To obtain the total peak heating requirements, it is neccessary to add the heat losses through the distribution network to the calculated peak heating loads of the buildings, as the district heating scheme has to be designed to provide the heat demanded plus the heat that is lost through the system.
The pipes selected are pre-insulated steel pipes, which include two layers of insulation covering the steel pipes. The first layer consist on a jacket of polyurethane foam and the second is an outer casing of HDPE (High Density Polyurethane).
To find the total heat loss through the pipes to the surrounding ground it is important to know the thermal conductivity and the different thicknesses of each layer. The ground that surrounds the piping will also act as a layer of insulation. For this reason, the thermal properties of the stoneless sand will be included into the heat loss calculation.
The total heat loss through the piping network will be added to the peak heating demand when sizing the electrode boiler and thermal storage.
The thermal conductivity of each layer of insulation including the ground are summarised below:
Thermal Conductivities:
- Steel Pipes (ks): 16 W/m.K
- Insulation (ki):0.0275 W/m.K
- Outer Jacket (kc):0.49 W/m.K
- Ground (kg):1.5 W/m.K
HEAT LOSS EQUATION
Then we can use the following equation to find the total heat losses in the pipes, in which case the temperature difference is taken for a design winter day.
Where,
- T1 = inner temperature of pipe
- T4 = ground temperature
- L = length of pipe
- ks = thermal conductivity of steel pipe
- ki = thermal conductivity of insulation
- kc = thermal conductivity of outer casing
- kg = thermal conductivity of ground surround stoneless sand
References:
- Poyry Energy Ltd. (2007). Community heating specifications for Barking Town Centre Energy Action Area. London.
- European Standard EN 10220:2002.
- Uponor Housing Solutions Ltd. (2007). Pre-insulated piping systems. Installation and technical guide.