Document mnZ0Q8DQZ111b83R4OVxwDE4

Heating VentiixAting Air Conditioning Guide 1938 appliance, because to do so it would be necessary to cool the products of combustion down below their dew point, which is ordinarily in the neighborhood of 130 F.. The actual dew point in the chimney is different from the theoretical value because excess air is admitted not only at the burner but also at the backdraft diverter which lowers the dew point. Natural gas is the richest of the gases and contains from 80 to 95 per cent methane, with small percentages of the other combustible hydrocarbons. In addition, it contains from 0.5 to. 5.0 per cent of COj, and from 1 to 12 or 14 per cent of nitrogen! The heat value varies from 700 to 1500 Btu per cubic foot, the majority of natural gases averaging about 1000 Btu per cubic foot. Table 4 shows typical values for the four main oil fields, although values from any one field vary materially. Table 4 also gives the calorific values of the more common types of manufactured gas. Most states have legislation which controls the distri bution of gas and fixes a minimum limit to its heat content. The gross or higher calorific value usually ranges between 520 and 545 Btu per cubic foot, with an average of 535. A given heat value may be maintained and yet leave considerable latitude in the composition of the gas so-that as distributed the composition is not necessarily the same in different dis- Table 4. Representative Properties of Gaseous Fuels. Based on Gas at 60 F and 30 in. Hg. (tAH Btu per Cu Ft High (Gross) (Net) Specific Ghavttt. . I.UU FOB COMBU3(Cu Ft) Products op Combustion Cubic Feet Total COi HiO with Ni Ulti- *0(5* Dry Basis Flams Tem(deo Fahb) Natural gas-- California 1200 1087 0.67 Natural gas-- Mid-Conti nental 967 873 0.57 Natural gas-- .Ohio 1130 1025 0.65 Natural gas-- Pennsylvania 1232 1120 0.71 Retort coal gas 575 510 0.42 Coke oven gas 588 521 0.42 Carburetted water gas 536 496 0.65 Blue water gas 308 281 0.53 Anthracite pro ducer gas 134 124 0.85 Bituminous producer gas 150 140 0.86 Oil gas 575 510 0.35 11.26 1.24 2.24 12.4 12.2 9.17 0.97 1.92 10.2 11.7 10.70 1.17 2,16 11.8 12.1 A 11.70 1.30 5.00 0.50 2.29 1.21 12.9 5.7 12.3 11.2 5.19 0.51 1.25 5.9 11.0 4.37 2.26 0.74 0.75 5.0 17.2 0.46 0.51 2.8 22.3 1.05 0.33 0.19 1.9 19.0 1.24 4.91 0.35 0.19 0.47 1.21 2.0 19.0 5.6 10.7 3610 ' 3580 3600 3620 3665 3660 3815 3800 3000 . 3160 3725 176 Chapter 9. Fuels and Combustion tricts, nor at successive times in the same district. There are limits to the variation allowable, because the specific gravity of the gas depends on its composition, and too great a change in the specific gravity necessitates a change in the adjustment of the burners of small appliances. Table 4 shows that a large proportion of the products of combustion when gas is burned may consist of water vapor, and that the greater the proportion of water vapor, the lower the maximum attainable COt by gas analysis. The table also shows that a low calorific value does not neces sarily mean a low flame temperature since, for example, natural gas has a theoretical flame temperature of 3600 F and blue water gas-of 3800 F, although it has a calorific value less than one third that of natural gas. The quantity of air given in Table 4 is that required for theoretical, combustion, but with a properly designed and installed burner, the excess air can be kept low. The division of the air into primary and secondary is a matter of burner design and the pressure of gas available, and also of the type of flame desired. PROBLEMS IN PRACTICE . 1 Differentiate between the general characteristics of hard and soft coals. Hard coals contain fixed carbon in large proportions and in addition more ash is present, especially in the smaller sizes. Soft coals have an increasing percentage of carbon in combination with hydrogen which is volatile and will distill on under high temperature, , producing smoke. 2 Name several important properties of coal from a utilization standpoint. a. Caking tendency, whether none, weak, or strong. b. Quantity of volatile matter. c. Friability. d. Fusibility of the ash. 3 What are the main data commonly available that fix the qualities of coal, and do these tell the whole story? a. Calorific value, Btu per pound. b. Proximate analysis giving percentages of moisture, volatile matter, fixed carbon, ash, and sulphur. " c. Temperature at which the ash softens. d. Screen sizes. Other important qualities not usually given are the friability of the coal, its caking tendency, and the qualities of the volatile matter. The percentage of ash and its fusion temperature do not tell how the ash is distributed or how much of it is less fusible lumps of slate or shale. 4 Are there available complete and sufficient data on gas and oils to fix their burning properties and furnace requirements? Yes. Because gas and oils are'of simple and uniform composition, data are available to fix their burning properties and furnace requirements, but the ability to control their combustion is somewhat less determinable. 5 What effect does moisture in fuels have on their efficiency? With any solid fuel, latent and sensible heat are lost at the stack when moisture is dried out of the fuel in burning, and when its hydrogen is burned. Therefore, such fuels as sub-bituminous coal and lignite, which are high in moisture content, have a low efficiency. However, these efficiencies may be improved if the stack gases are cooled to room tem perature, by heating the feed water, for example. 177