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, EstoniaTable 2 Selected pipe types and length in Case 1–3Figure 7 shows the pressure profile along the criticalroute. The network is designed for a 10 bar system.The minimum network static pressure is 2 bar and theminimum differential pressure at consumer is 50 kPa.The plant static supply pressure is 853 kPa in case 1 atdesign condition. In case 3, the designed plant supplypressure head rise to 917 kPa, which is due to theincreased flow rate indicated in Table 4. The pressuredrop along the reference line during summer is quitelow due to the reduced flow rate. However, extrapressure head has to be applied to overcome thepressure loss along the recirculation line in Case 2.The required static supply pressure is 800 kPa duringsummer as a result of small dimension recirculationline.Case 2 has higher return pipe heat loss comparing withcase 1 due to the introduction of recirculation line. Atconstant supply temperature 55 ºC, the heat transfercoefficient Us decreases with increase the return watertemperature. As shown in Table 3, the return watertemperature in case 2 (at 22 º C) is lower than that of incase 1. This leads to a higher heat loss in the supplypipe in case 2. As a consequence, more by-pass flowis required to compensate the extra supply pipe heatloss, therefore, the by-pass flow rate in case 2 is higherthan in case 1 in the summer season.Supply water in the recirculation pipeline in winterincreases the supply pipe heat loss in case 3. Theconcept of double pipe supply may not economicalfeasible, according to the simulation results. However,it may be used as an alternative solution to supplywater in the 3 rd pipeline under extreme whethercondition, which otherwise has to raise the plant supplytemperature to meet the increased heating demand.Furthermore, results in table 4 were limited to fixedrecirculation pipe diameters. The double pipe supplyconcept may be economical feasible by free selectionboth reference pipe and recirculation pipe diameterwith the objective to minimize the annual networkoperational cost or exergy consumption. This study isout of the scope of current paper due to the limitation ofthe simulation program.Table 3 shows the simulation results for case 1.By-pass is required when the heating load is smallerthan 1.53 kW. The return water temperature increasesalong with the increase of by-pass water flow rate. Insummer, the amount of by-pass water flow rateexceeds the actual flow rate passing through theconsumer, and the return temperature at the plantincreases up to 35.5 ºC. The heat loss in the returnpipe is accounted when the plant return temperature israised to higher than 30 ºC.Simulation results for case 2 and case 3 are shown inTable 4. They were put in the same table as case 2operates when there has by-pass requirement, whilecase 3 operates in the rest seasons. Italic is used forcase 3 to distinguish the two cases. Thanks to therecirculation line, the return temperature at the plant inthe reference line remains low at 22 ºC, while the returntemperature in the recirculation line can reach 44 ºC inthe summer, after deducting the single pipe heat loss.The low plant return temperature can help extract morepower in the CHP plant or be used in othercircumstance like direct flue gas condensation. On theother hand, high temperature return water in therecirculation pipe can be re-heated by an additionalheat exchanger or boiler with minimum energy input.Fig. 7 Pressure profile on the critical route in Case 1–377