Talk:Steam locomotive/Superheating

Superheating

Introduction

Aspects of superheating as applied to steam locomotives could fill entire libraries, but here is a summary. In the 19th century most steam locomotives used saturated steam. Archibald Sturrock, a locomotive superintendent of a British railway, put the design philosophy of the day in this way: "The power of a locomotive is its capacity to boil water." To make a locomotive more powerful, it was seen to be a simple matter of making the boiler larger and larger. This "bigger is better" treatment was applied especially to the fireboxes which were made wider and longer, thus necessitating some form of mechanical stoking. However, it was realised fairly early on in this process that contemporary steam locomotive development was heading up a blind alleyway. The boiler could only be made so big, as it had to fit under the bridges, past the stations & signals and through the tunnels. Long boilers tend to be overly heavy, or have an unacceptable overhang on the locomotive.

Successful researchers realised this in the late 1800s, and continuing all the way through the 1900s, and even into the 2000s, they pursued another line of investigation, one of the most successful ever applied to any steam installation, be it a locomotive, a ship or a power station: superheating. Swengel (1967:122) said that 'no single development ever equalled the superheater as a means of removing limitations from steam locomotive design'.

The Reasoning Behind Superheating

What is superheating?

In any steam boiler, a certain amount of heat energy is applied to the water to make it boil. The steam thus produced is at the same temperature as the water below it. This is called saturated steam. If the steam is taken away from the boiler, in a pipe, and then further heat is applied to that steam, the steam can rise above the temperature of the water in the boiler. This heated-above-the-boiler-temperature steam is called "superheated".

Saturated steam has some of the water in it, either as suspended droplets carried over from the boiler water, or as condensation from the steam being cooled slightly when it leaves the boiler and comes in contact with as-yet-unwarmed engine parts.

Dryness Fraction

If the steam is almost all water droplets, then it is said to have a low dryness fraction (say, 1%). If it has almost no condensate, then it is deemed to have a high dryness fraction. Dryness fraction is indicated in several ways, as a percentage (10% dryness fraction means that the steam is 10% pure steam and 90% water droplets) or as a fraction A dryness fraction of ½ means that ½ of the steam is pure steam and the other ½ is water droplets. Dryness fraction is also given as a unitless number: 0.5 is the same as ½.

It should be noted that the drier the steam, the better the performance of the locomotive or steam plant in general, since steam has expansive powers and water droplets do not. Supplying lots of water droplets into a steam cylinder would make it unable to work, or make it work very poorly. One pound of saturated steam at 200 p.s.i. occupies 2.134 cubic feet of volume. When admitted to the cylinders the water (condensation) content of this steam is usually about 7%.

Steam domes were provided on some locomotives in an attempt to take the steam from the highest point available, and thus have the driest steam available. The taller the steam dome, the drier the steam, or so says the theory. But as we saw in the introduction to this section, given that the boiler has to fit under the bridges, the bigger the boiler, the smaller the steam dome. Some locomotives had no steam dome.

The Dryness Fraction of the supplied steam is crucial to the efficiency of a steam plant. A low dryness fraction equals a very low efficiency. Superheated steam has a 100% dryness fraction.

Although water in its gaseous form (i.e., steam) can be compressed, liquid water cannot be. As a result, any liquid water trapped in the steam cylinders can cause great problems, especially if the cylinder has piston valves. If the cylinder has slide valves, they can "open" to allow the trapped water to escape. The higher the dryness fraction, the lower the amount of trapped water that will be in the cylinders. Trapped water that cannot escape from the cylinder can cause the engine to "hydraulically lock", thus bending the connecting rods, breaking the cylinder casting, shearing the piston retaining mechanism or destroying other vital components. Such a locomotive is in need of urgent major repairs. Avoiding trapped condensation water was thus of the highest priority for the drivers of the locomotives. Prior to superheating, and given the use of piston valves, a steam locomotive could not handle more than 7% water (from whatever source) in its cylinders, without sustaining significant damage.

Demand For Increasing Haulage Capacity of Locomotives

It was obvious by the 1880s that boilers could not go much larger without major expense of raising all of the bridges and re-boring all of the tunnels, yet the demand for more powerful locomotives was present on every continent. Thus, prior to superheating locomotive development had almost reached its practical capacity with saturated steam. This need for increasing power and tractive effort was especially true in the United States as the rail transport industry had just embarked on a project to vastly increase the carrying capacity of their railroads in concert with the breathtaking expansion of the needs of the oil industry. It was the increasing use of the products of the Oil Industry that paid for this, ironically a trend which later saw the eclipsing of the rail industry in the United States as the primary means of land transport.

