Calorifier

[smileypete]

When the pump cuts out at 45psia, the air volume of the EV doesn't change (ie: remains at 8 litres) because the EV's absolute pressure (50psia) is higher than the pump's cut-out absolute pressure (45 psia).

 

When, due to heating, the calorifier water expands by 2 litres, the EV air volume reduces to 6 litres and its pressure therefore increases to 8 x 50 / 6 =67 psia or 52 psig. Since the PRV's blow pressure is 42psig, the water should be p*ssing out of the PRV (as I stated before) but it's not (as I stated before). So there is still something that you (and I) are not taking into account.

See post #221:

 

However in practice, an undersized EV may work for a number of reasons, including the ones ChrisPy gave.

cheers,

Pete.

Edited November 12, 2008 by smileypete

[chris w]

See post #221.

 

cheers,

Pete.

I looked at post #221 and it adds nothing to resolving the apparent anomaly.

 

I deliberately used ChrisPy's calculation method (but substituted my actual real world figures for EV pressure etc) and the anomaly still exists. The calculation shows the PRV should be discharging water but the fact is that it is not and has not done so since I fitted the 8 litre, 35psig EV.

 

Chris

Edited November 12, 2008 by chris w

[smileypete]

I deliberately used ChrisPy's calculation method (but substituted my actual real world figures for EV pressure etc) and the anomaly still exists. The calculation shows the PRV should be discharging water but the fact is that it is not and has not done so since I fitted the 8 litre, 35psig EV.

 

I think Chris W's titanic intellect has struck a giant iceberg....

 

Or is a giant water filled expansion vessel, lurking just beneath the waves!

 

cheers,

Pete.

[chris w]

I think Chris W's titanic intellect has struck a giant iceberg....

 

Or is a giant water filled expansion vessel, lurking just beneath the waves!

 

cheers,

Pete.

I must have hit an iceberg, because I can't see the relevance of post #221 to my issue. I have done the calculations twice. My original way only using psig and ChrisPy's way using psia. Both give the same end result, which is that the EV should not be capable of preventing the PRV's opening using the pressures and volumes that are employed in my system.... but it does.

 

Show me the maths that reveals why it is working.....IMHO, there is an additional element at work here which is being disregarded. Those manufacturers that give specific EV instructions also say to set the EV between the WP's cut-out pressure and the PRV's blow pressure.

 

As I say, I am not an expert on fluid mechanics, but it occurs to me that maybe we should be using psia pressure solely for the PRV as that is surely the only part affected by atmospheric pressure (ie: the atmosphere is pushing down on the PRV and helping to keep it closed).

 

The EV and the calorifier are a closed system. If we took them into space (ie: a vacuum) as a system the internal pressures wouldn't change surely in the EV or calorifier because there is no connection to the outside world and it's a rigid framed system. One has to have an NRV of course to establish this isolation - which of course I do.

 

If we now run the calculations in psig pressure, then the EV's pressure will increase to 8 x 35 / 6 = 47psig. The PRV's blow pressure would be 42psig + 1 atmoshere = 57psia. Thus the calorifier's pressure would be well below the PRV's blow pressure and the PRV would not leak. I have no idea if my analysis is correct, but it would at least explain the result I am getting.

 

Further, if this were the correct explanation, then jigging the maths reveals that the maximum pressure to which the EV could be set and still just not blow the PRV would be 43 psig

 

I am willing to stand corrected but I need someone to do the (different) maths that gives the result I am seeing in practice.

Edited November 12, 2008 by chris w

[ChrisPy]

refer to post 218.

 

 

 

Also note that the use of absolute pressure is nothing to do with whether the atmosphere is pushing down on anything.

 

As I told you before, a fifteen-year-old knows that you always use absolute pressure for Boyle's Law. You just continue to demonstrate your ignorance. You should be embarrassed.

 

I suggest you go to night school and do basic physics again, and don't bother to add anything to this thread until the dawning of realisation hits you in the gob.

