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Condensation Control by the Numbers
Dew point, cold surfaces, breathing, heaters, desiccants and insulation for enclosures and cold pipes, worked through one coastal pump station
Keep water out of electrical enclosures and off cold pipes with numbers: dew point, night-sky cooling, breathing, heater sizing, desiccants, insulation and humidity logging.
Start readingAbout this book
Water causes more trouble in electrical and mechanical equipment than heat does, and most of it never arrives as rain. It arrives as vapour, carried in ordinary air, and turns to liquid on whatever surface happens to be colder than the air’s dew point. A control cabinet that passed its ingress test can hold a puddle on its gland plate. A cold-water main can drip a litre an hour onto the switchboard beneath it. A sealed instrument box can fog on the inside of its own window.
This book works through condensation with numbers. It starts with the physics of water in air: vapour pressure, relative humidity and dew point. It then finds the cold surfaces, including the ones a clear night sky makes, works out how enclosures breathe in moist air and through which paths, and compares the four strategies for keeping water out: seal, vent, ventilate or heat. It sizes anti-condensation heaters and desiccant charges, insulates a cold pipe so it stays dry, connects humidity to corrosion and salts, and ends with the measurements that prove a fix works. Every chapter follows one running case, includes a plausible mistake and how it was found, and ends with an exercise and its worked answer.
It is for electrical, mechanical and maintenance engineers, control-panel builders, technicians and technical students. It assumes school algebra and physics. Readers who want the design of a whole electronic product’s enclosure should read Electronics Packaging by the Numbers; readers who need humidity life testing should read Accelerated Life Testing by the Numbers. This book links to both rather than repeating them.
The running case
Every chapter follows one fictional site. This is an illustration. A water utility runs a small pump station, PS-7, a few kilometres from the coast in New South Wales. Its control cabinet, CC-7, is a powder-coated sheet-steel floor-standing enclosure 1,200 mm high, 800 mm wide and 400 mm deep, standing on a concrete plinth in the open, with an IP55 rating and a bottom gland plate. Its cables rise from an underground pit through four conduits.
The cabinet holds a variable-speed drive for the duty pump, which dissipates about 280 W while the pump runs, plus a controller, power supplies and a telemetry modem that dissipate about 30 W all the time. In summer the pump runs about 8 hours a day, in daylight. A separate polycarbonate telemetry box on the same pole holds a radio and a battery. Beside the cabinet, a small block-wall pump room houses the pumps and a DN100 steel delivery main that carries bore water at about 17 °C.
The utility, its site and its records are invented. None describes KEVOS® work or a real organisation. Weather conditions in the case are typical values chosen for the illustration, not records for any location.
How to use the book
Read Chapter 1 first: it sets out the quantities and formulas that every later chapter uses. Chapters 2 and 3 explain where condensation forms and how the water gets in. Chapter 4 chooses a strategy, and Chapters 5, 6 and 7 size the means: heaters, desiccants and pipe insulation. Chapter 8 explains why humidity matters below the dew point, through corrosion and salts, and Chapter 9 sets out how to measure, check and prove a fix. Every calculation was checked by running it. Vapour pressures come from the Magnus formula with the Alduchov–Eskridge coefficients, checked against the IAPWS-IF97 saturation equation and NIST data.
Source limits and professional responsibility
The physics is standard and the property data is published. Heat-transfer coefficients, sky temperatures, air-exchange rates and weather conditions in the running case are assumptions for the illustration, and a real installation should be judged on its own measurements. Electrical enclosures, their heaters and their contents are subject to electrical safety rules and the equipment makers’ instructions, and pump stations carry their own work health and safety hazards, including confined spaces in pits. Electrical work, the selection of rated equipment and the approval of designs are for qualified people working under the rules that apply. This book teaches how condensation happens and how to calculate the means of preventing it. It is not a design for any installation.
Who this book is for
Electrical, mechanical and maintenance engineers, control-panel builders, technicians and technical students who design, install or look after outdoor enclosures, pump stations, plant rooms and chilled or cold-water pipework.
What you will learn
- Calculate vapour pressure, relative humidity, dew point and absolute humidity, and know how accurate the formulas are.
- Predict which surfaces in an enclosure fall below the dew point, including roofs cooled by a clear night sky.
