Heat Exchanger for Down-Hole Condensation Process Theoretical and Experimental Investigation, Considering Surrounding Fluid Properties

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1 GRC Transactions, Vol. 40, 2016 Heat Exchanger for Down-Hole Condensation Process Theoretical and Experimental Investigation, Considering Surrounding Fluid Properties Benedict Holbein 1, Jörg Isele 2, Luigi Spatafora 3, Veit Hagenmeyer 4, and Thomas Schulenberg 5 1 Down-Hole Tool Engineering, Institute for Applied Computer Science (IAI) 2 Work Group: Geothermal Energy, IAI 3 Down-Hole Tool Engineering, IAI 4 Head of Institute, IAI 5 Head of Institute, Institute for Nuclear and Energy Technologies (IKET) Karlsruhe Institute of Technology (KIT), Eggenstein-Leopoldshafen, Germany Keywords Cooling system, deep geothermal energy, down-hole investigation and interaction, heat transfer simulation, thermal water properties, tool engineering ABSTRACT A major challenge for tool operations in deep geothermal boreholes is the combination of high ambient temperatures and the demand for electronic components for various measurement, investigation and data processing tasks. Different approaches to handle this include the usage of high temperature electronics in combination with heat shields and in some cases temporary cooling systems. However an alternative approach, pursued at KIT, is the development of a continuous cooling system, based on the cooling machine principle. It would allow the usage of standard electronics without time limitations, thus provide a widely extended range of performable operations. One of the critical parts for realization of a down-hole cooling machine is the external heat transfer from the used refrigerant to the surrounding fluid, which requires condensation to allow a closed thermodynamic cycle. Among pressure and corrosion conditions, the difficulties for the design of the external heat exchanger lay in the estimation of the outer heat transfer process, regarding the unknown properties of the surrounding fluid and their influence on the heat transfer process. For the consideration in the present contribution, a worst case situation with free convection of stagnant thermal water is assumed. Based on literature data on saline geothermal and sea water, the temperature and salinity dependence of important properties for the heat transfer are taken into account for the theoretical consideration. Additionally an approach for the experimental investigation of the heat transfer process at high temperatures, using a heat exchanger model and first results are presented. Figure 1. Cooling machine process and components in a 2-stage concept, [Holbein, 2015]. 949

2 1. Background for Heat Transfer Situation To explain the specific heat transfer situation, the overall concept for the down-hole cooling system is briefly described. The basic principle is dividable into four sub-processes: expansion, evaporation, compression and condensation which are processed continuously in sequence. During evaporation of a refrigerant in an insulated area, heat is taken from the electronic components. The vapor is compressed to a pressure at which the refrigerant condensation temperature is higher than the ambient temperature. Like this it can condense while transferring heat to the surrounding fluid. The condensate is brought back to evaporation pressure and temperature by expansion. Since a reliable condensation process is required, the possible ambient temperature has to be lower than the critical temperature of the refrigerant, because otherwise the liquid state is not reached. The foreseen approach to extend the usability of the cooling system beyond this limit is to perform a second stage process using a second refrigerant with higher critical temperature. This is beneficial in order to obtain a lower pressure gradient for the engineering of a suitable compressor and allows performing the evaporation process without vacuum in the evaporator. Figure 1 shows the principle of the cooling machine with its components and the exemplary thermodynamic process. As can be seen from the process in figure 1, to fulfill the condensation a certain heat amount has to be transferred to the surrounding, which can be estimated using equation (1) dq out dt dm dt * ( h h 2b 3b) dm dt * {( h h 2b * 3b ) + cp( 370C)* ( T 2b T 2b * )} (1) with dm/dt as refrigerant mass-flow, the enthalpy of condensation Δh, the heat capacity cp and the temperatures T around the condensation level at p 3 and 370 C [Baehr, 2013]. The condensation heat transfer of stage 1 at around 200 C is calculated similar using the values of the process points with the indexes a. The present contribution considers the heat