Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSMULTIPLE BOILER SYSTEMSGENERALBasic boiler connections for a three-zone hot water systemare shown in Figure 47. In this system, two boilers are connectedin parallel. Hot water from the top of the boilers moves to theair separator which removes any entrapped air from the water.The expansion tank connected to the separator maintainspressure on the system. The tank is about half full of waterunder normal operating conditions. Air pressure in the tankkeeps the system pressurized and allows the water to expandand contract as system water temperature varies. Water fromthe boiler moves through the separator to the three zone pumps,each of which is controlled by its own zone thermostat. In somesystems, each zone may have a central pump and a valve. Returnwater from the zones returns to the boiler in the return line.There several variations are possible with this type system butthe process is the same. There is no minimum boiler water flowlimit in this example.The Dual Boiler Plant Control example following is a dualboiler plant with high-fire, low-fire controlled boilers, 145Fminimum entering water temperature required prior to highfire,water flow must be maintained when the boiler is enabled,and a secondary hot water reset schedule of 110F water at 55FOA temperature and 180F water at 5F OA temperature. Theconcepts adapt well for single or multiple boiler systems.RELIEFVALVEPRESSUREGAUGEBOILERDRAINEXPANSIONTANKMAKE-UPWATERMANUALAIR VENTPRESSUREGAUGERELIEFVALVEBOILERDRAINRETURNLINEAIRSEPERATORDRAINZONEHWSZONEHWSZONEHWSZONEHWRZONEHWRZONEHWRFig. 47. Typical Piping for Multiple-Zone HeatingSystem.C2905NOTE: The primary/secondary decoupler is sized for the fullsecondary flow, and like the chiller plant decoupler,should be a minimum of 6 pipe diameters in length.Unlike the chiller decoupler, normal flow may be ineither direction.ENGINEERING MANUAL OF AUTOMATIC CONTROL309