Strojniški vestnik - Journal of Mechanical Engineering 62(2016)6, 381-388 © 2016 Journal of Mechanical Engineering. All rights reserved. D0l:10.5545/sv-jme.2015.3202 Original Scientific Paper Received for review: 2015-11-15 Received revised form: 2016-04-06 Accepted for publication: 2016-04-26 Measuring Selected Parameters of Polypropylene Fibre Heat Exchangers Maria Čarnogurska1* - Miroslav Prihoda2 - Marian Lazar1 -Natalia Jasminska1 - Robert Gallik1 - Michal Kubik1 technical University of Košice, Faculty of Mechanical Engineering, Slovakia 2Technical University of Ostrava, Faculty of Metallurgy and Materials Engineering, Czech Republic Heat exchangers whose heat exchange surfaces are made of the miniature polypropylene fibres are still under development. In the first phase of researching such exchangers, the possibility of attaching fibres into bundles was tested. The number of threads in one bundle ranges from several hundred to thousands of pieces. The sensitive element in the application of the fibres in a heat exchanger is the place where all the fibres are bundled together, into so-called potting, which must be perfectly tight. Pressure loss was measured in such bundles, depending on the water flow and the temperature at the entry and exit of the exchanger. The results allowed the definition of the overall heat transfer coefficient from one medium through the fibre wall to the other media and to determine the thermal performance of the exchanger. Experimental data shows that water-to-water heat exchanger reached the overall heat transfer coefficient value of to 800 W-m-2-K-1, depending on the release rate of the bundle. Subsequently, for the given conditions, the heat transfer rate depends only on the size of the heat transfer surface area, thus the number of fibres in the bundle. Keywords: polypropylene fibre, heat exchanger, measurement Highlights • A new type of heat exchanger whose heat exchange surface consists of polypropylene fibres has been designed and tested. • A testing seal potting apparatus has been developed. • Media temperature, flow rate and pressure drop in selected bundles of fibres with transparent and porous fibre were measured. • The impact of fibre length on the value of the overall heat transfer coefficient for this new type of heat exchanger was examined. 0 INTRODUCTION Currently, the proposed design of heat exchangers and other equipment must satisfy complex requirements in terms of production technology [1] and [2], trouble-free operation [3] and economic feasibility [4] and [5]. The main requirements for heat exchangers include a large overall heat transfer coefficient, low flow resistance, a simple process of cleaning the heat transfer surface area, corrosion resistance in various transported media, adaptation of the shape of the heat exchanger to the specified manufacturing needs, as well as price [6] and [7]. The critical operating parameters being monitored in heat exchangers include [8] and [9]: • Operating pressure: that can range from a deep vacuum in fractions of Pa to pressures reaching tens of MPa. • Operating temperature: from values close to absolute zero up to 1000 °C and higher. • Mass flow rate: from a few grams to hundreds of kilograms of media per second. • Good accessibility to a heat exchange area (necessary condition for cleaning and checking the status of working surfaces, especially when working with corrosive chemicals or a heterogeneous suspension with a tendency to form deposits, encrustations and corrosion). Decontamination and hygiene of a heat exchange surface is a specific condition for heat exchangers in the area of food and medical technologies. In addition to physical and chemical cleanliness, decontamination requirements also include bacterial cleanliness and harmlessness of heat exchange surfaces. In industrial practice, the most often used heat exchangers are constructed of steel or other metals; plastic is used less commonly as a construction material. As for plastic heat exchangers, the advantage is the durability of the material (up to 50 years), resistance to corrosion, chemicals and bacteria, high sanitary standards and low volume weight. One of the disadvantages of plastic heat exchangers is their lower material strength in comparison to metal exchangers, low resistance to higher temperatures (max. 150 °C), lower resistance to mechanical damage, risk of penetration of the diffused oxygen into the cooling medium, as well as a lower connection quality [10] and [11]. *Corr. Author's Address: University of Kosice, Faculty of Mechanical Engineering, Kosice, Slovakia, maria.carnogurska@tuke.sk 381 Strajniski vestnik - Journal of Mechanical Engineering 62(2016)6, 381-388 Individual parts