Steam, Its Generation and Use Part 11

A British Imperial gallon holds 277.42 cubic inches and weighs, at 62 degrees Fahrenheit, 10 pounds.

The above are the true weights corrected for the effect of the buoyancy of the air, or the weight in vacuo. If water is weighed in air in the ordinary way, there is a correction of about one-eighth of one per cent which is usually negligible.

TABLE 11

VOLUME AND WEIGHT OF DISTILLED WATER AT VARIOUS TEMPERATURES[12]

+-----------+---------------+----------+ |Temperature|Relative Volume|Weight per| | Degrees | Water at 39.2 |Cubic Foot| | Fahrenheit| Degrees = 1 | Pounds | +-----------+---------------+----------+ | 32 | 1.000176 | 62.42 | | 39.2 | 1.000000 | 62.43 | | 40 | 1.000004 | 62.43 | | 50 | 1.00027 | 62.42 | | 60 | 1.00096 | 62.37 | | 70 | 1.00201 | 62.30 | | 80 | 1.00338 | 62.22 | | 90 | 1.00504 | 62.11 | | 100 | 1.00698 | 62.00 | | 110 | 1.00915 | 61.86 | | 120 | 1.01157 | 61.71 | | 130 | 1.01420 | 61.55 | | 140 | 1.01705 | 61.38 | | 150 | 1.02011 | 61.20 | | 160 | 1.02337 | 61.00 | | 170 | 1.02682 | 60.80 | | 180 | 1.03047 | 60.58 | | 190 | 1.03431 | 60.36 | | 200 | 1.03835 | 60.12 | | 210 | 1.04256 | 59.88 | | 212 | 1.04343 | 59.83 | | 220 | 1.0469 | 59.63 | | 230 | 1.0515 | 59.37 | | 240 | 1.0562 | 59.11 | | 250 | 1.0611 | 58.83 | | 260 | 1.0662 | 58.55 | | 270 | 1.0715 | 58.26 | | 280 | 1.0771 | 57.96 | | 290 | 1.0830 | 57.65 | | 300 | 1.0890 | 57.33 | | 310 | 1.0953 | 57.00 | | 320 | 1.1019 | 56.66 | | 330 | 1.1088 | 56.30 | | 340 | 1.1160 | 55.94 | | 350 | 1.1235 | 55.57 | | 360 | 1.1313 | 55.18 | | 370 | 1.1396 | 54.78 | | 380 | 1.1483 | 54.36 | | 390 | 1.1573 | 53.94 | | 400 | 1.167 | 53.5 | | 410 | 1.177 | 53.0 | | 420 | 1.187 | 52.6 | | 430 | 1.197 | 52.2 | | 440 | 1.208 | 51.7 | | 450 | 1.220 | 51.2 | | 460 | 1.232 | 50.7 | | 470 | 1.244 | 50.2 | | 480 | 1.256 | 49.7 | | 490 | 1.269 | 49.2 | | 500 | 1.283 | 48.7 | | 510 | 1.297 | 48.1 | | 520 | 1.312 | 47.6 | | 530 | 1.329 | 47.0 | | 540 | 1.35 | 46.3 | | 550 | 1.37 | 45.6 | | 560 | 1.39 | 44.9 | +-----------+---------------+----------+

Water is but slightly compressible and for all practical purposes may be considered non-compressible. The coefficient of compressibility ranges from 0.000040 to 0.000051 per atmosphere at ordinary temperatures, this coefficient decreasing as the temperature increases.

Table 11 gives the weight in vacuo and the relative volume of a cubic foot of distilled water at various temperatures.

The weight of water at the standard temperature being taken as 62.355 pounds per cubic foot, the pressure exerted by the column of water of any stated height, and conversely the height of any column required to produce a stated pressure, may be computed as follows:

The pressure in pounds per square foot = 62.355 height of column in feet.

The pressure in pounds per square inch = 0.433 height of column in feet.

Height of column in feet = pressure in pounds per square foot 62.355.

Height of column in feet = pressure in pounds per square inch 0.433.

Height of column in inches = pressure in pounds per square inch 27.71.

Height of column in inches = pressure in ounces per square inch 1.73.

By a change in the weights given above, the pressure exerted and height of column may be computed for temperatures other than 62 degrees.

