Minimising the blowdown rate can substantially reduce energy losses as the temperature of the blowdown is directly related to that of the steam generated in the boiler.
As water vaporises in the boiler during steam generation, dissolved solids are left behind in the water, which in turn raises the concentration of dissolved solids in the boiler. The suspended solids may form sediments, which degrade heat transfer ( see Section 3.2.6). Dissolved solids promote foaming and carryover of boiler water into the steam.
In order to reduce the levels of suspended and total dissolved solids (TDS) to acceptable limits, two procedures are used, automatically or manually in either case:
●bottom bl owdown is carried out to allow a good thermal exchange in the bo iler. It is usually a manual procedure done for a few seconds every several hours
●surface or skimming blowdown is designed to remove the dissolved solids that concentrate near the liquid surface and it is often a continuous process.
The blowdown of salt residues to drain causes further losses accounting for between one and three per cent of the steam employed. On top of this, further costs may also be incurred for cooling the blowdown residue to the temperature prescribed by regulatory authorities.
In order to reduce the required amount of blowdown, there are several possibilities:
●the recovery of condensate ( see Sections 3.2.13 and 3 .2.15). This condensate is already purified and thus does not contain any impurities, which will be concentrated inside the boiler. If half of the condensate can be recovered, the blowdown can be reduced by 50 %
●depending on the quality of the feed-water, softeners, decarbonation and demineralisation might be required. Additionally, deaeration of the water and the addition of conditioning products are necessary. The level of blowdown is linked with the level of the more concentrated component present or added to the feed-water. In case of direct feed of the boiler, blowdown rates of 7 to 8 % are possible; this can be reduced to 3 % or less when water is pretreated
●the installation of automated blowdown control systems can also be considered, usually by monitoring conductivity. This can lead to an optimisation between reliability and energy loss. The blowdown rate will be controlled by the most concentrated component knowing the maximum concentration possible in the boiler ( TAC max. of the boiler 38ºC; silica 130 mg/l; chloride <600 mg/l). For more details, see EN 12953 – 10
●Flashing the blowdown at medium or low pressure is another way to valorize the energy which is available in the blowdown. This technique applies when the site has a steam network with pressures lower than the pressure at which steam is generated. This solution can be exergetically more favourable than just exchanging the heat in the blowdown via a heat exchanger (see Sections 3.2.14 and 3.2.15).
●Pressure degasification caused by vaporisation also results in further losses of between one and three per cent. CO2 and oxygen are removed from the fresh water in the process ( by applying slight excess pressure at a temperature of 103 °C). This can be minimised by optimising the deaerator vent rate (see Section 3.2.8).
●可考慮安裝自動沖放控制系統，用以觀測水中的導電度，如此可以在可靠度與能源損失間取得最佳化沖放率。可以鍋爐水中的溶解物最大濃度來控制其沖放率(TAC、最大38℃ 、矽130mg/公升、氯化物 <mg/公升)。
Discharges of treatment chemicals, chemicals used in deionizer regeneration, etc.
If blowdown is reduced below a critical level, the problems of foaming and scaling may return. The other measures in the description (recovery of condensate, water pre-treatment) may also be used to lower this critical value.
Insufficient blowdown may lead to a degradation of the installation. Excessive blowdown will result in a waste of energy.
A condensate return is usually standard in all cases except where steam is injected into the process. In this case, a reduction of blowdown by condensate return is not feasible.
Significant savings in energy, chemicals, feed-water and cooling can be achieved, and makes this viable in all cases, see examples detailed in Annex 7.10.1.
Energy Efficiency (2009) 3.2.7