Talking Thermodynamics

[crueby]

Hi MJM,...a friend has a flat in the attic of a victorian house and she is suffering from the high temps about 32 degrees. She is wondering how to keep it cool and says a fan doesn't make any difference. I have given her the info about closed windows   blankets etc. She says the window  perhaps is not glass but some sort of acrylic. it is a rented flat and there is possibly no insulation in the roof. I was wondering if muslin cloths in buckets of water might cause cooling by evaporation ?  Have you any ideas please....thanks

Willy

Basically the same setup as my upstairs shop, house is a Cape Cod style, with a 45 degree roof so the upstairs rooms only have windows in the end walls. When I bought the place, that space was unfinished, and the heat was awful up there in the summer. Insulation helped a lot, next step was to put one of those whole-house fans in one end window, with that on and the window at the other end open, it draws a tremendous amount of outside air through, cooling the space a lot - the sun on the outside roof and walls heats them up and turns them into a radiator on the inside. Short of air conditioning, it is the only thing that makes the space usable, and does not cost much to buy or run. I keep the drapes closed when the sun is on the windows as well.

[MJM460]

Hi Willy, I can always rely on you to take an unusual view on what was intended as a simple proposition.  It really tests my understanding and makes it all more interesting as there is normally something in what you say that needs to be understood.  In the context of reducing the fuel input to balance the reduced power output required to travel slower, you need to reduce the rate at which fuel is being burned to reduce the boiler energy input.  Now with gas or oil firing, you can usually reduce the firing rate by closing in on a fuel throttle valve, and get a reasonably quick response, though probably still slower than the driver may require.  With coal, the requirement is still to reduce the rate at which fuel is being burned.  I am not an expert on burning coal, and I will defer to Paul and others on the detail.  Once the coal is on the grate, I assume you can't get it back, so you can only let it die down a bit while perhaps the fireman gets a break in order to slow the fire.  And that is definitely too slow to please the driver.  The issue is more complex than that, with the need to ensure adequate air flow etc.  However, I would expect that while putting in more coal might help in the short term, when it heats up and starts burning, assuming adequate air flow, the rate the coal is burned might be higher if the fire is well managed.  But adding coal as you describe might well be in the fire man's bag of tricks to cope with rapidly changing situations.  The video Gas Mantle has posted might include useful information.  I am on very limited data at the moment, so am unable to watch it until later.

Hi Gas Mantle, thank you for posting that video.  Great to hear from you again.  I will come back and watch it when I am back on full data allowance.  These "pay by the Gb when you exceed the allowance plans" are really aggravating, especially when mostly you have plenty left over.  Unused data allowance should really carryover to be at all fair for the occasional month.  I hope your practice at coal burning with your vertical boiler is benefitting from this discussion.

Now, back to Willy's question about cooling the attic.  Putting blankets or curtains over the window and similar measures help reduce the heat input from the sun, but the heat is still coming in and with the roof not insulated, will not help enough in a long heat wave such as you are experiencing.

A fan works by increasing the evaporation of perspiration, but that same evaporation also increases humidity which reduces comfort by limiting the potential rate of evaporation.  A single person loses about 90 watts from normal life metabolism, so the fan helps the person feel cooler, but is still heating the room.  To improve comfort, you have to add ventilation, with open windows, on opposite sides of the apartment, if practical, and preferably with a fan placed to increase the air flow.  Best if the fan adds to any slight breeze rather than opposes it, so the best direction may change from time to time.  Again, to really work the process, if you know the difference between inside and outside temperatures, you might work the fans really hard at night if the outside air is much cooler, but turn them off during the heat of the day if it is hotter outside.

Your muslin cloth with water and a fan comes into the area of active cooling.  Here you can buy commercial units which optimise the process.  They are called evaporative air conditioners.  My son has one to serve his whole house, and I can assure you, it is very effective.  Much cheaper to run (and buy) than refrigerated air conditioning, as they only have air fans, a water pump and fancy muslin.  Providing you have plentiful water of course.  The cooling is done simply by evaporation of water.  However there are tricks to running them.  You need a window and door open, preferably on opposite sides of the house, and if there is really no breeze, a fan to help the air flow, so there is continuous air exchange with outside.  You also need some continuous water blowdown from the reservoir, as the salts in the water remain, just like in a boiler, so will salt everything up if you don't have blowdown.  If you don't have the necessary ventilation, the air inside gets very humid and uncomfortable, and everything goes mouldy.   And they do not work so well in the tropics with high humidity.

