{ "id": "eph19gb2n2", "slug": "flash-evaporation", "title": "Flash evaporation", "updated": "2026-10-15", "topic_path": [ { "id": "technology", "label": "Technology and the built world", "api_url": "https://www.edgepedia.org/api/v1/topics/technology" }, { "id": "technology.engineering", "label": "Engineering and manufacturing", "api_url": "https://www.edgepedia.org/api/v1/topics/technology.engineering" }, { "id": "technology.engineering.engineering.chemical.biomedical", "label": "Chemical, biochemical, and biomedical engineering", "api_url": "https://www.edgepedia.org/api/v1/topics/technology.engineering.engineering.chemical.biomedical" }, { "id": "technology.engineering.engineering.chemical.biomedical.distillation-and-evaporation-methods", "label": "Distillation and evaporation methods", "api_url": "https://www.edgepedia.org/api/v1/topics/technology.engineering.engineering.chemical.biomedical.distillation-and-evaporation-methods" } ], "geo": [ { "id": "geo.nongeo.t1800.technology.engineering.engineering.chemical.biomedical", "label": "Non-geographic · 1800 to 1945: Chemical, biochemical, and biomedical engineering", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.technology.engineering.engineering.chemical.biomedical", "path": [ { "id": "geo.nongeo", "label": "Non-geographic", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo" }, { "id": "geo.nongeo.t1800", "label": "Non-geographic · 1800 to 1945", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800" }, { "id": "geo.nongeo.t1800.technology", "label": "Technology and the built world", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.technology" }, { "id": "geo.nongeo.t1800.technology.engineering", "label": "Engineering and manufacturing", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.technology.engineering" }, { "id": "geo.nongeo.t1800.technology.engineering.engineering.chemical.biomedical", "label": "Chemical, biochemical, and biomedical engineering", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.technology.engineering.engineering.chemical.biomedical" } ] }, { "id": "geo.nongeo.t1946.technology.engineering.engineering.chemical.biomedical", "label": "Non-geographic · 1946 to 2000: Chemical, biochemical, and biomedical engineering", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.technology.engineering.engineering.chemical.biomedical", "path": [ { "id": "geo.nongeo", "label": "Non-geographic", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo" }, { "id": "geo.nongeo.t1946", "label": "Non-geographic · 1946 to 2000", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946" }, { "id": "geo.nongeo.t1946.technology", "label": "Technology and the built world", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.technology" }, { "id": "geo.nongeo.t1946.technology.engineering", "label": "Engineering and manufacturing", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.technology.engineering" }, { "id": "geo.nongeo.t1946.technology.engineering.engineering.chemical.biomedical", "label": "Chemical, biochemical, and biomedical engineering", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.technology.engineering.engineering.chemical.biomedical" } ] } ], "excerpt": "Flash evaporation is the partial vaporization of a liquid when its pressure drops below saturation pressure, a violent, pressure-driven boiling that underlies flash distillation and multi-stage flash desalination.", "snippet": "Flash evaporation is the partial vaporization of a liquid when its pressure drops below saturation pressure, a violent, pressure-driven boiling that underlies flash distillation and multi-stage flash desalination.", "node": "technology.engineering.engineering.chemical.biomedical.distillation-and-evaporation-methods", "markdown": "# Flash evaporation\n\nFlash evaporation is the partial vaporization of a liquid that occurs when its pressure drops below the saturation pressure corresponding to its temperature, so that a portion of the liquid boils without external heat supply. It is a violent, pressure-driven form of boiling, and it underlies flash steam recovery, flash distillation, and multi-stage flash (MSF) desalination.