Flash steam is formed when the condensate at high pressure is expanded. Once the condensate is at a lower pressure, part of the condensate will vaporise again and form flash steam. Flash steam contains both the purified water and a large part of the available energy, which is still present in the condensate.
Energy recovery can be achieved through heat exchange with make-up water. If the blowdown water is brought to a lower pressure in a flash tank beforehand, then steam will be formed at a lower pressure. This flash steam can be moved directly to the degasser and can thus be mixed with the fresh make-up water. The flash steam does not contain any dissolved salts and the steam represents a large portion of the energy in the blowdown.
Flash steam does, how ever, occupy a much larger volume than condensate. T he return pipes must be able to deal with this without pressure increases. Otherwise, the resulting backpressure may hamper the proper functioning of steam traps and other components upstream.
In the boilerhouse, the flash steam, like the condensate, can be used to heat the fresh feed-water in the degasser. Other possibilities include the use of the flash steam for air heating.
Outside the boilerhouse, flash steam can be used to heat components to under 100 °C. In practice, there are steam uses at the pressure of 1 barg. Flash steam can thus be injected into these pipes. Flash steam can also be used to preheat air, etc.
Low pressure process steam requirements are usually met by throttling high pressure steam, but a portion of the process requirements can be achieved at low cost by flashing high pressure condensate. Flashing is particularly attractive when it is not economically feasible to return the high pressure condensate to the boiler.
閃發蒸汽的產生是凝結水在高壓狀態下被膨脹。一旦凝結水在低壓狀態時，一部分凝結水又會再蒸發成閃發蒸汽(譯著：國立教育研究院譯成「二次蒸汽」)。
閃發蒸汽含有純化的水和大量可用的熱能。
其中熱能可以再熱交換器用鍋爐的補充水加以吸收，如果鍋爐沖放水被引到前方的低壓閃發槽內，則蒸汽就會變成低壓蒸汽。閃發蒸汽可以直接進入除氣器中，在除氣器中和補充水混合。閃發蒸汽不會含有任何溶解鹽性物質，而且蒸汽含有大部份沖放水的熱能。
閃發蒸汽不會佔據太多的體積，回流管線必須能承擔得起這些數量而且不會增生壓力，否則造成的背壓將衝擊蒸汽祛水器和上游其他組件的正常功能。
在鍋爐中閃發蒸汽和凝結水同樣可用來加溫除氣器中的生水。另外，也可用閃發蒸汽加熱燃燒用進氣。
在鍋爐外，閃發蒸汽可用來加熱低於100°C的元件。實務上，這些蒸汽的使用都在表壓力一大氣壓以下，以方便注入管線中，閃發蒸汽亦可用來預熱燃燒空氣。
低壓製程用的蒸汽要求條件通常是以高壓蒸汽經過節流閥降壓而來，但部分製程需要壓力可透過閃發高壓凝結水而得到便宜的蒸汽。
在回流高壓的凝結水到鍋爐中無經濟性時，閃蒸的方式特別有吸引力。
Where flash steam is produced from pressurised condensate, the temperature ( and energy content) of the condensate returning to the boiler is lowered. Where an economiser is fitted, this has the potential advantage that the economiser can then recover more energy from the exhaust stack into the return/feed-water stream, and the boiler efficiency will improve. This is the most energy efficient combination. However, there must be a use for the low pressure ( LP) steam from flashing, taking into account that LP steam (from all sources) can only be moved limited distances. In many cases (such as in refineries and chemical plants) there is a surplus of LP steam, and there is often no use for the steam from flashing. In such cases, the best option is to return the condensate to the deaerator, as flashing steam to the atmosphere is a waste of energy. To avoid condensate problems, condensate can be collected locally in a specific unit or activity and pumped back to the deaerator.
The installation of either option depends on the cost-benefit of installing the necessary pipework and other equipment (see Section 1.1.6).
當閃發蒸汽從高壓凝結水產生後，凝結水回到鍋爐的溫度(能含量)被降低，此時如有裝設節煤器，則節煤器可從廢氣中回收更多熱能交換給回流水和飼水而節能，而且鍋爐效率也可提升。這做法有助於提升能效，然而要利用閃發後的低壓蒸汽必須考慮其有距離的限制。在煉油廠或石化廠中會有過多的低壓蒸氣，而且往往沒有利用閃發蒸汽，這情況下最好的方式是將凝結水回送到除氣器，以免將閃發蒸氣放空，浪費能源。為避免凝結水出現問題，可在某一設備附近收集凝結水使用，或送回除氣器系統中處理。
選擇處理方式要看設置成本、效益或其他裝備而定。
This technique applies when the site has a steam network with pressures lower than the pressure at which steam is generated. Then, re-using flash steam can be exergetically more favourable than just exchanging the heat in the blowdown via a heat exchanger.
In theory, any energy use at a lower temperature can be a possible use for flash steam instead of fresh steam and there will be a range of opportunities on investigation, although implementation is not always easy. It is widely applicable in the petrochemical industry.
此技術應用在現場有一壓力較蒸汽產生時還低的蒸汽管網。再使用閃發蒸汽可以得到比以熱交換器將沖放蒸汽熱交換更具熱效及有利的熱能。
理論上，用在低溫場合的能量可用閃發蒸汽來取代新產生的蒸汽。雖然要引進這技術並不容易，但值得廣泛地研究。此方式廣泛地使用於石油化工業。
The recovery of flash steam saves on fresh top-up water and its treatment, although the main cost savings are in energy. The recovery of flash steam leads to much greater energy savings than with the simple collection of liquid condensate.
利用閃發蒸汽的主要目的固然在於節能，但也可節省生水添加量和水處理成本。利用閃發蒸汽回收能源效益會比單單收集凝結水的能源要大得多。
Energy Efficiency (2009) 3.2.14
