Expansion Vessel Settings

[watford 99]

Before I start flushing and refilling my Solar thermal system I wonder if someone could answer the follwing questions about expansion vessel settings please.

Perhaps I should firstly state that the system is of 20 evacuated tubes supplying heat to the bottom coil of a hot water cylinder. The expansion vessel capacity is 18 litre and the label on it says factory set to 3bar pressure. There is no heat dump and the 15mm pipework is not very extensive as I live in a bungalow

My questions are:
1. What quantity  of heat fluid should I put in the vessel. Is the amount critical? I suppose I can measure what comes out but I am not sure if I will lose some when draining down.

2. What pressure should the vessel be set at. I know the factory setting was 3bar and I could measure the pressure before I start but I would like to know what I should expect and again is the pressure critical.

Many thanks.

[desperate]

It's not about the amount of fluid you put in the vessel, rather than the correct relationship between the initial charge pressure and the working and maximum pressure of the system.

If you have a pressure release valve rated at 3 Bar, then the initial charge has to be somewhat lower otherwise if there is any increase in system volume when it heats up, the PRV will blow almost straight away. There is also the consideration of the height of the vessel in relation to the system, but in most domestic situations this is pretty small and not worth worrying about unduly.

Sooo, I would suggest that you reduce the initial charge pressure to about 1Bar, using the schrader type valve gizmo and a tyre pressure gauge, then charge the system to just above 1 Bar. This should result in the diaphragm inside the vessel being about in the middle of the cylinder, or in other words you will have 9litres of air or nitrogen on one side and 9 litres of working fluid on the other. As the working fluid expands and contracts it will result in only a relatively small pressure variation, say from 3/4Bar to 1 1/4 Bar, well away from the point that the PRV will lift.

 This should keep your system happy


Desperate

[watford 99]

Many thanks for your suggestions Desperate. In fact my prv is set at 6 bar. I guess this makes a 3 bar factory setting of the pressure vessel reasonable by your reasoning if I understand you correctly. I hope I don't have to put 9 litres of fluid into the expansion vessel having just bought 10 litres of ready mixed fluid at a cost of nearly £40 thinking this would be sufficient for the whole system

[Justme]

Why do you have an 18L vessel if your worked out that you needed less than 10L of fluid? Water expands about 4% if I remember right across the range we need.

[Philip R]

Justme,
the system is pressurised to allow it to have a raised boiling point,well above 100 degC, therefore the expansion of the water glycol could well rise well above 4%. If is flashes to steam, the whole collector manifold and part of the down pipes can steam lock. So extra expansion volume required. 18 litres is considderred the minimum size for a solar expansion vessel. If heat dumps are used, the vessel oversizing is less of an issue.

Watford 99
Also be aware that the corrugated stainless pipe to the vessel from the pump station may not be long enough to give a dead leg long enough to protect the vessel membrane from steam during stagnation conditions. Therefore connect the vessel well away from collectors on the cold side i.e. discharge side of cylinder coil. ( Can be achieved using additional metric pipe to 3/4 BSP non taper couplers and fibre washers ( type provided with vessel). Should steam fill the collector manifold, there remains sufficient vessel storage to not overpressurise the system, operating PRV and flashing steam to waste. Also vessel should be gas side down and water side up, so as to buffer the rubber membrane from steam. ( In a heating system arrangement, I would have this water side down, gas side up).
Have a read of CIBSE Solar heating Design guide or BPEC Domestic Hot water systems. Be aware, pressurised systems above 15 litres volume, require an unvented hot water ticket, so it can be signed off as a controlled service with building control.

PhilipR

[Philip R]

Everything I stated above is in addition to the spot on input from Desperate.

[Justme]

Justme,
the system is pressurised to allow it to have a raised boiling point,well above 100 degC, therefore the expansion of the water glycol could well rise well above 4%. If is flashes to steam, the whole collector manifold and part of the down pipes can steam lock. So extra expansion volume required. 18 litres is considderred the minimum size for a solar expansion vessel. If heat dumps are used, the vessel oversizing is less of an issue.

Even if the rate is 10% you still dont need an 18L vessel until you have a total solar capacity of 90L.

If it flashes over to steam your 18L is NOT going to contain any of it. Not sure of the exact value but its is more than 1500 to 1 expansion from liquid to gas. The PRV will blow if it goes to steam.

[Philip R]

Justme,
If the manifold overheated, boiled the water to steam, the increased pressure would operate the AAV if left open,( some folks leave it open all the time, others close it after the initial dissolved air is detrained). therefore depressurising the system. If not left on permanent open vent, the steam would raise system pressure by compressing the gas charge in the EV.

If say a manifold contains 2 litres of water, then 2 and a bit litres, say 3 litres gets pushed to the EV, the steam water interface will caused some steam flashing in the downpipes, thus requiring expansion volume.

So lets say we have 18litre EV, say 9liters water , 9 litres gas. adding 3 litres of water means 12 litres water and 6 litres of gas. Increaseing system pressure by 50% Depnding on your static head pressure, precharge pressure etc, you could find that your system operates the PRV in which case the contents will discharge and empty, to the point where on panel cooldown, there is no pressurised coolant to reach the panel manifold. Therefore some moderate vessel sizing is required.

As an aside many system boilers are sold with undersize EVs with respect to the water volume of rads and systems required to dissipate boilers heat output. That is why many heating problems are due to loss of pressure faults due to leaking/passing PRVs.
PhilipR 

[Justme]

If the manifolds 2 to 3 L contents all goes to steam you are not going to get 2 to 3L of displaced water going to the pressure vessel. You will have 3-5000L of steam. If just a small spot (about 2cc) of water goes to steam & empties the manifold then you would be right. I cant see any pressure vessel taking the volume increase due to steam. That job is for the PRV to handle. The pressure vessel is to handle the expansion & contraction of the liquid & to maintain a constant ish pressure over its normal working range.

