A Y Strainer is a straight through fitting that is installed on the return line at the chiller. Direction is important. Be sure to pay attention to the arrow of direction of flow when installing. On a 20 mesh Y strainer, there is an internal mesh that flow has to pass though before it can go anywhere. The screen catches any particles and/or contaminants floating around in the system and prevents it from getting back to the chiller. This protects the pumps, heat exchangers and all other necessary components of the chiller by ensuring that no particles or contaminants can get inside. It is a good idea to upsize your Y strainer about ½ a size from your mainline, that way there is no restriction on flow and everything moves freely.