Priming—Priming, or the passing off of steam from a boiler in belches, is caused by the concentration of sodium carbonate, sodium sulphate or sodium chloride in solution. Sodium sulphate is found in many southern waters and also where calcium or magnesium sulphate is precipitated with soda ash.

Treatment of Feed Water—For scale formation. The treatment of feed water, carrying scale-forming ingredients, is along two main lines: 1st, by chemical means by which such impurities as are carried by the water are caused to precipitate; and 2nd, by the means of heat, which results in the reduction of the power of water to hold certain salts in solution. The latter method alone is sufficient in the case of certain temporarily hard waters, but the heat treatment, in general, is used in connection with a chemical treatment to assist the latter.

Before going further into detail as to the treatment of water, it may be well to define certain terms used.

Hardness, which is the most widely known evidence of the presence in water of scale-forming matter, is that quality, the variation of which makes it more difficult to obtain a lather or suds from soap in one water than in another. This action is made use of in the soap test for hardness described later. Hardness is ordinarily classed as either temporary or permanent. Temporarily hard waters are those containing carbonates of lime and magnesium, which may be precipitated by boiling at 212 degrees and which, if they contain no other scale-forming ingredients, become "soft" under such treatment. Permanently hard waters are those containing mainly calcium sulphate, which is only precipitated at the high temperatures found in the boiler itself, 300 degrees Fahrenheit or more. The scale of hardness is an arbitrary one, based on the number of grains of solids per gallon and waters may be classed on such a basis as follows: 1-10 grain per gallon, soft water; 10-20 grain per gallon, moderately hard water; above 25 grains per gallon, very hard water.

Alkalinity is a general term used for waters containing compounds with the power of neutralizing acids.

Causticity, as used in water treatment, is a term coined by A. McGill, indicating the presence of an excess of lime added during treatment. Though such presence would also indicate alkalinity, the term is arbitrarily used to apply to those hydrates whose presence is indicated by phenolphthalein.

Of the chemical methods of water treatment, there are three general processes:

1st. Lime Process. The lime process is used for waters containing bicarbonates of lime and magnesia. Slacked lime in solution, as lime water, is the reagent used. This combines with the carbonic acid which is present, either free or as carbonates, to form an insoluble monocarbonate of lime. The soluble bicarbonates of lime and magnesia, losing their carbonic acid, thereby become insoluble and precipitate.

2nd. Soda Process. The soda process is used for waters containing sulphates of lime and magnesia. Carbonate of soda and hydrate of soda (caustic soda) are used either alone or together as the reagents. Carbonate of soda, added to water containing little or no carbonic acid or bicarbonates, decomposes the sulphates to form insoluble carbonate of lime or magnesia which precipitate, the neutral soda remaining in solution. If free carbonic acid or bicarbonates are present, bicarbonate of lime is formed and remains in solution, though under the action of heat, the carbon dioxide will be driven off and insoluble monocarbonates will be formed. Caustic soda used in this process causes a more energetic action, it being presumed that the caustic soda absorbs the carbonic acid, becomes carbonate of soda and acts as above.

3rd. Lime and Soda Process. This process, which is the combination of the first two, is by far the most generally used in water purification. Such a method is used where sulphates of lime and magnesia are contained in the water, together with such quantity of carbonic acid or bicarbonates as to impair the action of the soda. Sufficient soda is used to break down the sulphates of lime and magnesia and as much lime added as is required to absorb the carbonic acid not taken up in the soda reaction.

All of the apparatus for effecting such treatment of feed waters is approximately the same in its chemical action, the numerous systems differing in the methods of introduction and handling of the reagents.

The methods of testing water treated by an apparatus of this description follow.

When properly treated, alkalinity, hardness and causticity should be in the approximate relation of 6, 5 and 4. When too much lime is used in the treatment, the causticity in the purified water, as indicated by the acid test, will be nearly equal to the alkalinity. If too little lime is used, the causticity will fall to approximately half the alkalinity. The hardness should not be in excess of two points less than the alkalinity. Where too great a quantity of soda is used, the hardness is lowered and the alkalinity raised. If too little soda, the hardness is raised and the alkalinity lowered.

Alkalinity and causticity are tested with a standard solution of sulphuric acid. A standard soap solution is used for testing for hardness and a silver nitrate solution may also be used for determining whether an excess of lime has been used in the treatment.

Alkalinity: To 50 cubic centimeters of treated water, to which there has been added sufficient methylorange to color it, add the acid solution, drop by drop, until the mixture is on the point of turning red. As the acid solution is first added, the red color, which shows quickly, disappears on shaking the mixture, and this color disappears more slowly as the critical point is approached. One-tenth cubic centimeter of the standard acid solution corresponds to one degree of alkalinity.



Causticity: To 50 cubic centimeters of treated water, to which there has been added one drop of phenolphthalein dissolved in alcohol to give the water a pinkish color, add the acid solution, drop by drop, shaking after each addition, until the color entirely disappears. One-tenth cubic centimeter of acid solution corresponds to one degree of causticity.

The alkalinity may be determined from the same sample tested for causticity by the coloring with methylorange and adding the acid until the sample is on the point of turning red. The total acid added in determining both causticity and alkalinity in this case is the measure of the alkalinity.

Hardness: 100 cubic centimeters of the treated water is used for this test, one cubic centimeter of the soap solution corresponding to one degree of hardness. The soap solution is added a very little at a time and the whole violently shaken. Enough of the solution must be added to make a permanent lather or foam, that is, the soap bubbles must not disappear after the shaking is stopped.

Excess of lime as determined by nitrate of silver: If there is an excess of lime used in the treatment, a sample will become a dark brown by the addition of a small quantity of silver nitrate, otherwise a milky white solution will be formed.

Combined Heat and Chemical Treatment: Heat is used in many systems of feed treatment apparatus as an adjunct to the chemical process. Heat alone will remove temporary hardness by the precipitation of carbonates of lime and magnesia and, when used in connection with the chemical process, leaves only the permanent hardness or the sulphates of lime to be taken care of by chemical treatment.

TABLE 16

REAGENTS REQUIRED IN LIME AND SODA PROCESS FOR TREATING 1000 U. S. GALLONS OF WATER PER GRAIN PER GALLON OF CONTAINED IMPURITIES[16]

- - - Lime[17] Soda[18] Pounds Pounds - - - Calcium Carbonate 0.098 ... Calcium Sulphate ... 0.124 Calcium Chloride ... 0.151 Calcium Nitrate ... 0.104 Magnesium Carbonate 0.234 ... Magnesium Sulphate 0.079 0.141 Magnesium Chloride 0.103 0.177 Magnesium Nitrate 0.067 0.115 Ferrous Carbonate 0.169 ... Ferrous Sulphate 0.070 0.110 Ferric Sulphate 0.074 0.126 Aluminum Sulphate 0.087 0.147 Free Sulphuric Acid 0.100 0.171 Sodium Carbonate 0.093 ... Free Carbon Dioxide 0.223 ... Hydrogen Sulphite 0.288 ... - - -

The chemicals used in the ordinary lime and soda process of feed water treatment are common lime and soda. The efficiency of such apparatus will depend wholly upon the amount and character of the impurities in the water to be treated. Table 16 gives the amount of lime and soda required per 1000 gallons for each grain per gallon of the various impurities found in the water. This table is based on lime containing 90 per cent calcium oxide and soda containing 58 per cent sodium oxide, which correspond to the commercial quality ordinarily purchasable. From this table and the cost of the lime and soda, the cost of treating any water per 1000 gallons may be readily computed.

Less Usual Reagents—Barium hydrate is sometimes used to reduce permanent hardness or the calcium sulphate component. Until recently, the high cost of barium hydrate has rendered its use prohibitive but at the present it is obtained as a by-product in cement manufacture and it may be purchased at a more reasonable figure than heretofore. It acts directly on the soluble sulphates to form barium sulphate which is insoluble and may be precipitated. Where this reagent is used, it is desirable that the reaction be allowed to take place outside of the boiler, though there are certain cases where its internal use is permissible.

Barium carbonate is sometimes used in removing calcium sulphate, the products of the reaction being barium sulphate and calcium carbonate, both of which are insoluble and may be precipitated. As barium carbonate in itself is insoluble, it cannot be added to water as a solution and its use should, therefore, be confined to treatment outside of the boiler.

Silicate of soda will precipitate calcium carbonate with the formation of a gelatinous silicate of lime and carbonate of soda. If calcium sulphate is also present, carbonate of soda is formed in the above reaction, which in turn will break down the sulphate.

Oxalate of soda is an expensive but efficient reagent which forms a precipitate of calcium oxalate of a particularly insoluble nature.

Alum and iron alum will act as efficient coagulents where organic matter is present in the water. Iron alum has not only this property but also that of reducing oil discharged from surface condensers to a condition in which it may be readily removed by filtration.

Corrosion—Where there is a corrosive action because of the presence of acid in the water or of oil containing fatty acids which will decompose and cause pitting wherever the sludge can find a resting place, it may be overcome by the neutralization of the water by carbonate of soda. Such neutralization should be carried to the point where the water will just turn red litmus paper blue. As a preventative of such action arising from the presence of the oil, only the highest grades of hydrocarbon oils should be used.

