Flood Restoration Psychrometrics and Humidity Control 2 — Questions and Answers
Question 1: Which type of dehumidifier performs best in low-temperature environments below 50°F?
- Refrigerant (mechanical) dehumidifier
- Desiccant dehumidifier (Correct answer)
- Thermoelectric dehumidifier
- Evaporative cooler
Correct answer: Desiccant dehumidifier
Desiccant dehumidifiers use a moisture-absorbing rotor and regeneration heat rather than refrigerant coils, so they are not degraded by cold temperatures that would ice a refrigerant unit.
Question 2: Vapor pressure differential in the context of structural drying refers to:
- The difference in barometric pressure inside versus outside a structure
- The difference in moisture vapor pressure between wet building materials and the surrounding air (Correct answer)
- The pressure difference created by operating multiple air movers simultaneously
- The humidity gap between floor level and ceiling level of a structure
Correct answer: The difference in moisture vapor pressure between wet building materials and the surrounding air
Moisture migrates from areas of higher vapor pressure (wet materials) toward areas of lower vapor pressure (drier air), making this differential the driving force behind evaporation.
Question 3: In psychrometrics, 'humidity ratio' (specific humidity) is expressed as:
- Percentage of saturation at a given temperature
- Pounds of water vapor per pound of dry air (Correct answer)
- Grains of moisture per cubic foot of air
- Millimeters of mercury of vapor pressure
Correct answer: Pounds of water vapor per pound of dry air
Humidity ratio is the mass of water vapor relative to the mass of dry air, commonly expressed in grains per pound (gr/lb) in restoration work.
Question 4: During the evaporation phase of structural drying, what happens to the relative humidity of the air in the drying space?
- It decreases as moisture is drawn back into wet materials
- It stays stable if equipment is balanced correctly
- It increases as water evaporates from wet surfaces into the air (Correct answer)
- It drops immediately when air movers are first activated
Correct answer: It increases as water evaporates from wet surfaces into the air
As wet building materials release moisture through evaporation, that water vapor enters the air and raises the relative humidity of the drying space.
Question 5: The primary function of air movers (axial fans) in a structural drying system is to:
- Lower the temperature of the affected area to inhibit mold growth
- Create high-velocity airflow across wet surfaces to accelerate evaporation (Correct answer)
- Filter airborne contaminants and allergens released from flood-damaged materials
- Distribute dehumidified air evenly throughout the entire structure
Correct answer: Create high-velocity airflow across wet surfaces to accelerate evaporation
Air movers sweep away the saturated boundary layer of air clinging to wet surfaces, replacing it with drier air and dramatically increasing the rate of evaporation.
Question 6: Desiccant dehumidifiers have an advantage over refrigerant units specifically when:
- Temperatures are high (above 90°F) and relative humidity is very high
- The RH has already been reduced below 40% at normal room temperature
- Temperatures are very low or extremely low humidity levels must be achieved (Correct answer)
- Processing large volumes of standing water during the initial extraction phase
Correct answer: Temperatures are very low or extremely low humidity levels must be achieved
Desiccant units use hygroscopic material to absorb moisture and are unaffected by cold temperatures that cause refrigerant coil icing and loss of performance.
Question 7: The IICRC S500 standard reference drying conditions used as a psychrometric baseline are approximately:
- 70°F and 50% RH (Correct answer)
- 75°F and 55% RH
- 80°F and 40% RH
- 65°F and 60% RH
Correct answer: 70°F and 50% RH
IICRC S500 defines standard conditions as approximately 70°F (21°C) at 50% RH for baseline psychrometric calculations and equipment performance comparisons.
Flood Restoration Psychrometrics and Humidity Control 2
Which type of dehumidifier performs best in low-temperature environments below 50°F?