Pipes, valves & fittings
Cutting heat networks down to size
Pete Mills
Pete Mills, commercial technical operations manager at Bosch Commercial and Industrial outlines some of the potential pitfalls that need to be avoided when it comes to sizing a heat network
operators cannot afford to be complacent, as a small number of failing schemes have demonstrated. Many district heating designers oversize heat networks and the plant heating source as part of a conservative approach, sometimes overcompensating to allow for future expansion and the connection of further networks. Oversizing a network from the outset will affect capital expenditure and have huge ramifications for the long-term efficiency, overall performance, and return on investment of a system. An iterative approach is therefore necessary,
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refining the design to ensure realistic heating and DHW loads are used, which can mean spending more time modelling loads and accurately assessing heat losses.
Oversized, inappropriate boilers
A problem with the design of many smaller heat networks is a lack of renewable or low carbon energy heat sources, as it is the efficiency gains from these sources that deliver reduced energy costs. Peak load boilers should only operate when the load cannot be met by the renewable or low carbon heat sources and should be selected based on good turndown ratios. These heating systems spend much of their life operating at between 10% and 25% of their peak due to 24-hour operation with periods of low demand, so pumps and boilers must
18 August 2017
heat network promises heat provision at a lower cost and in an efficient way. However, the industry, from designers, contractors and
A well-designed heat network can deliver on efficiency and provide accurate metering for end users but de- signing these systems correctly and ensuring they deliver real savings is crucial
be sized correctly in order to perform efficiently. Opting for boilers with a large turndown ratio is a smart move as the wider the operational range of the boiler plant, the better it meets fluctuating requirements in the network. Cascades of smaller boilers can be particularly beneficial as the turndown ratio is based across the entire cascade rather than a single boiler. A cascade arrangement also ensures even load matching as each boiler only operates when required. Good turndown and control help ensure peak load boilers don’t flood a system with thermal energy that could
otherwise be picked up by a renewable or low carbon heat source. Wear and tear is shared between the boilers, enhancing efficiency, extending the operational life of the system, and reducing the likelihood of a complete breakdown. Rather than designing extra capacity into the
system, the CIBSE CP1 Code of Practice suggests using top-up standby boilers for additional capacity when needed, rather than continuously operating a boiler which is too large for the network. Common issues affecting the sizing of both the network pipework and heat producing plant are
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