If you consider the heat exchanger tubes and half the required distance between them as a single object, you are just asking how many circles of diameter 1.25 can be fit in a circle of diameter 20.
Your question does not have a provable answer. This website:
The best known packings of equal circles in a circle
has packings sorted by number of circles, and since your radius is 1.25/20 = 0.625, it looks like the maximum number is 213. It also looks like you might be able to download a file that shows you the pattern for 213.