Author Topic: Talking Thermodynamics  (Read 194778 times)

Offline MJM460

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Re: Talking Thermodynamics
« Reply #300 on: September 23, 2017, 02:44:46 PM »
Hi Willy, only a short time available tonight, so last question first.  I will try and get to the others tomorrow.  With AC, ohms law still applies though you must use impedance instead of resistance to be correct.  Fortunately your heater is almost certainly a pure resistive load so the values of resistance and impedance are the same.  Again the voltage we quote for AC, whether it be 230 or 250 volts is an RMS voltage, not the peak of the alternating voltage which is significantly higher, and the RMS voltage happens to have the same effect on a heating element as a DC voltage of that value.  So you need to use two formulae.  Ohms law V= I x R bit in the form I =  V/R.  Then power in watts, W = V x I = V^2/R.  You can calculate R = 250 ^2/ W, ie from rated voltage and rated power.  Then power at 230 V = 230^2/R. Because the voltage is squared, the effect of low voltage will be marked.

You also need to be aware that the resistance normally varies with the element temperature, usually increases with temperature.  I don't know of your element is rated based on the element hot or cold, but quite possibly cold at a defined temperature of perhaps 25 deg C. Because they don't know what temperature you will run at.  You can measure the resistance of the element cold and see if this agrees with your calculated figure from the rating.  You may be able to get a close hot reading if when your boiler is operating for some time and hot, switch off, and quickly unplug and measure.  It will cool a bit before you get a reading, but will give you an idea of the change.  Better would be if you are equipped with an AC current meter that clips on around the wires and measure the current hot and cold.  But don't fool with 230 V AC unless you really have the right equipment.  You can't measure resistance with power connected.

Finally, even with a rating of 1000 watts, unless you have a test certificate you still have to regard that as approximate.  I would hope it is within 10% of that, so in the range 900 to 1100, but I don't know what accuracy standard it is made to.  In the end measurements are necessary for accurate data, and even then the instruments should be calibrated.  Outside a formal lab environment there will be a measurement error in practice.

I will try and answer the other questions tomorrow.

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 #301 on: September 24, 2017, 01:02:09 AM »
hi, thanks for that the and here is some info about the Cartridge heaters. I did not use the sealing compound though , also some pics of the cartridge heaters with their housings, if they need to be taken into account. They are a pretty complex shape though to calculate the mass/volume !! Looking forward to more info ...and i hope i have not hilacked this thread too much .In the pictures are the electrical safety valve and the water level switch. this switch device is used with a 9 volt battery Cct ,to trigger the on/off mains Cct so as to be safe from electric shocks !! the insulation is PTFE.
« Last Edit: September 24, 2017, 01:10:08 AM by steam guy willy »

Offline MJM460

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Re: Talking Thermodynamics
« Reply #302 on: September 24, 2017, 06:01:27 AM »
More on electric boilers

Hi Willy, back to your questions in post #298.  First, you are not hijacking the thread, it is to answer questions like this that I am writing the thread, and I suspect there are many others with similar questions, and I hope they are finding it helpful and will join in when, between you and I, we miss something, or it is just still not clear to them.  And I hope also if they notice something I have got wrong.  Mistakes are just too easy in some of this stuff, I can only do my best, and correct things when they are noticed.

Does it matter whether you fill the boiler or not?  When you raise steam in an electric boiler, or any boiler for that matter, you perform two different processes in sequence.  First you heat up the whole system to your steaming temperature, then you evaporate liquid to produce steam at constant temperature.  When all the water is gone, the system temperature will start increasing if the energy input continues.  However, if you do continue with a dry boiler, or even a partially covered element or firebox, you will soon find some component reaches a mechanical strength limit at the higher than design temperature, something will melt.  On your case the element will probably burn out.  In a fired boiler, most likely the soldered joint will soften and release pressure before the copper softens enough to start yielding, as will be evidenced by bulging which will not spring back when the whole mess cools down.  It all depends on which limit is reached first in a particular boiler.

To understand the answer to your question, first consider each of those two basic processes.  While you are heating up, higher water level means more water to heat, so it will take longer for the constant energy input of your heater to get it all up to temperature.  Most of the energy input is stored in the water, copper and insulation, and a portion is lost to atmosphere as we have already seen.  This lost portion is initially low while all the temperatures are low, and increases with increasing temperature.

