The Polyethylene Casing as Diffusion Barrier for Polyurethane Insulated District Heating Pipes

Transcription

1 Polyethylene Casing as Diffusion Barrier for Polyurethane Insulated District Heating Pipes The Polyethylene Casing as Diffusion Barrier for Polyurethane Insulated District Heating Pipes Maria Olsson and Ulf Jarfelt Department of Building Physics Morgan Fröling and Olle Ramnäs Department of Chemical Environmental Science Chalmers University of Technology S Göteborg, Sweden Received: 19th December 2000; Accepted: 15th February 2001 SUMMARY Most district heating pipes are insulated with polyurethane foam in order to minimise heat losses. A high-density polyethylene (HDPE) casing protects the insulated pipe, and its permeability properties for the polyurethane cell gases, including air, play an important role for the long-term insulating capacity. The permeability of the HDPE casing of a district heating pipe was studied. Two methods were used to determine the permeability: I) by measuring the mass transfer through a sample, and II) by measuring the sorption and desorption of the gas in a sample. The experimental procedures are described. For carbon dioxide a permeability coefficient of about mole m -1 s-1 Pa-1 was found by both methods. For oxygen and nitrogen only method I was used and the permeability coefficients found were 1.9 and mole m -1 s-1 Pa-1, respectively. INTRODUCTION District heating pipes distribute hot water to heat consumers and are insulated with polyurethane (PUR) foam to minimise heat losses. A good long-term insulating capacity will result in energy savings and a subsequent lowering of the total environmental impact (1,2). The insulating capacity of the foam decreases over time when the blowing agent diffuses out of and air diffuses into the foam (3,4). The insulating capacity over time is not only influenced by the diffusion of the blowing agent but also by the diffusion of air and carbon dioxide. Due to the foaming process, carbon dioxide is always present in the PUR-foam of a district heating pipe. A district heating pipe has an outer casing of high-density polyethylene. Together with the PUR-foam, the polyethylene casing acts as a diffusion Cellular Polymers, Vol. 20, No. 1,

2 Maria Olsson, Ulf Jarfelt, Morgan Fröling and Olle Ramnäs barrier. For carbon dioxide, the polyethylene casing has been found to be the main diffusion barrier (1,4). To be able to predict the long-term thermal performance of a district heating pipe, knowledge of the diffusion properties of gases in the casing, as well as in the foam, is needed. Many studies on polymer permeability have been performed on films. Permeability coefficients for different gases in films of high-density polyethylene have been reported (5,6,7,8) and used in calculations for casing pipes, when predicting the long-term thermal performance (9,10). However, the permeability varies with different qualities of polyethylene and with different drawing ratios (6). Density, crystallinity, polymer chain length distribution and chain branching as well as additives are other factors influencing the permeability (4,5,7). The complexity of factors affecting the permeability gives reason for studying the specific polyethylene casing used for district heating pipes. EXPERIMENTAL Mass transfer of oxygen, nitrogen and carbon dioxide through polyethylene was studied in the diffusion chamber, and sorption and desorption of carbon dioxide were studied in the sorption chamber. Diffusion chamber The diffusion chamber consists of two volumes separated from each other by a sample of a polyethylene casing. The two volumes contain different gases. The mass transfer through the casing can be determined by gas chromatographic analyses of the gases in the volumes. The chamber was made by welding the walls of a steel box onto a steel pipe with sealed ends. The box covers an opening cut out of the pipe, where a piece of polyethylene casing can be placed, see Figure 1. The diameter of the steel pipe was chosen to fit a standard dimension of casing of a district heating pipe (diameter 172 mm). The volumes were filled with gas through metal tubings, which could be closed by ball valves. Experimental procedure Before application, the thickness of the casing sample was measured with a dial indicator. The sample was glued with epoxy and pressed with screws onto the opening. Finally, a steel cover was glued with epoxy to the box. 38 Cellular Polymers, Vol. 20, No. 1, 2001

3 Polyethylene Casing as Diffusion Barrier for Polyurethane Insulated District Heating Pipes Figure 1 Chamber for determination of casing permeability. The volume of the pipe is m 3 and the volume of the box is m 3. The area of the opening, where mass transfer is possible, is m 2. The screws help to provide gas tightness when the casing sample is glued onto the pipe The size of the large volume was determined from measurements with a calliper. The size of the more irregular small volume was determined by filling with water and weighing. The gas tightness of the volumes was checked by applying an over-pressure of 1 bar. In this study, the small volume was filled with carbon dioxide and the large volume was filled with oxygen or nitrogen. The chambers were stored at constant temperature, 23 C, during the experiments. Gas samples were taken immediately after filling and then at different times. The samples were analysed by gas chromatography. The sampling system connected to the diffusion chamber and the gas chromatograph is shown in Figure 2. Cellular Polymers, Vol. 20, No. 1,