Edited November 13, 2008 by ChrisPy

[RobinJ]

Just checked to make sure and the pump cut-out pressure is actually 2,5 Bar so there's not a lot of margin. Assuming I'm using Smileypete's calculator link correctly and their 'vessel operating pressure' equates to the PRV pressure, it gives me a pre-charge pressure of 2,8 Bar based on that PRV setting of 3 Bar. Expanded volume worked out at 3,64 litres and resulting pressure in the EV of 5,97 Bar. All this is based on 85 Deg water temp heated from the engine circuit and 10 Deg feed water. Go 0 - 100 Deg and an 8 litre EV isn't big enough.... Tight?

My figures weren't quite that accurate!

However, I estimate at that pressure it will only accomodate about 2l before the PRV goes off, so yes a bigger EV is required. If the 3.64 figure is right (I was using 4l) you will need at least 10l, probably better to find a 12 or 15l vessel?

As the pressure in the EV rises (due to raising the pump pressure) the amount of volume available for expansion goes down, since the pressure reaches the PRV pressure quicker.

[Dave Hannigan]

Chris,

 

your EV is precharged to 35psi. When you put it into your system and turn thr water on but no pump, then (ignoring any head of water for simplicity as the head could only be a few ft of pressure in a boat) the air side is at 35psi (you set it), the water side is 0.

 

When you turn the pump on the water side pressure climbs to 30psi (your figures). To do this the pump must force water into the system, that water can only be accomodated in the EV (assuming water is not compressible and no more air pockets in the system), therefore the diaphragm must move to accomodate this water, therefore its pressure must rise from 35psi. The water side is at 30psi as it is at the pump cut out pressure.

 

When you heat the calorifier then the water will expand into the EV but the starting pressure for water side is 30psi while the air side is 35psi + increase due to the pump.

 

The point I am trying to make is that the pressures will always be different with the water side being lower than the air side, with the result that although the EV is above PRV pressure it will not blow it.

 

The water added by the pump could be measured by taking out of circuit any accumulators and, from cold, turn the pump off, open a hot tap and measure the volume of water that comes from the tap. This will give you the volume of EV filled by the pumps action and therefore the starting point for the expansion calculations.

 

By the way according to Zilmet the design should be that the air pressure does not exceed PRV pressure.

 

Just a simple way please point out all my flaws

[ChrisPy]

Chris,

 

your EV is precharged to 35psi. When you put it into your system and turn thr water on but no pump, then (ignoring any head of water for simplicity as the head could only be a few ft of pressure in a boat) the air side is at 35psi (you set it), the water side is 0.

 

When you turn the pump on the water side pressure climbs to 30psi (your figures). To do this the pump must force water into the system, that water can only be accomodated in the EV (assuming water is not compressible and no more air pockets in the system), therefore the diaphragm must move to accomodate this water, therefore its pressure must rise from 35psi. The water side is at 30psi as it is at the pump cut out pressure.

 

When you heat the calorifier then the water will expand into the EV but the starting pressure for water side is 30psi while the air side is 35psi + increase due to the pump.

 

The point I am trying to make is that the pressures will always be different with the water side being lower than the air side, with the result that although the EV is above PRV pressure it will not blow it.

 

The water added by the pump could be measured by taking out of circuit any accumulators and, from cold, turn the pump off, open a hot tap and measure the volume of water that comes from the tap. This will give you the volume of EV filled by the pumps action and therefore the starting point for the expansion calculations.

 

By the way according to Zilmet the design should be that the air pressure does not exceed PRV pressure.

 

Just a simple way please point out all my flaws

When you turn the pump on the water side pressure climbs to 30psi (your figures). To do this the pump must force water into the system, that water can only be accomodated in the EV (assuming water is not compressible and no more air pockets in the system), therefore the diaphragm must move to accomodate this water, therefore its pressure must rise from 35psi. The water side is at 30psi as it is at the pump cut out pressure.