Once it is all up to steaming temperature, further heat input does not increase the temperature, it mostly goes into evaporating water into steam, although the heat loss to atmosphere continues, now at a constant rate due to the constant temperature.  How much steam is now found by an energy balance, and how much energy to evaporate a kg (or lbm) of steam.  We get this from the steam tables and it does not depend on how much water is available at saturated condition for the temperature, containing hf kJ/kg as enthalpy.  If you have less water during the steaming process, you will run out quicker, but it makes no difference to the steam production rate.  Steam production is completely determined by the energy input, the heat loss, and hg - hf for the water at that temperature.  And steam pressure is determined completely determined by the temperature, or vice versa.  And this pressure will be the pressure in the boiler once a little steam production has carried away the air that was in the boiler when it was filled.  Obviously less air if the boiler was more full.  But you cannot fill it completely as the steam needs some space to separate from the liquid, otherwise a lot of water will be carried over with the first steam.  Experiment is probably the best way to determine the maximum fill level before water carryover becomes too much of a problem.

The minimum water level must cover the element.  It does not matter whether it is a fired boiler or electric, the heating surface will get too hot if it is not covered by liquid as the heat transfer coefficient for vapour is very low compared with water, especially vigorously boiling water.  You could consider a little lower level and use a feed pump to maintain the level.  However, the engine will do work driving the pump before there is any available to do anything else, and the water entering the boiler will be at less than steaming temperature, so requires more heat to get to that dry saturated condition of steam.  You can only partly offset this by some boiler preheating the water (using exhaust steam for example).  You can't use an electric jug because the pump will not handle near boiling water due to its valve pressure drop and acceleration losses.

Will leaving the boiler open until at 100 deg help?  Remember the mountain top experiment?
  The pressure of the water vapour at 16 deg C is only 1.8 kPa, so 2% of the total boiler pressure at atmospheric pressure, and a nice vacuum of you can condense at that temperature.  The rest of the pressure is due to air.  Heating the air to 116 deg C increases the pressure in proportion to the absolute temperatures, so (273+116)/(273+ 16) = 1.34.  So the air at approximately 100 kPa in the cold boiler increases to 134 kPa in the sealed boiler when hot.  This is an increase in gauge pressure of 34 kPa or about 5 psi.  If this, plus the water vapour vapour pressure exceeds your safety valve setting, it will lift.  The air that escapes with that steam is not replaced, while the steam is quickly replaced by more evaporation so the air is soon gone, and the air heat content is negligible compared with the steam, so no big difference in the heat required to get to that point.  If you leave the boiler open while heating, yes the air escapes, with little saving in heat requirement, but steam escapes with it, taking away the latent heat.  So it would be slower to get to steam raising.  The air will help drive your engine, so not a significant problem.  Seal the boiler cold and start heating.

Sheet metal around the boiler.  You normally have sheet metal cladding around industrial insulation.  Yes, it reduces convection from within the insulation, but conducts heat pretty well so it gains heat from the loss through the insulation, and in turn looses that heat to the air with only minimal reduction in overall loss.  It is put there, not to increase heat conservation, but to provide weather protection and protection from mechanical damage.  Note that wet insulation is a very poor insulator, as the water has higher conductivity than insulation and higher specific heat so it takes more heat to get up to the temperature profile.  So add your metal sheet, but only to hide and protect the insulation you put under it.  But wood looks even better, even if it is brown stuff.

Wire drawing - I see this term used in two ways and I don't see it as a specific technical term.  I believe it is normally used to determine the erosion of steel by high velocity wet steam in a situation such as a gate valve, which is only designed to be fully open or shut, is used for throttling.  The pressure energy becomes high velocity at the small gap of a nearly closed valve, and if the steam is wet, it will, sooner rather than later, gouge a bigger path and the valve will no longer seal properly when shut.  A gate valve is best followed by a globe valve which is made for throttling, or at least only used in throttling mode as a controlled opening rather than sudden opening.  However, I have also seen the term used to describe reducing pressure during flow.  To me that is the cause rather than the result.

