Document 1QnMLngBv68NdQ3wOO1Jem5ra

396 CHAPTER 28 1959 Guide Lowar scale of chart it on 20-dog tomporator* difference between flow and return risers. To find friction when temperature . drop is other than 20 dog, auMpiy the adtiat heat conmyod by 120-roctoaf temp. drop) end rood fi>* corresponding friction. Ft per 100 It MiKnches per ft Conversion Ff/(I00 ft) to Alifioches/ft 0.5 1 2 3 ' 4 40 120 240 360 480 5 600 Fig. 6.... Friction Loss doe to Flow of Water in Type L Copper Tube Table 2 .... Iron and Copper Elbow Equivalents*........ Fitting Iroa Pip* Coppor Tubing 1.0 0.7' 0.5 0.5 0.4 1.0 2.0 3.0 1.0 0.7 0.5 0.4 1.0 3.0 4.0 3.0 0.5 12.0 * See-Teble S lor equivalent length of one elbow. 4.0 0.7 17.0 should be calculated. The friction loss at design flow for, all individual sections of pipe and for all fittings in the longest piping circuit should then be summarized. Table 2 shows the number of elbow equivalents for various fittings. These equivalents can be converted to equivalent feet of pipe by use of Table 3. Fig. 7 shows the elbow equiva lents for determining friction loss in tees. If the pipe size calculation indicates a required pump head different from that of a standard pump, pipe sizes in the system may be changed to establish a closer relationship between the two. The relationship between systems and pump heads is dis cussed in the section Circulating Pumps. If there are more circuits than one in a system, the fric tion loss for the longest circuit should be used to determine Table 3.... Equivalent Length of Pipe for 90-Deg Elbows . VW. fpt 5 6 8 10 Pip* Size XH 1 ix IK 2 2M 3 1.2 1.7 2.2 3.0 3.5 4.5 5.4 6.7 1-4 1.9 2.5 3.3 3.9 5.1 6.0 7.5 1.5 2.0 2.7 3.6 4.2 5-4 6.4 8.0 1.5 2.1 2.8 3.7 4.4 5.6 6.7 8.3 1.6 2.2 2.9 3.9 4.5 5.9 7.0 8.7 1.7 2.3 3.0 4.0 4.7 6.0 7.2 8.9 1.7 2.3 3.0 4.1 4.8 6.2 7.4 9.1 1.7 2.4 3.1 4.2 4.9 6.3 7.5 9.3 1.8 3.2 4.3 5.0 6.4 7.7 9.5 1-8 2.5 3.2 ' 4.3 - 5.1 6.5 7.8 9.7 3K 7.7 8.6 9.2 9.6 10.0 10.3 10.5 10.8 11.0 11.2 4 8.6 9.5 10.2 10.6 11.1 11.4 11.7 11.9 12.2 12.4 5 10.5 11.7 12.5 13.1 13.6 14.0 14.3 14.6 14.9 15.2 6 12.2 13.7 14.6 15.2 15.8 16.3 16.7 17.1 17.4 17.7 8 15.4 17.3 18.4 192 19.8 20.5 21.0 21.5 21.9 22.2 10 18.7 20.8 22.3 23.2 24.2 24.9 25.5 26.1 26.6 27.0 12 22.2 24.8 26.5 27.6 28.8 29.6 30.3 31.0 31.6 32.0 Hot-Water Heating Systems the pump head requirement.. Pining in the other circuits should be sized to obtain the same total friction loss. Effect* of Antifreeze Fluid*. Antifreeze solutions are some times used in heating systems when the danger of freezing exists. They should not be used in direct-fired boiler applica tion. Corrections must be made in flow rates, pipe size, and pump capacity relative to water when antifreeze solutions are used. See Chapter 49, Snow Melting. SaECTlON OF PIPING ARRANGEMENT .The type of distribution system selected for any particular building will depend primarily on the structural and archi tectural characteristics of the building, the need for separate control of different zones, the pressure head due to the height of the building, the space available for piping, and the rela tive cost of different arrangements of piping. Consequently, mains may be run in basement, separate floors, or in space above highest heating units. There are four distinct arrangements of main piping; (l) the two-pipe reversed-retum system, (2)'the one-pipe sys tem, (3) the series-loop system, and (4) the two-pipe direct return system. The last is not recommended unless elaborate pipe sizing technique and provision for adjustment are em ployed. Two-Pipe Reversed-Retum System--Description and Design The two-pipe reversed-retum main system. Fig. 8, has two mains, one supplying water to the heating units or to risers 397 to heating units, and one collecting water returned from these units. The piping is arranged in such a manner that the sum of the equivalent length of supply and return piping to any unit is approximately equal to that of any other unit. Compared with a one-pipe system, the two-pipe system may: 1. Have a smaller average pipe size. While two mains are re quired, the sizes of the mains vary as water passes from one main through the heating unit to the other. The maximum size of either main is no larger than the size of a one-pipe main. 2. Be more flexible in its application because-larger friction loss may be used in the heat-transfer unit circuits. 3. Require a minimum pumping head because the heating unit resistances are in paralleL 4. Permit adjustment of flow through individual units of ra diation over wider limits. Fig. 8.... A Two-Pipe Reversed-Retum System Notes: I. The chart it based oo straight tees, that it, branches A, 6, and C are (he same rise. 2. Head loss to desreddranl is obtained by wtecfmg proper curve accord<*>8 to Stesfroffoos, determining the flow at the cirded branch, and aattjptying Ibe head loss far (he same size elbow at the flow rate m (he circled branch by file equivalent elbows indicated. 3. When the sice of an oattet it reduced the equivalent betbows shown in (he chert do not apply. The meximom loss for any drcwl for any flow wifi not exceed 2 elbow equivalents at file maximum flow (gpm) occurring fat any branch of (he tee. 4. The (op curve of (he diarf is the overage of 4 curves, aae for ecdl of he tee dreuif* {Oustrated. Fig. 7.... Elbow Equivalents of Tees at Various Bow Conditions*- " Continuing the comparison, some disadvantages are: . 1. Two mains may require more pipe fitting labor tfran a single main. 2. Some additional pipe may be required to achieve a re versed return. 3. A two-pipe system will be slightly more expensive for mall systems. 4. The piping for two mains may be unsightly in occupied spaces. Important considerations in the design of a two-pipe reversed-retum distribution system are: 1. The head which the pump must generate is equivalent to the friction drop through the circuit having the highest resist ance. At design flow, the resistance through the distribution r' im to any one heating unit should be the same as that ugh any other unit. Where the length of pipe required to obtain distribution to a particular section must be shorter than to other sections, it is desirable to design that section with a higher friction tos. 2. A reversed return, as indicated in the section Adjustment of Flow and Capacity, will reduce the need for adjustments neces sary to obtain equal total head through each heating unit cir cuit. 3. Fan-type air-heating units with extended-surface coils can be used with a higher water temperature drop than that recom mended for direct radiation. It is necessary to avoid a difference . in air temperature across the face of the coil in.such units. One method of minimizing this difference is to install a minimum of two serpentine coils in each unit, one in front of the other. Coil