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American Society of Heating and Ventilating Engineers Guide, 1932 would never rain and the earth would be continually enveloped in a cloud of vapor. DUST AND CINDER ABATEMENT The logical solution of the problem of dust and cinder abatement is to prevent the discharge of the dust and cinders into the atmosphere. Efforts in this direction, however, have not been entirely satisfactory. Lower rates of combustion per square foot of grate area will reduce the quantity of solid matter discharged from the chimney with the gases of combustion. The burning of coke, coking coal, and sized coal from which the extremely fine coal has been removed will not as a general rule produce as much dust and cinders as will result from the burning of non-coking coals and slack coal when they are burned on a grate. Modem boiler installations are usually designed for high capacity per square foot of ground area because such designs give the lowest cost of construction per unit of capacity. Designs of this type, discharge a large quantity of dust and cinders with the gases of combustion, and if pollution of the atmosphere is to be prevented, some type of catcher must be installed. Dust and Cinder Catchers1 Many types of dust and cinder catchers of merit are on the market today. However, most of them are in the process of development and it will probably be some time before the best types for various classes of service are definitely determined. The various types of dust and cinder catchers available today can be divided into six general classes: 1. Settling chambers. 2. Dust and cinder traps. 3. Centrifugal separators. 4. Electrostatic precipitators. 5. Gas scrubbers. 6. Filters. The selection of the proper type of catcher calls for a careful study of the material to be caught and the draft and space available. After installation constant vigilance is necessary to keep the catchers in proper working condition if satisfactory operation is to be obtained. If possible, the dust or cinder catcher should be installed on the inlet side of the induced draft fans because the dust and cinders in the gases seriously erode the wheels of the fans, the inlet connections and the scrolls. Where the induced draft fans operate at high tip speeds and no catchers are installed, it is not uncommon for the fans to require major repairs within one year and complete replacement within five years. Disposal of Dust and Cinders Even after the dust and cinders have been caught the disposal of the material caught presents a serious problem. The cinders discharged with See Smoke and Dust Abatement, by M. D. Engle. February. 1931). 526 (Heating, Piping and Air Conditioning. Chapter 38--Smoke, Dust and Cinder Abatement the gases from stoker-fired boilers are usually very high in carbon and contain from 50 to 80 per cent as much heat per pound as the coal which is being burned. It is possible, and usually economical, to burn these cinders. They cannot be satisfactorily mixed with the coal in the stoker hopper but they can be blown into the furnace over the stoker fuel bed and burned satisfactorily. If a sufficient quantity of cinders is caught, a small unit pulverizer can be installed to prepare them for burning over the stoker fuel bed. The same pulverizer can be used for coal at times of peak load and will materially increase the capacity of the fuel burning equipment for the boiler to which it is connected. No satisfactory market has been developed for the dust caught from pulverized coal installations, but the possibilities are being investigated and it seems likely that in the future this material will have a market value that will go a long way toward paying the fixed charges on the cost of catching it. Testing of Dust and Cinder Catchers The problem of testing to determine the efficiency of a dust or cinder catcher after installation is extremely difficult. No test code is available at present but one is under development. The distribution of dust in the gas entering and leaving the dust and cinder catchers is not uniform and is different in practically every in stallation, and varies widely with changes in furnace conditions. In order to obtain a representative sample it is necessary to traverse the inlet and outlet of the catcher with a sampling tube which faces into the gas flow. The velocity of the gas into the sampling tube must be the same as the velocity of the gas in the duct at the instant the sample is taken. The swirls and eddy currents in the ducts make it difficult to obtain consistent readings, but if the test is conducted by some one of experience, an indication of the approximate efficiency can be obtained. Settling Chambers Probably the oldest form of dust catcher is the settling chamber, which generally consists of a large-sized, gas-tight space into which the dust-laden gases are discharged before being delivered to the chimney. The velocity of the gas should be reduced to a point where the larger and heavier particles will be precipitated by gravity. For good operation, the velocity of the gas should be reduced to a maximum of 2 fps. The bottoms of the chambers should be provided with dump plates through which the collected dust can be removed. Because these chambers are not effective in removing the finer dust particles they have been practically superseded by smaller and less costly devices. Dust and Cinder Traps Various types of traps have been devised. In general they all depend upon breaking the gas up into thin strata and subjecting those thin strata to several abrupt changes in direction. The dust is thrown out of the gas stream into specially shaped pockets or impinged against a roughened surface. The trapping pockets are drained into a hopper below with a small quantity of gas and the dust settles but by gravity due to the low velocity in the hopper. In the roughened surface type, various