- Estimate how much water an enclosure draws in by breathing and through cable conduits, and find the dominant path.
- Size and control an anti-condensation heater, size a desiccant charge, and choose insulation that keeps a cold pipe dry.
- Log humidity and surface temperatures, check the sensors, and prove that a condensation fix works.
Inside this book
Chapters
- 01Water in air: vapour pressure, humidity and dew pointWhat vapour pressure is, why it depends only on temperature, how relative humidity, dew point and absolute humidity follow from it, and why dew point is the number to watch.
- 02Cold surfaces and the night skyWhy surfaces fall below air temperature, how a clear night sky cools an enclosure roof by radiation, how much water then forms, and why a heater alone may not stop it.
- 03Breathing: how enclosures draw in waterThermal and barometric breathing, stack flow and cable conduits: how much moist air each path brings into an enclosure, and how a warm, wet pit turned out to be the real source.
- 04Seal, vent, ventilate or heat: choosing the strategyWhat an IP rating does and does not say about humidity, and how sealing, venting, fan ventilation and heating each change the dew point and the surfaces inside an enclosure.
- 05Sizing and controlling an anti-condensation heaterSet the temperature lift a heater must hold, size it with the wall-loss method and its outdoor allowance, place and control it, estimate its energy, and avoid the frost thermostat trap.
- 06Desiccants for sealed enclosuresHow desiccants hold water, what a unit of desiccant means, how to measure the water a sealed box takes in, and how to size a charge and set its replacement interval.
- 07Cold pipes, sweating and insulationHow much water a cold pipe gathers in humid air, the insulation thickness that keeps its surface above the dew point, why a vapour barrier matters, and the valves left bare for access.
- 08Corrosion, salts and the humidity that mattersWhy metals corrode below the dew point, how salts take up water at set humidities, how to count wet hours, measure corrosion with coupons, and why CC-7's hygrostat moved from 75% to 65%.
- 09Measuring humidity and proving the fixHumidity sensors and their errors, dew point uncertainty, checking a logger against salt solutions, measuring a heater's real performance, and the evidence that shows a fix works.
Apply This in Practice
Use these KEVOS® tools to try the method from this reading with your own numbers.
Explained in this book
Moist Air Dew Point and Surface Condensation Check (Humidity Ratio, Enthalpy, Dew-Point Margin)
Works out the dew point, humidity ratio and vapour pressure from temperature and humidity and checks a surface against the dew point with a margin.
Used in Water in air: vapour pressure, humidity and dew point, Cold pipes, sweating and insulation
Applies this method
Corrosion Rate from Mass Loss
Turns a corrosion coupon's mass loss, area, exposure time and density into a rate in mm a year, the measurement used to compare environments.
Applies this method
Enclosure Cooling Airflow (Heat Load, Wall Losses, Fan Airflow Needed)
Sizes an enclosure's ventilation fan from its heat load, wall losses and allowed temperature rise; its wall-loss term is also the first step of heater sizing.
Used in Seal, vent, ventilate or heat: choosing the strategy
Applies this method
Gas Volume at Pressure and Temperature
Converts a gas volume between temperatures at constant pressure, which gives the air an enclosure draws in each time it cools.
Applies this method
Heat Loss from an Insulated Pipe
Gives the outer surface temperature and heat flow of an insulated pipe; entered with a cold pipe, the surface temperature is compared with the room's dew point.
Applies this method
Instrument Calibration Check (Errors, Accuracy Class, Interpolated Correction)
Checks an instrument's errors at five reference points against an accuracy class and corrects an in-service reading; used here for a humidity transmitter against salt cells.
Worked example
Radiant Heat Loss
Gives the net radiant exchange between a surface and large surroundings; with the sky's effective temperature as the surroundings, it shows how much an enclosure roof radiates on a clear night.
Used in Cold surfaces and the night sky
Related Tools and Examples
Useful alongside
Vapour Pressure and Boiling Check (Clausius–Clapeyron, Boiling Point at Pressure, Margin)
Gives the vapour pressure of water or another liquid at any temperature from one known point, the quantity this chapter builds humidity on.
Related to Water in air: vapour pressure, humidity and dew point
Learning pathway
Check whether a cold surface will sweat
Learn how vapour pressure sets the dew point, then check your own surface against it with a margin.
