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  • 1.
    Vega, Alberto
    et al.
    RISE - Research Institutes of Sweden, Built Environment, Energy and Circular Economy.
    Yarahmadi, Nazdaneh
    RISE - Research Institutes of Sweden, Built Environment, Energy and Circular Economy.
    Jakubowicz, Ignacy
    RISE - Research Institutes of Sweden, Built Environment, Energy and Circular Economy.
    Optimal conditions for accelerated thermal ageing of district heating pipes2018In: Energy Procedia, ISSN 1876-6102, E-ISSN 1876-6102, Vol. 149, p. 79-83Article in journal (Refereed)
    Abstract [en]

    Technical lifetime prediction of polymeric materials is often based on accelerated ageing tests at elevated temperatures. Samples are exposed to relatively high temperatures to accelerate the natural degradation processes. For district heating pipes, accelerated thermal ageing is the ordinary method used to determine the lifetime of pipes. According to the Standard EN 253:2009 + A1:2013, the district heating pipes shall be subjected to an accelerated thermal ageing for a long period of time at 160 °C or 170 °C. The lifetime is determined by extrapolation using the Arrhenius relationship. However, papers published recently have questioned this method, especially the high temperatures used for ageing of the pipes and the use of Arrhenius equation to describe the complicated degradation mechanisms, which can result in the erroneous estimation of the technical lifetime. Our investigation has shown the complexity of the pipe's degradation mechanisms. The behaviour of mechanical shear strength at elevated temperatures (T > 130 °C), suggests an alteration rather than an acceleration of the degradation mechanisms. Accelerated ageing tests should reproduce the proper natural ageing mechanisms. The analyses of PUR's thermal conductivity and its chemical structure by FTIR confirmed the degradation patterns

  • 2.
    Vega, Alberto
    et al.
    RISE - Research Institutes of Sweden, Built Environment, Energy and Circular Economy.
    Yarahmadi, Nazdaneh
    RISE - Research Institutes of Sweden, Built Environment, Energy and Circular Economy.
    Jakubowicz, Ignazy
    RISE - Research Institutes of Sweden, Built Environment, Energy and Circular Economy.
    Determination of the long-term performance of district heating pipes through accelerated ageing2018In: Polymer degradation and stability, ISSN 0141-3910, E-ISSN 1873-2321, Vol. 153, p. 15-22Article in journal (Refereed)
    Abstract [en]

    The lifetime of pre-insulated district heating pipes (DHPs) is commonly evaluated using the method described in the normative European Standard EN 253. This lifetime is normally calculated using an Arrhenius equation, which makes use of test results from accelerated ageing tests at elevated temperatures. In this investigation, long-term accelerated ageing tests of DHPs at elevated temperatures were carried out. The ageing behaviour, especially at the interface between steel pipe and polyurethane (PUR) foam, showed several routes of degradation. It is clearly demonstrated using measurements of shear strength, thermal conductivity and alterations of chemical structure by Fourier Transform Infrared spectroscopy that the results of accelerated ageing at 170 and 150 °C significantly diverge from those obtained from the ageing test at 130 °C. It is therefore concluded that accelerated ageing at commonly used high temperatures does not create an acceleration of degradation processes at the steel/PUR interface relevant for the DHP application, but rather a significant alteration in mechanism. This finding is of crucial importance for the use of EN 253 and the development of future methods for lifetime prediction of DHPs.

  • 3.
    Yarahmadi, Nazdaneh
    et al.
    RISE - Research Institutes of Sweden, Built Environment, Energy and Circular Economy.
    Vega, Alberto
    RISE - Research Institutes of Sweden, Built Environment, Energy and Circular Economy.
    Jakubowicz, Ignacy
    RISE - Research Institutes of Sweden, Bioscience and Materials, Chemistry and Materials.
    Accelerated ageing and degradation characteristics of rigidpolyurethane foam2017In: Polymer degradation and stability, ISSN 0141-3910, E-ISSN 1873-2321, Vol. 138, p. 192-200Article in journal (Refereed)
    Abstract [en]

    The urgent need for revision of the normative test method (EN 253) for the lifetime prediction of district

    heating pipes requires a better understanding of the failure mechanisms involved. Therefore, various

    methods were used to study thermal degradation characteristics of rigid polyurethane (PUR) foam in

    both air and nitrogen atmosphere. Accelerated ageing in nitrogen caused insigni

    ficant changes, whereas

    ageing in air caused signi

    ficant changes in weight, dimensions, chemical structure and cell gas composition,

    indicating importance of the thermo-oxidative type of degradation. A clear indication of the

    thermo-oxidative type of degradation was the formation of new carbonyl groups in PUR together with

    the loss of CH

    2 groups after ageing in air. Another result of ageing in air was the loss of pentane and

    cyclopentane, and the formation of some new volatile compounds in the cells of PUR foam. However,

    despite a large difference in degradation characteristics between the samples aged in air and in nitrogen,

    no signi

    ficant difference in the flexural strength of PUR foam was recorded during the induction stage of

    the degradation process. Furthermore, it is shown that the signi

    ficant drop in shear strength, which

    re

    flects the adhesion force between PUR foam and steel pipe, observed during the early stage of

    accelerated ageing of district heating pipes is not caused by thermo-oxidative degradation.

