Heating Help

heat exchanger

I'm work in a building that has a steam boiler with a one pipe pipe steam system.  The store in the building has recently been renovated.  Another contractor came in and installed a steam to water flat plate heat exchanger.  The steam return line from the heat exchanger has an f&t trap and then ties into a wet return.  There is banging coming from the return line of the heat exchanger.  Some how i have gotten dragged into this.  I told the contractor to remove the f&t trap and let it go straight into the wet return.  He said the company requires an f&t trap on the return line of their heat exchanger.   The company said the same thing when i called, even when i told them it drains into a wet return.  Can someone please tell me why you need the f&t trap on the return of the heat exchanger if it ties into a wet return?  Thank you.  

pictures

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