transfer at an environment of around 200 C, because the actual experimental set-up is not usable for the 370 C process. This will be a task for further investigations. Nevertheless the investigation of the condensation process at around 200 C is already challenging, since water as surrounding fluid is not usable unless pressurized in an autoclave. Actually a suitable autoclave for 200 C and 600 bar pressure is available at the IAI but the respective experiments are more expensive and require a pressure resistant dimensioning of the condenser, thus they will be conducted in a next step. 2. Theoretical Investigation of the Condenser Heat Transfer 2.1 Procedure for the Process Investigation One problem for the investigation of the external condensation is that the properties of the surrounding geothermal water which are important for the heat transfer are strongly temperature dependent. Furthermore in many cases the thermal water also shows a high grade of salinity which also influences the important properties density, viscosity, heat capacity and heat conductivity [Hongbing, 2008]. In order to reliably dimension the condenser for all eventual borehole environments, these factors have to be taken into account. Nevertheless the available information on the thermal water properties, especially at high temperature is rare [Schroeder, 2015]. Therefore an estimation of the temperature and salinity dependence, based on existing literature data is presented in the following. Another problem for the investigation is a practical one. For a simple and therefore comprehensible experimental set-up which can be simulated, it is helpful to investigate the process without pressurization first to study other influencing factors besides pressure [Hongbing, 2008]. It is therefore planned to investigate the heat transfer at normal pressure first, before conducting autoclave experiments at higher pressure. For being able to do this at 200 C thermo-oil is used instead of water as surrounding fluid. It is obvious that the thermal properties of oil differ strongly from the water properties. It is therefore necessary to determine and quantify the differences between the regarded fluid properties and their impact on the heat transfer for a targeted interpretation of experimental results. Regarding the heat transfer coefficient α as it is defined and calculated in the literature, the important properties can be read from equation (2) [Churchill, 1975]: α i = Nu i * λ i L characteristic,i (2) The dimensionless Nusselt number Nu i is calculated for free convection with equation (3) as a function of density ρ, thermal expansion coefficient β, dynamic viscosity η, heat conductivity λ, heat capacity cp, all fluid properties 950

3 which are varying with the temperature. The correlation for a horizontal cross flowed pipe with free convection is used [Churchill, 1975]: g *β *( T pipe T surrounding )* π * d 9 16 outside Nu i = 0, ,387 * *η * cp 0,559 * 1+ 2 η λ η * cp (3) ρ λ The additional parameters in equation (3) are the pipe surface temperature T pipe, surrounding temperature T surrounding, pipe diameter d outside and gravity g. The heat transfer coefficient depending on the named properties directly influences the heat transfer in the fluid in theory. Heat transfer through the pipe wall with an outside temperature T outside and an inside temperature T inside is calculated as [Baehr, 2013]: dq transfer dt ( ) = T T inside outside π * L * ln d outside 1 d { } * d + inside + 2*λ inside wall α inside Nuinside 1 α outside { Nuoutside }*d outside with the pipe diameters d inside and d outside. It can be seen, that the heat transferability depends on the smaller heat transfer coefficient (α) on inside or outside, which is dominant then. The external single phase heat transfer is uncertain and can t be influenced actively. Therefore it is the critical factor for the total heat transfer between refrigerant and surrounding fluid in the down-hole condenser. To handle the uncertainties regarding the fluid properties, a stepwise approach, as illustrated in figure 2, is pursued. The approach contains the experimental investigation of the heat transfer in a thermal-oil environment and in parallel the simulation of the process with different surrounding fluids, namely pure water, saline water and thermo-oil. By comparison of the results, correlations between the heat transfer, characterized e.g. by temperature profiles of included elements and the properties of the surrounding fluid shall be defined. These correlations are used in a second step to compare experiments under pressure with water surrounding and simulated data. The adjusted correlations, including the pressure Figure 2. Stepwise approach for the investigation of the condenser heat transfer. influence are finally used to estimate the heat transfer in the real environment, where a combination of salinity, temperature and pressure is present. 