ASHRAE Psychrometrics and Moist Air Properties 2 — Questions and Answers
Question 1: Which psychrometric process occurs when air passes over a cooling coil whose surface temperature is below the air's dew point?
- Sensible cooling only, with no change in moisture content
- Sensible heating with increased relative humidity
- Cooling and dehumidification as moisture condenses on the coil (Correct answer)
- Humidification with simultaneous sensible cooling
Correct answer: Cooling and dehumidification as moisture condenses on the coil
When air contacts a surface below its dew point, both sensible cooling and dehumidification occur simultaneously as water vapor condenses out of the airstream onto the coil.
Question 2: What is the sensible heat ratio (SHR) of an air conditioning process?
- The ratio of latent heat change to total heat change in the process
- The ratio of sensible heat change to total heat change (sensible plus latent) in the process (Correct answer)
- The ratio of dry-bulb temperature change to wet-bulb temperature change
- The ratio of outdoor air enthalpy to return air enthalpy
Correct answer: The ratio of sensible heat change to total heat change (sensible plus latent) in the process
SHR is the fraction of total heat change that is sensible; it equals sensible heat divided by (sensible + latent heat) and determines the slope of the process line on the psychrometric chart.
Question 3: On a psychrometric chart, an adiabatic saturation (evaporative cooling) process follows which type of line?
- A line of constant dry-bulb temperature
- A line of constant dew point temperature
- A line of constant wet-bulb temperature (Correct answer)
- A line of constant relative humidity
Correct answer: A line of constant wet-bulb temperature
Because no heat is added or removed in an adiabatic process, evaporative cooling follows a constant wet-bulb temperature line, moving from lower humidity ratio toward the saturation curve.
Question 4: What happens to the humidity ratio during a pure sensible heating process?
- Humidity ratio increases proportionally with dry-bulb temperature
- Humidity ratio remains constant while dry-bulb temperature increases (Correct answer)
- Humidity ratio decreases as dry-bulb temperature increases
- Humidity ratio and dry-bulb temperature both remain constant
Correct answer: Humidity ratio remains constant while dry-bulb temperature increases
Sensible heating adds only thermal energy (no moisture) to the air, so humidity ratio remains unchanged while dry-bulb temperature rises and relative humidity decreases.
Question 5: What does bypass factor (BF) represent for a cooling and dehumidifying coil?
- Percentage of supply air that bypasses duct filters
- Fraction of air that passes through the coil without reaching the apparatus dew point condition (Correct answer)
- Ratio of outdoor air to return air in the mixed airstream
- Thermal efficiency rating of the cooling coil heat exchanger
Correct answer: Fraction of air that passes through the coil without reaching the apparatus dew point condition
Bypass factor is the fraction of airflow that passes through the coil without making effective thermal contact with the coil surface, remaining essentially at entering air conditions.
Question 6: How is the resulting condition of a mixed airstream determined when two airstreams at different psychrometric states are combined?
- By simple arithmetic average of the dry-bulb temperatures of both streams
- By the lever rule: the mixed point lies on the straight line between the two state points, divided by mass flow rate proportions (Correct answer)
- By taking the geometric mean of both inlet air conditions
- By selecting the dominant stream with the higher volumetric flow rate
Correct answer: By the lever rule: the mixed point lies on the straight line between the two state points, divided by mass flow rate proportions
The mixed air state is found using the lever rule on the psychrometric chart: the resulting point lies on the straight line connecting the two inlet state points, located proportionally by their mass flow rates.
Question 7: What is the apparatus dew point (ADP) of a cooling coil?
- The dew point temperature of supply air leaving the coil
- The hypothetical coil surface temperature on the saturation curve where all air would leave if contact were perfect (Correct answer)
- The minimum coil surface temperature required to prevent frost formation
- The temperature of condensate collected in the drain pan below the coil
Correct answer: The hypothetical coil surface temperature on the saturation curve where all air would leave if contact were perfect
The ADP is the theoretical point on the saturation curve representing the temperature at which all supply air would leave if every air molecule made perfect contact with the coil surface (i.e., bypass factor = 0).
ASHRAE Psychrometrics and Moist Air Properties 2
Which psychrometric process occurs when air passes over a cooling coil whose surface temperature is below the air's dew point?