In multiple-chiller applications, it is common to use parallel chillers in a primary, or production, loop, each with its own dedicated pump. The load is placed in a secondary, or distribution, loop, hydraulically decoupled from the production loop with its own pumping arrangement. If a heat exchanger is added to take advantage of free-cooling opportunities, extensive piping, valving, and pressure drop limitations have to be addressed or the distribution pump would have to carry the extra head loss imposed by the heat exchanger all the time. An alternative that offers both simplicity of control and diversity is to incorporate a tertiary, or free-cooling, loop with common piping with the distribution piping for heat transfer but hydraulically decoupled from the rest of the system. In the free-cooling loop, the chilled-water side of heat exchangers can be placed in parallel with their own dedicated pumps in a manner similar to that of the chillers in the production loop. The heat loss of the heat exchangers and their associated piping is addressed by their respective dedicated pumps, which only operate when free cooling takes place. That there is no pressure loss consequence to the distribution pump makes the tertiary free-cooling loop method ideal for most existing systems. By placing the cross-connection, or common piping, of the free-cooling loop upstream of the cross-connection for the production loop, free cooling of part of the load could be realised, conditions permitting. This extends the total amount of free cooling that can be obtained and accelerates the payback for the installation. The heat rejection side of the heat exchangers can be connected to cooling towers normally used by the chillers, ponds, reservoirs, or other acceptable sources of cooling medium.

KEYWORDS: free cooling, pumping, chillers, parallel, cooling towers, condensers, evaporators, heat flow, chilled water supply, heat exchangers, pumps, pressure drop, cooling load, energy conservation.