Author Topic: Talking Thermodynamics  (Read 88485 times)

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1065 on: October 02, 2018, 02:40:56 AM »
Hi MJM , thanks for this and i have another quote to ask people about ....NPSH !! I shall try it out at the club meeting soon !! :mischief: :naughty: .When the exhaust steam comes out of a really fast running engine is it that much cooler ?? or does it get cooler by the time it exits the chimney flue ?

willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1066 on: October 02, 2018, 12:59:53 PM »
Hi Willy, the NPSH required at the inlet of a good centrifugal pump with especially smoothed inlet passages to keep cavitation in the pump to an acceptable level is more than a meter.  A reciprocating pump does not have the smooth passage, the valve balls are in the way of the flow, and there is pressure required to accelerate the flow into each inlet stroke, so considerably more is required for a reciprocating pump.  When the water temperature is low enough, atmospheric pressure at the liquid surface does the job, but as the temperature increases, the vapour pressure increases, and this decreases the NPSH available, which is why our feed pumps do not like hot water.  Neither do injectors.  If we wanted to heat the water in the feed tank, the tank would have to be elevated more than would usually be acceptable in a model.  So feed water heaters go on the high pressure side of the pump.

If your friends want to be tricky, they might ask you how to calculate the NPSH available?  So be prepared, and remember it is the total head of liquid at the inlet nozzle after subtracting all friction losses is the inlet line, and most importantly, subtracting the vapour pressure of the fluid.  May be simpler to avoid the topic!

For a fully loaded up engine, it is not the engine running fast that cools the steam, it is the extraction of energy to produce work by the engine that reduces the temperature within the engine.  However, heat loss from the cylinder surface and from the exhaust system also help cool the steam.  And as Derek found, even the steam pipe heat losses contribute to cooling of the steam.  My measurements of the engine exhaust temperature are all very close to 100 degrees, which makes sense as the steam is at atmospheric pressure as it leaves the engine, so has to totally condense before it gets cooler.  However, as soon as some air is entrained in the exhaust steam it can quickly cool further, to the condensing temperature determined by the proportion of water vapour in the air.  (The proportion determines the vapour pressure of the water, hence its condensing temperature.)

For an engine running lightly loaded, heat losses from the cylinder and piping and mixing with atmospheric air are the main processes which soon cool the exhaust to the point where the familiar fog of condensed water is visible.

I hope that makes sense,

Thanks for following along,

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 #1067 on: October 14, 2018, 12:08:14 AM »
Hi MJM , sorry not to have been in touch but have been busy with the Freelance engine ..it is now finished and i have started on a new Eastons & Andersons beam engine , so more challenges and nifty file work to do.! In the meantime i have been insulating my new "kettle"... !!! here are a few pics ..I have been using 3 layers of 1/8" cork and some 1/4" mahogany slats .....so we shall see how this develops !!

Offline derekwarner_decoy

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Re: Talking Thermodynamics
« Reply #1068 on: October 14, 2018, 03:53:27 AM »
Certainly a different thermal experiment Willy   :facepalm:......have you conducted pre insulation trials?.......like timing a given volume of water from ambient....[recorded  temperature] to boiling....and how did you measure or determine boiling point?

Then repeat the identical tests with the recently insulated jug?.......

I think the actual ambient air temperature in your test room [kitchen] would also be needed for the calculation
______________________________

I see the determination of the commencement of the boiling point may be a little bit iffy in the calculation...so looking backwards....you could consider filling the un-insulated jug with a measured volume of water...............then bring and keep the water boiling for say 5 minutes...........pour out & measure the volume of the remaining water

Repeat the same tests  in the same manner with the insulated jug......

Then confirm the test results...

[this would take all of the scientific aberrations  :happyreader: out of the calculation ......you could simply confirm that the jug with the insulation provided a % variation (+/-) in boiling water {as the benefit of insulation} over a non insulated heating vessel]
______________________________

We will just be here  :cheers: awaiting an answer................. Derek
« Last Edit: October 14, 2018, 03:58:28 AM by derekwarner_decoy »
Derek Warner - Honorary Secretary [Retired]
Illawarra Live Steamers Co-op - Australia
www.ils.org.au

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1069 on: October 14, 2018, 07:42:16 AM »
Hi Willy, your freelance engine looked great in the show photos, beautifully finished.  Does the valve gear work over any range of intermediate positions, or is it effectively limited to F-S-R?

When you do that Eastern and Anderson, I hope you are planning to make the complete working valve gear!

On your jug experiment, it might be worth considering the controller part in the bottom, and only insulating the sides of the water containing part.  Otherwise the wiring, switch and shutoff mechanism will get hotter than intended, which may lead to problems.