Wet muslin cloth has also long been used here in the outback, in the form of the Coolgardie safe, which was used to keep food cooler before refrigeration became available, and also the hessian water bag that was hung from the frame of the buggy and I can even remember my father using one hanging from the car bumper in the early days on long trips, before cars had air conditioning.  The water was quite cool and refreshing even on the hottest days, so a break to get a drink from the water bag was always welcome.  So if you can arrange the muslin so it wicks up water from a bucket, or keep pouring it on regularly to keep the cloth wet, and a fan blowing over the cloth, you will get enough cooling to be worth trying out.  But you also need that through flow ventilation for air exchange with outside, otherwise the humidity increases to the point of insufficient further evaporation.  And the room is still being heated by you and the fan motor.  Evaporative cooling is self limiting in a closed space, due to that humidity and heat increase.

Best is to see if you can buy one of those portable evaporative coolers.  I have even seen battery operated ones in a camping store, but that was unusual.  If you go this way, make sure you have the required air exchange with the outside.

And you can also buy portable refrigerated air conditioners.  Again, tricks to the operation.  You need to reject the heat to outside.  Some models have only one air hose and actually exhaust the cooled air from the room, so overall are more expensive to run.  The preferred ones have two air hoses, a bit ungainly to install but by rejecting all the heat to air drawn on from the outside, instead of exhausting the cooled air, are more effective and economical to run.  Refrigerated air conditioning is the only exception to the ventilation requirement.  Ventilation is still required for healthy air, but not nearly so much as for simple air exchange or any form of evaporative cooling.

Hi Chris, thanks for joining in.  I have been in the roof space of our house on a hot day, above the insulation, and the roof tiles are really strong radiators.  Attic spaces definitely accumulate heat and  the first step is to increase ventilation as you say, to at least limit the temperature rise in the attic, by keeping it close to the outside temperature.  Roof insulation is very effective in these spaces, but of course not usually an option in a rental situation.  And of course closing window blinds when the sun is on the window.  Anything to reduce the incoming heat, and get rid of any air which is warmed by the remaining heat input.

Oh, and I have missed the comment about the fish!  Fish usually seek deep water where it is cooler, but can't do this in a shallow pond.  If I remember correctly, warm water holds less oxygen than cold, and rain should bring in oxygen saturated water, and disturb the surface to further increase oxygen content.  So I would expect the fish were weakened if not killed by the heat and lack of oxygen, and while they would normally seek the cold water, a sudden 10 degree change will kill them every time, as anyone who has had an aquarium well knows.  They do not tolerate changes of temperature very well.  But the weed will really take off in the heat, especially when the rain brings more oxygen.  Even barnacles and weedy fouling on a boat grow more slowly in the winter cold, and take off when warmer weather comes.

Thanks everyone for following along

MJM460

[paul gough]

Hi MJM & Willy, Just a comment regarding firing on locomotives. Firing rates for a class of locomotive,  hauling a particular load with certain characteristics, (Plain bearing 4 wheel stock and speed limited, air braked or not, roller bearing bogie stock etc.), on a particular section of track and the condition of the locomotive were all things known by crews. The were intimately familiar with the locos and the road over which they travelled, they had achieved their positions after decades of experience developed incrementally on various classes of loco and working, so 'knew' what would be needed. Any permanent way restrictions were all known as drivers would be informed of speed restrictions for any sections under repair before leaving the depot.

Oil firing was pretty responsive to fuel supply and coal firing a bit less immediate, adding water via the injector or starting a second one could also affect things. A loco doing proper work would soon 'die' if the inputs did not match demand, and fairly quickly. Firemen and drivers were always anticipating the conditions required ahead and were rarely caught out, and in the days of steam, for the most part they were constantly working quite hard to get the load to its destination. Conditions on preserved railways are usually a 'piece of cake' and even main line tour operations usually have more than enough people crammed into the cabs if the fireman needs help, so are not necessarily accurate guides to 'the good old days'. Hope this gives a sense of how things are/were on a steam loco. Paul Gough.

[MJM460]

Hi Paul, thanks for the interesting detail on the subtleties of engine firing.  Detail that most of us never appreciate.  Sounds like the voice of experience.

Also sheds some light on the challenges of firing a model at maximum efficiency.

Thanks for following along,

MJM460

[Gas_mantle]

A good signalman could also save the fireman a lot of work 

[paul gough]

Only too true! A speed check or halt had the potential to turn a portion of the journey, if not all of it, into something of a battle on tightly tabled sections or trains. Clawing back even a minute can create a testing time for the crew.  Paul Gough.

