Steam, Its Generation and Use Part 8

[Ill.u.s.tration: 1456 Horse-power Installation of Babc.o.c.k & Wilc.o.x Boilers at the Raritan Woolen Mills, Raritan, N. J. The First of These Boilers were Installed in 1878 and 1881 and are still Operated at 80 Pounds Pressure]

Durability--Babc.o.c.k & Wilc.o.x boilers are being operated in every-day service with entirely satisfactory results and under the same steam pressure as that for which they were originally sold that have been operated from thirty to thirty-five years. It is interesting to note in considering the life of a boiler that the length of life of a Babc.o.c.k & Wilc.o.x boiler must be taken as the criterion of what length of life is possible. This is due to the fact that there are Babc.o.c.k & Wilc.o.x boilers in operation to-day that have been in service from a time that antedates by a considerable margin that at which the manufacturer of any other water-tube boiler now on the market was started.

Probably the very best evidence of the value of the Babc.o.c.k & Wilc.o.x boiler as a steam generator and of the reliability of the apparatus, is seen in the sales of the company. Since the company was formed, there have been sold throughout the world over 9,900,000 horse power.

A feature that cannot be overlooked in the consideration of the advantages of the Babc.o.c.k & Wilc.o.x boiler is the fact that as a part of the organization back of the boiler, there is a body of engineers of recognized ability, ready at all times to a.s.sist its customers in every possible way.

[Ill.u.s.tration: 2400 Horse-power Installation of Babc.o.c.k & Wilc.o.x Boilers in the Union Station Power House of the Pennsylvania Railroad Co., Pittsburgh, Pa. This Company has a Total of 28,500 Horse Power of Babc.o.c.k & Wilc.o.x Boilers Installed]

HEAT AND ITS MEASUREMENT

The usual conception of heat is that it is a form of energy produced by the vibratory motion of the minute particles or molecules of a body. All bodies are a.s.sumed to be composed of these molecules, which are held together by mutual cohesion and yet are in a state of continual vibration. The hotter a body or the more heat added to it, the more vigorous will be the vibrations of the molecules.

As is well known, the effect of heat on a body may be to change its temperature, its volume, or its state, that is, from solid to liquid or from liquid to gaseous. Where water is melted from ice and evaporated into steam, the various changes are admirably described in the lecture by Mr. Babc.o.c.k on "The Theory of Steam Making", given in the next chapter.

The change in temperature of a body is ordinarily measured by thermometers, though for very high temperatures so-called pyrometers are used. The latter are dealt with under the heading "High Temperature Measurements" at the end of this chapter.

[Ill.u.s.tration: Fig. 11]

By reason of the uniform expansion of mercury and its great sensitiveness to heat, it is the fluid most commonly used in the construction of thermometers. In all thermometers the freezing point and the boiling point of water, under mean or average atmospheric pressure at sea level, are a.s.sumed as two fixed points, but the division of the scale between these two points varies in different countries. The freezing point is determined by the use of melting ice and for this reason is often called the melting point. There are in use three thermometer scales known as the Fahrenheit, the Centigrade or Celsius, and the Reaumur. As shown in Fig. 11, in the Fahrenheit scale, the s.p.a.ce between the two fixed points is divided into 180 parts; the boiling point is marked 212, and the freezing point is marked 32, and zero is a temperature which, at the time this thermometer was invented, was incorrectly imagined to be the lowest temperature attainable. In the centigrade and the Reaumur scales, the distance between the two fixed points is divided into 100 and 80 parts, respectively. In each of these two scales the freezing point is marked zero, and the boiling point is marked 100 in the centigrade and 80 in the Reaumur. Each of the 180, 100 or 80 divisions in the respective thermometers is called a degree.

Table 3 and appended formulae are useful for converting from one scale to another.

In the United States the bulbs of high-grade thermometers are usually made of either Jena 58^{III} borosilicate thermometer gla.s.s or Jena 16^{III} gla.s.s, the stems being made of ordinary gla.s.s. The Jena 16^{III} gla.s.s is not suitable for use at temperatures much above 850 degrees Fahrenheit and the harder Jena 59^{III} should be used in thermometers for temperatures higher than this.

Below the boiling point, the hydrogen-gas thermometer is the almost universal standard with which mercurial thermometers may be compared, while above this point the nitrogen-gas thermometer is used. In both of these standards the change in temperature is measured by the change in pressure of a constant volume of the gas.

In graduating a mercurial thermometer for the Fahrenheit scale, ordinarily a degree is represented as 1/180 part of the volume of the stem between the readings at the melting point of ice and the boiling point of water. For temperatures above the latter, the scale is extended in degrees of the same volume. For very accurate work, however, the thermometer may be graduated to read true-gas-scale temperatures by comparing it with the gas thermometer and marking the temperatures at 25 or 50 degree intervals. Each degree is then 1/25 or 1/50 of the volume of the stem in each interval.