[1](https://www.mdpi.com/1996-1073/16/21/7453) Because the vapor produced occupies far more volume than the liquid it leaves, roughly 1.67 m\\(^3\\)/kg of atmospheric steam against 0.00104 m\\(^3\\)/kg of water at 100 °C, a small flashed mass generates a large vapor volume.[2](https://www.engineeringtoolbox.com/flash-steam-generation-d_425.html)\n\n| Key fact | Value |\n|---|---|\n| Definition | Partial vaporization when liquid pressure falls below its saturation pressure[1](https://www.mdpi.com/1996-1073/16/21/7453) |\n| Flashed steam fraction (condensate) | \\( w = (h_{\\mathrm{il}} - h_{\\mathrm{fl}})/h_{\\mathrm{fe}} \\); 11% for 5 bar gauge condensate flashed to atmospheric pressure[2](https://www.engineeringtoolbox.com/flash-steam-generation-d_425.html) |\n| Vapor fraction calculation | Rachford-Rice equation, solved iteratively for \\( V/F \\)[3](https://www.myengineeringtools.com/references/pages/flash_distillation_material_balance.html) |\n| Thermodynamic status | A single-equilibrium-stage distillation[4](https://docs.intelligen.com/spd/SPDWebHelp/Bk7/AppendixD/The_Single-Stage_Flash.htm) |\n| Typical MSF plant | 19–28 stages, 10,000–40,000 m\\(^3\\)/day, 90–120 °C; GOR 8–12[5](https://www.mdpi.com/2227-9717/9/2/262) |\n| Installed capacity | RO 65%, thermal 28% (MSF 21%, MED 7%) of world capacity[6](https://mdpi-res.com/d_attachment/membranes/membranes-10-00265/article_deploy/membranes-10-00265-v2.pdf?version=1601522172) |\n| Flash chamber efficiency | Non-equilibrium factor \\( 1 - \\beta \\) of 0.24–0.66 at a top brine temperature of 112 °C[7](https://www.sciencedirect.com/science/article/abs/pii/S0011916498000186) |\n\n## How it works\n\nA flash unit is a single-equilibrium-stage distillation in which a feed is partially vaporized to give a vapor richer in the more volatile components; flash calculations combine a phase-equilibrium model with mass and energy balances.[4](https://docs.intelligen.com/spd/SPDWebHelp/Bk7/AppendixD/The_Single-Stage_Flash.htm) The flashed fraction is set by the enthalpy balance. For an adiabatic drum the feed enthalpy equals the enthalpy-weighted sum of the products,\n\n\\[ h_{\\mathrm{F}} = \\psi \\cdot h_{\\mathrm{V}} + (1 - \\psi) \\cdot h_{\\mathrm{L}} \\]\n\nwhere \\( \\psi \\) is the vapor fraction; the feed preheat temperature follows from \\( h_{\\mathrm{F}} \\) divided by the mixture heat capacity.[8](https://www.myengineeringtools.com/references/pages/flash_distillation_energy_balance.html) For flashing condensate or brine, the same balance gives the flashed fraction directly as \\( w = (h_{\\mathrm{il}} - h_{\\mathrm{fl}})/h_{\\mathrm{fe}} \\), with \\( h_{\\mathrm{il}} \\) and \\( h_{\\mathrm{fl}} \\) the initial and final liquid enthalpies and \\( h_{\\mathrm{fe}} \\) the evaporation enthalpy at the final condition. Condensate at 5 bar gauge (670.9 kJ/kg, 159 °C) flashed to 1 atm (419.0 kJ/kg, evaporation enthalpy 2257.9 kJ/kg) yields \\( w = 0.11 \\), i.e. 11 kg flash steam per 100 kg condensate.[2](https://www.engineeringtoolbox.com/flash-steam-generation-d_425.html) The single-stage problem has \\( 3n_{\\mathrm{c}} + 8 \\) variables and \\( 2n_{\\mathrm{c}} + 3 \\) equations, leaving \\( n_{\\mathrm{c}} + 5 \\) degrees of freedom; which two additional variables are specified defines the flash type (isothermal, adiabatic, PVf, bubble or dew point, and others).[4](https://docs.intelligen.com/spd/SPDWebHelp/Bk7/AppendixD/The_Single-Stage_Flash.htm)\n\n## How it is done\n\nIn flash distillation, the feed is throttled through a nozzle or valve into a flash drum where liquid and vapor separate under near-equilibrium conditions; a single stage gives limited separation.