KEVOS® MAKEOS · Engineering
Heating with Humidification (Heater Load, Water or Steam Needed, Equipment Check)
To bring cold, dry air to the temperature and humidity you need, how much heat must the heater supply and how much water or steam must the humidifier add, and are the heater and humidifier big enough?
Steady-flow balance for heating air and adding moisture: the moisture the humidifier must add, the energy that steam or spray water brings in, the heater load that remains, and both checked against the heater and humidifier capacities.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Total (barometric) pressure | kPa | Barometric pressure at the site: 101.325 kPa at sea level. |
| Air volume flow entering | m³/s | Volume flow of air entering the unit at the entering temperature. |
| Air entering | ||
| Entering dry-bulb temperature | °C | Dry-bulb temperature of the air entering, usually winter outdoor or mixed air. |
| Entering relative humidity | % | Relative humidity of the air entering. |
| Air leaving | ||
| Leaving dry-bulb temperature | °C | Dry-bulb temperature the leaving air must have. |
| Leaving relative humidity | % | Relative humidity the leaving air must have. |
| Humidifier and capacities | ||
| Humidifier type | choice | Steam injection adds moisture and heat; spray or pad humidification adds liquid water that cools the air as it evaporates. |
| Enthalpy of the steam or water added | kJ/kg | Enthalpy of the steam or water added: about 2,675 kJ/kg for steam near atmospheric pressure (steam table), or 4.18 × water temperature for liquid water. |
| Heater capacity | kW | Rated output of the heater or heating coil at these conditions. |
| Humidifier capacity | kg/h | Rated output of the humidifier in kilograms of water per hour. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Total (barometric) pressure
- What it is
- The total pressure of the moist air, the barometric pressure at the site, used in the humidity ratios and specific volume.
- Units and format
- Enter a number in kPa: kilopascals. The form accepts 50 kPa to 110 kPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 101.325 kPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Changes the humidity ratios and the dry-air mass flow slightly.
- Get it right
- Use about 101.325 kPa at sea level and the local value at altitude.
What this means: Air volume flow entering
- What it is
- The volume of air entering the unit each second at the entering condition, which gives the dry-air mass flow.
- Units and format
- Enter a number in m³/s: m³/s. The form accepts 0.001 m³/s to 1000 m³/s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 m³/s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The heater load and water added rise in proportion.
- Get it right
- Use the flow at the entering temperature; flows quoted at standard conditions differ.
Air entering
What this means: Entering dry-bulb temperature
- What it is
- The dry-bulb temperature of the air entering the heater, usually winter outdoor air or mixed air.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts 0 °C to 50 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Colder entering air raises the heater load.
- Get it right
- Use the coldest condition the system must meet; the tool accepts 0 to 50 °C.
What this means: Entering relative humidity
- What it is
- The relative humidity of the air entering the unit, which with its temperature sets the moisture it already carries.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0.1% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Drier entering air needs more water added.
- Get it right
- Cold air at high relative humidity still carries little moisture.
Air leaving
What this means: Leaving dry-bulb temperature
- What it is
- The dry-bulb temperature the conditioned air must have when it leaves the unit.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts 0 °C to 60 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 22 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher leaving temperature raises the heater load.
- Get it right
- It must be above the entering temperature for this process.
What this means: Leaving relative humidity
- What it is
- The relative humidity the conditioned air must have when it leaves the unit.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0.1% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 45% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher target raises the water added and, with spray, the heater load.
- Get it right
- The leaving air must hold at least as much moisture as the entering air.
Humidifier and capacities
What this means: Humidifier type
- What it is
- How moisture is added: steam injection brings latent heat with it, while spray or pad humidification adds liquid water that takes its latent heat from the air.
- Units and format
- Choose one of 2 options from the list: Steam injection; Water spray or evaporative pad (adiabatic).
- Example
- The form opens with “Steam injection” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Changes only the plausibility check; the enthalpy you enter sets the heater load.
- Get it right
- Match it to the enthalpy below: about 2,675 kJ/kg for steam, under 500 kJ/kg for water.
