STUDIES INTO THE THERMAL AND TRANSPORT PROPERTIES OF ICE SLURRIES FOR LOW ENERGY COOLING APPLICATIONS IN BUILDINGS

S A Tassou, I Chaer and I BellasMechanical Engineering

School of Engineering and DesignBrunel University

Uxbridge, Middlesex, UKTel: 01895266865; E-mail: Savvas.Tassou@brunel.ac.uk

1 BACKGROUNDWith increasing pressures on building owners and operators to adopt systems with minimum impact on the environment, building services engineers are faced with some difficult choices. Although in rural areas and greenfield sites it may be possible to avoid air conditioning through the design of buildings for natural ventilation, in built-up areas and buildings with high internal gains the avoidance of air conditioning is not always possible. It is therefore necessary to investigate alternative cooling technologies in terms of their energy and environmental performance and adopt systems which offer minimum impact on the environment.

Ice thermal storage and in particular ice slurry offers the potential of significantly reducing the energy consumption and emission of greenhouse gases arising from air conditioning installations. Ice slurry is a suspension of microscopic ice crystals in an aqueous solution. The ice crystals are typically 0.1 mm to 0.5 mm in diameter depending on the quality of the water in the solution. The solution is normally brine based allowing its freezing point to be suppressed and hence the temperature to be tailored for various applications. Temperatures as low as -40oC can be achieved for deep freeze and process applications. Ice-slurry may be readily pumped at ice concentrations up to 40%. Above 40% the fluid becomes too dense to pump using conventional rotodynamic pumps. The advantage of ice slurry based thermal storage systems over static ice storage systems is that the ice slurry can be pumped directly from the storage tank to the cooling coils without the need for a secondary fluid or a secondary heat exchanger, improving efficiency and reducing system cost. Advantages of ice slurries over chilled water storage and distribution systems include the ability to be used at temperatures below 0 oC for commercial refrigeration applications and their ability to provide a much higher heat transfer density due to the latent heat of ice, which can lead to much lower flow rates and pumping power compared to chilled water or other single-phase secondary distribution systems. Despite these advantages, however, ice slurry systems have not yet made inroads into the HVAC industry primarily due to the unavailability of information on thermophysical and transport properties of slurries at different concentrations. This information is essential to enable building services engineers to design and operate effectively ice slurry based air conditioning and refrigeration systems. For wider adoption of these systems by the building services and refrigeration industries there is a need, therefore, for engineering information regarding the thermophysical and transport properties of ice slurries and wide dissemination of design and operating guidance.

The main aim of this project was to develop experimental facilities for the investigation of the heat and fluid flow properties of ice slurries and provide engineering information to facilitate the design of efficient ice slurry systems. This report outlines the work carried out and presents the results obtained from the study of the thermal and transport properties of ice slurries. The report describes the test facility developed and the experimental investigations carried out and discusses the results obtained. The key advances and benefits to society are discussed together with recommendations for further work.

2 METHODOLOGY AND RESULTS2.1 Experimental Investigations The test facility developed for this project (Figure 1), is unique in the UK and enables a large number of different investigations with respect to ice slurries and other secondary refrigerants to be carried out. The facility consists of two independent circuits. The ice slurry formation circuit and the ice flow circuit. The ice formation circuit consists of a vertical scraper surface flooded shell and tube evaporator, a 25 kW condensing unit, pumps and a well insulated storage tank with a propeller type air motor driven mixer. This system is capable of producing ice crystals with diameters in the range between 170 μm and 600 μm (Figure 1c).

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The ice flow circuit consists of four unplasticized polyvinyl chloride (PVC-U)pipes mounted horizontally. All four pipes have a length of 5 m each and internal diameters of 31.75 mm, 25.40 mm, 19.05 mm and 12.70 mm respectively. The pipes are connected in parallel through two common headers of 31.75 mm internal diameter. Valves are installed at the inlet of each tube to enable pressure drop measurements in individual tubes.

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a) ice generator and flow circuit b) ice stoage tank

Figure 1. Experimental test facilities and ice slurry

A standard plate heat exchanger normally used in traditional secondary loop systems was also incorporated within the test facility to test its performance with ice slurries. The heat exchanger has 24 plates with 11 channels on each side. Each plate is 310 mm high and 112 mm wide. The hydraulic diameter is 4 mm. The hot (primary) fluid in the heat exchanger was mains water. Chilled water was also tested in the heat exchanger to form the basis for comparison with ice slurry.

