One of the key objectives of effective cooling water management is to prevent the precipitation of scale-forming minerals onto heat transfer surfaces. Scaling potential is highest at the hottest metal surfaces, and where the transfer of heat occurs from a process into the cooling water “ across plate/frame or shell and tube exchangers. However, it is also possible for scaling to occur at the other end of the process where heat is removed. Since cooling towers evaporate water to reject that heat, mineral deposition also happens within the tower itself. Some have dubbed this phenomenon Cold Crystallization, and owners will experience operational problems related to the condition. Reduced flow, congestion, accumulation of suspended matter, algae and biofilm can all happen within the scale matrix.
While mineral saturation is necessary for cold crystallization to manifest, another primary (but less obvious) reason is the intermittent flow of tower water. When flow is interrupted, and airflow continues, the cycled-up cooling water is evaporated away within the tower leaving the mineral salts behind to be air-dried onto tower surfaces causing unchecked deposition. In hard water areas, this can become a real nuisance. How can this challenge be avoided? By making one small but significant change in the sequence of events for an intermittent operation (or off-peak load). Rather than cycling water flow and running fans continuously, buildings should do the opposite. Run the cooling water pumps continuously, and cycle the tower fans “ until heat is fully dissipated and flow can safely be turned off. Too often a BMS or EMS (Building or Energy Management System) is programmed to stop pumps and operate fans “ and it is this sequencing that causes the problem. Keep the water flowing to eliminate cooling tower deposits.
Written by: John D. Caloritis, CWT, Technology Director The Metro Group, Inc.