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* Thermodynamics 201

Thermo 101

Thermodynamics 101…

You may have heard the word and thought -so what? Here I intend to open your eyes and maybe help you add these principles to your next locomotive. The principles may be stated in the following three lines.

Energy Input.

Work Out. →

Waste Energy.

Thermodynamic thinking will hopefully help you design and adapt your locos to make the first and third lines as small as possible and the second line as big as possible!

The Electric Motor.

Let us take the simple idea of an electric motor running at 12V. We provide electrical energy, the shaft spins providing work and the motor gets hot…

We fit a PWM ESC in front of the motor. This produces a square wave voltage at 0 to 12V rather than a linear voltage at 12V. True the motor only sees part of the full voltage, however the ESC requires electricity to run it. Thus the ESC is PARASITIC to the Energy In. The shaft provides rotational energy, the friction of the gears etc is PARASITIC to the Work Out. The Waste energy heats the motor, however there may be an internal fan to cool the armature which increases the Waste Energy of the system...

No electric motor (other than those running in liquid Helium…) is 100% efficient. The most that can be hoped for is 60% in a model motor. This means that a 12Volt motor pulling a 1Ampere load wastes 4.8 Watts -this is what causes the motor to get hot. Big motors are more efficient than small motors as the amount of flux is greater in the armature and the coil windings can be of thicker wire... This explains why a 3 pole motor is often vastly more powerful than it 5 pole cousin.

So, what can we do about the Waste Energy?

The answer in this case has been around since the NER EF-1 series of electric locos. The braking energy was used to turn the motors into dynamos and these charged the overhead lines. There exist several ESC systems † that can harvest the energy and use this to recharge the battery pack. But a good motor is a poor dynamo and a good dynamo is a poor motor…

In a large fan-fitted motor the amount of air that the fan will blow along the armature is dependant on the rotation of the shaft. This as explained above, is parasitic to the work of the motor. Your best bet might be to have a slow rotating big motor or a thermally triggered set of external blower motors. I have done both. I found that the external blowers are the more efficient as they only operate in hot weather with hot motors. My locos “Maude” and “Lady Jane Gray” both use this system. It works well but some thought has to be given to not turning the loco into a Hoover or a Leaf Blower…

† Dimension Engineering “SyRen” series are the most common.

The Steam cycle.

This is perhaps the most complex system to envisage… Water boils to dry steam in two distinct stages. The first is when the energy supplied to the water molecule is sufficient to break free of the molecular bonds. The second stage is the transferral from a vapour to a gas stage.

Let us imagine a pan of water on a Butane gas burner. The first time we do this is at the baking hot bottom of Death Valley. The water boils at 101˚ Centigrade, the flame is bright blue... The next time we boil our pan we are at Venice Beach on the sunny California coastline and the water boils at 100˚Centigrade, the flame is bright blue... Finally we boil the pan near the cold icy summit of Mount McInley where it boils at 88˚Centigrade, but the flame is PURPLE, and shot through with flashes of YELLOW, plus the pan takes far longer to boil even at the lower temperature... The decrease in atmospheric pressure reduces the amount of energy required to break the water molecule free from the other molecules, similarly the flame is now burning Butane vapour and this burns differently to a pure gas. The flame “chokes” itself as the Butane vapour expands to become Butane gas and requires far more Oxygen whilst producing large amounts of Carbon Monoxide at a lower energy release, an unfortunate cause of death for several generations of hikers.

An example of the importance of this would be the long iron ore and coal hopper pulling steam trains that used to have to pass over the Drakensberg mountain range in South Africa. The trains were typically 2Km long and had a loco at each end and loco in the middle. The cabs all had atmospheric pressure dials as well as boiler pressure dials. As the front loco climbed up it would have to progressively stop mechanically stoking the boiler as the atmospheric pressure dropped. Then the second and finally the trailing loco. Oxygen masks etc were used for the reasons above.

What defines a vapour? If I compress a volume of vapour eventually I will get a liquid. If I compress a gas I will never get a liquid. Water has two types of bonding, the Normal Attraction between molecules and Hydrogen Bonding. This is the bonding between adjacent Hydrogen atoms and accounts for the reason why ice expands at lower temperatures. This bonding also accounts for why Wet steam has so different characteristics to Dry steam. In wet steam there is insufficient energy to completely break the Hydrogen Bonds and they still stick the water molecules together making it a vapour. If we add sufficient energy to completely break the Hydrogen bonds and the water becomes a true gas. The advantage of this is that for the same temperature and volume of wet steam and then dry steam -we get the energy of the Hydrogen bonding BACK from the dry steam… Looking back at the Three line model, Super heating, i.e. turning the steam from a vapour to a gas, would seem to produce more energy into the First line -but it doesn’t. What has been done is to use some of the waste energy, from the hot flue gasses, to add the Hydrogen Bond energy to the steam. Thus we have decreased the waste energy not increased the input. How else can we harvest the waste energy? I think I am the only person at this scale to add the thinking of Andre Chapelon and Ing Da Porta into my design thinking. Both were late 20th Century designers and thinkers -both tried to maximise the efficiency of their locos. The other designer is Dr Karl Golsdorf who was late 19th Century.