A point about accuracy - your elevation, quoted to 7 decimal places of a meter implies that the last tenth of a micron is known to plus or minus less than 5 digits.  Measuring anything to this accuracy is a real challenge for even the very best micrometers, and I challenge any surveyor to measure elevation to that precision.  I thought you might have converted inches and feet with a calculator that gave you that many figures, but one inch is exactly 25.40 millimetres by definition, and dividing by 25.4 gives about 12 feet 8 1/4 inches, but not exactly, so I don't know how you arrived at the precise figure.  For its effect on the absolute pressure, the nearest 10 metres is probably more than accurate enough.  The weather bureau publishes atmospheric pressures all reduced to sea level, and I suggest that 3 metres is close enough to just look at the weather bureau latest observation, and assume it applies without correction to your location.  If you live in Denver, you would definitely have to calculate a correction.

Getting to be a long post, but I think that brings us to today's question.  I better get back to being sociable, might get back later, otherwise tomorrow.

I hope all that is helpful

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 #303 on: September 24, 2017, 02:03:59 PM »
Hi, Ok thanks for all those replies and observations, it helps out when trying to be too clever !! I have just thought about using boilers with steam engines and your word 'Vacuum' suddenly reminded me that when using a boiler one must be very careful to allow air to enter when the engine was stopped and the boiler shut done to stop the boiler imploding when it cools down !!

Offline paul gough

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Re: Talking Thermodynamics
« Reply #304 on: September 25, 2017, 01:08:22 AM »
Re wire-drawing, MJM is no doubt correct regarding the erosion by condensate in a seat or some such as being what should be the primary meaning for the term. However it has common usage in discussing steam circuits in locomotives, certainly in publications from Great Britain. It is normal, (or should be), in a publication to use "wire drawing of the steam" when first entering into any discussion and then shorten it to wire-drawing unless the context is already known. As MJM said, it is a pressure drop across an apparatus, eg., the drop in pressure between the entry of a superheater element or set of elements due to friction and their outlet, any steam circuit, or even can be used to discuss a loss associated with constricted ports, either their size or the opening at a specific point of the valves travel over them. In very general terms it is a lack of sufficient cross section area of the passage be it a pipe, orifice plate, valve, superheater element etc. This at least is my understanding from my reading over the years and hope it helps illuminate the phenomena. Regards Paul Gough. PS, I understand that the first or top portion of an indicator diagram where it slopes slightly can indicate wire-drawing to an experienced eye, maybe our marine engineers that are on board could expand on this.
« Last Edit: September 25, 2017, 01:33:04 AM by paul gough »

Offline paul gough

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Re: Talking Thermodynamics
« Reply #305 on: September 25, 2017, 08:09:29 AM »
Before someone jumps on me for lack of clarity in my PS of the preceding post. I am not referring to the normal slight down slope of the upper portion of the diagram that is due to the increase in cylinder volume from piston travel. I am trying to covey there is a departure in the trace from this expected and normal line that a discerning and experienced eye can infer wire-drawing. Unfortunately I am not able to elaborate further as I have no experience in taking extensive numbers of indicator diagrams and interpreting them, it might be that marine engineers who served on steamships might know of it and shed further light on this, I don't think there would be any steam loco designers or engineers from loco testing stations still extant let alone reading this thread to provide a clear explanation. I mention it as one example of a defect that could be seen on a diagram. Perhaps some other examples of deficiencies that appear on diagrams might be discussed as well. Regards, Paul Gough.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #306 on: September 25, 2017, 11:42:26 AM »
Home, sweet home -

Some of you have noticed that I have been travelling recently.  Just arrived home this afternoon, and it is always good to arrive safely home after a road trip of 12240 km, average fuel consumption 11.2 l/100 km and circumnavigating roughly a half of our continent.  Hooked up our little caravan to the Subaru Outback on July 13, and went, well, outback.  I think Hugh calls it Snow birding, we call it becoming grey nomads.  People from the southern states driving north to escape the winter and find some sunshine make for crowded roads and parks, well sometimes you might see at least 10 other cars in a day.  I suppose we all go at about the same speed, but one time we had to stop for around 5 minutes for roadworks, no one came up to wait behind us.   Then they mysteriously appear, one by one at the road houses and campgrounds, for overnight stops.  But is is a great way to get a real feel for the country, and to experience blue skies and clear starry nights in a way that is not possible in the city.  And to see some wild life in its natural habitat.  We were a little later than most in turning south to put the sun on our backs for the long road home, so perhaps that is why the road seemed less busy than normal.

