Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSThe Figure 70 supply piping runs out to the coils decreasingin size between AHU 1, 2, 3, 4, 5, and 6. The return linesbetween each AHU are typically sized the same as the respectivesupply lines. The drop across AHU 6 must be 16 ft in order toget the 200 gpm through the valve (8 ft) and the coil (8 ft). Toget the 200 gpm from AHU 5 to AHU 6, the drop across thepiping at AHU 5 is 18 ft (16 ft required to get the flow throughAHU 6 plus 2 ft to overcome the supply and return piping dropsbetween AHU 5 and AHU 6). The AHU 5 balancing valve isset to prevent the 18 ft drop across AHU 5 from forcing morethan 200 gpm to pass through the AHU 5 coil and control valve.The balancing valve B5 then is set to take a 2 ft drop at 200gpm. Similarly, set a 4 ft, 8 ft, 10 ft, and 12 ft drop respectivelyacross balancing valves B4, B3, B2, and B1.If this is a variable flow loop with a variable speed pump andthe pump is controlled to produce 16 ft across AHU 6, thecontrol issue here is: When AHUs 2, 3, 4, 5, and 6 are off (noflow beyond AHU 1), then, the drop across AHU 1 is only 16ft, 12 ft of which the AHU 1 balancing valve needs for designflow. Proper solutions are presented later in this section.Supply piping is the same for a reverse return system (Fig.71) as for the direct return system (Fig. 70). The return flow isreversed such that the return piping increases in size betweenAHUs 1, 2, 3, 4, 5, and 6. A full size return line is run back tothe source room from AHU 6. In this example, the pump is thesame as the direct return, since the return line from AHU 6 alsotakes a 6 ft drop. If the AHUs and source were positioned in acircular or hex pattern such that Coil 6 is closer to the pump,the run from AHU 6 back to the source would be shorter, andthe reverse return piping head would be less than for the directreturn, and the piping cost would be similar. In reverse returnsystems, balancing is usually only a trimming exercise.COUPLED VS DECOUPLED PIPING SYSTEMSPiping systems requiring constant flow through primaryequipment (chillers, boilers) and variable flow to AHUs may becoupled or decoupled. See CHILLER SYSTEM CONTROL andBOILER SYSTEM CONTROL for examples. Primary flowcontrol for coupled systems and secondary flow control fordecoupled systems are discussed later.SUPPLY4 FT1 FT1 FT2 FT1 FT1 FTDROPDROPDROPDROPDROPDROPHEATEXCHANGER12 FTDROP8 FTDROPHEAT/COOLCOIL1HEAT/COOLCOIL2HEAT/COOLCOIL3200 GPMPERAHU COILHEAT/COOLCOIL4HEAT/COOLCOIL5HEAT/COOLCOIL6DESIGN DROPS,ALL COILS &VALVESPUMP8 FTDROPV1V2V3V4V5V61200GPM48 FT HEAD0 FTDROPB1B2B3B4B5B61 FT1 FT2 FT1 FT1 FTDROPDROPDROPDROPDROP4 FT6 FTRETURNDROPDROPM15059Fig. 71 Reverse Return Piping SystemENGINEERING MANUAL OF AUTOMATIC CONTROL324