4 Maria Olsson, Ulf Jarfelt, Morgan Fröling and Olle Ramnäs Figure 2 System for sampling and analysis of gas samples from the diffusion chamber. A gas tight syringe (A) is connected to the diffusion chamber (F) with the casing sample (G) and to two gas sampling valves in series (B 1 and B 2 ). Gas samples are injected via the sample loops (C 1 and C 2, volumes around 200ml) and are separated on the gas chromatographic columns (D 1 and D 2 ) connected to the hot wire detector (E). CO 2 N 2 A He O 2 Gas chromatograph B 1 G C 1 He D 1 F B 2 E C 2 D 2 Before sampling, the system was flushed with the filling gas of the volume to be studied. The flushing proceeded until no other compounds but the flushing gas could be detected in samples taken by the gas tight syringe (SGE, 50 ml). When the system was clean, the syringe was filled with 30 ml of the flushing gas, and the valves into and out of the system were closed. The valve into the volume was opened, and the flushing gas was injected into it. The gas was allowed to diffuse out into the volume for half a minute with the valve closed before a sample (30 ml) was taken out with the syringe. The sample was then injected into the gas chromatographic columns via the gas sampling valves. The reason for injecting flushing gas before sampling was that repeated sampling should not affect the pressure inside the volume. In the gas chromatograph (Perkin-Elmer 3920B), the compounds were separated and detected. A molecular sieve column (13X, 170 cm x 1/8") was used for oxygen and nitrogen and a packed porous polymer column (HayeSep Q, 190 cm x 1/8") was used for carbon dioxide. The columns were connected to both sides of a hot wire detector (200 C, 150 må). Helium (30 ml/min) was used as a carrier gas. Calibration with pure gases 40 Cellular Polymers, Vol. 20, No. 1, 2001

5 Polyethylene Casing as Diffusion Barrier for Polyurethane Insulated District Heating Pipes allowed the concentration of each gas to be determined. The minimum detectable concentration was around 0.01% (vol) for the gases studied here. Sorption chamber The sorption chamber consists of a steel autoclave. It is sealed with a steel cover with an o-ring. The chamber is filled with a gas through metal tubings, which can be closed by ball valves. In the sorption chamber, pieces of polyethylene casing were kept in a certain gas. The sorption of gas into the polyethylene was determined by weighing, while the desorption of gas was determined by gas chromatographic analyses of the gases in the volume. Experimental procedure Pieces of polyethylene casing (totally about 100 g) were kept at 23 C and atmospheric pressure in the chamber filled with carbon dioxide. The purity of the gas was checked by gas chromatography. During the sorption phase, an analytical balance was used to determine the change in weight of the casing pieces at different times. After about a week, the carbon dioxide was replaced by air. The carbon dioxide dissolved in the polyethylene was then desorbed to the air. During the desorption phase, the concentration of carbon dioxide in the chamber was determined at different times by gas chromatography by the same procedure as described for the diffusion chamber. At the end of the desorption phase, the carbon dioxide concentration in the chamber was about 1% (vol). Since the total volume of the chamber (about m 3 ) was known, the total amount of desorbed gas could be calculated. The concentration of ambient carbon dioxide, 0.04% (vol), was considered in the calculations. Material The casing pipe studied was produced by a Swedish district heating pipe manufacturer from a polyethylene quality named HE2467-BL intended for jacketing of district heating pipes among other applications (11). According to the producer the density of the base resin is 943 kg m -3, and the density of the compound is 953 kg m -3. Carbon black is added as filler and the content is above 2% (11). According to the European standard EN253 (12) the density of the polyethylene polymer must be at least 935 kg m -3 and the density of the polyethylene casing at least 944 kg m -3. By Cellular Polymers, Vol. 20, No. 1,

6 Maria Olsson, Ulf Jarfelt, Morgan Fröling and Olle Ramnäs weighing samples in air and in 2-propanol, the density of the casing used in this study was found to be 960 kg m -3. When manufacturing the casing pipe, the plastic granules are melted at C and extruded to a pipe, which is cooled down to 20 C by direct water spraying. During the extrusion, the casing pipe is drawn approximately 20% to achieve the desired thickness of the pipe wall (13). PERMEABILITY The permeability coefficient is the material parameter used to describe mass transfer rate through a material of a certain thickness. p J = P L where J = mass transfer rate [mole m -2 s-1 ] P = permeability coefficient [mole m -1 s-1 Pa-1 ] p = partial pressure difference [Pa] L = thickness [m] (1) In the diffusion-chamber experiments, the amount of gas that had permeated through the casing from one volume into the other was determined, and the permeability coefficient was calculated as: P = n L A t p n = amount of gas permeated [mole] A = area [m 2 ] t = time [s] p = mean partial pressure difference [Pa] The equation assumes that the partial pressure difference is constant during the time considered. In fact, the difference decreased by less than two per cent from the start to the end of the experiments. Dilution due to the sampling procedure was considered when determining the amount of gas permeated. The results from the sorption study can also be used for determination of the permeability coefficient, under the assumption that Henry s law, Equation (3), is valid. (2) P = S D (3) 42 Cellular Polymers, Vol. 20, No. 1, 2001