Thanks Paul for some clarification of wire drawing.  It is consistent with some of my reading and it would seem to be a term more commonly used in the pressure drop sense in the marine industry.  In my industry, it was a term usually muttered while looking at the cutaway in the hardened satellite of a gate valve no longer sealing shut.  Cut like a water jet.  Like you, I would like to see someone start a thread on the indicator diagrams, they are only rarely used in my industry, then only on compressors, as the drivers are all turbines or electric motors.  So I have no experience at all on taking or interpreting them.  However perhaps it is a bit outside the scope of this forum, as I suspect that none of us have available an indicator device suitable for use on our models.

Thanks Willy for those pictures of your boiler and the screen shots of the data sheets.  It seems that the elements may actually be rated for 230 V, so the calculation would then indicate the higher power output at 250 V.  I also note that the tolerance on power is +5%, -10%, but the question of whether this applies to a hot or cold element is not answered, but I would assume cold.  So probably lower output when hot.  The grease is intended as a heat transfer compound rather than a lubricant.  I think we have discussed this before.  I do notice that your boiler seems to have flat ends without stays, though I notice the bushings have not been soldered in the picture, so you may have added stays later in the construction process.  Of course you now have the boiler pressure tested and steaming, so if it is now dimensionally stable, I guess that is practical evidence of its strength.  However, I keep wondering if your pressure housings for the elements could be extended right through and fixed at the other end, just like a large diameter hollow stay, you could then use grease on the sheaths without air displacement issues. 

I am not sure what external pressure the boiler could stand.  The effect of internal pressure being below atmospheric pressure, the condition commonly referred to as vacuum, is that the shell has the high pressure on the outside, that is why I refer to external pressure.  The commonly quoted formula for shell thickness is technically referred to as the thin shell formula, and it's derivation rests on the assumption that the material is in tension to contain the higher pressure on the inside.  Under external pressure, the shell is in compression, the failure mode is the collapse you refer to, and the strength very different, usually much less.  The boiler is relatively short, so the flat ends provide considerable stiffening against collapse, but as you say, it is good to open a path for air before the boiler cools down and avoid the issue.  And if this allows some extra steam to escape, it may speed the cooling.  You have given me the boiler dimensions, so when I get everything unpacked and put away, I will calculate the strength under external pressure for those dimensions.

I think that brings us up to date on the previous questions, so time for a good nights sleep.

MJM460
« Last Edit: September 25, 2017, 11:47:53 AM by MJM460 »
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Offline MJM460

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Re: Talking Thermodynamics
« Reply #307 on: September 26, 2017, 12:37:37 PM »
Boiler design considerations -

Yesterday I mentioned the thin shell formula often quoted for boiler shells, and noted that this formula applies for internal pressure of a cylindrical shell only.  It does not apply for external pressure such as the loading on the centre fire tube of a marine boiler.  It also does not apply to flat ends, or other shapes such as locomotive fire boxes.  It is obvious really, the diameter of curvature of a flat plate is infinite, so the thin shell formula gives an infinite thickness for any pressure.  But blank flanges for piping are flat, and not infinite thickness, there must be another formula.  And of course there is.  Blank flanges are quite thick when compared with the wall thickness of the circular  pipe, and we would not want to make our flat ends so thick.  The answer is usually found in the provision of stays which support the plate at intervals that leave the unsupported section strong enough in a reasonable thickness.  The designers of boilers carefully follow the rules for spacing of the stays.  I don't intend to branch into boiler design, but it is probably worth looking at some of the considerations involved, so it is clear why building to a published design supported by the appropriate calculations is the best course for a builder not experienced in pressure vessel design.  Boilers are even more complex to design because the surfaces subject to the combustion gases require additional calculations to determine the appropriate metal temperatures and design strength.  Allowance has to be made for the reduction in strength of the material at higher temperatures.  Hence there are separate codes for boilers and for unfired pressure vessels.

When a cylindrical shell has the high pressure on the outside, such as the marine boiler fire tube already mentioned, the failure mode is quite different from when the higher pressure is on the inside.  If subject to to great a pressure, the tube collapses, or squashes.  The collapse pressure is quite sensitive to any departures from true circular form such as dents, unlike the internal pressure case.  The formula are again quite complex, and the Australian Miniature Boiler Code for example refers to the pressure vessel code for these calculations.  Needless to say, the thickness required to resist buckling under external pressure is much greater than required for internal pressure.  This is normally reflected in the specified thickness for the design.  Again, specifying the thickness is best left to the experts.  Calculations for external pressure are included in the AS/NZ, BS and ASME pressure vessel codes.

