Total Dissolved Solids (TDS) are the impurities in boiler feed water that reduces the heat transfer rates, increase scale deposits, increase heat losses thereby reducing the boiler efficiency.
Total Dissolved Solids (TDS) comprises of both Suspended and Dissolved Solids. Substances that remain suspended in water and do not dissolve in it are called suspended solids usually mineral or organic in origin. Substances that are dissolved in water and cannot be seen by naked eye are called dissolved solids. Dissolved solids generally include sulfates and chlorides of sodium, magnesium, potassium and calcium and these are mainly responsible for water hardness.
Increase in the TDS concentration in boiler feed water leads to increased scale formation or deposition on the tube surface, which act as a resistance in transferring heat from flue gases to the water, which further leads to overheating of the tubes. Boiling is a bulk phenomenon that occurs throughout the liquid, so at initial stages bubbles are formed and then these bubbles reach the surface of the water and detach themselves from the surface. Due to the presence of TDS in water, these solids are carried over with the bubbles formed during boiling which reduces the heat transfer rates and more fuel will be needed to convert water into steam thereby reducing the boiler efficiency. Carryover of these solids with the steam reduces the quality of steam and also damages the control valves, heat exchangers and steam traps placed in the steam line.
If TDS concentration in feed water is maintained at 1000 ppm, steam dryness will reduce to 85% so this will lead to increase blowdown frequency to maintain the TDS levels under desired operating limits and inturn increases heat losses. If TDS concentration in feed water is maintained at 100 ppm then the steam dryness fraction will be around 98% thereby leading to reduced blowdown losses.
TDS concentration in feed water can be reduced by adopting various techniques:-
- Automatic Blowdown – This system senses the TDS concentration in feed water and automatically opens the blowdown valve when the TDS concentration exceeds the desired limit.
- Demineralisation Plant – In this, feed water is pretreated in the ion exchangers where dissolved solids such as sulfates and chlorides of sodium, magnesium, potassium and calcium get separated and washed out with the chemicals dosed during separation in an exchanger.
Also read – Why Do We Need A #BlowDown Valve?
It can be concluded that TDS concentration in feed water as low as possible so as to increase the efficiency of a boiler and the quality of steam produced.