LOGSTOR ROR A/S, DENMARK/ AREA SALES MANAGER Where, Ri = Thermal resistance of the insulation, mK/W Re = Thermal resistance of the carrier pipe, mK/W Rj = Thermal resistance of the jacket pipe, mK/W R s = Thermal resistance of the soil, mK/W R H = Thermal resistance of heat exchange between flow and return pi pes, mK/W �r'!tf-;�': !'\« t� ·� r�' '1��? '�-'\'1 i\_ı'«.� ·" ., °M.' Hoat Loss in Pipe Systems 120 '.§ 100 1 � 80 � 60 � � � � � � � � � � � � ◊� o� Q� <J� Q� ◊� Q'<!'-" <:i�... o� .,_ ◊'<!'� ◊'<;"� Q� .., Dlmons!on (mm] ııı Pre-insuıated System [ilTraditionalSystom // / Logstor Ror Figure 3: Heat Loss for Traditional lnsulation and Preinsulation [3] Consequently, the heat transmission co-efficient is the constant in the calculation, and the heat loss varies according to the temperatures. An example of the heat loss differences of the two different systems is shown in Table 3, where the heat loss is calculated for the steel pipe dimensions DN25 - DN300. The prerequisites for the calculation are: t f = 90°C t r = 50°C t s = 8°C As an example, the heat loss for a DN200 pi pe system is: • • Traditional: Pre-insulated: 81.5 W/m 55.8 W/m Resulting in approximately 30% lower heat loss i n a pre-insulated pipe system than in a traditional system. The important energy savings, which are gained by using pre-insulated pipes will be even more distinct after several years, as the reduction of the insulating property is insignificant in the pre-insulated pipes compared with the traditional district heating pipes [3]. An added advantage to the energy savings in the form of a slight loss of heat is the advantage of having practically no costs of mai ntenance and a life expectancy of about 30 years for pre-insulated pipes. Comparisons Between the Systems - Life time Experience has shown that the insulation properties of the traditionally insulated pipes will deteriorate over time. With time, the concrete ducts are no longer watertight and water starts penetrating into the ducts. The humidity in the ducts causes several problems. Firstly, the insulating properties of the cellular ıo ECOGrnERATION WORLO Transın :issi:m Coeffi:::imts J,00 � -- -----------� � ı• Pw-hsuhtw Pi;ıc � 2,50 111 Tcdit:onaln&ı.ıbton -Ncw 3. 11 't'ı:ı.ditbnalrıs�btim -OU ! ı.oo 8 ı.� ıs ı.oo � MO E 0,00 IMJ<Jli,JH4JIIJ,JIH.W.ı.ı.ıa,wı,LW114JUJL.AIJ.ıı,,,,.ııtı,llliJl'LL,llll.lj sıeclP,i:>cDumcoortrım J /// Logstor Ror Figure 4: Transmission Coefficients for Traditional lnsulation and Pre-insulation concrete and mineral wool are deteriorated because of the humidity and consequently the heat loss to the surrounding soil will be increasing. Secondly, the humidity will cause external corrosion on the steel pipe, which after some time may result in severe damage to the carrier pipe. These factors imply that the insulation values of the post-insulated system will decrease over time [4]. Figure 4 shows the transmission coefficient for pre-insulated, traditional and old traditional systems. Damage Frequency 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Yoars - - •Traditional lnsulation -P re-insulatlon // / Logstor Ror Figure 5: Damage Frequency [4] in addition to the energy loss the expenses to repair the pipes in concrete ducts are heavy and the risk of major water loss in the system due to corrosion of the pipes is evident. The Danish Technological lnstitute has collected information concerning the number of damages on various Danish district heating distribution networks. This has resulted in the facts of Figure 5 indicati ng the damage frequency as a function of the type and age of the network. The damage frequency figure points out that risks of damages in a traditional system is much more important than in a pre-insulated system. Furthermore it is worth noticing that after 16 years' lifetime of a traditional system, the number of damages per year rises strikingly.
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