Poll: Heating Concepts

[Harry Bloomfield]

This and this I would not expect to heat in the same way,

Why? Given enough time, all of the water above the bottom of the element, would IMHO, gain a similar level of temperature. Your wider cylinder, would simply take longer to get there.

 

[JohnW2]

John - it is so very simple, and obvious....

Harry - I do think that you should reconsider your use of the word "obvious". You're throwing it around all the time but, as I've said, although I don't regard myself as being particularly unintelligent and/or particularly lacking in knowledge about Physics, I personally find some of your assertions to be anything but 'obvious'.

In a normal cylinder, using a normal immersion heater, the heater will only heat the water, which is above the element, and perhaps slightly below - all due to the heat convection currents set up by the element, which begin soon after the element is switched on.

I totally agree, and it's what I've been saying all along. That is the question being asked by my Poll question and what you've just written appears to be a clear indication of a "No" answer from you, so I don't really understand why you appear to be 'frightened'/reticent to register such a vote

Now, imagine a vertical element ...

For a start, as I've said before, during this discussion I have never been thinking or writing about a vertical element. However ...

, contained within a narrow tube, only open to the cylinder at the top, and the bottom. The convection current then is rather different. The heated water flow has to exit at the top of the tube, and has to be drawn in at the bottom. That process will continue, until the water drawn in at the bottom, causes the stat to open, because the stat is measuring the temperature of the water going past it. Thus, the system has the ability to heat water in the cylinder to a lower depth, than the end of the element, almost down the cold inlet.

Do you mean "the cold inlet" of your tube (aka Willis heater housing)? If so, for a start, the dimensions of Willis heater sis such that I imagine the heater element must get down pretty close to its bottom.
However, if your tube were long, with the heater located, say, in the middle, then I would not expect appreciable heating of water in the tube below the element (since warmer/less dense water does not 'fall'). Let's face it, your 'tube' is merely a narrower version of the cylinder in which an immersion would be located

Now imagine that same tube, mounted alongside the main HW cylinder, with a pipe connecting it top and bottom - we get the very same flow of hot in the top, cold out the bottom, from the HW cylinder - that is a Willis. ...

As above, I don't really see that your 'tube' makes any appreciable difference to anything, whether within or outside of the cylinder. In fact, as I've just written, the cylinder is really just a 'wide tube'.

... Again the stat will remain closed, until it begins drawing hot water from the lower pipe, once the level of HW has 'spread' down that far.

This is where I have a problem. Per what you agreed at the start (above) I would not expect the water in your tube below the element to get appreciably heated (other than slightly/slowly, by conduction). Per EFLI's 'levelling' concept, that would mean that the heated water in the main cylinder would also be more-or-less restricted to the region above the Willis element. There would therefore not be any significantly heated water at the bottom of the main cylinder to be drawn into the bottom of the Willis (to 'open the thermostat'). In that situation, what I suggest would probably happen is that the water above the level of the Willis heater would get hotter and hotter until it eventually boiled, with that lower down still essentially 'cold' (until it eventually got heated by conduction).

That leads to a 'thought experiment', about which I suspect we will disagree. For the reasons we've discussed, I think that if one had a heating element (with NO thermostat) fairly high up in a vessel full of cold water, the water above the element would get hotter and hotter, and eventually boil, even whilst there was still much colder water below the element (and certainly near the bottom of the vessel) - as always, until water below the element eventually got slowly warmed by conduction. Do I take it that you disagree with me - and, if so, why?

A major difference, is that a normal immersion heater installation, when switched on, gradually heats the entire water contents, which is above the lower tip of the element - whereas, the Willis process does not quite so much, mix cold with its hot. The result is a gradually increasing store of HW, better defined, at the top of the HW cylinder.

Once I got my head around the basic concept of the Willis heater, I have repeatedly agreed that it will offer some 'benefit' (over an internal immersion), since heated water does not have to pass through cooler water whilst travelling up to the top of the cylinder. However, I'm not convinced that that 'benefit' will be particularly marked (since the heated water has to pass through cooler water only once), and hardly an issue at all when comparing a Willis with a 'high-up' horizontal immersion.

 

[JohnW2]

Why? Given enough time, all of the water above the bottom of the element, would IMHO, gain a similar level of temperature. Your wider cylinder, would simply take longer to get there.