At first, compounding was used to try and provide the increased power, by increasing efficiency in the use of steam. These locomotives went under many forms, the best known of which was the Mallets, a compounding and articulation system named after Anatole Mallet. He was a Swiss national working in France, who devised his system for narrow gauge locomotives to increase locomotive hauling capacity (and thus line capacity). Mallet's type of articulation was taken to its logical extremes in the US, which saw some gigantic locomotives culminating in the truly gargantuan Virginian Railroad's 800 class 2-10-10-2s which had their low-pressure cylinders at 48 inches in diameter (1.219 m) and the less-than-entirely-successful "triplexes" of the Erie railroad and the Virginian railroad.

Research leading to the development of Superheating

With the obvious need to provide more powerful locomotives, and the boilers now reaching their size limits, and the cylinders getting larger and larger (with all of the problems associated with balancing), research revealed that increased heating surface in the boiler was insufficient by itself. The free gas area, given as the sum of the cross-sectional areas of all of the fire tubes in the boiler, must be at least 16% of the grate area, and preferably a lot more. Research proved that the best arrangement was for a tube whose length was 100 times its internal diameter, for the best efficiency. Too small, and the gas flow is impeded. Impeded gas flow means the locomotive must pump more steam up the chimney to make the gas flow through the tubes, thus working the locomotive harder to produce the same steam. This greater pumping effort means increased back-pressure, which in turn reduces the power available for traction. Too large a tube and the flue gasses will form a cool "skin" along the surface of the tube which will allow most of the heat of the fire to shoot up the middle of the tube and not do any useful heat transference before being blasted out of the chimney. However, any boiler designed on the above idea of length being 100 times the diameter will be too long, too heavy and (likely) too expensive.

It was Dr Wilhelm Schmidt of Kassel, Germany, who undertook the follow-up research to the heat-transfer problems within boilers and it was he who developed the successful superheater.

The First Superheaters

Whilst the principle of superheating had been known for a very long time, no practical way had been found to make use of it. In the 1890s, Dr Schmidt began trials of his design of superheater, which was a form of "flame tube" superheater - a single, very large boiler tube of 17.5 inches (440 mm) across had a large number of steam pipes placed within it. These steam pipes were the superheater elements. The flame tube thus allowed the full force of the fire's burning to impinge upon the superheater elements, thus providing the greatest heat transference possible. A trial on a steam locomotive was arranged and two locomotives were so fitted, an S3 and a P4 of the Prussian railways. The S3 started it's trials on the 13th of April of 1898, and completing them in the same month. Results were very encouraging, although there were distortions in the flame-tube.

Two more designs were then produced; a type of smoke-box "superheater" which provided a very low superheat and a fire-tube superheater, in which mid-sized tubes were placed in the boiler and then the superheater elements placed within those. The first type (the one in the smoke box) turned out to be less than successful, and was eventually superseded by the second, which became standard on virtually all large steam locomotives until the very end of steam.

After the superheater became widely used, the normal method for superheating was set. This is to pipe the steam from the dome to a wet header in the smokebox; sometimes the throttle, or regulator, is in the dome, sometimes in this header. The steam is then directed through a set of smaller "U" shaped tubes running inside enlarged boiler fire tubes, and thence into a second dry header, where the throttle could be located in the case of some advanced locomotives. The now-superheated steam then passes on through to the cylinders and after doing useful work pushing the piston back and forth, exhausts up the chimney.

Superheating is also fitted to stationary steam plant and to marine steam engines.

Superiority of superheated steam locomotives

Dr Schmidt's road tests of his superheater designs showed that there would be a saving of 12% at least in coal consumption, with a possible 20% or more if high degrees of superheat were applied . Similar reductions in water consumption were also noted. The Tractive Effort of the locomotives also seemed to be increased, especially the continuous tractive effort when the locomotive was running at or near top speed. In short, it meant that at the same time as increasing the loads being hauled, the locomotives ran the same runs using less fuel and water.

To quote from T. Grime, in the Proceedings of the Institute of Locomotive Engineering:

When steam is superheated to a final temperature of 650°~700 °F (340 °C~370 °C), economies of 15 to 25% in fuel and 25 to 35% in water are attainable. With 300 °F (150 °C) superheat, the water and coal consumption may be as low as 16 pounds per indicated horsepower hour (9.7 kg/kWh) and 1.8 pounds per indicated horsepower hour (1.1 kg/kWh). In general, the steam economy is roughly proportional to the specific volumes of saturated and superheated steam at the pressure of generation.

