Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSEXPANSIONTANKMAKE-UPWATERPRESSUREREGULATINGVALVEPUMPSUPPLYHEAT/COOLSOURCEAIRSEPARATIONHEAT/COOLCOIL1HEAT/COOLCOIL2HEAT/COOLCOIL3RETURNM15053Fig. 51. Typical Water Distribution System.The expansion tank is charged with compressed air to placethe system under the minimum pressure required at the inlet tothe pump to prevent pump cavitation and the resultant impellererosion. The minimum inlet pressure required by a pump isreferred to as the net positive suction head (NPSH). Figure 54indicates the NPSH for a particular pump. The air volume inthe tank is sized, based upon the volume of water in the systemand the expected water temperature variations, to allow the waterto expand and contract as water temperatures vary throughoutthe year. The expansion tank static pressure does not effect theclosed system control valve differential close-off pressure, butmust be considered, in addition to the pump head, for valvebody and other piping system component pressure ratingselection.The air separator and expansion tank are omitted from theexamples in this section for simplicity.CONTROL REQUIREMENTS FOR WATERDISTRIBUTION SYSTEMSThe requirements of a properly applied distribution system are:1. Maintain controllable pressure drop across the controlvalves.2. Maintain required flow through the heating (or cooling)source.3. Maintain desired water temperature to the terminal units.4. Maintain minimum flow through the pump(s) that arerunning.5. Maintain NPSH.6. Stage the pumps of a multipump system to satisfyconditions 1 through 5.CENTRIFUGAL PUMPS USED IN HOT ANDCHILLED WATER SYSTEMSThe pump is a key component of a water distribution system.It is essential to understand pump characteristics in order tounderstand and design distribution systems and to understandpumping system control solutions. Centrifugal pumps arecommonly used to distribute hot and chilled water throughcommercial buildings. Many varieties of centrifugal pumps areavailable, as shown in Table 2. Figure 52 shows a typical basemountedpump.ENGINEERING MANUAL OF AUTOMATIC CONTROL313