Heating Help

What is this

I had a customer come in with this and tell me that it came off of a domestic hot water heat exchenger. He told me there was two of them. The direction of flow and that fact that it says it is a vacuum breaker doesn't make much sense. Any help would be great.

If it's a shell-and-tube heat exchanger

and if there's steam in the shell, the shell would require a vacuum breaker so the condensate could drain when the control valve shuts. Without the vacuum breaker, water hammer will destroy the tube bundle. 

Thank you sir.

That helps.