Water-Cooled Chillers

This link should take you to a page that will answer your question about approach temperatures. The site also contains other useful information.

http://www.air-conditioning-and-refrigeration-guide.com/water-cooled-chiller.html

The approach temperature is the difference between the leaving water temperature and the refrigerant temperature. This is true for both the chilled water side, and condenser water side. It is determined by the manufacturing process. An inexpensive chiller with a small heat exchanger may have an approach temperature of 6C (10F). A more efficient chiller with a better heat exchanger may have an approach temperature of 1C (2F). The approach is sometimes mentioned in manufacturers literature--but not often. However, if the machine is working at full capacity you can take measurements, and calculate the approach. As dirt and scale build up on a heat exchanger the approach gets higher (worse). Descaling may bring the approach back to its original condition--but not better. Regarding the PH value of water in a chiller plant. Both closed circui and open circuit water systems should really have some additives to prevent rust of iron pipes, kill any organisms, and protect against freeze-ups in cold weather.