A district heating network has a heat source supplying heating water and / or domestic hot water to two or more separate buildings in a project. This central heat source (gas boiler or a renewable technology) will then be connected to the various buildings in the scheme via an underground pre-insulated pipe network. This is designed to maintain the water temperature when in transit (avoiding heat losses to improve energy efficiency). At Mibec we can supply a wide range of pre-insulated pipe products from leading district heating pipe manufacturers like Rehau, Microflex or our own Mibec DHP pipe. In this short article we look at pre-insulated pipe layout principles when designing a district heating scheme and the pros and cons of the various options.
Designing the pipe network
At the largest scale there are two commonly used designs for the network, the ‘radial’ system and the ‘ring system’. The radial method has a central heat source with pre-insulated pipes branching (or radiating) away from it to supply each part of the network. This type of design is easier to plan than the alternative ring system (shown below) and has the advantage that individual pipe lengths are minimised, lowing initial capital costs and minimising heat losses when in use.
The ring method is often used to supply larger scale supply schemes with one or more heat sources. With the ring system not only can several heat sources be integrated, but it also results in higher supply security, as most customers can be reached via two piping paths. This makes future extension or adding new connections easy compared to the more limited radial method described above. However this system does have the disadvantage of needing longer pipe runs (not so good for construction costs and heat losses) and is more difficult to implement if the site topography is variable (slopes, hills, etc). However, for large projects higher capital costs can be offset through the effect of the diversity when in use – when there are a large number of buildings with a wide mix of customers in the project not all are likely to be at maximum heat loads at the same time giving a smoothing effect overall. In very large projects the diversity factor can be as low as 70% which could enable pipe sizes to be reduced accordingly (therefore reducing project costs).
Zooming in to the building connection scale, there are various methods for connecting properties to the heat network. The examples below show two of the options commonly used:
Firstly there is the branch method, this offers a flexible design, simple installation and is easy to extend at a later date. However this method does create more ‘T’ joints than the ‘chain’ method shown next which slightly increases the construction costs and installation time.
An alternative method is the Building-to-building (or chain) method, this approach is less flexible in design and is more difficult to extend at a later date (due to needing access to existing properties) but has the advantage of fewer ‘T’ connections in the ground potentially saving on construction costs.
When designing the building connections a decision will need to be taken around if the buildings are to be ‘directly’ connected to the heat network (where the water from the pre-insulated pipe network flows directly through the buildings heating systems) or ‘indirectly’ where a Heat Interface Unit (HIU) is used as a heat exchanger to provide a physical barrier to the water and each building will have its own sealed system. At Mibec we offer a range of HIUs, which can be supplied in a number of permutations to suit the project and budget. Models are designed to offer both supply and very importantly individual metering of domestic hot water (DHW) and / or primary heating water.
We offer a full free of charge specification service covering the whole of the UK, designed to support architects, specifiers or contractors, helping you to select the optimum piping solution to meet your district heating needs. Please feel free to email or call our Mibec support department on 01948 661639 where one of our trained advisers will be more than happy to help you with your next project.