Acidity will occur where sea water is present in a boiler. There is the possibility of such an occurrence in marine practice and in stationary plants using sea water for condensing, due to leaky condenser tubes, priming in the evaporators, etc. Such acidity is caused through the dissociation of magnesium chloride into hydrochloride acid and magnesia under high temperatures. The acid in contact with the metal forms an iron salt which immediately upon its formation is neutralized by the free magnesia in the water, thereby precipitating iron oxide and reforming magnesium chloride. The preventive for corrosion arising from such acidity is the keeping tight of the condenser. Where it is unavoidable that some sea water should find its way into a boiler, the acidity resulting should be neutralized by soda ash. This will convert the magnesium chloride into magnesium carbonate and sodium chloride, neither of which is corrosive but both of which are scale-forming.

The presence of air in the feed water which is sucked in by the feed pump is a well recognized cause of corrosion. Air bubbles form below the water line and attack the metal of the boiler, the oxygen of the air causing oxidization of the boiler metal and the formation of rust. The particle of rust thus formed is swept away by the circulation or is dislodged by expansion and the minute pit thus left forms an ideal resting place for other air bubbles and the continuation of the oxidization process. The prevention is, of course, the removing of the air from the feed water. In marine practice, where there has been experienced the most difficulty from this source, it has been found to be advantageous to pump the water from the hot well to a filter tank placed above the feed pump suction valves. In this way the air is liberated from the surface of the tank and a head is assured for the suction end of the pump. In this same class of work, the corrosive action of air is reduced by introducing the feed through a spray nozzle into the steam space above the water line.

Galvanic action, resulting in the eating away of the boiler metal through electrolysis was formerly considered practically the sole cause of corrosion. But little is known of such action aside from the fact that it does take place in certain instances. The means adopted as a remedy is usually the installation of zinc plates within the boiler, which must have positive metallic contact with the boiler metal. In this way, local electrolytic effects are overcome by a still greater electrolytic action at the expense of the more positive zinc. The positive contact necessary is difficult to maintain and it is questionable just what efficacy such plates have except for a short period after their installation when the contact is known to be positive. Aside from protection from such electrolytic action, however, the zinc plates have a distinct use where there is the liability of air in the feed, as they offer a substance much more readily oxidized by such air than the metal of the boiler.

Foaming—Where foaming is caused by organic matter in suspension, it may be largely overcome by filtration or by the use of a coagulent in connection with filtration, the latter combination having come recently into considerable favor. Alum, or potash alum, and iron alum, which in reality contains no alumina and should rather be called potassia-ferric, are the coagulents generally used in connection with filtration. Such matter as is not removed by filtration may, under certain conditions, be handled by surface blowing. In some instances, settling tanks are used for the removal of matter in suspension, but where large quantities of water are required, filtration is ordinarily substituted on account of the time element and the large area necessary in settling tanks.

Where foaming occurs as the result of overtreatment of the feed water, the obvious remedy is a change in such treatment.

Priming—Where priming is caused by excessive concentration of salts within a boiler, it may be overcome largely by frequent blowing down. The degree of concentration allowable before priming will take place varies widely with conditions of operation and may be definitely determined only by experience with each individual set of conditions. It is the presence of the salts that cause priming that may result in the absolute unfitness of water for boiler feed purposes. Where these salts exist in such quantities that the amount of blowing down necessary to keep the degree of concentration below the priming point results in excessive losses, the only remedy is the securing of another supply of feed, and the results will warrant the change almost regardless of the expense. In some few instances, the impurities may be taken care of by some method of water treatment but such water should be submitted to an authority on the subject before any treatment apparatus is installed.



Boiler Compounds—The method of treatment of feed water by far the most generally used is by the use of some of the so-called boiler compounds. There are many reliable concerns handling such compounds who unquestionably secure the promised results, but there is a great tendency toward looking on the compound as a "cure all" for any water difficulties and care should be taken to deal only with reputable concerns.

The composition of these compounds is almost invariably based on soda with certain tannic substances and in some instances a gelatinous substance which is presumed to encircle scale particles and prevent their adhering to the boiler surfaces. The action of these compounds is ordinarily to reduce the calcium sulphate in the water by means of carbonate of soda and to precipitate it as a muddy form of calcium carbonate which may be blown off. The tannic compounds are used in connection with the soda with the idea of introducing organic matter into any scale already formed. When it has penetrated to the boiler metal, decomposition of the scale sets in, causing a disruptive effect which breaks the scale from the metal sometimes in large slabs. It is this effect of boiler compounds that is to be most carefully guarded against or inevitable trouble will result from the presence of loose scale with the consequent danger of tube losses through burning.

When proper care is taken to suit the compound to the water in use, the results secured are fairly effective. In general, however, the use of compounds may only be recommended for the prevention of scale rather than with the view to removing scale which has already formed, that is, the compounds should be introduced with the feed water only when the boiler has been thoroughly cleaned.



FEED WATER HEATING AND METHODS OF FEEDING

Before water fed into a boiler can be converted into steam, it must be first heated to a temperature corresponding to the pressure within the boiler. Steam at 160 pounds gauge pressure has a temperature of approximately 371 degrees Fahrenheit. If water is fed to the boiler at 60 degrees Fahrenheit, each pound must have 311 B. t. u. added to it to increase its temperature 371 degrees, which increase must take place before the water can be converted into steam. As it requires 1167.8 B. t. u. to raise one pound of water from 60 to 371 degrees and to convert it into steam at 160 pounds gauge pressure, the 311 degrees required simply to raise the temperature of the water from 60 to 371 degrees will be approximately 27 per cent of the total. If, therefore, the temperature of the water can be increased from 60 to 371 degrees before it is introduced into a boiler by the utilization of heat from some source that would otherwise be wasted, there will be a saving in the fuel required of 311 / 1167.8 = 27 per cent, and there will be a net saving, provided the cost of maintaining and operating the apparatus for securing this saving is less than the value of the heat thus saved.

The saving in the fuel due to the heating of feed water by means of heat that would otherwise be wasted may be computed from the formula:

100 (t - t{i}) Fuel saving per cent = ———————- (1) H + 32 - t{i}

where, t = temperature of feed water after heating, t_{i} = temperature of feed water before heating, and H = total heat above 32 degrees per pound of steam at the boiler pressure. Values of H may be found in Table 23. Table 17 has been computed from this formula to show the fuel saving under the conditions assumed with the boiler operating at 180 pounds gauge pressure.

TABLE 17

SAVING IN FUEL, IN PER CENT, BY HEATING FEED WATER GAUGE PRESSURE 180 POUNDS

+ -+ -+ Initial Final Temperature Degrees Fahrenheit Temperature -+ -+ -+ -+ -+ -+ - Fahrenheit 120 140 160 180 200 250 300 + -+ -+ -+ -+ -+ -+ -+ -+ 32 7.35 9.02 10.69 12.36 14.04 18.20 22.38 35 7.12 8.79 10.46 12.14 13.82 18.00 22.18 40 6.72 8.41 10.09 11.77 13.45 17.65 21.86 45 6.33 8.02 9.71 11.40 13.08 17.30 21.52 50 5.93 7.63 9.32 11.02 12.72 16.95 21.19 55 5.53 7.24 8.94 10.64 12.34 16.60 20.86 60 5.13 6.84 8.55 10.27 11.97 16.24 20.52 65 4.72 6.44 8.16 9.87 11.59 15.88 20.18 70 4.31 6.04 7.77 9.48 11.21 15.52 19.83 75 3.90 5.64 7.36 9.09 10.82 15.16 19.48 80 3.48 5.22 6.96 8.70 10.44 14.79 19.13 85 3.06 4.80 6.55 8.30 10.05 14.41 18.78 90 2.63 4.39 6.14 7.89 9.65 14.04 18.43 95 2.20 3.97 5.73 7.49 9.25 13.66 18.07 100 1.77 3.54 5.31 7.08 8.85 13.28 17.70 110 .89 2.68 4.47 6.25 8.04 12.50 16.97 120 .00 1.80 3.61 5.41 7.21 11.71 16.22 130 .91 2.73 4.55 6.37 10.91 15.46 140 .00 1.84 3.67 5.51 10.09 14.68 150 .93 2.78 4.63 9.26 13.89 160 .00 1.87 3.74 8.41 13.09 170 .94 2.83 7.55 12.27 180 .00 1.91 6.67 11.43 190 .96 5.77 10.58 200 .00 4.86 9.71 210 3.92 8.82 + -+ -+ -+ -+ -+ -+ -+ -+

Besides the saving in fuel effected by the use of feed water heaters, other advantages are secured. The time required for the conversion of water into steam is diminished and the steam capacity of the boiler thereby increased. Further, the feeding of cold water into a boiler has a tendency toward the setting up of temperature strains, which are diminished in proportion as the temperature of the feed approaches that of the steam. An important additional advantage of heating feed water is that in certain types of heaters a large portion of the scale forming ingredients are precipitated before entering the boiler, with a consequent saving in cleaning and losses through decreased efficiency and capacity.

