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, EstoniaNetwork Heat Loss CalculationThe reference network was designed with twin pipes byplacing the supply and return pipe in the same casing.Two types of twin pipes were considered in thesimulation: AluFlex multilayer flexible pipe and straightsteel pipe. The pipes were selected with continuousdimension ranging from Alx14 to 32 for AluFlex pipeand DN 32 to DN40 for steel pipe, based on the marketavailable products [8]. Single AluFlex pipe is selectedfor the 3rd recirculation line. This 3rd pipeline can beassumed being placed in the same trench along thetwin pipes. The thermal interaction between the twinand the single pipe is assumed negligible.The heat loss in the twin pipe was calculated accordingto the reference [7,9][1][2]coefficients corresponding to the temperaturedifference between the flow and the ground.The temperature variation along the pipeline wascalculated as internal flow with isothermal boundarycondition. The downstream temperature in the pipe isexpressed as [4]:T d , T u and T a represent the downstream fluidtemperature, upstream fluid temperature, and ambienttemperature respectively. M and K are parametersinclude the overall heat transfer coefficient. As theoverall heat transfer coefficients have to be calculatedbeforehand, the influence of flow temperature variationon U s and U r along the pipeline is neglected. It is areasonable assumption when the thermal by-passtemperature is set close to the plant temperature,however, may cause appreciable errors if thetemperature drop along the network is high.It is worth to be noted that though the design returntemperature (22 o C) is higher than ground temperature,the net heat transfer in the return pipe may absorb heatfrom surrounding which makes U r negative. However,negative U r has to be set to zero as the simulationprogram cannot handle negative heat transfercoefficient.[3]RESULTS AND DISCUSSIONFig. 6 District heating networkThe supply and return pipe are assumed identical andplaced horizontally in the same depth from the ground.The linear thermal transmittance U ij reduces toU 11 =U 22 =U 1 and U 12 =U 21 =U 2 . In addition, the thermalconductivity of insulation foam was assumed constant.U 1 and U 2 were then calculated with the analyticalsolution developed from the multi-pole method [10].The simulation program cannot handle two heattransfer coefficients in the same pipe, U s and U r werederived to represent the overall heat transferHeat ExchangerNetwork simulation starts from proper selection of pipedimension, based on the design condition and thedesign criteria introduced in the previous section.Table 2 shows the selected pipe types andcorresponding length for three different cases. Case 1is the reference case. Flexible twin pipe Alx 20 to 32and steel twin pipe DN32 and DN 40 were selected.The third recirculation pipe was designed in case 2based on the summer by-pass flow rate. Pressuregradient 1500 pa/m for street pipes and 500 pa/m formain pipes were set as the dimension criteria. Thoughsmaller pipe was suggested by the program, the Alx16single pipe was selected as the minimum diameter pipeavailable on the market. It was assumed that therecirculation pipe can be used as water supply in winterin case 3. Therefore, the main pipes in the referenceline were re-designed with considering that a portion ofsupply water goes through the recirculation line. It canbe seen that the supply pipe has smaller diameter thanreturn pipe in some sections in the twin pipe line.76