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, Estonia excavated material mixed with water and specialadditives in order to get a self-compactingbehaviourThe use of self-compacting materials offers a widerange of advantages and applications:it is possible to dig out a narrower trench, becauseno machines are needed for the critical compactionaround the pipesthe backfill process is significant faster – after thetrench is filled up, it takes normally only one dayuntil the material is hard enough to walk onself-compaction is more reliable within difficultconditions (many crossing pipes etc)without the use of compaction machines, buildingsnearby the construction site are stressed less (novibrations)there is less inconvenience for residents livingnearby the construction site, because of the noisereductionin combination with the pipeline laying technique,the sheeting can be omitted, because nobodyneeds to work in the trenchA common problem is the local availability of thetechnology. The price is also an issue, if the reason ofthe application is the approach to save money.Another problem concerning the dimensioning of thecompensation measures is the bad predictability of thefriction between the jacket pipe and the selfcompactingmaterial. Depending on whether the pipesare taken into service during or after the hardeningtime, which is about a month long, a more or lesscrucial tunnel effect is observed [4].The reuse of the excavated soil as base material ismore elegant, than the stabilised sand mix, because ofthe recycling aspect. Research projects have evenshown that sharp particles are less problematic,because they are enclosed in the self-compactingmass. An advantage of the stabilised sand mix is theeasier application.If the district heating line does not run under a street,compaction measures around the pipes can be avoidedsimply by watering the cable sand, which is filled inlayers into the trench.Cost saving potentials of backfill material withinsulation propertiesIf a reduction of the heat losses comes into consideration,the change to a higher insulation series isevaluated. Calculations show that in most cases aneconomical justification is not given for this measure.The idea of filling the trench around the pipes withmaterial that provides an additional insulation seems tobe promising. Like in the case of the self-compactingmaterial, the local availability is the greatest problem.An economical justification is only achievable, if thetransport costs are low and the heat price is high. Froma technical point of view, the compaction behaviour hasto meet the requirements and regulations. The jackettemperature must not exceed the maximum of 50 °Cand the friction between pipe and the material shouldbe in the common range.A calculation method for heat losses of plastic jacketpipes is described in EN 13941 ANNEX D [2]. Figure 1shows the influence of the thermal conductivity of soilλ s on the heat losses. Normally the value of λ s lies inbetween 1,0 and 2,0 W/m*K [2]. The curve becomesvery non-linear below a value of 1,0 W/m*K. Thisindicates that it is necessary to customise thecalculation method in order to get realistic results. Theheat losses are cut down by 30%, if the λ s is reducedfrom 1,5 to 0,35 W/m*K.heat losses of the flow and return pipeΦf + Φr [W/m]75706560555045400.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00Thermal conductivity of the soil λ S [W/m*K]Fig. 1 Heat losses of a district heating line as a function ofλ s (DN 250, 120/50 °C, Z = 0,6 m, C = 0,55 m,λ i = 0,03 W/m*K)The insulation material should solely be integrated inthe calculation as an additional thermal resistivity(R λ,embedment ), since the soil around the pipes is notmade completely out of it. The heat dependency of theinsulation foam‘s thermal resistivity should also betaken into account. Figure 2 illustrates, what is meantwith ―addi tional insulation layer‖.Fig. 2 The different layers of the heat conductivity problem298