Handbook for heating and ventilating engineers . velocities, it may be said that on accountof the low rate of transmission of heat to or from thegases, long flue passages are necessary, so that gases mov-ing at a normal rate will have time to give off or to takeup a maximum amount of heat before leaving the furnace. Air is heated chiefly by actual contact with heated sur-faces and not much by radiation. Consequently, the ef-ficiency of a furnace is increased when it is designed sothat the gases and air in their movement impinge perpen- 68 HEATING AND VENTILATION dlcularly upon the heated surfa
Handbook for heating and ventilating engineers . velocities, it may be said that on accountof the low rate of transmission of heat to or from thegases, long flue passages are necessary, so that gases mov-ing at a normal rate will have time to give off or to takeup a maximum amount of heat before leaving the furnace. Air is heated chiefly by actual contact with heated sur-faces and not much by radiation. Consequently, the ef-ficiency of a furnace is increased when it is designed sothat the gases and air in their movement impinge perpen- 68 HEATING AND VENTILATION dlcularly upon the heated surfaces at certain places. Thispoint sliould not be so exaggerated that there would beserious interference with the draft. The efldciency is alsoincreased if the general movement of the two currents bein opposite directions. Furnaces for residences are usually of the portable 17, the same being enclosed in an outer shell composedof two metal casings having a dead air space or an asbes-tos Insulation between them. Some of the larg^er sized. Fig. 17. plants, however, have the furnace enclosed in a permanentcasement of brick work, as in Fig. 18. Each of the twotypes of furnaces give good results. The points usuallygoverning the selection between portable and permanentsettings are price and available floor space. The cylindrical fire-pot is probably better than a con-ical or spherical one, there being less danger of the fireclogging and becoming dirty. A lined fire-pot i-s betterthan an unlined one, because a hotter fire can be maintainedin it with less detriment to the furnace. There is of coursea loss of heating surface in the lined pot, and in some forms FURNACE HEATING 69 of furnaces the fire-pot is unlined to obtain this increasedheating surface. It seems reasonable to assume, however,that the lined pot is longer lived and contaminates the airsupply less.
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectventila, bookyear1913