[9](https://moodle2.units.it/pluginfile.php/389467/mod_resource/content/3/01_Flash.pdf) The material balance is \\( F = V + L \\) with \\( F z_{i} = V y_{i} + L x_{i} \\), and for multicomponent mixtures the vapor fraction follows from the Rachford-Rice equation,\n\n\\[ \\sum_{i} \\frac{z_{i} \\cdot (K_{i} - 1)}{1 + (V/F) \\cdot (K_{i} - 1)} = 0 \\]\n\nwhich determines \\( V/F \\) without prior knowledge of the phase compositions and converges quickly with Newton-Raphson iteration.[3](https://www.myengineeringtools.com/references/pages/flash_distillation_material_balance.html)\n\nIn an MSF desalination plant the same step is repeated: brine passes through a sequence of chambers at successively lower pressure, flashing a little vapor at each stage. Brine-recirculation MSF is the most widely used configuration for large plants, normally with 15–40 stages.[10](https://www.deswater.com/DWT%5Farticles/vol%5F177%5Fpapers/177%5F2020%5F14.pdf) Modern plants use 19–28 stages, produce 10,000–40,000 m\\(^3\\)/day, and operate at 90–120 °C.[5](https://www.mdpi.com/2227-9717/9/2/262)\n\n## Origin\n\nThe multi-stage flash process was developed in Glasgow in 1956–57, and the first commercial units were commissioned in 1960; by 1965 installed MSF capacity had already exceeded the total installed capacity of all previous desalination methods.[11](https://www.desware.net/sample-chapters/d01/d01-010.pdf) A British patent application, No. 829820, was filed in September 1957 for an MSF process, and in 1958 two units were designed, one with 24 stages and a performance ratio of 5.8 and one with 40 stages and a performance ratio of 10.5, both commissioned successfully in 1960.[12](https://www.eolss.net/sample-chapters/c07/E6-144-28.pdf) An early engineering assessment of the flash evaporator appeared in 1960, when A. Frankel published \"Flash Evaporators for the Distillation of Sea-Water\" in Proceedings of the [Institution of Mechanical Engineers](https://www.edgepedia.org/institution-of-mechanical-engineers).[13](https://doi.org/10.1243/pime_proc_1960_174_030_02) Fundamental laboratory experiments that quantified flashing efficiency through the non-equilibrium fraction were published in 1972 by Osamu Miyatake and colleagues in Nippon Kaisui Gakkai-Shi.\n\n## Variants\n\nBrine-recirculation versus once-through MSF is the main configuration choice. Once-through MSF (MSF-OT) performs identically to brine-recirculation MSF as long as feed seawater stays above 25 °C, making it the preferred large-scale thermal scheme in equatorial regions where seawater holds near 28 °C year-round; where winter seawater falls to 5–15 °C, performance deteriorates unless low-temperature stage volumes are greatly increased.[14](https://journals.sagepub.com/doi/10.1243/095765002320183559) Enhanced MSF extracts up to 0.773 of the flash vapor, raising GOR by 74.1% with an average 21.8% reduction in stage brine concentration.[10](https://www.deswater.com/DWT%5Farticles/vol%5F177%5Fpapers/177%5F2020%5F14.pdf) A 2025 loop-configured MSF with reservoir tanks raises the final flashing-stage outlet temperature to 65–70 °C against about 40 °C in classical MSF, keeping last-stage pressure near 30 kPa and cutting the required vacuum by 19–25%; it eliminates thermal vapor compressors and internal heat-transfer tubes by condensing vapor through direct contact with cooler saline water.[15](https://www.nature.com/articles/s41598-025-18858-2) On the modeling side, a modified pressure-driven Lee model combined with the VOF method computes the phase-change driving force from local temperature and static pressure rather than a fixed saturation temperature, capturing the transient from initial rapid boiling to dynamic equilibrium.