Fixed Primary and Variable Secondary Flow System
Primary-Secondary chilled water piping system includes two sets of pumps i.e. system/load side pumps called secondary pump and chiller pumps called primary pumps. The secondary pumps distributes water to the terminal/load units and the primary pumps circulate water through a loop that serves the chillers only. The two piping systems are decoupled through what is commonly referred to as a “De coupler line”, Traditionally, Primary-Secondary piping arrangements have meant full flow in the chiller loop and variable flow in the system/load loop. Primary-Secondary concept is widely used because it ensures proper flow through the chiller minimizing risk during low flow and also ensure adequate and required flow rate to meet the building load, thereby optimizing power consumption. This concept is also used where the service room is located at one place in the building campus and the building is slightly away from the plant room and /or well spread out.
The Secondary pump ON/OFF and loading is based on the load demand. Based on the DPT installed on the load side the secondary pumps shall be operated. DPT (Differential Pressure Transducer) must be installed across the inlet and outlet head of the farthest equipment in the system to ensure the minimum/proper flow rate.
The primary chilled water flow should always be higher than the secondary requirement. Based on the temperature sensor located on the de-coupler line, the load requirement is assessed. If the temperature is equal to the chiller outlet water temperature, then system function is ok (the primary cooling production is higher than the secondary requirement). If the decoupler line temperature is equal to the return chilled water temperature or higher than the supply water temperature, then the chilled water production is lesser, which calls for switching on additional chiller and a pump.