The efficiency of a boiler is a matter of paramount importance. Extensive test are made to find out if an installation is working at maximum efficiency, the amounts of steam generation and of fuel consumption being carefully recorded
COAL AND WATER are fed automatically to many boiler installations. From the huge bunkers above the boilers, coal is fed down separate chutes into each firegrate. The pumps to the right of the boilers feed water to the drums. On the extreme right is a water softening plant which helps to increase the boilers’ efficiency by removing scale-forming substances from the feedwater.
IT is a matter of considerable importance to make sure that a new boiler installation, whatever its cost, will work with safety and with efficiency. Large quantities of coal are consumed, and the greater the cost of that commodity the more need there is to see that none of it is wasted. A small installation, costing some £4,000, consumes about 4,000 tons of coal a year if working night and day. This, with coal at, say, 25s. a ton, means an annual coal bill of about £5,000. An increase of 10 per cent in efficiency would save £500 a year.
The main test of the safety of a boiler is generally carried out at the maker’s works, where it is put under a hydraulic pressure at least 50 per cent higher than that at which it will have work. The testing of boiler drums by X-rays is described in the chapter “X-Rays in Industry”. There are also various physical tests of the steel plates and other parts of which the boiler is made. These parts have to conform to certain strict regulations issued by the Government and by the insurance companies concerned.
There are many factors affecting the efficient daily working of the boiler. It is the practice to carry out more or less elaborate tests on the site, after the installation has been completely erected, with all its auxiliary machinery and accessories. The principal tests are made to discover how many pounds of steam the boiler will evaporate for each pound of coal burned; if it will maintain the heaviest demands for steam which are likely to be put upon it in normal working conditions, as well as during periods of overloads ; whether it will, at the same time, produce steam without priming, or give what is known as dry steam; and so on.
There are many types of boilers, which have been described in the chapter “The Story of the Boiler”. The type most commonly used, however, in thousands of industrial works all over Great Britain is the Lancashire boiler, with its cylindrical shell from 7 feet to 9 feet in diameter and from 25 feet to 30 feet long.
The main principles and features of a steam test of such a plant are similar for all types, except for the differences of higher pressures, greater capacities and added refinements. In large power stations the equipment is more elaborate, for the annual coal bill may run into tens of thousands of pounds.
In the common industrial installation the plant to be tested generally includes an economizer, which is a nest of cast-iron pipes of about 4 in. diameter, through which the feedwater to the boiler is pumped. The economizer is placed in the flues, in the way of the stream of hot gases leaving the boiler. Sometimes the water is heated by exhaust steam in a feed heater. There is also frequently a superheater, which is another nest of smaller tubes, of the best mild steel, fitted at the back end, or downtake of the boiler.
Then there are the necessary feed pumps, which may be driven by steam if they are of the reciprocating type, or by electric motors, if they are of the centrifugal turbine type, for higher pressures. If natural chimney draught is not sufficient, forced or induced draught fans are provided; further, if hand firing is not adopted, the mechanical stoker may be driven by a small steam engine, or by an electric motor. All these accessories take their share in contributing to the efficiency of the boiler plant. Larger plants may include air heaters.
For the purpose of carrying out the tests, certain measuring instruments are required. These are for weighing the coal and for measuring the quantity of water pumped into the boiler and the amount of steam generated by and leaving the boiler. The temperature of the gases in the flues and of the water entering its first heating stage, as well as during its entry into the boiler, and the temperature of the steam before and after it is superheated must also be recorded. The draught is an important item, as it affects the amount of coal which can be burned. The chemical contents of the gases and products of combustion have to be analysed, chiefly to ascertain the amount of carbon dioxide (CO2), which has its bearing on efficiency of combustion. Further refinements include the analysis of the coal used and of the amount of moisture in the steam, and the analysis of the water and of climatic conditions during the period of test.
Removal of Scale
If the boiler is not new, but has been working for some time, it should previously have been thoroughly scaled and cleaned. Scale forms on the water side of the shell plates, just as an ordinary house kettle becomes furred up. This scale, which may reach any thickness up to an inch, is a bad conductor of heat, and it will reduce the evaporative power, as well as causing local overheating. The economizer must also be free from internal scale, and the outer surfaces of its pipes must be scraped clean of soot. The flues must also be cleaned out. The duration of the test is generally from about eight to ten hours; sometimes, with large installations, the test may last two or three days. The longer the test, the more accurate the result obtained, as any fluctuations in the load, or changes in the working conditions, will have less effect on the final result and more accurate average figures for all readings will be secured.