[1](https://www.mdpi.com/1996-1073/16/21/7453)\n\n## Applications\n\nDesalination dominates: MSF accounts for about 35% of water desalination facilities worldwide by one count[16](https://pmc.ncbi.nlm.nih.gov/articles/PMC9457763/), and produces about 94% of thermal-desalination water in the GCC countries and 53% of total capacity in MENA.[10](https://www.deswater.com/DWT%5Farticles/vol%5F177%5Fpapers/177%5F2020%5F14.pdf) Beyond desalination, flash evaporation is used in wine processing, geothermal energy, and waste-heat recovery,[1](https://www.mdpi.com/1996-1073/16/21/7453) and flash steam from condensate letdown can be recovered to heat consumers demanding less than 100 °C, such as HVAC and hot-water service systems.[2](https://www.engineeringtoolbox.com/flash-steam-generation-d_425.html)\n\n## Limitations and alternatives\n\nScaling and corrosion are the most costly operational problems in thermal desalination; a 90% increase in brine-heater fouling lowers the heat-transfer coefficient and top brine temperature, cutting the desalination rate by 5.5% while raising steam consumption.[17](https://www.desware.net/sample-chapters/d06/E6-107-11.pdf) Flashing itself is incomplete: at a top brine temperature of 112 °C the non-equilibrium factor \\( 1 - \\beta \\) ranges from 0.24 to 0.66, and reaching equilibrium would require more superheat, flashing surface area, active nucleation sites, and brine residence time.[7](https://www.sciencedirect.com/science/article/abs/pii/S0011916498000186) The average flashing heat flux is 100–200 kW/m\\(^2\\), correlated as \\( Q'' = 0.055 (T_{\\mathrm{b(av)}} - T_{\\mathrm{sat}})^{3} \\).[7](https://www.sciencedirect.com/science/article/abs/pii/S0011916498000186) Designers correct for the gap between real and ideal evaporation with the non-equilibrium allowance (NEA), estimated for one plant model as \\( \\mathrm{NEA} = 195\\, h_{\\mathrm{b}}^{1.1} (\\mathrm{SL}_{\\mathrm{st}} \\times 10^{-3})^{0.5} (\\Delta T_{\\mathrm{B}})^{0.25} (\\mathrm{TV})^{2.5} \\).[18](https://www.degruyterbrill.com/document/doi/10.1515/cppm-2021-0040/html)\n\nAgainst alternatives, MSF's GOR of 8–12 kg distillate per kg steam trails MED's 10–16.[5](https://www.mdpi.com/2227-9717/9/2/262) SWRO energy demand is about 3–4 times lower than MSF and 2–3 times lower than MED; typical water costs are 0.8–1.5 USD/m\\(^3\\) for MSF, 0.7–1.2 for MED, and 0.5–1.2 for RO, and in a comparative simulation of 45,000 ppm wastewater both MSF and RO reached 500 ppm, with MSF giving higher yield but RO being simpler with lower energy demand.[6](https://mdpi-res.com/d_attachment/membranes/membranes-10-00265/article_deploy/membranes-10-00265-v2.pdf?version=1601522172) RO is nonetheless not recommended for feed above 45,000–47,000 ppm salinity, although thermal processes, which can handle higher-salinity inputs than conventional reverse osmosis, are not strictly required at that threshold.[17](https://www.desware.net/sample-chapters/d06/E6-107-11.pdf) • [19](https://www.energy.gov/cmei/ito/desalination-basics)\n\n## References\n\n1. [Numerical Simulation Method for Flash Evaporation with Circulating Water Based on a Modified Lee Model (Energies, 2023)](https://www.mdpi.com/1996-1073/16/21/7453)\n2. [Flash Steam Generation - Fundamental Physics (The Engineering ToolBox)](https://www.engineeringtoolbox.com/flash-steam-generation-d_425.html)\n3. [Flash Distillation Material Balance | Process Engineering Reference Sheets](https://www.myengineeringtools.com/references/pages/flash_distillation_material_balance.html)\n4. [The Single-Stage Flash (SuperPro Designer documentation)](https://docs.intelligen.com/spd/SPDWebHelp/Bk7/AppendixD/The_Single-Stage_Flash.htm)\n5. [Commercial Thermal Technologies for Desalination of Water from Renewable Energies: A State of the Art Review (Processes, MDPI)](https://www.mdpi.com/2227-9717/9/2/262)\n6. [Modelling and Optimisation of Multi-Stage Flash Distillation and Reverse Osmosis for Desalination of Saline Process Wastewater Sources (Membranes)](https://mdpi-res.com/d_attachment/membranes/membranes-10-00265/article_deploy/membranes-10-00265-v2.pdf?version=1601522172)\n7. [The non-equilibrium factor and the flashing evaporation rate inside the flash chamber of a multi-stage flash desalination plant (Desalination)](https://www.sciencedirect.com/science/article/abs/pii/S0011916498000186)\n8. [Flash Distillation Energy Balance | Process Engineering Reference Sheets](https://www.myengineeringtools.com/references/pages/flash_distillation_energy_balance.html)\n9. [La Simulazione di Processo, Flash Distillation lecture notes (University of Trieste)](https://moodle2.units.it/pluginfile.php/389467/mod_resource/content/3/01_Flash.pdf)\n10. [MSF challenges and survivals (I.S. Al-Mutaz, Desalination and Water Treatment 177 (2020) 14–22)](https://www.deswater.com/DWT%5Farticles/vol%5F177%5Fpapers/177%5F2020%5F14.pdf)\n11. [The Development of Multistage Flash Distillation (R.S. Silver)](https://www.desware.net/sample-chapters/d01/d01-010.pdf)\n12. [Multi-stage Flash Desalination (M.A. Darwish), UNESCO-EOLSS sample chapter](https://www.eolss.net/sample-chapters/c07/E6-144-28.pdf)\n13. [A. Frankel (1960). Flash Evaporators for the Distillation of Sea-Water. Proceedings of the Institution of Mechanical Engineers.](https://doi.org/10.1243/pime_proc_1960_174_030_02)\n14. [Performance of the once-through multistage flash desalination process (El-Dessouky, Ettouney, Al-Juwayhel)](https://journals.sagepub.com/doi/10.1243/095765002320183559)\n15. [Design and optimization of a novel loop configured MSF desalination system with reservoir tanks for minimal liquid discharge (Scientific Reports, 2025)](https://www.nature.com/articles/s41598-025-18858-2)\n16. [Water Desalination Using the Once-through Multi-Stage Flash Concept: Design and Modeling](https://pmc.ncbi.nlm.nih.gov/articles/PMC9457763/)\n17. [A Perspective of Thermal Type Desalination: Technology, Current Development, and Thermodynamics Analysis (DESWARE/EOLSS)](https://www.desware.net/sample-chapters/d06/E6-107-11.pdf)\n18. [Modelling and simulation of industrial multistage flash desalination (Azzour plant, Kuwait)](https://www.degruyterbrill.com/document/doi/10.1515/cppm-2021-0040/html)\n19. [Desalination Basics](https://www.energy.gov/cmei/ito/desalination-basics)\n\n---\n*Topic: Encyclopedia › Technology and the built world › Engineering and manufacturing › Chemical, biochemical, and biomedical engineering › Distillation and evaporation methods*\n\n*Initially written Sep 29, 2026 · Edited: Oct 15, 2026*\n\n*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*\n\nLicense: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license\n", "same_as": [], "url": "https://www.edgepedia.org/flash-evaporation", "markdown_url": "https://www.edgepedia.org/flash-evaporation.md", "license": { "name": "Edgepedia Community License 1.0", "url": "https://www.edgepedia.org/edgepedia/license", "summary": "Free with credit, commercial use included. AI training is open to everyone. For other uses, organizations over USD 100M in revenue or 100M monthly users license separately.", "spdx": "LicenseRef-Edgepedia-Community-1.0" }, "credit": "\"Flash evaporation\", Edgepedia (EdgeChat), https://www.edgepedia.org/flash-evaporation. Edgepedia Community License 1.0.", "credit_md": "\"[Flash evaporation](https://www.edgepedia.org/flash-evaporation)\", Edgepedia (EdgeChat), [https://www.edgepedia.org/flash-evaporation](https://www.edgepedia.org/flash-evaporation). [Edgepedia Community License 1.0](https://www.edgepedia.org/edgepedia/license).", "credit_html": "\"Flash evaporation\", Edgepedia (EdgeChat), https://www.edgepedia.org/flash-evaporation. Edgepedia Community License 1.0.", "speakable": "Flash evaporation is the partial vaporization of a liquid when its pressure drops below saturation pressure, a violent, pressure-driven boiling that underlies flash distillation and multi-stage flash desalination." }