Chapelon may be described as a designer who emphasised the importance of streamlining the steam passages and the use of compounding to extract the last ergs of energy from his steam.

Da Porta may be described as a designer who saw the emphasis on waste energy with its prevention and recycling it into the system as his priority.

Gőlsdorf may be seen as a designer who emphasised high pressure (for the time), lightness of parts and simplicity of steam pathways.

The Champelon ideal.

In this all the energy from the steam leaving the superheaters is used -even if some of the dry steam has to be reheated by the hot exhaust gases in the smoke box prior to being used for compounding. Steam should flow smoothly without frictional losses and wall effect losses.

The Champelon ideal can be approached as having what would be for a Gauge ‘3’ locomotive massive ports compared with the LBSC and HG designs, and then “polishing” them to a matte finish not shiny. A matte surface sticks a layer of air to it and the steam slides over it. Try to ensure that routes are direct and not too convoluted. Whether you use the steam for compounding at this stage is moot. The used steam is then exhausted through an exhaust system.

Forget everything you have read by HG and LBSC on the subject of smoke box calculations!

The simple rules as laid out by those two gentlemen are correct -but wrong in this case… They can only deal with small amounts of gases passing through the smoke box, the aim is to have LARGE volumes of gas “screaming” out of the smoke stack. This uses the waste energy from the steam to pull more air through the fire box, increasing the temperature of the exhaust gases and thus the temperature of the super heaters. The trick to doing this is called “The Kyla” principle. This is a group of four nozzles that fire into a cone each. This group of four then fires into another cone with a venturi and then this expands into the smoke stack. Being deaf I have been informed that a working 3.5” gauge Kyla is very noisy!!! There have been several variations on the Kyla -but all begin with Kyla.

The Da Porta ideal.

All the energy from the combustion of the fuel is used to provide energy for the locomotive; all waste energy must be reused and there should little or no parasitic losses.

The prime parts of any Da Porta system are the water pre-heater and the firebox. The water pre-heater simply uses the waste heat from the gases to preheat any mechanically injected water to the boiler. This simple device will harvest around 10% of the waste energy or use less fuel for the same amount of steam. Super heaters project well into the firebox. These may have “Los Turbulatori” which are spiral deflectors along their length to force exhaust gasses against the fire tube walls as well as acting as heat absorbers for the super heater spears.

The other classic Da Porta device is the Gas Producer Firebox (GPF). Classically the coal burns incandescently in a thin layer with a fierce draft. In a GPF the thick layer of coal burns at red heat and waste steam and compressed air are blown through it. The coal, (or llama dung!), does not “burn to completion” (i.e. CO2) but produces H2, CO, and CH4 . These are Hydrogen, (from the steam), Carbon Monoxide, (from the coal and air), and Methane, (from the coal and steam). These then burn as gases within the fire tubes mixed with a secondary air feed, directly heating the fire tubes and super heaters. This means that the amount of heat from the firebox to the side walls is higher as there is more burning material at a lower temperature next to it in a thick dense layer, rather than a thin incandescent line.

In a Da Porta system there should be no steam leaks to maximise the thermal effects.

The Gőlsdorf ideal.

Here the emphasis is on lightness of oscillating parts, as the lighter the part the less energy is needed to make it move. Steam paths should be short, simple, large and direct.

The prime part of any Golsdorf system is the steam drier, even if the later designs used super heating and compounding -the steam drier was always there. The typical steam drier used is the one designed by Clench. This is rather like a flute except the steam blows through the holes into the pipe. The steam “notes” are optimised for generation of standing waves at various rates through the rate of steam passage through the drier. It is possible to hit a higher note on a flute by blowing too hard across it -the same is true in reverse with a Clench. The compression waves tend to push the droplets to the side of the drier and these drain back to the boiler. Less energy is lost in reboiling the droplets.

The steam is at a high temperature and pressure. This is because the higher the pressure of the boiler, the easier it is to get the water to boil and the more steam per volume unit volume you get. The very low grade fuel used by his locomotives forced him into systematic design lightening, “I cannot take a tonne off a loco -but I can shave a kilogramme off each part of it”…

The steam passageways are very short, very large and at a high pressure. The large steam chest volume, compounding and superheaters, were a few of his ideas -as was the use of piston valves.

Here we have seen how three designers each strove to increase the efficiency of their locomotives by different techniques but all by tapping into the waste energy at the bottom of the system. Hopefully these will encourage you to think about how you might tap into the waste energy -this will make your run times longer and be cheaper to run your locos.

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Page last modified on April 30, 2018, at 08:56 AM