Bottom blowdown control typically involves periodically opening a bottom blowdown valve to discharge a small portion of the water and sludge from the bottom of the boiler, which is then replaced with fresh feedwater.
The frequency and duration of the blowdown cycles can be controlled based on various factors such as boiler operating conditions, water quality, and boiler manufacturer's recommendations. Even if the boiler system has an automated blowdown system to monitor and control boiler water dissolved solids content, a bottom blowdown is still necessary, but at a more infrequent rate.
The benefits of bottom blowdown control in terms of improving boiler efficiency include:
Reduction of scale and sludge buildup:
Accumulated scale and sludge on the bottom of the boiler can reduce heat transfer efficiency by acting as an insulating layer, inhibiting heat transfer from the burner flame to the water. Regular bottom blowdown can help remove these deposits, maintaining clean heat transfer surfaces and improving boiler efficiency.Prevention of carryover:
Carryover is the entrainment of water droplets or sludge particles in the steam leaving the boiler. It can cause problems such as water hammer, erosion of steam turbine blades, and reduced heat transfer in downstream heat exchangers. Bottom blowdown can help reduce carryover by removing sludge from the bottom of the boiler, preventing it from being carried over with the steam.Control of dissolved solids:
Dissolved solids, such as salts and minerals, can accumulate in boiler water during continuous evaporation and concentration. Excessive levels of dissolved solids can cause scale formation, corrosion, and reduced boiler efficiency. Regular bottom blowdown can help control the level of dissolved solids by removing a small portion of the concentrated water and replacing it with fresh feed-water.Energy savings:
By maintaining clean heat transfer surfaces and preventing scale buildup, bottom blowdown control can improve heat transfer efficiency in the boiler, resulting in energy savings. Improved boiler efficiency can lead to reduced fuel consumption and operating costs.
Improper blowdown practices, such as excessive blowdown or inadequate blowdown, can lead to wastage of water, energy, and chemicals, and may even cause damage to the boiler. Therefore, it's essential to follow best practices and guidelines for bottom blowdown control to optimise boiler efficiency.