Now Willy mentioned the possibility of collapse of his boiler due to the low pressure if it is allowed to cool to atmospheric temperature.  This is a real possibility for industrial steam containing vessels, and now days the full scale pressure vessels that I am familiar with are designed for full vacuum as well as for the required internal pressure.  The relatively small diameter of Willy's boiler, combined with the fact that the shell is not fired, so the metal temperature is well controlled, means that the design check is relatively easy.  The internal design pressure is probably the overriding consideration, especially as the maximum possible vacuum is only a little over 100 kPa or 14.7 psi.  A quick first pass on the dimensions indicates it is probably quite strong enough to resist collapse, however I would still recommend a formal check by the boiler designer, as it is never wise to commit to the results of an unchecked calculation.  The best procedure is always to admit air while the vessel cools, at least until you have the formal calculation.  Collapse under vacuum used to be demonstrated in every junior science program using a square thin walled metal can.  I presume it still is, the result is quite spectacular but not particularly dangerous as the can is usually surrounded by the sink when the cold water is splashed over it to condense the steam.  And of course steam is not escaping as the pressure is reducing, dramatically in this case, but it is a lot of work to rebuild a boiler that collapses.  Again the failure mode is much different from failure under internal pressure test.

Another case where the thin wall, formula does not give all the answers is when you put a hole in the shell for inlets, outlets level glasses and so on.  We all know the normal design solution is a bush soldered into the shell.  The dimensions of the bushes are carefully specified in the model boiler codes.  These bushes are in fact designed to properly compensate for the missing part of the shell.  So long as you don't cut down on the amount of metal, you can use the standard bush designs with confidence.  Even make them a bit bigger, but no smaller.  The problem comes when you want a larger opening, perhaps for a steam turret.  The opening has to be properly reinforced and the design supported by appropriate calculations.  This even applies to tee fittings in tubing and piping, where the metal thickness is carefully controlled so the cylindrical tube or pipe can safely contain the internal pressure.  In a forged pipe fitting, the metal thickness is so carefully controlled that it is not immediately obvious that the extra thickness is there.  But it is.

So a boiler has many components which all contribute to pressure containment, and only the circular shell with internal pressure is described by that thin shell formula.  Even then, allowance has to be made for corrosion, and fabrication techniques.  Even welded joints are not considered as strong as the base metal unless they are proven by x-ray examination.  Tennessee Whiskey is well familiar with these inspections and his certifications are testament to his skill with welding equipment.  For most of us a satisfactory silver soldered joint in a copper boiler is easier to achieve, and the code requirements for the joint design are intended to allow for the expected fabrication methods, when supported by knowledgable inspection.

If this is of any interest, perhaps a little on testing tomorrow.

Thanks for following along.

MJM460
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Offline paul gough

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Re: Talking Thermodynamics
« Reply #308 on: September 26, 2017, 02:08:19 PM »
Re -thinking about efficiencies in our system as a measure of heat in, (boiler fuel) to work done, (engine output) it is known that higher pressures and superheat give better efficiencies even in models. Logically it then should be the case that pre-heating the feed water by means of exhaust steam or exhaust gases would do likewise. But to what extent??

The question arises for us modellers and particularly for rather small models is there any point in feed water heating? Is something primitive like running a feed water line through the firebox, (I'm talking metho or small gas fired boilers here), or some similar arrangement going to be practically worth it and enhance performance, eg. save fuel or reduce the firing rate. If it is a worthy theoretical proposition then we need to ask/find out what minimum temperature increase is needed to make a feed water heater a worthwhile modification. Is there any way to work this out, other than the obvious build one and try it method? Regards Paul Gough.

