As for running a chiller in free-cooling, that is usually not worth the effort, and it is much simpler/cheaper to run a plate-frame exchanger. A better opportunity is to run the plate and frame as its own primary loop from the chilled water return, and then through the chillers. Great for those situations where you need a 44°CHWS, and have a 45°F tower water. It allows you to get as much of the free cooling direct from the condenser loop, and then top the cooling off with the chiller. Best of all, it's not hard to add to an existing plant, as long as you can find space for the heat exchanger and its own primary pump.