What this means: Enthalpy of the steam or water added
- What it is
- The specific enthalpy of the steam or water entering the airstream from the humidifier, which is the energy it brings in per kilogram added.
- Units and format
- Enter a number in kJ/kg: kJ/kg. The form accepts 0 kJ/kg to 3500 kJ/kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2675 kJ/kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher enthalpy lowers the heater load needed.
- Get it right
- For liquid water use about 4.18 × its temperature in °C; for steam read the steam table at its pressure.
What this means: Heater capacity
- What it is
- The heat output the heater or heating coil can deliver at these air conditions.
- Units and format
- Enter a number in kW: kilowatts. The form accepts 0 kW to 1000000 kW. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 kW as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the pass or fail line of the heater check.
- Get it right
- Coil ratings depend on water or steam supply temperature; use the selection at your conditions.
What this means: Humidifier capacity
- What it is
- The mass of water or steam per hour the humidifier can deliver into the air at full output.
- Units and format
- Enter a number in kg/h: kg/h. The form accepts 0 kg/h to 100000 kg/h. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30 kg/h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the pass or fail line of the humidifier check.
- Get it right
- Steam humidifier ratings are in kilograms of steam per hour; check the rating at your supply pressure.
Method and formulas
- Each state: p_v = φ p_sat(T) (Hyland–Wexler), ω = 0.622 p_v ÷ (P − p_v), h = 1.005 T + ω (2,500.9 + 1.82 T); dry-air mass flow ṁ_a = V̇₁ ÷ v₁.
- Water added ṁ_w = ṁ_a (ω₃ − ω₁).
- Heater load Q = ṁ_a (h₃ − h₁) − ṁ_w h_w; when the steam brings more heat than needed, the heater load is zero.
- Example: 1 m³/s of air from 5 °C and 40 % to 22 °C and 45 % needs 23.87 kg/h of steam at 2,675 kJ/kg and a 20.81 kW heater; a 25 kW heater and 30 kg/h humidifier meet it.
Limitations
- All added water is assumed absorbed; spray humidifiers that leave droplets or wet the duct deliver less, and their water treatment matters.
- Air temperatures from 0 to 60 °C; relative humidity is taken over liquid water.
- Steady design point only; controls, duct losses and fan heat are not included.
- Example values are illustrations. Replace them with values for your actual system.
Typical uses
- Size the heater and humidifier for a cleanroom, laboratory, museum or printing room
- Check winter humidification for static control or comfort
- Compare steam injection with spray humidification for the same duty
Who it is for
- Mechanical services designers
- HVAC technicians
- Facility engineers
What you get
- Heater load
- Water or steam added and the energy it brings
- Humidity ratio, enthalpy and dew point in and out
- Heater and humidifier capacity checks
Interpreting the result
- Spray humidification needs more heater capacity than steam for the same leaving state, because the water takes its latent heat from the air.
- Cold outdoor air holds little moisture, so even modest indoor humidity targets need a lot of added water in winter.
Before relying on the result
- Check the humidifier maker’s absorption distance in the duct before placing coils, filters or bends downstream.
References
- Çengel Y. A. and Boles M. A., Thermodynamics: An Engineering Approach, 5th edition (McGraw-Hill, 2006), chapter 14 (section 14-7: heating with humidification). Method reference; no tables, figures or examples are reproduced.
- ASHRAE Handbook—Fundamentals (SI), chapter 1 Psychrometrics: Hyland–Wexler saturation pressure.
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
Method explained here · Library collection
Psychrometrics for HVAC and process air: humidity, dew point and the loads you cannot feel
In Fluids, HVAC and Building Services
What it covers: A practical guide to moist air: dry-bulb, wet-bulb, dew point, humidity ratio and enthalpy, the main processes on the psychrometric chart, sensible and latent loads, and condensation.
Method explained here · KEVOS® original book
Heating and humidifying in winter
In Humidity Control by the Numbers
What it covers: Size 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.
KEVOS® tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.