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Solvent Vapours by the Numbers

Vapour pressure, mixtures, flammable headspaces, evaporation, blush, ventilation, ovens and solvent recovery for workshops that clean, paint and bond

How solvents evaporate, mix, burn and condense in a workshop, and how to calculate headspaces, evaporation, ventilation, oven exhaust and recovery to control them.

11 chapters · About 110 minutes · Free to read · Intermediate operations and process engineering

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KEVOS® LibraryOPERATIONSIMPACTLIKELIHOODSolvent Vapoursby the NumbersKEVOS® ORIGINAL BOOK11 CHAPTERS

About this book

Organic solvents are among the most useful liquids in a workshop. They clean oil from metal, carry paint to a surface, thin adhesives and wash spray guns. They are also volatile, flammable and harmful to breathe, and most of what a workshop buys ends up in its air. Controlling them well is a matter of numbers: how fast a solvent evaporates, how much vapour sits above it, at what concentration the vapour can burn, how much air dilutes it, and how much can be recovered.

This book teaches that arithmetic. It starts with where purchased solvent goes and with vapour pressure, the property that governs nearly everything else. It covers blends, flammable headspaces in drums and tanks, evaporation from open surfaces, and the cooling that makes coatings blush. It then turns to control: dilution and purge ventilation, local exhaust, ovens kept below the flammable limit, recovery by condensing and distilling, and substitution with less hazardous solvents.

It is for production, maintenance and process engineers, workshop supervisors and safety advisers in businesses that clean, paint, coat or bond with organic solvents. It assumes algebra, logarithms and the ideal gas law. Each chapter works the running case with real property data, includes one plausible mistake and how it was found, and ends with an exercise and its worked answer.

The running case

Every chapter follows one fictional workshop. This is an illustration. Plant S makes painted steel machine frames and guards, about 60 frames a week. Its solvent use has grown with the business:

  • Acetone for wiping parts before painting and for cleaning spray guns, bought in 205 L drums;
  • A paint thinner, a blend of toluene, 2-butanone (methyl ethyl ketone, MEK) and ethyl acetate, used to thin a two-pack polyurethane paint and to flush the spray equipment;
  • The paint itself, whose solvent evaporates in the spray booth and in a low-temperature curing oven.

The story begins with three events in one summer: a fire-service inspection that asked how the plant knew its oven was safe, a near miss when a worker began to cut an “empty” drum, and a batch of frames whose gloss finish came out milky. The plant’s operations engineer is asked to put numbers on the plant’s solvents and to cut their use.

The plant, its products, its recipes, its suppliers and its measurements are invented. None describes KEVOS® work or a real company. Solvent properties are real values from the NIST Chemistry WebBook and the NIOSH Pocket Guide, cited where used; the thinner’s composition is illustrative.

How to use the book

Read Chapters 1 to 3 in order: they cover where solvent goes, vapour pressure and blends, which every later chapter uses. Chapter 4 deals with flammable headspaces, Chapter 5 with evaporation and Chapter 6 with evaporative cooling and blush. Chapters 7 to 9 cover ventilation of rooms, local exhaust and ovens. Chapter 10 covers recovery and Chapter 11 substitution and proving a reduction.

KEVOS® tools are linked where they perform a step of the method, with a note of exactly what each one calculates and where its model stops. Where a tool solves a related problem rather than the chapter’s own, the text says so.

Source limits and professional responsibility

The relations used here are the established ones of phase equilibrium and mass transfer: the Clausius–Clapeyron relation, Raoult’s law, Le Chatelier’s mixing rule, diffusion-limited evaporation and well-mixed dilution. Each chapter states the assumptions they need, and real solvents and real rooms often depart from them. Standards and codes are named by number and scope only.

Flammable atmospheres and airborne contaminants are regulated hazards. In Australia, the work health and safety laws, Safe Work Australia’s model codes and workplace exposure limits, the hazardous area classification and flammable liquid storage standards, and the solvent’s safety data sheet govern real decisions, together with competent people: a hazardous area classification by a qualified person, ventilation designed and tested by a competent engineer, and exposure monitoring by an occupational hygienist where needed. This book is educational. It explains how the numbers are made and how far to trust them; it is not a risk assessment, a hazardous area classification or a substitute for any of these.

Who this book is for

Production, maintenance and process engineers, workshop supervisors and safety advisers in businesses that clean, paint, coat or bond with organic solvents.

What you will learn

  • Account for where purchased solvent goes, and estimate vapour pressure at any temperature from one known point with the Clausius–Clapeyron relation.
  • Work out the vapour above solvent blends with Raoult's law and their lower flammable limit with Le Chatelier's rule.
  • Screen closed containers and headspaces against the lower explosive limit, and explain why empty drums and over-rich spaces are dangerous.
  • Estimate evaporation, evaporative cooling and blush, and size dilution, purge and local exhaust ventilation and oven exhaust.
  • Plan solvent recovery by condensing or vacuum distillation, choose substitutes with a solubility screen and prove a reduction in use.

