Variable Flow Temperatures in District Heating

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This article aims to put some figures to the benefits of using variable flow temperatures on a heat network.

The key to making this work is our range of electronic HIUs, that track the incoming primary temperatures and adjust the flow to radiators accordingly. So with a 65°C flow temperature radiators run at 60°C, while a flow temperature of 85°C will provide a radiator flow temperature of 80°C.

Capital cost saving comes from been able to size radiators much smaller (the extra load for peaks comes from increased temperature rather than increased radiator size), and from significantly smaller network pipe sizing.

There is also the opportunity to use plastic pipework where before it would be impossible. This is because plastic does not like been held at high temperatures - it shortens the life considerably. However, if the higher temperatures are only seen for brief periods - at the points of highest load - then the life expectancy increases considerably. To find out more on this please talk to any plastic pipe manufacturer.

Radiator Sizing

Traditional Sizing

Taking a nominal peak load for a radiator as 1kW, when it is -5°C outside (a 25°C difference between the property and external temp).

http://www.stelrad.com/wp-content/uploads/2014/08/Stelrad-Compact-Horizontal-Product-and-Technical-Information.pdf

Radiator output is based on average temperature, and the difference between this average and the surrounding air. So a radiator with a flow of 75°C and a return of 65°C has an average of 70°C. With the air temperature at 20°C, this gives a ΔT of 50°C, as seen in the sizing tables on the radiator details linked above. This will require a network flow temperature of around 80°C.

A K2, 450mm x 800mm radiator gives an output of 1.097kW (10% more than needed).

So a standard (old fashioned) approach would be to setup the network to always deliver 80°C, and to setup HIUs to deliver 75°C.

For normal heating loads, where it is 10°C outside, the difference between inside and outside is only 10°C compared to the 25°C peak - so the heating load may actually be closer to 0.4kW.

Low Temperature Fixed Temperature Sizing

With a desire to run heat networks cooler we may tend to see these days radiators sized more on a network temperature closer to 70°C, and a flow to the radiators on 65°C. We may also want cooler return temperatures so we base sizing on a 20C drop rather than a 10C drop, giving us a return of 45°C, an average of 55°C, and a ΔT of 35°C.

A 450x1400mm radiator at ΔT of 30°C is 980W, or a 450x1000mm at ΔT of 40°C is 1026W.

As we are between these two we could select a radiator of 450mm x 1200mm to achieve our 1kW load.

Variable Temperature Sizing

Now, the same system, if we allow the flow to rise to 80°C when its -5°C outside (a few days a year at best), then the radiator flow will be at 75°C.

We wish for the same return temperature at peak of 45°C, so the average radiator temp becomes 60°C, and the ΔT is now 40°C.

Therefore we can use the 450mm x 1000mm radiator.

Variable Temperature Sizing to 90C

Now, the same system, if we allow the flow to rise to 90°C when its -5°C outside, then the radiator flow will be at 85°C.

We wish for the same return temperature at peak of 45°C, so the average radiator temp becomes 66°C, and the ΔT is now 45°C.

Therefore we can use the 450mm x 900mm radiator.