Document 5b3kM8GaD5qD5QRnnJNbp73LN

128 CHAPTER 8 1962 Guide And Data Book 210 ft. Hie preliminary design friction less is,then found in Table 1, as in Example /, to be 294 ft per 100 ft or 270 mi per ft. At this average design friction loss (see Table l)a 1-in. main will be used for the required flow of & gpm. Prom manufacturers' data at 5 gpm flow, the pressure drop from a 1-in. fitting is.established at 3000 mi or 0.25 ft. If any heating units are located below the main the gravity head due to temperature difference should be subtracted from available fitting pressure drop. In this example, the riser and convector friction pressure drop should act exceed 3000 mi or 0.25 ft. The friction loss through the 94-in. risers .to the 98 MBh convector was calculated to be 028 ft or 3450 mi in Example 1. This loss plus the 0.1-ft (1200-mi) loo in the convector, equals 028 ft (4650 mi) which exceeds the capacity of the fitting. If 94-tn. risers are used,, however, the riser pressure drop is con siderably reduced. For 98 MBh flow the pipe friction drop is 034 ft for 100 ft (40 mi per ft). The equivalent length changes from 22 to 26 ft. The riser pressure drop is then 28 X 034 or 0.09 ft (28 X 40 or 1040 mi).The head loss through the convector circuit is then 009 + 0.1 or 0.19 ft (2240 mi). Since this is be low the available fitting pressure drop, the riser sise is satis factory. Other upfeed risers are sued in a similar manner. The piping to the 63 MBh convector below the main is sited as follows: The water in this circuit will cool to room temperature during off periods. The center line of the convector is 514 ft below the rpm The thermal head opposing circulation in the convector circuit when one riser is filled with 210 F water and the other has 70 F water is found to be 405 mi per ft or 465 X 53 -- 2560 mi total. Subtracting 2560 from the 3000 mi pressure drop in the . fittings leaves 440 mi available to start circulation. The com bined convector and riser pressure drop equals 1380 mi even when a 1-in. riser is used. 8iaee the convector and riser pressure drops are greater than the available fitting pressure loss,- a change must be made. The total fitting pressure drop could be increased by adding a supply one-pipe fitting, or by increasing fitting pressure drop through use of a larger pump. In this case, the riser is aired on the basis of an added fitting. In accordance with the rising procedure followed in this example, a 94-in. riser is adequate. The pipe sixes selected should be recalculated at the design flow rates either in milinches as shown in the upper part of the following table or in feet per hundred feet as shown in the Lower part of the table. Secfloo Pipe Size fa. Load MBh 1m per Foot of Pipe Mi tengfa--ft Linear Elbow Tote! Pipe Eqelv. Equlv. Total Friction loo Mi* quirements of the system based on the assumptions stated for the problem. Series-Loop System--Description and Design This system, illustrated in Fig. 11, consists of one or more loops or circuits. In each circuit, as the same water is circulated through the heating units in succession, the rise of pipe should not vary materially in any portion of a circuit. The system circuit length thus becomes very important be cause it influences directly the water flow rate, pressure drop, and temperature drop. Since the water temperature in each successive heating unit decreases progressively, some de signers consider it necessary to increase progressively the size of heating units sufficiently to compensate for the drop in temperature. If a small design temperature drop, such as 20 deg or less is used, it may not be necessary to increase the heating units in size toward the end of the circuit. Various considerations affecting the design of a series-loop system are given tn the following paragraphs: Main... 1 50 8 ose-pir>e fitting: at 30C Total................... 240 140 87h 227 54,500 24,000 78,500 - 6.55 ft SeeHoe Pip* Size fa. leflgfa--ft tood MBh 100 Ft of Pipe ft linear Elbow Tola} Pipe Equiy. Eqwlv. Main... 1 30 2.0 8 one-pipe fittings at 03. ft bead* 140 87* 227 Total....................... Total Friction Lot* ft 4.54 2.0 6.54 Tte acted value tor ranataaee ei the ote-pipe fitting abootd be obtained bam the fitting manufartarete. They az frequently lower then the mine aeeuiMd h uuBpie The pump capacity required is 5.0 gpm at 634 ft bead. The in. pump as shown in Fig. 9 would meet the re 1. It may be possible to eliminate all or large sections of the distribution main and thereby reduce cost. In large buildings it may be necessary to use a partial two-pipe main in-combina tion with the series-loop system for certain sections or divisions of the building. 2. The practicability of the system will depend upon the SuanCity of water to be circulated through the heating units and ae friction loss. Baseboard or finned-type pipe elements for series-loop systems are usually manufactured from standard steel pipe or copper tubing. The friction loss can be determined from Figs. 3 and 4. 3. If a series-loop system serves more than a single space, and if the design temperature drop for the system is in exces of 20 deg, the sue of the heating unit should be determined on the basis of the temperature of the water entering.the particular heating unit and the temperature drop acres the unit. The tem perature drop through a particular heating unit is equal to the capacity of that unit divided by the capacity of the total radia tion circuit multiplied by the total temperature drop of the circuit. 4. Since the temperature of the water supplied to heating units in the series-loop system cannot be regulated, it is ad visable to provide dampers in the cabinets containing finned pipe or baseboard radiation to permit adjustment of the heat delivery by each unit. Valvea.or adjusting fittings, if used in the water circuit, would affect the performance of the entire loop and would not be a satisfactory means of controlling the heat output of the units- - Hot Water Heating Systems 129 Certain disadvantages of the series-loop system are: ft, the flow in the system will increase until pump bead and flow equal system pressure drop and flow. Construction of a system 1 The difficulty of.making suitable provision for adjustment and of the system. 