Expansion Vessel Settings

[SimonHobson]

What is the purpose of the precharge pressure
 You cant get all of the air out of anything but the most simple system. That remaining air expands with heat ...

It's not air that's the issue - air in the system will compress. But, the water (or pretty much any other liquid) will change volume with temperature and this volume change needs somewhere to go. In an open vented system the level in the feed&expansion tank will rise and fall a bit to suit, in a closed system the change is accommodated by the gas compressing in the EV. Without an EV, a closed system would rise rapidly in pressure until something "gives" - a safety valve opens or something bursts.
The pipes etc will also expand with rising temperature, but for all combinations* I'm familiar with, the liquid expands more.

Why the precharge pressure ?
Consider if you had no initial pressure in the EV. You charge the system to (say) 2 bar (which is 3 bar absolute, ie relative to a perfect vacuum). The gas in the EV will now be compressed down to 1/3 of it's original size. The effective size of the EV is now quite small.
Now suppose we pre-charge the EV to 1 bar (2 bar absolute) first ? As we fill the system, until the pressure is up to 1 bar then no water enters the EV, and when we reach 2 bar then the gas will be 2/3 of it's original size - we've raised it's pressure from 2 to 3 bar absolute (ie 3/2) so it's volume reduces to 2/3.

In this simple case, by pre-charging the EV, the gas volume at working pressure is double that from not pre-charging it - for the same size of EV. That means a given change in liquid volume will have half the effect on the system pressure.

The downside is that the volume of liquid available to make up the system volume (eg if we have a leak) is reduced (we have 1/2 the liquid in the EV), and when the system pressure drops to 1 bar the EV will be empty and the system pressure will drop off very quickly to nothing.


* Except ice and very cold water, which isn't relevant to this thread.

[derekmt]

I'm very familar with PV=RT,  the expansion of solids and liquids,  relative incompressiblity of liquids and the change in vapour pressure with temperature for water.
By precharge I mean the system pressure not the expansion chamber pressure with the system empty

What is it you are trying to achieve with the static pressure in the system of 2 to 3 bar relative (3 to 4 absolute) at 20C (293K)?

• Changes in the boiling point of the liquid?
• reduction in volume of the static trapped air in the system?
• reduction in volume  of the circulating air in the system?

[SimonHobson]

By precharge I mean the system pressure not the expansion chamber pressure with the system empty

What is it you are trying to achieve with the static pressure in the system of 2 to 3 bar relative (3 to 4 absolute) at 20C (293K)?

Changes in the boiling point of the liquid?

Yes, that's a factor - especially for solar thermal systems where heat input is uncontrolled and temperatures can get quite high.

reduction in volume of the static trapped air in the system?
reduction in volume  of the circulating air in the system?

There shouldn't be any - in theory. If air is able to remain in bubbles at high points, then clearly the flow rate is low enough that the presence of the air is unlikely to be an issue. If the air is entrained and carried round the system, then eventually it ought to find a vent and get released - though unless there are sections with low flow velocities to promote separation (eg radiators on a CH system), it can take some time.

Another factor is maintaining a positive pressure in all parts of the system, and especially at pumps. From what I've read, the choice of system pressure is normally determined by setting a minimum pressure at the top of the system and adding on the static head from there to the fill point/gauge. So if your design is for a minimum of 0.5 bar at the highest point, and the filling point is 15 foot below that, then you'd fill to 1 bar on the gauge. If you fill point is near enough at the high point (ie in the attic), then you'd only fill to 0.5 bar.

The manual for the Aplha2 pump I put in the flat has a chart that sets different maximum temperatures depending in the inlet head. The higher the head, the higher the permitted temperature - all to do with the decreasing margin against cavitation as the temperature is raised.
At my last job, I know they had problems with some lift pumps which struggled lifting hot water from a sump tank - not a large lift, but at high temperatures they could cavitate and stop pumping. The same model of pump never had a problem lifting from the hot sump.

[derekmt]

typical pump static head requirements are 0.4 bar at 90c,  but we fill at 20c, so we dont need  even 0.4 bar at the pump, as the system will pressurise as it warms.

[SimonHobson]

You are probably right that the pressure requirements probably aren't that great. What I'd suggest though is that most people aren't in a position to do detailed assessment of what their system requires - so in general it's easiest to just work to rules of thumb and apply generic settings that will be "safe" for all but the extreme installations. As long as you don't go too far, then having "a tad more" pressure is unlikely to cause any problems - eg if someone ends up with 1 instead of 1/2 bar then it's not going to damage anything. Ending up with too low a pressure is more likely (more being relative to "not a lot to start with") to cause them problems.

