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486 CHAPTER 21 1950 Guide milinches per foot, or 600 milinches for 4 ft. If the first floor radiators are located 3 ft above the mains, second floor radiators 12 ft above the mains, third floor 21 ft, and fourth floor 30 ft, the heads are 450, 1800, 3150, and 4500 milinches, respectively. In forced circulation systems flow is produced mechanically by means of a pump driven by electricity, steam, or other source of energy. As Hot Water Heating Systems and Piping 487 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. Open gate valve__________ Open globe valve________ 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: 100. . ___ _ . 50. .. _ 25._ Iron Pipe. 2.0 3.0 3:0 1:8 4.0 1610 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 lehgth'of straight pipe of the same nominal size and 25 diam long. Hence, one elbow equivalent in feet of pipe 25 (in inches) divided by 12. j F R IC T IO N ---------- M IL IN C H E S PER FO O T OP P IP E Fig. 1. Heads Resulting prom Temperature Difference (Gravity Systems) forced circulation velocities are higher than those in gravity systems* and as the friction in a heating system varies almost as the square of the velocity, a given error in the calculation or assumption of the velocity is less im portant in a forced circulation system than in a gravity circulation system, 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 heat ing system must be known in order to design either gravity or forced circu lation systems. The friction loss of fittings is customarily expressed inequiv- Fig. 2. Friction in Black Iron Pipes Based on Flow in Pounds per Hour alent number of elbows of the same pipe size which would have the same 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.