Author Topic: Talking Thermodynamics  (Read 154538 times)

Offline Ye-Ole Steam Dude

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Re: Talking Thermodynamics
« Reply #195 on: August 02, 2017, 01:38:16 AM »
Thanks Bob for giving us this information.

Thomas

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #196 on: August 02, 2017, 02:15:30 AM »
Hi all thanks for the info ,it all makes sense actually. I don't know why the HP cylinder looks different with no cylinder head bolts though. there is a wonderfull model made by someone in Germany on the web Utube that is really superb,

Offline MJM460

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Re: Talking Thermodynamics
« Reply #197 on: August 02, 2017, 11:41:21 AM »
More on thermal expansion and triple expansion engines.

Thanks Willy for those great pictures, they make the arrangement very clear.  Thanks Dan for coming in.  From the pictures we can see that not only are the cylinders separate as you have noted, but also the supports, rather like three separate engines with a common crankshaft.  Because the lp cylinders each expand the same amount, the pipe can be attached to both as you have observed.  Thanks also to Thomas for the observations on the angles.  Maryak, you must have worked on those engines, thank you for all the detail.  The turbine makes great sense as it is so much more suitable than reciprocating engines for the very large volume of low pressure steam before it is condensed.

I think balancing is probably quite complex for these engines.  There is not only the the normal balancing of a reciprocating masses, and the inertia forces for such large pistons, also balancing the power strokes and also the steam distribution.  Perhaps you will be able to explain it to us all.  I can see that the steam exhaust for the hp cylinder must be taken in by the ip cylinder, but I quickly get lost extending it to the lp stage, particularly when the two cylinders are not in line or even at 180 degrees to each other.

In terms of the thermodynamics, this is a great example of the problem with trying to extract that expansive power of the steam, the volume gets huge.  Adding a turbine is a great way to expand a large volume of steam to the lowest possible pressure, which is of course limited by the temperature of the sea water available for condensing.  Even the course into northern waters would have contributed to better efficiency due to cooler water available for condensing.  Or would have done, if it was not for a small miscalculation of the risk associated with icebergs.  Obviously the advantage of the turbine was enough to justify the cost of the complexity, telling us something about the economics of shipping.  Pity the accountants never had to swing those valves, even once would probably justify the installation of power operators for that job.

A great introduction to my intended return to the topic of condensing, but I would like to know more about that valve timing and influence of the volume of the steam chests and crossover piping for that triple expansion engine.

Apologies for the walls of text.  I know this thread needs pictures, so just a little update on the adventures in the long paddock.  The long paddock seems to have been joined by the long rail.  The train in the picture was as best I could measure it with the car speedo, around 1800 metres long, about 1.1 miles.  It was stationary, took 70 seconds to pass it at 92 kph.  How about one of you people who like repetitive work modelling this one?  Simple enough, only four electric locomotives, two at the front, one in the middle and one at the tail end, and a few identical cars in between.  Might have to scale down the voltage for your club track though.  The signs only said high voltage.  I don't know what the locomotive power rating is, but my compressors with several megawatt size motors were 11000 V.  Perhaps the trains accept higher currents.  Interesting that each car was marked with direction of travel, I presume something to do with the unloading method, but also implies that it is turned around by a large loop, or perhaps the direction is only relevant to the fully loaded cars.

Thanks for looking in

MJM460
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Offline Ye-Ole Steam Dude

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Re: Talking Thermodynamics
« Reply #198 on: August 02, 2017, 01:19:12 PM »
Triple Expansion Steam Engines

1. Cylinder Volumes:
HP = 1.0
IP = 2.6
LP = 7.0

To keep the LP Cylinder a manageable diameter it was split into 2 cylinders at 3.5 x HP volume giving a 4 cylinder triple expansion engine normally arranged from front to back LP,HP,IP,LP.

With this arrangement balance was achieved by setting the cranks at:
Between Forward LP and HP 1660
Between Forward LP and IP 2700
Between Forward and Rear LPs 700
In addition suitable counterweights were added to the crankshaft.

With Titanic and Olympus, not shown is a turbine which utilised the LP exhaust to drive the centre propeller. For manoeuvering the exhaust was switched over from this turbine and sent directly to the condenser. This made emergency reversing/stopping difficult as until this exhaust was removed, the turbine continued to move the centre propeller ahead. Transfer was accomplished by some big hand operated valves!