To facilitate the design of ice slurry systems for food retail applications, an integral multi-deck vertical display cabinet was also installed in the ice flow circuit. Tests were performed on the unit with primary refrigerant R22, a propylene glycol water mixture as a secondary fluid and with ice slurry for comparison purposes.

Temperature sensing in the ice slurry flow section was performed with mineral insulated (type T) thermocouples placed in the inlet and the outlet flow streams of each tested component. Pressure drop in each tube and other components was measured with a combination of pre-calibrated pressure and pressure differential transducers. The ice slurry flow rate was measured upstream of the test section using an electromagnetic flow meter.

During each test the ice fraction was measured using the filtration method. With this method, the flowing mixture was sampled near the inlet and outlet of each component tested and a filter cloth with 100 μm pores was used to filter out ice crystals from the chilled liquid. The ice fraction was determined from the corresponding weight of the ice collected with the filter and the chilled solution passed through the filter.

2.2 Thermophysical properties of ice slurriesAn extensive literature survey of the thermophysical and transport properties of ice slurries was carried out. Properties studied include density, viscosity, thermal conductivity, enthalpy and specific heat. The literature survey also includes past experimental work and technologies used for ice slurry production and distribution (Bellas 2002).

Several experimental methods to measure the density and ice concentration of ice slurries have been reported but research is continuing to develop on-line methods for ice concentration measurement (Dickey et al 1989, Kauffeld et al 1999, Fournaison 2001).

Correlations for the calculation of the physical properties of ice slurries are either based on semi-empirical methods which rely on the determination of the ice content of the ice slurry from on-line density measurements, along with measurements of the melted slurry at its melting temperature, or the assumption that the slurry is an immiscible multiphase mixture. The properties of the mixture are then determined based on the weight fraction and the property of each component of the mixture (Wasp et. al. 1975, Guilpart et. al. 1999).

The majority of thermophysical property correlations found in the literature are based on the assumption that ice slurry is a suspension of pure ice crystals in a residual liquid phase (Bell et al 1999). Experimental investigations have shown that this assumption may lead to significant errors in the calculations and more experimental work is needed to develop more accurate correlations for ice water mixtures based on different freeze depressant fluids Thomas 1965, Melinder 1997).

2.3 Experimental Results

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e) Ice slurry in tankc) Ice particles

The experiments performed produced several results for pressure drop and heat transfer specific to 5% propylene/water ice slurry at different ice concentrations and flow rates in straight pipes, pipe fittings, plate heat exchangers and plate fin cooling coils. Some of these results are summarised below. More comprehensive data and results are given in the publications of the investigators listed in the Reference section. 2.3.1 Pressure Drop in Straight pipes The experimental investigations produced several results for ice concentrations between 0% and 37% for flow in two PVC-U pipes with internal diameters of 12.70 mm and 19.05 mm respectively (see Figure 2). The pressure drop was found to increase with increasing ice concentration and flow rate for both pipes (Chaer et al 2000)

The results of experimental investigations into the melting heat transfer and pressure drop of ice slurry flowing in a commercial plate heat exchanger were obtained for mean ice fractions between 0 and 20 % by weight, and flow rates between 1.0 m3/h and 3.7 m3/h. Increasing the ice fractions from 0% to 20 % caused around a 15% increase in the pressure drop over the flow range tested, see figure 3. The overall heat transfer coefficient, based on the logarithmic mean temperature difference, was found to remain fairly constant as the ice fraction increased from 5% to 20%. The heat transfer capacity of the heat exchanger was found to increase by more than 30% with melting ice slurry flow compared to chilled water flow, as can be seen in Figure 3. This increase can lead to more than 60% reduction in the secondary fluid flow rate for the same load, leading to significant reductions in both the size of the heat exchangers and pumping power (Bellas et al 2001).

