How to calculate industrial humidification load

Start with the air entering your building

Humidification load is the amount of water vapor a system must add each hour to maintain the intended indoor conditions. Floor area alone is not enough to size an industrial humidifier: ceiling height, outdoor-air exchange, indoor temperature and target relative humidity all matter.

The FanMist Practical Method starts with the moisture deficit between incoming outdoor air and the target indoor air. It then applies that difference to the volume of air being replaced each hour.

Collect five inputs

Gather the following for each area or zone. Record where each value came from so the sizing can be checked against actual operating conditions.

• Building length, width and ceiling height, in feet.
• Process exhaust and makeup-air flow, in cubic feet per minute (CFM). Include dust-collection air that is actually lost outdoors.
• Target indoor relative humidity and temperature.
• Local outdoor winter design conditions.
• Moisture-content values from the guide’s psychrometric table, expressed in grains per cubic foot.

Choose the governing airflow

For the guide’s Practical Method, use the greater of total exhaust CFM or total makeup-air CFM. Do not add the two together: they are related sides of the building’s air balance.

If measured airflow is unavailable, the guide uses building volume multiplied by an estimated air-change rate (ACH). Its initial estimates range from 1.0 ACH for tight construction to 2.0 ACH for loose construction or frequent door traffic. An estimate needs checking when process exhaust or unusual building conditions dominate.

Calculate the moisture deficit

Find the target indoor moisture content at the chosen temperature and RH. Subtract the incoming-air moisture allowance using the same units. The supplied guide uses a conservative allowance equal to 50% of the outdoor saturation-table value at the design temperature. That is a method-specific sizing assumption, not a claim that heating removes half the water vapor.

Keep grains per cubic foot separate from grains per pound of dry air. They are different quantities and cannot be substituted into the same formula.

Worked example: a woodworking plant

This is the illustrative plant in the IHC woodworking guide, not a customer case study. The building is 250 × 300 × 25 feet. Exhaust is 30,750 CFM and makeup air is 35,000 CFM, so the guide uses 35,000 CFM.

At 35% RH and 69°F, the guide’s target moisture content is 2.73 grains/ft³. Its outdoor allowance is 0.24 grains/ft³, giving a deficit of 2.49 grains/ft³.

Turn the load into a system design

The moisture load is a starting point for equipment selection. Fan airflow, nozzle output, pump capacity, zoning and unobstructed evaporation space must work together. High product throughput, process moisture and other secondary loads may call for a more detailed calculation.

Send IHC your building dimensions, airflow information, location and target conditions. The full guide includes the tables, assumptions and equipment-selection steps needed to review the result.