of a heat exchanger are connected by glue or are welded. Welded constructions of heat exchangers are of a better quality and also find application in highly aggressive environments. A particular feature of plastic heat exchangers is that they are not produced in a gilled configuration because of the low thermal conductivity of the plastic. A separate group of plastic heat exchangers are heat exchangers in which the heat exchange surface is made of polypropylene fibres (capillaries) [12] and [13]. Other cited works state the results of experiments by individual authors related to, for example, the possibility of using heat exchangers with polypropylene fibres to obtain waste heat from domestic and industrial water was assessed [14] and [15], the studies of the affinity of polypropylene fibres for biological pollution in comparison with stainless steel (Escherichia coli bacteria) [16], the use of exchangers with polypropylene fibres (as a source of heat) for thermal pumps was investigated [17], the interaction between the movement of fibres and heat transfer was monitored, the impact of the shape and size of the heat exchanging area upon the output of the exchanger was assessed the use of exchangers with polypropylene fibres (as a source of heat) for thermal pumps was investigated [18] and [19], etc. The geometry of polypropylene fibres is shown in Fig. 1. Fig. 1. The polypropylene fibres 200x magnifications Table 1. Basic physical characteristics of propylene [21] Measured parameter Value Density [g-cm-3] 0.95 (crystal) 0.895 (amorphous) Melting point [°C] 173 Decomposition point [°C] 286 Thermal expansion [K-1] (100 to 180)-10-6 Specific thermal capacity [kJ-kg-1 -K-1] 1.70 to 1.90 Water absorption [%] 0.03 Hardness [Rockwell] R80 to R100 Thermal conductivity [W-m-1-K-1] 0.16 to 0.25 Polypropylene is a thermoplastic polymer selected from the group of polyolefins; it is colourless and odourless [20] and [21]. It has good stability, hardness and strength, but low impact strength. It is not susceptible to internal stress and can be appropriately welded. It has excellent electrical and chemical resistance. It becomes fragile at temperatures below zero. The basic physical properties of polypropylene are shown in Table 1. 1 STUDY AREA In the first phase of designing a heat exchanger surface made of polypropylene fibres, research was carried out to see if there was the possibility of joining polypropylene fibres into bundles. A created bundle (Fig. 2) consists of 1400 pieces of fibre. Those fibres are inserted into a 20 mm diameter and 40 mm long PVC tube. The part of the fibres that is glued into a single unit, and forms a compact unit in a PVC tube, is the so-called potting. Fig. 3 shows a sectional view of an array of 18 pottings in a single shell of a heat exchanger. The manner in which the bundles are placed, and especially by their number, makes it possible to influence the thermal performance of the heat exchanger. Fig. 2. Fibre bundle in a potting Fig. 3. System of pottings side cut The fibres are produced by continuous casting of liquid polypropylene from an extruder. Subsequently, the fibre is cooled in an air stream, thereby shrinking into the given dimension. Fibres suitable for the use in a heat exchanger should have intact and smooth walls, i.e. so-called transparent fibres. In contrast, research is underway to examine the possibility of using a porous fibre mainly for medical purposes. The wall of such fibres consists of 30 % to 50 % air. The second phase of the design tested the potting leaks. Table 2 contains the selected technical parameters of a bundle of polypropylene fibres. The bundle is tested for the so-called collapse pressure and burst pressure. Collapse pressure is an overpressure from the outside of the fibre wall 382 Čarnogurska, M. - Prihoda, M. - Lazar, M. - Jasminska, N. - Gallik, R. - Kubik, M. Strojniški vestnik - Journal of Mechanical Engineering 62(2016)6, 381-388 ActV ^.jm.Kjn Dil W I« t- lOOtViO IMO« 51 r? WIM £1 towards the inside wall, following which the fibre wall collapses. Burst pressure indicates the overpressure inside of the capillary against the external pressure, following which there is a rupture of the capillary wall. Table 2. Technical parameters of a polypropylene fibre bundle Parameter Value Capillary material Polypropylene Potting material Polyurethane Number of capillarie 1400 Total length 750 mm Capillaries inner diameter 0.225 mm Outer diameter of capillaries 0.275 mm Collapse pressure 0.2 MPa Burst pressure 0.4 MPa Maximal operating temperature 70 °C Approx. heat transfer area 1 m2 Fig. 5. Microsection of the wall along the length of the fibre -1500x magnification 3 TESTING POTTING INTEGRITY 2 CHARACTERISTICS