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, EstoniaComfort level is increased by applying a higher bypassthermostat setting and/or a ―hot‖ heat exchanger duringidle. Fig. 5 shows an example of flat station with ―hot‖heat exchanger and thermostatic controlled heatexchanger [7]. Idle temperature is approx. 50 °Ccorresponding to dhw tapping temperature.For simulations a branch pipe flow (Q1) of 800 l/h isassumed. This represents a situation where thethermostat is fully open until the desired set temperatureis reached. Further a step vice flow change from zero toQ1 or zero to Q2 is assumed. Tapping flow is assumedto be on a high level flow for one tap, which is typicallyapplied when opening the dhw. Q2=400 l/h for allsimulations.161412Delay until reaching 45°CL2=5m & 10m (internal ø10mm) - Heat Exchanger hot & cold at idlehot - dt at T2 - L2=5mhot - dt at T4 - L2=5mcold - dt at T3 - L2=5mcold - dt at T4 - L2=10mhot - dt at T3 - L2=5mhot - dt at T4 - L2=10mcold - dt at T4 - L2=5mdT [sec]10864Fig. 5. Dynamic control performance (idle recovery) forthermostatic controlled heat exchanger for dhw production.Heat exchanger is warm during idle. [7]Fig. 5 shows a flat system with ―hot‖ heat exchanger atidle. Bypass temperature setting corresponds to aprimary supply temperature (T11) of 58 °C and primaryreturn temperature (T12) of 44 °C. This setting is thehigh end, meaning in ―high‖ end regarding comfort. Forthis system there are no primary delays, and dhwtapping temperature at the flat station is available afterapprox. 2 sec. Additional delay due to dhw pipingtowards tap would be similar to previous example.In many practical matters a compromise between thetwo examples regarding idle temperature setting fulfilsdemands for good comfort with reasonable energyconsumption.In the following a general trade off is included betweenbranch pipe length, dhw pipe length, idle condition forheat exchanger and temperature delay on dhw, basedon dynamic simulations. Pipes are simplified by simpledelay models with no heat loss. Heat exchanger isbased on a lumped capacity model described in [5].200 1 2 3 4 5 6L1 [m] (internal ø20mm)Fig. 7. Dynamic simulation for hot and cold heat exchangerduring idle. Delay (dt) for dhw temp. of 45 °C.Heat exchanger simulated is Danfoss XB06H-40 [6]. Itcan be seen from figure 7, that influence on hot or coldheat exchanger is in the range of 2 sec. delay. Branchpipe length (L1) has minor impact on time delay. This isdue to the fact that temperature is maintained with atemperature gradient along pipe during idle, reflectingT1 to T2. Basically water in branch pipe is heated to acertain level already before tapping. Anyhow, due toenergy loss and return temperature, idle bypasstemperature is lower than dhw tapping temperature inthis case.Main influence on time delay is dhw pipe diameter andlength (L2). Connection in flats shall be of ―starcoupling‖ principle where every tap has its own supplypipe with a small inner diameter. Temperature in dhwpipe water is assumed to be room temperature prior totapping. In general, additional delays of typically 3 to 6seconds shall be expected due to thermal interactionwith thermal capacities along the way to tap andhydraulic dynamics on branch pipe side and hydraulicdynamics on dhw side.Simulated waiting time for a dhw temperature of 40 °Cis included in figure below:Fig. 6. Basic application for flat station, including boundaryconditions for dynamic simulations.19