Document MMmMX07Gvjnk0NdQNr1vmJrjy

400 CHAPTER 28 1959 Guide 210 ft. The preliminary design friction loss is then found in Table 1, as in Example 1, to be 294 It per 100 ft or 270 mi per ft. At thin average design friction loss (see Table 1) a 1-in. main will be used for the required flow of 5 gpm. From manufacturers' data at 5 gpm flow, the pressure drop from a 1-in. fitting is established at 3000 mi or 025 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 not exceed 3000 mi or 025 ft. The friction loss through the 14-in. risers to the 9 MBh convector was calculated to be 028 ft or 3450 mi in Example 1. This loss plus the 0.1-ft (1200-mi) loss in the convector, equals 0.38 ft (4650 mi) which exceeds the capacity of the fitting. If 94-in. risers are used, however, the riser pressure drop is con siderably reduced. For 92 MBh flow the pipe friction drop is 024 ft for 100 ft (40 mi per ft). The equivalent length changes from 22 to 26 ft. The riser pressure drop is then 26 X 024 or 0.09 ft (26 X 40 or 1040 mi). The head loss through the convector circuit is then 0j09 + 0.1 or 0J9 ft (2240 mi). Since this is be low the available fitting pressure drop, the riser sue is satis factory. Other upfeed risers are sued in a similar manner. ' The piping to the 62 MBh convector below the main is sized as follows: The water in this circuit will cool to room temperature during off periods. The center line of the convector is 594 ft below the mam. 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 465 mi per ft or 465 X 52 = 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. Since 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 thin case, the riser is sized on the basis of an added fitting. In accordance with the sizing procedure followed in this example, a 94-in. riser is adequate. The pipe sizes selected should be recalculated at the design flow rates either in milincbes 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. Section Pipe Site la. Main. .. 1 Load MBh Lots per feel of Pipe Mi length -ft Elbow Total Equiv. Total Friction less Mi* SO 240 140 $7b 227 54,500 24,000 78,500 6.55ft Section Pipe Size In. load MBh 100 Ftof Pipe Ft Langfh--Ft Linear Elbow Total Pipe Equiv. Total Lon Ft Main. .. 50 2.0 140 87b 227 4.54 8 one-pi) >e fitting! at 0.2.5 ft head 2.0 6.54 * JO v tHtliimhi* b The elbow equivalent length is based on: S elbows, 1 tees, J gate valves, 1 Baw check valve end 1 boiler. ` * The actus! value (or reeistaaee ot the one-pipe fittings should be obtained (rum the fitting manufacturers. They are frequently lower than tltt value aeturned in nnmplfi The* pump capacity required is 5.0 gpm at 624 ft head. The 194 in. pump.as shown in Fig. 9 would meet the re 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 size 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 mrm.ll design temperature drop, such as 20 deg or less is used, it may not be necessary to increase the hmting units in size toward the end of the circuit. Various considerations affecting the design of a series-loop system are given in the following paragraphs: 1. It may be poanble 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 quantity of water to be circulated through the heating units and the frictioQ 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 excess of 20 deg, the size 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 across 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. Valves or adjusting fittings, if used in the water circuit, would affect the performance of toe entire loop and would not -be a satisfactory means of controlling the heat output of the units. fHof Wafer Heating Systems 401 Certain of the series-loop system are: 1. The difficulty of making suitable provision for adjustment utnrf balancing of the system. 2. The total circulating head required may be excessive. 3. The design of the system may be more difficult than for one-pipe ot two-pipe main systems. Example S: A series-loop system is to be designed for the .residence used in Examples 1 and t for two-pipe and one-pipe design. No heat will be supplied to the basement. The total heat loss equals 45,500 Btu. Baseboard-type radiation having a rating of 02 MBh per tinner foot at an average temperature of 190 F, is selected as the radiation used. The heating, units are of baseboard type having a rated output of 02 MBh per linear foot at an average temperature of 190 F. They are connected by 94-in. pipe. A 20-deg design' temperature drop will be used. Solution: For a 452 MBh load 425 gpm must be circulated for a 20-deg temperature drop. For a single looped main the measured length equals 140 ft. The approximate equivalent length equals' 140 X 12 or 210 ft. If the 194-in. pump, Fig. 9, were initially selected, it would develop a 6-ft head at 425 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 pump may be tried. The 3-in. pump. Fig. 9, has 12.7 ft available head at 425 gpm delivery. Ii 12-ft available head and 250-ft equivalent length are used as the. nearest available values. Table 1 indicates that a flow of 42 gpm at a velocity of 22 fps will be established through the 94-in. main. However, because of cost, 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. The 194-in. pump at 6-ft head for 425 gpm is again selected for preliminary sizing of piping. The measured supply main length plus the length of the long est loop circuit is 106 ft. The equivalent length equals 106 X 12 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 toe supply main would be of 1-in. size for a flow of 7 gpm (which is more than the re- ?uired 42) and the 94-in. looped main would deliver 3.7 gpm more than toe required 225 gpm). Because the flows are_rnore than required, the system temperature drop would be les than 20 deg. A smaller pump could therefore be used if desired. The pipe sizes should be recalculated at the design flow rates as follows: Meet load MBh Design Haw Ft/100 Ft Pipe Size In. fqu Total Ft Press Drop Ft AB 45.5 4.55 m 1 36.0* 2 elbows 2 gate valves 1 flow valve 1 boiler 5.0 2.5 50.0 7.5 . 101' 1.77 BCA .22.5 | 2.25 | 1M | X . 70* 8 elbows 1 cock 2 tees @ 50% 13^6 1.7 13.6 99 1.5 Total........................ ...................................................... 3.27 * Actual pipe fe&sth. Since the total system head loss equals 327 ft at a flow- of 425 gpm and the 194-in. pump is a satisfactory Selection. If desired, toe actual design temperature drop can be deter mined. Because the pump head at 425 gpm is greater than 327 ft, the flow in the system will increase until pump head and flow equal system pressure drop and flow. Construction of a system curve, as described under centrifugal pumps, shows that for the system with the pipe size selected and with toe lV4-in. pump, equilibrium will be reached at 6-ft head and 62 gpm flow. The actual temperature drop for design conditions would then be (42/62) X 20 or 142 deg, approximately. Combination Systems Three types of systems have been illustrated, namely: one-pipe, two-pipe reversed-return, and series-loop. Actually the design for a particular building may be a composite of two or even all three of these types of systems. For example, a two-pipe reversed-return system of distribution may be used with a number of one-pipe loops in a single building such as an apartment house, or a two-pipe reversed-return system may be used with a main riser at each end of a build ing with a series-loop system used to serve the space on 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 used. -Adjustment of Row and Capacity Where the heating elements and piping are properly 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 water is within 5 to 15 percent of the correct value for a given heating unit, the difference in mean temperature will 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. 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 lically unbalanced, because the water for'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 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 sued so that the pressure drop through each piping circuit is the same at the design flow rate. These troubles are minimized by the reversed-return sys tem, Fig. 8. In this system the water for the first heating load taken off the roain is the last to be returned, and all circuits ' 'are of approximately equal length. Proper hy draulic balance is therefore easily obtained and the system is inherently self-adjusting. While the reversed-return piping circuit is much preferred, in some large installations a saving in pipe can be effected by rise of a direct-return system. When selecting the type of piping arrangement-such as one-pipe, two-pipe, series-loop, upfeed, and downfeed, the