OF POLYPROPYLENE FIBRES Polypropylene fibres can deform or even break when cut, or after being glued in the potting. A rupture occurs when the adhesive does not completely fill the empty space between the fibres. A rupture is a permanent deformation of the fibre cross-section, i.e. reduced cross-section (Fig. 4). ggjggaat :•'»•. SF-' 1 h i if. I- -Kil t.v Fig. 4. Polypropylene fibre cut- 200x magnification Wall thickness changes along the length of an intact fibre. Fig. 5 shows a microsection of the wall at 1500* magnification. The changing of the wall thickness is chaotic; it is not subject to any dependency, not only along the cross-section but also along the length of the capillary. The wall thickness of the presented capillary ranges from 30 ^m to 33 ^m. As a potting is made of three different materials with differing properties, it was necessary to conduct an integrity test. The test device is shown in Fig. 6. Water coming from a pressurized water source passes through a water filter (1), continues through a closing valve (2) and after the control pressure gauge (3) it enters the first potting. The water flows through the fibre bundle and enters the second potting (5). Subsequently, the water flows through the control pressure gauge (6) and the closing valve (7), which flows into a collecting tank. The water pressure at the inlet of the water filter was set to 0.3 MPa. Water flowing under this pressure for 30 s expelled the air contained in the fibres of the bundle. Subsequently, the valves were closed before and after the bundle; the pressure drop in the fibre bundle was observed for 5 min. Fig. 6. Scheme of a potting's' integrity measurement This particular experiment was conducted in order to identify a possible problem with the tightness of the bundle. A fibre bundle with detected leakage was immersed into a water tank at 50 °C to 70 °C together with a protective package perforated with about 30 holes, 3 mm in diameter. The packaging was tasked to protect fibres from mechanical damage during the repeat experiment. After cooling the fibre to the level of the inlet temperature, the fibre bundle 383 Measuring Selected Parameters of Polypropylene Fibre Heat Exchangers Strajniski vestnik - Journal of Mechanical Engineering 62(2016)6, 381-388 was again exposed to a water pressure of 0.3 MPa. The potting was tested in this manner over several cycles. The number of cycles at the time of individual testing increased with the increasing success of previous pressure tests. Table 3 shows the results of the tested fibre bundle. The same procedure was used for testing the tightness of the pottings at a high water temperature in a water bath into which the bundle was immersed after the test. The results showed that the potting's exhibited leakage at a temperature above 52 °C. 4 MEASUREMENT OF PRESSURE LOSS FIBRE BUNDLES The pressure drop was measured on six selected fibre bundles. Three fibre bundles consisted of transparent fibres and the other three with porous ones. Potting tightness before the measurement was 100 %. All parameters of the tested bundles (outer diameter, number of fibres in a bundle, and their length) were identical. A source of the pressurized water was represented by a pressurized water supply network with a pressure of 0.45 MPa and a temperature of 18 °C. The fibre bundle was connected to the rubber hoses via an inlet (4) and an output potting (5) (Fig. 7). Fig. 7. Schematic diagram of a fibre bundle during the measurement of a pressure loss The water flowed into the bundle through a flow metre (1) and a water filter (2). Water pressure was measured with a type PM 03.05 pressure gauge (3), with an amplitude between 0 MPa and 0.6 MPa and with an accuracy of ± 0.3 %. Fig. 8 shows the dependency of the pressure loss and water flow through the fibre bundle. The bundles of transparent fibres (t1 to t3) and bundles of porous fibres (p1 to p3) show an almost linear progression of the change in a pressure drop in the flow rate. 0.5 1 1.5 2 2.5 Water flow of fiber bundles Qv [dm3/min] Fig. 8. Pressure drop fibre 5 MEASUREMENT OF TEMPERATURE AND FLOW An experiment with an integrated bundle of transparent polypropylene capillaries was conducted on a heat exchanger that was constructed in a counter-current wiring with a tangential inlet of the secondary medium (Fig. 9). Fig. 9. Heat exchanger with a tangential inlet and sliding head Table 3. Integrity test - measurements results test number Interval between individual tests [h] QV [dm3-min-1] p [MPa] t [°C] l [m] nc [l] tw [°C] basic test - 1.61 0.3 19.2 0.726 - - 2 18 1.61 0.3 19.1 0.724 3 52 3 15 1.58 0.3 18.8 0.722 13 52 4 151 1.63 0.3 18.3 0.720 13 52 5 151 1.66 0.3 18.1 0.716 18 52 Note: nc the number of reheating cycles of the bundle and Its subsequent re-cooling down, l bundle length, p pressure of water In the fibres during the test, t temperature of the test water in the capillaries before the test, tw the temperature of the water in which the bundle was submerged. 