Document NEq4e41j1KEx95D635pLZoEQy

456 CHAPTER 24 ~_________________________________ 1948 Guide En forced circulation systems flow is produced mechanically by, means of a pump driven by electricity, steam, or other source of energy. As forced circulation velocities are higher than those in gravity systems, and-as the friction in a heating system varies almost as the square of die velocity,; a given error in the calculation or. assumption of die velocity is less important in a forced circulation system than in a gravity circu- . Hot Water Heating Systems and Piping 457 Table 1. Iron and Copper Elbow Equivalents Fitting Elbow, 90-deg ........ . ... Elbow, 45-deg____________ Elbow, 90-deg long turn__ Elbow, welded, 90-deg___ Reduced coupling________ Open return band. _____ Iron Pipe 1.0 0.7 0.5 0.5 0.4 1.0 0.5 12.0 Copper Tubing Fitting 1.0 0.7 0.5 0.5 0.4 1.0 0.7 17.0 Angle radiator valve.____ Radiator or convector...... Boiler or heater__________ Tee, per cent flowing through branch: t no SO 25. .._ ______ . Iron Pipe 2.0 3.0 3.0 1.8 4.0 16.0 Copper Tubing 3.0 4.0 4.0 1.2 4.0 20.0 The friction in one 90 deg standard elbow is approximately equal to the friction of a length of straight pipe of the same nominal size and 25 diam long. Hence one elbow equivalent in feet of pipe equals 25 diam (in inches) divided by 12. F R IC T IO N --------- M IL IN C H E S PER FO O T OF P IP E 130 140 150 160_ 170 180 190- 200 gin TEMPERATURE OF WATER IN aOW RISER, DEG FAHR ; - ., Fig. 1. Heads Resulting from Temperature Difference (Gravity Systems) lation systeru, and, consequently, it is easier to design a satisfactory forced circulation system than a satisfactory gravity circulation system. FRICTION LOSS Values of friction loss due to flow of water in the various parts of a' heating system must be known in order to design either'gravity or forced circulation systems. The friction loss of fittings is customarily expressed in equivalent' number of elbows of the same pipe size which would have the same Fig. 2. Friction in Black Iron Pipes Based on Flow in Pounds per Hour friction loss. An elbow is assumed to have the same friction loss as a straight pipe having a length equal to 25 diameters (nominal) of the elbow. The resistance of various types of fittings expressed in equivalent elbow resistance is shown in Table 1. The friction loss in black iron pipes for a determined weight of water may be obtained from Fig. 2. For the frequently used temperature drop of 20 deg the friction loss may be determined directly from the heat