Inside this book

Chapters

  1. 01Where the solvent goesA solvent mass balance for a workshop: what is bought, what leaves as waste and what evaporates, the three hazards of solvent vapour, and the order in which to control them.
  2. 02Vapour pressure and boilingWhat vapour pressure is, how it rises steeply with temperature by the Clausius–Clapeyron relation, checking it against Antoine data, boiling, and ranking solvents by volatility.
  3. 03Blends, Raoult's law and mixed vapoursThe vapour above a solvent blend by Raoult's law, why it is richer in the volatile parts, the flammable limit of mixed vapours by Le Chatelier's rule, and how blends change as they evaporate.
  4. 04Flammable headspacesThe saturated vapour in a closed drum or tank against its explosive limits, how it compares with the flash point, why empty drums and over-rich spaces are dangerous, and purging.
  5. 05Evaporation from open surfacesHow fast solvent evaporates from open containers, wipes and spills, the mass transfer coefficient, measuring it by weighing, and what lids, freeboard and cooler liquid save.
  6. 06Evaporative cooling, blush and condensationHow evaporating solvent cools a surface, the limiting temperature of a wet film, how much a real part cools, and why humid air condenses on fresh coatings and makes them blush.
  7. 07Dilution and purge ventilationHow much air dilutes a vapour release below a fire limit and below a health limit, mixing factors, build-up and purge times, and why the air at a worker's face can be far above the room average.
  8. 08Capturing vapour at the sourceLocal exhaust hoods for solvent work, capture velocity and how fast it falls with distance from a hood, sizing hood airflows, checking the duct and fan, and commissioning by measurement.
  9. 09Ovens and dryers below the LELThe solvent load an oven receives, sizing its exhaust from the peak release to stay below a fraction of the lower explosive limit, the effect of oven volume, and why not to throttle it.
  10. 10Recovering solventWhen condensing solvent from an air stream can and cannot work, the saturation limit on recovery, distilling dirty thinner under vacuum to keep it cool, and what a small still saves.
  11. 11Substitution, reduction and proofChoosing a less volatile substitute that still cleans and spares plastic parts, measuring use per part, proving a reduction with a fair trial, and the solvent balance before and after.

Apply This in Practice

Use these KEVOS® tools to try the method from this reading with your own numbers.

References and source notes

The manuscript and worked examples are original KEVOS® writing. References inform particular concepts; their authors do not endorse this book. Web references were consulted on 31 October 2026.

  • L. D. Landau and E. M. Lifshitz; revised by E. M. Lifshitz and L. P. Pitaevskii; translated by J. B. Sykes and M. J. Kearsley. Statistical Physics, Part 1, third edition (Course of Theoretical Physics, Volume 5) (1980). Pergamon Press, all rights reserved. Only the title pages and contents were read, as a subject map for the chapters on phase equilibrium, the Clapeyron–Clausius formula, solutions and mixtures of ideal gases. No text, equations, figures or problems are taken from it.
  • National Institute of Standards and Technology. NIST Chemistry WebBook, NIST Standard Reference Database 69 (current edition). Phase-change data for acetone, toluene, ethyl acetate, 2-butanone and isopropanol: Antoine coefficients and their temperature ranges, normal boiling points and enthalpies of vaporisation, used for the vapour pressures in Chapters 2 to 10.
  • National Institute for Occupational Safety and Health. NIOSH Pocket Guide to Chemical Hazards (current edition). Vapour pressures at 20 °C, lower and upper explosive limits, flash points and specific gravities for the five solvents, and the recommended exposure limit for acetone quoted in Chapter 7.
  • D. Mackay and R. S. Matsugu. Evaporation rates of liquid hydrocarbon spills on land and water (1973). Canadian Journal of Chemical Engineering 51, 434–439. A widely used mass transfer correlation for evaporation from pools, described in Chapter 5.
  • C. M. Hansen. Hansen Solubility Parameters: A User's Handbook, second edition (2007). CRC Press. Source of the solvent solubility parameters used in Chapter 11.
  • H. Le Chatelier. Estimation du grisou par les limites d'inflammabilité (1891). Annales des Mines. Origin of the mixing rule for the flammable limits of gas mixtures used in Chapter 3; cited from secondary accounts.
  • Safe Work Australia. Managing risks of hazardous chemicals in the workplace, model Code of Practice (current edition). Named for scope in Chapters 1 and 7: the model code under the work health and safety laws for identifying, assessing and controlling risks from hazardous chemicals. Workplace exposure limits for airborne contaminants are also published by Safe Work Australia and are not reproduced here.
  • Standards Australia; Standards New Zealand. AS/NZS 60079.10.1 (classification of areas: explosive gas atmospheres), AS 1940 (storage and handling of flammable and combustible liquids) and AS/NZS 4114 (spray painting booths and designated spray painting areas) (current editions). Named for scope only in Chapters 4, 7 and 9. Their requirements and values govern real installations and are not reproduced.
  • National Fire Protection Association. NFPA 86, Standard for Ovens and Furnaces (current edition). Named for scope only in Chapter 9, as a standard that sets ventilation requirements for ovens that release flammable vapours.

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