A pressure of one pound per square inch is exerted by a column of water 2.3093 feet or 27.71 inches high at 62 degrees Fahrenheit.

Water in its natural state is never found absolutely pure. In solvent power water has a greater range than any other liquid. For common salt, this is approximately a constant at all temperatures, while with such impurities as magnesium and sodium sulphates, this solvent power increases with an increase in temperature.

TABLE 12

BOILING POINT OF WATER AT VARIOUS ALt.i.tUDES

+--------------+----------------+-------------+---------------+ |Boiling Point | Alt.i.tude Above | Atmospheric | Barometer | | Degrees | Sea Level | Pressure | Reduced | | Fahrenheit | Feet | Pounds per | to 32 Degrees | | | | Square Inch | Inches | +--------------+----------------+-------------+---------------+ | 184 | 15221 | 8.20 | 16.70 | | 185 | 14649 | 8.38 | 17.06 | | 186 | 14075 | 8.57 | 17.45 | | 187 | 13498 | 8.76 | 17.83 | | 188 | 12934 | 8.95 | 18.22 | | 189 | 12367 | 9.14 | 18.61 | | 190 | 11799 | 9.34 | 19.02 | | 191 | 11243 | 9.54 | 19.43 | | 192 | 10685 | 9.74 | 19.85 | | 193 | 10127 | 9.95 | 20.27 | | 194 | 9579 | 10.17 | 20.71 | | 195 | 9031 | 10.39 | 21.15 | | 196 | 8481 | 10.61 | 21.60 | | 197 | 7932 | 10.83 | 22.05 | | 198 | 7381 | 11.06 | 22.52 | | 199 | 6843 | 11.29 | 22.99 | | 200 | 6304 | 11.52 | 23.47 | | 201 | 5764 | 11.76 | 23.95 | | 202 | 5225 | 12.01 | 24.45 | | 203 | 4697 | 12.26 | 24.96 | | 204 | 4169 | 12.51 | 25.48 | | 205 | 3642 | 12.77 | 26.00 | | 206 | 3115 | 13.03 | 26.53 | | 207 | 2589 | 13.30 | 27.08 | | 208 | 2063 | 13.57 | 27.63 | | 209 | 1539 | 13.85 | 28.19 | | 210 | 1025 | 14.13 | 28.76 | | 211 | 512 | 14.41 | 29.33 | | 212 | Sea Level | 14.70 | 29.92 | +--------------+----------------+-------------+---------------+

Sea water contains on an average approximately 3.125 per cent of its weight of solid matter or a thirty-second part of the weight of the water and salt held in solution. The approximate composition of this solid matter will be: sodium chloride 76 per cent, magnesium chloride 10 per cent, magnesium sulphate 6 per cent, calcium sulphate 5 per cent, calcium carbonate 0.5 per cent, other substances 2.5 per cent.

[Ill.u.s.tration: 7200 Horse-power Installation of Babc.o.c.k & Wilc.o.x Boilers and Superheaters at the Capital Traction Co., Was.h.i.+ngton, D. C.]

The boiling point of water decreases as the alt.i.tude above sea level increases. Table 12 gives the variation in the boiling point with the alt.i.tude.

Water has a greater specific heat or heat-absorbing capacity than any other known substance (bromine and hydrogen excepted) and its specific heat is the basis for measurement of the capacity of heat absorption of all other substances. From the definition, the specific heat of water is the number of British thermal units required to raise one pound of water one degree. This specific heat varies with the temperature of the water.

The generally accepted values are given in Table 13, which indicates the values as determined by Messrs. Marks and Davis and Mr. Peabody.