Before starting it is necessary to make sure that a plentiful supply of coal and water for the duration of the test is available, and that there are sufficient observers to take the periodical readings at all points where automatic recording instruments are not in use. Coal and water are the two main items, for they will tell what the boiler really does; the other details show how the boiler does its work.
The coal may be weighed by ordinary scales as it is used, or a known quantity, which has been weighed by the truckload, may be deposited on the firing floor. One disadvantage of this method is that if some is left over at the end, or has to be made up, that quantity has to be weighed and adjustments must be made accordingly.
Before beginning to take readings it is necessary to see that the load, or steam demand, is about normal, that the condition of the boiler fires is satisfactory, that the coal hoppers of the mechanical stokers are full, and that the steam pressure is at or near that at which the test is to be carried out. The pressure is generally just below that at which the safety valves are set to blow off. The water level in the boiler should also be about normal, the feed pump running steadily and the mechanical stokers working.
INSTRUMENT PANELS of boiler installations contain gauges, recording instruments and meters which instantly show whether any part of the system is working at maximum efficiency. In addition to the usual gauges, there are instruments which record steam flow temperature and pressure, feedwater meters and handwheels for the control of combustion.
The same conditions should obtain also at the end of the test — particularly in the water level of Lancashire and other boilers which have a big water volume. The reason for this is that a difference of an inch in the water level of such a large area might mean about 120 gallons, or 1,200 lb. of water for a boiler 9 feet by 30 feet. The steam pressure should be as nearly as possible the same at the beginning and at the end.
The temperature of steam — when it is not superheated — has a direct relation to the working pressure; in other words, the temperature at which water boils varies with the pressure. At atmospheric pressure the temperature of boiling point is 212° Fahrenheit, but at 150 lb. steam pressure the temperature of the steam is 358°. A cubic foot of water at 62° Fahrenheit weighs 62| lb., but at 358° it weighs only about 55 lb. The effect of this is that a difference of one inch in the water level will represent more or less weight according to the temperature.
Observations Every Half-Hour
When everything is ready, the test may be started at, say, nine o’clock. The works plant and machinery will have got into their stride for the day, the boiler will have been well warmed up and will have settled down to its work. We will assume this work to be the supply of steam to one large main steam engine driving the works or factory. A start at this hour will enable an eight hours’ run to be made with only one break for a meal. The test would not stop during that time, but as the works would generally be shut down there would be little load on the boiler and the test conditions would vary.
The object of the test, however, might be to determine how much coal is used over a full working week. The testers would then take into consideration all meal-time stoppages and night working conditions. This is what really counts, because, though a boiler may give satisfactory results when all conditions are favourable, the results may be unsatisfactory when all the factors which tend to reduce that efficiency are taken into account. If there is much difference between day and night conditions it is advisable to carry out separate tests for each to obtain correct figures for the respective performances.
The principal readings to be recorded for the data from which the results are worked out are numerous. Among the most important are: boiler steam pressure and water level as shown by gauges; gas temperatures at various points in the flues; feedwater temperatures at the inlet to and outlet from the “hotwell” from which the pump draws its supply; economizer water temperature at inlet and outlet; superheater temperature at inlet and outlet; the quantity of steam passing out of the boiler as shown by a steam flow meter; the coal used and the ashes left over at the end of the test. The amount of smoke issuing from the chimney must also be noted.
Readings are taken on log-sheets, and observations are made every half-hour if the load on the boiler is fairly steady. If, however, it fluctuates much from the normal full load, it is advisable to take readings more frequently. To synchronize the readings taken by the observers, some of whom may be out of sight, it is sometimes necessary for the chief observer to indicate the time by a loud whistle. When, however, several automatic recording instruments are used there is no need for such measures, especially if the instruments are fitted with an integrator which adds up the totals as they accumulate, by means of several dials resembling those of an ordinary house gas or electricity meter, divided into units, tens, hundreds, thousands and so on.
