Poor pressure on a megaflo??

[ChrisR]

Because of the business of whether the pressures are adiabatic or not.

When you set your precharge to 0.5 bar, the other end of the vessel is open to air. So the precharge is 1.5 bar absolute.

The mains pressure is, again, measured relative to 1 atmosphere.
So the absolute mains pressure in your example is 2.5 bar.

SO the air bag squishes to 1.5/2.5 of what it was.
0.6 x 85 is 51. 85-51 is 34.

 

[Agile]

I dont agree.

The pressure relative to the atmosphere is only 0.5 bar when its charged and so it will squash to 0.333 of the original volume.

Tony

 

[ChrisR]

You're perhaps forgetting that the vessel when full of air (think of it when the water has just made it wet..) isn't in contact with the atmosphere any more. It's sitting there at an absolute pressure of 1.5 bar.

If you pump up a balloon in free air and take it to outer space it gets bigger!

I repeat, the mains pressure, measured on a gauge is 1.5 bar, but its abs p is 2.5 bar.
Think of it as a column of water 15metres high - but don't forget it has the atmosphere pressing down on the top

 

[Agile]

I will agree with you as thats probabably correct.

Lets use your figures of absolute pressures of water at 2.5 bar and pressure charge air at 1.5 Bar.

The volume of static water in the 85 li vessel becomes 51 litres. That does not seem to have been agreed by some of the other contributors!

Tony

 

[Crystal]

Agile

Same issue. From another forum (it is the same Agile isn't it ?).

I once checked the working gas pressure at an Indian Takeaway ( for free ) and it was only 14 mB whereas it should have been 21 mB.

Quite apart from any safety issues the Owner was paying 40% extra for gas he was not using.

Tony

I didn't comment at the time as I am not a member.
More like .7% ?

 

[ChrisR]

The volume of static water in the 85 li vessel becomes 51 litres. That does not seem to have been agreed by some of the other contributors!

85 - 51 gives the 34 litres CH quoted.

Even the best people in the world forget about absolute versus relative pressures .

Crystal:
Re the gas pressure thing. I stick my hand up as one who did exactly that, getting the gas cost thing severely wrong

There ARE issues around the way the gas meter works, but the gas pressure gauge measures pressure relative to the atmosphere.
So we're talking about 1.014 bar versus 1.021 bar for the stuff in the cubic metre which is being paid for. A difference of 0.690%.
Yes to the next question, there IS an influence on how much you pay for your gas, by your altitude!

 

[Crystal]

Not raising a gas issue here. Just thought it illustrates a point with respect to the pressure vessel issue. I think it a very good and clear example of the counter-intuitive.

 

[Agile]

We can all be wrong sometimes particularly when we can mostly ignore the atmospheric pressure all around us until we go on an Everest climbing holiday where five percent of those who go dont come back ( alive ).

I well remember that posting and I knew that it was stated wrongly and was waiting for everyone to comment but not a single person did ! As Crystal says its counter intuitive. It reads as if its correct! Get gas at 14 mB instead of 21 mB and the immediate thought is that you are only getting 14/21 of the amount that you are paying for.

The actual fallacy in that case is that at a lower burner pressure the gas comes out slower so you dont use up as much and dont pay for such a large volume of gas.

Tony

 

[ChrisR]

I well remember that posting and I knew that it was stated wrongly and was waiting for everyone to comment but not a single person did !

I remember agile posting similar remarks and I also remember his defending strongly when questioned about it. So this would appear to be a covering-up exercise. If I could be bothered I'd find the link, but we must excuse an old codger with a "failing memory"...

 

[chrishutt]

Using Precharge = lowest dynamic pressure P,
Vessel volume V and
Static Mains pressure M, (all pressures as measured on gauge)
Stored volume is given by V(1 - (P+1)/(M+1))

Thanks for that ChrisR. That's essentially the arithmetic I was doing but I hadn't yet reduced it to a nice, neat formula. Using that it's much clearer how any unknown quantity can be calculated from other known values.

I'm relieved that I thought of the absolute versus gauge pressure thing in the first place. If Tony had read my specimen example post he wouldn't have fallen into the trap of using gauge pressures since I went to some trouble to make the distinction plain, having very nearly made the same error myself.

When I started to think about Boyle's Law I realised (as one does) that a pressure of zero would require an infinite volume to produce a finite number constant, so gauge pressures obviously wouldn't do! A bit of Googling (and logic) then confirmed that absolute pressures must apply.

I found this site quite useful - http://www.arca53.dsl.pipex.com/. Click on Science and then go down to Fluids and click on Gas Laws.

 

[toasty]

Interesting topic chaps, keep it up.

I personally can see how an oversized expansion vessel could improve matters flow wise if incoming pressure is limited - but (and it is a big but) you would need to use a massive vessel at which point the costs would become prohibitive, not to mention the space required to house the vessel.

For me the maths just don't add up (and this isn't a dig at anyone, I'm just an interested bystander)

Take this example, the guy who originally posted has 1.5bar and 17litres/m
I would say he's actually looking for 30litres per minute, so the vessel must surely have to supply the additional 13litres per minute.

