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, EstoniaThe governing equations then change to:[8]Table 8 Pipe temperature predication comparison (supplyoutlet temperature is controlled at 50 o C)WhereThe boundary conditions change to :[9]The system linear ordinary differential equations can besolved with Eigen value method or with Laplacetransformation. The Laplace transformation wasapplied in this study. Eq. 8 is transformed to:The final solutions are given as:Where :[10][11][12], [13], [14]Tws- DN32, which is the longest main pipe in HE ofcase 1, is selected for the assessment with U 1 =0.141and U 2 =0.0523. The pipe length is assumed 500 m.Ground temperature ranges from 0 to 15 o C. The inletof supply and return temperatures are known as 55 o Cand 22 o C respectively. The outlet temperature ofsupply pipe is controlled as 50 o C and 45 o C,respectively.Table 8 shows the temperature prediction based onsingle pipe simplification and the coupled pipeequations. T_Difference represents the coupledsolution minus the single pipe solution. When thetemperature drop along the supply pipe is controlled at5 o C, the prediction between the single pipe and thecoupled pipe is very close. The prediction errorsincrease with increase the ground temperature. Thesingle pipe approach predicts lower supply watertemperature and higher return temperature than thoseof coupled pipe solutions. It was also observed thatwhen the ground temperature is higher than 4 o C, thenet heat transfer effect in the return pipe is to absorbheat to the surrounding.79The by-pass water temperature in this study was set ina conservative way. In many practices, the by-passwater can be set 10 °C lower than the supply watertemperature. Even lower by-pass temperature isproposed for the low temperature district heatingnetwork [3]. Table 9 shows the simulation results basedon a10 °C temperature drop along the supply pipe. Itshows the prediction errors increase in both supply andreturn pipes. The heat transfer was predicted in areverse trend in the return pipe at 4 °C. Considerableprediction error was found in the return pipe at highground temperature.It is worth to be noted that the increase of supplytemperature drop has more influence on the return pipetemperature prediction than that of supply pipe. Thereason can be explained from the expression of U s andU r in Eq. 1–2. As the magnitude of T s -T g is higher thanT r -T g , the same amount of return water temperaturevariation will have more influence on U r than U s ,therefore causes a larger prediction error in the returnpipe than in the supply pipe.Table 9 Pipe temperature predication comparison (supplyoutlet temperature is controlled at 45 o C)CONCLUSIONIn this paper, a preliminary study was conducted on theinfluence of by-pass flow on the network return watertemperature in a designed low temperature DHnetwork. The concept of supply water recirculation was