7 Polyethylene Casing as Diffusion Barrier for Polyurethane Insulated District Heating Pipes S = solubility coefficient [mole m -3 Pa-1 ] D = diffusion coefficient [m 2 s-1 ] The solubility coefficient is obtained from the weight of the sample at saturation. The diffusion coefficient is obtained from the following relationship: k ct () Dt( ( 2 1 ) π ) = 1 e L c 2 0 π ( 2k 1) k c = mean concentration in sample [mole m -3 ] c 0 = initial concentration in sample [mole m -3 ] (4) RESULTS Four samples of polyethylene casing of the original thickness (around 4 mm) and two samples that had been turned thinner (around 2 and 3 mm), were used in the diffusion-chamber experiments. Figure 3 shows the amount of carbon dioxide, which had permeated through the casing into the large volume, and the amount of either oxygen or nitrogen, which had permeated through the casing into the small volume when the samples of original thickness were studied. After three weeks, the concentrations of permeated gases were % (vol). The results from measurements of the carbon dioxide permeation through three polyethylene samples of different thickness are shown in Figure 4. The results from the sorption and desorption studies are summed up in Figure 5. Table 1 shows the permeabilities obtained from the different experiments. Equation (2) was used to calculate the permeabilities from the results of the diffusion chamber experiments. From the results of the sorption chamber experiments, a solubility S = mole m -3 Pa-1 was evaluated from the asymptote of the curve in Figure 5. The diffusion coefficient was evaluated from Equation (4), and the permeabilities were calculated according to Henry s law (Equation (3)). DISCUSSION Values found in the literature for the permeability of carbon dioxide, oxygen and nitrogen through polyethylene are not unambiguous (5,6,7,8). Cellular Polymers, Vol. 20, No. 1,

8 Maria Olsson, Ulf Jarfelt, Morgan Fröling and Olle Ramnäs Figure 3 The increasing amount of carbon dioxide, oxygen and nitrogen over time due to permeation through the polyethylene casing of a district heating pipe at 23 C, atmospheric pressure (casing thickness: 3.8 mm, diffusion area: m 2 ) 3.0E-04 CO 2 Amount of gas (mole) 2.0E E-04 O 2 N 2 0.0E Time (h) Figure 4 Carbon dioxide permeation rate as a function of polyethylene thickness at 23 C, atmospheric pressure. Mass transfer rate (mole m -2 s -1 ) 6.0E E E E /L (m -1 ) Figure 5 Sorption and desorption of carbon dioxide in samples of polyethylene casing at 23 C, atmospheric pressure (casing thickness 3.95 mm). 0.5 Change in weight (mg g -1 ) sorption, CO 2 desorption, CO Time (h) 44 Cellular Polymers, Vol. 20, No. 1, 2001

9 Polyethylene Casing as Diffusion Barrier for Polyurethane Insulated District Heating Pipes Table 1 Permeability coefficient for carbon dioxide, oxygen and nitrogen in samples from the polyethylene casing of a district heating pipe at 23 C, atmospheric pressure Experimental method Sample thickness [m] Diffusion chamber P ermeability coefficient [mole m 1 Carbon dioxide Sorption chamber Oxygen s -1 Pa- 1] Nitrogen The values found are within the ranges: (carbon dioxide); (oxygen) and (nitrogen) mole m -1 s -1 Pa -1. The values found in this study are within these ranges, namely: about 9 (carbon dioxide); 1.9 (oxygen) and (nitrogen) mole m -1 s -1 Pa -1, see Table 1. The HDPE casing studied here was drawn approximately 20% at extrusion (13). However, relaxation in the plastic due to elevated temperature during production may decrease the influence of drawing shown by Webb et al. (6). The same authors (6) also found differences in permeability between two different trade marks of polyethylene plastic (approx. 10%), which support the suggestion to study the specific polyethylene used in an application. The uncertainties of the permeation coefficients calculated from the diffusion chamber experiments can mainly be ascribed to the gas chromatographic analysis and corresponding determination of amount of moles. The permeability given for nitrogen is the most uncertain (in this case estimated to 20%) because of the difficulty to determine the small change in concentration that the very slowly permeating nitrogen will give rise to in the diffusion chamber. In this case even a very minor leakage of air will influence the result. The uncertainty of the value calculated from the sorption chamber experiment is estimated to be of the same order of magnitude, mainly because of the difficulty to determine the small change in weight caused by the sorption or desorption of carbon dioxide in polyethylene. The experimental methods used provided results within two to four weeks. In the diffusion chamber, two gases were studied at the same time. The duration of an experiment may increase if gases of high solubility in the polyethylene are involved. Oxygen and nitrogen should preferably not be studied at the same time, since it is advisable to use one of these gases for the detection of air leakage. Cellular Polymers, Vol. 20, No. 1,

