What is the difference between bypass and decoupler?

There is the primary pump and loop, which circulates water through the chiller at a constant speed. Then there is the secondary loop to the building, which has it's own pump, and typically VFD. They share a common pipe, or decoupler pipe, which basically connects between the supply and return of these two loops. As the secondary loop pump increases speed and requires cold water, it will pull the cold water out of the primary loop. As it pulls more and more water out of the primary loop, the flow through the decoupler pipe is less and less. It will reach a point where the building return is equal to the primary pump GPM, and there will be no flow in the decoupler pipe. So in essence, there will be no circulation, but all return water going into the chiller. In fact, the decoupler pipe can actually flow backwards, depending on the GPM of each pump and the mixing of bulding return with chiller supply.

Bypass is the connection between CHWS&CHWR pipes for FCU OR AHU, the purpose of this bypass is power and load saving after the required temperature for the required space hits the set point.

Decoupler (Decoupler bridge) used for the piping design between the primary and secondary loops, the purpose of it is to have a minimum pressure drop, thus any change in flow rate in one loop will not affect the flow rate at the other.

after attaining the required temperature bypass  is activated (bypass= thermostat in refrigerators) in fcus

decoupler link between secondary and primary chilled water loop

>the bypass is between the supply and return water to AHU .

>the decoupler is between the entering  of the main chiller pump (big system) and the entering of the small chiller pump (boaster pump)in a small (new)building