By coils, I assume that you mean the ones in closed circuit coolers (please correct me if not). Aside from using a glycol mixture in your loop to give freeze protection, the best way that I have found is to keep the water moving at all times. I always design bypass loops around my water cooled equipment, with solenoid valves that open to maintain a minimum flow through the loop (and therefore through the coil) even when the water cooled equipment is shutdown. Because the water is spending most of its time in the heated equipment room, and because it is picking up heat from the pump, it doesn't lose enough heat in the coil to ever reach freezing in my climate.
For tower sump water, electric sump heaters work well. In very cold climates or where the tower will be shutdown for much of the winter, a remote sump located inside of the building is the way to go.
