test section. All tests were performed at a fixed refrigerant saturation temperature of 40 °C with mass fluxes of 100, 200, 300 kg m⁻² s⁻¹ and heat flux of 7.3–7.7 kW m⁻². Experimental results showed that flow condensation HTCs increase as the quality and mass flux increase. At the same mass flux, the HTCs of R142b and R32 are higher than those of R22 by 8–34% while HTCs of R134a and R123 are similar to those of R22. On the other hand, HTCs of R12 and R125 are lower than those of R22 by 24–30%. Previous correlations predicted the present data satisfactorily with mean deviations of less than 20% substantiating indirectly the reliability of the present data. Finally, a new correlation was developed by modifying Dobson and Chato's correlation with an introduction of a heat and mass flux ratio combined with latent heat of condensation. The correlation showed a mean deviation of 10.7% for all pure halogenated refrigerants' data obtained in this study.     

Acoustic resonance in plate heat exchangers: Résonance acoustique des échangeurs de chaleur à plaques

A ‘whistling' sound, which develops under transient conditions in some automobile air conditioning systems equipped with plate type evaporators, was identified as acoustic resonance. The ‘whistle' was reproduced in the laboratory under steady-state conditions. Testing of these heat exchangers was done first with R134a and then with nitrogen. Nitrogen testing proved to be much faster and easier and provided results comparable to the results obtained using R134a. The evidence presented in this work suggests that the acoustic resonance in this type of heat exchanger is similar in nature to the acoustic resonance problems reported for tube array in duct type heat exchangers. This is to the authors' knowledge the first time that acoustic resonance problems have been reported in the literature for plate heat exchangers.     

Optimized transcritical CO2 heat pumps: Performance comparison of capillary tubes against expansion valves

A capillary tube based CO₂ heat pump is unique because of the transcritical nature of the system. The transcritical cycle has two independent parameters, pressure and temperature, unlike the subcritical cycle. In the present study, a steady state simulation model has been developed to evaluate the performance of a capillary tube based transcritical CO₂ heat pump system for simultaneous heating and cooling at 73 °C and 4 °C, respectively against optimized expansion valve systems. Capillary tubes of various configurations having diameters of 1.4, 1.5 and 1.6 mm along with internal surface roughness of 0.001–0.003 mm have been tested to obtain the optimum design and operating conditions. Subcritical and supercritical thermodynamic and transport properties of CO₂ are calculated employing a precision in-house property code.

It is observed that the capillary tube system is quite flexible in response to changes in ambient temperature, almost behaving to offer an optimal pressure control. System performance is marginally better with a capillary tube at higher gas cooler exit temperature. Capillary tube length turns out to be the critical parameter that influences system optimum conditions. A novel nomogram has been developed that can be employed as a guideline to select the optimum capillary tube.     

Nucleate boiling heat transfer coefficients of mixtures containing HFC32, HFC125, and HFC134a

Nucleate boiling heat transfer coefficients (HTCs) of binary and ternary mixtures composed of HFC32, HFC125, and HFC134a on a horizontal smooth tube of 19.0 mm outside diameter were measured. 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 kW m⁻² to 10 kW m⁻² with an interval of 10 kW m⁻² in the pool temperature at 7 °C. HTCs of nonazeotropic mixtures of HFC32/HFC134a, HFC125/HFC134a, and HFC32/HFC125/HFC134a showed a reduction of HTCs as much as 40% from the ideal values while the near azeotropic mixture of HFC32/HFC125 did not show the reduction. Four of the well known correlations were compared against the present data for binary mixtures. Stephan and Körner's and Schlünder's correlations yielded a good agreement with a deviation of less than 10% but they can not be easily extended to multi-component mixtures of more than three components. A new correlation was developed utilizing only the phase equilibrium data and physical properties. A regression analysis was carried out to account for the reduction of HTCs and the final correlation, which can be easily extended to multi-component mixtures of more than three components, yielded a deviation of 7% for all binary and ternary mixtures.     

The role of surfactant adsorption rate in heat and mass transfer enhancement in absorption heat pumps

The importance of heat and mass transfer additives in absorption chillers and heat pumps has been recognized for over three decades. However, a universally accepted model for the mechanisms responsible for enhanced absorption rates has yet to be proposed. The Marangoni effect—an instability arising from gradients in surface tension at the liquid-vapor interface—is generally accepted as the cause of the convective flows that enhance transfer rates. Certain surfactant additives can significantly improve absorption rates and thus reduce the overall transfer area required by a given machine. Any means available that can increase the efficiency and acceptability of absorption machines is to be welcomed, as this technology provides an alternative to vapor compression systems which is both environmentally friendly and more versatile with regards to energy sources. This study investigates the rate at which a surfactant additive adsorbs at a liquid-vapor interface. The residence time of the falling liquid solution in an absorber is quite short. An effective additive must not only reduce the surface tension of the solution; it must do so quickly enough to cause the Marangoni instability within the short absorption process time. The entrance region of an absorber features a freshly exposed interface at which no surfactant has adsorbed. A numerical model is used to analyze surfactant relaxation rates in a static film of additive-laced solution. Kinetic parameters for the combination of the working pair LiBr-H₂O and the additive 2-ethyl-1-hexanol are derived from data in the literature for static and dynamic surface tension measurements. Bulk, interfacial and boundary parameters influencing relaxation rates are discussed for surfactant adsorption occurring in the absence of absorption, as well as for concurrent adsorption and stable vapor absorption. Initial solution conditions and absorption driving force are shown to impact the potential for instability in the effect they have on the rate of interfacial additive adsorption.     

Degradation of polyethylene terephthalate films in the presence of lubricants for HFC 134a: a critical issue for hermetic motor insulation systems

Earlier studies conducted in the use of sealed tubes with polyalkylene glycol lubricants and polyethylene terephthalete (PET) films revealed that the PET films exhibited embrittlement and (visual) degradation. This led to an investigation of PET embrittlement mechanisms with the new lubricants used with HFC 134a. The lubricants studied were three polypropylene glycols (the monol, the diol and the completely end-capped glycols), pentaerythritol ester and a blend of monol and ester. The effects of moisture content, temperature and lubricant structure were studied. All lubricants in this study were of viscosity grade ISO-32 (150 SUS). The results were compared to PET film embrittlement in the presence of CFC 12 and mineral oil. This study reconfirmed the earlier findings that the PET films must be dried to lower than 0.1 wt.% moisture content for use in hermetic systems. This paper discusses the effect of the moisture content of the lubricant and the effect of the lubricant structure on PET films. The dependence of the various mechanisms on temperature is shown. Esters and end-capped polyalkylene glycols are recommended for use with HFC 134a.