Industry & humidity target
Start with the material or process requirement.
Choose the indoor conditions
The guide uses 35–45% RH around 69–72°F as a common woodworking starting range, with some processes calling for conditions closer to 50% RH. Select the actual target for the species, product, finish and required equilibrium moisture content.
Wood absorbs or releases moisture toward equilibrium with its surroundings. Dry winter air can change dimensions, open joints, distort stock and disrupt machining or finishing.
A wood product’s response takes time. Acclimate incoming stock, measure wood moisture and coordinate storage, machining, assembly and finishing conditions.
Count the portion of filtered dust-collector airflow actually lost outdoors. The source illustrates 25,000 CFM × 15% loss = 3,750 CFM, added to 27,000 CFM of other exhaust. Verify the return-air fraction rather than assuming all collector airflow is lost.
Reference equilibrium moisture content
These reference ranges illustrate the relationship, not a product guarantee. Species or paper stock, temperature, coatings and exposure time affect equilibrium. Confirm with material suppliers and plant measurements.
| Room RH | Approximate material moisture content |
|---|---|
| 30% | 6.2–6.3% |
| 35% | 6.9–7.1% |
| 40% | 7.7–7.9% |
| 45% | 8.5–8.7% |
| 50% | 9.2–9.5% |
What the calculation represents
Cold outdoor air contains relatively little water vapor. Heating it lowers its relative humidity without itself removing water. Replacing plant air with that winter air creates the humidification load.
Atomization creates small droplets; airflow carries them into open space where they evaporate. Dalton’s law describes vapor’s partial pressure, diffusion follows concentration gradients, and temperature affects saturation pressure. Airflow and mixing determine distribution; there is no universal fixed speed for vapor diffusion.
Building & air balance
Find the outdoor-air exchange that governs the winter load.
Collect the building data
| Input | Where to obtain it |
|---|---|
| Dimensions and served zones | Plans and measured areas/heights; multiply length × width × height. |
| Outdoor design temperature | Project design conditions for the location. |
| Indoor temperature and RH | Material and process requirements. |
| Exhaust CFM | Fans, dust collection and process extraction; distinguish outdoor discharge from return air. |
| Makeup-air CFM | MAUs, rooftop units and actual outdoor-air fractions. |
| Operating modes | Production schedule, door openings, economizer minimum and freeze protection. |
| Secondary moisture loads | Product regain, process evaporation and occupancy when material. |
Use the governing airflow once
Do not add exhaust and makeup air together as though they were independent losses. The practical method uses the larger value to represent replacement-air demand.
If measured CFM is unavailable, estimate outdoor-air exchange from building volume and an explicit ACH assumption. The source’s screening values are 1 ACH for a tight building, 1.5 for average conditions and 2 for a loose building. Replace estimates with verified air-balance data when possible.
Avoid using total HVAC supply or recirculating cleanroom airflow as outdoor exchange. Confirm each stream’s actual destination and operating condition.
Economizers and multiple HVAC units
The source uses roughly 400 CFM per ton as an initial comfort-HVAC estimate, with a typical 350–450 range. Actual fan data, a test-and-balance report and BAS trends are preferable.
Example: 100 tons × 400 × 0.20 = 8,000 CFM of outdoor air. For three 50-ton units at 15%, 20% and 20%, calculate each separately: 3,000 + 4,000 + 4,000 = 11,000 CFM.
Confirm winter damper settings and low-temperature lockouts against the design-day condition. Do not assume a 20% minimum or zero outside air without checking the actual sequence. Classified process air systems require their own design data.
Process-specific verification
Document larger margins when frequent dock openings, rough-mill/finishing differences or planned expansion justify them.
Confirm winter dust-collection return fractions and any economizer lockouts with the facility’s mechanical team.
Moisture & calculation
Convert the target conditions and outdoor-air volume to pounds per hour.
Selected saturation values
These are selected values from the source’s Table 2-A, sufficient for its worked examples and common indoor temperatures. Use the original table for other listed temperatures or obtain appropriate psychrometric data.
The units here are grains per cubic foot. Do not substitute a chart value expressed in grains per pound of dry air. One pound of water equals 7,000 grains.
| Temperature · °F | Water vapor at 100% RH · grains/ft³ |
|---|---|
| −10 | 0.29 |
| −5 | 0.35 |
| 0 | 0.48 |
| 5 | 0.61 |
| 10 | 0.70 |
| 15 | 0.99 |
| 20 | 1.24 |
| 25 | 1.56 |
| 30 | 1.95 |
| 35 | 2.38 |
| 40 | 2.86 |
| 45 | 3.44 |
| 50 | 4.11 |
| 55 | 4.89 |
| 60 | 5.80 |
| 65 | 6.85 |
| 66 | 7.07 |
| 67 | 7.31 |
| 68 | 7.57 |
| 69 | 7.80 |
| 70 | 8.10 |
| 71 | 8.32 |
| 72 | 8.59 |
| 73 | 8.87 |
| 74 | 9.15 |
| 75 | 9.45 |
| 80 | 11.04 |
Calculate the moisture deficit
- Multiply saturation grains/ft³ at the indoor temperature by the target RH expressed as a decimal.