HTH
Regards
Bob

G' day Maryak,

I am a bit confused with order of arrangement for each cylinder, in the attached first photo Cylinder No.-1 is not at the same timing as Cylinder No.-4. Your numbers show each LP ( 1 and 4 ) are each at 70 degrees. In the photo, No.-4 appears to be close to 160 degrees ( or so ) opposite from No.-1. It looks like No.1 and No.3 are the same. Am I looking at this incorrectly?

Thank you,
Thomas

Offline MJM460

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Re: Talking Thermodynamics
« Reply #199 on: August 03, 2017, 12:59:22 PM »
More on Titanic engines

Hi Thomas, I can see what you mean about the angles, though I think I have interpreted them slightly differently from what you have described.  The penny dropped when I read Maryak's post, that the angles are only nominally about balance, but the odd angles come from the need to progressively expand the steam a bit in the hp cylinder, then transfer it to the ip cylinder and expand it a bit more, then finally transfer it to the lp cylinders where we not only expand it a bit more, but also provide as steady a flow as possible to the turbine which is a continuous steady flow machine.  There are compromises along the way, especially if the cut off varies as speed is reduced to cruising speed from maximum.  I will leave it to Maryak to add a bit more explanation, I have attached a little sketch showing my interpretation of his figures.  You will note I have drawn it with the hp cylinder at top dead centre.  If you think about the hp cylinder as the engine rotates, you can see that steam admission to cut off follows much the same sort of pulsation says a single cylinder engine as you would expect, with steam into the top of the cylinder for the first half rotation, and into the bottom of the cylinder for the second half.

Then if you look at the lp cylinders, the rear lp which I have labelled RLP, is exhausting at max volume, while the FLP so just starting to exhaust at minimum flow.  The combination gives a fairly steady flow to the turbine for the first half revolution.  Similarly the bottom end of the lp cylinders gives reasonable steady flow for the second half.  The angular difference between them seems to give quite good flow for the whole revolution, another reason apart from sheer size for having two lp cylinders.

So it remains to see if the ip cylinder properly accepts exhaust from the hp cylinder and exhausts it to the lp cylinders in turn.  Its a bit mind bending, but if you turn the drawing around, or redraw it so the ip cylinder is at top dead centre, and follow the similar thought process, I think you will find it works.  The complication is when the cut off varies, when the process might be a bit less smooth, and the volume of the steam crossover piping and steam chests become important.  I am hoping that Maryak will shed some light on that as well.

I am suspicious that the artist might have taken some liberties for artistic presentation, not expecting anyone to analyse it in this much detail, or maybe the artists angles will also work, I just don't know.

But my area is thermodynamics and I look for what thermodynamics tells us that makes it worthwhile building such a complex arrangement.  Why not just a three cylinder triple to provide more power than a simple single or twin cylinder engine?

Basically there are two ways that expansion to a low pressure (which can only be achieved by condensing) provides more power output from the engine.  The first we have covered earlier, the force on the piston is due to the difference in pressure between the top side and the bottom side of the piston.  More differential pressure gives more work output.  Even on a simple oscillating engine.  But then, if the valve gear can cut off admission, the steam trapped in the cylinder is able to do more work by continuing to expand to a lower pressure, to provide even more work output, providing this lower pressure can be exhausted to a lower pressure in the condenser.  Theory might let us expand to a very low pressure, and get even more work out, but as you can see in the Titanic engine example, the volume becomes huge.  Now the arrangement with the turbine following the low pressure stage is very clever, because a turbine is easily able to handle a much larger volume of steam, and expand it to a lower pressure in a reasonably sized machine.  With two large triple expansion engines exhausting to a turbine, the scale is clearly such that economics support installing the turbine.  Otherwise they would not have done it.

Of course, condensing also adds further complexity.  We need a heat exchanger.  We have already see that the heat rejected in condensing is close to the same as the heat added in the boiler for evaporation.  The heat transfer equation is the same, Q=U x A x dT.

