Document 0EX08Q25EEX3LwM0dR9vwJ5x

American Society of Heating and Ventilating Engineers Guide, 1924-25 quantity of air. Table 78 contains a comparison of the two ducts, on a percentage basis, for three'temperatures. It is significant to note that the improved duct has a center-line length of 11 ft. as against 14 ft. for the rectanglar duct. On the other hand, the improved duct was handi capped by having a register grille, whereas the rectangular duct had none. This has been found to be a considerable handicap, amounting to 4 per cent of the furnace capacity at moderate register temperatures. The failure of the rectangular duct to handle the same quantity of air as the round duct may be ascribed to sharp right angle turns, greater length, and greater frictional surface for the same cross-sectional areas. TABLE 78. COMPARISON OF FURNACE CAPACITIES FOR TWO TYPES OF RECIRCULATING DUCT Air Temperature Average at Registers 130 (Low) 160 (Moderate) 190 (High) Capacity B.t.u. per Hour* Rectangular Duct 62,500 97,500 136,000 Capacity B.t.u. per Hour* Round Duct 71,000 116,000 167,000 Per Cent Increase Round Duct 13.6 19.0 22.4 These values were selected from the curves of Fig. 46. Capacity means B.t.u. (heat units) per hour added to the air as it passed through the furnace and measured at the furnace bonnet, just as the air enters the leaders. SIGNIFICANCE OF THE AIR TEMPERATURE AT THE REGISTER ON THE RATING AND CAPACITY OF A WARM-AIR FURNACE3 One of the most important objects of the research work of the warm air furnace heating investigation, which is now in progress at the University of Illinois, has been the determination of the factors affecting the rating of a warm air furnace. Every manufacturer and installer is vitally concerned with the basis upon which furnace ratings are determined. Most manufacturers are agreed that the square inches of leader pipe area which a furnace can supply is a satisfactory basis for expressing the heating capacity of a furnace. Unfortunately, this is not the end of the story by any means, as such a basis of rating is still indefinite, unless the ait temperature at the registers is also stated. The real significance of this air temperature at the registers can be shown very easily by reference to any series of tests run at the University on piped furnaces. In fact, it is a very simple matter to show that a given furnace, connected to a given system of leaders, stacks, and registers as shown in Fig. 47 can be made to develop several different heating capacities when operating with the same number of square inches of leader pipe area. It is only necessary to increase the draft, thereby burning more coal per square foot, of grate, and sis a consequence secure a higher air tempera ture at the register face. That the heating capacity of the furnace will be increased by such a procedure is obvious to anyone, and it should be equally obvious that any attempt to express the rating of a furnace in square inches of leader pipe, area means nothing unless the register tem perature is stated at the same time. *From an article by A. C. Willard in the American Artisan and Hardware Record, December 30, 1922. 120 American Society of Heating and Ventilating Engineers Guide, 1924-25 Fig. 47. Elevation of Piped Furnace Testing Plant--Note Alternative and Better Arrangement of Recirculating Duct in Fig. 45 121