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, EstoniaHEAT LOSS ANALYSIS AND OPTIMIZATION OF A FLEXIBLE PIPING SYSTEMJ. Korsman 1 , I.M. Smits 1 and E.J.H.M. van der Ven 21Liandon B.V.2 Thermaflex <strong>International</strong> Holding B.V.ABSTRACTThe object of this paper is to evaluate heat losses of aflexible PB-PE-PE piping system in the field, comparedto a conventional rigid Steel-PUR-PE piping system.The flexible system is optimized in both insulationquantity (thickness) and quality.The heat loss for pairs of pipes in the field, with 70 °Csupply and 40 °C return temperature, is based on heatloss measurements in the laboratory and has beenevaluated using the multipole method.Since the hydraulic properties of Polybutylene andsteel medium pipes differ, hydraulic calculations of ademonstration distribution network, fitted with eithersystem, are made.Total system heat losses for this demonstrationnetwork are calculated by summing the product of theheat loss per pair of pipe and the amount of pipe used.INTRODUCTIONFlexible piping systems for district heating and coolinghave several advantages when compared to rigidpiping, mainly during installation, some even in use.Flexible pipe can be utilized much like cable, arrives onlarge reels, requires less engineering and fewer hasjoints. However, flexibility comes at a price. It seemsharder to reach comparable levels of insulation, seeSmits et al. 2010 [1].The reason for this lies in the specific properties of thematerial most commonly used for insulation:Polyurethane foam. PUR foam has a crystallinestructure and tends to be quite rigid. It is not verysuitable for flexible applications. Bending may lead to abreakdown of the crystalline structure and may alsocompromise the bonding between foam and mediumpipe, thus creating a channel. This channel mayaccelerate the exchange of foaming agent and air withthe environment, thereby speeding up the ageingprocess. Flexible variants of PUR are available, but donot seem quite as good. Insulation foam made ofpolyolefins show ample flexibility and quite goodinsulation properties for small diameters. Furthermore,aging typically is a faster process than in rigid systems.As for the medium pipe, metals may be flexible enoughfor the smaller diameters, but are too rigid for thebigger pipes. Again, using polyolefins like Polyethylene(PE), cross linked Polyethylene (PE-X), Polypropylene310(PP) or Polybutylene (PB) improves flexibility. Fromthese, PE does not have adequate strength at highertemperatures and PP is rather stiff. This leaves PE-Xand PB, of which the latter can be welded withoutdifficulty. This is therefore the material of choice for thisstudy. In accordance with the temperature durationprofile mentioned in the BRL5609/EN15632, PB issuitable up to a maximum temperature of 95 °C.As with other plastics, PB is prone to some diffusion ofoxygen and water vapor. These effects have beeninvestigated by Korsman et al. 2008 [7]. To preventoxygen diffusion, an EVOH oxygen diffusion barriermay be used. Unless fully submerged for years, thediffusion of water vapor will not be much of a problem.When kept completely under water, it will take at least30 years for all cells to fill with condensate.It is not all that easy to compare the heat loss of aconventional Steel–PUR–PE piping system to a flexiblePB–PE–PE piping system. Internal diameters differ, asdoes the friction coefficient, because PB is smootherthan steel. A pipe for pipe comparison yields skewedresults. One way around this problem is to comparecomplete distribution systems, as was done inKorsman et al. 2008 [2]. A demonstration (or reference)network is used to design and compare similarnetworks. For reference purposes, the same network isused in this study.The differing properties that complicate comparisonbetween piping systems, can also be used to minimizedistribution system heat loss. The object of this study isto reach comparable heat loss for the flexible system,by exploiting specific properties, whilst transporting thesame amount of heat with comparable pressure losses.1. HEAT LOSS IN THE GROUNDHeat losses have been measured on test rigs asdescribed by van Wijnkoop et al. 2010 [3] and havebeen evaluated by van der Ven et al. 2010 [4].With the results of these tests, the in-ground heatlosses are calculated using the multipole method byJohan Cleasson and Camilla Persson in 2005 [5]. Notethat the mentioned heat losses are calculated for a pairof pipes, run at 70 °C supply and 40 °C returntemperature.For rigid piping, some room between the pipes isrequired for welding, see Fig. 1a.