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #309 on: September 26, 2017, 03:10:07 PM »
Hi MJM , I was talking to the club members today about the cooling of model boilers in locomotive and one chap said that as the boiler cooled it actually sucked new feedwter in through the clacks. so there was no need for a 'snifting' valve. Also where is the best place to have the inlet water valve ? Taking into account if the valve fails ....under  the water level or above it?
Willbert.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #310 on: September 27, 2017, 11:10:39 AM »
More on feedwater heating -

Hi Paul, consideration of feedwater heating is an area where perhaps logic lets us down.  But let's work through two, perhaps three situations.  First without a feed pump, as in my models so far, and I think Willy's electric boiler.  Preheating in this case means boiling the jug, and filling the boiler from the jug before the plug is tightened.  Now you can see what happens.  With the water at say 80 deg by the time the plug is in and the burner lit, the enthalpy of the water is 334 kJ/kg instead of 63 kJ/kg at 15 deg C.  If our chosen pressure is say 0.198 MPa (approx 2 bar, or 30 psi) the enthalpy of saturated water is 503.7 kJ/kg then that heating phase before steaming begins has to contribute (503.7-334)/(503.7-63) or close to 40 % of the heat required to heat cold water to the steaming point.  Clearly reduces startup time, and either saves fuel or extends the run time from a fixed amount of fuel, with the attendant risk of low water level.  However, once that steaming point is reached, the heat required to produce steam is entirely determined by that chosen pressure, and the heat available from the burner, the actual steam rate is not affected by the preheating.

If the steam plant has a feed pump, then some cold water is continually added to the boiler, presumably the quantity adjusted to match the steam rate so the level remains close to constant.  This means the average temperature of the water in the boiler near the entry point, is a little below the steaming temperature.  Mixing quickly brings the feedwater up to steaming temperature, but at the expense of the quantity of steam produced.  Conservation of energy and the saturated steam pressure-temperature relationship both still apply, and the energy to heat the feedwater must come from somewhere.  Now you can see that if you have a feed water heater using exhaust steam energy which is otherwise lost to the process, less energy is needed to heat the incoming water so some of the lost steam production is restored.  As the steam production in the end has to be balanced against consumption, and assuming the load and speed is the same for both cases, the fuel consumption is slightly reduced in response to the preheating.  When you are shovelling coal in a small locomotive, it is probably hard to tell, but the theory depends only on conservation of energy which is a fundamental law of physics and always applies.  With a small Meths burner, there is probably no adjustment, so a little more steam is produced and the locomotive possibly sees slightly higher pressure and goes a little faster.  If you use a radio to throttle the steam and maintain constant speed, the boiler pressure will increase a little, as will the stack temperature and a new equilibrium is reached.  Of course the differences might be masked by the other variables, however, it is still useful to understand the theory.

Now that third case, what if you have a hand pump?  First, I suggest that the hand pump will be an intermittent operation, you are unlikely to sit there steadily working the pump continuously.  Two feedwater heater designs are possible.  You could top up the feed water tank from that electric jug, useful things those, and if all else fails you can make a cup of tea.  Probably even worth insulating the tank.  If you plan on this one, it is best to have a raised tank, so you always have a positive head on the pump suction valve, otherwise you may get a vapour lock in your pump.  But that will reduce the additional heat needed to heat the makeup water.  Alternatively you could use some exhaust heat, either by a coil in the tank, again it needs to be raised, or you could even make a little heat exchanger and pump the water through this.  A little more difficult to control, probably more practical with an engine driven pump.  The water in the exchanger will get quite close to exhaust temperature while there is no flow, then the warmer water will be moved into the boiler when pump operation resumes.

In summary, using exhaust steam to preheat the feedwater does in principal reduce fuel consumption or increase steam production, however it has no effect on the engine inlet or exhaust, unless of course it imposes a significant back pressure on the exhaust.

There is a limit as to how much exhaust heat can be recovered.  You will remember that heat only flows from a high temperature to a lower temperature.  The exhaust temperature will be at about 100 deg if you do not have a superheater, or a bit above if you do.  The boiler temperature will be something above the exhaust temperature, and we have looked at a few cases earlier, say 116 - 120 degrees.  But then we actually need a temperature difference to drive the flow, so we can't get the feedwater temperature to the exhaust temperature of 100 degrees.  And the heat required to heat the water to that temperature is a very small portion of the total heat in the exhaust steam.

Remember the equation for heat transfer, Q = U x A x (T2 - T1). To get a given heat transfer, as the temperature difference reduces, the required area increases.  Practical area in most large industrial installations rarely achieve less than 10 deg approach, and in a small model feed water heater, almost certainly a very much bigger gap. 

So there you can see the effect of feedwater heating, you know what you are trying to achieve, but as U is notoriously difficult to predict, a little test rig would be desirable to see if you get a worth while extra run length with a practical heater size.

