Book - School of Science and Technology

External distribution systems 175Table 6.16 Heat loss from pipe-in-pipe system buried 600 mm below ground levelSize ofservice pipes(mm)Insulation thicknesson pipes(mm)Diameter ofenclosing conduit(mm)Heat loss (W/m run of enclosingconduit for following meanwater temperatures)75 C 100 C 125 CTwo 20 25 250 39 53 63Two 25 25 250 42 58 73Two 32 25 300 50 65 83Two 40 25 300 54 71 89Two 50 25 300 58 79 99Two 65 25 350 69 92 117Two 80 37 350 62 84 108Two 100 37 400 73 98 123Two 125 37 450 83 110 140Two 150 37 600 96 130 165Single pipe withinouter plastic cover(foam insulation)Twin insulated pipes withinheavily protected steelouter pipeFigure 6.24 Pipe-in-pipe systems for underground heat distribution(c) Construction as (a), but with a loose granular insulating fill poured round the barepipes.(d) Construction as (a), but with a fill of aerated concrete poured round the bare pipes.(e) Raw excavation, sometimes lined with tar paper, where bare pipes are laid forinsulation by a hydrophobic powder which cures when it is heated to form a protectiveskin on the pipe.Instances of failure of each of the constructions shown have convinced many engineersthat use of one of the factory formed pipe-in-pipe systems (Figure 6.24) is the way forward.Table 6.16 lists heat loss data for one such system. The Achilles heel of site application ofthese is the point at which they enter a valve pit or other site-formed structure, where thewatertight continuity of the outer pipe is broken. A sceptic would emphasise that thebetter patterns of pipe-in-pipe are provided with devices of one sort or another, arrangedto provide waming of the presence of vagrant moisture!