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American Society of Heating and Ventilating Engineers Guide, 1937 neys (in inches) for various amounts of heat supplied- to the appliance, and for various chimney heights. This is in accordance with American Gas Association recommendations. PROBLEMS IN PRACTICE 1 What are the principle factors influencing the intensity of natural draft? The intensity of natural draft depends largely upon the height of chimney above the grate bar level and the temperature difference between the chimney gases and the atmosphere. 2 What two kinds of draft need be considered? Natural draft caused by temperature differences, and artificial draft caused by me chanical forcing. 3 What is the effective height of a chimney? The height from the grate level to the top of the chimney is the effective height in pro ducing natural draft. 4 What dual purpose does a tall chimney fulfill? A tall chimney primarily creates the necessary draft to move the air required for the combustion process and to move the products of combustion, and secondarily it dis charges the gases at a high elevation to prevent them from becoming a nuisance. 5 What is the direct influence of the height on the design of a chimney? The immediate purpose of height is to provide that draft intensity under the conditions of chimney gas temperature such that it will be adequate to overcome all the frictional resistances of the installation, as well as to provide for the actual gas movement. 6 Of what importance is chimney cross-sectional area in stack design? The area should be as large as is economically feasible in order that the frictional loss for the chimney height should not destroy the effectiveness of the height-created draft in overcoming the necessary frictional resistances of the boiler and its flue connections. 7 Of what importance is the Reynolds number in chimney design? It permits the selection of a more specific value of the chimney friction factor, rather than a general one, to correspond with conditions of size, nature of the gas, rate of gas flow, and condition of the surface. 8 a. Name the principal advantages of natural draft. b. Name the principal disadvantages of natural draft. a. Simplicity, reliability, freedom from mechanical parts, low cost of maintenance, relatively long life, relatively low depreciation, operation with no power requirement. b. Lack of flexibility, irregularity, dependence on surroundings, susceptibility to tem perature changes. 9 9 How is mechanical draft created? By forced draft, by induced-draft fans, or by a Venturi chimney. XO # Distinguish between theoretical and available draft. Theoretical draft is the difference in pressure inside and outside the base of a chimney when it is under operating temperatures but when there are ho gases flowing. Available draft is less than theoretical draft by the friction loss due to the flow of gases througn the chimney. Chapter 27 FUELS AND COMBUSTION Classification of Coal, Airfor Combustion, Draft Required, Com bustion of Anthracite, Firing Bituminous Coal, Burning Coke, Hand Firing, Classification and Use of Oil, Classification and Use of Gas THE choice of fuel for heating is a question of economy, cleanliness, fuel availability, operation requirements, and control. The principal fuels to be considered are coal, oil, and gas. CLASSIFICATION OF COALS The complex composition of coal makes it difficult to classify it into clear-cut types. Its chemical composition is some indication but coals having the same chemical analysis may have distinctly different burning characteristics. Users are mainly interested in the available heat per pound of coal, in the handling and storing properties, and in the burning characteristics. A description of the relationship between the qualities of coals and these characteristics requires considerable space; a treatment applicable to heating boilers is given in U. S. Bureau of Mines Bulletin 276. A classification of coals is given in Table 1, and a brief description of the kinds of fuels is given in the following paragraphs, but it should be recognized that there are no distinct lines of demarcation between the kinds, and that they graduate into each other: Anthracite is a clean, dense, hard coal which creates very little dust in handling. It is comparatively hard to ignite but it burns freely when well started. It is non-caking, it bums uniformly and smokelessly with a short flame, and it requires little attention to the fuel beds between firings. It is capable of giving a high efficiency in the common types of hand-fired furnaces. Semi-anthracite has a higher volatile content than anthracite, it is not as hard and ignites somewhat more easily: otherwise its properties are similar to those of anthracite. Semi-bituminous coal is soft and friable, and fines and dust are created by handling it. It ignites somewhat slowly and burns with a medium length of flame. Its caking prop erties increase as the volatile matter increases, but the coke formed is relatively weak. Having only half the volatile matter content of the more abundant bituminous coals it can be burned with less production of smoke, and it is sometimes called smokeless coal. The term bituminous coal covers a large range of coals and includes many types having distinctly different composition, properties, and burning characteristics. The coals range the high-grade bituminous coals of the East to the poorer coals of the West. Their caking properties range from coals which completely melt, to those from which the volatiles and tars are distilled without change of form, so that they are classed as non caking or free-burning. Most bituminous coals are strong and non-friable enough to Permit of the screened sizes being delivered free from fines. In general,, they ignite 483