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Humidity Control by the Numbers
Moist air for air-handling plant: properties, the chart, loads, mixing, humidifying, cooling coils, reheat and evaporative cooling, in one print hall
Size and check air-handling processes with moist-air balances: properties, the chart, room loads, mixing, heating and humidifying, cooling coils, reheat, evaporative cooling and condensation.
Start readingAbout this book
An air-handling unit is a row of simple processes: two airstreams mix, a coil heats, a humidifier adds water, another coil cools and wrings water out, a fan adds a little heat. Each process obeys a mass balance for the dry air, a mass balance for the water and an energy balance, and the whole art of psychrometrics is keeping those three balances on the right basis. Most humidity problems in real plant trace back to one of them being skipped.
This book works through the processes one at a time. It sets out the properties of moist air and the chart that maps them, shows how to measure humidity well enough to trust, and turns a room’s sensible and latent loads into the air the unit must supply. It then calculates mixing, winter heating and humidifying, summer cooling and dehumidifying, reheat and humidity control, and evaporative cooling, and finishes with condensation, commissioning and the checks that keep a system honest through the seasons.
It is for mechanical and building services engineers, HVAC and process technicians, facility and production managers and students who know what relative humidity is and want to calculate with it. It assumes algebra and the idea of specific heat. Each chapter works one running case with real numbers, includes one plausible mistake and how it was found, links the KEVOS® tool that performs the chapter’s calculation where one exists, and ends with an exercise and its worked answer.
The running case
Every chapter follows one fictional plant. This is an illustration. A carton printer runs sheet-fed presses in a hall 60 m long, 30 m wide and 8 m high in an inland suburb of a large Australian city. The printer specifies 22 ± 2 °C and 50 ± 5% relative humidity all year, because paperboard takes up and gives off moisture with the air around it and changes size, flatness and static charge as it does.
- The air-handling unit: a mixing box taking 3.0 m³/s of outdoor air and 5.0 m³/s of return air, filters, a steam humidifier, a hot-water heating coil, a chilled-water cooling coil and a draw-through supply fan absorbing 15 kW.
- Summer loads: 80 kW sensible and 9 kW latent on the humid design day.
- Winter loads: a 25 kW sensible surplus from the presses and 4 kW latent.
- Design weather: a humid summer day of 29 °C and 70%, a hot dry day of 38 °C and 20%, a winter day of 5 °C and 80%, and a frosty morning of 0 °C and 95%.
- The warehouse: an adjoining finished-goods store cooled in summer by direct evaporative coolers handling 6.0 m³/s.
The plant, its loads, the weather figures and any prices are invented. None describes KEVOS® work or a real business. Standard atmospheric pressure, 101.325 kPa, is used throughout.
How to use the book
Read Chapters 1 to 4 in order: they set out the properties, the chart, measurement and the room balance that every later chapter uses. Chapters 5 to 9 each take one process and can be read in any order once the supply air state of Chapter 4 is understood. Chapter 10 brings the processes together for commissioning and the seasons.
Every property is calculated on the basis of one kilogram of dry air, with the same formulas the linked KEVOS® tools use: the Hyland–Wexler saturation pressure, the ideal-gas humidity ratio and the standard moist-air enthalpy. Values are given to more figures than a measurement could justify, so that a reader can follow each step.
Source limits and professional responsibility
The balances in this book are exact within their stated assumptions of steady flow and ideal-gas moist air. Room loads, design weather, equipment ratings and sensor behaviour are illustrations; real values come from a load calculation for the actual building, the site’s weather records, manufacturers’ data and measurement on site.
Humidifiers, evaporative coolers, cooling towers and other water-using air-handling plant carry a risk of Legionella and other microbial growth, and their installation and maintenance are regulated by state and territory health law and by AS/NZS 3666. Refrigeration, steam and electrical work need licensed trades. This book teaches how to calculate and check air-handling processes. It is educational and is not a design for any building.
Who this book is for
Mechanical and building services engineers, HVAC and process technicians, facility and production managers, and students who need to design, check or commission air-handling plant that controls humidity.
What you will learn
- Calculate humidity ratio, enthalpy, specific volume and dew point for any air state, on a dry-air basis.
- Plot air-handling processes on the psychrometric chart and read sensible heat ratios from it.
- Turn room sensible and latent loads into the supply air temperature, moisture content and airflow.
- Calculate mixing, heating, humidifying, cooling and dehumidifying processes with mass and energy balances.
- Judge reheat, evaporative cooling and condensation risks, and commission an air-handling unit with measurements that prove it works.
Inside this book
Chapters
- 01Moist air in numbersThe properties an air-handling engineer works with: humidity ratio, enthalpy, specific volume, dew point and relative humidity, on a dry-air basis, from temperature, humidity and pressure.
- 02Reading and drawing on the psychrometric chartHow the psychrometric chart is built, the six air processes as directions on it, mixing lines, sensible heat ratio and the room ratio line, plotting a real design day, and chart pressure.
- 03Measuring humidity on site: wet bulb, dew point and sensorsHow wet-bulb psychrometers, chilled-mirror hygrometers and capacitive humidity sensors work, how to calibrate them with saturated salts, and where to put them so they read the space and not a press.
- 04Room loads and the supply air stateHow a room's sensible and latent loads fix the temperature and moisture of its supply air, why the sensible load sets the airflow, and the errors of fan heat and volume at the wrong state.
