Spa,

The key to draining a line is slope. The water must travel to the drain and out of the drip tube by gravity. Therefore the drains only need be at the low end of the system(could be in the middle). They will drain when the zone valves are closed and the pressure is decreasing to zero. The slope should be more than 1% to be efficient in draining for winter.

Water hammer only occurs when the pressure is too high for the wall of the tube and causes the valves to slam shut and send a shock wave that will hit the closed drain, which is under pressure, and cause it to split open. The number of drains per line is irrelevant. Hope this made sense. JMHO Aloha

Sounds good. Thanks for the info.