Now the heat transfer coefficient for condensing is not as high as for boiling, but it is still quite high.   The temperature difference is the real problem.  Even in winter the water temperature is always above zero, and for most of us, at the club pond, it will be quite a bit higher.  Winter is building time, rather than sailing time.  As the water takes up heat, it's temperature rises.  It can't get above the steam inlet temperature, and realistically, even in full scale industrial condensers, it will at best get to a maximum 10 - 20 degrees below the steam temperature, and that requires a lot of tubes.  You can see it if you find a ships condenser picture.  We can probably anticipate an LMTD less than 50 degrees.  Compare that with the firing temperature in a boiler.  I really don't know the fire temperature, but I am guessing that the temperature difference is more than 200 degrees, remembering that the flue gasses are much lower than the fire temperature at the chimney end of the boiler.  So with lower heat transfer coefficient and much lower temperature difference, we need considerably more heat transfer area in the condenser than the boiler.  However, providing we use enough tubes, we can condense the steam.  And this is done on ships so that water can be reused, instead of using the considerably more expensive process of desalination of sea water for boiler feed.

Now while it is possible, I don't advocate trying to calculate the area needed for a condenser.  I would use a published design, scale it to my estimated engine steam consumption, preferably by testing, then make it as large as I can accommodate in the model.  Then I suspect my time would be more enjoyably spent building a new condenser, with the size adjusted as necessary to condense all the steam if it turns out to be insufficient.  There is actually no real problem in making the condenser too large.  You just get a nearer approach temperature, or even sub cool the water a bit.  So long as it is not too heavy, or takes too much space in your model, it will be ok.

By building a condenser, you have the possibility of increasing your engine power output by increasing the differential pressure on the piston or pistons.  Unfortunately there is another problem we have to deal with, and that will be the topic for next time, or when we deal with some of the interesting little side tracks on our project.

Thanks for looking in

MJM460
« Last Edit: August 03, 2017, 01:04:16 PM by MJM460 »
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Offline Ye-Ole Steam Dude

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Re: Talking Thermodynamics
« Reply #200 on: August 03, 2017, 01:20:42 PM »
Hello MJM460,

I appreciate your explanation and drawing, this design is so interesting and I wish that I knew more about the complete process. This kind of work is very impressive when I think about the time and era it was conceived, when you factor in what tools and knowledge was available to the designers.

Enjoying following this thread and thanks again,
Thomas

Offline Ye-Ole Steam Dude

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Re: Talking Thermodynamics
« Reply #201 on: August 03, 2017, 04:55:33 PM »
I found this information and drawing on the internet and it is pretty straight forward in explaining the cylinder arrangement. Hope this will help.

Titanic’s 4-cylinder reciprocating engines were balanced on what was called the Yarrow, Schlick, and Tweedy system. The crank throws were not arranged at 90-degree intervals as one might assume. Instead, vibration was reduced by adjustment of the relative crank angles and crank sequence being used. Beginning with the HP cylinder piston at top dead center (TDC), the crank sequence and angles of the engines were: HP at TDC, then a 106° rotation for the IP to TDC, then a 100° rotation for the forward LP to TDC, then a 54° rotation for the aft LP to TDC, then a 100° rotation for the HP to return to TDC. This is the link to the website: http://www.titanicology.com/Titanica/TitanicsPrimeMover.htm
 

This crank arrangement is shown in the diagram below.

 

« Last Edit: August 03, 2017, 05:00:39 PM by Ye-Ole Steam Dude »

Offline MJM460

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Re: Talking Thermodynamics
« Reply #202 on: August 04, 2017, 12:27:03 PM »
Hi Thomas, I am glad that you are enjoying the thread, it is all about sharing knowledge, so it's great to have some interest.

That was a great find on the titanic engines.  Balance is not so easily achieved when pistons are different sizes, but I think the sequencing of valve events also has an influence on the angles between cranks.  It also appears that slightly different angles might be selected, depending on just what the designer is trying to achieve.  I am still fascinated that the two low pressure pistons with the angular displacement between the cranks are able to produce a steady enough flow to the turbine, instead of the normal zero flow at top and bottom dead centre, I had not thought of that.  But I will let Bob or others tell us more about triple expansion engines, while I return to the condenser topic.

