Decoupler line or bypass line is of crucial importance in primary-secondary arrangement of chilled water systems.
In primary-secondary chilled water systems, the flow rate in the primary loop is maintained constant and the pumps (primary pumps) are NOT driven through VFDs. Assume that flow rates in this loop are reduced based on some parameter, possibilities are there for the pumps to NOT satisfy the min flow requirement of chillers. This will cause the chiller to shut down and therefore the primary loop is always equipped with constant speed pumps giving constant flow rate at all times.
Then, we have the Secondary loop or the DISTRIBUTION LOOP. This distributes chilled water to various loads per the demand. Obviously, these pumps are driven through VFDs to vary the flow based on the demand using liquid differential pressure transmitter at critical circuit(s).
The point to be noted here is both primary and secondary pumps are arranged in series. But, primary pumps are of constant flow rates and secondary pumps are of variable flow rates. Anyone now might get a question how is this even possible? They are arranged in series, where will the difference in flow rate goes? The answer is through DECOUPLER LINE which bypasses the secondary loop in the system.
The doupler line is a bypass line that is connected between the suction points of primary pumps and secondary pumps. Please note that chiller outlet point and secondary suction point are one and the same.
Consider a situation,where the system is designed for 100 GPM flow. Assume the system has a demand 50% of design. Then, the primary pump will have 100 GPM flow trough the chiller. The secondary pumps operating at lower speeds will now suck only 50 GPM for distributing to all AHUs and thereby meeting the demand. Another 50 GPM will now flow into the decoupler line at the junction in secondary suction point. Now at the other decoupler junction (primary suction point), we have two in-flows. 50 GPM from AHUs and 50 GPM from decoupler line. Thus making it to 100 GPM and completing the cycle.
So, without decoupler line, it is impossible to establish the primary-secondary arrangement. By default, it is understood that the secondary loop is of variable flow.
