EKS-HUM-013 / Moisture load and airflow
Moisture Load
湿负荷
Moisture load is the rate at which water vapour must be removed from a defined space or process to maintain the selected humidity condition.
- Aliases
- water-vapour load, humidity load
QUANTITY, SYMBOL AND UNIT
Quantity / objectMoisture load rate
Symbolṁ_w,load
SIkg/skilogram of water per second
Commonkg/hkilogram of water per hour
CommonL/daylitre of condensate per day: Requires an explicit water-density basis
Definition
Moisture load is the rate at which water vapour must be removed from a defined space or process to maintain the selected humidity condition.
It is a mass-flow quantity, usually built from internal generation, outdoor-air ventilation and infiltration, wet materials, openings and process releases. The boundary, schedule and design condition belong to every load value.
Formula and variables
ṁ_w,load = Σ ṁ_w,source Conditions: Steady-state mass balance for a defined control boundary. Each source term must use the same time basis and sign convention.
| Symbol | Meaning | Unit |
|---|---|---|
ṁ_w,load | water-vapour removal rate required to hold the target condition | kg/s or kg/h |
ṁ_w,source | water-vapour source rate crossing or generated within the boundary | kg/s or kg/h |
Relations and distinctions
Moisture load and latent load
Moisture load is a water mass rate. Latent load is the associated energy rate under a stated phase-change and reference-state basis.
Moisture load and rated capacity
Load belongs to the application; rated capacity belongs to equipment tested at stated conditions. Capacity must be evaluated against the actual load and operating state.
Engineering meaning
- Build the load by source and schedule before selecting dehumidification capacity.
- Use humidity-ratio and dry-air mass-flow balances for ventilation, infiltration and process-air contributions.
Common misconceptions
MisconceptionRoom volume and target RH are enough to determine moisture load.
Accurate explanationVolume affects transient storage, but steady load is governed by water-vapour sources, air exchange and the control boundary.
Application boundaries
- A peak, average and startup moisture load can be different and must not be mixed.
- The load calculation does not by itself include equipment derating, defrost time, air distribution or control reserve.
Supporting sources
-
ASHRAE / SRC-ASHRAE-F25-CH18
ASHRAE Handbook - Fundamentals, Chapter 18: Nonresidential Cooling and Heating Load Calculations ↗
2025 SI edition / Chapter 18, Section 1, heat flow rates and moisture added to a space
Supports: Moisture added within a space creates latent heat gain; At constant humidity ratio, water vapour must be removed at the rate it is added; Internal and outdoor-air sources must be included in load work -
ASHRAE / SRC-ASHRAE-F17-CH16
ASHRAE Handbook - Fundamentals, Chapter 16: Ventilation and Infiltration ↗
2017 SI edition / Chapter 16, air exchange effects on moisture and latent load, Equations 35 and related discussion
Supports: Outdoor-air exchange can add or remove moisture; Humidity-ratio difference and airflow determine the air-exchange moisture contribution
Maintenance record
Moisture Load
- Maintained by
- Yakeclimate Engineering Team
- Authoritative sources verified
- August 3, 2026
- Terminology checked
- August 3, 2026
- Last updated
- August 3, 2026