Author Topic: Talking Thermodynamics  (Read 194498 times)

Offline AVTUR

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Re: Talking Thermodynamics
« Reply #1395 on: December 29, 2020, 12:30:01 PM »
A little aside.

We used very fancy thermocouples (Platinum, Platinum/Rhodium) at work to measure very high gas temperatures. The actual measurement was fraught with problems and someone would always ask what are you actually measuring. The answer was always the same same - the temperature of the thermocouple bead.

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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1396 on: December 30, 2020, 03:16:57 AM »
thanks  MJM. and  AVTUR for the reply and  trusting the actual readings must be quite stressful especially when things dont seem to add up !! still that is research

Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1397 on: December 30, 2020, 10:43:26 AM »
Hi Avtur, thanks for commenting.  Those certainly are exotic thermocouples. 

I guess in the situation you were using them, in addition to conduction and convection, you might have radiation between the thermocouple and hotter and cooler components in the gas path.  These would also affect the temperature reached by the thermocouple. 

Willy, the other important point that is highlighted by your question is the time required for the heat transfer to take place.  This is the reason that temperature measurement is inherently slow, so difficult in situations involving rapidly changing temperatures.

MJM460



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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1398 on: December 31, 2020, 02:11:04 AM »
Hi MJM ..also I think that when I remove the thermometer from the water ..because the water evaporates it cools it.untill it gets back to ambient, there is so much going on with the TH laws that we are possibly still learning about ? Especially in space where it is very cold /hot but all the parts of the equipment still seems to operate ?!! I think one has to wait for things to settle down to get the true readings as your last paragraph infers ?!!

Thanks and a prosperous and productive new year

Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1399 on: December 31, 2020, 10:37:16 AM »
Hi Willy, you are quite right about there being so much thermodynamics going on.  It is indeed very difficult to frame these questions really completely.

When you take the thermometer out of the water, it is wet, and the water starts to evaporate.  But remember your description of the problem.  The air and water are at the same temperature! 

So why does the water evaporate?  And yes, when it evaporates, it does cool the sheath.

We should tie up one loose end in the problem description, the air humidity was not defined.  I would expect that the air humidity in the room was somewhat below 100%, probably less than 50%, so the water vapour pressure in the air was somewhat below the saturation vapour pressure.  Consequently, some of the water molecules with something above the average energy level will escape the water surface, leaving the remaining water a little cooler. The thermometer will then provide some heat to the now cooler water, and the air will supply heat to return the whole to the original average temperature.  We could carry this on to conclude that the room will end a little cooler, or the system maintaining the room temperature will supply some extra heat.  And of course the bath is also evaporating a little if the water vapour content of the air near the surface is less than the equilibrium vapour pressure for the temperature.  If the air is very still, the humidity builds near the water surface and the evaporation stops.  However air movement near the surface removes those freshly evaporated water molecules and so the process continues further.  But you might disturb this still air by entering the room to conduct the experiment.

The description can go on, but the important points are first that truly reaching thermal equilibrium, when there is no further heat transfer is a slow process, and difficult to achieve.  In practical terms we are normally dealing with close enough rather than exact.

Second, the thermometer is only measuring the temperature of the thermometer, and we need to place the thermometer in close thermal contact with the object or fluid we are measuring, and give them time to reach the same temperature before we take the reading.

Thirdly, trying to answer this question also involves understanding of evaporation of fluids and the associated heat transfer.

Things operating in space involve a whole extra level of thermodynamics, but it is getting very far from my experience.  Also, like the errors in the assumptions of conservation of mass, not likely to be of much practical use to most of us as model engine builders.  So I will use that as an excuse to call the thread back to topic!

So time for me to wish you and yours a very happy and safe new year.  Let’s hope it is a bit easier than this year for all.

Thank you for all the interesting questions that have kept my mind active throughout the year.  I hope I have been able to help you and others who have been reading the thread to understand a little more thermodynamics.

The 9pm fireworks have just happened in Sydney so the kids can go to bed.  The midnight ones will be a little shorter than usual, and people are being encouraged to watch from home.  The Melbourne one have been cancelled to avoid tempting people to gather in large numbers to see the spectacle.  A very different New Year’s Eve for most this year.  Time for us all to imagine alternative ways to enjoy seeing the new year 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 #1400 on: July 01, 2021, 12:46:24 PM »
Hi Willy, good to have you thinking about thermodynamics again, but I don’t want to hijack your build thread, I will address your question about the bath water temperature here.

Of course it’s not really about engines, but what is a water heater but a poorly designed boiler which they omitted to design for sufficient pressure, and with a thermostat set too low to generate steam?  Of course this latter fault compensates for the first, fortunately, otherwise they would be very dangerous devices.