Quite so - I don't see why whether a wide cylinder, a narrower cylinder or a very narrow cylinder ('pipe') makes any appreciable different to the concepts.

 

[ericmark]

Why?

I was trying to point out how a room is so much different to a cylinder. I am sure we all at school put some solder around ice in a test tube to hold ice at the bottom of the tube, and boiler water in the top of the test tube, but that is long and thin.

As the tank becomes squatter, then the whole situation changes, we have all bathed in the sea and realised how much colder the sea gets to swim in as we get further from the shore, and my own measurements in my living room have shown how the temperature in the same room can vary a lot as it does not mix. Not top to bottom but side to side at the same height.

If we heat a radiator to 70°C or more, it will start a thermal movement and the air circulates around the room, but heat a radiator to just 40°C, and it will often not start that movement. So if we don't get radiators as hot, then we need to include some fans to move the air. We are seeing more and more of these for sale, as the radiators with TRV control are not running as hot, we have always had these fan assisted radiators, but they need a good flow, and with condensing boilers, the return hot water turns the boiler output down, they would be better plumbed in series rather than parallel when fan speed controls how much heat into each room. With the old non-condensing boilers we could mix radiator types, but the new boilers monitor the return water temperature.

I use to for stress relief and pre-heating on welds, and if you got a draft started in a pipe, "the chimney effect" it could cause rapid cooling of the pipe, so we had to be careful to block the ends up to stop the draft, I have had on a 36" pipe the cover come off, and trying to get it back on, was a real problem, didn't make that mistake twice.

 

[Harry Bloomfield]

As above, I don't really see that your 'tube' makes any appreciable difference to anything, whether within or outside of the cylinder. In fact, as I've just written, the cylinder is really just a 'wide tube'.

Then you are wrong! The element will act like a pump, drawing cold in at the bottom, hot out the top, by convection currents. Which is the basic principle to the Willis system.

It makes little difference to the Willis system, how long the element is - the heated water will still be 'pumped' out of the top, cold drawn in at the bottom, and will repeat it - it is blatantly obvious to me.

If you doubt thee physics, the principle John, then try this.....

A kettle, or even a pan filled with water. Find a funnel, of similar size to the base. Placed it in the pan, such that it is fully covered by the water. You will find that the heated water rapidly flows out the top of the funnel. Depending on how gently the heat is applied, you may get a level of heated water, appear at the top surface.

As the tank becomes squatter, then the whole situation changes, we have all bathed in the sea and realised how much colder the sea gets to swim in as we get further from the shore, and my own measurements in my living room have shown how the temperature in the same room can vary a lot as it does not mix.

That, is because where the sea is shallow, it has less depth in for the mixing to take place from wave action, less cold water to mix with.

Not top to bottom but side to side at the same height.

That variation, is due to uneven mixing of the air, heat sources, and perhaps solar gain. It is the reason why they suggest that even in cold weather, that a fan can help, especially a ceiling fan, pushing the warmer air at the ceiling, down.

 

[ericmark]

That variation, is due to uneven mixing of the air, heat sources, and perhaps solar gain. It is the reason why they suggest that even in cold weather, that a fan can help, especially a ceiling fan, pushing the warmer air at the ceiling, down.

I have in the past wanted the variation, having the air around the windows cold, is not a problem unless sitting at the window, my first house had picture windows front and back, and the hot air central heating pushed the hot air to the cold single glazed windows, latter houses had radiators and circulating water, and the air at the windows remained cold.

That, is because where the sea is shallow, it has less depth in for the mixing to take place from wave action, less cold water to mix with.

That was my point.

 

[Harry Bloomfield]

I use to for stress relief and pre-heating on welds, and if you got a draft started in a pipe, "the chimney effect" it could cause rapid cooling of the pipe, so we had to be careful to block the ends up to stop the draft, I have had on a 36" pipe the cover come off, and trying to get it back on, was a real problem, didn't make that mistake twice.

Good point! That is basically how a chimney works too. Fire at the base, warm air rises, takes the products of combustion up and out.