[steam guy willy]

Ni MJM , with a  locomotive oil fired boiler is the oil and water consumption closely correlated ? and what is the saving in fuel costs with summer temps of 30 + degrees and winter's of say -20 degrees ??  just wondering !!

willy

[steam guy willy]

Only too true! A speed check or halt had the potential to turn a portion of the journey, if not all of it, into something of a battle on tightly tabled sections or trains. Clawing back even a minute can create a testing time for the crew.  Paul Gough.

Yes ,at 60 mph or 1 mile a minute ..that last mile when you are slowing down for a station will take quite a while ?

[MJM460]

Hi Willy, sorry I was absent last night.  Unexpectedly found ourselves at a remote site with no internet service.  Should be ok tomorrow also, but after that, if I am missing a day or so occasionally, you will understand why.  But glad to see that the conversation continued.  We were rewarded last night with a magnificent view of the Milky Way, which in our southern skies can be so brilliant a stripe across the sky that it hides all but the brightest stars.  And again tonight, even clearer.

I will leave Paul to talk about the time and effort to slow down for a station, but I think his comment was more about a signalman showing an unnecessary orange signal, causing an unscheduled slowdown which puts the train a minute behind schedule, then taking a lot of work to catch up.

With regard to the air temperatures, of course the water cannot be at -20, but the air can be.  So let's assume the water is kept at zero instead of +30 by a little feed water heating using exhaust steam, and the steam raised at 1000 kPa, it takes about 4% more energy to heat the water from zero to boil and evaporate to dry saturated steam at 180 degrees C which is the saturation temperature of 1000 kPa steam.  This pressure is just an example, I don't know what pressure a mainline locomotive would operate at.  The figures come simply from reading the steam tables.

However, the air for combustion also has to be heated.  And it can certainly be at -20 in northern parts.  I don't think there are any railways in southern parts that are far enough south to get those temperatures.  Unless Southern Chile and Argentina, perhaps.

I really don't know the typical flue gas outlet temperature for a locomotive, so let's assume for example stack gas at 300 C.  With air entering at 30 and leaving at 300, the heat lost to atmosphere is due to 270 degrees temperature rise.  On the other hand, if the air enters at -20, that temperature rise is 320.  As the specific heat of air is roughly constant, the heat lost to atmosphere is increased by about 20%, and this all has to be supplied by burning more coal.

With 4% extra heat for the water, we can assume 4% extra air required, so the total extra heat is approximately 25%, (1.04 X 1.2).  And of course we should expect more heat losses from the boiler to the air rushing past.  That extra heat all comes from extra coal, so we would expect to need a bit over 25% extra coal, which might require an extra fireman.  A reasonable guess would assume running cost proportional to coal consumption, so about 25% higher cost due to the low temperature. 

I don't know the cost per mile at 30 degrees, as it almost certainly involves more than the straight cost of coal, but whatever the figure at 30 degrees, you could reasonably assume it would increase by about 25% at -20.

I assume you don't really get -20 C in your country, but I can assure you it was common enough in Southern Ontario when we lived in Canada, and almost certainly in Scandinavia, Russia, and northern parts of China.  But I am sure there are other countries too.

So in summary, we need a bit more information on steam conditions, coal and other proportional costs, and coal energy values, then the steam tables tell is the energy requirements, so estimates of cost differences can be relatively simply calculated.

Thanks for following along,

MJM460

[steam guy willy]

HI MJM, more interesting stuff...so overnight and daytime they must have had the means to keep the water above freezing......also the coal at --20 would also need to be heated up to reach combustion point.....Re- friends cooling problem ..it would appear that some expense would be incurred ...so what about in the 3 storey BLDG to leave the letter box open and have the muslin/water contraption next to her slightly ajar door ?? would there be natural convection up the stairs ?? possibly not if heat travels from hot to cold......i have noticed in my ground floor flat when i leave the letterbox open and the sash window open at the top there is a draft...?  Here in blighty they have painted the railway tracks white ,but not all of them, also the chap from the railway said they "stretch" the tracks before they weld them so they can expand/contract.....he was wearing a suit however.......Also they said that they had to shut down one of the Nuclear reactors in Switzerland because the water temp in the lake was too high......so lots of talk in the MSM about the currant high temps...

Willy

[paul gough]

From memory exhaust gas temps. on a loco are 450F. to 550F. Say 500F. for arguments sake. So your speculation of 300C. is pretty close. Obviously these figures varied somewhat over the course of the journey, but the testing/design staff would have averaged it out unless looking for a particular performance aspect. A caveat to this is, these temps. apply on the older style locos I am familiar with, I probably don't have any test details for modern Post WW1 era locos any more as I have cut down my library to pre 1900 books. However I have in mind there are some data charts in one of my remaining books that may be of interest. I shall look for it tonight and post photos if I find them.

