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, Estoniacontrol logics for building automation, which are todayoften used for controlling DH substations. The controlmethod suggests how the flow can be determined foreach heat load. The flow is regulated by adjusting thepump‘s rotating speed. Speed-controlled pumps arecommonly used nowadays and they provide a superiorcontrollability [1], [10].Table 1: A summary of flow-weighted mean primary returntemperatures (bold) and resulting reduction for varioustemperature programmes.Let us first study an example of an optimal controlcurve for a 100 % oversized system. Such a curve ispresented in Fig. 1, which also shows the relativemagnitude of the varying radiator flow in relation to therequired flow. The blue dashed line in the diagramcorresponds to the primary return temperature. For thesake of comparison, the primary return temperature fora 55/45 °C system is also shown (gray dashed line).Temperature [ C]Rel. flow [%]10090807060504030755025Primary return temperature reduction0-15 -10 -5 0 5 10 15Outdoor temperature [C]T p,sT p,r,optT s,s,optT s,r,optT p,r,55/45Fig. 1 Temperatures with an optimised temperature curveand a variable flow in a 100% oversized system. Theprimary return temperature from a 55/45 °C programme isshown for comparison.Flow-weighted, yearly mean primary returntemperatures from the radiator HEX have beencalculated with regard to the outdoor temperatureduration. Above the dashed line in Table 1, results areshown for a correctly dimensioned system, with an80/60°C programme as well as with an optimisedprogramme. The gain is estimated to just under twodegrees C. The last column shows how the primaryreturn temperature is affected when the length of theHEX is doubled. This comparison can be justified bythe fact that the primary return temperature issignificantly influenced by the lower secondary flow thatthe optimisation entails, while the pressure drop andheat transfer rate in the HEX can remain at amagnitude close to the original ones.m sUnder the dashed line, results are shown for a systemthat is oversized by 100 %. The first three temperatureprogrammes are 55/45, 60/40 and 80/30 °C, whereasthe last two are optimised ones with variable flow.The following conclusions could be drawn from thetable: The oversizing of a radiator system leads, in itself,to a significant reduction of the primary returntemperature, provided that some kind ofcompensation has been made in order for thesystem to work properly, i.e., that an accurateindoor temperature has been provided. By optimising the system (through the use of avariable secondary flow), the primary returntemperature can be further reduced, especially ifthe system is oversized. By extending the radiator HEX, the returntemperature can be further reduced with thetemperature programmes that employ a relativelylow flow. Regardless of the degree of oversizing, acombination of an optimised temperatureprogramme and an extended HEX provides asubstantially reduced primary return temperature.The values presented in the table have been calculatedonly for the radiator HEX. When considering thesubstation‘s total return temperature, it can be said tobe smoothed by the DHW consumption. Calculationscorresponding to those in Table 1 for a parallel and a 2-stage substation for 20 flats (based on the SwedishDistrict Heating Association‘s recommendations forsizing) result in reductions in the return temperaturethat are approximately 20 % lower than the valuesshown in the table. The difference between the paralleland the 2-stage connection is negligible when thereturn temperature from the radiator HEX is low ormoderate, a fact that has been previously207