Machinery's Handbook, 31st Edition
Heat
369
Heat and Combustion Related Properties Latent Heat.— When a body changes from the solid to the liquid state or from the liq- uid to the gaseous state, a certain amount of heat is used to accomplish this change. This heat does not raise the temperature of the body and is called latent heat. When the body changes again from the gaseous to the liquid, or from the liquid to the solid state, it gives out this quantity of heat. The latent heat of fusion is the heat supplied to a solid body at the melting point; this heat is absorbed by the body although its tem- perature remains nearly stationary during the whole operation of melting. The latent heat of evaporation is the heat that must be supplied to a liquid at the boiling point to transform the liquid into a vapor. The latent heat is generally given in British thermal units per pound, or kilojoules per kilogram. The latent heat of evaporation of water is 966.6 Btu/pound, or 2248 kJ/kg. This means that it takes 966.6 Btu to evaporate 1 pound, or 2248 kJ to evaporate 1 kilogram, of water after it has been raised to the boiling point, 212 ° F or 100 ° C. When a body changes from the solid to the gaseous state without passing through the liquid stage, as solid carbon dioxide does, the process is called sublimation . Table 2. Latent Heat of Fusion Substance Btu per Pound kJ/kg Substance Btu per Pound kJ/kg Substance Btu per Pound kJ/kg Bismuth 22.75 52.92 Paraffine 63.27 147.17 Sulfur 16.86 39.22 Beeswax 76.14 177.10 Phosphorus 9.06 21.07Tin 25.65 59.66 Cast iron, gray 41.40 96.30 Lead 10.00 23.26Zinc 50.63 117.77 Cast iron, white 59.40 138.16 Silver 37.92 88.20 Ice 144.00 334.94 Table 3. Latent Heat of Evaporation Liquid Btu per Pound kJ/kg Liquid Btu per Pound kJ/kg Liquid Btu per Pound kJ/kg Alcohol, ethyl 371.0 863 Carbon bisulfide 160.0 372 Turpentine 133.0 309 Alcohol, methyl 481.0 1119 Ether 162.8 379 Water 966.6 2248 Ammonia 529.0 1230 Sulfur dioxide 164.0 381 Table 4. Boiling Points of Various Substances at Atmospheric Pressure
Boiling Point
Boiling Point
Boiling Point
Substance
° F
° C
Substance
° F
° C
Substance
° F
° C
Aniline Alcohol Ammonia Benzine Bromine
363 183.9 Chloroform 140 60.0 Saturated brine
226 107.8 833 445.0 590 310.0 212 100.0 213.2 100.7
173 78.3 Ether − 33.3 Linseed oil 176 80.0 Mercury 145 62.8 Napthaline 118 47.8 Nitric acid − 28
100 37.8 Sulfur 597 313.9 Sulfuric acid 676 357.8 Water, pure 428 220.0 Water, sea 248 120.0 Wood alcohol
Carbon bisulfide
150
65.6
Oil of turpentine 315 157.2 Specific Heat.— The specific heat of a substance is the ratio of the heat required to raise the temperature of a certain weight of the given substance 1 ° F to the heat required to raise the temperature of the same weight of water 1 ° F. As the specific heat is not constant at all temperatures, it is generally assumed that it is determined by raising the temperature from 62 to 63 ° F. For most substances, however, specific heat is practically constant for temperatures up to 212 ° F. In metric units, specific heat is defined as the ratio of the heat needed to raise the temperature of a mass by 1°C to the heat needed to raise the temperature of the same mass of water by 1°C. In the metric system, heat is measured in joules (J), mass is in grams (g), and measurements are usually taken at 15°C. Because specific heat is a dimensionless ratio, the values given in Table 5 and Table 6 that follow are valid in both the US system and the metric system.
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