[Thus] the operating range of any given [steam] engine is increased; this is of especial benefit to tank engines and to all engines working in areas where fuel is expensive or water is scarce. The proportion of time spent in taking water and fuel, or alternatively, the frequency with which these operations are necessary, is therefore reduced.

Degrees of superheat

Given that superheating produces an increase in efficiency of 10-15% for an increase in temperature of 100 to 150 F° (55 to 85 C°), some locomotives have a greater degree of superheat applied. In a traditional steam locomotive, sometimes higher temperatures did not offer proportionally greater efficiencies. The reasons for this were studied extensively by André Chapelon in his work, which showed why this was the case. His rebuilt locomotives made excellent use of very high superheat, and were the more efficient because of it. His student, namely Livio Dante Porta, took up Chapelon's work and extended it. The degree of superheat applied to Livio Dante Porta's locomotives would have been considered "prolific" even bordering on the "insanely high" by the designers of traditional steam locomotives. Superheated locomotives were sometimes fitted with pyrometers to indicate the steam temperature which, towards the end of the steam era, was typically around 600 °F (315 °C).

To quote from 'Steam Locomotive Design: Data and Formulae":

there is an unfortunate lack of uniformity in the practical application of the descriptions given to steam in its various states. The acceptance of the following is, however, fairly general:- Any apparatus giving a superheat of 10° - 20 °F (5.5 °C - 11.5 °C) is termed a steam drier.

• A low degree of superheat is one giving a superheat of 50°F to 100°F;
• A moderate degree implies a superheat of about 100 °F-200 °F;
• A high degree superheat is that in excess of 200 °F.

Livio Dante Porta started his superheat at 450 °C (842 °F), which would probably come in under the general heading of "excessive" in the above scheme of things and his locomotives were very successful with a greatly increased efficiency. This was due to many other changes that Livio Dante Porta realised had to be made in order to take advantage of the increased superheat. In one locomotive, he increased the superheat by the simple expedient of hammering tapered wooden dowels into the smokebox end of the non-superheater boiler tubes thus stopping any flue gasses from passing into them. This reduced to zero the heating surface of the non-superheater flues, while vastly increasing the heat and flow through the superheater elements. The locomotive so treated was noted for it's increased overall efficiency.

Characteristics of Superheated Steam

To quote from 'Steam Locomotive Design: Data and Formulae":

Compared with saturated steam, superheated steam:

1. Possesses a larger volume and greater total heat value per unit of mass
2. More nearly approaches the condition of a perfect gas. It is therefore more fluid; hence, frictional resistance to flow is reduced and, consequently, a higher flow velocity attained under given conditions [this has been disputed, but it's part of the original quote so I have left it as is]
3. Of itself possesses no lubricating qualities whatsoever, and is intensely cutting in action. This explains the necessity for the adoption of cast iron, or, preferably, steel for piping, together with the other measures previously mentioned [better lubrication is needed because the steam temperature is higher]
4. Does not increase the load starting capacity of any given locomotive, but provides a considerable increase in the power output when running
5. Gives a more rapidly falling expansion curve on the indicator diagram, but coincidentally the back pressure is reduced and the compression curve improved, to such an extent that the mean effective pressure is not impaired and is, in fact, higher at piston speeds of 1,000 feet (300 m) per minute and over
6. Has a lower heat conductivity [which means the cooling near the walls of the piping does not propagate as fast across the steam volume as in saturated steam - the middle of the flow remains hotter, longer]
7. Must lose all superheat before condensation can commence
8. Enables the water to be carried at a lower level in the boiler as the evaporation [in the boiler] is not so rapid and high rates of combustion are unnecessary [thus reducing fuel wastage]. Incidentally, this procedure also conduces to a higher dryness fraction, and therefore tends to raise the degree of superheat

Applications of Superheated Steam to Locomotives

The first applications of the superheater were to stationary practice, then marine practice, then the locomotives in Germany, namely the S3 class 4-4-0s from the class build of 1899 onwards. The S3 class appeared from 1893 onwards, thus the earlier ones had no superheater; this was fitted later on when the locomotives were given major overhauls. In Europe and Britain, and especially France, the use of the superheater spread rapidly.