 

[ebee]

I can’t help thinking that’s perhaps everybody thought it obvious that an apple would fall down from a tree quite naturally all on its own unless it was physically attached to the tree and that was the only thinking keeping it up there until a bloke called Newton started to wonder what actually pulled that apple downwards rather than leaving it happily floating in the air . Folk probably thought him to be crackers until they realised that he was not . I think that each one of us could have been quite happily on either side of that particular stance all of those years ago . So I am glad this discussion is progressing from each direction and each opposing view back and forth is an absolute delight.
Will any of us change our minds either slightly or significantly? I do not know. Yet it certainly is interesting whatever outcome

 

[Harry Bloomfield]

I can’t help thinking that’s perhaps everybody thought it obvious that an apple would fall down from a tree quite naturally all on its own unless it was physically attached to the tree and that was the only thinking keeping it up there until a bloke called Newton started to wonder what actually pulled that apple downwards rather than leaving it happily floating in the air . Folk probably thought him to be crackers until they realised that he was not .

Which reminds of the lads toddler, clumsier days - I taught him 'What makes things fall down', gravity was the taught answer. 'Who discovered gravity', Newton was the taught answer - until one day I told him no, 'Terry knocks things down'.

 

[JohnW2]

Then you are wrong! The element will act like a pump, drawing cold in at the bottom, hot out the top, by convection currents. Which is the basic principle to the Willis system. ....

I do not yet understand the alleged mechanism of 'drawing cold in at the bottom'.

... It makes little difference to the Willis system, how long the element is - the heated water will still be 'pumped' out of the top, cold drawn in at the bottom, and will repeat it - it is blatantly obvious to me.

Fair enough, but, as I recently wrote, something is only "obvious" to someone if they believe it to be "obvious" - and, of course, any individual's 'beliefs' may be incorrect!

You are sounding increasingly like that maths teacher of mine! Maybe someone else here may be a better teacher than you, in that they can help me understand rather than just assert their own views?

I would like to think that, in the course of this discussion, I have never made 'unsupported assertions'. Rather, I have been explaining 'how I currently see things', explaining what I don't understand (and why) and asking countless questions, regularly intersperced with admissions that I may well be 'missing something'.

On the basis of your assertions, do I take it that you agree with Sunray that a Willis heater will 'work' ('as intended') even if it is located well above the top of the main cylinder?

If you doubt thee physics, the principle John, then try this..... A kettle, or even a pan filled with water. Find a funnel, of similar size to the base. Placed it in the pan, such that it is fully covered by the water. You will find that the heated water rapidly flows out the top of the funnel. Depending on how gently the heat is applied, you may get a level of heated water, appear at the top surface.

I'm not sure that I will be able to find a funnel of an appropriate size. However, I have to say that if I'm understanding correctly what you are suggesting would happen, I would regard that as extremely 'strange'!

 

[JohnW2]

I can’t help thinking that’s perhaps everybody thought it obvious that an apple would fall down from a tree quite naturally all on its own unless it was physically attached to the tree and that was the only thinking keeping it up there until a bloke called Newton started to wonder what actually pulled that apple downwards rather than leaving it happily floating in the air .

Quite so. "Obvious" is very much in the eye of the beholder (sometimes 'most beholders') at a particular point in time, is not necessarily "obvious" to everyone and does not necessarily remain "obvious" with the passage of time.

 

[Harry Bloomfield]

Fair enough, but, as I recently wrote, something is only "obvious" to someone if they believe it to be "obvious" - and, of course, any individual's 'beliefs' may be incorrect!

If you accept that warm air, or water, naturally rises then there is nothing more to discus.

You are sounding increasingly like that maths teacher of mine! Maybe someone else here may be a better teacher than you, in that they can help me understand rather than just assert their own views?

Because the theory, and physics is so very simple, and cannot be questioned. The Willis system works, in many thousands of applications, so how can it not work. You seem to be suggesting it cannot possibly not work, and exactly the way I describe.

On the basis of your assertions, do I take it that you agree with Sunray that a Willis heater will 'work' ('as intended') even if it is located well above the top of the main cylinder?

As I have said before, in regard to that - yes I do believe it will work, but I am less certain of how well. The flow down to the main cylinder, is dependent on the water cooling somewhat, in the long hot pipe, feeding the main cylinder, enough to draw the water down to the main cylinder.

I'm not sure that I will be able to find a funnel of an appropriate size. However, I have to say that if I'm understanding correctly what you are suggesting would happen, I would regard that as extremely 'strange'!

