Document 85pGOYGoRdzzkDnDJKx8xOKea

American Society of Heating and Ventilating Engineers Guide, 1934 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. 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. Nature's Dust Catcher Nature has provided means for catching solid particles in the air and depositing them upon the earth. A dust particle forms the nucleus for each rain drop and the. rain picks up dust as it falls from the clouds to the earth. In fact, without dust in the air to form the nuclei for rain drops it would never rain, and the earth would be continually enveloped in a cloud of vapor. 216 Chapter 16 AIR CLEANING EQUIPMENT Requirements of an Air Cleaner, Types, Air Washers and Scrubbers, Viscous Type Filters, Dry Air Filters, Air Filter Installations AIR cleaning devices are intended to remove impurities in air brought into a building for ventilating or air conditioning purposes. These impurities include carbon (soot) from the incomplete combustion of fuels burned in furnaces and automobile engines, particles of earth, sand, ash, automobile tires, leather, animal excretion, stone, wood, rust and paper, threads of cotton, wool and silk, bits of animal and vegetable matter, bacteria and pollen. Microscopic examination shows that the character of the impurities varies with the locality, but as a rule carbon forms the greater part of them while the total is somewhat proportional to the state of industrial activity and the wind intensity. Additional information on sources of air pollution will be found in Chapter 15. Observations have shown that practically all atmospheric impurities are less than 5 microns in size. (One micron equals 0.001 millimeter or , approximately 0.00004 in.). The size and composition of each individual particle determines its buoyancy and consequently the length of time it will remain in suspension. The chart, Fig. 1, shows graphically the sizes of impurities found in the air, and other related data. To estimate the probable dust load for air filter installations, the following approximate averages of atmospheric dust concentration may be used (7000 grains equal 1 lb): Rural and suburban districts......... ............ ............. .......... 0.2 to 0.4 grains per 1000 cu ft Metropolitan districts..........................................................0.4 to 0.8 grains per 1000 cu ft Industrial districts.................................................................0.8 to 1.5 grains per 1000 cu ft REQUIREMENTS OF AN AIR CLEANER To fulfill the essential requirements of clean air, an air cleaner should: 1.Be efficient in the removal of harmful and objectionable impurities in the air, such as dust, dirt, pollens, bacteria. 2. Be efficient over a considerable range of air velocities. 3. Have a low frictional resistance to air flow, that is, the pressure drop across the filter, measured in inches of water, should be as low as possible. 4.Have a large dust-holding capacity without excessive increase of resistance, or have ability to operate so as to keep the resistance constant automatically. 5. Be easy to clean and handle, or clean itself automatically. 6. Leave the air passing through the cleaner free from entrained moisture or charging liquids used in the cleaner. 217