Hi Derek, you certainly hit the point in talking about controlling all the variables.  I have found that the mass of water is very important, obvious really, but it does not take much variation to confuse the results.  Weighing the water before and after is definitely the best way to go, but still hard to get an accurate small difference between two much larger weighings.

On the issue of determining the point of boiling, I would suggest the thermocouple placed in the spout to measure the water temperature and time to say 95 deg, or a bit higher if it gives consistent results.  Normally you can see the boiling occurring through the glass sides but not through the insulation.  And of course the ambient temperature will have an effect as you say, but not easy to control unless the house has central heating.

I have also been silent lately, but not totally inactive.  I found that with my centreflue boiler, it is not easy to consistently remove the water to the same point each time so I can tell how much was used in steam production, and gives highly variable results.  I need to do some more runs to get either consistency or an idea of the scatter.  But of course the temperature in the shop today would have been about 28, if I had been able to get out there, so another variable compared with my results earlier in the year.

To overcome this, I took the opportunity to purchase a scale that could measure the whole steam plant when I noticed a suitable scale on sale recently,  I should have been less generous with the heavy chipboard base!  The reading also seems to be a little dependent on the positioning of the weight on the scale.  Working on a method to balance the whole unit with the centre of gravity in the same place each time.  Unfortunately, adding water to the boiler shifts the centre of gravity.  But I have a plan, and when family commitments (life, really) allow me some time, I will do more runs and post up a photo and some results.  I am close to the maximum for the scale so the arrangement must not be too heavy.

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 #1070 on: October 15, 2018, 02:36:37 AM »
Hi Derek and MJM,  Ok I have looked at the construction of the kettle and i have done a boil test I have timed the test and waited till i could here the water boiling/bubbling. You can see the inner construction of the kettle that has a flat shape where the handle is although the outside is more curved. the kettle does switch off automatically but i do not know the exact temp it will do this !? It has tried to switch off before it is aurally boiling and i think that is because i have carried the insulation around the kettle rather than only covering the actual part that contains the water . the handle and the inner part is where the steam/heat operates the switching off mechanism. I have tried a preliminary test and it boils 1 Litre of water at ambient 20  cent in 3 mins 05 seconds and the kettle is rated at 1Kwatt....the outside only went up to 22 degrees !! so more  testing ... I have not tried the test with no insulation as i was going to do this afterwards and doing 4 tests by removing successive layers of the insulation !!?  This post is a bit longer as i wanted to give all the info. i usually give a short post and this does not give all the info that i should include in the question...so .. this is just an academic exercise really using the electric element to give a really reliable constant heat input. ........
Willy.........

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1071 on: October 15, 2018, 12:09:44 PM »
Hi Willy, it sounds like you have it covered.  I suspect that insulating the part below the water container might interfere with the switch off point.  I don't know how those kettles work, whether they are just temperature which would mean a switch point a little below 100 or it might never get there, or some sort of vapour pressure measurement in a bulb deep inside.  However as long as you measure the quantity as accurately as you are able and do several runs under each condition to demonstrate a level of consistency (or the degree of spread) your results will be quite interesting.  Your thermometer might help.

Watching with interest

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 #1072 on: October 16, 2018, 02:41:42 AM »
Hi MJM, Ok so....I thought originally that my kettle with its water at a certain temp with a fixed input heat would boil at the same amount of time  ??   And ..I have done an initial test with the different amounts of insulation  and the time taken to boil and switch itself off was virtually the same. However the temp outside of the kettle increased dramatically with no insulation in the final test. So was the time taken to boil the same because the 1 litre of water with a 1 Kw heater took quite a short time to boil....??  And i was thinking about formulae .. 1X1X1 = 1 however 2x2x2 -= 8  and 3x3x3 = 27 etc or is this intuitive thinking a bit silly ??!!! If 2 litre with 2Kw is heated will it take longer ??  so what is the  verdict with this experiment, and would actuall a longer time , using the steam produced in a closed vessel driving an engine give more pertinent information....... so the conclusion i have is that there would be no saving of electricity to have an insulated kettle ??!!!! :-\....

Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1073 on: October 16, 2018, 11:00:25 AM »
Hi Willy, quite a lot of information there, so let's go through it one thing at a time.

Kettle plus water at the same temperature with the same heat input should take the same amount of time, provided only that the quantity of water is close enough to the same.

The outside temperature of the insulation or jug decreases with the amount of insulation.  With more insulation, there is more temperature drop through the insulation, so quite as expected.  The method of attaching the thermocouple to the outside is an important factor in the actual readings, but as it was the same in each case, the error is probably about proportional so the direction you have observed is consistent.