TABLE 13

SPECIFIC HEAT OF WATER AT VARIOUS TEMPERATURES

+----------------------+--------------------------------+ | MARKS AND DAVIS | PEABODY | | From Values of | From Values of | | Barnes and Dieterici | Barnes and Regnault | +-----------+----------+---------------------+----------+ |Temperature| Specific | Temperature | Specific | +-----------+ Heat +----------+----------+ Heat | | Degrees | | Degrees | Degrees | | |Fahrenheit | |Centigrade|Fahrenheit| | +-----------+----------+----------+----------+----------+ | 30 | 1.0098 | 0 | 32 | 1.0094 | | 40 | 1.0045 | 5 | 41 | 1.0053 | | 50 | 1.0012 | 10 | 50 | 1.0023 | | 55 | 1.0000 | 15 | 59 | 1.0003 | | 60 | 0.9990 | 16.11 | 61 | 1.0000 | | 70 | 0.9977 | 20 | 68 | 0.9990 | | 80 | 0.9970 | 25 | 77 | 0.9981 | | 90 | 0.9967 | 30 | 86 | 0.9976 | | 100 | 0.9967 | 35 | 95 | 0.9974 | | 110 | 0.9970 | 40 | 104 | 0.9974 | | 120 | 0.9974 | 45 | 113 | 0.9976 | | 130 | 0.9979 | 50 | 122 | 0.9980 | | 140 | 0.9986 | 55 | 131 | 0.9985 | | 150 | 0.9994 | 60 | 140 | 0.9994 | | 160 | 1.0002 | 65 | 149 | 1.0004 | | 170 | 1.0010 | 70 | 158 | 1.0015 | | 180 | 1.0019 | 75 | 167 | 1.0028 | | 190 | 1.0029 | 80 | 176 | 1.0042 | | 200 | 1.0039 | 85 | 185 | 1.0056 | | 210 | 1.0052 | 90 | 194 | 1.0071 | | 220 | 1.007 | 95 | 203 | 1.0086 | | 230 | 1.009 | 100 | 212 | 1.0101 | +-----------+----------+----------+----------+----------+

In consequence of this variation in specific heat, the variation in the heat of the liquid of the water at different temperatures is not a constant. Table 22[13] gives the heat of the liquid in a pound of water at temperatures ranging from 32 to 340 degrees Fahrenheit.

The specific heat of ice at 32 degrees is 0.463. The specific heat of saturated steam (ice and saturated steam representing the other forms in which water may exist), is something that is difficult to define in any way which will not be misleading. When no liquid is present the specific heat of saturated steam is negative.[14] The use of the value of the specific heat of steam is practically limited to instances where superheat is present, and the specific heat of superheated steam is covered later in the book.

BOILER FEED WATER

All natural waters contain some impurities which, when introduced into a boiler, may appear as solids. In view of the apparent present-day tendency toward large size boiler units and high overloads, the importance of the use of pure water for boiler feed purposes cannot be over-estimated.

Ordinarily, when water of sufficient purity for such use is not at hand, the supply available may be rendered suitable by some process of treatment. Against the cost of such treatment, there are many factors to be considered. With water in which there is a marked tendency toward scale formation, the interest and depreciation on the added boiler units necessary to allow for the systematic cleaning of certain units must be taken into consideration. Again there is a considerable loss in taking boilers off for cleaning and replacing them on the line. On the other hand, the decrease in capacity and efficiency accompanying an increased incrustation of boilers in use has been too generally discussed to need repet.i.tion here. Many experiments have been made and actual figures reported as to this decrease, but in general, such figures apply only to the particular set of conditions found in the plant where the boiler in question was tested. So many factors enter into the effect of scale on capacity and economy that it is impossible to give any accurate figures on such decrease that will serve all cases, but that it is large has been thoroughly proven.

While it is almost invariably true that practically any cost of treatment will pay a return on the investment of the apparatus, the fact must not be overlooked that there are certain waters which should never be used for boiler feed purposes and which no treatment can render suitable for such purpose. In such cases, the only remedy is the securing of other feed supply or the employment of evaporators for distilling the feed water as in marine service.