I mentioned last time that there was more to a condenser than just removing heat from the steam.  The problem is air.  So first where does the air come from?  Two sources, each in fact quite small.  There is always some dissolved air in water.  In industry, this air is first boiled out in a vessel called a deaerator.  This is followed by a chemical oxygen scavenger.  With steel boilers air removal is particularly important as oxygen causes corrosion in the warm wet environment in the boiler.  But you can safely assume there will be some air in your model boiler feed water.  The second source comes in as soon as you are able to maintain some vacuum.  There will be some air in leakage.  The quantity is small, but it accumulates in a condenser so its presence soon becomes important.

If we remember Willy's mountain top experiences, we talked about some air in the boiler when it was sealed up.  The air and water vapour in the vapour space act independently, so the total pressure is the sum of the air pressure plus the water vapour pressure.  We had to heat the air, as well as heat the water.  Fortunately, in the boiler, the air is entrained in the steam production, so it does not accumulate and is soon near enough to zero.

In a condenser, air is a non-condensable and unless we make special provision, there is no path out, so it accumulates.  And in accumulating, the partial pressure of air increases, and hence the total pressure in the condenser increases.  Even though the steam condenses at quite a low pressure due to the cooling water temperature, the piston sees the total pressure during the exhaust stroke, so there is no advantage in the low water condensing temperature.

You can see where this is leading, prototype engines with condensers also have an air pump.  The job of the air pump is to remove the air down to the low pressure of the condensing steam, so the condenser truly does provide the low exhaust back pressure we are looking for.

In many of the prototype machines often selected for modelling, the air pump is some sort of diaphragm pump with sufficient displacement to deal with a volume of low pressure air, remembering that some of the low pressure steam will also enter the air pump with the air.  The pump then has to compress the air and steam mixture to above atmospheric pressure so it can be discharged to the exhaust stack.  An air pump is in fact an air compressor, much like a bike pump, and clearance volume is critical, unlike the water pump for which clearance volume is not so important. 

You will also recognise that the condensed water is also at low pressure compared with atmospheric pressure, so normally a water pump is used to remove the condensate.  There are even some quite ingenious designs where the air and water pumps are combined.  When you remember the purpose, and what the design is trying to achieve, these designs become a little easier to understand.

In summary, adding condensing to an engine, increases the power output due to the reduced back pressure on the piston.   We achieve this advantage even on simple engines without early cutoff and expansion.

We can condense at atmospheric pressure simply to recover the water, but there is no real power advantage to this over a simple atmospheric exhaust.  To achieve the advantage of low back pressure due to condensing, we need a condenser, a condensate pump and an air pump.

The low temperature difference inherent in condensing means we need a large heat transfer area, more than required in the boiler that produces the steam, usually provided by means of a large number of tubes.

I believe I have some questions from Willy and Paul which have not been addressed, so I will have a go at those next time.

Thanks for reading,

MJM460
The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #203 on: August 04, 2017, 02:38:53 PM »
Hi, more explanations all good ...I was wondering ,if i may enquire about the jet condensing arrangement on the Woolf compound engine i am making. The cylinders are double acting but the air pump is single acting? and i was thinking that when the exhaust steam is condensing and forming a vacuum ,is this vacuum impeding the opening of the flap valves to the air pump at various places in the beam engine cycle ? Here are a few pics and drawings of the Beeleigh Mill engine, I made the drawings showing the use of hippopotamus hide valves as used in some engines before they took the air pump apart and discovered ordinary metal to metal flap valves. Just a thought and i may be wrong , however the force produced by the Newcommen atmospheric engines was quite considerable, and could this force be calculate into horses power as they used to say.....Also the jet condenser valve is always open rather than connected to the valve events ?

Offline Dan Rowe

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Re: Talking Thermodynamics
« Reply #204 on: August 04, 2017, 02:56:59 PM »
MJM, I have to admit that I struggled with thermodynamics in school. You mentioned the engine book by K.N. Harris but I think his boiler book is more likely far better known. I think a good topic for this thread would to explain how to size a boiler to an engine using the boiler book as a starting point.

K.N. Harris was an engineer and had a lot of practical knowledge.

