Double wall heat exchangers are design to keep the working fluids from cross contaminating if a tube wears or fails. Each tube is actually a tube within a tube and there are two tubesheets. The tubes are sealed to the tubesheet that it is in contact with and a gap between the tubesheets lets a leaking tube drip between the tubesheets to signal a tube failure. Many plumbing codes require this for heat exchangers being used to heat domestic water. Check your local codes to see if it affects your application.