Humidifier Capacity (RH Target)
Moisture addition (lb/hr, gal/day) and latent load from supply CFM and the entering-to-target RH rise.
Example
You enter
- Supply airflow (CFM) 1000
- Supply dry-bulb (°F) 70
- Entering RH (%) 20
- Target RH (%) 40
You get
- Moisture addition 13.99
- Daily water 40.3 gal/day
- Latent load added 14843 BTU/hr
Checked against published worked examples from: ASHRAE.
Details, formula, and sources
ScopeWith altitude-corrected humidity ratios. Per ASHRAE Fundamentals 2021 Ch. 1 psychrometrics.
Formula or tableMoisture addition (lb/hr) = 60 × CFM × rho × (W_target − W_entering); W = 0.621945 × Pw / (P − Pw) with Pw = RH × Pws(T_db) and P the altitude-corrected pressure. Daily water = lb/hr × 24 / 8.34. Latent load = addition × 1061 BTU/lb.
Source and editionASHRAE Fundamentals 2021 Chapter 1 (psychrometrics) by name.
Free accessASHRAE Handbook TOC free at ashrae.org; full handbook licensed.
What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
API field namescfm, supply_db_F, entering_rh_pct, target_rh_pct, addition_lb_hr, gpd, latent_btu_hr
- Latent heat of vaporization 1061 BTU/lb (near room temperature)ASHRAE Fundamentals 2021 Ch. 1
- Dry-air gas constant 287.055 J/(kg·K)physical fact
- Water density 8.34 lb/galphysical fact for water