Every thermometer, especially if intended for use above the boiling point, should be suitably annealed before it is used. If this is not done, the true melting point and also the "fundamental interval", that is, the interval between the melting and the boiling points, may change considerably. After continued use at the higher temperatures also, the melting point will change, so that the thermometer must be calibrated occasionally to insure accurate readings.

TABLE 3

COMPARISON OF THERMOMETER SCALES

+---------------+----------+----------+----------+ | |Fahrenheit|Centigrade| Reaumur | +---------------+----------+----------+----------+ |Absolute Zero | -459.64 | -273.13 | -218.50 | | | 0 | -17.78 | -14.22 | | | 10 | -12.22 | -9.78 | | | 20 | -6.67 | -5.33 | | | 30 | -1.11 | -0.89 | |Freezing Point | 32 | 0 | 0 | |Maximum Density| | | | | of Water | 39.1 | 3.94 | 3.15 | | | 50 | 10 | 8 | | | 75 | 23.89 | 19.11 | | | 100 | 37.78 | 30.22 | | | 200 | 93.33 | 74.67 | |Boiling Point | 212 | 100 | 80 | | | 250 | 121.11 | 96.89 | | | 300 | 148.89 | 119.11 | | | 350 | 176.67 | 141.33 | +---------------+----------+----------+----------+

F = 9/5C+32 = 9/4R+32

C = 5/9(F-32) = 5/4R

R = 4/9(F-32) = 4/5C

As a general rule thermometers are graduated to read correctly for total immersion, that is, with bulb and stem of the thermometer at the same temperature, and they should be used in this way when compared with a standard thermometer. If the stem emerges into s.p.a.ce either hotter or colder than that in which the bulb is placed, a "stem correction" must be applied to the observed temperature in addition to any correction that may be found in the comparison with the standard. For instance, for a particular thermometer, comparison with the standard with both fully immersed made necessary the following corrections:

_Temperature_ _Correction_ 40F 0.0 100 0.0 200 0.0 300 +2.5 400 -0.5 500 -2.5

When the sign of the correction is positive (+) it must be added to the observed reading, and when the sign is a negative (-) the correction must be subtracted. The formula for the stem correction is as follows:

Stem correction = 0.000085 n (T-t)

in which T is the observed temperature, t is the mean temperature of the emergent column, n is the number of degrees of mercury column emergent, and 0.000085 is the difference between the coefficient of expansion of the mercury and that in the gla.s.s in the stem.

Suppose the observed temperature is 400 degrees and the thermometer is immersed to the 200 degrees mark, so that 200 degrees of the mercury column project into the air. The mean temperature of the emergent column may be found by tying another thermometer on the stem with the bulb at the middle of the emergent mercury column as in Fig. 12. Suppose this mean temperature is 85 degrees, then

Stem correction = 0.000085 200 (400 - 85) = 5.3 degrees.

As the stem is at a lower temperature than the bulb, the thermometer will evidently read too low, so that this correction must be added to the observed reading to find the reading corresponding to total immersion. The corrected reading will therefore be 405.3 degrees. If this thermometer is to be corrected in accordance with the calibrated corrections given above, we note that a further correction of 0.5 must be applied to the observed reading at this temperature, so that the correct temperature is 405.3 - 0.5 = 404.8 degrees or 405 degrees.

[Ill.u.s.tration: Fig. 12]

[Ill.u.s.tration: Fig. 13]

Fig. 12 shows how a stem correction can be obtained for the case just described.

Fig. 13 affords an opportunity for comparing the scale of a thermometer correct for total immersion with one which will read correctly when submerged to the 300 degrees mark, the stem being exposed at a mean temperature of 110 degrees Fahrenheit, a temperature often prevailing when thermometers are used for measuring temperatures in steam mains.

Absolute Zero--Experiments show that at 32 degrees Fahrenheit a perfect gas expands 1/491.64 part of its volume if its pressure remains constant and its temperature is increased one degree. Thus if gas at 32 degrees Fahrenheit occupies 100 cubic feet and its temperature is increased one degree, its volume will be increased to 100 + 100/491.64 = 100.203 cubic feet. For a rise of two degrees the volume would be 100 + (100 2) / 491.64 = 100.406 cubic feet. If this rate of expansion per one degree held good at all temperatures, and experiment shows that it does above the freezing point, the gas, if its pressure remained the same, would double its volume, if raised to a temperature of 32 + 491.64 = 523.64 degrees Fahrenheit, while under a diminution of temperature it would shrink and finally disappear at a temperature of 491.64 - 32 = 459.64 degrees below zero Fahrenheit. While undoubtedly some change in the law would take place before the lower temperature could be reached, there is no reason why the law may not be used within the range of temperature where it is known to hold good. From this explanation it is evident that under a constant pressure the volume of a gas will vary as the number of degrees between its temperature and the temperature of -459.64 degrees Fahrenheit. To simplify the application of the law, a new thermometric scale is constructed as follows: the point corresponding to -460 degrees Fahrenheit, is taken as the zero point on the new scale, and the degrees are identical in magnitude with those on the Fahrenheit scale.

