CPR Psychrometric Drying Principles 2 — Questions and Answers
Question 1: On a psychrometric chart, what does the curved line on the far left represent?
- The dew point line
- The saturation curve (100% RH) (Correct answer)
- The wet bulb temperature line
- The enthalpy baseline
Correct answer: The saturation curve (100% RH)
The curved line on the far left of a psychrometric chart is the saturation curve, representing 100% relative humidity where air is fully saturated.
Question 2: A restorer measures dry bulb temperature at 75°F and wet bulb temperature at 62°F. What does the difference between these two readings indicate?
- The dew point temperature
- The degree of evaporative cooling, reflecting lower relative humidity (Correct answer)
- The enthalpy of the air
- The specific volume of the air
Correct answer: The degree of evaporative cooling, reflecting lower relative humidity
The wet bulb depression (difference between dry and wet bulb temperatures) indicates how much evaporative cooling occurred, which is directly related to the air's relative humidity.
Question 3: During a structural drying job, the dew point inside the structure is 55°F and the surface temperature of an exterior wall is 50°F. What is the likely consequence?
- Accelerated drying of the wall material
- Condensation will form on the wall surface (Correct answer)
- The dehumidifier will operate more efficiently
- Relative humidity will drop below target
Correct answer: Condensation will form on the wall surface
When a surface temperature falls below the dew point of the surrounding air, moisture condenses on that surface, potentially worsening damage.
Question 4: Which psychrometric property is most directly used to calculate the moisture-removal capacity of a refrigerant dehumidifier?
- Dry bulb temperature
- Specific volume
- Grain depression (inlet vs. outlet humidity ratio) (Correct answer)
- Enthalpy at saturation
Correct answer: Grain depression (inlet vs. outlet humidity ratio)
Grain depression — the difference in humidity ratio (grains per pound) between the dehumidifier's inlet and outlet — directly quantifies how much moisture the unit is removing.
Question 5: In psychrometrics, 'enthalpy' refers to:
- The weight of moisture per unit volume of air
- The total heat content (sensible + latent) of moist air (Correct answer)
- The temperature at which condensation forms
- The percentage of moisture relative to saturation
Correct answer: The total heat content (sensible + latent) of moist air
Enthalpy is the total heat energy in moist air, combining both sensible heat (temperature) and latent heat (moisture content).
Question 6: A desiccant dehumidifier is preferred over a refrigerant dehumidifier when ambient temperatures are:
- Above 80°F with high humidity
- Between 60°F and 70°F
- Below 45°F (Correct answer)
- Above 90°F with low humidity
Correct answer: Below 45°F
Refrigerant dehumidifiers lose efficiency at low temperatures because the evaporator coil can frost over, making desiccant units the preferred choice below approximately 45°F.
Question 7: The specific humidity (humidity ratio) of air is expressed in which unit?
- Percent relative humidity (%RH)
- Grains of moisture per pound of dry air (Correct answer)
- BTUs per pound
- Cubic feet per minute (CFM)
Correct answer: Grains of moisture per pound of dry air
Humidity ratio, or specific humidity, is expressed as grains of moisture per pound of dry air (gr/lb), with 7,000 grains equaling one pound.
CPR Psychrometric Drying Principles 2
On a psychrometric chart, what does the curved line on the far left represent?