Regarding stopping a train; this is perhaps the most important skill any driver has, again this skill is built up through years of experience, if you ever travelled in a sleeping car you will know what really skilled braking is, a good sleep and not jolted into wakefulness at every stop. Coming over the top of a rise and feeling the mild surge when a couple of thousand tons, especially at speed, becomes the propelling force, serves to remind one of the tremendous forces involved. In the steam days of paramount importance was NOT to make repeated light brake applications. I'm talking air brakes not vacuum. Braking had to be done in a fashion that did not risk depletion of the locomotives capacity to recharged the brake pipe, so goods trains were held in check by bringing their speed right down with one application and then recharging the break pipe while it accelerated again so as to have all the auxiliary reservoirs on each waggon recharged, repeated applications could drain them and then you would have no brakes! If coming down a mountain range,  a number of hand brakes might be set based on the ruling gradient which meant that steam may have to be worked on any light grade or level sections. Coming to a stop at a particular point, a station platform or gently rolling into a loop, was about being at an appropriate speed before making the final stop, all known from experience, knowing the load and what was required ahead as well as the braking characteristics of the rolling stock. Paul Gough.

[MJM460]

Hi Willy, you are quite right, the coal would also have to be brought up to temperature, and that takes from the energy left for raising steam, so further increases the required amount of coal to be burned.   I don't know if it has to be specifically heated in the tender to stop lumps from freezing together, or if it is dry enough and warm enough for that not to be a problem.  It will soon heat once on the grate, but the heat ultimately comes from burning more coal, unless it can be obtained from waste heat such as exhaust steam or even some flue gas.  It also has to be kept cool enough not to ignite in the tender, so more to it than it looks to the casual observer.  And well out of my field of knowledge.

Returning to the cooling problem, I hope it all made sense, even though installing air conditioners may not be practical.  While heat travels from hot to cold, warm air is less dense so it rises, and definitely will give a draft up through a three storey building so long as you let outside air in at the bottom and out at the top.  Even this natural air ventilation will help limit humidity rise and keep the air more fresh, so will improve comfort.  You can assist the draft by pointing a fan out wards through the upper story window, so long as replacement air is able to flow in downstairs.

Why outwards?  Because the energy consumed by the fan ends up as heat in the air, so if you are trying to cool the room, expel the air that is heated by the fan first.

A separate issue is the muslin.  In the upstairs flat, with a draft over it, it will tend to cool the air as you expect.  But cooler air is more dense, so will tend to oppose the natural convection due to the warmer downstairs air rising.  Still possible to achieve your aim, but even more important to have that fan assist by expelling air from the upstairs window.

Ideally you want that natural convection draft up the stairs, over the wet muslin, through the upstairs living/sitting area and out through the upstairs window, assisted by the fan pointing outwards through the window.  And of course plenty of open windows, doors or letterboxes downstairs to let outside air flow in to replace what the fan blows out.

Painting rails white?  I don't really know why they would do that, unless to reduce the heat input from the sun.  As we saw earlier in this thread the white paint does reduce the heat absorbed by the rails but only for the part of the sun's heat in the visible spectrum.  Still, it all counts.  But I would be concerned about the effect of the paint on wheels slipping during acceleration or braking.  Again, not something I know much about.  Continuously welded track is another area which I don't know much about.  There is quite a bit of science in tying down the track so it does not buckle on expansion when welded into a continuous long length.  A really specialist area that I have not read much about.  It is not much different from long continuously welded pipelines.  Inside a plant we deliberately install extra bends where necessary to take account of expansion and contraction, but that is not a practical solution for railway track.  But stretching the rails takes a lot of force.  A straight track is very stiff in tension or compression. 

The nuclear reactor question might have a simpler explanation.  Unlike the board meeting discussing the nuclear reactor and the bicycle shed, I would rather talk about the nuclear reactor.  Generally they use water for cooling because it has higher specific heat, so requires much smaller heat exchangers than air cooling.  And water usually achieves a lower temperature, partly because of this.  In inland installations, the plant often uses a lake or river as a heat sink for cooling.  The heat taken up by the lake or river causes an increase in water temperature until it is balanced by evaporative cooling from the surface.  This has an adverse effect on the environment, fish life etc. so usually environment authorities specify a maximum allowable discharge temperature from the plant.  If the incoming water is already hotter than normal, it may not be practical to reject enough heat from the plant within the allowable maximum discharge temperature.  I suspect this is the reason the plant had to be shut down.  Alternatively they could invest more money in the larger heat exchangers required for air cooling.