Temperatures referred to this new scale are called absolute temperatures and the point -460 degrees Fahrenheit (= -273 degrees centigrade) is called the absolute zero. To convert any temperature Fahrenheit to absolute temperature, add 460 degrees to the temperature on the Fahrenheit scale: thus 54 degrees Fahrenheit will be 54 + 460 = 514 degrees absolute temperature; 113 degrees Fahrenheit will likewise be equal to 113 + 460 = 573 degrees absolute temperature. If one pound of gas is at a temperature of 54 degrees Fahrenheit and another pound is at a temperature of 114 degrees Fahrenheit the respective volumes at a given pressure would be in the ratio of 514 to 573.

[Ill.u.s.tration: Ninety-sixth Street Station of the New York Railways Co., New York City, Operating 20,000 Horse Power of Babc.o.c.k & Wilc.o.x Boilers.

This Company and its Allied Companies Operate a Total of 100,000 Horse Power of Babc.o.c.k & Wilc.o.x Boilers]

British Thermal Unit--The quant.i.tative measure of heat is the British thermal unit, ordinarily written B. t. u. This is the quant.i.ty of heat required to raise the temperature of one pound of pure water one degree at 62 degrees Fahrenheit; that is, from 62 degrees to 63 degrees. In the metric system this unit is the _calorie_ and is the heat necessary to raise the temperature of one kilogram of pure water from 15 degrees to 16 degrees centigrade. These two definitions lead to a discrepancy of 0.03 of 1 per cent, which is insignificant for engineering purposes, and in the following the B. t. u. is taken with this discrepancy ignored.

The discrepancy is due to the fact that there is a slight difference in the specific heat of water at 15 degrees centigrade and 62 degrees Fahrenheit. The two units may be compared thus:

1 Calorie = 3.968 B. t. u. 1 B. t. u. = 0.252 Calories.

_Unit_ _Water_ _Temperature Rise_ 1 B. t. u. 1 Pound 1 Degree Fahrenheit 1 Calorie 1 Kilogram 1 Degree centigrade

But 1 kilogram = 2.2046 pounds and 1 degree centigrade = 9/5 degree Fahrenheit.

Hence 1 calorie = (2.2046 9/5) = 3.968 B. t. u.

The heat values in B. t. u. are ordinarily given per pound, and the heat values in calories per kilogram, in which case the B. t. u. per pound are approximately equivalent to 9/5 the calories per kilogram.

As determined by Joule, heat energy has a certain definite relation to work, one British thermal unit being equivalent from his determinations to 772 foot pounds. Rowland, a later investigator, found that 778 foot pounds were a more exact equivalent. Still later investigations indicate that the correct value for a B. t. u. is 777.52 foot pounds or approximately 778. The relation of heat energy to work as determined is a demonstration of the first law of thermo-dynamics, namely, that heat and mechanical energy are mutually convertible in the ratio of 778 foot pounds for one British thermal unit. This law, algebraically expressed, is W = JH; W being the work done in foot pounds, H being the heat in B. t. u., and J being Joules equivalent. Thus 1000 B. t. u.'s would be capable of doing 1000 778 = 778000 foot pounds of work.

Specific Heat--The specific heat of a substance is the quant.i.ty of heat expressed in thermal units required to raise or lower the temperature of a unit weight of any substance at a given temperature one degree. This quant.i.ty will vary for different substances For example, it requires about 16 B. t. u. to raise the temperature of one pound of ice 32 degrees or 0.5 B. t. u. to raise it one degree, while it requires approximately 180 B. t. u. to raise the temperature of one pound of water 180 degrees or one B. t. u. for one degree.

If then, a pound of water be considered as a standard, the ratio of the amount of heat required to raise a similar unit of any other substance one degree, to the amount required to raise a pound of water one degree is known as the specific heat of that substance. Thus since one pound of water required one B. t. u. to raise its temperature one degree, and one pound of ice requires about 0.5 degrees to raise its temperature one degree, the ratio is 0.5 which is the specific heat of ice. To be exact, the specific heat of ice is 0.504, hence 32 degrees 0.504 = 16.128 B. t. u. would be required to raise the temperature of one pound of ice from 0 to 32 degrees. For solids, at ordinary temperatures, the specific heat may be considered a constant for each individual substance, although it is variable for high temperatures. In the case of gases a distinction must be made between specific heat at constant volume, and at constant pressure.

Where specific heat is stated alone, specific heat at ordinary temperature is implied, and _mean_ specific heat refers to the average value of this quant.i.ty between the temperatures named.