By Chad Edmondson
Every cooling tower design requires a strategy for supplying make-up water to replace water that is lost due to evaporation, drift or blowdown. This is necessary to maintain the condenser water volume and keep dissolved minerals sufficiently diluted. There are two basic make-up water strategies to choose from – mechanical and electric.
Mechanical Water Make-Up
Most cooling towers rely on a mechanical float valve to supply make-up water directly into the cold-water basin. These valves, which are often included as standard on cooling towers, operate similarly to the float valve in a toilet tank. Installed inside the cooling tower, the float portion of a mechanical make-up valve floats atop the water in the cold-water basin. As the water level drops so does the float, which is mechanically linked to a plunger inside the valve. This causes the plunger to become unseated, thus opening the valve and allowing make-up water to flow into the basin. The water level inside the basin then rises until the float reaches a predetermined level and the plunger inside the valve returns to its seat, shutting off the flow of water.
Although the operation is remarkably simple, there are a few things to keep in mind when opting for mechanical make-up. First, mechanical valves require 15 to 50 psi to operate properly, otherwise damage may occur. Most mechanical valves cannot close against water pressures above 50 psi. Freeze protection may also be necessary since the make-up water piping has to run to the cold-water basin. Finally, the mechanical valve and components should be designed to withstand the wet and turbulent environment inside the tower. Valve bodies are typically made of PVC or bronze to resist corrosion.
Electronic Water Make-up
Sometimes mechanical valves are not an option because there is no water makeup source at the tower. In these cases, an electric solenoid valve located in the mechanical room can be installed to deliver water directly into the condenser water piping. Typically these valves would not require any freeze protection because they are located indoors.
Electronic make-up includes two main parts: (1) the solenoid valve, which is located remotely from the cooling tower and wired to (2) a set of probes which are located in a piece of PVC pipe inside the tower. The probes signal various water level statuses to the solenoid. Often there are five probes, each of a different length to indicate a specific water level condition. The solenoid responds by opening or closing the valve based on which probes are or are not submerged in water. The probes can also indicate high or low water alarm conditions.
The solenoid valve is typically designed to operate with make-up water line pressures of 10 to 125 Psi. Higher pressures should be avoided as they can cause water hammer. Some valves also include a slow closing feature to help prevent water hammer.
Other Design Considerations
Regardless of which type of valve you choose, there are a few rules which should always be observed:
Valves and make-up water piping should be sized based on the amount of evaporation and blowdown that is anticipated to occur.
A bypass line should be included to facilitate fill at start-up as well as refill following blowdowns or bleeds.
An air gap should be included between the basin and the supply pipe or the supply pipe and condenser water pipe to prevent any vapor from the treated condenser water from entering into the municipal water supply.
A backflow preventer should be installed between the municipal water supply and the tower to prevent tower or condenser water from contaminating the water supply. The type of backflow preventer should be specified based on municipal requirements and meet all relevant codes and regulations.