dogtop,
(funny, what's in top of a dog??! let me guess...)
The rating of a heat exchanger is almost the same process as designing it. You need to be very good at understanding the thermal properties of the process fluis flowing through an exchanger (including the air used for coolers) and have access (and understanding) of a complex design software like HTRI. Then you'll be able to perform exchanger rating calculations (HTRI does rating calcs for TEMA exchangers, plate exchangers, air coolers, etc).
The google might be a way to go, but I suspect you have already checked out this venue and not much joy was offered. All those good advices require some engineering experience and a lot of reading in the heat transfer books.
Alternatively, if you need a quick answer, come back with a specific question, perhaps the process datasheet and you might get lucky, someone will run a short rating calculation for you. Otherwise, I can give you a list of boock, manuals, standards you can study, then you can do your own calculations.
Cheers,
gr2vessels