I know this is a huge fear in the industry. But,you have to take a close look at what your working on. For Instance where is the liquid port located? How far away from the tubes is it? You are not going to freeze a tube on a RTHD by dumping liquid into the evap liquid port at the bottom of the barrel. And I dont see a reason to shutdown or pumpdown a 30h (However I am not that familiar with this machine). Refrigerant is going to migrate to the coldest place so why not use this same port for adding ref. Then you dont risk carry over(slugging) or shutting down the machine. You also dont need water running through the barrel When removing refrigerant once you get all refrigerant isolated into the evap/cond. when pulling off the bottom. That way with multiple chiller plants you are not mixing water for very long and therefore not loosing the temp/humidity control of the building. Now If we are talking brazed plate and frame heat exchangers you better have water running at all times. And have fun adding refrigerant to these. They are a pain.