Document wD64YL6oqvJvVGwq79xNm941o

Heating Ventilating Air Conditioning Guide 1939 2 Does the size of a fuel affeet the quantity of air required to burn it at a given rate? The total air required to give the same gas analysis at the stack is independent of the size of the fuel burned, but for non-caking fuels the ratio of the air passing through the fuel bed to the total air entering the burner base decreases, for the same thickness of bed as the size of the fuel becomes smaller; this decrease is very rapid for sizes less than one inch. For coals that cake, this ratio will depend on the way the caked bed is broken up and on the size of the resulting pieces. 3 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, and d. fusibility of the ash. 4 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' and 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. 5 4 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 off under high temperature, producing smoke. 6 Is the volatile matter which is given off when coals are burned of the same nature in all coals? No. The products given off by coals when they are heated differ materially in the ratios by weight of the gases to the oils and tars. No heavy oils or tars are given off by anthra cite, and very small quantities are given off by semi-anthracite. As the volatile matter in the coal increases to as much" as 40 per cent of ash-free and moisture-free coal, in creasing amounts of oils and tars are given up. For coals of higher volatile content, the relative quantity of oils and tars decreases, so it is low in the sub-bituminous coals and in lignite. 7 Is smoke a primary product in the burning of fuels? Visible smoke may include very small particles of carbon, oil, tar, water (condensed steam), and ash. Of these, the oils, tars, and ash are mainly primary products, and thewater is partly primary. The carbon, which usually comprises the greater part of the smoke, results.from the breaking up by heat of oils, tars, and such gases as methane, so it may be considered a secondary product. 8 Is the sulphur in coals detrimental to combustion? Not so far as is known, but its complete combustion gives only 25 per cent as much heat as is given by the same weight of carbon. Sulphur is undesirable because it causes cor rosion of flues and stacks, and also because its gases pollute the atmosphere, and damage buildings and vegetation. 9 How do deposits of soot on the surfaces of a boiler or heater affect the quantity of fuel burned? There are two effects. The soot acts,as an insulating layer over the surface and reduces the heat transmission to the water or air; the Bureau of Mines Report of Investigations No. 3272 shows that the loss of seasonal efficiency is not as great as has been believed and should not be over 6 per cent because the greater part of the heat is transmitted through the firepot. The soot clogs the passages and reduces the draft; the loss of efficiency from this action may be much more, and also the lack of draft results in ^unsatisfactory heating. .ft 184 Chapter 10 CHIMNEYS AND DRAFT CALCULATIONS Natural Draft, Mechanical Draft, Characteristics of Natural Draft Chimneys, Determining Chimney Sizes, General Equa tion, Chimney Construction, Chimneys for Gas Heating THE design and construction of a chimney is so important a part of the heating engineer's work that a general knowledge of draft characterictics and calculations is essential. Draft, in general, may be defined as the pressure difference between the atmospheric pressure and that at any part of an installation through which the gases flow. Since a pressure difference implies a head, draft is a static force. While no element of motion is inferred, yet motion in the form of circulation of gases throughout an entire boiler plant installation is the direct result of draft. This motion is due to the pressure difference, or unbalanced pressure, which compels the gases to flow. Draft is often classified into two kinds according to whether it is created thermally or artificially, viz, (1) natural or thermal draft, and (2) arti ficial or mechanical draft. Natural Draft Natural draft is the difference in pressure produced by the difference in weight between the relatively hot gases inside a natural draft chimney and an equivalent column of the cooler outside air, or atmosphere. Natural draft, in other words, is an unbalanced pressure produced thermally by a natural draft chimney as the pressure transformer and a temperature difference. The intensity of natural draft depends, for the most part, upon the height of the chimney above the grate bar level and. also the temperature difference between the chimney gases and the atmosphere. A typical natural draft system consists essentially of a relatively tall chimney built of steel, brick, or reinforced concrete, operating with the relatively hot gases which have passed through the boilers and accessories and from which all the heat has not been extracted. Hot gases are an essential element in the operation of. a natural draft system, although inherently a heat balance loss. A natural draft chimney performs the two-fold service of assisting in the creation of draft by aspiration and also of discharging the gases at an elevation sufficient to prevent them from becoming a nuisance. Natural draft is quite advantageous in installations where the total loss of draft due to resistances is relatively low and also in plants which have practically a constant load and whose boilers are seldom operated above