Flash Boilers, or "Proportionate" Generators.
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By D. H. Simpson.
The introduction of the Clarkson and Darracq-Serpollet motor-omnibuses, fitted with flash boilers, brings the question of this type of generator prominently before the public. The flash boiler, as now used, owes its existence to the perfection to which small liquid-fuel burners have been brought, to the production of nickel-steel tubes, and to efficient cylinder lubrication at high steam temperatures. Its successful application to road vehicles for public service can be credited to the skill displayed, by the respective makers, in the design of the apparatus controlling the supply of fuel and water to meet the varying demand for power.
A flash boiler is totally different, in the way it turns water into steam, from an ordinary water-tube or fire-tube boiler. Any one of the present-day boilers, of the second and third patterns named, is, to all intents, a vessel containing water which, on the application of heat, is brought up to boiling point and converted into steam. The water is turned into steam, which rises, and which, if not withdrawn, exerts pressure on the water, the effect being that the water must be further raised in temperature to make steam against this increase in pressure : for any given steam pressure, the water must be at a certain temperature. Hence, in course of ordinary service, a boiler contains
A CONSIDERABLE VOLUME OF WATER,
at a high temperature, which has taken a certain amount of heat to raise it to that temperature. When the steam pressure falls, a portion of this heat is given out, and, if it rises, some more heat is stored in the water, whilst, if a certain rate of steam production is required for an engine, heat must be supplied to the boiler at the exact rate necessary to keep both the steam pressure and the water level constant. If insufficient heat is given, more steam will be drawn out of the boiler than is being produced, with a consequent loss of heat, causing a loss of pressure, or a reduction of the quantity of water in the boiler. If too much heat is given, more steam will be made than is used, which will raise the pressure ; or, more cold water can be pumped in, so as to raise the water level. The water in the boiler can circulate, rising, mixed with steam, up certain tubes, and, when relieved of the buoyant steam bubbles, can descend. The steam and water are in the same chamber, and in intimate contact, over a large area.
A flash boiler, on the other hand, is, practically, one long tube, which is wound into a convenient shape so that it can be heated in the most suitable manner when placed over either a solid-fuel fire or a liquid-fuel burner. The water to be turned into steam is pumped in at one end of this tube, and is pushed forward along it, during which course it is, gradually, warmed up to boiling point : it then, more rapidly, moves forward by its expansion, due to bubbles of steam increasing its volume : later, as steam of larger bulk, it goes, rapidly and finally, as extra heat is taken up, and issues in a highly-superheated state. Thus, while passing once through the boiler, the water must be turned into superheated steam. It is the
ABSENCE OF RE-CIRCCI ATION
of any unevaporated water that constitutes the vital difference between flash and ordinary tube boilers. In the latter, a portion of the water may circulate through the tubes many times, before being turned into steam and delivered to the engine ; on the other hand, in a flash boiler, every drop of water is forced through, whether it be converted into steam before leaving the boiler or no, and, if complete conversion is not effected during ene passage of the coil's length, wet steam, i.e., priming, will result.
In order to be certain that no water is allowed to be delivered by the boiler unevaporated, the steam must be superheated. As steam, once made, requires very little additional heat to increase its temperature, it is evident that, if too much heat is given to the tubes in proportion to the water flowing through them, the steam will be excessively superheated, with resultant trouble in the engine. In a flash boiler the water contained in the tubes bears such a small relation to the heating surface, being, roughly, one-twentieth, or less, of that of a water-tube or fire-tube boiler of equivalent evaporative capacity, that it requires very little additional heat to raise the temperature of this water. With an ordinary boiler, it is perfectly easy for the driver to ascertain, by means of the pressure and water gauges, whether the boiler is receiving sufficient heat to meet the momentary clemand for steam. If the heat supplied by the fire is too much, either the steam pressure or the water level will rise. If, to take the opposite case, too little heat is given to the boiler, the pressure, or the height of water in the glass, will fall accordingly. Thus two simple instruments, which were originally intended to warn the driver of dangerous extremes, show, in combination, the ratio between the heat given to the boiler and the heat delivered in the steam. In a flash boiler, this
RATIO IS NOT ASCERTAINABLE,
as .there is no variable amount of stored heat, although, as the amount of stored heat is so slight, it is all the more necessary to maintain the correct relation between the supply and demand. This accounts for the very rapid rise of pressure on a flash i boiler; when the' consumption of steam is ; ds, for instance, when the engine is stopped, provided the fire is not reduced. If, on the other hand, the water pumped into the boiler is less than the amount of steam being taken by the engine, or is stopped completely, a rise in pressure, due to the fact that the heat normally required to vaporise the incoming water is exclusively given to the water already in the tubes, takes place. When the water contained in the tubes is evaporated, a sudden fall in the steam pressure results, due to the absence of any water to make steam.
