The shroud that closes the gap between the fan blade tip and the cabinet opening. Without it a fair share of the air just recirculates around the blade instead of moving through the condenser, so a missing ring quietly costs cooling capacity.
Fit it whenever the fan or fan motor is replaced.
Teco is made in Italy and its spare parts are model-specific — check the model plate on the back of your unit before ordering. See the full Teco range, including the Teco TK aquarium chillers themselves.