In a recent blog article, we established the importance of mechanical filtration and reviewed a few common types for cooling tower applications. One of the key considerations for a tower owner when going through the selection process is to determine the best-suited installation method for their application. In the world of evaporative cooling, the three main filtration installation methods are basin sweeping, side stream, and full stream.
A basin sweeping filtration installation focuses on cleaning settled solids from the basin floor of a cooling tower, independent of the process cooling loop. Water is drawn directly from the cooling tower basin, filtered, then pumped back to the tower.
Typically accompanying this installation is a specifically designed grid of PVC pipes and spray nozzles that covers the cooling tower basin floor. The spray nozzles, commonly referred to as “venturi” or “eductor” nozzles, take advantage of the venturi principle to discharge up to 5x the flow that enters the nozzle. It is important to ensure the pressure to the nozzles is as designed to achieve the full capability of the nozzle.
A basin sweeping unit is typically sized by the total area the basin covers. A rule of thumb for basin water levels less than 3 feet is to use 1 GPM/ft2 of basin area. The recommended GPM value can vary for water levels greater than 3 feet or in applications that have heavy solids accumulation.
This installation option will not affect, disrupt, or add additional pressure requirements to the main flow. Basin sweeping filtration can drastically cut down on the frequency, downtime, and man hours required to manually clean a cooling tower basin.
A side stream installation sends a small percentage of the main recirculation flow through the filtration system. Typically, this range is between 10-20% of the main flow rate, although some manufacturers recommend percentages as low as 3%. This system filters the water going directly to the process cooling loop, independent of the cooling tower basin.
The goal of determining a side stream percentage is to filter at a rate greater than the debris entering your system. To avoid any operating concerns or flow disruption, it is best practice to install both the suction and return lines of a side stream filter downstream of the main discharge pump.
Side stream installations are advantageous for their relative simplicity of connection. They simply require two “tees” to be cut, installed into the main line at a distance of at least ten pipe diameters apart. This installation option will not affect, disrupt, or add additional pressure requirements to the main flow. A cost-effective option, side stream installation helps to reduce the required maintenance intervals of downstream heat exchange equipment.
As the name implies, a full stream installation is installed such that 100% of the main recirculation flow is sent through the filtration system. Like side stream systems, this filters water going directly to the process cooling loop, independent of the cooling tower basin. Inherently, being sized to handle the full flow will require a considerably larger filter unit and overall footprint.
Full stream installation can be very difficult for existing systems and are typically recommended for new installations. A considerable pressure drop is added with a full stream installation, which makes incorporation to an existing system difficult. If this route is chosen, it will often require an increase in pump/motor size to overcome the additional pressure requirements.
While not the most cost-effective solution, a full flow installation will drastically reduce the required maintenance intervals of downstream heat exchange equipment.