TABLE 14

APPROXIMATE CLa.s.sIFICATION OF IMPURITIES FOUND IN FEED WATERS THEIR EFFECT AND ORDINARY METHODS OF RELIEF

+-----------------------+--------------+-----------------------------+ | Difficulty Resulting | Nature of | Ordinary Method of | | from Presence of | Difficulty | Overcoming or Relieving | +-----------------------+--------------+-----------------------------+ | Sediment, Mud, etc. | Incrustation | Settling tanks, filtration, | | | | blowing down. | | | | | | Readily Soluble Salts | Incrustation | Blowing down. | | | | | | Bicarbonates of Lime, | Incrustation | Heating feed. Treatment by | | Magnesia, etc. | | addition of lime or of lime | | | | and soda. Barium carbonate. | | | | | | Sulphate of Lime | Incrustation | Treatment by addition of | | | | soda. Barium carbonate. | | | | | | Chloride and Sulphate | Corrosion | Treatment by addition of | | of Magnesium | | carbonate of soda. | | | | | | Acid | Corrosion | Alkali. | | | | | | Dissolved Carbonic | Corrosion | Heating feed. Keeping air | | Acid and Oxygen | | from feed. Addition of | | | | caustic soda or slacked | | | | lime. | | | | | | Grease | Corrosion | Filter. Iron alum as | | | | coagulent. Neutralization | | | | by carbonate of soda. Use | | | | of best hydrocarbon oils. | | | | | | Organic Matter | Corrosion | Filter. Use of coagulent. | | | | | | Organic Matter | Priming | Settling tanks. Filter in | | (Sewage) | | connection with coagulent. | | | | | | Carbonate of Soda in | Priming | Barium carbonate. New feed | | large quant.i.ties | | supply. If from treatment, | | | | change. | +-----------------------+--------------+-----------------------------+

It is evident that the whole subject of boiler feed waters and their treatment is one for the chemist rather than for the engineer. A brief outline of the difficulties that may be experienced from the use of poor feed water and a suggestion as to a method of overcoming certain of these difficulties is all that will be attempted here. Such a brief outline of the subject, however, will indicate the necessity for a chemical a.n.a.lysis of any water before a treatment is tried and the necessity of adapting the treatment in each case to the nature of the difficulties that may be experienced.

Table 14 gives a list of impurities which may be found in boiler feed water, grouped according to their effect on boiler operation and giving the customary method used for overcoming difficulty to which they lead.

Scale--Scale is formed on boiler heating surfaces by the depositing of impurities in the feed water in the form of a more or less hard adherent crust. Such deposits are due to the fact that water loses its soluble power at high temperatures or because the concentration becomes so high, due to evaporation, that the impurities crystallize and adhere to the boiler surfaces. The opportunity for formation of scale in a boiler will be apparent when it is realized that during a month's operation of a 100 horse-power boiler, 300 pounds of solid matter may be deposited from water containing only 7 grains per gallon, while some spring and well waters contain sufficient to cause a deposit of as high as 2000 pounds.

The salts usually responsible for such incrustation are the carbonates and sulphates of lime and magnesia, and boiler feed treatment in general deals with the getting rid of these salts more or less completely.

TABLE 15

SOLUBILITY OF MINERAL SALTS IN WATER (SPARKS) IN GRAINS PER U. S. GALLON (58,381 GRAINS), EXCEPT AS NOTED

+------------------------------+------------+-------------+ |Temperature Degrees Fahrenheit| 60 Degrees | 212 Degrees | +------------------------------+------------+-------------+ |Calcium Carbonate | 2.5 | 1.5 | |Calcium Sulphate | 140.0 | 125.0 | |Magnesium Carbonate | 1.0 | 1.8 | |Magnesium Sulphate | 3.0 pounds | 12.0 pounds | |Sodium Chloride | 3.5 pounds | 4.0 pounds | |Sodium Sulphate | 1.1 pounds | 5.0 pounds | +------------------------------+------------+-------------+

CALCIUM SULPHATE AT TEMPERATURE ABOVE 212 DEGREES (CHRISTIE)

+------------------------------+----+----+-------+----+---+ |Temperature degrees Fahrenheit|284 |329 |347-365| 464|482| |Corresponding gauge pressure | 38 | 87 |115-149| 469|561| |Grains per gallon |45.5|32.7| 15.7 |10.5|9.3| +------------------------------+----+----+-------+----+---+

Table 15 gives the solubility of these mineral salts in water at various temperatures in grains per U. S. gallon (58,381 grains). It will be seen from this table that the carbonates of lime and magnesium are not soluble above 212 degrees, and calcium sulphate while somewhat insoluble above 212 degrees becomes more greatly so as the temperature increases.

Scale is also formed by the settling of mud and sediment carried in suspension in water. This may bake or be cemented to a hard scale when mixed with other scale-forming ingredients.