standard setup

[alankelly]

Hi everybody

Hope you are all well

Just about to finish my install and I am a little confused about prv pressures / ev precharge pressures and general setup static pressures.

My pump station has a 6 bar prv fitted and the expansion vessel supplied by navitron has a precharge pressure of 2.5bar, so going by information read on the forum my static pressure should set to about 2.7 bar to ensure I meet the EV pressure plus a little.

But where the confusion lies is most people seem to have a 3 bar PRV with an EV precharged to about 1.5 bar and a setup pressure of about 1.7bar

What do you guys recommend

Do I go along with the original setup ie 2.7 bar static pressure with a EV precharge of 2.5 bar and a PRV at 6 bar or do I change the PRV to 3 bar and reduce the precharge in the EV to 1.5 bar

(My biggest concern is the Newark cylinder twin coil, as reading the specifications the solar coil is only rated to 7 Bar)

Any advice greatfully received

Best and kindest regards Alan

[forever green]

my system is 1.3 bar and my expansion vessel is 1.1 bar. The 6 bar PRV is fine.   Worked absolutely fine for the last 4 years

[KLD]

Alan,

In the event of stagnation, some flat-plate collectors don't empty quite so  gracefully as a heat-pipe collector manifold does. One variable to play with in such a system is the pressure: the higher this is, the higher can the panel temperatures rise to before the system fluid starts to evaporate. OTOH, if you have a heat-pipe collector, and the system layout is correct, then I can't think of a reason to put the pressures up unnecessarily high. In that case, lower the EV pre-charge to say 1 bar, and pressurise the system to 1.2bar. If the EV volume is sized appropriately, the PRV should never blow and is a real safety device. Personally I'd change it to a 3bar type.
 
Klaus

[alankelly]

Hi Guys

Sorry for the late reply

I have now fitted a 3 bar safety valve and have done as recommended by Klaus and lowered the EV precharge to 16psi and set the system pressure to 20 psi. Also regarding the size of the expansion vessel, I have fitted a 18 litre version, and the total volume of the solar fluid is about 10 litres (due to 15 m pipe runs of 12mm twinsol, so do you feel this size EV be suitable?

Many thanks in advance

Best and kindest regards Alan.

[KLD]

Alan,

your EV volume of 18L should be fine with a 3bar PRV.
This is why:

In good approximation the gas in the EV can be treated as an ideal gas. Boyle's Law then states: pV = constant, where p is the absolute pressure, and V the volume filled by the gas. In your case, the initial volume is 18L at p=2.1 bar absolute (using 14.5 psi / bar). When you charged your system to 20psi (2.4 bar absolute), the gas volume in the EV is compressed to 15.9L, i.e. you have 2.1L of solar circuit fluid in the EV.
Now, at your maximum pressure of 3bar above ambient (i.e. 4 bar absolute), the gas volume in the EV is compressed further, down to 9.5L. The vessel has now taken up an extra (15.9 - 9.5) L = 6.4 L of fluid. Compare this to the manifold volume of just over 1L each.

Klaus

PS  What is your arrangement for heat dumping ? Just seen your other thread, where you say you're using an "Antman" solution.