Part 14

SPECIFIC HEATS OF VARIOUS SUBSTANCES +--------------------------------------------------------------------+ | SOLIDS | +-------------------------------+----------------+-------------------+ | | Temperature| | | | Degrees | Specific | | | Fahrenheit | Heat | +-------------------------------+----------------+-------------------+ | Copper | 59-460 | .0951 | | Gold | 32-212 | .0316 | | Wrought Iron | 59-212 | .1152 | | Cast Iron | 68-212 | .1189 | | Steel (soft) | 68-208 | .1175 | | Steel (hard) | 68-208 | .1165 | | Zinc | 32-212 | .0935 | | Brass (yellow) | 32 | .0883 | | Glass (normal ther. 16^{III}) | 66-212 | .1988 | | Lead | 59 | .0299 | | Platinum | 32-212 | .0323 | | Silver | 32-212 | .0559 | | Tin | -105-64 | .0518 | | Ice | | .5040 | | Sulphur (newly fused) | | .2025 | +-------------------------------+----------------+-------------------+ | LIQUIDS | +-------------------------------+----------------+-------------------+ | | Temperature| | | | Degrees | Specific | | | Fahrenheit | Heat | +-------------------------------+----------------+-------------------+ | Water | 59 | 1.0000 | | Alcohol | 32 | .5475 | | | 176 | .7694 | | Mercury | 32 | .03346 | | Benzol | 50 | .4066 | | | 122 | .4502 | | Glycerine | 59-102 | .576 | | Lead (Melted) | to 360 | .0410 | | Sulphur (melted) | 246-297 | .2350 | | Tin (melted) | | .0637 | | Sea Water (sp. gr. 1.0043) | 64 | .980 | | Sea Water (sp. gr. 1.0463) | 64 | .903 | | Oil of Turpentine | 32 | .411 | | Petroleum | 64-210 | .498 | | Sulphuric Acid | 68-133 | .3363 | +-------------------------------+----------------+-------------------+ | GASES | +--------------------------+---------------+--------------+----------+ | | | Specific | Specific | | | Temperature| Heat at | Heat at | | | Degrees | Constant | Constant | | | Fahrenheit | Pressure | Volume | +--------------------------+---------------+--------------+----------+ | Air | 32-392 | .2375 | .1693 | | Oxygen | 44-405 | .2175 | .1553 | | Nitrogen | 32-392 | .2438 | .1729 | | Hydrogen | 54-388 | 3.4090 | 2.4141 | | Superheated Steam | | See table 25 | | | Carbon Monoxide | 41-208 | .2425 | .1728 | | Carbon Dioxide | 52-417 | .2169 | .1535 | | Methane | 64-406 | .5929 | .4505 | | Blast Fur. Gas (approx.) | ... | .2277 | ... | | Flue gas (approx.) | ... | .2400 | ... | +--------------------------+---------------+--------------+----------+

Latent heat is not lost, but reappears whenever the substances pass through a reverse cycle, from a gaseous to a liquid, or from a liquid to a solid state. It may, therefore, be defined as stated, as the heat which apparently disappears, or is lost to thermometric measurement, when the molecular constitution of a body is being changed. Latent heat is expended in performing the work of overcoming the molecular cohesion of the particles of the substance and in overcoming the resistance of external pressure to change of volume of the heated body. Latent heat of evaporation, therefore, may be said to consist of internal and external heat, the former being utilized in overcoming the molecular resistance of the water in changing to steam, while the latter is expended in overcoming any resistance to the increase of its volume during formation. In evaporating a pound of water at 212 degrees to steam at 212 degrees, 897.6 B. t. u. are expended as internal latent heat and 72.8 B. t. u. as external latent heat. For a more detailed description of the changes brought about in water by sensible and latent heat, the reader is again referred to the chapter on "The Theory of Steam Making".

Ebullition--The temperature of ebullition of any liquid, or its boiling point, may be defined as the temperature which exists where the addition of heat to the liquid no longer increases its temperature, the heat added being absorbed or utilized in converting the liquid into vapor. This temperature is dependent upon the pressure under which the liquid is evaporated, being higher as the pressure is greater.

TABLE 5

BOILING POINTS AT ATMOSPHERIC PRESSURE

+---------------------+--------------+ | | Degrees | | | Fahrenheit | +---------------------+--------------+ | Ammonia | 140 | | Bromine | 145 | | Alcohol | 173 | | Benzine | 212 | | Water | 212 | | Average Sea Water | 213.2 | | Saturated Brine | 226 | | Mercury | 680 | +---------------------+--------------+

Total Heat of Evaporation--The quantity of heat required to raise a unit of any liquid from the freezing point to any given temperature, and to entirely evaporate it at that temperature, is the total heat of evaporation of the liquid for that temperature. It is the sum of the heat of the liquid and the latent heat of evaporation.

