Low Carbon System Design – a whole system approach “70ºC flow / 40ºC return”
The main characteristic of a “70/40” system is a large Delta T and a low system return temperature.
CIBSE AM12:2013 (9.16, p 49) states:
“It is recommended that, for new systems, radiator circuit temperatures of 70ºC (flow) and 40°C (return) are used with a maximum return temperature of 25°C from instantaneousdomestic hot water heat exchangers.”
To achieve 70ºC flow and 40°C return temperatures calls for a ‘whole system approach’ by the specifier
Properly designed and commissioned CHP District Heating schemes have the potential to significantly reduce energy consumption and carbon emissions. However, traditional design practices are often based on flow/return temperatures of 80/60°C. A return temperature of 60°C is sufficiently high to prevent the Combined Heating and Power system from delivering maximum cost-savings, as this temperature only allows limited cooling of the generator and can result in the CHP shutting down.
If system circulating temperatures of 80/60 are used instead of 70/40, heat losses from the distribution system can only increase.
The most effective way to reduce return water temperatures is to reduce water flow rates.
Time for a re-think
The latest version of CIBSE AM12 ‘Combined Heat and Power for Buildings’ is designed to address these issues and provides new best practice guidance for communal heating systems.
A project that does not achieve recommended return water temperature, therefore, does not follow best practice. This can have serious implications for the system designer.
District Heating & Central Plant Design Challenges:
Plant Energy Efficiency
CIBSE AM12:2013 (9.16, p 50):
“It is preferable to adopt a variable volume control system for the DH system and all of the building heating systems connected. This will ensure that pumping energy is minimised through reducing the volume of water to be pumped and the pressure drops to be met, and also reduces heat losses through ensuring that return temperatures remain low under part load conditions.”
SAV have put together a CIBSE-accredited CPD Seminar, which explores the main issues arising from a 70/40 approach to design. The agenda for this is given under the CPD Seminars tab further along this website. The 70/40 method can be expected to have far-reaching implications, and discussions which are a normal part of each Seminar will sharpen your insight into developments in this field.
If you would like to explore the potential offered by the 70/40 method to any building services project, please feel free to provide some information via the Contact Us page, or by phone / e-mail. We would then assess how best to develop the ideas further, either by CPD Seminar, round table discussion, site visit or other communication.
We look forward to hearing from you.
Please contact SAV Systems using the details below or by going to the Contact Us page.
By e-mail: [email protected]
And we’ll be happy to help!