In general, feed water heaters may be divided into closed heaters, open heaters and economizers; the first two depend for their heat upon exhaust, or in some cases live steam, while the last class utilizes the heat of the waste flue gases to secure the same result. The question of the type of apparatus to be installed is dependent upon the conditions attached to each individual case.

In closed heaters the feed water and the exhaust steam do not come into actual contact with each other. Either the steam or the water passes through tubes surrounded by the other medium, as the heater is of the steam-tube or water-tube type. A closed heater is best suited for water free from scale-forming matter, as such matter soon clogs the passages. Cleaning such heaters is costly and the efficiency drops off rapidly as scale forms. A closed heater is not advisable where the engines work intermittently, as is the case with mine hoisting engines. In this class of work the frequent coolings between operating periods and the sudden heatings when operation commences will tend to loosen the tubes or even pull them apart. For this reason, an open heater, or economizer, will give more satisfactory service with intermittently operating apparatus.

Open heaters are best suited for waters containing scale-forming matter. Much of the temporary hardness may be precipitated in the heater and the sediment easily removed. Such heaters are frequently used with a reagent for precipitating permanent hardness in the combined heat and chemical treatment of feed water. The so-called live steam purifiers are open heaters, the water being raised to the boiling temperature and the carbonates and a portion of the sulphates being precipitated. The disadvantage of this class of apparatus is that some of the sulphates remain in solution to be precipitated as scale when concentrated in the boiler. Sufficient concentration to have such an effect, however, may often be prevented by frequent blowing down.

Economizers find their largest field where the design of the boiler is such that the maximum possible amount of heat is not extracted from the gases of combustion. The more wasteful the boiler, the greater the saving effected by the use of the economizer, and it is sometimes possible to raise the temperature of the feed water to that of high pressure steam by the installation of such an apparatus, the saving amounting in some cases to as much as 20 per cent. The fuel used bears directly on the question of the advisability of an economizer installation, for when oil is the fuel a boiler efficiency of 80 per cent or over is frequently realized, an efficiency which would leave a small opportunity for a commercial gain through the addition of an economizer.

From the standpoint of space requirements, economizers are at a disadvantage in that they are bulky and require a considerable increase over space occupied by a heater of the exhaust type. They also require additional brickwork or a metal casing, which increases the cost. Sometimes, too, the frictional resistance of the gases through an economizer make its adaptability questionable because of the draft conditions. When figuring the net return on economizer investment, all of these factors must be considered.

When the feed water is such that scale will quickly encrust the economizer and throw it out of service for cleaning during an excessive portion of the time, it will be necessary to purify water before introducing it into an economizer to make it earn a profit on the investment.

From the foregoing, it is clearly indicated that it is impossible to make a definite statement as to the relative saving by heating feed water in any of the three types. Each case must be worked out independently and a decision can be reached only after an exhaustive study of all the conditions affecting the case, including the time the plant will be in service and probable growth of the plant. When, as a result of such study, the possible methods for handling the problem have been determined, the solution of the best apparatus can be made easily by the balancing of the saving possible by each method against its first cost, depreciation, maintenance and cost of operation.

Feeding of Water—The choice of methods to be used in introducing feed water into a boiler lies between an injector and a pump. In most plants, an injector would not be economical, as the water fed by such means must be cold, a fact which makes impossible the use of a heater before the water enters the injector. Such a heater might be installed between the injector and the boiler but as heat is added to the water in the injector, the heater could not properly fulfill its function.

TABLE 18

COMPARISON OF PUMPS AND INJECTORS _____________ Method of Supplying Feed-water to Boiler Relative amount of Saving of fuel over Temperature of feed-water as coal required per the amount required delivered to the pump or to unit of time, the when the boiler is injector, 60 degrees Fahren- amount for a direct- fed by a direct- heit. Rate of evaporation of acting pump, feeding acting pump without boiler, to pounds of water water at 60 degrees heater per pound of coal from and without a heater, Per Cent at 212 degrees Fahrenheit being taken as unity _____ ____ ____ Direct-acting Pump feeding water at 60 degrees without a heater 1.000 .0 Injector feeding water at 150 degrees without a heater .985 1.5 Injector feeding through a heater in which the water is heated from 150 to 200 degrees .938 6.2 Direct-acting Pump feeding water through a heater in which it is heated from 60 to 200 degrees .879 12.1 Geared Pump run from the engine, feeding water through a heater in which it is heated from 60 to 200 degrees .868 13.2 _____ ____ ____

The injector, considered only in the light of a combined heater and pump, is claimed to have a thermal efficiency of 100 per cent, since all of the heat in the steam used is returned to the boiler with the water. This claim leads to an erroneous idea. If a pump is used in feeding the water to a boiler and the heat in the exhaust from the pump is imparted to the feed water, the pump has as high a thermal efficiency as the injector. The pump has the further advantage that it uses so much less steam for the forcing of a given quantity of water into the boiler that it makes possible a greater saving through the use of the exhaust from other auxiliaries for heating the feed, which exhaust, if an injector were used, would be wasted, as has been pointed out.

In locomotive practice, injectors are used because there is no exhaust steam available for heating the feed, this being utilized in producing a forced draft, and because of space requirements. In power plant work, however, pumps are universally used for regular operation, though injectors are sometimes installed as an auxiliary method of feeding.

Table 18 shows the relative value of injectors, direct-acting steam pumps and pumps driven from the engine, the data having been obtained from actual experiment. It will be noted that when feeding cold water direct to the boilers, the injector has a slightly greater economy but when feeding through a heater, the pump is by far the more economical.

Auxiliaries—It is the general impression that auxiliaries will take less steam if the exhaust is turned into the condensers, in this way reducing the back pressure. As a matter of fact, vacuum is rarely registered on an indicator card taken from the cylinders of certain types of auxiliaries unless the exhaust connection is short and without bends, as long pipes and many angles offset the effect of the condenser. On the other hand, if the exhaust steam from the auxiliaries can be used for heating the feed water, all of the latent heat less only the loss due to radiation is returned to the boiler and is saved instead of being lost in the condensing water or wasted with the free exhaust. Taking into consideration the plant as a whole, it would appear that the auxiliary machinery, under such conditions, is more efficient than the main engines.



STEAM

When a given weight of a perfect gas is compressed or expanded at a constant temperature, the product of the pressure and volume is a constant. Vapors, which are liquids in aeriform condition, on the other hand, can exist only at a definite pressure corresponding to each temperature if in the saturated state, that is, the pressure is a function of the temperature only. Steam is water vapor, and at a pressure of, say, 150 pounds absolute per square inch saturated steam can exist only at a temperature 358 degrees Fahrenheit. Hence if the pressure of saturated steam be fixed, its temperature is also fixed, and vice versa.

Saturated steam is water vapor in the condition in which it is generated from water with which it is in contact. Or it is steam which is at the maximum pressure and density possible at its temperature. If any change be made in the temperature or pressure of steam, there will be a corresponding change in its condition. If the pressure be increased or the temperature decreased, a portion of the steam will be condensed. If the temperature be increased or the pressure decreased, a portion of the water with which the steam is in contact will be evaporated into steam. Steam will remain saturated just so long as it is of the same pressure and temperature as the water with which it can remain in contact without a gain or loss of heat. Moreover, saturated steam cannot have its temperature lowered without a lowering of its pressure, any loss of heat being made up by the latent heat of such portion as will be condensed. Nor can the temperature of saturated steam be increased except when accompanied by a corresponding increase in pressure, any added heat being expended in the evaporation into steam of a portion of the water with which it is in contact.

Dry saturated steam contains no water. In some cases, saturated steam is accompanied by water which is carried along with it, either in the form of a spray or is blown along the surface of the piping, and the steam is then said to be wet. The percentage weight of the steam in a mixture of steam and water is called the quality of the steam. Thus, if in a mixture of 100 pounds of steam and water there is three-quarters of a pound of water, the quality of the steam will be 99.25.

Heat may be added to steam not in contact with water, such an addition of heat resulting in an increase of temperature and pressure if the volume be kept constant, or an increase in temperature and volume if the pressure remain constant. Steam whose temperature thus exceeds that of saturated steam at a corresponding pressure is said to be superheated and its properties approximate those of a perfect gas.

As pointed out in the chapter on heat, the heat necessary to raise one pound of water from 32 degrees Fahrenheit to the point of ebullition is called the heat of the liquid. The heat absorbed during ebullition consists of that necessary to dissociate the molecules, or the inner latent heat, and that necessary to overcome the resistance to the increase in volume, or the outer latent heat. These two make up the latent heat of evaporation and the sum of this latent heat of evaporation and the heat of the liquid make the total heat of the steam. These values for various pressures are given in the steam tables, pages 122 to 127.

The specific volume of saturated steam at any pressure is the volume in cubic feet of one pound of steam at that pressure.

The density of saturated steam, that is, its weight per cubic foot, is obviously the reciprocal of the specific volume. This density varies as the 16/17 power over the ordinary range of pressures used in steam boiler work and may be found by the formula, D = .003027p^{.941}, which is correct within 0.15 per cent up to 250 pounds pressure.

The relative volume of steam is the ratio of the volume of a given weight to the volume of the same weight of water at 39.2 degrees Fahrenheit and is equal to the specific volume times 62.427.