NOT TRUE. THE DIAPHRAGM WILL NEVER MOVE UNTIL THE WATER PRESSURE RISES ABOVE 35. THE VOLUME OF WATER PUMPED WILL BE A FEW DROPS, JUST ENOUGH TO PRESSURISE THE SYSTEM. COME ON, THIS IS SIMPLISTIC BASIC PHYSICS.

 

The point I am trying to make is that the pressures will always be different with the water side being lower than the air side, with the result that although the EV is above PRV pressure it will not blow it. UNLESS THE DIAPHRAGM HAS SOME STRUCTURAL ELASTIC STRENGTH THERE WILL NEVER BE A DIFFERENCE IN PRESSURE ACROSS IT, EXCEPT WHEN IT IS COMPLETELY FILLING THE VESSEL (NO WATER PRESENT).

[dor]

I totally agree with Chris PY here. In all these threads, I find it remarkable that people seem to think the pressure can be different either side of a flexible diaphragm (assuming it is in a position to move). The only time the pressure will be different is if the diaphragm is pressed against the bottom of the reservoir. Then it cannot move until the pressure in the system exceeds the pressure on the other side of the diaphragm.

Edited November 13, 2008 by dor

[Denis R]

My figures weren't quite that accurate!

However, I estimate at that pressure it will only accomodate about 2l before the PRV goes off, so yes a bigger EV is required. If the 3.64 figure is right (I was using 4l) you will need at least 10l, probably better to find a 12 or 15l vessel?

As the pressure in the EV rises (due to raising the pump pressure) the amount of volume available for expansion goes down, since the pressure reaches the PRV pressure quicker.

'Vessel operating pressure' should be set to pump cut-out pressure.

 

'Vessel precharge pressure' can be set the same as pump cut-out pressure too, to give maximum expansion.

'Security valve pressure' is PRV release pressure, which is almost always 3 bar.

 

Edit: Pump cut out pressure looks pretty high at 2.5 bar, is it really that high and if so is it adjustable down to say 2.0 bar? Otherwise you'll need a pretty big expansion vessel.

 

cheers,

Pete.

Thanks for the heads up on my misuse of the calculator Pete. Once I'd cricked my neck trying to read the spec plate on the pump, I was surprised too at the 2,5 Bar setting. I've lost the blimmin' instructions for it so I'll need to do a bit of research to see if it's adjustable - there are no useful knobs or screws apparent....

 

Robin, my calculations weren't that accurate either!! Now Pete's put me straight on the formula it appears that even a 12 litre EV is too small. It's up to 80 litres!!!

Supplementary question to all. As it's downstream of a NRV do I need an EV that's suitable for potable water still?

 

Edited 'cos once again I F*****d up the calculation. Jeez. Pete, see what you mean about lowering the pump cut-out pressure!!!

Edited November 13, 2008 by Denis R

[Gibbo]

So, on my boat I have 2 X 60 litre calorifiers and no EV.

 

According to this thread I should lose about 4 litres of water through the PRV when they are heated. But I don't. I lose maybe a cup full. So where is it going? Expanding the tanks?

 

But I am now going to fit an EV as I'm not convinced all that pressure is going to be good for the calorifiers.

 

What size do I need

 

Gibbo

[blackrose]

So, on my boat I have 2 X 60 litre calorifiers and no EV.

 

According to this thread I should lose about 4 litres of water through the PRV when they are heated. But I don't. I lose maybe a cup full. So where is it going? Expanding the tanks?

 

But I am now going to fit an EV as I'm not convinced all that pressure is going to be good for the calorifiers.

 

What size do I need

 

Gibbo

 

I think my EV is 8 litres for a 60 litre calorifier. I'm not sure but I think that allows 4 litres of expansion or 6% of calorifier volume (I was told 4% was the minimum).

 

Very easy to fit, just T it in either just after the NRV on the cold water inlet side or on the hot water side of the calorifier. I can't see that there's a real difference because essentially any location after the NRV is part of the same pressure system.

