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, EstoniaEXPERIMENTAL MEASUREMENTSThe experimental set-up is similar to that used whendetermining the thermal conductivity of single districtheating pipes in [6]. A flexible twin pipe of about 18meters coiled with a diameter of 1.8 meters isimmersed into a pool with circulating water. In thisexperiment, the pool water was about 17 ºC. Previoustests show that air is unsuitable as surrounding mediadue to difficulties in keeping stable temperatures. Thesupply and return service pipes are connected in a loopcirculating water at a temperature of about 80ºC. Whensteady-state is established in the insulation, at timet = 0, the circulation is stopped. Then, the temperaturedecline of the stagnant loop water is measured at oneposition in the coil. The thermocouples are placed atthree positions of each service pipe, see Fig. 2. One isplaced on top of the service pipe, one on the side andunderneath the service pipe. The insulation is peeledoff at the positions of the thermocouples and then putback and sealed to be water proof. The reattachment ofthe insulation was probably insufficient and it appearsas if pool water permeated after about 5.5 hours anddisturbed the measurements.The twin pipe studied in this paper has two copperservice pipes, semi-flexible polyurethane foam and aslightly corrugated LDPE casing. The pipe is ofdimension DN 20 with the pipe dimensions described inTable 1. The pipe producer declares a thermalconductivity of the semi-flexible polyurethane foam ofλ50 = 0.0255 W·m-1·K-1of a newly produced pipe ofthis kind. The pipe has no diffusion barrier. The densityof the polyurethane foam was ρ = 60 kg/m 3 .Table 1. Dimensions of the twin pipe studiedCasing outer diameter (mm) 91Casing thickness (mm) 2.2Service pipe outer diameter (mm) 22Service pipe thickness (mm) 1.0The initial coil temperature was T 0 = 81.3ºC. The watertemperature at the service pipe (T w,meas (t)) decreasesduring the 16 hours of measurements. The pooltemperature was initially T 1 = 17.4 ºC and increasedslightly to T 1 =17.9 ºC during the 16 hours. Themeasured coil and pool temperatures are showed inFig. 3.A sawtooth disturbance of about T = 0.07 ºC and asmall noice of about T = 0.015ºC can be seen in thepool water measurements in Fig. 4. A detailed study ofthe coil temperatures T w,meas (t) shows that thetemperature of some thermocouples decreases abruptoccurred at about t = 5.5 hours. The marked choseninterval in Fig. 3 and Fig.4 is chosen to minimize theerrors.Fig. 2 Experimental set-up and positions of thermocouplesat the service pipes91