[EccentricAnomaly]

If the manifolds 2 to 3 L contents all goes to steam you are not going to get 2 to 3L of displaced water going to the pressure vessel. You will have 3-5000L of steam. If just a small spot (about 2cc) of water goes to steam & empties the manifold then you would be right.

This is something I've wondered about. Are the manifolds (for the Navitron ETs) designed in such a way that as the water boils the steam forces the remaining liquid out? In other words, are they designed so there isn't a "sump" of water which can't be pushed out and which will therefore contain liquid which boils?

A related point is whether draining the system removes all the liquid from the manifold (other than a small amount wetting the surfaces, of course).

Of course, even if there is a noticeable amount of liquid in the manifold which is not pushed out by steam it doesn't follow that the total volume of fluid (liquid and vapour) in the system will increase by the steam volume of that water because a lot will condense in the rest of the system.

However, from the point of view of preserving anti-freeze you'd presumably prefer to have as small a quantity of liquid boil as possible. Ideally just a little (JustMe's 2cc) will boil and push all the remaining liquid out of the manifold in liquid form.

[KLD]

We've marveled about the dynamics of a stagnation event many times. The conclusion IIRC was that boiling will start most likely at the "hot" end of the manifold, but in any case will be localised. This will produce some steam that will start pushing liquid out of the manifold. In those places where there is only steam in contact with the heat pipe ends, no more (or very little) energy input happens. Eventually, the manifold is mostly empty, and steam has pressed into the connecting pipes for maybe 1 meter. This means that the volume the expansion vessel has to take up is in the order of 2L to 5L. It is prudent to design the system such that it self-recovers after stagnation, i.e. the PRV should not release any fluid.

The initial pressure settings are such that there is some extra volume of fluid in the EV to serve as "spare" when the temperature drops below that of the fluid during filling. Say you initially fill with room temperature water; then you'd get at most 20°C minus 4°C = 16°C temperature drop; the aforementioned 4% volume variation are for a temperature change between 4°C and 100°C, so for our 16°C change we'll have a 0.66% volume change. Say 10L total system volume, delta V = 0.066L. This is the minimum "spare" you need.
The relation between volume and pressure in the EV can be assumed linear: p*V = const. Take your 18L vessel, pre-charge it to 1bar. Now, fill your system to 1.2bar, and you end up with 15L of gas + 3L of water in the EV  (18L * 1 bar = 15L * 1.2bar ).

Klaus

[watford 99]

I have found the information provided in the replies to my query very interesting. My problems arise from the fact that the system was already installed when I moved in in 2007. It was, in fact, installed in 2006. I received no information to tell me what I needed to do to run or maintain the system and I therefore rather ignored it until the circulation pump became noisy. I do know, however, that the system was installed by a firm called Solarbank which is no longer in business I am told.

I drained and flushed the system last weekend and refilled with new heat transfer fluid using my old Hozelock garden sprayer which I was able to connect with a purchased adapter. The old transfer fluid was very brown. I hope I got it all out by flushing through with water. It seems that the liquid in the manifold needs to be forced out which means that, having got the old fluid out by pumping in water, some new transfer fluid needs to be wasted to get the flushing water out of the manifold. 

I find my pressure gauge never reads below 0.8 bar even when the system is depressurised so my next job is to get a new one as I am having to hope I have topped up to somewhere near the correct operating pressure. From other answers I have received, and from reading other websites,  I understand I should be aiming for a pressure between 1 to 2 bar when the system is cold and the expansion vessel pressure should be approximately 0.3 bar below this. My Resol controller is set to stop pumping when the bottom temperature sensor in the HW cylinder registers 60 deg.C. It has reached this a couple of times this week so I have had to reduce the time my gas boiler is operating. I only wish I had known what I know now as I am sure I have not got the best out of the system during the past 4 years.

I do not think these thermal solar systems are  simple  or reliable enough for the general householder. I know one can always call in a plumber to do the maintenance but at £70 an hour I wonder if this makes these systems uneconomic.

[2807]

Hello watford99

I do not think these thermal solar systems are  simple  or reliable enough for the general householder. I know one can always call in a plumber to do the maintenance but at £70 an hour I wonder if this makes these systems uneconomic.

I disagree, when the basic operating principals are understood, the systems are very simple.  Reliability depends very much on the weather, but once fitted, I can't think of a more cost effective way of collecting energy and virtually no cost.

When the North Sea gas becomes uneconomic to pump & the Russians decide that they no longer want to sell gas cheaply, I am sure that many more people in the UK will decide that solar thermal is the economic way forward.

Yes, you can call in a plumber, but it isn't really necessary & unless the plumber understands the theory, then they are not going to be able to sort any problems anyway.

2807

[watford 99]

    "Reliability depends very much on the weather">>>>> "unless the plumber understands the theory"

I think, 2807, you make my point rather better than me.  At present these systems are neither simple nor reliable enough for a non-technical householder. The cost of fuel is increasing by leaps and bounds as we all know, but then so is the cost of installation and maintenance of such systems.

[martin]

"At present these systems are neither simple nor reliable enough for a non-technical householder" - couldn't be further from the truth - if the installation has been properly done, and the system correctly pressurised (to around one bar), it's a "walk away and forget it" system - I have one myself, and won't touch it at all unless I want to replace the antifreeze/water mix, when it's incredibly simple  - repressurise until it hits "1 bar", shut the valve, disconnect the pressurising device, walk away...........

Incredibly simple and reliable - if you're having problems, grab a new Navitron expansion vessel with the built-in gauge, which makes things really simple. Over the time I've had my system it's saved a small fortune while the Rayburn's been off!