Edited November 13, 2008 by blackrose

[Gibbo]

I think my EV is 8 litres for a 60 litre calorifier. I'm not sure but I think that allows 4 litres of expansion or 6% of calorifier volume (I was told 4% was the minimum).

 

Very easy to fit, just T it in either just after the NRV on the cold water inlet side or on the hot water side of the calorifier. I can't see that there's a real difference because essentially any location after the NRV is part of the same pressure system.

 

Yeah I can't see that it makes a difference. I think the argument discussion regarding its position was if there's no NRV installed, which I intend to keep.

 

Now, have I got room under the bed for 2 X 8 litre EVs???????????

 

Gibbo

[RobinJ]

Robin, my calculations weren't that accurate either!! Now Pete's put me straight on the formula it appears that even a 12 litre EV is too small. It's up to 80 litres!!!

I thought the calorifier was 80l?

You shouldn't need to go bigger than about 16l for the EV.

Supplementary question to all. As it's downstream of a NRV do I need an EV that's suitable for potable water still?

Not unless you plan to drink hot water?

I suppose you want one that won't grow nasty things without the presence of air?

Most people seem to flush their systems out with some sort of steriliser once in a while.

[Denis R]

I thought the calorifier was 80l?

You shouldn't need to go bigger than about 16l for the EV.

Thanks for the feedback Robin. Yes the calorifier is about 80 litres, but playing with the calculator, having the pump at 2,5 Bar, the PRV at 3,0 Bar and the EV pre-charge pressure at 2,3 Bar is a crap combination. Depending on the temp range it comes out with an EV of 50 litres plus.

If I can get the pump cut-off down to 2,0 Bar as Pete suggests and keep the water temp range 10 - 85 Deg C it comes out where you're at. I think the nearest standard size EV is 18 litres, still physically challenging to find somewhere to fit it!

Apparently the FloJet is adjustable so it's beginning to sound do-able. Cheers.

 

Edited for crap syntax.

Edited November 13, 2008 by Denis R

[RobinJ]

According to this thread I should lose about 4 litres of water through the PRV when they are heated. But I don't. I lose maybe a cup full. So where is it going? Expanding the tanks?

Thats the amount of expansion you need to accomodate to stop the PRV operating everytime you heat it up!

Without an EV it will depend on the pressure of pump PRV and capacity of the calorifier as to how much water will actually escape.

A bit like a combi, you can put the pressure up to 2 bar, go and bleed a radiator (get out a cupful if you are lucky) and the pressures back down to sod all (mind you thats fitted with an EV too)!

You will need to accomodate about 5l with a total of 120l, so depending on your pump cut out pressure and PRV pressure, your probably looking minimum 10l.

[Guest TerryL]

refer to post 218.

 

 

 

Also note that the use of absolute pressure is nothing to do with whether the atmosphere is pushing down on anything.

 

As I told you before, a fifteen-year-old knows that you always use absolute pressure for Boyle's Law. You just continue to demonstrate your ignorance. You should be embarrassed.

 

I suggest you go to night school and do basic physics again, and don't bother to add anything to this thread until the dawning of realisation hits you in the gob.

He told me he was a maths and physics "major" whatever that is! You have to wonder about his other claims as well!

[smileypete]

But I am now going to fit an EV as I'm not convinced all that pressure is going to be good for the calorifiers.

 

What size do I need

 

See post #179 and #274:

 

http://www.canalworld.net/forums/index.php...st&p=295604

http://www.canalworld.net/forums/index.php...st&p=297017

 

cheers,

Pete.

[Gibbo]

See post #179 and #274:

 

http://www.canalworld.net/forums/index.php...st&p=295604

http://www.canalworld.net/forums/index.php...st&p=297017

 

cheers,

Pete.

 

Aye, but I still haven't seen any agreement!

 

I think I'll go overboard and use 2 X 8 litres.

 

Gibbo

[chris w]

refer to post 218.