  • 4.
    Yarahmadi, Nazdaneh
    et al.
    RISE - Research Institutes of Sweden, Built Environment, Energy and Circular Economy.
    Vega, Alberto
    RISE - Research Institutes of Sweden, Built Environment, Energy and Circular Economy.
    Jakubowicz, Ignacy
    RISE - Research Institutes of Sweden, Built Environment, Energy and Circular Economy.
    Determination of essential parameters influencing service life time of polyurethane insulation in district heating pipes2017In: Energy Procedia, Elsevier Ltd , 2017, p. 320-323Conference paper (Refereed)
    Abstract [en]

    Pre-insulated district heating pipes (DHP) have been in use during the last forty years. Many improvements and development have been done in the system. However, life-time prediction is still an uncertain issue. This paper is a part of a bigger project with the objective to determine mechanisms related to the deterioration of the mechanical and insulation properties of pre-insulated heating pipes as a result of ageing. The focus in this project is on degradation mechanisms of the PUR material at high temperatures. In this paper some results of the two types of exposure are presented. The first type comprises a condition where the new pipes are subjected to accelerated ageing at three different temperatures. The second type comprises condition, when the PUR material itself is aged in different atmospheres in order to identify different degradation mechanisms. The chosen ageing temperatures in the first condition were 130°C, (close to the supply temperature), 150°C and 170°C, (accelerated ageing temperature in EN 253 [1]). Changes in thermal insulation and the adhesion force between the PUR and the steel pipe were evaluated using the transient plane source (TPS) technique and the SP plug method respectively. The results of ageing show that the degradation of PUR is a multi-stage process composed of a rapid change in properties followed by a plateau phase which changes later to a gradual deterioration of the properties. The results of the PUR material exposure at 150°C in air and in nitrogen showed significant differences in the degradation characteristics between the two environments as were revealed by DSC and FTIR methods. © 2017 The Authors. Published by Elsevier Ltd.

  • 5.
    Yarahmadi, Nazdaneh
    et al.
    RISE - Research Institutes of Sweden, Built Environment, Energy and Circular Economy.
    Vega, Alberto
    RISE - Research Institutes of Sweden, Built Environment, Energy and Circular Economy.
    Jakubowicz, Ignacy
    RISE - Research Institutes of Sweden, Bioscience and Materials, Chemistry and Materials.
    Livslängd för fjärrvärmerör: Fas 1 - Nedbrytningsmekanismer2017Report (Other academic)
    Abstract [sv]

    Dagens standard för prefabricerade fjärrvärmerör SS-EN253 innehåller olika testmetoder och krav där ett av de viktigaste är kravet på livslängd på minst 30 år vid en kontinuerlig driftstemperatur på 120 °C. Bestämningen av tekniska livslängden är baserad på mätningar av förändringar i skjuvhållfastheten efter accelererad termisk åldring. Den beräknade livslängden för ett fjärrvärmerör är baserad på extrapolering med hjälp av Arrhenius ekvation. Det finns en stor enighet kring uppfattningen att Arrhenius modellen inte ger en god överensstämmelse med de observerade åldringsdata och att det följaktligen saknas tillförlitliga metoder för livslängdsbestämning av fjärrvärmerör. Trots många forskningsrapporter finns det fortfarande skilda teorier om hur snabbt PUR-isoleringar bryts ner och vilka nedbrytningsmekanismer som dominerar. Syftet med det här projektet är att kartlägga de komplicerade sambanden och förstå vilka nedbrytningsprocesser som dominerar under olika betingelser och utifrån denna kunskap utveckla tillförlitliga metoder för livslängdsprediktering.

    Allt testmaterial utgjordes i denna studie av polyuretan (PUR) cellplastisolering som togs ut från nytillverkade fjärrvärmerör. Accelererad åldring genomfördes i luft och i kväveatmosfär vid 150 °C för upp till 8 veckor. Det visade sig att exponering i kväveatmosfär inte orsakar någon mätbar förändring av egenskaper. En motsvarande åldring i luft orsakar däremot stora förändringar i både dimensioner, viktförlust och färg. IR (infraröd) analys kunde också bekräfta att materialet bryts ner genom termo-oxidativa processer. Termo-oxidativ nedbrytning syns också i analyser av cellgassammansättningen. Materialet efter åldring i luft innehåller betydligt mindre halt cyklopentan samtidigt som fyra nya flyktiga ämnen kunde detekteras. Trots alla indikationer på materialets termo-oxidativa nedbrytning har böjhållfastheten inte förändrats hos materialet. Detta måste utredas vidare för att kunna förklara mekanismer bakom resultatet.

    Förutom beständighetsstudier av PUR materialet genomfördes långtidsåldring av fjärrvärmerör vid olika temperaturer. Resultaten av skjuvhållfasthet mätningar indikerar att det åtminstone finns tre olika faser i nedbrytningsprocessen. Initialt sjunker hållfastheten troligen på grund av fysikaliska effekter orsakade av de höga temperaturerna. Därefter ökar skjuvhållfastheten troligen beroende på sidoreaktioner och fortsatt tvärbindning. Det tredje steget är en termo-oxidativ nedbrytning som såsmåningom leder till förlust av vidhäftningen. Även värmeledningsförmågan ökar betydligt som ett resultat av nedbrytningsprocesser. Det finns behov av ytterligare forskning för att kunna föreslå en ny modell för livslängdsberäkningar baserade på experimentella resultat.

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