- 05Mixing outdoor and return airThe mass, water and energy balances for mixing outdoor and return air, outdoor-air share by mass, the straight-line rule, fog when cold and humid streams meet, minimum outdoor air and economy cycles.
- 06Heating and humidifying in winterSize the winter heater and humidifier from a mass and energy balance, compare steam injection with water spray, see where the energy goes, and check absorption, water quality and frosty mornings.
- 07Cooling coils and dehumidificationCalculate a cooling coil's total, sensible and latent load and condensate, find its apparatus dew point and contact factor, compare coil and room sensible heat ratios, and avoid sensible-only sizing.
- 08Reheat, overcooling and humidity controlWhy a dehumidifying coil must over-cool and what the reheat costs at full and part load, how dew-point control sequences work, ways to recover reheat energy, and how a drifting sensor wastes both.
- 09Evaporative cooling for the warehouseSize direct evaporative coolers from the wet-bulb limit and pad effectiveness, find their water use and real cooling to the room, see why humid days defeat them, and keep them hygienic.
- 10Condensation, commissioning and seasonal checksCheck pipes, ducts and cold reels against the dew point, insulate ducts in humid spaces, commission an air-handling unit by measurement, and keep it right with trend logs and seasonal checks.
Apply This in Practice
Use these KEVOS® tools to try the method from this reading with your own numbers.
Applies this method
Adiabatic Mixing of Two Air Streams (Mixed Temperature, Humidity, Dew Point, Fog Risk)
Mixes two moist-air streams by dry-air mass: mixed temperature, humidity ratio, enthalpy, relative humidity, dew point, outdoor-air share and a fog check.
Used in Mixing outdoor and return air
Applies this method
Cooling Coil Load with Dehumidification (Psychrometric Balance, Condensate, Sensible Heat Ratio)
Balances a cooling coil that dehumidifies: total, sensible and latent load, sensible heat ratio, condensate, and the inlet and outlet humidity and dew point against coil capacity.
Applies this method
Direct Evaporative Cooler Check (Wet-Bulb, Supply Temperature, Water Use, Cooling Capacity)
Models a direct evaporative cooler: wet-bulb limit, leaving temperature and humidity at the pad effectiveness, water evaporated and sensible cooling against a target.
Applies this method
Heating with Humidification (Heater Load, Water or Steam Needed, Equipment Check)
Balances heating with steam or spray humidification: water to add, energy it brings, heater load left, and both against the heater and humidifier capacities.
Applies this method
Moist Air Dew Point and Surface Condensation Check (Humidity Ratio, Enthalpy, Dew-Point Margin)
Gives humidity ratio, enthalpy, specific volume and dew point from temperature and humidity, and checks a surface temperature against the dew point plus a margin.
Used in Moist air in numbers
Related Tools and Examples
Useful alongside
Linear Calibration Check (Straight-Line Fit, Calibrated Value, Calibration Interval)
Fits a calibration line through readings at several saturated-salt fixed points, giving the offset and slope this chapter’s check looks for and the uncertainty of corrected readings.
Related to Measuring humidity on site: wet bulb, dew point and sensors
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 30 October 2026.
- ASHRAE. ASHRAE Handbook—Fundamentals, Chapter 1: Psychrometrics (current edition). Named as the reference for the Hyland–Wexler saturation-pressure equations and the moist-air relations that the book and the linked KEVOS® tools use. Paywalled; no tables, figures or text are reproduced.
- R. W. Hyland and A. Wexler, ASHRAE Transactions 89(2A). Formulations for the thermodynamic properties of the saturated phases of H2O from 173.15 K to 473.15 K (1983). Named as the origin of the saturation-pressure equations over water and ice used for every property calculation in the book.
- L. Greenspan, Journal of Research of the National Bureau of Standards 81A(1). Humidity fixed points of binary saturated aqueous solutions (1977). Cited in Chapter 3 for the principle of checking humidity sensors over saturated salt solutions. Readers take the equilibrium humidity at their temperature from the paper itself; no values are reproduced.
- E. M. Greitzer, Z. S. Spakovszky and I. A. Waitz, Massachusetts Institute of Technology. 16.Unified: Thermodynamics and Propulsion (lecture notes) (2026). Open university notes used as the checkable reference for the steady-flow mass and energy balances applied to every air-handling process.
- Standards Australia and Standards New Zealand. AS/NZS 3666 Air-handling and water systems of buildings: Microbial control (Parts 1 to 3) (2011). Named by number and scope in Chapters 6 and 9 for humidifiers and evaporative coolers. No clauses or values are quoted; the book does not cover its requirements.
- Standards Australia. AS 1668.2 The use of ventilation and air conditioning in buildings, Part 2: Mechanical ventilation in buildings (current edition). Named by number and scope in Chapter 5 as the standard by which a designer determines outdoor air rates. No clauses or values are quoted.
- NSW Health. Managing cooling water systems throughout their lifecycle (2026). Cited as an example of state health guidance on Legionella control in water systems; readers check the rules of their own state or territory.
- Australian Institute of Refrigeration, Air Conditioning and Heating (AIRAH). Design data for air conditioning in Australian locations (current publications). Named generically in Chapters 1 and 7 as a source of design weather conditions for real projects. No specific publication or value is relied on; the book's design conditions are illustrations.
- Bureau of Meteorology. Climate data online (2026). Named as a source of the site weather records from which real design conditions are derived. The book's design conditions are illustrations, not Bureau data.