Technical methods of effective and timely drying
Technical methods of effective and timely drying
This domain tests the physical reasoning behind drying: how air movement, temperature, and dehumidification change vapor pressure, and how to apply those principles to assemblies and occupied spaces. Expect questions on psychrometrics, containment, pressure, and specialty drying.
Core rules
- Evaporation is driven by vapor pressure deficit, not by “dry air” in general. Warm air holds more moisture; lowering specific humidity or raising temperature increases the driving force.
- Air movement removes the saturated boundary layer. It does not lower dew point. If a surface is below dew point, condensation continues until humidity falls or the surface warms.
- Refrigerant and desiccant dehumidifiers solve different conditions. Refrigerant/LGR units are efficient in moderate temperatures; desiccant units perform well in cool, low-humidity conditions but add heat.
- Containment and pressure control keep drying energy in the wet zone and protect unaffected areas. Monitor pressure differentials and make-up air.
- Use the dry standard and material EMC to decide when drying is complete. Daily readings should trend toward the goal, not merely fluctuate.
- Specialty assemblies need targeted methods: cavity injection, negative air, floor mats, and controlled heat for hardwood, plaster, and concrete.
Worked example
An open office is at 80°F dry-bulb and 60% RH. The dew point is about 65°F. A gypsum wall surface near a cold exterior corner reads 62°F by infrared. Because the surface is below dew point, moisture will condense there even with strong air movement. The correct response is to lower the space dew point with more dehumidification, warm the surface if practical, and verify with a thermo-hygrometer and moisture meter. Adding air movers alone would only circulate air across a condensing surface.
Traps
Confusing relative humidity with specific humidity; assuming air movement changes dew point; overusing desiccant heat in a warm occupied space; verifying by appearance instead of readings.