As vapors are liquids in their gaseous form and the boiling point is the point of change in this condition, it is clear that this point is dependent upon the pressure under which the liquid exists. This fact is of great practical importance in steam condenser work and in many operations involving boiling in an open vessel, since in the latter case its altitude will have considerable influence. The relation between altitude and boiling point of water is shown in Table 12.

The conditions of feed temperature and steam pressure in boiler tests, fuel performances and the like, will be found to vary widely in different trials. In order to secure a means for comparison of different trials, it is necessary to reduce all results to some common basis. The method which has been adopted for the reduction to a comparable basis is to transform the evaporation under actual conditions of steam pressure and feed temperature which exist in the trial to an equivalent evaporation under a set of standard conditions. These standard conditions presuppose a feed water temperature of 212 degrees Fahrenheit and a steam pressure equal to the normal atmospheric pressure at sea level, 14.7 pounds absolute. Under such conditions steam would be generated at a temperature of 212 degrees, the temperature corresponding to atmospheric pressure at sea level, from water at 212 degrees. The weight of water which would be evaporated under the assumed standard conditions by exactly the amount of heat absorbed by the boiler under actual conditions existing in the trial, is, therefore, called the equivalent evaporation "from and at 212 degrees."

The factor for reducing the weight of water actually converted into steam from the temperature of the feed, at the steam pressure existing in the trial, to the equivalent evaporation under standard conditions is called the factor of evaporation. This factor is the ratio of the total heat added to one pound of steam under the standard conditions to the heat added to each pound of steam in heating the water from the temperature of the feed in the trial to the temperature corresponding to the pressure existing in the trial. This heat added is obviously the difference between the total heat of evaporation of the steam at the pressure existing in the trial and the heat of the liquid in the water at the temperature at which it was fed in the trial. To illustrate by an example:

In a boiler trial the temperature of the feed water is 60 degrees Fahrenheit and the pressure under which steam is delivered is 160.3 pounds gauge pressure or 175 pounds absolute pressure. The total heat of one pound of steam at 175 pounds pressure is 1195.9 B. t. u. measured above the standard temperature of 32 degrees Fahrenheit. But the water fed to the boiler contained 28.08 B. t. u. as the heat of the liquid measured above 32 degrees Fahrenheit. Therefore, to each pound of steam there has been added 1167.82 B. t. u. To evaporate one pound of water under standard conditions would, on the other hand, have required but 970.4 B. t. u., which, as described, is the latent heat of evaporation at 212 degrees Fahrenheit. Expressed differently, the total heat of one pound of steam at the pressure corresponding to a temperature of 212 degrees is 1150.4 B. t. u. One pound of water at 212 degrees contains 180 B. t. u. of sensible heat above 32 degrees Fahrenheit. Hence, under standard conditions, 1150.4 - 180 = 970.4 B. t. u. is added in the changing of one pound of water into steam at atmospheric pressure and a temperature of 212 degrees. This is in effect the definition of the latent heat of evaporation.

Hence, if conditions of the trial had been standard, only 970.4 B. t. u. would be required and the ratio of 1167.82 to 970.4 B. t. u. is the ratio determining the factor of evaporation. The factor in the assumed case is 1167.82 / 970.4 = 1.2034 and if the same amount of heat had been absorbed under standard conditions as was absorbed in the trial condition, 1.2034 times the amount of steam would have been generated. Expressed as a formula for use with any set of conditions, the factor is,

H - h F = ——- (2) 970.4

Where H = the total heat of steam above 32 degrees Fahrenheit from steam tables, h = sensible heat of feed water above 32 degrees Fahrenheit from Table 22.

In the form above, the factor may be determined with either saturated or superheated steam, provided that in the latter case values of H are available for varying degrees of superheat and pressures.

Where such values are not available, the form becomes,

H - h + s(t{sup} - t{sat}) F = —————————————— (3) 970.4

Where s = mean specific heat of superheated steam at the pressure existing in the trial from saturated steam to the temperature existing in the trial, t{sup} = final temperature of steam, t{sat} = temperature of saturated steam, corresponding to pressure existing, (t{sup} - t{sat}) = degrees of superheat.

The specific heat of superheated steam will be taken up later.

Table 19 gives factors of evaporation for saturated steam boiler trials to cover a large range of conditions. Except for the most refined work, intermediate values may be determined by interpolation.

Steam gauges indicate the pressure above the atmosphere. As has been pointed out, the atmospheric pressure changes according to the altitude and the variation in the barometer. Hence, calculations involving the properties of steam are based on absolute pressures, which are equal to the gauge pressure plus the atmospheric pressure in pounds to the square inch. This latter is generally assumed to be 14.7 pounds per square inch at sea level, but for other levels it must be determined from the barometric reading at that place.

Vacuum gauges indicate the difference, expressed in inches of mercury, between atmospheric pressure and the pressure within the vessel to which the gauge is attached. For approximate purposes, 2.04 inches height of mercury may be considered equal to a pressure of one pound per square inch at the ordinary temperatures at which mercury gauges are used. Hence for any reading of the vacuum gauge in inches, G, the absolute pressure for any barometer reading in inches, B, will be (B - G) / 2.04. If the barometer is 30 inches measured at ordinary temperatures and not corrected to 32 degrees Fahrenheit and the vacuum gauge 24 inches, the absolute pressure will be (30 - 24) / 2.04 = 2.9 pounds per square inch.

TABLE 19

FACTORS OF EVAPORATION CALCULATED FROM MARKS AND DAVIS TABLES

____________ Feed Temp- erature Degrees Steam Pressure by Gauge Fahren- heit __ ___________ 50 60 70 80 90 100 110 __ __ __ __ __ __ __ __ 32 1.2143 1.2170 1.2194 1.2215 1.2233 1.2233 1.2265 40 1.2060 1.2087 1.2111 1.2131 1.2150 1.2168 1.2181 50 1.1957 1.1984 1.2008 1.2028 1.2047 1.2065 1.2079 60 1.1854 1.1881 1.1905 1.1925 1.1944 1.1961 1.1976 70 1.1750 1.1778 1.1802 1.1822 1.1841 1.1859 1.1873 80 1.1649 1.1675 1.1699 1.1720 1.1738 1.1756 1.1770 90 1.1545 1.1572 1.1596 1.1617 1.1636 1.1653 1.1668 100 1.1443 1.1470 1.1493 1.1514 1.1533 1.1550 1.1565 110 1.1340 1.1367 1.1391 1.1411 1.1430 1.1448 1.1462 120 1.1237 1.1264 1.1288 1.1309 1.1327 1.1345 1.1359 130 1.1134 1.1161 1.1185 1.1206 1.1225 1.1242 1.1257 140 1.1031 1.1058 1.1082 1.1103 1.1122 1.1139 1.1154 150 1.0928 1.0955 1.0979 1.1000 1.1019 1.1036 1.1051 160 1.0825 1.0852 1.0876 1.0897 1.0916 1.0933 1.0948 170 1.0722 1.0749 1.0773 1.0794 1.0813 1.0830 1.0845 180 1.0619 1.0646 1.0670 1.0691 1.0709 1.0727 1.0741 190 1.0516 1.0543 1.0567 1.0587 1.0606 1.0624 1.0638 200 1.0412 1.0439 1.0463 1.0484 1.0503 1.0520 1.0535 210 1.0309 1.0336 1.0360 1.0380 1.0399 1.0417 1.0432 __ __ __ __ __ __ __ __ ____________ Feed Temp- erature Degrees Steam Pressure by Gauge Fahren- heit __ ___________ 120 130 140 150 160 170 180 __ __ __ __ __ __ __ __ 32 1.2280 1.2292 1.2304 1.2314 1.2323 1.2333 1.2342 40 1.2196 1.2209 1.2221 1.2231 1.2241 1.2250 1.2259 50 1.2093 1.2106 1.2117 1.2128 1.2137 1.2147 1.2156 60 1.1990 1.2003 1.2014 1.2025 1.2034 1.2044 1.2053 70 1.1887 1.1900 1.1911 1.1922 1.1931 1.1941 1.1950 80 1.1785 1.1797 1.1809 1.1819 1.1828 1.1838 1.1847 90 1.1682 1.1695 1.1706 1.1717 1.1725 1.1735 1.1744 100 1.1579 1.1592 1.1603 1.1614 1.1623 1.1633 1.1642 110 1.1477 1.1489 1.1500 1.1511 1.1520 1.1530 1.1539 120 1.1374 1.1386 1.1398 1.1408 1.1418 1.1427 1.1436 130 1.1271 1.1284 1.1295 1.1305 1.1315 1.1324 1.1333 140 1.1168 1.1181 1.1192 1.1203 1.1212 1.1221 1.1230 150 1.1065 1.1078 1.1089 1.1099 1.1109 1.1118 1.1127 160 1.0962 1.0975 1.0986 1.0997 1.1006 1.1015 1.1024 170 1.0859 1.0872 1.0883 1.0893 1.0903 1.0912 1.0921 180 1.0756 1.0768 1.0780 1.0790 1.0800 1.0809 1.0818 190 1.0653 1.0665 1.0676 1.0687 1.0696 1.0706 1.0715 200 1.0549 1.0562 1.0573 1.0584 1.0593 1.0602 1.0611 210 1.0446 1.0458 1.0469 1.0480 1.0489 1.0499 1.0508 __ __ __ __ __ __ __ __ ____________ Feed Temp- erature Degrees Steam Pressure by Gauge Fahren- heit __ ___________ 190 200 210 220 230 240 250 __ __ __ __ __ __ __ __ 32 1.2350 1.2357 1.2364 1.2372 1.2378 1.2384 1.2390 40 1.2267 1.2274 1.2282 1.2289 1.2295 1.2301 1.2307 50 1.2164 1.2171 1.2178 1.2186 1.2192 1.2198 1.2204 60 1.2061 1.2068 1.2075 1.2083 1.2089 1.2095 1.2101 70 1.1958 1.1965 1.1972 1.1980 1.1986 1.1992 1.1998 80 1.1855 1.1863 1.1869 1.1877 1.1883 1.1889 1.1895 90 1.1750 1.1760 1.1766 1.1774 1.1780 1.1786 1.1792 100 1.1650 1.1657 1.1664 1.1671 1.1678 1.1684 1.1690 110 1.1547 1.1554 1.1562 1.1569 1.1575 1.1581 1.1587 120 1.1444 1.1452 1.1459 1.1466 1.1472 1.1478 1.1484 130 1.1341 1.1349 1.1356 1.1363 1.1369 1.1375 1.1381 140 1.1239 1.1246 1.1253 1.1260 1.1266 1.1272 1.1278 150 1.1136 1.1143 1.1150 1.1157 1.1163 1.1169 1.1176 160 1.1033 1.1040 1.1047 1.1054 1.1060 1.1066 1.1073 170 1.0930 1.0937 1.0944 1.0951 1.0957 1.0963 1.0969 180 1.0826 1.0834 1.0841 1.0848 1.0854 1.0860 1.0866 190 1.0723 1.0730 1.0737 1.0745 1.0751 1.0757 1.0763 200 1.0620 1.0627 1.0634 1.0641 1.0647 1.0653 1.0660 210 1.0516 1.0523 1.0530 1.0538 1.0544 1.0550 1.0556 __ __ __ __ __ __ __ __