 

 

 

Also note that the use of absolute pressure is nothing to do with whether the atmosphere is pushing down on anything.

 

As I told you before, a fifteen-year-old knows that you always use absolute pressure for Boyle's Law. You just continue to demonstrate your ignorance. You should be embarrassed.

 

I suggest you go to night school and do basic physics again, and don't bother to add anything to this thread until the dawning of realisation hits you in the gob.

Well I still haven't seen the maths out of you to explain the situation with my set up. Anyone can read Google which is all you have done.

 

I gave you an opportunity in my last post and instead you just throw insults. If you can explain it mathematically, then I would respect your argument. At the moment you are a clone of TerryLicense(sic) who just makes statements but can't justify them mathematically.

 

To repeat:

 

EV = 8 litres, precharged to 35 psi

Calorifier = 60 litres heated to 85degC (so water expansion ~2 litres)

NRV fitted on inlet to Calorifier

WP cut-out pressure = 30psi

 

Go ahead, do the maths using those figures. Your previous calculations, but substituting my pressures etc, would indicate the PRV should p*ss water out.

 

However, the PRV does not drip a drop, but did, before the EV was fitted.

[smileypete]

Aye, but I still haven't seen any agreement!

Well, you asked and I replied.

 

I'm intrigued to see how great university educated minds come unstuck on a basic plumbing problem.

 

I could post further info but you still wouldn't quite be convinced, would you...?

 

cheers,

Pete.

Edited November 13, 2008 by smileypete

[ChrisPy]

I gave you an opportunity in my last post and instead you just throw insults. If you can explain it mathematically, then I would respect your argument. At the moment you are a clone of TerryLicense(sic) who just makes statements but can't justify them mathematically.

I justified a typical case way back. If, as a chartered engineer or whatever, you can't apply the calculations to your own case then I certainly can't be bothered to do it for you.

Especially as you labelled me a prat.

Am I bothered ? ..............

[Gibbo]

Well, you asked and I replied.

 

I'm intrigued to see how great university educated minds come unstuck on a basic plumbing problem.

 

I could post further info but you still wouldn't quite be convinced, would you...?

 

cheers,

Pete.

 

Where have I said anything to imply that? Or have you, yet again, attributed something someone else has said to me?

 

My contribution to this thread has been not much more than "where does all that water go?" and "I'll put 2 X 8 litres in".

 

Gibbo

[chris w]

I justified a typical case way back. If, as a chartered engineer or whatever, you can't apply the calculations to your own case then I certainly can't be bothered to do it for you.

Especially as you labelled me a prat.

Am I bothered ? ..............

I know you did. I did apply your example (and gave the figures in an earlier post for my pressures) and showed it didn't work for my pressure settings. It indicates that my calorifier pressure will be well-above the PRV blow-off pressure, which it can't be in reality because the PRV doesn't leak a drop.

 

I suspect that you have also secretly redone the calculation yourself and find yourself in the invidious position of agreeing with me but not understanding why. Otherwise, you would not want to appear such a fraud as well as a prat.

 

Here's YOUR calculation again with MY pressure settings.........

 

EV = 8 litres, precharged to 35 psig (50psia)

Calorifier = 60 litres heated to 85degC (so water expansion ~2 litres)

NRV fitted on inlet to Calorifier

WP cut-out pressure = 30psig (45psia)

PRV setting 42psig (57psia)

 

After heating the calorifier water, the EV volume reduces by 2 litres rom 8 litres to 6 litres, so its pressure increases to 8 x 50 / 6 = 67 psia (52psig)

 

This is way above the PRV pressure (either psig or psia) so how come the PRV is not blowing? Without the EV, the PRV dribbles everytime the water is heated.

[smileypete]

Where have I said anything to imply that?

where you said...

 

Aye, but I still haven't seen any agreement!

The only one not agreeing with the calculator AFAICT is Chris W

 

Do you agree with it? If so, good! If not...

 

cheers,

Pete.

Edited November 13, 2008 by smileypete