2.2 Surrounding Fluid Properties and Their Impact on the Heat Transfer The comparison of the fluid properties shows a recognizable impact on the heat transfer of the condenser. Although in terms of mechanical design, this could only mean a larger dimensioned heat transfer surface, it is of scientific interest and important for an optimized component embodiment. For the calculation of the pure water properties at increased temperatures, empirical polynomial equations are used where density, heat capacity, heat conductivity and dynamic viscosity are approximated as temperature (but not pressure) dependent [Kleiber, 2013]. The salinity influence is calculated using empirical equations for sea-water collected by Mostafa et al [Mostafa, 2010]. It is clear, that real geothermal water contains many different substances as salts and ions in different quantities and furthermore in an individual composition for each region. Nevertheless the salinity influence within the present contribution is reduced to certain mass fractions of sodium chloride (NaCl), because many geochemists identify it as the main impact 1 (4) 951

4 [Schroeder, 2015] [Francke, 2010]. Additionally the most widespread information of the salinity influence on fluid properties is found for sea-water, where NaCl is dominant. In figure 3 the the mentioned fluid properties which influence the heat transfer coefficient (α) are shown for the example of pure water. The temperature dependent property profiles are supplemented by linear approximations. It has to be pointed out, that the focus for this consideration lays on the qualitative impact on heat transfer. It can be seen, that there is a clear correlation between changing density and heat conductivity with the heat transfer coefficient, which are strongly decreasing when moving towards high temperatures above 200 C. The heat capacity is increasing with the temperature but the decreasing factors are dominant, till a turn of α at around 350 C. The viscosity is also decreasing with increasing temperature, which indicates, that a correlation between decreasing viscosity and increasing heat transfer coefficient exists (see equation (3)). In fact this effect is clarified if the other properties are hold constant and the heat transfer coefficient is plotted for decreasing dynamic viscosity, as in figure 4. Dynamic viscosity and the other properties are compared by extrapolating the polynomial approximation equations for pure-water, salt-water with different concentrations of NaCl which have limited validity, covering salt-concentrations up to 150 g/kg and temperatures up to 180 C [Mostafa, 2010]. The profiles for the used thermooil are extrapolated using the substance data from the supplier and according to literature information [Julabo, 2016], [DOC, 1929], [Wrenick, 2005]. Figure 5 shows the example of dynamic viscosity for the named fluids. As can be seen, the profiles for different waters run nearly parallel, while a higher salt fraction results in slightly higher viscosity. The same is true for the density while for heat capacity and heat conductivity higher salt fraction results in slightly lower values. The original equations carry uncertainties around 2.5%. Due to the extrapolation for higher temperatures this uncertainty is surely increased. Additionally the calculation of the heat transfer coefficient is based on approximations; hence the absolute values of the fluid properties and resulting heat transfer parameters are not reliable. However, they give useful information about the difference compared to pure-water, for which the available literature is comprehensive. For the experiments using thermo-oil, the differences are much bigger, which doesn t change the basic approach for their comparison with water. The experiments which will be conducted afterwards, with a pressurized water-surrounding will represent a useful data base for a reliable evaluation of the experimental data. At the end the target of this approach is to make the Figure 3. Impact of fluid properties on the heat transfer coefficient for free convection, equation (3). Figure 4. Correlation between dynamic viscosity and heat transfer coefficient, equation (3). Figure 5. Extrapolated temperature dependence of viscosity for different surrounding fluids. experimental results comparable and interpretable and to get information for an optimized dimensioning of the condenser. The correlations regarding fluid properties are translated into their impact on changing dimensioning parameters. Therefore the heat transfer situation is simplified in a way that the focus lays on the external and uncertain heat transfer. In table 1, the used parameters for the simplified calculation are given. The heat transfer coefficients based on the mentioned free convection approximation for the surrounding fluids lay between 400 and 2500 W/m^2/K, as figures 6 & 7 show. 952