384 Čarnogurska, M. - Prihoda, M. - Lazar, M. - Jasminska, N. - Gallik, R. - Kubik, M. Strojniški vestnik - Journal of Mechanical Engineering 62(2016)6, 381-388 The construction of the shell of heat exchanger allowed the release of fibres in a range from 0 % to 10 %. The exchanger has been studied for the impact of the fibres' tension on the intensity of the heat transport. A sliding head (4) placed on the shell of the heat exchanger (7) together with a rubber seal (5) and a thrustwasher (6) were used for this purpose. The potting of the investigated bundle itself (1) was sealed using a so-called O-ring (2) and pressed with a washer (3). The experiment used three different versions of a fibre bundle. Each version had a different number of fibres, different outer fibre diameter and a variety of tensions. The parameters of the heat exchangers are shown in Table 4. At zero tension, the fibre is at 100 % of its length, and the fibre axis is a straight line. When released, the bundle changes the distance between the clamping of the pottings; the fibre length remains constant. The bundle release Alu [%], expressed as a percentage of the total length of the fibres, is described by Eq. (1): Al = 100 - (h -V -100 i ' (1) Given the diameter and the size of fibres, the release of individual bundles was chosen as per Table 5. View of the experimental stand is shown in Fig. 10. Table 4. Parameters of used bundles Type of a bundle Number of fibres [pc] Outer diameter Fibre length [mm] [m] Heat transfer area [m2] A 400 0.65 0.71 0.580 B 900 0.35 0.70 0.682 C 1385 0.275 0.70 0.834 Table 5. Release values for different types of bundles Type of a bundle Alu [%] A 0 0.7 1.4 B 0 1.5 3 C 0 1.4 3.7 During the experiment, the temperatures were measured at the inlet and outlet of the primary and secondary circuits of the heat exchanger as well as the water flow through the fibres. The measured data was to define the overall heat transfer coefficient k (Wm-2K-1) in accordance with Eq. (2): k = - P S -AT, (2) where S is the total heat exchanger area of the bundle of fibres [m2], P is the heat transfer rate of the exchanger [W] and ATln is the logarithmic mean temperature difference (LMTD) [K]. Fig. 10. Heat exchanger with tangential inlet and sliding head The heat exchanger's performance is determined from the heated water in the secondary branch, i.e. in fibres. This medium herein is indicated with an index 2. Eq. (3) is valid: P = Qm2 ■ Cp TO. (3) The mean temperature logarithmic gradient is calculated from the Eq. (4): _ AT0 - AT AT -S ln AT In AT0-AT (4) At the counterflow heat exchanger, the individual differences of temperatures are determined from the Eqs. (5) and (6): AT = TT" AT = T T ' s 2, (5) (6) ln where index 1 is the primary medium; one bar represents the incoming temperature and two bars the outgoing temperature of the medium [K]. Measuring Selected Parameters of Polypropylene Fibre Heat Exchangers 385 Strajniski vestnik - Journal of Mechanical Engineering 62(2016)6, 381-388 6 DISCUSSION Bundle type A was characterized by a large outer diameter of fibres (0.65 mm) and the fibres were relatively solid. When released by 0.7 %, the fibres were evenly deployed over the cross section and they were washed thoroughly. When released by 1.4 %, the fibres already touched the wall of the heat exchanger, which resulted in a decrease in the overall heat transfer coefficient. The course of the factor k according to the flow rate of the fibre bundle (bundle A) is shown in Fig. 11. The course of the heat transfer coefficient for bundle B is shown in Fig. 12 and for bundle C in Fig. 13. In all three bundle types, the smallest overall heat transfer coefficient values were obtained at 0 % release. In a larger flow cross-section of fibres, higher overall heat transfer coefficients are reached when releasing a bundle up to circa 1 %. In smaller flow cross-sections of fibres, a greater release is more suitable (over 3 %). For example, with a flow of 100 dm3-h-1 and approximately the same bundle release (1.4 % to 1.5 %), a greater overall heat transfer coefficient value is shown by a B-type bundle (circa 590 W-m-2-K-1). With the same flow, bundles A and C reach value k of circa 420 W-m-2-K-1. In terms of evaluating the influence of measurement inaccuracies upon the results obtained, the following can be stated. The measured values included temperature, flow and pressure. The instruments that were used for