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2.3.2 Heat Transfer and Pressure Drop of Ice Slurries in Plate Heat Exchangers

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a) 12.7 mm dia pipe b) 19.05 dia pipe

Figure 2. Pressure drop versus flow velocity for ice slurries flowing in a 12.7 mm and 19.05 mm internal diameter pipe

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2.3.2 Performance of Ice Slurry supermarket refrigeration applicationsThis part of the experimental investigations produced several results specific to 5% propylene glycol/water slurries melting in the plate fin cooling coil of a vertical display cabinet at mean ice fractions between 0% and 30 % and flow velocities between 0.54 m/s and 0.88 m/s. The pressure drop was found to increase with increasing flow rate and ice fraction (Figure 4) Increasing the ice fraction from 0% to 30% resulted in a 11% increase in the pressure drop at the higher flow rate of 0.88 m3/h and 28 % increase at the lower flow rate of 0.54 m3/h.

The air on and air off temperature profiles across the evaporator coil were found to be more stable when the coil was operated with ice slurry compared to operation with a primary refrigerant. Despite the fact that ice slurry entering the coil had at least 2 oC higher temperature than that of R22, it was still capable of providing an average air outlet temperature similar and even lower than those obtained with R22 (Tassou et al 2001).

3 PROJECT PLAN REVIEW, KEY ADVANCES AND BENEFITS TO SOCIETY3.1 Project plan and expenditureThe project was carried out in line with the plan outlined in the Case for Support. The project aims and objectives have been satisfied but further work is still being carried out on a number of areas of ice slurry flow and heat transfer using a PhD research student, Mr Ioannis Bellas, funded by the Department. There was no significant difference between the original spending plans and the actual overall project expenditure.

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Figure 3. Variation of pressure drop and heat transfer with flow rate in plate heat exchanger

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Figure 4. Variation of pressure drop and fluid temperature in a plate fin heat exchanger

3.2 Key Advances and Benefits to SocietyThe work carried out at Brunel through the EPSRC support and the support of the industrial partners, as well as work that is currently being pursued with support from the Department of Mechanical Engineering, is contributing to the international research effort on ice slurries which is coordinated by the International Institute of Refrigeration IIR). The Brunel Team is a member of the International Consortium on Ice slurries which consists of industrial, research and academic groups working on ice slurries around the world. The findings of all the participating research groups are reported at six-monthly international meetings and will lead to the preparation of a Handbook on Ice Slurries which will be published by the IIR in 2003.

The Brunel Team has presented and published results of their work on heat transfer and pressure drop in heat exchangers and pipes and fittings at international symposia, conferences and scientific journals. The paper `Heat Transfer and Pressure Drop of Ice Slurries in Plate Heat Exchangers’ presented at the 7 th UK National Conference on Heat Transfer held in Nottingham, in September 2001 received the HTFS prize for the best paper presented at the conference. This paper will also appear in the Journal of Applied Thermal Engineering in 2002.

The heat transfer and pressure drop data obtained from the experimental programme as well as results from continuing investigations at Brunel, will provide valuable engineering information for the design of effective ice-slurry based cooling systems and should lead to the wider application of the technology.

As shown by the results of this study, ice slurry systems have provide substantially increased heat transfer rates in plate heat exchangers compared to single phase secondary refrigerants, leading to substantially reduced flow rates and pumping power even though the pressure loss is slightly increased compared to single phase flows. The heat transfer performance of ice slurries in plate fin coils is also comparable to the performance of traditional refrigerants which make them an attractive alternative technology for commercial refrigeration applications.

4 FURTHER RESEARCH AND DISSEMINATION ACTIVITESThe results obtained in this study are specific to 5% propylene glycol/water slurries with ice crystal size between 0.1 mm and 0.6 mm. Further work is required to quantify the effect of the ice crystal size, freezing point depressant agent, the Reynolds number and flow geometry on ice slurry heat transfer and pressure drop. The heat exchanger pressure drop and heat transfer results also indicate that for each application there may be an optimum combination of flow rate and ice concentration to maximise heat transfer without substantially increasing friction losses. To enable such optimisations to be carried out substantially more experimental data is required for different freeze depressant/water mixtures.

Other issues also need to be addressed before ice slurries can be widely adopted by industry. These include the design of storage systems to prevent stratification and ice particle agglomeration and the development of techniques to control the ice fraction in the flow to suit different applications and varying loads. These areas will be the subject of further investigations at Brunel University using the unique facilities developed through the support of the EPSRC and the industrial collaborators.

