Some questions from someone woefully ignorant
1) Do clamshell HX's any better/worse than tube HX's?
2) Would stainless steel HX's be advantageous over aluminized steel and worth any extra costs incurred
3) Can anyone comment on the lifespans of said types of HX's
I have seen nothing that would indicate that either style of HX would outperforn the other for heat transfer. Tubular heat exchanger will often times allow for a smaller cabinet size though.
Tubular heat exchangers should be more durable compared to a clamshell heat exchanger using like materials due to the way that stesses are distributed through their structures.
Stainless steel is a much more durable material than aluminized steel but doesn't transfer heat as well. This requires a larger heat exchanger in return and also a larger cabinet. One manufacturer overcomes this by copper cladding their stainless heat exchangers.
The only issue we've had with clamshell is setting the furnace directly on the coil box on a downflow application. The top of the A coil can sometimes block airflow through the center of the heat exchanger. We've had a few cases where we've had to add a spacer to get better flow through the center where the limit is located.
I would say we've found almost no problems with anyone's residential tubular HX vs the crimped clamshell type of either metal.