As fluid is lost from a steam or hot-water loop through steam traps, valves, and fittings, it must be replaced with clean water. This replacement water is not pure but has chemicals that it introduces into the system. As a result, over many cycles of operation, the boiler water has more and more dissolved chemicals. These solids can precipitate on pipes, boiler tubes, and valve surfaces, severely impeding the water flow and reducing the heat-transfer rate at the boiler tubes.
A high-capacity boiler often is installed with a constant-discharge of fluid, to keep the dissolved solids under control. This is costly in water charges, sewer fees, and in the cost of expensive chemicals to treat the replacement water. It also has the potential of introducing damaging chemicals into the environment.
An improvement on this system is a device that constantly monitors the chemistry of the boiler water and discharges water only when the dissolved solids exceed the setpoint. This simple device minimizes the thermal losses from the system as well. These losses can be quite high because the blowdown water is usually at a very high temperature, and it’s replaced with cold water from the city mains.
Another system further reduces the volume of blowdown fluids by adjusting the surface blowdown rate to a low-flow or high-flow rate. The blowdown flow is high when the conductivity of the fluid is above the setpoint. When the conductivity is below the setpoint, the flow rate is at the low position. The latter happens as fresh makeup water is added to the boiler blowdown, decreasing the solids concentration of the discharge water, even while the overall system content is above the setpoint. The low flow rate allows mixing in the boiler drum to overcome this balance more quickly, while eliminating a lower volume of water and all its costly consequences.
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Excerpt from Energy Conservation Projects.