There are so many different styles of heat exchangers (HE) that this will be just a simple start.
There are chemical washes, but I don't recommend them due to the longevity of your job. There’s nothing worse then a mechanical break down or contamination of your fresh water
system. And the chemicals may just eat thru the wall of one of the tubes.
I would recommend removing the core
of the HE if possible. If not, there will be end caps that can be taken off. And others are just throw-aways.
If you can get the core
out, it's a well-built system. You can inspect the tubes for erosion or other possible leakage.
To clean, it best to start with a piece of brass rod half the diameter of the inside of the tubes. With the end ground square to the length, push it through gently. It's best to have the core mounted to something solid so you can get straight smooth strokes. DO NOT dent the tubes!!!
If it's fairly clean then use a larger brass rod that will just slide easily thru the tubes. Chances are they will not be perfectly straight. And you do not want to gouge the metal but just scrape out the alkali build up. It doesn't have to be a perfect job, but just so you can see thru clearly.
I've seen people run long drills thru with damaging results. If it's so bad you have to use a drill then do it by hand and gently. Also, grind a radius on the corners of the drill. That keeps it from digging into the metal allowing the flutes do the cleaning
. The tubes are between .015" to .060" wall thickness depending on the size of your HE.
Well, that's a start. Enjoy................................_/))