Nowt strange about it at all - try it. Even try a drop of food colouring on the inside of the funnel, then you will see the flow.

 

[JohnW2]

If you accept that warm air, or water, naturally rises then there is nothing more to discus.

I have to disagree, because I think there is a lot that needs to be discussed ...

... it sounds as if you are perhaps one of those people who takes the ('colloquial') phrase "hot air/water rises" far to literally. As I have repeatedly pointed out, that 'rising' does not usually result in any 'net physical movement' of air or water.

Consider a vertical glass tube with a closed bottom (e.g. a long test tube) filled with cold water. Heat a little of the water at the very bottom. The region where water is 'hotter' will rapidly 'rise' to the top of the tube, but there will be virtually no change in level of water at the top (only a tiny amount due to expansion) (i.e. no water will 'rise up') and there certainly will not (could not possibly be) any cold water 'drawn in at the bottom.

Because the theory, and physics is so very simple, and cannot be questioned.

That's silly. The amount of discussion indicates that it's not all that 'simple', and some aspects have certainly been questioned.

The Willis system works, in many thousands of applications, so how can it not work. You seem to be suggesting it cannot possibly not work, and exactly the way I describe.

Not true. I did initially wonder how it could work at all, but with the help of a couple of people here eventually understood how it was working. I therefore now have no problem in agreeing that it 'works' in the sense of allowing a small proportion of water at the top of the main cylinder to be heated without having to rise through cooler water - hence a little more efficient than an internal immersion, but probably not appreciably so in the case of a 'high up' horizontal immersion.

What I still do not understand, hence am still not convinced is true, is whether the above 'working' can result in appreciable heating of water in the main cylinder below the level of the heat source (whether the heat source be a Willis or an internal immersion),

As I have said before, in regard to that - yes I do believe it will work, but I am less certain of how well.

As above, I remain far from convinced.

Nowt strange about it at all - try it. Even try a drop of food colouring on the inside of the funnel, then you will see the flow.

Still sounds very strange to me, but I'll defer further comment until I've seen if I can find the needful to enable me to conduct the experiment!

 

[ericmark]

but there will be virtually no change in level of water at the top

I seem to remember the experiment when doing 'O' level physics. I remember the experiment as I had an expensive fountain pen, and the experiment was done with acetone, and I got it on my fingers, and melted my expensive pen. But I failed my 'O' level physis.

Also failed to pass 'A' level, but that was due to a mix-up in exam dates.

I will admit expansion due to heat, and the results did mess up my brewing. Near all hydrometers are calibrated at 20°C, and most come with a chart to correct the reading at a different temperature. But what I did not realise was the chart is for a hydrometer is dependent on what the hydrometer is made from, so a plastic one is different to glass, and pyrex glass is different to standard glass.

It seems the reason why pyrex can stand heat, is it does not expand as much as normal glass. Pyrex is a real problem with recycling, as if put in with standard glass, it does not mix, and has to be removed from the batch, this is why recycling will not accept broken glass, as they can't identify any pyrex.

So a hydrometer made of pyrex does not really need any correction chart.

So heat water in a standard glass test tube may not show much expansion as the tube is also expanding, but do the same experiment with a pyrex test tube and the results will be very different.

I note even making a cup of coffee, pour in boiling water to near the top, and there is still room for milk, as the cup expands and the liquid level drops.

 

[JohnW2]

.... But I failed my 'O' level physis. ... Also failed to pass 'A' level, but that was due to a mix-up in exam dates.

I have to admit that, albeit a very long time ago, my education/'qualifications' in Physics did quite a lot better than that

.... Pyrex is a real problem with recycling, as if put in with standard glass, it does not mix, and has to be removed from the batch, this is why recycling will not accept broken glass, as they can't identify any pyrex.

That always makes me laugh! Have you never heard the loud sound of smashing glass when they tip the contents of bins into recycling trucks? I'm sure that some people will put Pyrex into recycling bins, so, whether they like it or not, by the time the truck gets back to its depot, it must contain a lot of bits of smashed up borosilicate glass in amongst bits of 'standard glass'

I note even making a cup of coffee, pour in boiling water to near the top, and there is still room for milk, as the cup expands and the liquid level drops.

I think you probably have a good imagination (or super-human eyesight!), because I would think that the amounts of expansion we're talking about are probably pretty miniscule