Buffer Logic

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Plantcp.jpg

Screenshot 2019-12-21 Thermal Integration HIU Desk.png

The four lines represent sensor readings from temperature sensors in a buffer store at regular distances apart. It is an example of a properly controlled and stratified store, with return temperatures to boilers maintained at network return temperatures. The buffer store is acting as the 'low loss header' for the boilers.

What can be seen is loading in two states. First one can see boiler(s) cycling to satisfy controls to the left. This is fixed logic, with a fixed on and off temperatures.

The position of sensors is fixed, limiting the logic that can be applied using fixed logic.

The final reheat in the graph shows the same situation with recovery held off for a period to increase buffer volume and time between boiler startup.

The improved logic is to use the rate of temperature drop to estimate load, then calculate the time and number of boilers that should then fire. The aim is to vary the buffer volume between fires so as to maintain minimum required storage volumes, and maximise the vvolume for buffering - thereby reducing the overall number of boiler startups (inefficiency).

Terms

  • Storage - volume of water that is reserved for use.
  • Buffer - volume of water that can drop in temperature between boiler cycles.

The storage volume required will vary during the day, following known patterns that will require additional storage in preparation for.

The buffer volume is decided by controls - when to fire the boilers, and how many.

Example

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4000 litres storage made up of 2 x 2000 litres in series.

2000 litres to be maintained at temperature. 2000 to be used for buffering boilers and for additional storage ahead of peak loads.

Buffer store is fitted with 4 x sensors in lower half, used to determine rate of discharge.

Time is measured between lowest sensors dropping to 45C and a higher sensor dropping to 45C. This time is multiplied by the height of the store divided by sensor distance apart to determine the time it will take for the buffer to deplete fully.

The power of depletion determines the number of boilers to fire. Boiler input must be greater than the depletion power.

A sensor at the top of the buffer store enables higher loads to be caught, bringing in boilers earlier if temperature drops.