Use of lower flow and return temperatures in district heating

District heating schemes make use of underground pre-insulated pipes to connect the various buildings on the site to a centralised heat source. The pre-insulated pipes are designed to maintain the water temperature when in transit (avoiding heat losses to improve energy efficiency). When planning a district heating project the specifier will often have a choice of going down the steel pipe or flexible plastic pipe route. In this short article we take a look at some of the recommendations made in the 2015 CIBSE guide ‘Heat Networks: Code of Practice for the UK’ that relate to this key decision in terms of energy efficiency and project costs.

Traditionally district heating schemes in the UK have used steel pipework to meet the high operational temperatures that are needed to compensate for their heat losses over large distances. Plastic pipes however, despite lacking the very high operating temperature of steel pipes, are starting to become very popular due to their improved flow characteristics and superb insulation performance allowing for lower temperatures to be used at both the heat source and for flow and return in the pipes – improving energy efficiency across the system.

The CIBSE guide contains some very useful information on this topic including the following key design consideration on page 5 – “For a given flow temperature, a low design return temperature will reduce peak volume flow rates leading to smaller pipes and lower costs. Maintaining low return temperatures under part-load conditions is important to keep heat losses and pumping energy low. Designing for lower operating temperatures will result in higher efficiencies with some types of heat sources, e.g. heat pumps”.

The CIBSE guide also goes on to state (on page 24) that “In general, polymer pipes, at least for smaller diameters, will have lower installation costs because of the greater flexibility and because fewer joints are needed. A further option to the designer is the use of twin pipe systems which offer lower heat losses and lower installation costs”.

As can be seen, plastic pipes have some great benefits for district heating projects in terms low design temperatures (improving efficiency) and costs. At Mibec, we offer a wide range of pre-insulated pipe products from leading manufacturers like Rehau, Microflex or our own Mibec DHP pipe to suit your requirements. Pre-insulated pipes are extremely flexible and lightweight allowing for rapid installation and the ability to easily navigate over or around obstacles (such as trees and existing buildings) on site. Due to the flexibility of the pipe they are supplied in coils (up to 250 metres for some sizes) allowing for easy transport, delivery and storage on site. The pipe is also easy to handle; the installers can roll out the pipe straight into the pre-prepared trench directly from the coil. The ability to keep the pipe in a single long flexible piece has many advantages in terms of limiting the number of joints and connections required which saves on time, costs and reduces the likelihood of any future leaks. A reduction in joints will also mean less pipe cutting and less fittings to individually insulate to maintain the heat loss performance. Pre-insulated pipes are also very easy to cut (needing only a normal hand saw) and because the pipes have such a high degree of flexibility it easy to make the building connections.

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 right district heating solution to meet your needs. Please feel free to email or call our support department on 01948 661639 where one of our trained advisers will be happy to help you.