5 The lowest values are estimated for thermo-oil. Figure 7 shows that the salinity has only a small influence on the resulting heat transfer coefficients for temperatures up to 200 C. This is also caused by previously described contrary correlations of fluid properties with temperature and impact on the heat transfer. The stronger deviation of the values between 200 C and 350 C could be partly caused by the extrapolation of the literature equations beyond their validity. No literature was found covering the salt water properties significantly higher than 200 C. The heat conduction value for the pipe wall, depending on the wall thickness and the heat conductivity of the pipe material, is around W/m^2/K for a steel pipe, thus around 7 times higher than the external coefficient. In figure 8 the results for a pipe based condenser with axial throughflow are shown. It contains the external heat transfer coefficients and the required pipe length for pure water and water with different salinities. The required heat transfer surface represented by the number and length of transfer pipes for thermo-oil is around 2 times higher than for the water surroundings. For the calculation, the parameters listed in Table 1 and the surrounding heat transfer coefficients are used and a required heat flow of 500 W is assumed [Numrich, 2013]. The diagram in the upper right of figure 9 shows the correlation of the external with the total heat transfer coefficient, calculated with equation (3). For comparison, the internal heat transfer coefficient assumed is 2034 W/(m^2*K). 3. Experimental Investigation of the Condensation Heat Transfer For testing the condenser heat transfer and the condenser prototypes under realistic condition, meaning in a hot water environment, a high pressure has to be provided and managed as well. For the design of the condenser, including material choice and dimensions it is a cost and time saving approach to perform preliminary experiments at normal pressure first. Table 1. Parameters for the simplified calculation of the condenser heat transfer. Inner heat transfer coefficient calculated with film condensation approach for overheated steam in vertical pipes [Numrich, 2013]. Axial flowed pipe with Ethanol vapor as refrigerant. Inner heat transfer coefficient α inside W m 2 * K Outer pipe diameter Inner pipe diameter Total pipe length Input vapor temperature d outside ( mm) d inside ( mm) L ( mm) T in ( C) Refrigerant vapor mass-flow Figure 6. Comparison of heat transfer coefficients of pure-water and thermo-oil. Figure 7. Comparison of heat transfer coefficients of pure-water and saline water. dm dt kg h 3.1 Experimental Set-Up at Atmospheric Pressure For the experiments at atmospheric pressure, a large container which is filled with thermo-oil and enclosed by a heating system is used. The oil is heated up using an oil heater and Figure 8. Results of the heat transfer calculation for the pipe condenser. 953

6 Figure 9. Experimental set-up for condensation tests at 1 atm. Figure 10. Set-up of the high pressure experiment using an autoclave. the container is heated up as well from outside. This way a liquid surrounding with up to 250 C for testing the condenser is provided. Figure 9 shows the experimental set-up for the experiments at 1 atm. The refrigerant vapor is produced and compressed externally and flows in the condenser through an inlet fitting. The condensate flows out through an outlet fitting. At the inlet and outlet the mass flow and pressure are measured. Inside the heat transfer pipes and the oil container, thermocouples are mounted to measure the temperature time profiles at different spots. 3.2 Experimental Set-Up at Over Pressure In a next step, the condenser is adjusted according to the results of the experiments at 1 atm and dimensioned to stand surrounding pressures of 600 bar. This way it can be tested in a hot water and high pressure environment. The autoclave which is erected in the geothermal test laboratory at the IAI, realizes surrounding temperatures of up to 200 C and pressures up to 600 bar. It will serve as realistic test environment for further experiments on condenser and condensation process. In figure 10, the planned experimental setup for the high pressure testing is shown. As can be seen, the general structure with the condenser inside a heated container is similar. However the condenser has to be mounted at the connection flange at the top of the autoclave. Temperature measurements are done inside the surrounding water and inside the condenser. Additionally the pressures inside and outside the condenser are measured. 4. Construction and Assembly of Condenser Prototype A prototype for the condenser has been manufactured and mounted (see figure 12). It is designed with the aim to provide a simple geom- Figure 11. Condenser prototype, complete design a), locking threats b) and installation threats c) of flanges. 954

