Expansion Vessel Sizing

There are two ways you can size an expansion vessel for a sealed solar circuit:

  1. Typical rule of thumb
  2. Full calculation

Rule of Thumb

18l will be sufficient for any domestic installation with less than 30m of pipe and 40 tubes. 12l will be sufficient for most installations, e.g up to 30 tubes with 10mm pipe.

A suggested rule is: system volume * 1.04 (liquid expansion) * 2.5 (to allow for steam volume).

A typical system volume is 4l for 20 tubes, solar coil and 10mm pipe, or 5l for 40 tubes.

Full Calculation

This paper from the EU task 26 solar programme, by Hausner and Fink discusses stagnation behaviour of systems in detail, and provides calculations to work out how large a vessel you need to avoid the pressure release valve (PRV) dumping when the panel stagnates.

You need to know these values:

  • Number of tubes (to calculate manifold volume)
  • Length of pipe (typically 5-30m)
  • Diameter of pipe (normally 10 or 15mm)
  • Heat exchanger volume (typically 1.5-2l for coil, 0.5l for PHE)
  • PRV setting (normally 3-4 bar)
  • Expansion Vessel precharge pressure (normally 1-1.3 bar)
  • Height of PRV above EV. (normally they are close together, so about 0 metres)

Strictly speaking you also need to know the 'maximum specific steam power' (MSP) for your panel at stagnation, and the power dissipation of your pipe at the stagnation steam temperature. These two things determine how for down the pipe the steam will extend. Using the defaults in the spreadsheet of 30W/m2 for MSP and 25W/m for pipe power dissipation should be fine for Navitron panels. We hope to generate data to confirm these numbers at some point.

Fill the above values into the provided spreadsheet to get the size of EV you will need. Then buy the next available size (12l, 18l, 35l): http://wookware.org/heating/EVsizing.ods (Openoffice version) http://wookware.org/heating/EVsizing.xls (Excel version) http://wookware.org/heating/EVsizing.gnumeric (Gnumeric version)

Here is another example of calculating the EV volume, by a slightly different method: http://www.fes-web.co.uk/Files/FES%20expansion%20vessel%20calc%20iss2.pdf