Humour aside, the same physics applies to water heaters and boilers and every other example of heat and energy transfer, and understanding the thermodynamics helps in so many situations.

So let’s have a look at what factors affect the temperature delivered to the bath.  The thermostat controls the heat input to the system to raise the incoming cold water to the set temperature.  Then there are heat losses from the hot water pipe, which are proportional to the temperature difference between the water and the surrounding air.  The cold water inlet temperature determines how much heat is required to reach the set temperature, but not any losses after the water is heated.

The basic control is the thermostat which controls the heat addition to raise the water in the area of the thermostat to the set temperature.  If the incoming water is cooler, as it is in winter, it will require more heat to get to the set temperature, but in principle it heats to the same temperature.

But winter operation also affects the heat losses.  I suggest that there is likely high heat losses between the heater unit and the bath.  I don’t know about your building standards, but here, in all the systems I have seen the insulation on the hot water lines is pretty minimal at best, and the heat loss from a poorly insulated pipe means lower temperature delivered to your bath, particularly if the heater unit is in an unheated part of the house, and/or part of the piping is outside.  The houses I lived in in Canada, the heater was in the basement which was heated, so not far different between summer and winter, and all the piping was also within the heated walls.  Hence the water delivery temperature was not affected much.  Of course, if the heater or piping had been outdoors, it would have frozen solid and burst!

There is one other factor which can affect the delivered water temperature, and that is the thermostat itself.   Nearly all thermostats depend on a differential temperature.  Part of the actual hardware is cold.  If there was no temperature difference between parts of the thermostat, it would not work very well.  Even the thermocouple in your meter is paired with a cold junction, somewhere in the circuit.  So if the cold junction of the thermostat is outdoors and hence cooler, the thermostat would likely reach its setting at a lower than normal temperature.

In summary however, I would suggest that the primary explanation for the lower water temperature at your bath is the heat loss from the hot water pipe.  If the pipe is accessible, and you are sufficiently keen, you might improve the situation by better insulating the hot water piping.   You would then require less hot water to get a comfortable path, so save a little on the energy cost for heating.

I hope this answers the question.

Thanks everyone for looking in

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

Offline crueby

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Re: Talking Thermodynamics
« Reply #1401 on: July 01, 2021, 04:59:26 PM »
+1 on insulating the pipes - in my house the kitchen is a long run from the water heater, and before I insulated the pipes I had to run the hot water tap for a lot longer before I got hot water in the kitchen sink, the length of pipe from the water heater was acting as a radiator to heat the basement. Also did the shorter runs to the bath just above the heater while I was at it. In the winter, with the basement colder than in summer, the effect was magnified. Also, once you have the tub or sink filled, in winter with very low humidity there would be more evaporation quicker, cooling the water faster, correct?

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1402 on: July 01, 2021, 06:42:01 PM »
Hi MGM and Chris .. I live in a medieval council house .(low rent) and the uninsulated pipe from the combi boiler to the bath is about 7.5 meters in length with a 90 degree bend......The pipe runs under the floor in the cellars and there are  also air bricks under the floor that ventilate the cellars with a through draft.  There is no heating in the cellars so this is why there is so much heat loss. ??
Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1403 on: July 02, 2021, 10:57:43 AM »
Hi Chris, it’s good to know that we are not the only ones where hot water pipes are not well insulated.  I guess there is not much incentive for the builder to spend money on insulation that no one will admire, just to save the owner on heating costs.

Part of the reason it takes so long for the hot water to reach the kitchen is that with, or without insulation, when there is no water flow, the pipe cools down to the basement temperature, and when water flows, it has to first heat the pipe on the way through.  Cooler basement requires more heat to get the metal up to temperature.  Then under constant flow, the heat loss still means the eventual water temperature is a bit lower than in summer.  Heating the pipe may be the biggest problem for the kitchen sink, but a bigger volume required for a bath means the steady heat loss becomes more important.

Humidity is a less obvious but more interesting factor.  Humidity is defined as the vapour pressure of the water in the air as a percentage of the saturation vapour pressure at the prevailing at temperature.  The saturation pressure can be found in the first section of steam tables, those one or two pages that give the saturation pressure for temperatures up to the atmospheric boiling point, 100 C or 212 F.

It is easy to see that the vapour content (which is proportional to the partial pressure) is much less at lower temperature than warmer temperature, so much less vapour in the air, even at the same measured relative humidity. 

Certainly if the humidity of air in the bathroom or kitchen is low, evaporation will be higher and carry away more heat, at least if the ventilation is working.  Otherwise the mirror steams up, indicating humidity near 100%, so no more evaporation.  It certainly contributes to how quickly the bath cools after the tap is turned off.

In summer the humidity might be near what the weather bureau announces for your area.  But if the air conditioning is on, the outside humid air has been cooled, and some water drops out as we know, but the air discharged into the room can be very high humidity.

