Document Rj0pdaOyJNONQbGbRb0VnO9Ez

American Society of Heating and Ventilating Engineers Guide, 1937~~~ the air stream must change direction or shape, streamline fittings should be employed^ Horizontal ducts should pitch at least V> in. ner f,w upward from the furnace. 1 The recirculating grilles (or registers) should have a free area at least equal to the ducts to which they connect,, and their free area should never be less than 50 per cent of their gross area. The location and number of return grilles will depend on the size, details and exposure of the house. Small compactly built houses may frequently be adequately served by a single return effectively placed in a central hall More often it is desirable to have two or more returns, provided, however that in two-story residences one return is placed to effectively receive the cold air returning by way of the stairs. S[ %j n I^ ?^ 1I BI m II Fig. 4. Heating Effect at Registers for Various Stacks with 10-in. Leader Where a divided system of two or more returns is used, the grilles must be placed to serve the maximum area of cold wall or windows. Thus in rooms having only small windows the grille should be brought as close to the furnace as possible, but if the room has a bay window, French doors, or other large sources of cooling or leakage of cold air, the grille should be placed close by, so as to collect the cool air and prevent drafts. When long ducts of this type are employed they must be made oversize. This precaution is particularly important when long ducts and short ducts are used in the same system. The long ducts must be over size, if they are to operate satisfactorily in parallel with short ducts. Return ducts from upstairs rooms may be necessary in apartments or other spaces which are closed off or badly exposed. Metal linings are 416 Chapter 23--Gravity Warm Air Furnace Systems Hvisable in such ducts. It is important that these ducts be free from nnecessary friction and turbulence, and that they be located to prevent preheating of the air before it reaches the furnace. furnace Return Connection Circulation of the air is accelerated if the return connection to the furnace is through a round inclined pipe connected to two 45 deg elbows rather than through a vertical pipe connected to two 90 deg elbows. The top of the return shoe should enter the casing below the level of the ate in the furnace. In order to accomplish this the shoe must be wide as is indicated in Fig. 6, No. 1 arrangement. Tests of six different systems of cold air returns, Fig. 6, made at the University of Illinois2, resulted in the following conclusions: to of stacks sqm tDlaederarea 2 0 2r*!ir 23J5 0.473 3 S r*l2" 36J) 4 o r*io- 3oo 0.717 0598 *4 ity <#l ... I' 4 14 y y *S sj. 6 s ruir 390 7 o r*13' 39X1 AH stacks 10 6'from boot* to register ( Leader slope. 1 in. per R Wet diameter. 6 in. 8 m. feeder 0.777 0.777 v* ^ | Uj J 9.00 r- ,,n / y . s y / y* / * s' ,1 1M ' y / <y 11 iK 720 --130 y* ^* 140 150 160 170 ISO 190 200 210 220 EQUIVALENT REGISTER UR TEMPERATURE<T(lG-TLCT+65 F) 0G F 230 240 Fig. 5. Heating Effect at Registers for Various Stacks with 8-in. Leader 1. In general, somewhat better room temperature conditions may be obtained by returning the air from positions near the cold walls. 2. Friction and turbulence in elaborate return duct systems retard the flow of air, and may seriously reduce, furnace efficiency, and lessen the advantages of such a design. 3. The cross-sectional duct area is not the only measure of effectiveness. Friction and turbulence may operate to make the air flow out of all proportion to the various duct areas. FURNACE CAPACITY The size of furnace should, of course, be such as will provide the necessary air heating capacity, usually expressed in square inches of leader pipe area, and at the same time provide a grate of the proper area- to burn the necessary fuel at a reasonable chimney draft. The total leader pipe area required is obtained by finding the sum of the leader pipe areas as already designated. ~ ^Investigation of Warm-Air Furnaces and Heating Systems. Part IV, by A. C. Willard, A. P. Kratz, and V, S. Day (University of Illinois Engineering Experiment Station Bulletin No. 189). 417