A more descriptive name than " Flash Boiler" would be " Proportionate Generator," as the quantity of steam produced, from instant to instant, is (or should be) in strict relation to the heat given to the boiler in the same periods. In an ordinary boiler, the contained water is a store of heat, and this water content can receive, or give up, heat within a certain range ; this quality, lacking in the flash boiler, enables the older types to deal with differences between the rate at which they receive heat and have to part with it in the steam they deliver. Thus, if more steam is required than the fire is evaporating, the contained water can be drawn upon to give up some of its heat ; or, stopping the supply of additional water relieves the boiler of the work of heating up the entering water. This "loaning " action is strictly limited, and the" accommodations" must be repaid out of a surplus of heat when the boiler is receiving more heat than is necessary to make the required amount of steam. The absence of stored heat in a flash boiler, besides preventing ready ascertainment of its working state, makes it necessary that the amount of heat, and also the amount of water, shall be both proportionate to each other, and exactly sufficient for the requirements of the engine at the time of its demand. We have here the great difficulty of the satisfactory application of the flash type of boiler, to supply steam to an engine, even on a constant load, and it will be clear that the variable loads, which are incidental to the propulsion of road vehicles, make the problem of the greatest difficulty. In order to show how the present successful systems have been evolved, I will name the methods that have preceded them. Serpollet, for locomotive purposes, started with coke firing, and a simple by-pass valve, the latter of which could vary the amount of water discharged by the feed pump, or, if kept open when the %Thiele was standing, could empty all water out of the boiler into the water tank. The bore of the tubes was reduced, by a longitudinal indent,
to a crescent form, giving a large surface and small volume. To strengthen the tubes, and to store a certain reserve of heat, which should be instantly available, the tubes were covered with cast iron, or, alternatively, made very thick, and these tubes were placed over a fire contained in a box lined with fire-brick. When, in the driver's estimation, firing had proceeded sufficiently to make the tubes hot enough, a small quantity of water was pumped into the tubes by a hand pump. If the tubes were, in fact, J fficiently hot, the water pumped in would be " flashed '' into steam, and the vehicle would start, after which the pump driven by the engine would keep up the water supply. The quantity of steam sent to the engine was controlled by regulating the amount of water fed to the boiler the pressure of steam was that necessary to drive the engine. The very small cubic contents of the tubes, which held only a few cylinder-fulls of steam, yielding this readily. To stop, the feed supply to the boiler was entirely diverted, and the boiler emptied. This meant re-filling the boiler by the hand pump, and, to avoid the exercise involved, some vehicles, steam trams for example, were stopped by brakes, steam being left on : when a re-start was required, the brakes were released, and the vehicle proceeded without the necessity for use of the hand pump.
A further improvement was the introduction of a stop valve in the main steam-pipe. Some form of safety valve was necessary, so a spring-loaded valve was substituted for the hand-release valve. Means were also provided for the driver to regulate this valve, so that the pressure in the boiler at which the feed water was returned to the tank could be increased to meet hills or other heavy calls. The driver, by this means, could anticipate an increase in the demand, and prepare for it, by raising the pressure before the engine required it, so saving a drop in speed, as otherwise the engine would slow down till the pressure of steam in the boiler increased hy its not being withdrawn so rapidly.
In 1898 the writer replaced the water-tube boiler fitted to an experimental steam car with a flash boiler made up of weldless steel tubes t 5-16ths inch in external diameter by ths inch bore. These were connected up in series, and heated by a coke fire. The tubes were indented under Row's-patent to get the water and steam broken up in their course, and to prevent a solid core of water passing through the tube, and issuing as such, without being converted into steam. The advantages and disadvantages of the flash boiler were soon apparent, and we were con
fronted by the problem of whether to control the temperature of the steam, and to use it in the engines we had (compound, double-acting, piston-valve), or to design fresh engines to cope with the high superheat. The former course was chosen. The first plan tried was the injecting of a portion of the feed water into a drum, through which the steam passed to the stop valve. This worked all right, but led up to the principle of passing the feed water inside a tube through the steam drum, a practice which reduced the steam temperature fairly proportionately and worked well. Innumerable arrangements of valves and feed pumps were tried with the objcct of controlling automatically the water supply, but all proved incapable of dealing with this complex problem. An integral feed pump, giving fixed volumes of water per engine revolution, variable from about one-half to four times the normal consumption, under the driver's control, proved to be the best under working conditions. A thermometer, placed about half-way along the circuit, gave readings which approximated to those of the pressure gauge on an ordinary boiler, so that the driver could regulate his fire by them. Great trouble was experienced with donkey pumps for use when standing; also the burning away of the tubes (nickel steel being then unprocurable) was serious.
The defects of the flash boiler, when fired with solid fuel, completely
OUTWEIGH THE ADVANTAGES,
as compared with an ordinary boiler fitted with a superheater. The fuel consumption is greater, and the water consumption, by reason of the amount of heat that has to be wasted through the impossibility of exact adjustment of the lire to the demand, is only very slightly lower. The combination of a liquid-fuel type of flash boiler and a solidfuel fire will be a failure, and only a source of continuous trouble in regular everyday work on commercial vehicles.
So far, only solid-fuel firing has been described; this necessitated the heavy tubes, to store some of the heat to tide over firing-up, and the variations inseparable from a tire. A certain amount of automatic control of the heat was obtained by the discharge of the exhaust steam into the funnel or doWn-flue : as more steam was used, more draught was given to the. fire.
In a continuation it will be explained why flash boilers, when fitted with liquid-fuel burners, have met with greater success, in England, France, and America alike.