The temperature, pressure and other properties of steam for varying amounts of vacuum and the pressure above vacuum corresponding to each inch of reading of the vacuum gauge are given in Table 20.

TABLE 20

PROPERTIES OF SATURATED STEAM FOR VARYING AMOUNTS OF VACUUM CALCULATED FROM MARKS AND DAVIS TABLES Heat of Latent Total Temp- the Liquid Heat Heat erature Above Above Above Density or Absolute Degrees 32 De- 32 De- 32 De- Weight per Vacuum Pressure Fahren- grees grees grees Cubic Foot Ins. Hg. Pounds heit B. t. u. B. t. u. B. t. u. Pounds 29.5 .207 54.1 22.18 1061.0 1083.2 0.000678 29 .452 76.6 44.64 1048.7 1093.3 0.001415 28.5 .698 90.1 58.09 1041.1 1099.2 0.002137 28 .944 99.9 67.87 1035.6 1103.5 0.002843 27 1.44 112.5 80.4 1028.6 1109.0 0.00421 26 1.93 124.5 92.3 1022.0 1114.3 0.00577 25 2.42 132.6 100.5 1017.3 1117.8 0.00689 24 2.91 140.1 108.0 1013.1 1121.1 0.00821 22 3.89 151.7 119.6 1006.4 1126.0 0.01078 20 4.87 161.1 128.9 1001.0 1129.9 0.01331 18 5.86 168.9 136.8 996.4 1133.2 0.01581 16 6.84 175.8 143.6 992.4 1136.0 0.01827 14 7.82 181.8 149.7 988.8 1138.5 0.02070 12 8.80 187.2 155.1 985.6 1140.7 0.02312 10 9.79 192.2 160.1 982.6 1142.7 0.02554 5 12.24 202.9 170.8 976.0 1146.8 0.03148

From the steam tables, the condensed Table 21 of the properties of steam at different pressures may be constructed. From such a table there may be drawn the following conclusions.

TABLE 21

VARIATION IN PROPERTIES OF SATURATED STEAM WITH PRESSURE _________ Pressure Temperature Heat of Latent Total Pounds Degrees Liquid Heat Heat Absolute Fahrenheit B. t. u. B. t. u. B. t. u. __ __ __ __ __ 14.7 212.0 180.0 970.4 1150.4 20.0 228.0 196.1 960.0 1156.2 100.0 327.8 298.3 888.0 1186.3 300.0 417.5 392.7 811.3 1204.1 __ __ __ __ __

As the pressure and temperature increase, the latent heat decreases. This decrease, however, is less rapid than the corresponding increase in the heat of the liquid and hence the total heat increases with an increase in the pressure and temperature. The percentage increase in the total heat is small, being 0.5, 3.1, and 4.7 per cent for 20, 100, and 300 pounds absolute pressure respectively above the total heat in one pound of steam at 14.7 pounds absolute. The temperatures, on the other hand, increase at the rates of 7.5, 54.6, and 96.9 per cent. The efficiency of a perfect steam engine is proportional to the expression (t - t{1})/t in which t and t{1} are the absolute temperatures of the saturated steam at admission and exhaust respectively. While actual engines only approximate the ideal engine in efficiency, yet they follow the same general law. Since the exhaust temperature cannot be lowered beyond present practice, it follows that the only available method of increasing the efficiency is by an increase in the temperature of the steam at admission. How this may be accomplished by an increase of pressure is clearly shown, for the increase of fuel necessary to increase the pressure is negligible, as shown by the total heat, while the increase in economy, due to the higher pressure, will result directly from the rapid increase of the corresponding temperature.

TABLE 22

HEAT UNITS PER POUND AND WEIGHT PER CUBIC FOOT OF WATER BETWEEN 32 DEGREES FAHRENHEIT AND 340 DEGREES FAHRENHEIT Temperature Heat Units Weight Degrees per per Fahrenheit Pound Cubic Foot 32 0.00 62.42 33 1.01 62.42 34 2.01 62.42 35 3.02 62.43 36 4.03 62.43 37 5.04 62.43 38 6.04 62.43 39 7.05 62.43 40 8.05 62.43 41 9.05 62.43 42 10.06 62.43 43 11.06 62.43 44 12.06 62.43 45 13.07 62.42 46 14.07 62.42 47 15.07 62.42 48 16.07 62.42 49 17.08 62.42 50 18.08 62.42 51 19.08 62.41 52 20.08 62.41 53 21.08 62.41 54 22.08 62.40 55 23.08 62.40 56 24.08 62.39 57 25.08 62.39 58 26.08 62.38 59 27.08 62.37 60 28.08 62.37 61 29.08 62.36 62 30.08 62.36 63 31.07 62.35 64 32.07 62.35 65 33.07 62.34 66 34.07 62.33 67 35.07 62.33 68 36.07 62.32 69 37.06 62.31 70 38.06 62.30 71 39.06 62.30 72 40.05 62.29 73 41.05 62.28 74 42.05 62.27 75 42.05 62.26 76 44.04 62.26 77 45.04 62.25 78 46.04 62.24 79 47.04 62.23 80 48.03 62.22 81 49.03 62.21 82 50.03 62.20 83 51.02 62.19 84 52.02 62.18 85 53.02 62.17 86 54.01 62.16 87 55.01 62.15 88 56.01 62.14 89 57.00 62.13 90 58.00 62.12 91 59.00 62.11 92 60.00 62.09 93 60.99 62.08 94 61.99 62.07 95 62.99 62.06 96 63.98 62.05 97 64.98 62.04 98 65.98 62.03 99 66.97 62.02 100 67.97 62.00 101 68.97 61.99 102 69.96 61.98 103 70.96 61.97 104 71.96 61.95 105 72.95 61.94 106 73.95 61.93 107 74.95 61.91 108 75.95 61.90 109 76.94 61.88 110 77.94 61.86 111 78.94 61.85 112 79.93 61.83 113 80.93 61.82 114 81.93 61.80 115 82.92 61.79 116 83.92 61.77 117 84.92 61.75 118 85.92 61.74 119 86.91 61.72 120 87.91 61.71 121 88.91 61.69 122 89.91 61.68 123 90.90 61.66 124 91.90 61.65 125 92.90 61.63 126 93.90 61.61 127 94.89 61.59 128 95.89 61.58 129 96.89 61.56 130 97.89 61.55 131 98.89 61.53 132 99.88 61.52 133 100.88 61.50 134 101.88 61.49 135 102.88 61.47 136 103.88 61.45 137 104.87 61.43 138 105.87 61.41 139 106.87 61.40 140 107.87 61.38 141 108.87 61.36 142 109.87 61.34 143 110.87 61.33 144 111.87 61.31 145 112.86 61.29 146 113.86 61.27 147 114.86 61.25 148 115.86 61.24 149 116.86 61.22 150 117.86 61.20 151 118.86 61.18 152 119.86 61.16 153 120.86 61.14 154 121.86 61.12 155 122.86 61.10 156 123.86 61.08 157 124.86 61.06 158 125.86 61.04 159 126.86 61.02 160 127.86 61.00 161 128.86 60.98 162 129.86 60.96 163 130.86 60.94 164 131.86 60.92 165 132.86 60.90 166 133.86 60.88 167 134.86 60.86 168 135.86 60.84 169 136.86 60.82 170 137.87 60.80 171 138.87 60.78 172 139.87 60.76 173 140.87 60.73 174 141.87 60.71 175 142.87 60.69 176 143.87 60.67 177 144.88 60.65 178 145.88 60.62 179 146.88 60.60 180 147.88 60.58 181 148.88 60.56 182 149.89 60.53 183 150.89 60.51 184 151.89 60.49 185 152.89 60.47 186 153.89 60.45 187 154.90 60.42 188 155.90 60.40 189 156.90 60.38 190 157,91 60.36 191 158.91 60.33 192 159.91 60.31 193 160.91 60.29 194 161.92 60.27 195 162.92 60.24 196 163.92 60.22 197 164.93 60.19 198 165.93 60.17 199 166.94 60.15 200 167.94 60.12 201 168.94 60.10 202 169.95 60.07 203 170.95 60.05 204 171.96 60.02 205 172.96 60.00 206 173.97 59.98 207 174.97 59.95 208 175.98 59.93 209 176.98 59.90 210 177.99 59.88 211 178.99 59.85 212 180.00 59.83 213 181.0 59.80 214 182.0 59.78 215 183.0 59.75 216 184.0 59.73 217 185.0 59.70 218 186.1 59.68 219 187.1 59.65 220 188.1 59.63 221 189.1 59.60 222 190.1 59.58 223 191.1 59.55 224 192.1 59.53 225 193.1 59.50 226 194.1 59.48 227 195.2 59.45 228 196.2 59.42 229 197.2 59.40 230 198.2 59.37 231 199.2 59.34 232 200.2 59.32 233 201.2 59.29 234 202.2 59.27 235 203.2 59.24 236 204.2 59.21 237 205.3 59.19 238 206.3 59.16 239 207.3 59.14 240 208.3 59.11 241 209.3 59.08 242 210.3 59.05 243 211.4 59.03 244 212.4 59.00 245 213.4 58.97 246 214.4 58.94 247 215.4 58.91 248 216.4 58.89 249 217.4 58.86 250 218.5 58.83 260 228.6 58.55 270 238.8 58.26 280 249.0 57.96 290 259.3 57.65 300 269.6 57.33 310 279.9 57.00 320 290.2 56.66 330 300.6 56.30 340 311.0 55.94

