Project
Screw chillers and primary pumps .
Secondary pumps .
station which services each building . This includes each residential block , Hotel block and retail .
Three identically sized pumps with variable speed controllers , two run and one standby , are provided for each . Chilled water shall then be circulated within its own loop per building / user thereby eliminating the risk of a water leak affecting the entire main plant and avoiding unnecessary pumping power losses .
An energy meter is installed in the primary leaving chilled water piping to record the amount of chilled water supplied to the building by the chillers and to track the thermal energy used by the plant .
CHILLER CONTROL
• Primary Chilled Water Circuit : All four chillers are able to run simultaneously to provide the full load for the chilled water system to the building and controlled by a central chiller plant controller as well as an electronic BMS control system . An excess / deficit meter is installed in the primary / secondary decoupler piping which dictates how many chillers are required via the electronic control system . On start-up of a chiller , its primary chilled water pump starts up . After a 20-minute time delay the excess / deficit meter dictates if the primary flow is still in deficit and whether another chiller is required to start up . This process is repeated to start all four chillers if required . While chillers are in the run mode they maintain a chilled water leaving temperature of 5 ° C . When the excess / deficit meter is in excess mode ( too much chilled water in the primary circuit ) it stops the chillers as required in a timed delay manner . When a chiller stops its primary and condenser water pumps also stop .
• Secondary Chilled Water Circuits : When the primary chilled water system is requested to start up the chillers by the electronic control system , the first secondary chilled water pump starts up . When flow is established , and after a short time delay , the second and third secondary chilled water pumps start up in a similar manner . Only a maximum of three pumps run and one pump is a standby . The secondary chilled water pumps provide chilled water to various heat exchangers in the building through a system of insulated steel piping . The heat exchangers have two-way or three-way control valves in the piping system to regulate the amount of secondary chilled water flow through them to maintain a constant chilled water leaving temperature . To accommodate the varying secondary chilled water flow , the secondary chilled water pumps have VSD drives for the pump motors which are controlled by a differential pressure monitoring system in the secondary chilled water piping and this varies the speed of the pumps . Each heat exchanger system has an energy meter installed in the secondary chilled water piping to record the amount of chilled water supplied to the heat exchanger .
• Tertiary Chilled Water Circuits : The tertiary chilled water is provided from the heat exchangers and provides chilled water to the various air handling units and fan coil units serving the various buildings . Tertiary chilled water is pumped by a pump adjacent to the heat exchangers through an insulated steel piping system to the various units . Pumps on the tertiary loop are in a standby and duty configuration . The air handling units and fan coil units have a two-way valve control valve at each unit to vary the flow of tertiary chilled water through them to control the air temperature leaving the units . Some units at the ends of the tertiary chilled water pipe runs have three-way control valves to ensure that there will always be water flow at the ends of the piping system . Since the water flow through each unit shall vary according to their load requirement the tertiary chilled water pumps have VSD drives on the motors to vary the speed of the pumps according to the varying flow requirements .
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RACA Journal I April 2023 www . refrigerationandaircon . co . za