Book - School of Science and Technology

652 Automatic controls and building management systemscosts. With a variable flow system, provision must be incorporated to ensure that the flowrate through the chiller evaporator can never fall below the manufacturer's recommendedminimum and thus incur a risk of freezing.Chiller controlSingle chillers will normally be supplied by the manufacturer with integral safety andcapacity controls. Multiple chillers may be arranged in either series or parallel withrespect to the chilled water circuit, see Figure 22.33. The points to be aware of whenselecting the more appropriate arrangement are that:. Since the frequency of starting must be limited, the minimum output (turn down) ofone machine must be less than the lowest operating load of the system, or inertia vesselsincorporated into the chilled water system to reduce the frequency of starting.. In a parallel arrangement, it must be established that the sequence control will not leadto sub-cooling and that the water flow rates through each evaporator are regulated tothe correct quantity. Difficulties with part load operation of multiple chiller cannormally be best overcome by the use of the chillers manufacturers own multiple chillercontrol system or `chiller manager'. With modern digital control of chillers, this willFan(s)Single or multiple dry coolersor cooling towersconnected in parallelTower bypasscontrol valveCondenserwater pumpChiller 1 Chiller 2Condensers connectedin parallelEvaporators connectedin parallelAlternative arrangementwith evaporatorsin seriesFigure 22.33 Typical condenser water circuit