However, I would disagree with your logic. Yes, you may well fill cold - but then after that, you will vent air and perhaps have leaks. Sooner or later, I'd think it's probably a rare system that won't need topping up when warm. I'd imagine having a closed system with a negative pressure would be "a bad thing" and best avoided. Thus you need to have some minimum pressure to charge the system to even if it's just to allow for the drop when it cools.

[derekmt]

However, I would disagree with your logic. Yes, you may well fill cold - but then after that, you will vent air and perhaps have leaks. Sooner or later, I'd think it's probably a rare system that won't need topping up when warm. I'd imagine having a closed system with a negative pressure would be "a bad thing" and best avoided. Thus you need to have some minimum pressure to charge the system to even if it's just to allow for the drop when it cools.

 you fill , bleed, cold run for a short while, top up, bleed , topup,run,bleed,topup, run bleed, run....
This is all SOP for those who have built cars. especially mid engined front radiator cars.
A solar system is not very different from a car cooling system, they are both  pumped, self pressurising, water glycol , heat transfer systems.

I think you are alluding that the expansion vessel isnt just for expansion, but is a fluid reservior as well (pressurised systems can these), and  this precharge pressure is really to put fluid in  the fluid reservoir.

This makes sense for an installer who wants to leave site quickly.

[SimonHobson]

I think you are alluding that the expansion vessel isnt just for expansion, but is a fluid reservior as well (pressurised systems can these), and  this precharge pressure is really to put fluid in  the fluid reservoir.

This makes sense for an installer who wants to leave site quickly.

Quite possibly, but the inference that it's only for "get done and leave ASAP" people - and that's bad - is a bit ingenious.

Back to your car analogy. Yes, I'm familiar with the issues of getting air out of cooling systems  , but the thing you neglect to mention is that invariably there is a reservoir of "spare" coolant in the expansion vessel when cold - to allow for that bit of air we couldn't shift, or a slight leak, or a bit of evaporation, or ... And of course, either by marking put there by the manufacturer or by empirical observation, we can also check the level when hot and allowing for the difference caused by expansion.

Ultimately, whatever words one wants to use to describe it, systems are generally setup the way they are because ... over time it's been found to work reasonably reliably. It probably would be true to say that many systems could be setup in a more optimal way if someone wants to analyse every last detail, but since there's really no gain* in doing so then just working to a rule of thumb (pressurise to "<some default> plus the head to the top of the system") is good enough. Obviously there is a limit, but unless you intend pressurising close to the PRV limit and/or there are badly specced components in the system then that's not an issue.

* I can see no gain to be had from running a system at (say) 1/2 bar if it's happy running at (say) 1 bar.

[derekmt]

...
but the thing you neglect to mention is that invariably there is a reservoir of "spare" coolant in the expansion vessel when cold - ...

Sorry thats exacty what I did say...

this is like extracting teeth.. So we can deduce that:

 "pre charge is to insert extra liquid into the system to allow for losses and remove the need for intensive  bleeding of the system...."

at last. *(when  the pre conditions of preventing boiling / cavitation have already been met)

  Derek the ingenious
p.s. its not ignieious to suggest installers look for methods that save time .. their time is money

pp.s I dont buy arguments that are based on the premise that if its been done that way often enough it must be right.
pps 1/2 bar less pressure is half a bar more safety in the best designed systems failures still occur.  
that extra 1/2 bar is an extra  1.5kj in a 20 litre epxansion vessel

[SimonHobson]

p.s. its not ignieious to suggest installers look for methods that save time .. their time is money

Sorry, it's just how I read it.

pp.s I dont buy arguments that are based on the premise that if its been done that way often enough it must be right.

True, but I didn't quite say it that way - or at least, it wasn't meant to sound like that. Given the number of ways things can be done, this is one that's been found to work quite well. As I said, you could sit down and do all the calcs - decide on elevation of BP, satisfy pump requirements, etc and then set a pressure for each installation. I suspect that the majority of systems would end up with similar requirements - which means it's a reasonable approach to work on simple "rules of thumb" for most "standard" setups. The practice is common with heating systems, and there you need to have something that even a below average plumber can cope with

pps 1/2 bar less pressure is half a bar more safety in the best designed systems failures still occur.  
that extra 1/2 bar is an extra  1.5kj in a 20 litre epxansion vessel

Well yes, there is always the argument that by <some incremental change> you can (incrementally) reduce risks. Personally I'd consider the change in risk between 1/2 and 1 bar, or 1 and 1.5 bar to be insignificant in the grand scheme of things. Better still would be to eliminate pressurised systems altogether (ie go open vented) - but apart from the practicality problems for many, that introduces it's own different (but also small) risks.