THE CONTROL AISLE of the boiler house at Battersea Power Station (see pages 164 and 165). In this huge power house, where the steam is used to generate electricity, perfect control of the boiler installation is necessary. The boilers can evaporate 330,000 lb. of water in an hour, the steam having a maximum temperature of 900 degrees Fahrenheit.
Every reasonable effort, especially in the matter of the demand for steam from the boiler, is taken to preserve as steady a load as possible, thereby allowing more easy control of the fires and more uniform conditions all round. When the end of the test is approaching care must be taken that all the main conditions of pressure, water level, state of fires and so forth are similar to those in force at the start. The more important readings at start and finish should be checked by two observers. For tests of long duration, the readings need not be taken so frequently and every hour might be sufficient for the purpose, with observers working in two or three shifts.
Next comes the process of working out the results. Possibly the boiler is specified to evaporate 10,000 lb. of steam an hour, at 150 lb. working pressure, and 8 lb. of steam are expected to be evaporated for every pound of coal consumed. Coal has been burnt at a stated rate of so many pounds for each square foot of firegrate area an hour, with cold feed at 60° Fahrenheit.
The main calculations are quite simple if the boiler has fulfilled these requirements. Dividing by the number of test hours the total weight of steam generated and passed off by the boiler during the whole test period gives the amount of steam generated in an hour. The amount of coal consumed can be measured. Then dividing the amount of steam generated per hour by the amount of coal burnt per hour gives the evaporation of steam generated per pound of coal.
This is by no means all, though it tells the owner the bare facts of what the boiler does. There are many smaller factors affecting the results. To enable all boilers to be put on the same footing for comparisons of efficiency, certain standards have been fixed. Other things being equal, if one boiler were fed with water at 60° Fahrenheit, and another had the feedwater heated up to nearly boiling point, the second boiler would evaporate more water per pound of coal than the first, even though it were otherwise identical.
For this particular comparison the results have to be corrected to that standard. The amount of heat required to convert water at 212° Fahrenheit into steam at the same temperature-is made the basis of this standard. With a pressure of 150 lb., when the temperature is 358° Fahrenheit, account has to be taken of the extra heat in steam at that temperature, and of the extra water which that excess quantity of heat units would evaporate at 212°. The total evaporation found by this calculation is known as the “evaporation from and at 212°”. The relationship between this and the actual evaporation of the boiler is called the “factor of evaporation”.
Then, again, one boiler may be using inferior coal, and the calorific value of the coal has to be ascertained by analysis. Such a test determines the number of standard heat units, known as British Thermal Units, which the coal possesses. For example, the calorific values of coal may vary from over 15,000 B.Th.U.s for a good Welsh anthracite down to 12,000 B.Th.U.s for a cheaper coal of inferior quality.
One British Thermal Unit, or B.Th.U,, represents the amount of heat required to raise one pound of water at its maximum density by 1° Fahrenheit, that is, from 60° to 61°. It will thus be seen that one pound of coal of 15,000 B.Th.U.s calorific value should theoretically raise the temperature of 150 lb. of water by 100°. The total heat in one pound of steam, raised, from water at 32° Fahrenheit to steam at 150 lb. pressure, is 1,193 B.Th.U.s. Theoretically one pound of coal of 15,000 B.Th.U.s should evaporate 12| lb. of steam in those conditions. It does not, because of various losses in the process. If a boiler, burning coal of this calorific value, were to evaporate 10 lb. of steam per lb. of coal, its efficiency would be 80 per cent. The more accurate tests of power station installations generally give the result as the number of pounds of steam generated for each thousand B.Th.U.s of the coal. In industrial concerns the results are sometimes given as so many shillings cost per 1,000 lb. of steam raised, the chief item of the cost being coal.
Some boilers, if forced hard to give the required output, will perhaps evaporate 10,000 lb. of steam, but when the moisture content of the steam is tested — by instruments called calorimeters — it may be found that, because of priming in the boiler, the steam contains as much as 5 per cent of water. This water has not been evaporated and it is worse than useless, for it tends to create more moisture as it passes along the pipes. Moreover the water is definitely injurious, sometimes dangerous, to the piping and the engine that it is supplying. This is one of the reasons why superheaters are used.
