IF small chiller run more then 100% delta T or actual temperature above set point ,then stop small chiller ELSE big chiller.
IF big chiller run then set big chiller condenser & evaporator pump 1 ELSE 0.
IF small chiller run then set small chiller condenser & evaporator pump 1 ELSE 0.
IF duty off time ,then small chiller .
above description is a simple program example to reduce energy consumption program.
this is a so simple principle,assume there have two chiller for provide chill water.one is 500RT and another is 200RT.
500RT chiller with 50hp&50hp condenser & evaporator pump,200RT chiller with 20hp&20hp condenser & evaporator pump.
if the cooling load is down to low load,perhaps 200RT,if we still run the 500RT chiller,the condenser & evaporator pump also need to be run.it can run normally,but not efficiency,because chiller must be unload to adoptive this situation,chiller actual motion is close unload valve,move running area to high head cause valve close action.
meanwhile,the pump consumption power same with full load condition,if use VFD to reduce pump rpm too low,there may be cause chiller trip,because chiller have low water flow limit,if water flow less 50% design flow rate,they would trip.
if same load(200RT)switch to small chiller,the chiller run at full load condition,it mean the regulator or unload valve is near full open cause lowest friction or compressor lift and both two pump is smaller power consumption.
this is very simple method to reduce power consumption just use simple controller to do that like above control logic step .