Another heat source for feedwater heating that could potentially give a higher boiler inlet temperature is the flue gas.  The main issue is to ensure that feedwater heating only gets heat that is left after the boiler and superheater have used all that is possible.  Otherwise the feed water heating is at the expense of steam production, and I suspect that would be counterproductive.  And at the end of the day, you don't want to cool the flue gas to the point where the water component starts to condense, as that nearly always leads to corrosion problems.

Willy, the feed check valve admitting water if the pressure falls below atmospheric is a good point.  Atmospheric pressure drives the water in the direction that lifts the ball and permits flow.  If you have a feed pump.  Of course the boiler could end up over full as the cool water lowers the temperature and hence vapour pressure of the water.  The only air admitted to the boiler would be the amount dissolved in the cold water.

We discussed the location for the feedwater inlet back in post #117 when Maryak provided a very interesting picture of full size practice.  It is on page 8 of 21 of the thread (based on my forum settings).  The recommendation was not too close to the shell, and normally near but below the surface.  The second part of your question, taking into account the possibility of check valve failure,  I am sure you are thinking of whether water or steam could escape from the boiler.  There are a few things to consider.  First, if you have a feed pump, you actually have three check valves in a row.  As any one will prevent back flow, all three have to be in bad shape, and any leakage should be quite small.  I suspect an issue with the feed pump might be noticed before there was too much problem, but perhaps others have some relevant experience they can contribute.  Note that when water escapes through the leaking valves it is at boiler temperature, so superheated water when the pressure is reduced.  This means a portion will flash to steam until the excess heat is absorbed, and this will tend to spit hot water around.  Definitely not desirable.

Next time I suggest looking at the potential work output from your engine, and the differences in potential work output for the three temperatures.

Thanks for looking in,

MJM460
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #311 on: September 27, 2017, 01:24:09 PM »
Hi, May i ask about flash steam plants with regards to feed pumps.....I don't know much about them and have always wondered how you introduce water into the coil when the pressure in the coil is very high to drive the engine ? are you trying to pump fresh water into the coils above the pressure already there. and does this require a lot of energy that reduces the efficiency of the plant? Of course with an injector the water does have to be cold.! Also is there an actual temperature difference between the steam and the water level in a boiler? If the temp is the same is it steam or water ? or have i missed something here. Are there any pictures of unclad boilers that show the temperatures with the different colours as are used with buildings.? I do have a handdraulic feed pump on my boiler, but the sight glass does not work any more as i think it is scaled up.!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #312 on: September 27, 2017, 01:32:47 PM »
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<a href="https://www.youtube.com/watch?v=63c9KR0bqb8" target="_blank">http://www.youtube.com/watch?v=63c9KR0bqb8</a>
 
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electrically heated steam engine


Here is a video talking about the steam plant.....This is on Utube if you can find it .
« Last Edit: September 27, 2017, 01:50:58 PM by steam guy willy »

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #313 on: September 27, 2017, 08:10:07 PM »
Hi ,MJM Here is a vid of my steam crane also electrically heated........and a quick disappearing trick at the end !!..I will be doing another steam up of the boiler with some Rockwool. does this need to be fairly loose or quite tightly packed ?? I would imagine a bit loose to allow the air to do its part. I shall also hold it in place withe some plywood. The contemporary thinking from the club boiler inspector is to have the flanged end plates showing outwards so one can see the depth of them, however this will give more area of copper exposed to the outside to conduct the heat into the air. !https://youtu.be/2EkjRslFgSM
<a href="https://www.youtube.com/watch?v=2EkjRslFgSM" target="_blank">http://www.youtube.com/watch?v=2EkjRslFgSM</a>

Offline MJM460

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Re: Talking Thermodynamics
« Reply #314 on: September 28, 2017, 01:15:20 PM »
Hi Willy, more very interesting and relevant questions.  I hope that I can clarify a few points for you, so here goes.

Flash steam plants.  First fluids always flow from high pressure to low pressure, you need a pump or compressor to make them flow the other way, so yes, the feed pump must provide enough pressure to exceed the highest pressure in the coils, and that occurs right at the pump piston face.  The pressure is already lower on the coil side of the discharge valve and continues to drop through the coil to the engine.  The pump does indeed use power from the engine, but this is a small portion of the power developed by the engine.  Of course in our models friction is probably a greater loss in proportion to the work done than in a full size plant, but providing the packing and piston ring are not too tight it is still a reasonably small portion.  This is because the volume change of the water is very small with pressure increase, which means that work for compression is also very small.