Dan
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Offline Maryak

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Re: Talking Thermodynamics
« Reply #205 on: August 05, 2017, 01:14:09 AM »
G' day Maryak,

I am a bit confused with order of arrangement for each cylinder, in the attached first photo Cylinder No.-1 is not at the same timing as Cylinder No.-4. Your numbers show each LP ( 1 and 4 ) are each at 70 degrees. In the photo, No.-4 appears to be close to 160 degrees ( or so ) opposite from No.-1. It looks like No.1 and No.3 are the same. Am I looking at this incorrectly?

Thank you,
Thomas

Hi Thomas, sorry to be late in responding below is a diagram which I hope explains the differences



Regards
Bob
Если вы у Тетушки были яйца, она была бы Дядюшкой

Offline MJM460

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Re: Talking Thermodynamics
« Reply #206 on: August 05, 2017, 12:35:47 PM »
Out on the plains, the brolgas are dancing

Hi Willy, Just in case I have created some confusion, I should make a small clarification on jet condensing.  I mentioned that steam industrial steam plants sometimes use a jet ejector instead of an air pump.  Much the same principle as the injectors used for boiler feed water on some locomotives.  It literally uses a steam jet and Venturi arrangement to create a low pressure so that air flows in, at which point an diverging nozzle increases the pressure enough to discharge the air to the atmosphere.  This is quite different to the jet condenser on your engine, which uses a spray of cool water directly injected into the exhaust steam in order to condense the steam.  No tubes involved.  Saves soldering in all those tubes.  Basically the latent heat in the steam heats the water by direct contact with the water spray.  The end result is dependent on the amount of water sprayed in.  You could use just enough to condense the steam and the whole mass of steam plus water ends up as saturated water at the steam condensing pressure, or you can add extra water and sub cool the whole mixture to some extent.

The actual condensing pressure depends on your air pump.  The air pump removes the mixture of air plus water vapour from the condenser space, thus lowering the pressure in that space.  It then compresses the air and vapour to a pressure high enough to discharge to the atmosphere.  In your engine, the air pump handles both the air/vapour mixture and the condensed water.  While it is handling both, it is strictly a compressor.  The difference is basically in the ratio of swept volume to clearance volume.  In a typical feed pump, and also on a dedicated condensate pump, the swept volume or displacement of the piston is relatively small, while the clearance volume, basically dependant on the valve chamber arrangement does not really matter and can be relatively large.  As soon as the piston starts moving towards the valve chamber, a water filled cylinder rapidly increases in pressure until the discharge valve opens and the liquid is discharged into the outlet pipe, in the case of a boiler feed pump, right up to boiler pressure.  Even a quite small bubble of air in the water pump causes a real problem.  In the presence of a small bubble, the bubble must be reduced in volume, or compressed, until the pressure is sufficient to open the discharge valve.  If the clearance volume is significant, the bubble is compressed in the clearance space but not enough to open the discharge valve.  When the piston starts moving down again, the bubble just expands to its original volume and there is no discharge flow.  In your pump, the displacement is quite large.  When the air pump piston moves up, it reduces the pressure so that all the water flows past the first flap (or ball) valve, and the stroke is such that a volume of air and water vapour also passes the first valve.  When the piston then starts moving down, the pressure is increased so that the valve in the piston opens and air and water flows to the top side of the piston.  In fact you have a two stage compressor, and on the next upstroke, not only does more air and water flow past the lower valve, the water on top of the piston is lifted, and the air above the piston compressed until the top valve opens and water and air are discharged into the top chamber.  The gland in the top plate prevents air leaking back past the pump rod and increasing the volume that has to be discharged.  When the piston is at its lowest point, the remaining volume is quite small so sufficient of the vapour is compressed to a high enough pressure to flow through the valve vent holes.  (Don't make them too small!). Not important that air pump is single acting while the engine is double acting.  Some variation or fluctuation in the condenser pressure will not matter.  Compressibility of the air, vapour will smoothe the pulsations considerably.

By the way, vacuum is a useful concept in conjunction with a pressure gauge that measures only the difference between the measured pressure and atmospheric pressure.  However, there is no such thing as negative pressure.  Zero pressure is the vacuum of deep outer space, or in your equipment, but only if you a have a really, I mean super really good vacuum pump.  Atmospheric pressure is about 14.7 psi or 101.3 kPa, just over 1 bar.  The pressure in your jet compressor, is unlikely to ever get to zero but easily somewhere in the range 10 to 14 psi, and at best possibly even lower.  The discharge of your pump must be something above 14.7 psi in order to discharge to atmosphere.  Remember the molecular model of gases.  Pressure comes from the change of momentum when the gas molecules hit a surface and bounce off.  There is no negative pressure.