The gain due to superheat cannot be predicted from the formula for the efficiency of a perfect steam engine given on page 119. This formula is not applicable in cases where superheat is present since only a relatively small amount of the heat in the steam is imparted at the maximum or superheated temperature.

The advantage of the use of high pressure steam may be also indicated by considering the question from the aspect of volume. With an increase of pressure comes a decrease in volume, thus one pound of saturated steam at 100 pounds absolute pressure occupies 4.43 cubic feet, while at 200 pounds pressure it occupies 2.29 cubic feet. If then, in separate cylinders of the same dimensions, one pound of steam at 100 pounds absolute pressure and one pound at 200 pounds absolute pressure enter and are allowed to expand to the full volume of each cylinder, the high-pressure steam, having more room and a greater range for expansion than the low-pressure steam, will thus do more work. This increase in the amount of work, as was the increase in temperature, is large relative to the additional fuel required as indicated by the total heat. In general, it may be stated that the fuel required to impart a given amount of heat to a boiler is practically independent of the steam pressure, since the temperature of the fire is so high as compared with the steam temperature that a variation in the steam temperature does not produce an appreciable effect.

The formulae for the algebraic expression of the relation between saturated steam pressures, temperatures and steam volumes have been up to the present time empirical. These relations have, however, been determined by experiment and, from the experimental data, tables have been computed which render unnecessary the use of empirical formulae. Such formulae may be found in any standard work of thermo-dynamics. The following tables cover all practical cases.

Table 22 gives the heat units contained in water above 32 degrees Fahrenheit at different temperatures.

Table 23 gives the properties of saturated steam for various pressures.

Table 24 gives the properties of superheated steam at various pressures and temperatures.

These tables are based on those computed by Lionel S. Marks and Harvey N. Davis, these being generally accepted as being the most correct.

TABLE 23

PROPERTIES OF SATURATED STEAM

REPRODUCED BY PERMISSION FROM MARKS AND DAVIS "STEAM TABLES AND DIAGRAMS" (Copyright, 1909, by Longmans, Green & Co.) Pressure, Temper- Specific Vol- Heat of Latent Heat Total Heat Pounds ature De- ume Cu. Ft. the Liquid, of Evap., of Steam, Absolute grees F. per Pound B. t. u. B. t. u. B. t. u. 1 101.83 333.0 69.8 1034.6 1104.4 2 126.15 173.5 94.0 1021.0 1115.0 3 141.52 118.5 109.4 1012.3 1121.6 4 153.01 90.5 120.9 1005.7 1126.5 5 162.28 73.33 130.1 1000.3 1130.5 6 170.06 61.89 137.9 995.8 1133.7 7 176.85 53.56 144.7 991.8 1136.5 8 182.86 47.27 150.8 988.2 1139.0 9 188.27 42.36 156.2 985.0 1141.1 10 193.22 38.38 161.1 982.0 1143.1 11 197.75 35.10 165.7 979.2 1144.9 12 201.96 32.36 169.9 976.6 1146.5 13 205.87 30.03 173.8 974.2 1148.0 14 209.55 28.02 177.5 971.9 1149.4 15 213.0 26.27 181.0 969.7 1150.7 16 216.3 24.79 184.4 967.6 1152.0 17 219.4 23.38 187.5 965.6 1153.1 18 222.4 22.16 190.5 963.7 1154.2 19 225.2 21.07 193.4 961.8 1155.2 20 228.0 20.08 196.1 960.0 1156.2 22 233.1 18.37 201.3 956.7 1158.0 24 237.8 16.93 206.1 953.5 1159.6 26 242.2 15.72 210.6 950.6 1161.2 28 246.4 14.67 214.8 947.8 1162.6 30 250.3 13.74 218.8 945.1 1163.9 32 254.1 12.93 222.6 942.5 1165.1 34 257.6 12.22 226.2 940.1 1166.3 36 261.0 11.58 229.6 937.7 1167.3 38 264.2 11.01 232.9 935.5 1168.4 40 267.3 10.49 236.1 933.3 1169.4 42 270.2 10.02 239.1 931.2 1170.3 44 273.1 9.59 242.0 929.2 1171.2 46 275.8 9.20 244.8 927.2 1172.0 48 278.5 8.84 247.5 925.3 1172.8 50 281.0 8.51 250.1 923.5 1173.6 52 283.5 8.20 252.6 921.7 1174.3 54 285.9 7.91 255.1 919.9 1175.0 56 288.2 7.65 257.5 918.2 1175.7 58 290.5 7.40 259.8 916.5 1176.4 60 292.7 7.17 262.1 914.9 1177.0 62 294.9 6.95 264.3 913.3 1177.6 64 297.0 6.75 266.4 911.8 1178.2 66 299.0 6.56 268.5 910.2 1178.8 68 301.0 6.38 270.6 908.7 1179.3 70 302.9 6.20 272.6 907.2 1179.8 72 304.8 6.04 274.5 905.8 1180.4 74 306.7 5.89 276.5 904.4 1180.9 76 308.5 5.74 278.3 903.0 1181.4 78 310.3 5.60 280.2 901.7 1181.8 80 312.0 5.47 282.0 900.3 1182.3 82 313.8 5.34 283.8 899.0 1182.8 84 315.4 5.22 285.5 897.7 1183.2 86 317.1 5.10 287.2 896.4 1183.6 88 318.7 5.00 288.9 895.2 1184.0 90 320.3 4.89 290.5 893.9 1184.4 92 321.8 4.79 292.1 892.7 1184.8 94 323.4 4.69 293.7 891.5 1185.2 96 324.9 4.60 295.3 890.3 1185.6 98 326.4 4.51 296.8 889.2 1186.0 100 327.8 4.429 298.3 888.0 1186.3 105 331.4 4.230 302.0 885.2 1187.2 110 334.8 4.047 305.5 882.5 1188.0 115 338.1 3.880 309.0 879.8 1188.8 120 341.3 3.726 312.3 877.2 1189.6 125 344.4 3.583 315.5 874.7 1190.3 130 347.4 3.452 318.6 872.3 1191.0 135 350.3 3.331 321.7 869.9 1191.6 140 353.1 3.219 324.6 867.6 1192.2 145 355.8 3.112 327.4 865.4 1192.8 150 358.5 3.012 330.2 863.2 1193.4 155 361.0 2.920 332.9 861.0 1194.0 160 363.6 2.834 335.6 858.8 1194.5 165 366.0 2.753 338.2 856.8 1195.0 170 368.5 2.675 340.7 854.7 1195.4 175 370.8 2.602 343.2 852.7 1195.9 180 373.1 2.533 345.6 850.8 1196.4 185 375.4 2.468 348.0 848.8 1196.8 190 377.6 2.406 350.4 846.9 1197.3 195 379.8 2.346 352.7 845.0 1197.7 200 381.9 2.290 354.9 843.2 1198.1 205 384.0 2.237 357.1 841.4 1198.5 210 386.0 2.187 359.2 839.6 1198.8 215 388.0 2.138 361.4 837.9 1199.2 220 389.9 2.091 363.4 836.2 1199.6 225 391.9 2.046 365.5 834.4 1199.9 230 393.8 2.004 367.5 832.8 1200.2 235 395.6 1.964 369.4 831.1 1200.6 240 397.4 1.924 371.4 829.5 1200.9 245 399.3 1.887 373.3 827.9 1201.2 250 401.1 1.850 375.2 826.3 1201.5