10 Maria Olsson, Ulf Jarfelt, Morgan Fröling and Olle Ramnäs A certain time has to pass before a gas entering a polymer sample at one surface can be detected at the opposite surface. For the 3.8 mm thick sample of HDPE, exposed to carbon dioxide at one side, the time lag to response at the opposite side can be calculated to about 100 hours (15). If the sample is exposed to the gas before the start of an experiment, the time lag will be reduced. Figure 3 indicates that carbon dioxide could be detected after 50 hours, which is due to the fact that the polyethylene sample was exposed to carbon dioxide during the start-up phase of the experiment. Use of the permeabilities found in this study increases the possibilities of making accurate determinations of the long-term thermal performance of polyurethane-insulated district heating pipes, though different casing qualities should preferably be studied to reveal whether or not significant deviations in the permeability between different producers exist. Other issues of interest are to study the temperature dependence of permeability, since the casing is subjected to different temperatures depending on the climate, temperature loads and dimensions of the pipe, and to study the permeation of cyclopentane and other, new polyurethane blowing agents. REFERENCES 1. Svanström M., Fröling M., Ramnäs O. and Jarfelt U., Carbon Dioxide Diffusion in District Heating Pipes, Cell. Pol., 18, 2, (1999), p Fröling M., Jarfelt U., Ramnäs O., Insulation of district heating pipes environmental aspects of the blowing agent of polyurethane foam, Proceedings of the 7 th International symposium on district heating and cooling, May, Lund, Sweden (1999) 3. Olsson Maria, Long-Term Thermal Performance of Polyurethane Foam -measurements and modelling, Licentiate Thesis, Department of Building Physics, Chalmers University of Technology, Göteborg, Sweden (1999) 4. Svanström M., Ramnäs O., Olsson M. and Jarfelt U., Mass Transfer of Carbon Dioxide through the Polyethylene Casings of District Heating Pipes, J. Thermal Ins. Build. Env., 21 (1997), p Branderup J. and Immergut E.H. (editors): Polymer Handbook, 3 rd edition, Wiley-Interscience, John Wiley & Sons, New York (1989) 6. Webb J.A., Bower D.I., Ward I.M. and P.T. Cardew: The Effect of Drawing on the Transport of Gases through Polyethylene, J. Poly. Sci. Part B: Poly. Physics., 31, (1993), pp Cellular Polymers, Vol. 20, No. 1, 2001

11 Polyethylene Casing as Diffusion Barrier for Polyurethane Insulated District Heating Pipes 7. Michaelis A.S. an d Bixler H.J., Flow of gases through polyethylene, J. Poly. Sci., 50, (1961), pp Kjeldsen, Peter: Evaluation of gas diffusion through plastic materials used in experimental and sampling equipment, Wat. Res., 27,1, (1993), pp Smidt H.D. and Daugaard J., Long-term insulating properties of preinsulated district heating pipes, Euroheat and Power Fernwärme International, 4-5, (1997), pp Eriksson D. and Sundén B., "Heat and mass transfer in polyurethane insulated district cooling and heating pipes, J. Thermal Env. Building Science, 22 (1998), pp Borealis A/S (1995), Polyethylene HE2467-BL High density polyethylene for pipes, p /2, Borealis A/S, Lyngby, Denmark. 12. European Standard EN 253 (1994), Preinsulated bonded pipe systems for underground hot water networks Pipe assembly of steel service pipes. Polyurethane thermal insulation and outer casing of polyethylene 13. Göran Johansson (2000), personal communication, Powerpipe Systems AB, Göteborg, Sweden 14. Viallulenga, J.P.G. and Seoane B., Permeation of carbon dioxide through multiple linear low-density polyethylene films, European Polymer Journal 36 (2000) 15. Carslaw H.S. and Jaeger, J.C., Conduction of heat in solids, 2 nd edition, Oxford Science Publications, Oxford (1988), pp Cellular Polymers, Vol. 20, No. 1,

12 Maria Olsson, Ulf Jarfelt, Morgan Fröling and Olle Ramnäs 48 Cellular Polymers, Vol. 20, No. 1, 2001