There are many applications in the HVAC industry where a chemical must be added to a volume of water. Condenser water, in an open cycle that passes through a cooling tower, must be chemically treated to minimize corrosion and fouling of the piping and the tubes in the condenser side of the chiller. The water circulating in closed systems, if to a lesser extent, must also be chemically treated.
An effective way to maintain the chemical balance of fluid systems is an automated one that introduces chemicals to the flow stream based on a signal from a pH or other sensor in the fluid path. The sensor must be downstream of the point of introduction, a sufficient distance for mixing to occur. When a deficiency is signaled to the solenoid, the valve cracks open and a metered flow of chemical enters the fluid.
Another metering application is for boiler blowdown. When the chemical concentration in the boiler basin exceeds a specific pH or conductivity, a valve is open for a fixed amount of time to discharge a small volume of fluid. This is called automatic blowdown. The alternative is continuous blowdown, which removes a steady volume of water from the boiler. This wastes the thermal energy in the blowdown fluid and also wastes the chemicals used to treat the feedwater.
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Excerpt from Energy Conservation Projects.