- For the source’s practical design method, multiply saturation grains/ft³ at the outdoor design temperature by 0.50.
- Subtract the outdoor allowance from the indoor target to obtain the deficit.
CFM and ACH formulas
The CFM formula converts minutes to hours. The ACH alternative already gives an hourly air volume, so it does not multiply by 60 again.
The source uses a 15% margin. Any larger allowance for door traffic, uncertainty or expansion should be an explicit design decision. Preserve calculation precision until the final selection; small differences from rounded source examples are expected.
When to use the comprehensive method
Do not assume secondary factors cancel or assign a universal percentage. Estimate them from the actual process before changing equipment capacity.
| Factor | Effect |
|---|---|
| Product regain | Dry incoming stock absorbing moisture adds to demand. |
| Process water vapor | Documented moisture released into the served space can reduce humidifier demand. |
| Occupancy | Moisture from people may reduce the net demand. |
| Unusual operational changes | High throughput, frequent doors or tight tolerances warrant a more detailed balance. |
Worked example & equipment
Follow the source’s illustrative facility from inputs to equipment selection.
Worked example: 859 lb/hr
This is the guide’s sizing example, not a customer project or a recommendation for every facility in the industry.
| Step | Value / calculation |
|---|---|
| Building | 250 × 300 × 25 ft = 1,875,000 ft³; 75,000 ft² |
| Exhaust / makeup air | 30,750 / 35,000 CFM |
| Governing airflow | 35,000 CFM |
| Indoor target | 35% RH at 69°F |
| Saturation at indoor temperature | 7.80 grains/ft³ |
| Indoor target moisture | 7.80 × 0.35 = 2.73 grains/ft³ |
| Outdoor design | 0°F; 0.48 grains/ft³ at saturation |
| Outdoor moisture allowance | 0.48 × 0.50 = 0.24 grains/ft³ |
| Moisture deficit | 2.73 − 0.24 = 2.49 grains/ft³ |
| Hourly air volume | 35,000 × 60 = 2,100,000 ft³/hr |
| Base load | 2.49 × 2,100,000 ÷ 7,000 = 747 lb/hr |
| Capacity with 15% margin | 747 × 1.15 ≈ 859 lb/hr |
| Water equivalent | Approximately 1.72 gpm |
Translate capacity into fans and pumps
A 27-inch ring with fourteen 7.94 lb/hr blue-band nozzles supplies 111.16 lb/hr at the stated pressure. Recalculate if the nozzle size or count changes.
- Eight fans, each with fourteen 7.94 lb/hr nozzles: 8 × 111.16 = 889.28 lb/hr.
- The reference pairs the example with a 2 gpm station. Verify the selected station’s output, nozzle demand and pump bypass capacity.
- Provide eight matched rings/valves, 112 nozzles, appropriate fan-relay capacity, tubing, fittings, filtration and treatment.
Sizing worksheet and bill of materials
In a spreadsheet, calculate governing CFM as MAX(exhaust cell, makeup-air cell). Keep it in a separate cell; do not include the result cell inside its own MAX formula.
Useful conversions: CFM × 60 = ft³/hr; 1 gallon ≈ 8.34 lb of water; 1 gpm ≈ 500.4 lb/hr; °C = (°F − 32) ÷ 1.8. Send the completed inputs to IHC for the final layout and equipment selection.
| Record | Required value |
|---|---|
| Building | Dimensions, volume and served zones |
| Design conditions | Outdoor temperature; indoor temperature and RH |
| Air balance | Exhaust, makeup air, governing CFM or documented ACH |
| Moisture | Indoor target, outdoor allowance and deficit |
| Capacity | Calculated load, chosen margin and final lb/hr / gpm |
| Equipment | Fan count, nozzle size/count, ring output and airflow ratio |
| Pump selection | Station flow, zone split and pump bypass capacity |
| Installation | Relays, tubing, fittings, supports, filtration, treatment and drain |
| Verification | Source of each input, date and responsible reviewer |
Parts & references
Continue with related guides, parts and the original PDF.
Original & related PDFs
This web guide is adapted from the IHC reference linked below. The original PDF retains its diagrams, document revision and printable layout.