Drying System Design
Design sequence
- Set the dry standard and target MC for each material.
- Classify the loss to estimate the evaporation load.
- Choose open or closed drying.
- Size dehumidification and airflow.
- Establish containment and pressure relationships.
Open drying ventilates with outside air and works only when outdoor dew point is below the target indoor dew point and weather, security and air quality allow. Closed drying seals the space and uses dehumidifiers; it is required when outside air is hot and humid or contaminated.
Dehumidifier capacity is rated at AHAM conditions, 80°F and 60% RH. At cooler or drier inlet air, actual pints removed fall sharply - often to half of nameplate or less. Size from the performance curve at expected inlet conditions, not from the box. Typical starting targets are 70–90°F inside the chamber with an indoor humidity ratio low enough to keep vapor pressure below that of the wet materials.
Worked example
A 500 ft² room with 8 ft ceilings is 4,000 ft³, a Class 2 loss. One LGR rated 130 pints/day at 80°F/60% RH might remove only 55–70 pints/day at 70°F/40% RH. If the calculated evaporation load is near 100 pints/day, a single unit is undersized; add a second unit or warm the inlet air.
Traps
Sizing from nameplate pints; sealing a chamber with inadequate dehumidification and creating a greenhouse; placing containment so return air is blocked; running open drying when outdoor dew point exceeds indoor conditions.