Variable Primary Flow System Control Strategies (Dedicated pumping type)

Primary Chilled Water Variable Flow Control Strategy (CHP)

  1. Chilled Water Supply Main Header Temperature Sensor (T1), Chilled Water Return Main Header Temperature Sensor (T2) and Flow Meter at Return Main Header (FM1)shall be used to calculate Instantaneous Building Thermal Load (IBTL = Actual Chilled Water Flow (FM1) x Constant xΔT) for Chilled Water System Efficiency calculations(*Note: ΔT = T2 – T1).
  1. Chilled Water Actual System Load Required (ALR) = FM1 x Constant x (T2 –Fixed Set-point) to calculate. This ALR calculation is used for Cut-In and/or Cut-Out of Chiller when Building Load increases or decreases to matched the actual building load demand. (*Note: Fixed Set-point shall base on design Supply Chilled Water Temperature of 6.7 degree Celsius.)

(SW1 Chilled Water System have total of 4 Chillers after full installation, thus set-points for Cut-In of chillers shall have 3 set-points for cut-in more chillers when building load increases, and 3 set-points for cut-out extra chillers when building load decreases. When operating schedule activated always one chiller system will be in operation at all-time till schedule “Off”. Increasing or decreasing of chiller systems shall be by the chiller cut-in and cut-out set-points.

When Chilled Water System “ON” with 1 Chiller and building load increases to 495RT and stays for more than 10 minutes confirmation delay time, cut-in 2nd chiller system.

When 2 Chillers are operating and building load increases to 990RT and stays for more than 10 minutes conformation delay time, cut-in 3rd chiller system.

When 3 Chillers are operating and building load increases to 1485RT and stays for more than 10 minutes conformation delay time, cut-in 4thchiller system.

When 4 Chillers are operating and building load decreases to 1100RT and stays for more than 10 minutes confirmation delay time, cut-out one chiller system to 3 Chillers.

When 3 Chillers are operating and building load decreases to 825RT and stays for 10 minutes confirmation delay time, cut-out one chiller system to 2 Chillers.

When 2 Chillers are operating and building load decreases to 440RT and stays for more than 10 minutes confirmation delay time, cut-out one chiller system to 1 chiller. The last chiller system in operation will operate till schedule “Off”.)

  1. ALR dependent on the air side terminals’ thermal load requirements.
  1. ΔP1 & 2 is dependence on the air side terminal’s chilled water flow intakes to handle the thermal load requirements.
  1. Chilled Water By-Pass Valvesat chillers Inlet pipes are crossover valves at normally closed positions (used for manual by-passing only).
  1. Two-Way Modulating Valve (M) is an automatic safety/pressure-relief by-pass valve controlled by ∆P22ndDP control set-point to maintain chillers’ min. flow/∆P.
  1. If cooling load decreases/increases at terminal’s ends, modulating valves close/open and flow regulation occurred; If chilled water headers ΔP increases/decreases above ∆P11stDP control set-point, VSD CHW pumps modulate flow to maintain ΔP set-point.
  1. Chilled Water Pump (CHP) min. flow is set to ensure min. chillers’ ΔP/flow across each chiller to eliminate nuisance tipping.(*Note: Alternatively for Item 7 to 9 you may want to use 2 DP Transducers to provide these functions.)
  1. If CHP is at min. it will turn constant (not regulating) speed, and if ΔP continues to increase, then Valve-M opens and modulate to relief and regulates according to ΔP changes when it hits the ∆P 2nd DP control set-point.
  1. If CHP at max design flow and ALR hits or exceeds the ALR cut-in set-points over a specified time-lag, then next Chiller Set cut-in respectively till ALR is maintained. (Refer to Item-2)
  1. If CHPs speed/flow decreases and ALR drop below ALR cut-out set-points over a specified time-lag, then Chiller Sets will cut-out respectively until only 1 Chiller Set operating to maintain the ALR. (Refer to Item-2)

Condensing Water Variable (CWP) Flow include Variable Speed Cooling Towers (CT) Control Strategy

1.CWPs’ design flow-rate shall be by the Variable Speed Drives (VSD) to set and calibrate the design required condenser flow-rate.

2.The numbers of CWP & CT in operation shall base on the numbers of Chiller sets in operation.

3.The CWP’s VSDs shall be controlled by the CT water Inlet/Condenser Outlet Header’s temperature sensor (T3). VSD will slow down to reduce flow-rate when water temperature is below set-point, and will speed up to increase flow-rate when water temperature is higher than set-point.

4.CTs shall equip with VSDs to modulate the CT fan speeds to maintain the CT outlet/Condenser inlet Header temperature by means of the temperature sensors at the condenser inlet header(T4).The VSD will increase CT’s fan speed when temperature is higher than set-point, and reduce speed when temperature is below set-point.

5.The numbers of CT in operation shall depend on the CT Water Outlet/Condenser Inlet Header’s temperature(T4) against the CT inlet air wet-bulb (WS-1 & 2) temperature (e.g. System will operate more CTs if CT water outlet temperature is higher than CT inlet air wet-bulb temperature and shall stop operating extra CT when CT water outlet temperature is near or equal or lower than the CT inlet air wet-bulb temperature.).

(SW1 CHILLED WATER CHILLER System, Cooling Tower to be activated during operation shall follow these strategies. Minimum CT shall operates according to number of chiller activated, more CT shall be activated when CT outlet water temperature higher than ambient weather station wet bulb temperature of 0.8 degree Celsius and extra CT shall be cut-out when CT outlet water temp drops to 0.2 degree Celsius higher than weather station wet bulb temperature. Another CT Activation Set-point shall be created to override turning on of more CTs when CT outlet water temperature have drops to 26 degree Celsius or lower. Turning on more CTs function shall be in action only CT outlet water temp is above 26 degree Celsius. This to avoid wasting extra energy when outside wet bulb is low and CT outlet water temp is low.)

6.CT fans speed will modulate base on the set-point set for the CT water outlet or condenser water inlet headers temperature. CT fans will slow down when temperature drop below set-point and speed up when temperature is higher than set-point. (refer to Item-5)

Note:

Highest Ambient DB in Singapore is at max 32.8 0C & min 26.5 0C,

With Highest Ambient WB at max 26.1 0C & min 24.5 0C during month of August