You mentioned injectors, there is a lot of heat added to the water that enters the injector, and if it gets up to the point where it starts to vapourise too soon, the injector passages choke up and it stops working.  With a pump, the suction stroke of the pump lowers the pressure for the incoming water, and again if due to inlet temperature, the water vapourises under the lower pressure, the pump becomes vapour locked.  However there is more latitude than with an injector.

Water and steam are nominally at the same temperature.  I say nominally, because the boiler is not normally in true equilibrium while heating is going on.  The heating surface is at a higher temperature than the water, otherwise heat would not flow.  The water starts to vapourise, but it's pressure is slightly higher than at the surface due to the water depth, very small depth in a model boiler, and the expansion to steam means the density is very low so it is rapidly displaced to the surface.  If you turn off you element when the boiler is up to pressure, boiling will quickly cease, and condition will get quite close to equilibrium, especially with good insulation.  When all the water in the boiler is in equilibrium, the liquid and vapour are at the same temperature. 

Liquid and vapour are two separate phases, which under certain conditions can exist at the same time for most fluids.  Our boilers have only water in them once the air which was initially in the boiler is displaced.  The gas bottle used by many has perhaps propane, a chemical substance classed as a hydrocarbon, which is highly flammable and a useful fuel.  Propane also has a range of conditions where liquid and vapour exist at the same time, just the pressure and corresponding temperature are different from those for water.  Similarly, most things we might normally consider gases due to their nature at atmospheric temperature, such as butane, air, carbon dioxide and even natural gas all can exist as liquid at some temperature, very low in all those cases, and have a two phase region where liquid and vapour exist in equilibrium at the same time.

So liquid and vapour (or gas) are terms which refer to the phase state of a substance.  However, water is a substance whose molecules contain two hydrogen atoms and one oxygen atom chemically bonded together.  The substance water can exist as solid, liquid or vapour, and we are all familiar with all three of these phases.  There are quite a range of conditions over which any two of those phases can exist in equilibrium at the same time, however all three can only exist at the same time and in equilibrium at one specific temperature and pressure known as the triple point.

By the way, even in your cup of tea, liquid water and vapour are approximately in equilibrium, and of course there is also air at the surface.  When the water is hotter than the equilibrium temperature for the water vapour pressure in the air, some of it evaporates, and under many conditions you can see this as a steam cloud rising from the cup.  If you blow the steam away, more liquid will evaporate to replace it, and this evaporation takes heat out of the tea.  If the tea is cooler than the equilibrium temperature for the vapour pressure in the air, some of the vapour will condense as it does on any cool surface.

In the boiler, there is liquid water and water vapour at the same time, and when in equilibrium they are at the same temperature and pressure.  However liquid water has a much higher density than water vapour, so gravity and surface tension combine to place all the liquid in a continuous phase at the bottom of the boiler, while the vapour fills all the vapour space, and there is a clear visible boundary between them.  Vigorous boiling is not true equilibrium, but the equilibrium temperature is considered the best estimate of the average temperature of the whole.

I have not seen any thermal pictures showing the temperature difference in a boiler, but I am sure that they will exist.  I don't know what temperature range is normal, certainly it will depend on the energy density which determines the necessary temperature difference for heat flow.  One of my catalogues has an instrument for viewing the temperature colours, but it is around $1000 so I will not be picking one up any time soon.

Thank you for those two videos, really excellent productions.  Clearly your safety valve works well and switching off the power reduces the steam production very quickly.  I love that crane.  It just needs to be nearer the edge of the table so it can raise and lower things from the floor.  Is the control valve just a disk valve to reverse the directions?

If you are adding rock wool, it should be packed as tight as practical.  Air doing its thing means transferring heat by convection, so you want to limit the air movement.   But rockwool is quite a suitable insulating material.  For a more permanent job it needs a metal or preferably wood cladding for appearance and mechanical protection.  Not sure if it is as bad as glass fibre, but worth taking the normal precautions for loose fine fibres when you are working with it.  Saves the itchy fingers at least.

I will come back to those flanged end plates tomorrow as it is getting late.  I suspect the above will raise more questions, but the post is long enough so I will finish off and return to the topic if there is any clarification required.

Thanks for following along,

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

 

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