7 etry, which can be realized using standard forms. This is advantageous regarding the fabrication with special non-corrosive materials with high strength, which are expensive and difficult to acquire. The condenser is designed in a way to provide different possibilities for adjustment of heat transfer surface, flow directions and temperature measurement spots. The heat transfer pipes can be closed using locking screws which can be mounted in the flange. Since they are connected to the flanges and storages via fittings, they can also be replaced by longer or shorter pipes or pipes with different wall thickness and material. The threads in the top and bottom flange are used to mount thermocouple fittings and allow measuring the temperature inside the pipes and storages in different depths. The flange threats are placed and dimensioned in order to be able to mount the inlet pipes from both sides. This allows different configurations for the flow direction of vapor and condensate and that the condenser prototype can be installed hanging or standing, depending on the used experiment environment. 5. Conclusion of Present Results and Outlook The condenser prototype is mounted into the experimental set-up as illustrated in figure 9 and the experiment setup is completed. In the next months the test under ambient pressure will be run and evaluated. After that the results are used to make prognoses about the condensation process in a water environment as preparation for the high pressure experiments. Through the presented approach it will be possible to achieve an optimal condenser design for the conditions in geothermal boreholes. An optimized condenser design will also be beneficial for the design of other downhole cooling machine components as the compressor, as pressure losses can be reduced. This will be an important step for the realization of the described cooling machine for downhole tool electronics. References [Baehr, 2013] H. Baehr, K. Stephan: Wärme- und Stoffübertragung, 8th edition, Springer Vieweg, p. 311ff, Berlin, Germany, (2013). [DOC, 1929] [Francke, 2010] [Holbein, 2015] United States Department of Commerce, Bureau of Standards: Thermal Properties of Petroleum Products, Government Printing Office, Washington, p.24ff, Nov. 9 th, (1929). H. Francke, M. Thorade: Density and viscosity of brine: An overview from a process engineers perspective, Chemie der Erde, Geochemistry, 70, Suppl. 3, 23-32, (2010). B. Holbein et al: Integrated Cooling Systems for an Extended Operation Range of Borehole Tools, Transactions, Geothermal Resources Council, Annual Meeting, Reno, Nevada, Sep , (2015). [Hongbing, 2008] S. Hongbing et al: New equations for density, entropy, heat capacity, and potential temperature of saline thermal fluid, Deep-Sea Research I, 55, ELSEVIER, p , June 1 st, (2008). [Julabo, 2016] [Mostafa, 2010] Julabo GmbH: Temperierflüssigkeit Thermal HL60, product data sheet, website: temperierfluessigkeiten/thermal-hl60, last assessed Mar. 18 th, (2016). S. Mostafa et al: Thermophysical properties of seawater: a review of existing correlations and data, Desalination and Water Treatment, 16:1-3, , Aug. 03 rd, (2012). [Schroeder, 2015] E. Schröder et al: Measuring techniques for in situ measurements of thermodynamic properties of geothermal water, Proceedings, World Geothermal Congress 2015, Melbourne, Australia, April 19-25, (2015). [Numrich, 2013] R. Numrich, J. Müller: Filmkondensation reiner Dämpfe, Verein Deutscher Ingenieure: VDI Wärmeatlas, 11 th edition, Springer Vieweg, p. 1011ff, Berlin, Germany, (2013). [Churchill, 1975] Churchill SW, Chu HHS: Correlating equations for laminar and turbulent free convection from a vertical plate, Int J Heat Mass Transfer 18: , (1975). [Kleiber, 2013] [Wrenick, 2005] M. Kleiber, R. Joh: Flüssigkeiten und Gase, Verein Deutscher Ingenieure: VDI Wärmeatlas, 11 th edition, Springer Vieweg, p. 357ff, Berlin, Germany, (2013). S. Wrenick, et al: Heat Transfer Properties of Engine Oils, Proceedings, World Tribology Congress III, Washington D.C:, USA, Sep , (2005). 955

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