So your valves will open when the pressure on the underside is higher than the pressure on the top side, so the force is available to open the valve.  When the higher pressure is on top, the valve leather or plate moves down onto the support place and closes off the vent holes, so the water and air cannot flow back.  To understand how they work, just look at the direction of pressure difference, and don't worry about vacuum.  When your engine starts, the condenser part will be full of air.  So long as the air pump removes more than is introduced by the feed water plus the inevitable air inleakage, the pressure will, fall to something near the saturation pressure of the condensed steam after a short run time and give you quite a good vacuum to increase your engine output.

The horses power of the engine can be calculated, but in addition to the force, it requires the stroke length and number of strokes per minute to complete the calculation.  I will explain that another time, as I have a few units not addressed so far.

Hi Dan, thanks for looking in.  I intend to go on to boilers when we get past condensing, and I will include your suggestion of looking at analysing Mr Harris' boiler capacity method as you suggest.  I have the book and will try and make some notes in the mean time.

Hi Maryak, thanks for that explanation.  I wonder if optimising the flow for that turbine might be one factor in the choice of crank angles for the Titanic, while for more typical engine arrangements, exhaust pulsations would not be so important so other factors such as balance can be given more importance.

A wild bird, voluntarily in the park stands about 5 ft high and has around 6 ft wingspan!

Thanks for looking in,

MJM460
The more I learn, the more I find that I still have to learn!

Offline MJM460

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Re: Talking Thermodynamics
« Reply #207 on: August 06, 2017, 12:30:05 PM »
Hi Willy, I have held over some of your questions from earlier posts, so I think it is time to talk about them.

In post #174, you asked about some ways of raising steam a bit quicker.  I think the first step is to think about how much energy is required to raise steam, then where the heat goes, and then what is the effect of the procedures that you propose.

Your boiler is a great example, as you can insulate it very well, and then assume that all the energy from your electric element goes into the boiler and its contents.  So the heat goes into the copper of the boiler, the water you put in the boiler, and the air that is in the boiler when you tighten the plug.

I did some rough calculations assuming the empty boiler has a mass of 1 kg and holds 0.5 kg of water.  It depends on all the dimensions, but I guessed the mass of air at around 0.01 kg, but it's probably quite a bit less.  I also assumed the whole lot starts at 15 deg C.

To get the whole lot to 100 deg, but without making steam, remember that constant volume process at the top of the mountain?  The copper will absorb 33 kJ, the water about 178 kJ, and the air about 0.08 kJ.  You can see the water takes much more heat than the copper, while the air is absorbing a negligible proportion of the heat.  Your 500 watt heating element provides 500 J/sec, or 0.5 kJ/s, so requires about 7 min to get everything to 100 deg C.  Obviously plus or minus a bit depending on the actual mass of your boiler and water.  The element is surrounded by the water, and air is only heated via the water and top of the boiler shell, so does not really have any effect on  heat transfer.  Of course, if you leave the plug loose, air will escape as the heating progresses, but more importantly, some water vapour will escape with it.  Now, heating 1 kg of water from 15 to 100 deg C requires 356 kJ, but evaporating this 1 kg to steam requires 2676 kJ.  Remember I assumed the boiler only contained 0.5 kg.  You can see it takes much more heat to evaporate water than simply to heat it.  The escaping steam will take away with it much more heat than the energy absorbed by the tiny mass of air.  Clearly not the way to go.  Please also remember that while the water expands on heating, and so does compress the air, the volume change is tiny.  You can use the steam tables to find the difference in the specific volume of water at each temperature.  You will see the compression is negligible.  Similarly the steam does not compress the air.  The steam and air molecules occupy the space essentially independently, and the measured gauge pressure is the sum of the separate pressure of each component.  The air pressure does increase, but due to its increase in temperature, and the energy is accounted for in calculating the heat absorbed by air.