When superheaters are fitted such moisture content is evaporated, but at the expense of the final temperature desired for the steam. There is also the disadvantage of scale being formed in the superheater tubes. The presence of scale tending to reduce their efficiency and to cause overheating.
Various anti-priming devices, or separators, are used to catch such water from the steam before it leaves the boiler.
The heating of the feedwater not only gives higher efficiency and greater evaporation, but also makes the life of the boiler much less subject to strains. Such heating may be carried out by a variety of methods. For the Lancashire type and for many water tube boiler installations the economizer plays a great part. This accessory is placed in the flue between the boiler and the chimney, so as to make use of the gases which leave the back end of the boiler at about 600° to 650° Fahrenheit and would otherwise go to waste up the chimney.
OPERATION OF A COAL METER is illustrated by this diagram. A continuous chain runs over a sprocket, geared to a mechanical counter, and down the chute leading_from the coal bunkers to the firegrate of the boiler. The chain, being immersed in the coal, is carried down with it at the same speed. If the capacity of the chute is, say, 10 cubic feet, this quantity of coal moves the chain once down the chute, the counter thus recording the proportionate number of units.
The saving in coal consumption, or the approximate increase in the boiler efficiency due to the economizer, varies from 10 to 14 per cent. When coal is dear this means that the first cost of the economizer — some hundreds of pounds — may be saved in two years or so. If a superheater is fitted also, the effect of the economizer is not quite so great, because the superheater has already extracted some of the heat.
Further economy can be secured, with a still higher feed temperature into the boiler, by making use of the exhaust steam from the boiler pump, as well as from any other steam-driven auxiliaries such as the stoker engine and the economizer engine. Instead of allowing the exhaust from these engines to go to waste, it may be led into a tank of water called the “hotwell”, from which the pump draws it and passes it on to the economizer for the second stage of heating. The hotwell may raise the water temperature from its normal atmospheric condition to as much as 150°. Further economies are effected by lagging the steam pipes, or covering them with a thick layer of some non-conducting material.
Although a boiler may give high efficiency figures when new, it will not continue to do so for long. The depreciation will depend on the water used and on the amount of soot deposited in the flues and on the outer surface of the economizer pipes. Hard water, which contains considerable quantities of lime in the form of carbonates and sulphates, causes scale on the inside water surfaces of the plates. Scale and soot are bad conductors of heat, and the longer and harder the boiler is worked the more serious does the accumulation of scale become.
A deposit of even one-eighth of an inch of scale appreciably affects the heat transfer to the water. The soot on the economizer pipes is kept down to reasonable limits by the use of mechanical scrapers which are driven up and down the outside of the pipes. For the boiler, water-softening plant may be installed, or certain chemical ingredients which prevent the formation of hard scale may be introduced with the feedwater.
These leave a soft sludge, some of which can be easily discharged each time the boiler is - blown down. There are certain contrivances for blowing the soot out of the flues, though this is not quite so easy in the Lancashire type as in the water tube boiler, where it is deposited on the outside of the tubes, and can be swept off by steam jet soot blowers applied while the boiler is working.
Accurate automatic measuring instruments play an important part in the satisfactory testing of a boiler installation. When the coal is fed on to the fires by mechanical stokers, with hoppers overhead from which the coal runs down chutes to the firegrate, it is possible to introduce a coal meter. This is generally based on the principle that a given weight of coal occupies a certain volume, no matter how big or small the separate lumps may be. A measurement of the volume of coal passing down the chute is also a measure of its weight, and can be automatically recorded.
A Paradox Demonstrated
This question of the respective weights of two receptacles of equal volumetric capacity but containing coal in lumps of widely different sizes has puzzled many. Most people, including many engineers and others who handle coal, are firmly convinced that the receptacle containing small coal will weigh more than the one filled with large lumps. Some have refused to believe that the two receptacles will weigh the same.
This statement was disputed by responsible officials of a large steel works. Tests lasting over a week were carried out by the laboratory staff, who made several special screens with meshes varying from 1¼ in. down to 1/16 in. After equal volumes of the various sizes had been weighed, it was found that they all weighed the same within limits of not more than about 1 per cent. Another method tried was to fill up the air spaces of these two similar containers with water, after they had been filled with the different coals, and it was found that the same amount of water was poured in either instance.