TABLE 24

PROPERTIES OF SUPERHEATED STEAM

REPRODUCED BY PERMISSION FROM MARKS AND DAVIS "STEAM TABLES AND DIAGRAMS" (Copyright, 1909, by Longmans, Green & Co.) Degrees of Superheat Pressure Pounds Saturated Absolute Steam 50 100 150 200 250 300 t 162.3 212.3 262.3 312.3 362.3 412.3 462.3 5 v 73.3 79.7 85.7 91.8 97.8 103.8 109.8 h 1130.5 1153.5 1176.4 1199.5 1222.5 1245.6 1268.7 t 193.2 243.2 293.2 343.2 393.2 443.2 493.2 10 v 38.4 41.5 44.6 47.7 50.7 53.7 56.7 h 1143.1 1166.3 1189.5 1212.7 1236.0 1259.3 1282.5 t 213.0 263.0 313.0 363.0 413.0 463.0 513.0 15 v 26.27 28.40 30.46 32.50 34.53 36.56 38.58 h 1150.7 1174.2 1197.6 1221.0 1244.4 1267.7 1291.1 t 228.0 278.0 328.0 378.0 428.0 478.0 528.0 20 v 20.08 21.69 23.25 24.80 26.33 27.85 29.37 h 1156.2 1179.9 1203.5 1227.1 1250.6 1274.1 1297.6 t 240.1 290.1 340.1 390.1 440.1 490.1 540.1 25 v 16.30 17.60 18.86 20.10 21.32 22.55 23.77 h 1160.4 1184.4 1208.2 1231.9 1255.6 1279.2 1302.8 t 250.4 300.4 350.4 400.4 450.4 500.4 550.4 30 v 13.74 14.83 15.89 16.93 17.97 18.99 20.00 h 1163.9 1188.1 1212.1 1236.0 1259.7 1283.4 1307.1 t 259.3 309.3 359.3 409.3 459.3 509.3 559.3 35 v 11.89 12.85 13.75 14.65 15.54 16.42 17.30 h 1166.8 1191.3 1215.4 1239.4 1263.3 1287.1 1310.8 t 267.3 317.3 367.3 417.3 467.3 517.3 567.3 40 v 10.49 11.33 12.13 12.93 13.70 14.48 15.25 h 1169.4 1194.0 1218.4 1242.4 1266.4 1290.3 1314.1 t 274.5 324.5 374.5 424.5 474.5 524.5 574.5 45 v 9.39 10.14 10.86 11.57 12.27 12.96 13.65 h 1171.6 1196.6 1221.0 1245.2 1269.3 1293.2 1317.0 t 281.0 331.0 381.0 431.0 481.0 531.0 581.0 50 v 8.51 9.19 9.84 10.48 11.11 11.74 12.36 h 1173.6 1198.8 1223.4 1247.7 1271.8 1295.8 1319.7 t 287.1 337.1 387.1 437.1 487.1 537.1 587.1 55 v 7.78 8.40 9.00 9.59 10.16 10.73 11.30 h 1175.4 1200.8 1225.6 1250.0 1274.2 1298.1 1322.0 t 292.7 342.7 392.7 442.7 492.7 542.7 592.7 60 v 7.17 7.75 8.30 8.84 9.36 9.89 10.41 h 1177.0 1202.6 1227.6 1252.1 1276.4 1300.4 1324.3 t 298.0 348.0 398.0 448.0 498.0 548.0 598.0 65 v 6.65 7.20 7.70 8.20 8.69 9.17 9.65 h 1178.5 1204.4 1229.5 1254.0 1278.4 1302.4 1326.4 t 302.9 352.9 402.9 452.9 502.9 552.9 602.9 70 v 6.20 6.71 7.18 7.65 8.11 8.56 9.01 h 1179.8 1205.9 1231.2 1255.8 1280.2 1304.3 1328.3 t 307.6 357.6 407.6 457.6 507.6 557.6 607.6 75 v 5.81 6.28 6.73 7.17 7.60 8.02 8.44 h 1181.1 1207.5 1232.8 1257.5 1282.0 1306.1 1330.1 t 312.0 362.0 412.0 462.0 512.0 562.0 612.0 80 v 5.47 5.92 6.34 6.75 7.17 7.56 7.95 h 1182.3 1208.8 1234.3 1259.0 1283.6 1307.8 1331.9 t 316.3 366.3 416.3 466.3 516.3 566.3 616.3 85 v 5.16 5.59 6.99 6.38 6.76 7.14 7.51 h 1183.4 1210.2 1235.8 1260.6 1285.2 1309.4 1333.5 t 320.3 370.3 420.3 470.3 520.3 570.3 620.3 90 v 4.89 5.29 5.67 6.04 6.40 6.76 7.11 h 1184.4 1211.4 1237.2 1262.0 1286.6 1310.8 1334.9 t 324.1 374.1 424.1 474.1 524.1 574.1 624.1 95 v 4.65 5.03 5.39 5.74 6.09 6.43 6.76 h 1185.4 1212.6 1238.4 1263.4 1288.1 1312.3 1336.4 t 327.8 377.8 427.8 477.8 527.8 577.8 627.8 100 v 4.43 4.79 5.14 5.47 5.80 6.12 6.44 h 1186.3 1213.8 1239.7 1264.7 1289.4 1313.6 1337.8 t 331.4 381.4 431.4 481.4 531.4 581.4 631.4 105 v 4.23 4.58 4.91 5.23 5.54 5.85 6.15 h 1187.2 1214.9 1240.8 1265.9 1290.6 1314.9 1339.1 t 334.8 384.8 434.8 484.8 534.8 584.8 634.8 110 v 4.05 4.38 4.70 5.01 5.31 5.61 5.90 h 1188.0 1215.9 1242.0 1267.1 1291.9 1316.2 1340.4 t 338.1 388.1 438.1 488.1 538.1 588.1 638.1 115 v 3.88 4.20 4.51 4.81 5.09 5.38 5.66 h 1188.8 1216.9 1243.1 1268.2 1293.0 1317.3 1341.5 t 341.3 391.3 441.3 491.3 541.3 591.3 641.3 120 v 3.73 4.04 4.33 4.62 4.89 5.17 5.44 h 1189.6 1217.9 1244.1 1269.3 1294.1 1318.4 1342.7 t 344.4 394.4 444.4 494.4 544.4 594.4 644.4 125 v 3.58 3.88 4.17 4.45 4.71 4.97 5.23 h 1190.3 1218.8 1245.1 1270.4 1295.2 1319.5 1343.8 t 347.4 397.4 447.4 497.4 547.4 597.4 647.4 130 v 3.45 3.74 4.02 4.28 4.54 4.80 5.05 h 1191.0 1219.7 1246.1 1271.4 1296.2 1320.6 1344.9 t 350.3 400.3 450.3 500.3 550.3 600.3 650.3 135 v 3.33 3.61 3.88 4.14 4.38 4.63 4.87 h 1191.6 1220.6 1247.0 1272.3 1297.2 1321.6 1345.9 t 353.1 403.1 453.1 503.1 553.1 603.1 653.1 140 v 3.22 3.49 3.75 4.00 4.24 4.48 4.71 h 1192.2 1221.4 1248.0 1273.3 1298.2 1322.6 1346.9 t 355.8 405.8 455.8 505.8 555.8 605.8 655.8 145 v 3.12 3.38 3.63 3.87 4.10 4.33 4.56 h 1192.8 1222.2 1248.8 1274.2 1299.1 1323.6 1347.9 t 358.5 408.5 458.5 508.5 558.5 608.5 658.5 150 v 3.01 3.27 3.50 3.75 3.97 4.19 4.41 h 1193.4 1223.0 1249.6 1275.1 1300.0 1324.5 1348.8 t 361.0 411.0 461.0 511.0 561.0 611.0 661.0 155 v 2.92 3.17 3.41 3.63 3.85 4.06 4.28 h 1194.0 1223.6 1250.5 1276.0 1300.8 1325.3 1349.7 t 363.6 413.6 463.6 513.6 563.6 613.6 663.6 160 v 2.83 3.07 3.30 3.53 3.74 3.95 4.15 h 1194.5 1224.5 1251.3 1276.8 1301.7 1326.2 1350.6 t 366.0 416.0 466.0 516.0 566.0 616.0 666.0 165 v 2.75 2.99 3.21 3.43 3.64 3.84 4.04 h 1195.0 1225.2 1252.0 1277.6 1302.5 1327.1 1351.5 t 368.5 418.5 468.5 518.5 568.5 618.5 668.5 170 v 2.68 2.91 3.12 3.34 3.54 3.73 3.92 h 1195.4 1225.9 1252.8 1278.4 1303.3 1327.9 1352.3 t 370.8 420.8 470.8 520.8 570.8 620.8 670.8 175 v 2.60 2.83 3.04 3.24 3.44 3.63 3.82 h 1195.9 1226.6 1253.6 1279.1 1304.1 1328.7 1353.2 t 373.1 423.1 473.1 523.1 573.1 623.1 673.1 180 v 2.53 2.75 2.96 3.16 3.35 3.54 3.72 h 1196.4 1227.2 1254.3 1279.9 1304.8 1329.5 1353.9 t 375.4 425.4 475.4 525.4 575.4 625.4 675.4 185 v 2.47 2.68 2.89 3.08 3.27 3.45 3.63 h 1196.8 1227.9 1255.0 1280.6 1305.6 1330.2 1354.7 t 377.6 427.6 477.6 527.6 577.6 627.6 677.6 190 v 2.41 2.62 2.81 3.00 3.19 3.37 3.55 h 1197.3 1228.6 1255.7 1281.3 1306.3 1330.9 1355.5 t 379.8 429.8 479.8 529.8 579.8 629.8 679.8 195 v 2.35 2.55 2.75 2.93 3.11 3.29 3.46 h 1197.7 1229.2 1256.4 1282.0 1307.0 1331.6 1356.2 t 381.9 431.9 481.9 531.9 581.9 631.9 681.9 200 v 2.29 2.49 2.68 2.86 3.04 3.21 3.38 h 1198.1 1229.8 1257.1 1282.6 1307.7 1332.4 1357.0 t 384.0 434.0 484.0 534.0 584.0 634.0 684.0 205 v 2.24 2.44 2.62 2.80 2.97 3.14 3.30 h 1198.5 1230.4 1257.7 1283.3 1308.3 1333.0 1357.7 t 386.0 436.0 486.0 536.0 586.0 636.0 686.0 210 v 2.19 2.38 2.56 2.74 2.91 3.07 3.23 h 1198.8 1231.0 1258.4 1284.0 1309.0 1333.7 1358.4 t 388.0 438.0 488.0 538.0 588.0 638.0 688.0 215 v 2.14 2.33 2.51 2.68 2.84 3.00 3.16 h 1199.2 1231.6 1259.0 1284.6 1309.7 1334.4 1359.1 t 389.9 439.9 489.9 539.9 589.9 639.9 689.9 220 v 2.09 2.28 2.45 2.62 2.78 2.94 3.10 h 1199.6 1232.2 1259.6 1285.2 1310.3 1335.1 1359.8 t 391.9 441.9 491.9 541.9 591.9 641.9 691.9 225 v 2.05 2.23 2.40 2.57 2.72 2.88 3.03 h 1199.9 1232.7 1260.2 1285.9 1310.9 1335.7 1360.3 t 393.8 443.8 493.8 543.8 593.8 643.8 693.8 230 v 2.00 2.18 2.35 2.51 2.67 2.82 2.97 h 1200.2 1233.2 1260.7 1286.5 1311.6 1336.3 1361.0 t 395.6 445.6 495.6 545.6 595.6 645.6 695.6 235 v 1.96 2.14 2.30 2.46 2.62 2.77 2.91 h 1200.6 1233.8 1261.4 1287.1 1312.2 1337.0 1361.7 t 397.4 447.4 497.4 547.4 597.4 647.4 697.4 240 v 1.92 2.09 2.26 2.42 2.57 2.71 2.85 h 1200.9 1234.3 1261.9 1287.6 1312.8 1337.6 1362.3 t 399.3 449.3 499.3 549.3 599.3 649.3 699.3 245 v 1.89 2.05 2.22 2.37 2.52 2.66 2.80 h 1201.2 1234.8 1262.5 1288.2 1313.3 1338.2 1362.9 t 401.0 451.0 501.0 551.0 601.0 651.0 701.0 250 v 1.85 2.02 2.17 2.33 2.47 2.61 2.75 h 1201.5 1235.4 1263.0 1288.8 1313.9 1338.8 1363.5 t 402.8 452.8 502.8 552.8 602.8 652.8 702.8 255 v 1.81 1.98 2.14 2.28 2.43 2.56 2.70 h 1201.8 1235.9 1263.6 1289.3 1314.5 1339.3 1364.1

