ATM guns have their internal, hard-coded cooling.
Coolant racks, boards and cells are used exclusively for generators.
Each generator produces heat, with amount based on current rate and amount of generators/fuel chambers around it.
Coolant boards hold coolant cells. Those are filled with coolant that can store heat. They can be recharged (cooled down) by placing them in a recharge rack that (afaik) have to be connected with radiators.
Radiators can be also piped to generator to get rid of heat. ATM they are 100% efficient (so you don't need cells at all) but afaik plan is to use some coolant even with radiators.