To recapitulate, the heat added to a body is divided as follows:

Total heat = Heat to change the temperature + heat to overcome the molecular cohesion + heat to overcome the external pressure resisting an increase of volume of the body.

Where water is converted into steam, this total heat is divided as follows:

Total heat = Heat to change the temperature of the water + heat to separate the molecules of the water + heat to overcome resistance to increase in volume of the steam, = Heat of the liquid + internal latent heat + external latent heat, = Heat of the liquid + total latent heat of steam, = Total heat of evaporation.

The steam tables given on pages 122 to 127 give the heat of the liquid and the total latent heat through a wide range of temperatures.

Gases--When heat is added to gases there is no internal work done; hence the total heat is that required to change the temperature plus that required to do the external work. If the gas is not allowed to expand but is preserved at constant volume, the entire heat added is that required to change the temperature only.

Linear Expansion of Substances by Heat--To find the increase in the length of a bar of any material due to an increase of temperature, multiply the number of degrees of increase in temperature by the coefficient of expansion for one degree and by the length of the bar. Where the coefficient of expansion is given for 100 degrees, as in Table 6, the result should be divided by 100. The expansion of metals per one degree rise of temperature increases slightly as high temperatures are reached, but for all practical purposes it may be assumed to be constant for a given metal.

TABLE 6

LINEAL EXPANSION OF SOLIDS AT ORDINARY TEMPERATURES

(Tabular values represent increase per foot per 100 degrees increase in temperature, Fahrenheit or centigrade)

+-------------------+--------------+----------------+----------------+ | | Temperature | | | | | Conditions|Coefficient per |Coefficient per | | Substance | Degrees | 100 Degrees | 100 Degrees | | | Fahrenheit | Fahrenheit | Centigrade | +-------------------+--------------+----------------+----------------+ |Brass (cast) | 32 to 212 | .001042 | .001875 | |Brass (wire) | 32 to 212 | .001072 | .001930 | |Copper | 32 to 212 | .000926 | .001666 | |Glass (English | | | | |flint) | 32 to 212 | .000451 | .000812 | |Glass (French | | | | |flint) | 32 to 212 | .000484 | .000872 | |Gold | 32 to 212 | .000816 | .001470 | |Granite (average) | 32 to 212 | .000482 | .000868 | |Iron (cast) | 104 | .000589 | .001061 | |Iron (soft forged) | 0 to 212 | .000634 | .001141 | |Iron (wire) | 32 to 212 | .000800 | .001440 | |Lead | 32 to 212 | .001505 | .002709 | |Mercury | 32 to 212 | .009984 | .017971 | |Platinum | 104 | .000499 | .000899 | |Limestone | 32 to 212 | .000139 | .000251 | |Silver | 104 | .001067 | .001921 | |Steel (Bessemer | | | | |rolled, hard) | 0 to 212 | .00056 | .00101 | |Steel (Bessemer | | | | |rolled, soft) | 0 to 212 | .00063 | .00117 | |Steel (cast, | | | | |French) | 104 | .000734 | .001322 | |Steel (cast | | | | |annealed, English) | 104 | .000608 | .001095 | +-------------------+--------------+----------------+----------------+

High Temperature Measurements--The temperatures to be dealt with in steam-boiler practice range from those of ordinary air and steam to the temperatures of burning fuel. The gases of combustion, originally at the temperature of the furnace, cool as they pass through each successive bank of tubes in the boiler, to nearly the temperature of the steam, resulting in a wide range of temperatures through which definite measurements are sometimes required.

Of the different methods devised for ascertaining these temperatures, some of the most important are as follows:

1st. Mercurial pyrometers for temperatures up to 1000 degrees Fahrenheit.

2nd. Expansion pyrometers for temperatures up to 1500 degrees Fahrenheit.

3rd. Calorimetry for temperatures up to 2000 degrees Fahrenheit.

4th. Thermo-electric pyrometers for temperatures up to 2900 degrees Fahrenheit.

5th. Melting points of metal which flow at various temperatures up to the melting point of platinum 3227 degrees Fahrenheit.

6th. Radiation pyrometers for temperatures up to 3600 degrees Fahrenheit.

7th. Optical pyrometers capable of measuring temperatures up to 12,600 degrees Fahrenheit. For ordinary boiler practice however, their range is 1600 to 3600 degrees Fahrenheit.

Table 7 gives the degree of accuracy of high temperature measurements.