t = Temperature, degrees Fahrenheit. v = Specific volume, in cubic feet, per pound. h = Total heat from water at 32 degrees, B. t. u.

[Graph: Temperature of Steam—Degrees Fahr. against Temperature in Calorimeter—Degrees Fahr.

Fig. 15. Graphic Method of Determining Moisture Contained in Steam from Calorimeter Readings]



MOISTURE IN STEAM

The presence of moisture in steam causes a loss, not only in the practical waste of the heat utilized to raise this moisture from the temperature of the feed water to the temperature of the steam, but also through the increased initial condensation in an engine cylinder and through friction and other actions in a steam turbine. The presence of such moisture also interferes with proper cylinder lubrication, causes a knocking in the engine and a water hammer in the steam pipes. In steam turbines it will cause erosion of the blades.

The percentage by weight of steam in a mixture of steam and water is called the quality of the steam.

The apparatus used to determine the moisture content of steam is called a calorimeter though since it may not measure the heat in the steam, the name is not descriptive of the function of the apparatus. The first form used was the "barrel calorimeter", but the liability of error was so great that its use was abandoned. Modern calorimeters are in general of either the throttling or separator type.

Throttling Calorimeter—Fig. 14 shows a typical form of throttling calorimeter. Steam is drawn from a vertical main through the sampling nipple, passes around the first thermometer cup, then through a one-eighth inch orifice in a disk between two flanges, and lastly around the second thermometer cup and to the atmosphere. Thermometers are inserted in the wells, which should be filled with mercury or heavy cylinder oil.



The instrument and all pipes and fittings leading to it should be thoroughly insulated to diminish radiation losses. Care must be taken to prevent the orifice from becoming choked with dirt and to see that no leaks occur. The exhaust pipe should be short to prevent back pressure below the disk.

When steam passes through an orifice from a higher to a lower pressure, as is the case with the throttling calorimeter, no external work has to be done in overcoming a resistance. Hence, if there is no loss from radiation, the quantity of heat in the steam will be exactly the same after passing the orifice as before passing. If the higher steam pressure is 160 pounds gauge and the lower pressure that of the atmosphere, the total heat in a pound of dry steam at the former pressure is 1195.9 B. t. u. and at the latter pressure 1150.4 B. t. u., a difference of 45.4 B. t. u. As this heat will still exist in the steam at the lower pressure, since there is no external work done, its effect must be to superheat the steam. Assuming the specific heat of superheated steam to be 0.47, each pound passing through will be superheated 45.4/0.47 = 96.6 degrees. If, however, the steam had contained one per cent of moisture, it would have contained less heat units per pound than if it were dry. Since the latent heat of steam at 160 pounds gauge pressure is 852.8 B. t. u., it follows that the one per cent of moisture would have required 8.5 B. t. u. to evaporate it, leaving only 45.4 - 8.5 = 36.9 B. t. u. available for superheating; hence, the superheat would be 36.9/0.47 = 78.5 degrees, as against 96.6 degrees for dry steam. In a similar manner, the degree of superheat for other percentages of moisture may be determined. The action of the throttling calorimeter is based upon the foregoing facts, as shown below.

Let H = total heat of one pound of steam at boiler pressure, L = latent heat of steam at boiler pressure, h = total heat of steam at reduced pressure after passing orifice, t{1} = temperature of saturated steam at the reduced pressure, t{2} = temperature of steam after expanding through the orifice in the disc, 0.47 = the specific heat of saturated steam at atmospheric pressure, x = proportion by weight of moisture in steam.

The difference in B. t. u. in a pound of steam at the boiler pressure and after passing the orifice is the heat available for evaporating the moisture content and superheating the steam. Therefore,

H - h = xL + 0.47(t{2} - t{1})

H - h - 0.47(t{2} - t{1}) or x = —————————————- (4) L

Almost invariably the lower pressure is taken as that of the atmosphere. Under such conditions, h = 1150.4 and t_{1} = 212 degrees. The formula thus becomes:

H - 1150.4 - 0.47(t_{2} - 212) x = ——————————————— (5) L

For practical work it is more convenient to dispense with the upper thermometer in the calorimeter and to measure the pressure in the steam main by an accurate steam pressure gauge.

A chart may be used for determining the value of x for approximate work without the necessity for computation. Such a chart is shown in Fig. 15 and its use is as follows: Assume a gauge pressure of 180 pounds and a thermometer reading of 295 degrees. The intersection of the vertical line from the scale of temperatures as shown by the calorimeter thermometer and the horizontal line from the scale of gauge pressures will indicate directly the per cent of moisture in the steam as read from the diagonal scale. In the present instance, this per cent is 1.0.

Sources of Error in the Apparatus—A slight error may arise from the value, 0.47, used as the specific heat of superheated steam at atmospheric pressure. This value, however is very nearly correct and any error resulting from its use will be negligible.