1 x 1 x 1 = 1, 2 x 2 etc. the maths is correct for that sum but consider a different sum -

1 / 1 = 1, 2 / 2 = 1, 3 / 3 etc, so the answer depends on which sum you do.  The question is which sum should we have done? (The / symbol meaning divide)

It takes energy to raise the temperature of the water (and the jug).  We can work out how much energy is required for the water by looking up the enthalpy column in the steam tables which give us the energy in a mass of one kilogram.  Using 20 C as the start point (just to be able to find the figures directly listed) and 100 C.  The figures are 83.96 at 20 C and 419.04.  We can subtract these to get 335.08 to heat water from 20 to 100.  The units are KJ/kg, so are for 1 kg, so to find the answer for 2 litres, close enough to 2 kg, we multiply that by 2 so 670 KJ is the amount of heat required.  It is a bit more complicated to work out how much the jug material absorbs, but let's accept that as a constant error, so leave it out.

Now if the jug has a 2 kW element, it gives 2000 W.  One Watt is one Joule/second, so 2 kW supplies 2000 kJ/s, or 2 kJ/s. ( k being used to mean thousand)

So to find the time to heat the water, we want 670 KJ supplied at 2kJ/s, so we divide 670 by 2, or 335 seconds or 5 min 35 seconds.  (Remembering that the jug will absorb some so prolong the time, as will any heat loss from the surface of the jug.)

If you have a smaller jug, which contains 1 litre, but has a 1 kW element, it requires half the energy for the smaller mass, but has half the heat input, so the time will be the same, 5 min 35 sec.

You didn't say what capacity or heating element your jug has, so I have just taken nice easy figures to illustrate.

I have mentioned that it will take extra time due to the heat absorbed by the jug and also due to the heat loss to the atmosphere.  The actual jug should have the same effect on each of your experiments.  However that outside temperature determines the heat loss to the air, which will also prolong the time.  However, the heat loss is proportional to the heat transfer coefficient, and area as well as the temperature.  The area is quite small (in square meters) and the heat transfer for natural convection is also relatively low.  And remember that the heat loss at first, when the outside temperature is very close to the ambient temperature, is almost the same in each case, then slowly increases as the water temperature increases.  But the effect of that heat loss depends on how it compares with the element heat output.  If it is a very small proportion, say around 50 watts, it is only about 2%.  But that same loss will be more significant if you only have a 1 kW jug.  If the measured time is very similar for each of the experiments it suggests that the heat loss is small compared with the element output, and so not detected by the accuracy of your experimental technique.

The other factor in trying to actually predict the answer is the element itself.  You will remember from the discussion of your electric boiler, that the rating is a nominal figure, and the exact heat output depends on the element tolerances and temperature, as well as your exact supply voltage.

Overall an interesting experiment, even if your conclusion is that the manufacturer saving money by not insulating the jug is not costing you a lot of electrical energy, compared with the cost of the insulation. 

You might be interested to find, if you look around a hike equipment store, that hikers gas stoves can be supplied with heat fins that keep the combustion gasses close to the walls of the jug, and increase the heat transfer rate, and insulation around the outside of the fins.  This is quite expensive but saves sufficient gas in boiling the jug to be worth it, not only for the cost of the gas, but because the mass of gas has to be carried.  I guess the fins and insulation both contribute to the savings.

I hope this contributes to understanding the results of the experiment.  I am sure you can see the comparison with your electric boiler.  Just allowing for the different layout and quantities.

Thanks to everyone 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 #1074 on: October 17, 2018, 02:35:26 AM »
Hi MJM, thanks for this, and i suppose that just doing this relatively quick boil up does not give as much info needed for using a boiler as a source of power to drive an engine for an extended period of time. I was wondering if some of the heat used to boil the water actually goes into heating up the insulation which is why the last heat up was a bit less than the others ? Also i think that the radiation / conduction / convection all have different effects with the different amounts of insulation. All in all a worthwhile academic exercise that has some relevance to thinking about everyday energy conservation in the home. !! With an externally fired boiler the insulation would be far more important as you have found with your experiments.

Willy


Offline MJM460

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Re: Talking Thermodynamics
« Reply #1075 on: October 17, 2018, 12:41:57 PM »
Hi Willy, I tend to agree that there is not much more that can be gained by further experiments with the kettle in terms of boiler performance, as in a boiler we are more interested in steam production, the heat up time is simply a start up delay, possibly a bit annoying but not really important.  But the experiment with the jug does tell us a lot about heat transfer that is of interest, so don't dismiss it lightly.  Let's summarise it.

First yes, some heat is stored in the cork during heat up.  To understand whether this is important, let's assume 9 mm of cork.  We need the density of cork, about 150 kg/cubic meter, and the specific heat, about 1880 J/kg.C. 

From the density we find that the mass of 9 mm of cork is 0.009 X 150 = 1.35 kg per square meter.  If the kettle area is about 0.2 square meters, that would be 0.27 kg.

