A cooling jacket, also referred to as a flow generator, ensures that there is always sufficient water flow across the motor surface in order to cool the motor.
In a typical well installation, the pump is surrounded by the walls of the well. The motor is installed below the pump. Because the pump draws in water from below, the water flows along the well walls and over the surface of the motor, cooling the motor in so doing.
If the pump is installed in open water, there will be no well walls to guide the water flow over the surface of the motor, thus necessitating a cooling jacket. Please enquire with your Grundfos dealer for more information about cooling jackets.