1 Learn
Read Water in air: vapour pressure, humidity and dew point in Condensation Control by the Numbers.
What vapour pressure is, why it depends only on temperature, how relative humidity, dew point and absolute humidity follow from it, and why dew point is the number to watch.
2 Apply
Open Moist Air Dew Point and Surface Condensation Check (Humidity Ratio, Enthalpy, Dew-Point Margin) and enter your own figures.
At this air temperature, relative humidity and pressure, what are the dew point, humidity ratio and enthalpy of the air, and will a surface at this temperature stay dry with the margin you need?
3 Verify
Read the tool’s feasibility and verification checks under the result, and test your answer against its stated limitations:
- Uniform air and steady conditions; real condensation often happens where air is trapped against a cold surface, at night, or when warm humid air enters a cool enclosure.
- Air temperatures from 0 to 90 °C; relative humidity is taken over liquid water.
- Hygrometers are often only accurate to a few per cent of relative humidity, which moves the dew point by about a degree; the margin should cover this.
- Example values are illustrations. Replace them with values for your actual site.
4 Improve
Download the decision report or copy the link with your inputs to keep a record, then change one assumption at a time and compare.
Next reading: Cold pipes, sweating and insulation in Condensation Control by the 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 29 October 2026.
- C. Kittel and H. Kroemer. Thermal Physics, second edition (1980). Supplied source, © W. H. Freeman and Company, in copyright. Read as a subject map only: thermal radiation and the Stefan–Boltzmann law, chemical potential, phase equilibrium, the vapour pressure curve and the Clausius–Clapeyron relation, and kinetic theory. No text, derivations as set out, figures, tables or problems are reproduced; the treatment, case, numbers and exercises in this book are its own.
- O. A. Alduchov and R. E. Eskridge, Journal of Applied Meteorology 35, 601–609. Improved Magnus form approximation of saturation vapor pressure (1996). Source of the Magnus-form coefficients used for every vapour pressure and dew point in the book: 610.94 Pa, 17.625 and 243.04 °C.
- International Association for the Properties of Water and Steam. Revised release on the IAPWS Industrial Formulation 1997 for the thermodynamic properties of water and steam (2007). The saturation-pressure equation of IAPWS-IF97 was used to check the Magnus formula; it was itself checked against the NIST Chemistry WebBook water saturation data. The Magnus values agree within 0.25% from 0 to 50 °C.
- National Institute of Standards and Technology. NIST Chemistry WebBook, SRD 69: thermophysical properties of water (2026). Reference saturation pressures of water used to confirm the formulas above.
- AutomationDirect. Enclosure heating and heater selection (2026). Maker's heater sizing method used in Chapter 5: heat transmission coefficients of 5.5 W/m²·K for painted steel, 3.7 for stainless steel, 12 for aluminium and 3.5 for plastic, the formula P = A × k × ΔT − P(internal), and the doubling of the first term for outdoor enclosures.
- Campbell Scientific. Desiccant packs for enclosures (MIL-D-3464E unit definition) (2026). Source of the definition of a unit of desiccant used in Chapter 6: at least 3 g of water adsorbed at 20% relative humidity and at least 6 g at 40%, at 25 °C, citing MIL-D-3464E. The specification itself was not read.
- L. Greenspan, Journal of Research of the National Bureau of Standards 81A, 89–96. Humidity fixed points of binary saturated aqueous solutions (1977). Source of the equilibrium relative humidities over saturated salt solutions at 25 °C used in Chapters 8 and 9 (lithium chloride 11.30%, magnesium chloride 32.78%, magnesium nitrate 52.89% and sodium chloride 75.29%).
- International Organization for Standardization. ISO 9223 Corrosion of metals and alloys: corrosivity of atmospheres (editions as called up). Named by number and scope. The conventional estimate of time of wetness as the hours with relative humidity above 80% and temperature above 0 °C is described from published secondary accounts; no values from the standard are quoted.
- Standards Australia. AS 60529 Degrees of protection provided by enclosures (IP Code) (edition as called up). Named by number and scope as the basis of IP ratings. The book explains what an IP test does and does not show about humidity; it quotes no test details.