In winter, the outside air might be quite high humidity at the outside temperature, but when we bring it into the house and heat it up to a more comfortable temperature, unless we add moisture with a humidifier, the humidity at room temperature will be very low.  So as you suggest evaporation will be higher than typical for summer, which will contribute to cooling the water, and steaming up the mirror.

Hi Willy, that well ventilated basement certainly helps cool your bath water in winter as already described.  The 90 degree elbow doesn’t have much effect on heat loss, unless the fitting has a lot more surface area for heat loss than plain pipe, but still not big in the scheme of things.  More effect on the pressure drop due to flow.  Remember the fundamental law is that heat loss is proportional to temperature difference and surface area.  It is hard to estimate how much difference you can make by wrapping the accessible sections of pipe with a suitable insulating material, but it will still take more time for the first hot water to reach the bath, as the pipe will pretty much reach air temperature between baths.  Might be worth a try if most of the pipe is accessible.

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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Pulsations and water hammer -
« Reply #1404 on: August 18, 2021, 11:03:46 AM »
Pulsations and water hammer -

Some time back, when Chris was describing the water headers for the pump section of the Holly engine, he made a comment in passing that perhaps I could explain water hammer in the Talking Thermodynamics thread.  In particular, it was noticed that there were no pulsation chambers on the inlet headers, only on the discharge headers of the Holly engine, and the questions were raised regarding what exactly are they for, and why not on the inlet header.

That was a big challenge, Chris, and I am not sure that I can do it justice, but I decided to give it a try.  After all I had some involvement with pulsation dampeners on pumps in my work, so I should be able to contribute a little.  So here goes.

Pulsations and water hammer are associated with unsteady flow situations, either a significant change in flow, such as suddenly turning off a tap, or repetitive flow changes as produced by a reciprocating pump, the primary interest in the Holly engine thread.

I generally think of pulsations as the pressure changes arising from the inertia forces involved when the velocity of the fluid, water in this case, changes.  Water pipes are generally sized to give velocities less than 10 m/s.  Around 2000 fpm if you prefer those units. 

In comparison, I tend to think of water hammer as being due to the pressure change, originating from the origin of the flow change, travelling as a wave through the fluid at the speed of sound in the fluid, for water about 1200 m/s.  The actual figures vary a bit, but you can see there is a difference of several orders of magnitude.  However, I am not sure that these definitions are universal and in everyday parlance, they seem to be used interchangeably, and I will probably follow suit.  But I am open to more information if someone would be kind enough to contribute.

The important feature of the pressure pulsations in water hammer is that as they travel along a pipe, and at any restriction or change of direction, and even the pipe outlet, reflections occur which travel back through the system.  As the reflections meet the next pressure pulse, they add up.  Moreover if the distances are just right, they can result in a standing wave, and the repetitive pulsations at that stationary point can excite a natural frequency of the pipe work, and you will definitely hear and feel the result.  The same phenomenon in a gas system is the reason for the tone in an organ pipe and all wind instruments, and can break the piping in large industrial gas compression systems if not properly considered in the piping design.    Even in the case of shutting a tap in your home, if those reflections reach a bend where the pipe is not well supported, the pulse can move the pipe with quite a bang.  The solenoid valves on a washing machine can cause a severe case, but a manual tap can be shut more slowly to silence the bang until a few more pipe clips can be added in appropriate places.

The maths of these travelling pressure waves gets very heavy very quickly, and I certainly don’t understand it well enough to try and describe here.  But it’s worth understanding that there are really two different mechanisms involved.

To give an idea of the inertia forces that are involved due to the acceleration of the fluid in the system, the film from the museum that was linked to describe the engine mentioned that at each revolution, the three cylinders combined moved approximately four tonnes of water, this being the combined displaced volume of the three cylinders.  Actually 1.19 m^3 per cylinder, or 3583 kg ( or 3.95 tons) for three cylinders.  It sounds like a lot, but relative to the size of the engine, it is like any of us playing with a half kilo weight, or about 1 lb. relative to our body weight

Four tons is a correct calculation of the displacement of three cylinders but it greatly underestimates  the actual mass of water moved by the engine.  When the flow velocity changes in the course of each piston stroke, all the water in the pipe from the inlet out in the lake has to move.    At the same time, all the water from the pump to the surge tower in the water system also has to move in response to the engine piston movement on the discharge stroke.  For it to be otherwise, the water would have to compress or expand, or a gap in the water column would have to open up and close with the velocity changes.

Actually the water does compress and expand, but only a very little bit, which is the source of those pressure waves which travel at the speed of sound in the water, but not enough to significantly change the volume of water affected by the pump.  Water is not generally considered a compressible fluid for this purpose.