Now those calculations were based on raising the temperature to 100 deg C.  At this temperature, the water vapour pressure will be 101 kPa, or I atmosphere.  The air will be about 130 kPa, giving a total of 230 kPa(absolute) or 130 gauge pressure.  We could start releasing the steam air mixture, however the pressure will rapidly drop as the air escapes.  Not easy to see, as we expect the pressure to drop when some steam is let out to the engine.  And at 100 deg C we will then only produce steam at atmospheric pressure, not real useful unless we have a condenser.  But we should also be aware that while air and steam are both present in the boiler, the pressure gauge will show a higher pressure than you would expect from the steam tables.  If necessary, the safety valve will release a little, or if you open the steam valve, the engine will start but rapidly slow down and possibly stop.  About another 4 minutes will raise the temperature of water plus boiler to 150 deg, so somewhere in that 4 minutes, you will start getting enough steam to run your engine, especially if it is unloaded.

The experiment is not difficult to try, so it is worth checking the time to raise steam with the plug loose, and comparing the time to raise steam with it tight from the same temperature.  The time consuming bit is waiting for the boiler to cool between trials.  Possibly only practical to do one or two tests each day.  A brief break in machining time on separate days is probably more efficient than waiting for it all to cool.

You could of course compress the air in the boiler with a bike pump.  This would give an initial pressure reading that might imply you could open the steam valve.  If you have a suitable fitting, try it.  However, my guess would be that the engine will run for a few seconds while the air pressure reduces, and you then have to wait for the heat input to get the water up to temperature.  I would not recommend spending time making the fitting unless you want it for other purposes.

It would be interesting to know the actual mass of your boiler, and the quantity of water you use to fill and you normal heat up time to compare with the calculation.  But the more important question is whether you can make the boiler produce steam a bit quicker.  You can see now where the heat goes, and how much is required.  Your vibration idea, I assume is with the thought of increasing the heat transfer.  I think I may have already answered this one, the calculation did not need to know the heat transfer coefficient, only copper specific heat, mass of water and copper, and steam tables.  You heating element only provides so much energy, 500 watts, and just gets as hot as it needs to, to achieve adequate heat transfer.  I suspect the heating element insulation is the main limit, but if there is any effect of vibrating the boiler during heating, it is only to reduce the temperature of the element.

So is there anything that can be done to speed up the process?  Well, first we could reduce the heat required, by starting at say 80 degrees instead of 15 deg.  This is relatively easy if you fill the boiler from a boiling kettle instead of cold water.  You might go a step further by pouring in some boiling water, letting it sit for a few minutes to heat the copper, then tipping it out and refilling with water from the freshly boiled kettle.  However all of this takes time, so you would probably not achieve much difference in time from just filling with hot water in a single step, may even be slower.

You could increase the heating element power output by using a higher voltage.  Power output is just as sensitive to an increase in voltage as to a low voltage, so with a variable transformer, you could increase the voltage and shorten the heat up time.  I do not recommend this.  It would work, but you would run a real risk of exceeding the temperature at which your element burns out.  Better to stay within the rating.  However, it's worth making sure that your power cords have the current carrying capacity, so that you are not working on low voltage.

The next trick involves modifying the boiler, or making a new one, to use an element with a higher rating, or even two elements (which you connect in parallel) so you put in the required energy in a shorter time.  Probably the best solution, though a long power cord will need an even heavier rating.

I think that leaves only some outstanding questions from Paul that have not been addressed.  I will have a go at those next time.

Thanks for looking in,

MJM460
The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #208 on: August 06, 2017, 01:37:59 PM »
Thanks for that, so we use copper as it is easier to work with and join together with silver solder, and also it is traditional and not too prone to deterioration. Could one however line the inside with a stainless steel sheath the ends as well as the tube, or even plate it with something ,cadmium or nickel ?? Just thinking about this !! Also in my boiler there are two 500 watt elements actually. I was thinking with the vibration that this would help the release of the bubbles a bit quicker? So, more questions .......and will they ever end !! I do find all this quite fascinating actually and of course this is why with modern engines of all sorts there are so many additional bits and pieces that fill up the engine compartments and spaces around the engines.!!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #209 on: August 06, 2017, 04:37:46 PM »
This is the video of this boiler from a few years ago