In the United States, the first wide-spread use of the superheater seems to have been in the D16sb class 4-4-0s of the Pennsylvania Railroad in 1905. The better known E3sd 4-4-2's of the Pennsylvania Railroad were superheated and these locomotives formed the trial for the better-known K28's and K4s' Pacifics (4-6-2's)

The last known large American main-line steam locomotive built new without a superheater seems to have been the F15 4-6-2's of the Chesapeake and Ohio Railroad of 1902. They were all superheated following rebuilds during the 1920s, and some seemed to have survived in that form until the dieselisation efforts of the 1950s. After these locomotives, it would seem all new built locomotives for mainline passenger or express use had superheaters fitted from new.

On freight engines, this was the era of the Drag Freight, a reference to the practice of attaching as many wagons behind the engine as could be dragged up the steepest of grades. This was slow, plodding, tough and very high powered haulage, which was seen as not benefiting from superheating. Thus large saturated steam locomotives for freight duties appeared as late as the early 1920s. After that date, the superheater was deemed to be proven technology and thus the more recently built freight engines were slowly superheated as major overhauls fell due. Freight engines built new after the mid 1920s appear to have been superheated from the start, especially those locomotives which hauled the newly-appeared fast freight. The Era of Drag Freight was ending as competition with the motor vehicle became more apparent and rail began to lose out on the more time-sensitive freight to the highways. It should be noted that the way the rail companies had built up their systems to handle these enormous, slow freight trains was one of the reasons they were simply not agile enough to counter the threat from road transport.

Balancing disadvantages and advantages of Superheating

• Superheated steam is a most erosive substance. This means the parts exposed to it are likely to wear out faster, unless made of higher tensile substances. While this meant the superheater and all of the piping and valving needs to be made of more expensive materials, the savings in fuel and water are such to more than compensate for the increased costs. Also, the better the materials, the lower the maintenance costs.

• It also requires special lubrication because of the increased heat. Given that the problem of adequate lubrication was only resolutely tackled right at the end of steam, the use of superheated steam meant this problem, which could be more-or-less ignored on saturated steam engines, became acute.

• Related to the above, saturated steam will provide lubrication for moving parts; superheated steam will not.

• The fitting of any new piece of equipment, such as a superheater, will require more maintenance. However the superheater will reduce the maintenance costs of the boiler, because the boiler is less stressed.

On balance, the superheater is a highly desirable device, except where the use of the locomotive is such that the superheater would not get the time to warm up (say, a shunting or switching locomotive). It will reduce costs, fuel, water and maintenance. It increases power and range of the locomotives.

Conclusion to Superheating

The benefits of superheating are many.

One of these is in the reduction of this heat loss through condensation. There are references to an increase in starting tractive effort due to superheating, however such was not strictly true. The early practice following the initial introduction of superheating involved lowering the boiler pressure (thus lowering boiler maintenance costs) while increasing the cylinder size to obtain to achieve the same tractive effort. At the same time the removal of many small tubes and replacing them with larger diameter flues, to hold the superheaters, lessened the amount of evaporate surface, compared with the old saturated boiler. While this limited the capacity of the locomotive's boiler, the locomotive itself had a higher power, due to the presence of the superheater. Some superheater-fitted engines did not perform as well as was expected, entirely due to a misunderstanding of the needs of a superheated engine By the mid-1920s, designers understood that superheating, large fire-spaces and a good boiler capacity were the key to successful locomotives.

Many locomotives had a superheater fitted, without these additional desiderata, and the superheater was subsequently blamed for the lack of performance. It was the study of these failures and the application of the lessons learnt in the design work of André Chapelon, sometimes ignored in the United States, that showed how to use the superheater effectively. These modifications included the use of very large pipes to conduct the steam to where it was needed; overly large valves with a long lap and a locomotive exhaust system that could handle the greatly increased through-put of steam. Chapelon's final rebuilds in 1935 had unbelievable power-to-weight ratios of around 42 draw bar horse power per ton. To give a salient example, if a New York Central Niagara 4-8-4 had had a similar power-to-weight ratio to that of André Chapelon's last set of rebuilds, the New York Central's locomotives would have been capable of producing over 17,000 rail horse power.

Even today, André Chapelon's lesson has not been well learnt, if at all. It should be noted that the best a modern-day diesel locomotive can achieve (and at 214% the initial purchase price of a steam locomotive) is a power to weight ratio of 22.84 draw-bar horse power per ton.