The fact thus proved has made possible the use of a simple form of meter. It consists of an endless chain which passes down the column of coal, entering it at the top and leaving it at the bottom. At the same time the chain operates a counter mechanism giving the volume in cubic feet, or the equivalent weight. For example, if the area of the chute is one square foot and the coal, taking the chain with it, travels 10 feet downwards each minute, the volume which has passed in that time is 10 cubic feet. When coal is fired by hand this method cannot be so easily applied, unless the coal is fed from an overhead bunker through a chute on to the firing floor, and is from there shovelled up by the fireman. There is also the simple but more laborious method of weighing the coal on scales as it is used.
Water is most conveniently measured by a reliable water meter placed in the pipe line, but for greater accuracy a system of measuring tanks is sometimes fitted up. Three tanks are used, or two of these may be replaced by one which is divided into two compartments. The upper two tanks, into which the main water supply is led (with an arrangement for switching the pipe over from one to the other as each is filled) are placed above the third. From the bottom of the lowest tank the feed pipe goes to the pump. The upper tanks are graduated in gallons or they may be provided with a gauge glass showing the quantity drawn off at each filling.
COAL METER FITTED in a chute of 15 in. diameter, for testing purposes. Only about 4 feet of chain were immersed in the coal, but tests proved eminently satisfactory. The diagram on the previous page explains the principle of operation.
In the process of the combustion of coal the carbon in the coal combines with the oxygen in the air admitted to the furnaces and forms carbon dioxide, or carbonic acid gas, represented by the symbol CO2. The maximum theoretical possible proportion of CO2 is about 19 per cent. Too much or too little air is detrimental to the best efficiency. A fair average for various coals is about 12 per cent. In practice this may be anything from 5 to 15 per cent, according to the firing conditions and the skill of the operators.
This percentage is a definite indication of combustion efficiency. Two different methods of measuring the CO2 are used; one by electrical instruments, based on the principle of the Wheatstone Bridge acting in changing gas conditions. The Wheatstone Bridge is an electrical device for measuring an unknown electrical resistance. Another commonly used instrument automatically abstracts small samples of the gas from the base of the chimney, and by chemical means determines and records the percentage on a continuous recording chart.
The steam flow meter is an instrument for measuring the amount of steam — in pounds — which passes through any given pipe, no matter how the amount fluctuates. The design is based on the principle of producing a differential pressure on the two sides of a thin plate with a central hole through which steam may flow in large quantities. In place of an orifice plate a special cone-shaped nozzle may be used. When a fluid passes through such an orifice plate, inserted between the flanges of two adjacent pipes, a differential pressure is set up on the two sides of the plate which varies in a definite and known relation to the rate of flow. By applying this varying pressure through connecting pipes to certain mechanism in the instrument, indicating pointers, or recording inked pens are caused to move over a flow scale or chart.
This differential pressure is proportional to the square of the flow through the pipe line, and is translated by the mechanism into terms of flow such as pounds, gallons or cubic feet, as may be required. The chart generally takes a circular form on the face of the instrument and is rotated by clockwork. It is divided into time units over the twenty-four hours, and a continuous graph is marked by the pen on the chart, giving the reading in pounds of steam passing through the pipe at any minute through the round of the clock. Thus it is possible to see at a glance how much steam is passing at any moment. The boiler attendant often knows better than those in the works about the fluctuations in load that go on during the day. As, it would be difficult to read the small figures on such a chart from a distance, meters are made with big figures on a large segmental scale, and a moving pointer shows the amount passing at any moment.
It is possible to combine several different readings on one instrument, such as the steam flow from the boiler, the amount of air being supplied for combustion and the temperature of the flue gases — three important factors which have a joint bearing on the efficiency of the boiler. Such instruments are called boiler meters. There are also automatic steam-temperature recorders, so that a permanent robot sentinel can be provided to keep a tireless watch on everything that happens. Some firemen do not like these instruments, feeling that they are spying on their work, but the instruments give valuable and helpful information which would otherwise not be available.
Many other measurements of various kinds may be made in a boiler test. A full report made on the lines of the Code adopted by the Institution of Civil Engineers would contain more than ninety separate entries.
X-RAY EXAMINATION of a high-pressure vessel in the works of a boiler manufacturer. The rays will reveal the slightest defect or flaw in the metal which otherwise might escape an ordinary inspection.