measuring the stated values caused a so-called primary inaccuracy. A secondary inaccuracy resulted from the measurement of individual values of the given parameters due to the instruments used. A so-called tertiary inaccuracy should show what influence a particular parameter has upon the calculated value, i.e. upon overall heat transfer coefficient k. If, in Eq. (2), we take into account the Eqs. (3) and (4), for overall heat transfer coefficient k we obtain an equation in the shape: k = Qm2 ■ Cp ■(('- T) ln (T'- T?) S ■[(( ' - TVC- T) (t-T') (7) The equation shows that the k value is influenced by an inaccuracy in measuring temperature and flow. The development of a percentage deviation (error) in the final value of overall heat transfer coefficient k depend on the measurement error of the given parameters (each individually) is shown in Fig. 11. Overall heat transfer coefficient for bundle type A Fig. 12. Overall heat transfer coefficient for bundle type B Fig. Overall heat transfer coefficient for the bundle type C Fig. 14 on the vertical axis. The horizontal axis shows the measurement error for the appropriate temperature and flow. The diagram shows a strong dependence of the overall heat transfer coefficient k upon the temperature of outlet water from the exchanger T" and flow Qm2 and relatively weaker dependence upon temperatures T[, T" and T2'. In order to achieve the required accuracy when determining the overall heat transfer coefficient at a level of 3 %, thermal sensors with an accuracy of 386 Čarnogurska, M. - Prihoda, M. - Lazar, M. - Jasminska, N. - Gallik, R. - Kubik, M. Strojniški vestnik - Journal of Mechanical Engineering 62(2016)6, 381-388 Error in determining temperatures T Error in determining mass flow Qn Fig. 14. The influence of inaccuracies in measured values upon the heat transfer coefficient Error in determining temperatures '/',", '/V. 7'/ Fig. 15. Overall heat transfer coefficient error depending upon the measured temperature consists of three varying materials (metal, adhesive, polypropylene) whose change in shape depending upon temperature is expressed in different ways (each material has different thermal conductivity). At the same time, measurements confirmed that a heat exchanger using fibres of a certain degree of release have a better performance in comparison to a design where fibres are under tension. To a certain degree, this result is related to the slight movement of the fibres along the cross-section which, at a certain release of fibre tension, causes a more intensive exchange of heat between the primary and secondary medium. Testing the tightness of pottings took place at a pressure of 0.3 MPa. This pressure represents a sufficient reserve during application of the given exchangers in real operation when using low potential heat from waste water. Finally, it is important to emphasize that the use of the heat exchangers of the polypropylene fibres is limited by the thermal, pressure, or strength resistance, respectively. 8 ACKNOWLEDGEMENTS This paper was written with the financial support of Project VEGA No.1/0004/14 and SP2016/110-FMMI at Technical University of Ostrava. ±0.1 °C must be used. This can be seen in Fig. 15 where there is a visible influence of deviations of the red values of all temperatures upon the k value. For example, if temperature T" were read with an inaccuracy of +0.5 °C and other temperatures with an inaccuracy of -0.5 °C, the resulting deviation in the overall heat transfer coefficient k would be as much as ±10 %. 7 CONCLUSIONS Examination of the heat exchangers made of polypropylene fibres showed that one of the fundamental problems in their application is the tightness of fibres in the potting. It has been established that at a water temperature above 52 °C, it is very difficult to ensure a leak-proof performance of the exchanger. It is clear from the above that heat exchangers made of polypropylene fibres may only achieve faultless operation up to 50 °C. Above this temperature, a breakdown in potting occurs, since it 9 NOMENCLATURES lz is the length of the bundle without tension, [m] Alz the size of release, [m] S the total heat exchange area of given fibre bundle, [m2] P heat transfer rate, [W] ATln logarithmic mean temperature difference, [K] Qm2 mass flow, [kgs-1] cp mean specific heat capacity, [Jkg-1 K-1 ] T2 incoming water temperature, [K] T" temperature of the exiting water, [K] AT0 temperature difference between the primary and the secondary medium at the beginning of the exchanger, [K] ATS similar difference but at the end of the exchanger, [K] 10 REFERENCES [1] Brestovič, T., Čarnogurska, M., Prihoda, M., Kubik, M. (2015). 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