This problem is not unique to nuclear reactors, and conventional power plants can also have operation curtailed by the same constraint.  The nuclear reactor is actually cooled by raising steam, which in turn drives turbines to drive electric power generating machinery.  The turbines need a condenser for the exhaust to maximise the power generated.  The condenser is normally water cooled, but can be air cooled.  Air cooled condensers are common in the Middle East, and I know of one in Australia, though there may be others.  They are a very special beast, but they work reliably even though the minimum condensing temperature, and hence minimum turbine exhaust pressure, is a bit higher than with water cooling.

Hi Paul, I am glad that 300 was a reasonable guess, but more good luck than knowledge I am afraid.  I don't really know locomotive operating pressures either, so my logic was drawing a long bow as they say.  Modern locos might operate at higher pressures, and they would then require higher stack temperatures to transfer the necessary heat to the steam.

I was most interested to read your description of the braking process.  Obviously much more to it than appreciated by most of us.  I could not help thinking about how much greater problem those iron ore trains in the west must pose.  You only have to look at the track and sleepers to feel that those trains are really heavy compared with standard freight trains.  And the train seems to stretch from horizon to horizon.  So the driver and fireman have to manage all of that as well as managing the grate so that it neither goes out, not lifts the safety valve.  And of course does not over fill the boiler or run it dry.

What do you suggest the driver of a small gauge (3 1/2 or 5 inch gauge) could learn from full size practice?

Thanks everyone for looking in,

MJM460

[derekwarner]

Digressing a little..& apologies to Paul, MJM & Willy, however just a few weeks ago....in the West of Western Australia, Rio Tinto achieved it's first 28,000 tonne delivery of iron ore by a driverless train set

https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=2ahUKEwiQs8WO_ODcAhVCgbwKHXAMAUIQFjABegQICRAB&url=https%3A%2F%2Fwww.railwaygazette.com%2Fnews%2Ftechnology%2Fsingle-view%2Fview%2Fdriverless-heavy-haul-train-complete-first-iron-ore-delivery.html&usg=AOvVaw1ktGlg84pQjQNB2gwV3K7X

[paul gough]

I have in mind a figure of around 400C. max. for smokebox temp. on a hard working engine of the older types I am familiar with. I'll try to find something on 'modern' engines. Boiler pressures of modern locos was in the range of 250-300 psi, there were some locos that exceeded this a bit, from memory the 38 class of N.S.W. railways had Australia's highest at 245 P.S.I.

At the time I was in the railroad department of Mt. Newman Mining Co.,(1972), the trains were hauled by three 3600 H.P. Alco locos with 100 cars each of 100 tons capacity. Thus 10,000 tons of ore in each train and at the time 10 trains a day. When seen for the first time they were quite awe inspiring as they were about a mile long, huge by Australian standards for the time. After I left, radio control came in, this allowed two locos to be put in the middle of 50% larger trains for better power distribution but also to relieve the load on the draw gear. Braking back then was by conventional air brakes, but I have heard they use radio controlled braking now, but I know nothing of it. These were all diesel electric hauled trains so no grate or fireman, the crew being a driver and observer. Well, I suppose there was one grate on the locos, it was the crews toilet in the nose of the loco. Due to environmental requirements flushing 'deposits' onto the tracks was not permitted so 'electrically incinerated' toilets were fitted. Trouble was they weren't incinerators they were 'slow bakers' and the odour entering the cab was overwhelming. No-one wanted to open the cab widow, air conditioned cabs, as it was often 40 C. outside. The answer to this when on the move was to go to a trailing unit and relieve oneself or if stationary climb up into the first ore car and contribute something to japanese metallurgy.

Model operation is somewhat different to full size and don't know if there is anything specifically to be learned. Mostly it is about knowing the loco intimately and to do this means familiarity with it under various conditions of operation. The more one knows about how things work, the more one knows what is going on, then combining this with experience produces the 'feel' that good operators have.

Still looking for the tables I referred to previously. Paul Gough.

[paul gough]

Derek, Only just saw your post, I'll have a look at the article. I'm afraid I am very out of date nowadays, it is sobering to think a lot of my experiences, at the time, related to up to date locos and operation and now it is all well and truely historical and pretty much 'ho hum' by many contemporary standards. Paul Gough.