A nice long shower is say 15mins, so that it approx 200litres that the vessel must provide (13litres x 15), presumably then a 250 ish litre vessel would be required (the rubber bag must take up some room) and I'd have to accept that following a 15min shower the vessel would take about 10-15mins to refill itself.
Plus I'd have to find a space to fit something about as big as the megaflow it was feeding.

Roughly speaking expansion vessels seem to be about £1 per litre so it would cost £300, plus fitting and of course would probably last 5-10years before needing replacement (I assume that the rubber bag in them can't be repaired)

So as I say, I think it will work to increase flow but only for very large expansion vessels.

Oh and of course there is no way a vessel can improve his pressure, he's surely stuck with that at 1.5bar.
I'm 100% sure short of pumping there is nothing he can do about that.

 

[chrishutt]

Take this example, the guy who originally posted has 1.5bar and 17litres/m
I would say he's actually looking for 30litres per minute, so the vessel must surely have to supply the additional 13litres per minute.

Toasty, you haven't been paying attention, have you? Write out 50 times "I must remember dynamic pressure".

The OP, we are told, has a static (mains) pressure of 1.5 bar and a maximum flow rate of 17 litres/minute. But he doesn't tell us the dynamic pressure, i.e. the pressure when the 17 litres is flowing. This will be much lower due to the pressure drop on his long, undersized supply pipe from the mains. Depending on where the pressure measurement is taken, it might be around 0.5 bar. It's this pressure, as well as the flow rate, that will be boosted by an accumulator.

Using the formula kindly supplied by ChrisR, we can work out what an accumulator of a given size would achieve on those figures. For example a 200 litre accumulator would store 80 litres of water at 1.5 bar. However this would not be released to the tap in a steady stream as you seem to think.

Initially the tap drawing say 30 litres/minute would take most of its water from the accumulator. As the pressure drops and the flow decreases it would take more form the mains and less form the accumulator. The various pressures and flow rates are constantly changing so no simple arithmetic - more like calculus I should think!

In that case the 80 litre boost would be used up in a few minutes, but if we assume a more modest flow rate (say 20 litres/min) the boost could last much longer - perhaps enough to cover a 15 minute shower.

 

[toasty]

Errm...

Fair point about the tricky maths, you are right, we are dealing with differential equations here, but I don't think we need to go into that right now.

Good point about the water being used up quickly, although I suspect a shower would actually restrict this to a sensible level especially at 1.5bar.

My point still stands though (I think) in that a small accumulator is not really going to be much use, it will empty too quickly, and as you say a 200litre accumulator will only hold 80 at 1.5bar so we'd need (for my example) a 500 litre vessel for 200 litres of usable water.

So, no Chris, I have to disagree with you, I have been paying attention, but you don't get something for nothing, sure the dynamic pressure will be better than it was, but it won't be better than the static pressure - that's what I was trying to say. Badly probably!

Thanks for the response too, nice to see an interesting topic from time to time

 

[ChrisR]

The maths for the discharge of the vessel isn't too fearsome, it's basically logs, exponential decays etc. I know I could do them ok once. But rather a long time ago I have to admit.

Back at the 85 litre vessel:

Simplifying rather, the storage at 1.5 bar is 34l, and at 1 bar 21l. Which means that if you had constant-flow taps and nothing coming in from the mains, you could have say 10 litres per minute at above 1 bar (double the previous dynamic pressure) , for 1.3 minutes, from the accumulator alone.

If the mains were 3 bar, it works out to about 3.4 minutes with the same assumptions.
In practice the flow would be higher to start with, but as it dropped the mains would be providing water all the time too, so you get (a lot) longer. Say double, 7 minutes. Wow.
Say 255 litre pressure vessel, 21 minutes – getting better. I’m sure GAH could furnish tables, but you really need to know the resistance of the inlet and outlet pipes, or just put huge tanks in. (Could measure “Time Constants” once a tank was in for a trial, and work from there).

They do smooth out steps in water flow when extra taps are turned on too, which is a benefit for a flat..

 

[doitall]

So is it worth the outlay chris

Thank you for your email, and the interest in our product ,but our systems are designed to increase water volume but not pressure, so I can increase your flow rate to the desired volume you want but not the pressure. However in the next 4-6 weeks we will have a mains on-line pump that can increase pressure to 3.4-4 bar that will then deliver 12 litres a minute ( water board requirements) so this pump linked to one or two accumulators would work, we normally sell the accumulators as a package with an unvented cylinder, but I guess from your comments that you have the hot water cylinders in place.

I would recommend two model 500 accumulators which hold approximately 250 litres of water in each + the pump, I can give you the retail cost of the accumulators but the pump has not been released as yet

500 model Coldstream Accumulator 905.00 less merchant terms each, available from all leading merchants
estimate for the pump would be 600.00 net trade price.



Mains pressure was stated as 1.5bar, and the drawoff was 40 litres per minute 20/20 hot and cold over a ten minute time period.