Smart Fog · Engineering tools
Commercial Humidifier Sizing Calculator
Estimate the water evaporation rate your commercial humidification application needs, in kg/h and lb/h, from actual airflow and entering and target air conditions.
This is a steady-state moisture balance—not an equipment selection. Use outdoor air for a facility ventilation load, or a measured entering airstream for a known air-treatment process. Add uncounted moisture loads and subtract known moisture gains separately.
2. Estimated evaporation capacity
Including your selected allowance
Water evaporated per hour
Not nozzle count, feedwater flow or a final equipment rating. Confirm delivery efficiency, water quality, turndown, controls, temperature effects and installation constraints with an application review.
3. Save your sizing brief or request a review
Opens your email application with this brief addressed to info@smartfog.com. Nothing is sent until you send the email. No automatic submission. If your email app truncates a long draft, copy the brief or attach the downloaded text file.
Humidification load calculation: the method
The calculator finds humidity ratio at both states, then uses specific volume at the entering condition to convert actual volume into dry-air mass flow.
W = 0.621945 × pᵥ / (p − pᵥ), where pᵥ = RH/100 × pws(T). W is kg water/kg dry air; pressures are kPa.
v = 0.287042 × (Tenter + 273.15) × (1 + 1.607858 × Wenter) / p (m³/kg dry air). ṁda = actual m³/h ÷ v.
Signed net = ṁda × (Wtarget − Wenter) + additional demand − moisture gains. Evaporation demand is the greater of zero and signed net. The selected percentage allowance is added to that demand. 1 ACFM = 1.69901079552 m³/h.
Worked example: the default conditions
At 101.325 kPa, 1,000 actual m³/h entering at 5 °C and 50% RH, with a target of 21 °C and 40% RH, gives entering W ≈ 2.689 g/kg, target W ≈ 6.168 g/kg and dry-air mass flow ≈ 1,263.63 kg/h. With zero added load, gains or allowance, the evaporation estimate is 4.40 kg/h (9.69 lb/h). This example was independently checked against PsychroLib.
Method and references
Saturation vapor pressure uses ASHRAE water/ice equations, switching at the water triple point (0.01 °C), as implemented in PsychroLib. Humidifier application guidance: ASHRAE Handbook: Humidifiers (PDF). These references do not constitute an endorsement of this tool.
From humidification load to a system specification
Humidification load is how much water vapor must be added per hour to close the moisture deficit under your design conditions. The humidity ratio difference—not the relative humidity difference alone—determines that demand. Use a winter design condition or measured operating case appropriate to your project; this tool does not choose the design weather for you.
A steam humidifier and an evaporative humidifier can serve the same moisture requirement, but their heat balance, water treatment, distribution and control strategy differ. This calculator estimates moisture addition rate only. It does not compare energy costs or guarantee that a particular system can reach the required leaving condition.
What to bring to an engineering review
- The calculated brief, plus the source of your airflow and weather assumptions.
- Operating schedule, startup needs and expected minimum and maximum load.
- For duct installation: duct dimensions, air velocity, available straight run and absorption distance constraints.
- Available water quality, compressed-air supply, drainage and electrical service.
- Humidity sensor placement, control-system requirements and the process being protected.
Unknown values are engineering questions, not reasons to guess. Start with Smart Fog’s TS100 in-duct humidifier for duct applications, or request an application review for other delivery options. Use the enthalpy calculator to explore moist-air properties and return to all humidity calculators for supporting checks.
Commercial humidifier sizing FAQ
Should I use outdoor CFM or total supply CFM?
For a facility outdoor-air load, use only outdoor ventilation air and quantified infiltration, at entering conditions. Recirculating air is already part of the indoor moisture balance. For a known airstream, use that airstream’s actual flow and its measured entering condition, which may be mixed air. Both modes use the same mass-balance equation; the physical boundary differs.
Can I size a humidifier from square footage?
Not reliably. Floor area alone omits outdoor air exchange, climate, indoor design conditions, pressure, process demand and internal moisture gains. Room volume with air changes can help estimate outdoor airflow only when the air-change assumption actually represents outdoor replacement air.
Does heating cold air add or remove moisture?
Sensible heating without moisture exchange changes relative humidity, not humidity ratio. The calculator compares humidity ratios, so warming cold outdoor air is not treated as adding water.
Does this cover startup, absorption distance or adiabatic cooling?
No. This is a steady-state balance, not a startup or recovery model. Moisture uptake by dry materials, intermittent operation and transient loads need separate assessment. No saturation or absorption path is modeled. In particular, an adiabatic humidifier cools air, so the entered leaving temperature and RH may require preheat or other conditioning and may not be achievable by the humidifier alone. Check condensation risk, duct velocity, absorption distance and controls before selection.
What if the signed moisture load is negative?
No humidification is required by this balance. A negative result indicates a net moisture surplus under the entered assumptions; it is not a dehumidifier sizing result. Cooling-coil and latent-load analysis require a separate model.