Psychrometrics and Moist Air Properties Flashcards
7 cards from real ASHRAE practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Psychrometrics and Moist Air Properties flashcards as text
Which psychrometric process occurs when air passes over a cooling coil whose surface temperature is below the air's dew point?
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.
What is the sensible heat ratio (SHR) of an air conditioning process?
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.
On a psychrometric chart, an adiabatic saturation (evaporative cooling) process follows which type of line?
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.
What happens to the humidity ratio during a pure sensible heating process?
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.
What does bypass factor (BF) represent for a cooling and dehumidifying coil?
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.
How is the resulting condition of a mixed airstream determined when two airstreams at different psychrometric states are combined?
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.
What is the apparatus dew point (ADP) of a cooling coil?
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).