The results of the work have been disseminated through the IIR workshops on ice slurries, presentations at International Conferences and Journal publications. A list of publications is given below.

REFERENCES/PUBLISHED PAPERSDickey L. C., Radewonuk E. R. and Dallmer M. F., “Determining Ice Content of a Fine Ice Slurry from Density Measurements”, AICHE Journal, Vol. 35, No. 12, pages 2033-2036, 1989.

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Bell I. O. and Lallemand A., 1:“Study of a Two Phase Secondary Refrigerant 1: Intrinsic Thermophysical Properties of an Ice Slurry”, 2:“Thermal Study of an Ice Slurry used As Refrigerant in A cooling Loop”, International Journal of Refrigeration, Vol 22, pp164-174,1999.Guilpart J., Fournaison L. and Ben Lakhdar M. A., “Calculation Method of Ice Slurries Thermophysical Properties Application To Water/Ethanol Mixture”, 20th International Congress of Refrigeration, IIR/IIF, Sydney. 1999.Melinder A., “Thermophysical properties of Liquid Secondary Refrigerants”, IIR, 1997.Thomas D. G., “Transport Characteristics of Suspension: VIII. A Note on the Viscosity of Newtonian Suspensions of Uniform Spherical Particles”, Journal of Colloid Sci. 20 pages 267-277, 1965 . Wasp E. J., Kenny J. P. and Gandhi R. L., “Solid-Liquid Flow: Slurry Pipeline ransportation”, Series on Bulk Material Handling , Vol. 1, No. 4, 1975/77.Turian R. M., Dong-Jin S. and Feng-Laung H., ”Thermal Conductivity of Granular Coals, Coal-Water Mixtures and Multi-Solid/Liquid Suspensions”, Fuel, Vol. 70, No. 10, pages 1157-1172, 1991.Bellas, I. (2002). Thermophysical and transport properties of ice slurries, Bunel University Internal Report, 17 pgs. Fournaison L, Chourot, J. M. and Guilpart J. (2001) Different Ice Mass Fraction Measurement Methods, , 4th , IIR Workshop on Ice Slurries, Osaka, 12 – 14 November 2001, pp. 41-48.Guilpart J., Fournaison L. and Ben Lakhdar M. A. (1999), Calculation Method of Ice Slurries Thermophysical Properties Application To Water/Ethanol Mixture, 20th International Congress of Refrigeration, IIR/IIF, Sydney. 1999. Kauffeld M., Christensen K. G., Lund S. and Hansen T. M.,” (DTI RESEARCH GROUP)) Experience with Ice Slurry”, Proccedings of the 1st Workshop on ice slurries pages 42-73, Switzerland 1999.Wasp E. J., Kenny J. P. and Gandhi R. L.(1975/77), Solid-Liquid Flow: Slurry Pipeline Transportation, Series on Bulk Material Handling , Vol. 1, No. 4.Chaer I. , Bellas J. and Tassou S. A. (2001), Flow Characteristics and Pressure Drop of Ice Slurries in Straight Tubes, Clima 2000/Napoli 2001 World Congress, Napoli, 15-18 September 2001.Bellas J., Chaer I. And Tassou S. A. (2001), Heat Transfer and Pressure Drop of Ice Slurries in Plate Heat Exchangers, 7th UK National Conference on Heat Transfer, Nottingham, 11-12 September 2001.Tassou S. A., Chaer I., Bellas, J. And Terzis, G. (2001), Comparison Of The Performance Of Ice Slurry And Traditional Primary And Secondary Refrigerants In Refrigerated Food Display Cabinet Cooling Coils, 4th , IIR Workshop on Ice Slurries, Osaka, 12 – 14 November 2001, pp. 87-96.Tassou S. A., Chaer I., Bellas J. (2002), Flow Characteristics and Pressure Drop of Ice Slurries in Bends and pipe fittings, to be presented at the 5th IIR Workshop on Ice Slurries, Stockholm, 30-31 May 2002.Bellas J., Chaer I. And Tassou S. A. (2002), Heat Transfer and Pressure Drop of Ice Slurries in Plate Heat Exchangers, Applied Thermal Engineering, Volume 22, Number 7, May 2002, pp. 721-732(12)

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