Also, a gap can open up when the pressure is sufficiently low, (when the pressure is below the vapour pressure, water boils to fill the space with vapour), and collapse when the pressure rises above the vapour pressure.  Unfortunately, the collapsing of the gap is very damaging and can damage the pipes in the area where the collapse occurs, or even burst the pipe, so definitely to be avoided.

However, if a gap in the water column is deliberately introduced, in the form of an air filled pulsation chamber, the water column accelerations can be greatly reduced in compressing and expanding the air in the chamber, and so minimise the effect of the long pipes on the inlet and outlet side of the pump.  The important difference is that the air in the pulsation dampeners will not condense, but simply compress and expand in a manner that slows the acceleration of the fluid, and hence reduces the forces on the system.  The air volume undergoes significant volume change as its pressure changes.

Well, that’s more than enough words to introduce the subject of water hammer and pulsations.   To make more progress on understanding pulsations in particular, we need to do some calculations.  So next post I will present the results of some maths that will allow us to quantify the accelerations and forces involved.

Thanks for looking in,

MJM460

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

Offline Admiral_dk

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Re: Talking Thermodynamics
« Reply #1405 on: August 18, 2021, 11:51:06 AM »
Nice explanation - but I see a few errors in your 'constants'.

The speed off sound in water is 1400-1580 m/s. dependent on temperature, salinity and pressure .... not 1200 m/s.

One ton is 1000 Kg. .... (thought the Yanks got that one wrong too with their 907.185 Kg.).

Per

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1406 on: August 18, 2021, 01:28:28 PM »
Hi Admiral, thanks for looking in and the kind words.  Regarding the speed of sound in water, I do believe that you are correct.  The figure that I obtained from an old text book on water hammer appears to be a misprint of some kind.  Fortunately the value does not come into my calculations, and the importance is just the comparison of typical velocity of water in pipe, generally less than 10 m/s (in my experience, this is a high figure for general pipe liquid flow) compared with the velocity a pressure change is propagated along the pipe, clearly not much chance of mistaking which one is intended.  So thanks for pointing that out, I have noted the correct value in my text book.

Regarding the ton, the situation is less clear.  The term used in this country is the tonne, or 1000 kg. also called a metric ton.  Quite confusing, as 1000 kg is also referred to as a short ton, or 2204 lb. compared with 2240 lb. also called a long ton.  I should have been precise and stayed with kg, or used tonne, or metric ton.  But then the calculated mass was 3583 kg, so about four tons, but not 4.0 and definitely not 4.00.  I am usually more pedantic about stating “approximately” or “about” when I make such approximations, I must be slipping.  Thank you for helping ensure clarification.  I will try and be more consistent.

MJM460

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

Offline crueby

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Re: Talking Thermodynamics
« Reply #1407 on: August 18, 2021, 02:55:10 PM »
Hi MJM,


Great start on the water hammer topic, I'll be following along closely on this one.

EDIT: corrected my wording on the air chambers below.

On the Holly pumping engine, they do have one air chamber on the outlet pipe, where the outlet pipes come together, and none on the inlet side before any of the check valves. On the Allis pumping engine, which I am drawing now from plans of the one in Boston, they put air chambers on every pump inlet and outlet, plus one at the very end of both inlet and outlet pipes. Interesting contrast. Both brand pumps are same era, about the same size, but do have interesting differences in design choices.
The Holly has an inlet pipe down each side of the pump chambers at the bottom, and an outlet down each side at the top, with large banks of check valves between. On the Allis, the one inlet is on one side, one outlet on the other, each with one bank of check valves. Given the 48" pipes, both have a Lot of check valves, total of 1200 and 1400 valves, several inches across each, arranged in beehive towers.


Chris
« Last Edit: August 18, 2021, 04:10:58 PM by crueby »

Offline AVTUR

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Re: Talking Thermodynamics
« Reply #1408 on: August 18, 2021, 03:57:01 PM »
MJM mentioned that water, or any liquid, is slightly compressible. Really, elastic is a better word since compressible fluids are assumed to be gases.

A water hammer can be used to transmit power similar to hydraulics although I don't think it has actually been used. A Romanian, George Constantinesco, came to London before WW1 and tried to interest the world with this method of power transmission. Unfortunately he said that liquid was compressible, was laughed at and ignored. In the end someone realised what he was saying. His power transmission system came to nothing but the principle was used, with great success, to synchonise a machine gun so that it fired through a rotating propeller.

AVTUR
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1409 on: August 19, 2021, 03:31:19 AM »
Hi All, good see some more content on this thread .. I can remember sometime ago seeing a programme from the USA about using the water hammer to heat water ..? this was filmed in a fire station and this was used with great success to provide them with hot water ??!!! this was possibly about 20 years ago . so does anyone remember this or have any info on it ??!!!

Thanks

Willy

 

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