It's good to reduce risks, but you need to keep things in proportion.
A quick search turned up this report - and the graphic puts things into perspective. 491,348 deaths in England and Wales during 2009 - that's an average of over 1300/day. Apparently 33 drowned in the bath - against that, I think the risks from failure of one of our low pressure systems are quite small. I've no statistics on that, but I can't recall hearing a single story/tale/anecdote related to it.

[desperate]

...
but the thing you neglect to mention is that invariably there is a reservoir of "spare" coolant in the expansion vessel when cold - ...

Sorry thats exacty what I did say...

this is like extracting teeth.. So we can deduce that:

"pre charge is to insert extra liquid into the system to allow for losses and remove the need for intensive  bleeding of the system...."

at last. *(when  the pre conditions of preventing boiling / cavitation have already been met)

  Derek the ingenious
p.s. its not ignieious to suggest installers look for methods that save time .. their time is money

pp.s I dont buy arguments that are based on the premise that if its been done that way often enough it must be right.
pps 1/2 bar less pressure is half a bar more safety in the best designed systems failures still occur.  
that extra 1/2 bar is an extra  1.5kj in a 20 litre epxansion vessel

I disagree with that Derek,

 any loss will soon reduce the pressure to zero (Gauge) which would lower the boiling point unacceptably, allow for air to be drawn in through minute leaks*, increase the risk of cavitation in the pump plus a few other disadvantages I could think of. Also I see no reason why an expansion vessel reduces the need to remove air from a system, it may provide a space where trapped air could lurk and hence not cause circulation problems, but air should always be purged from any plumbing system designed to be closed to minimise corrosion. Having an air space inside a steel expansion vessel is the last thing you want.

THE overriding reason for the vessel is to provide a sealed volume to allow for the thermal expansion of the working fluid, and hence maintain the fill pressure at a reasonably stable level. Most domestic size systems using 10 or 12 mm  pipework will contain between 5-10 litres of fluid, and on an everyday basis will fluctuate between say 20 and 70 degrees C. I would guess that 10 litres of water heated from 20 to 70 would expand by 1/2 litre ish ( cant be bothered to look it up) and if accommodated in say a 6 litre vessel would only result in a rise from about 1 to 1.5 bar, thus leaving plenty of leeway before the relief valve opens.

* It is perfectly possible to have a "leak" that is big enough to admit air (assuming below atmospheric pressure in the pipework) that would not show as a leak of water at raised pressure. Most if not all plumbing systems are not hermetically sealed, no matter how well we think we have put them together.

I accept that just because something has been done for a long time one way it's right, technically that is, and to the "N"th degree of accuracy maybe, but by the same token if it has been done like that for some time without problems, on a simple system like this, it cant be terribly wrong either. As Simon said you could work out the precise optimum pressure and set it as so and your system will work, but what significance is that on a system that is designed to cope with widely varying conditions of energy input, and hence temperature and pressure, after all that's why we use an expansion vessel.

I think I'm rambling now.

Desp

[derekmt]

It's good to reduce risks, but you need to keep things in proportion.
A quick search turned up this report - and the graphic puts things into perspective. 491,348 deaths in England and Wales during 2009 - that's an average of over 1300/day. Apparently 33 drowned in the bath - against that, I think the risks from failure of one of our low pressure systems are quite small. I've no statistics on that, but I can't recall hearing a single story/tale/anecdote related to it.

and yet we have schedule Part P to protect us from electrocution fatality risks smaller than the the risks from Wellington boots...

[derekmt]

I disagree with that Derek,

 any loss will soon reduce the pressure to zero (Gauge) which would lower the boiling point unacceptably, allow for air to be drawn in through minute leaks*, I....Desp

I dont agree with it either.  I am just trying to find out the justification for system fill pressure over and beyond providing suction head requirement, and positive head in all parts of the system.

A reason could  to de skill the design/installation... no longer any thought required just bung in a couple of extra bar and she'll be apples?
a reason could be you dont need to bleed the system to such a high degree. (residual air in the system affects the self pressurisation )