2 The total circulating head required may be excessive. x The of the system may be more difficult- than for curve, as described under centrifugal pumps, shows that for the system with the pipe sise selected and with the 194-in. pump, equilibrium will be reached at 6-fl bead and 63 gpm flow. The actual temperature drop for design conditions would then be (43/63) X 20 or 143 oeg, approximately. one-piP or two-pipe main systems. Example 3: A series-loop system is to be designed for the used in Examples 1 and B for two-pipe and one-pipe Combination Systems No heat will be supplied to the basement. The total Three types of systems have been illustrated, namely: heat loss equals 45,500 Btu. Baseboard-type radiation having a rating of 03 MBh per linear foot at an average temperature of 190 F, is selected as the radiation used. The beating units are one-pipe, two-pipe reversed-retum, and series-loop. Actually the design for a particular building may be a composite of of baseboard type having a rated output of 03 MBh per linear two or. even all three of these types of systems. For example, - foot at an average temperature of 190 F. They are connected by 94-in. pipe. A 20-deg design temperature drop will be used. a two-pipe reversed-retum system of distribution may be used with a number of one-pipe loops in a single building Solution: For a 453 MBh load 435 gpm must be circulated for a 20-deg temperature drop. For a single looped main the measured length equals 140 ft. The approximate equivalent such as an apartment house, or a two-pipe reversed-retum system may be used with a mam riser at each end of a build length equals 140 X 13 or 210 ft. If the 114-in. pump. Fig. 9, ing with a series-loop system used to serve the space on were initially selected, it would develop a 6-ft head at 435 gpm delivery. For an available head of 6 ft and an equivalent length of 218 ft (nearest to 210 ft) Table 1 shows that only 3 gpm will circulate through the 94-in. main. A larger, and more costly each floor of the building. In any case, it is well to bear in mind the principles of design involved in each particular type of system for that portion of the installation where it is np may be tried. The 3-in. pump, Fig. 9, has 12.7 ft available used. at 435 gpm delivery- If 12-ft available head and 250-ft equivalent length - are used as the nearest available values,'Adjustment of Row and Capacity Table 1 indicates that a flow of 43 gpm at a velocity of 23 fpa will be established through the 94-in. main. However, because of Where the heating elements and piping are properly appearance, and temperature drop considerations, this may not be desirable. If the system is divided into two loops, as shown in Fig. 11, a smaller pump may be used and the system temperature drop decreased. sized and arranged, a hot water system is, to a large ex tent, self-adjusting. This is so' because the mean tempera ture of the water flowing inside the heating unit is the most important factor in determining its output. If the flow of The 194-in. pump at 6-ft head for 435 gpm is again selected water is within 5 to 15 percent of the correct value for a for preliminary suing of piping. given heating unit, the difference in mean'temperature will' The measured supply main length plus the length of the long est loop circuit is 106 ft. The equivalent length equals 106 X 13 or 159 ft. For 6-ft available head and 160 ft equivalent length. Table 1 indicates that the friction loss would be 394 ft per 100 ft or 450 mi per ft. At this friction loss the supply main would be of 1-in. rite for a flow of 7 gpm (which is more than the re quired 43) and the 94-in. looped main would deliver 3.7 gpm (more than the required 235 gpm). Because the flows are more than required, the system temperature drop would be law than 20 deg. A smaller pump could therefore be used if desired. not exceed 1 or 2 deg and, consequently, the effect on heat output will be negligible. However, the limitations of com mercial, pipe sizes and pipe arrangement sometimes result in flow rates considerably different than those required to develop the proper output of the heating units. A means of adjusting the heat output of heating elements and branch circuits must be provided in the system to correct these effects. The pipe sites should be recalculated at the Herign flow rates as follows: ' Adjusting means may be necessary due to the type of sys tem, for example, the two-pipe system using direct-return mains. In this system the water flow circuits are hydrau Station Hoot Load MBh Oerign Flow Gpm Friction Lon Ft/100 Ft Pipe Size Ini ftftpar. Total Eqviv. Ft Pten Drop Ft lically unbalanced, because the water lor the first heating load taken off the main is the first to be returned. As a re sult, the first heating unit will receive a much greater flow than is required to develop its rated output. The flow through the last heating unit can be so low that practically no heat- AB 45.5 4.55 iH 2 elbows 2 gate valves 1 flow valve 1 boiler 1 36.0* 5.0 2.5 50.0 7.5 101 1.77 is delivered. The poor distribution of flow in the directreturn system can be corrected by the installation of orifices or of adjusting fittings or valves, by means of which the system is balanced after it is placed in operation. In some cases, proper flow distribution can also be established by careful pipe size selection. That is, the piping can be sized *BCA. 22.5 2.25 1H 70* so that the pressure drop through each piping circuit is the same at the design flow rate. 8 elbows 1 cock 2 tees @ 50% 13.6 1.7 13.6 99 1.5 These troubles are minimized by the reversed-retum sys tem, Fig. 8. In this system the water for the first heating load taken off the main is the last to be returned, and all' Total.. -- 'Actual pip, length. 3.27 circuits are of approximately equal length. Proper hy draulic balance is therefore easily obtained and the system is inherently self-adjusting. While the reversed-retum piping circuit is much preferred, in some large installations a Since the total system head toes equals 337 ft at a flow of 435 gpm and the 194-in. pump is a satisfactory selection. saving in pipe can be effected by use of a direct-return system. If desired, the actual design temperature drop can be deter mined. Because the pump head at 435 gpm is greater than 337 When selecting the type of piping arrangement such as one-pipe, two-pipe, series-loop, upfeed, and downfeed, the in i it A -a l! i| -