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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).

減少鍋爐沖放次數基本上可減少能源的損失，因為沖放水的溫度直接與鍋爐生產的蒸汽有關。
當鍋爐水在鍋爐內蒸發產生蒸汽的時候，其中溶解的固態物就留在鍋爐水中，久而久之其溶解固態物的濃度就會升高，成懸浮狀的固態物可能形成沉澱而降低熱傳導力。
溶節的固態物也會形成泡沫狀而把鍋爐水帶進蒸汽中。為了要減少懸浮物及總溶解物(TDS)到可接受的程度，下列兩種方式可以利用：
●底部沖放鍋爐水：允許鍋爐有較好的熱交換，通常是以人工方式執行，其頻率大約每幾小時充放幾秒鐘。
●水面刮除後沖放：用來除去集合在鍋爐水液面的溶解物，通常是一種連續性作業。
沉積的塩類會造成蒸汽1~3%的損失。此外，為了冷卻沖放出的餘渣到主管當局允許的溫度會產生一些成本。
為減少沖放量有幾項可能做法：
●回收凝結水:這些凝結水已經淨化，不會帶有不純物。如果有一半的凝結水可以回收，則鍋爐沖放水可以減少50%。
●飼水要經過軟化、去碳水化合物及除去礦物質，另外飼水除氣及加入調節品質的化學藥劑是有必要的。沖放的程度與飼水中物質濃度和添加物有密切關係，如果飼水直接進入鍋爐中，沖放率要7%~8%；但如先經過水質處理，沖放率只要3%以下就可以了。
●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)。
●利用中壓或低壓以閃發方式是另一種回收沖放水中能源的方式。這技術的使用必須在現場有一比鍋爐產生的蒸汽壓力還要低的蒸汽管網輸送到需要的使用端。這項技術比將沖放水以熱交換器更能有效回收熱能。
●透過蒸發造成的壓力除氣方式會產生1~3%的能量損失。CO₂和O₂利用這種方式從供水中釋放(利用103 °C的溫度時稍微超壓)，這技術可以優化除氣器通氣率，減少能量損失。

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.

如果沖放頻次達到一臨界水準時，發泡和結垢形成現象又會再出現。其他方法也可採用以降低這臨界值，例如回收凝結水、飼水預處理等。
沖放量不足可能導致設備效率降低；過度的沖放反而會造成能源浪費。
在所有的案例中鍋爐水沖放是一標準動作。除非蒸汽是用來引入生產製程裡頭使用，在這情況下利用凝結水回收的方式來減少沖放並不可行。