Document vVRdOBBJj6JQNzj71pe81KDxZ

American Society of Heating and Ventilating Engineers Guide, 1926-27 use of a trap at the junction of the hood and branch pipe is permissible, provided it is not allowed to fill up completely. The passing of pipes through fire-walls .should be avoided wherever possible, and sweep-up connections should be so arranged that foreign material cannot be easily introduced into them. Where considerable quantities of explosive dust or inflammable materials pass through the exhaust fan, the blast wheel should be con structed of brass composition, copper or other soft metal arid in all cases ample clearance should be provided between blast wheels and housings. Where stringy or fibrous material is to be handled through the fan be sure to employ a fan wheel especially designed for that purpose. MAINTENANCE OF SYSTEM Because of its simplicity the exhaust system usually receives but little attention once it is installed; however, to obtain the best results, it should be inspected at suitable intervals and necessary adjustments made. The exhaust fan should be given proper attention the same as any other high-speed machine. It should be kept in proper alignment and tightly bolted to its foundation. Suction hoods, which have been removed to adjust the machines, should be replaced, as soon as the adjustments are completed. Never start a machine with the blast gate closed, as the slight air leak-' age past the blast gate may draw material into the pipe and clog it. Disconnect furnace feeders from the furnaces when not in operation, and do not overload the system by ill-advised additions. 284 Chapter XXV MECHANICAL DRAFT THERE is a certain draft which will give the best results for every kind of fuel and rate of combustion. The amount of fuel that can be burned per hour per square foot of grate surface is governed by the quality, and the type of fuel as well as by the draft obtainable. Mechanical draft is used to obtain economy of operation, increased capacity or both and may be accomplished by either the forced or induced method. The two common methods of producing mechanical draft are by means of fans or steam jets. Each method has its advantages and design conditions will govern the choice of apparatus. Steam jets are reliable, have nothing to break or wear out and are more economical to install. On the other hand fans usually take more power to operate and in cases where fan engines or turbines are used steam can be recovered in the water heater or condensers. Mechanical draft fans are usually either disc or centrifugal type and because of the severe service to which they are subjected they must be of rugged construction, well balanced, must be able to operate continuously, withstand high stresses, maintain the proper pressure and horse power characteristics and show a good efficiency over a wide range of operative conditions. The amount of coal that can be burned per hour per square foot of grate surface is governed by the quality and type of coal as well as the amount of draft available. Mechanical draft fans should be of such capacity that they will be able to handle the quantity of gases produced. If it is assumed that 5 lb. of coal per boiler horse power at 24 lb. of flue gases per pound of coal there would be 120 lb. of flue gases per hour to handle. The volume of flue gases may be easily computed from the density of gases at the flue temperature and the size of fan to be provided may be obtained from the maker's table. No attempt should be made to put more air through existing boilers by speeding up the fans as the power consumption will be increased too rapidly. With the fuel bed at constant thickness doubling the weight of air requires about 3J4 times the draft pressure difference, and trebling the weight of air about 6 times. Under the first condition the fan would require times the weight of steam so it can be seen that the most efficient fan must be selected in order for boilers to be driven at a high rating. The effects of running a fan under conditions other than those for which it is designed are graphically shown by the performance curves of any given fan. -- Where the size of the openings and the length of the pipe remain constant the volume of air handled by any fan increases almost directly, 285