Document x1OYVYMmgOK3a9OEgO2DoOarG

"American Society of Heating and Ventilating Engineers Guide, 1935 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'1 distributed the composition is not necessarily the same in different dis 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 Table 4. Representative Properties of Gaseous Fuels, Based on Gas at 60 F and 30 in. Hg. Ga8 Btu per Cu Ft Hii?h (Gross) (Net) Specific Gratot, =Air roa Combus tion, (Cu Ft) Products op Combustion Cubic Feet Total COj HtO with Nt Ulti mate COj Dry Basis Theoretical Flame Tt*. pbbaturk (deq Fahr) Natural gas-- Mid-Conti nental 967 873 b.57 9.17 0.97 1.92 10.2 11.7 . 3580 Natural gas-- Ohio 1130 1025 0.65 10,70 Natural gas-- Pennsylvania 1232 .1120 Retort coal gas 575 510 Coke oven gas ~588 521 0.71 . 11.70 0.42 5.00 0.42 5.19 1.17 2.16 11.8 12.1 3600 1.30 .0.50 0.51 2,29 12.9 1.21 . 5.7 1.25 5.9 12.3 11.2 11.0 3620 3665 : 3660, Carburetted water gas Blue water gas Anthracite pro ducer gas Bituminous producer gas Oil gas 536 496 0.65 308 281 .0.53 134 124 0.85 150 : 140 0.86 575 510 0.35 4.37 2.26 1.05 1.24 4.91 0.74 0.75 0.46 0.51 5.0 17.2 ' 3815 2.8 22.3 7800 0.33 0.19 1.9 19.0 3000 o735 "0.19 0.47 1.21 2.0 19.0 5.6 10.7 3160. 3725 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 C02 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. 454 Chapter 27--Fuels and Combustion PROBLEMS IX PRACTICE 1 Name several important properties of coal from a utilization standpoint. 0. Caking tendency, whether none, weak, or strong, j Quantity of volatile matter. c. Friability. 1. Fusibility of the ash. 2 0 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. i. 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. . 3 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. 4 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. 5 What are the advantages of a sized fuel for heating furnaces? Because a sized fuel encourages a more uniform flow of air through the bed, the burning will be more uniform, and the bed will be less liable to develop holes and will require less attention. Uniformity of fuel size is more desirable as the area of the bed becomes smaller; it is less important with fuels that cake, but with sized fuels the caking will be more uniform and the air flow through the bed will be steadier. In addition, ash and pieces of slate are less likely to be segregated and to form lumps of clinker. 6 Does the size of a fuel affect 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 thefuel 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. 7 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- 455