12th International Symposium on District Heating and Cooling

The <strong>12th</strong> <strong>International</strong> <strong>Symposium</strong> on District Heating and Cooling,September 5 th to September 7 th , 2010, Tallinn, EstoniaThis system is called a rigid system with a tightconnection between the service pipe, the foam andouter casing. Once it is formed, this product cannot bere-used in own process.Fig. 4. Section view of Steel/PUR pipeextruder. Once the material flows out of the extruder,the pressure drop causes the expansion of thehydrocarbon. The aggregation state of the moleculechanges from liquid into gas.Examples of hydrocarbon gases that can be used are:LPG, Butane or Isobutane. And just like the PUR foamthere is a degassing effect: the exchange of theblowing agent with air will increase the heat loss of theproduct. This effect is shown in ‗Heat loss of flexibleplastic pipe systems, analysis and optimization‘ byE.J.H.M. van der Ven et al. [4].Foam production processesThe insulation foams described in this paper are madeof Poly-urethane (PUR) foam, Poly-ethylene (PE) foamor cross linked Poly-ethylene foam (PE-Xa). Thesefoams have different properties. Some of theseproperties influence the heat loss properties of thecomplete product.Polyurethane (PUR) foamPUR foam is a thermo-set foam. It is made out of twochemicals, a Poly-alcohol and an Iso-cyanate. Thesematerials react and the Polyurethane is formed. Thisreaction is irreversible, so the material can never returninto its original chemicals. The blowing agent for thiskind of foam can be Carbon Dioxide, Nitrogen orHydrocarbon molecules, for instance Cyclopentane orButane.If Hydrocarbon gases are used, these gases stronglyinfluence the heat loss performance of the preinsulatedsystem. These gases have different thermalconductivities compared with air. After production of thefoam an exchange with air starts. A product that isfreshly made contains a high percentage ofHydrocarbon gases. At this point in time the productwill have the lowest heat loss possible. If the sameproduct is for instance three years old it contains moreair and less Hydrocarbon gases due to gas diffusion.And so the product will have a higher heat losscompared to the fresh product. The process ofdegassing is described in research papers ‗Long termheat loss of plastic Polybutylene piping systems‘ by S.de Boer et al. [3].Poly-ethylene (PE) foamPE foam is a thermoplastic foam. Once it is formed, itcan go back to its original state by heating it above itsmelting point. Because of this property, it is possible tore-use these kinds of foams.The foaming process to make PE foam is called the―physical foaming process‖. A Hydrocarbon molecule ismixed into the PE matrix under high pressure in an121Cross linked Poly-ethylene foam (PE-Xa)Although cross linked PE foam (x-PE) is also made ofPE, there is a big difference compared to PE foam: thetype of Blowing agent.The foaming process to make x-PE foam is called the―chemical foaming process‖. In this case a chemical ismixed into the PE matrix. The blowing agent can forinstance be Azodicarbonamide. While heating thematrix, the chemical starts decomposing and gases arereleased. These gases are Carbon dioxide andNitrogen. The thermal conductivity of these gases ismore or less equal to the thermal conductivity of air. Sothe aging effect of this product in relation to the heatloss is less.To make this foaming process possible, it is necessaryto connect the Poly-ethylene chains with each other.This is called the cross link. The complete process tomake x-PE foam is called the ―chemical foamingprocess with cross link‖. To make the comparison withPE foam complete: this process is called the ―physicalfoaming process without cross link‖.The blowing agent is not the only additive thatinfluences the thermal conductivity of the foam andtherefore the heat loss properties of the pre-insulatedsystem. Also other additives can influence the thermalconductivity of PE foam.Nucleating agents will influence the cell structure of thefoam. As a basic rule: the finer the foam the lower thethermal conductivity. With this additive the convectionpart of the insulation material will be influenced.Another additive that influences the thermalconductivity is an anti-radiation additive. By using thisspecial kind of additive it is possible to create areflection of radiation energy.HEAT LOSS TEST METHODThis chapter briefly describes the test rig and testmethod used to determine the absolute heat losses ofthe different types of pre-insulated pipes.