frog5150, standardizing the details of construction as required by ASME Code is one thing and fairly easily done.
Standardizing the VESSEL DESIGN is another thing altogether and depends on the market you're in and so forth. Most of the features that perform the intended function are not subject to Code rules. For example small package lube oil coolers are widely standardized, while process heat exchangers are generally specific to a plant, process etc. Tube layout, diameter, length, materials, general flow and baffle arrangement, nozzle diameters and arrangement, none of these thing is specified by Code.
I am more familiar with S & T exchangers, but I'm sure the same type of things could be said of other vessels.
What is your product line?
Regards,
Mike