Now we use the specific heat to work out how much heat is required to heat that mass of cork by one degree C.  0.27 X 1880 = 508 J/Deg C.

The average temperature of the cork when the kettle boils is approximately 60 deg, so from the stat at ambient temperature until the kettle boils, the heat absorbed by the cork is 508 X 40 = 20304 J, or 20.3 kJ.  This would add about 10 seconds, about 2%, to the time required to come to the boil.   For best accuracy, it should all be taken into account.  The specific heat of cork is relatively high but because the density is low, the actual heat required is quite low.  For comparison, the specific heat of water is 4217 J/kg, but the density is 1000 kg/cu.m.  These figures are all readily available in almost any book on heat transfer, though recent ones are likely to be more accurate than very old texts due to improvement in measurement techniques.

If your jug is glass say 2 mm thick, the density is 2800 kg/kg, the specific heat is 800 J/kg.C.  The similar procedure gives 4480 J / square meter per Deg C, and of course the glass is all heated very close to 100 C or 80 deg rise.  So for 0.2 square meters, takes 71680 J or 71.68 KJ.  Your 2 kW element has to run 35 seconds just to heat the glass.  I don't know how close my guesses are to the actual mass of glass in your jug.  Of course, if your jug is plastic, the density will be much less, but I don't have figures for specific heat.

The other thing your experiment does is gives us some real figures to use in exploring how the theory works.  We have discussed the theory previously, but let's remember the formula and put in some real figures.

I want to recheck my calculations before posting that part, so perhaps tomorrow as it is getting late here.

Thanks to everyone 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 #1076 on: October 18, 2018, 01:50:43 AM »
Hi MJM, the jug is a plastic item and the element is 1KW . I have just got the 2 volumes of this book and i think it was the precursor to the Cassells book. It has a vast amount of info including the 20" of rainfall in Hunstanton  AKA Sunny Hunny !! there are lots of new words in it that i have never herd of before and the original came out about 1890. this edition is about 1918 !! The info therein is mostly in imperious measurements and i don't know if the values for everything has changed after 100 years of further investigations ?? It gives formulas for so many things  Chimney dimensions etc etc. So here are a few pages from the 780 ??
Willy..........

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1077 on: October 18, 2018, 01:52:18 AM »
More pics.

Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1078 on: October 18, 2018, 11:47:16 AM »
Hi Willy, what an interesting book.  You might notice how many of the topics are still the stuff of Thermodynamics and Engineering data books today, and that the laws of thermodynamics were known and being formulated in words even then.

The differences to today are in detail more than substance, and reading the words that reflect the understanding of the time might well be helpful as an intermediate step to understanding the words used today.  The differences seem in the main to be in detail rather than substance.  The information on parallel motion mechanisms was interesting.  Even then it was known that the Watts linkage is only an approximation, understanding that is almost lost today.

I have not checked all the material properties, but I would expect that differences from the values tabulated today would be mostly due to developments in measuring instruments and experimental technique.  So I would expect that today's values are a little more accurate, but probably only enough to give a little more precision rather than a significantly different answer.  And of course if they were using slide rules or manual calculation, they would probably not notice the differences.

Modern computers facilitate doing many more calculations which can sometimes be important.  With manual calculations, and I can assure you even with a slide rule, which would have been a great time saver when it came into general use in science and engineering, the temptation is to minimise the number of calculations and use hand drawn graphs and artistic licence to estimate values in between.

The other thing you will notice with those inferial units is the various constants in the formula.  This comes about from the somewhat arbitrary definition of units of force, and the calorie, which were defined for everyday commercial purposes well before the fundamental relationships between them were understood.  The SI system which I prefer for these calculations has the advantage of being developed later and so the unit of force has been defined in therms of Newton's laws, which results in a rational system of units, where for most formulae, the constant of proportionality is 1, much easier to remember and use in multiplication and division than the ones which crop up in other unit systems, not only ft.-lb.-second, but also metric systems such as centimetre-gram-second, or meter-kg-second.  At least they all use seconds for time.

Have not made any progress on the calculations today.  We are supposed to be retired, sitting around wondering what to do.  Hah!  We have been flat out for a 12 hour day, pausing only for a short lunch and late afternoon break.  Among other things, regrouped the tiles in a bathroom.  So much for nothing to do.  I don't know where all that extra free time is supposed to come from.  So perhaps tomorrow for calculations.

I expect the plastic jug would absorb much less heat than glass, but unfortunately my book does not see to list likely plastics.  It is still worth exploring a little further.

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 #1079 on: October 18, 2018, 11:48:34 PM »
Hi MJM , what i really like about this book is that it gives the names of the people that discovered and recorded the actual topics that are explained. as it says on the title page   A good book !!

Willy