313D Psychrometrics and Humidity Control 1 — Questions and Answers
Question 1: What does the term 'psychrometrics' refer to in HVAC?
- The study of chemical reactions in refrigerants
- The study of thermodynamic properties of moist air (Correct answer)
- The measurement of refrigerant pressures in a system
- The analysis of duct static pressure losses
Correct answer: The study of thermodynamic properties of moist air
Psychrometrics is the engineering science that studies the thermodynamic properties of moist air — mixtures of dry air and water vapor.
Question 2: What is the dew point temperature?
- The temperature at which air reaches maximum velocity through a duct
- The temperature differential across the evaporator coil at design conditions
- The temperature to which air must be cooled for water vapor to begin condensing (Correct answer)
- The lowest allowable refrigerant suction temperature in the system
Correct answer: The temperature to which air must be cooled for water vapor to begin condensing
Dew point is the temperature at which air becomes saturated and water vapor condenses into liquid, assuming constant pressure and moisture content.
Question 3: On a psychrometric chart, wet-bulb temperature lines run in which direction?
- Horizontally across the chart
- Diagonally downward from the saturation curve toward the lower right (Correct answer)
- Vertically parallel to the dry-bulb temperature axis
- In concentric arcs around the origin point
Correct answer: Diagonally downward from the saturation curve toward the lower right
Wet-bulb temperature lines slope diagonally downward from upper left to lower right on a psychrometric chart and nearly coincide with lines of constant enthalpy.
Question 4: What is relative humidity (RH)?
- The actual mass of water vapor in air measured in grains per pound of dry air
- The ratio of actual moisture content to the maximum moisture air can hold at that temperature (Correct answer)
- The temperature difference between wet-bulb and dry-bulb readings
- The temperature at which condensation forms on interior surfaces
Correct answer: The ratio of actual moisture content to the maximum moisture air can hold at that temperature
Relative humidity is the ratio of actual water vapor pressure to saturation water vapor pressure at the same temperature, expressed as a percentage.
Question 5: What is the key difference between sensible heat and latent heat in air conditioning?
- Sensible heat causes a measurable temperature change; latent heat involves phase change without temperature change (Correct answer)
- Sensible heat is removed at the condenser; latent heat is added at the compressor
- Sensible heat relates to refrigerant flow; latent heat relates to air velocity
- There is no practical difference — both describe the same heat transfer mechanism
Correct answer: Sensible heat causes a measurable temperature change; latent heat involves phase change without temperature change
Sensible heat changes the temperature of air (measurable with a thermometer), while latent heat is absorbed or released during phase changes like evaporation or condensation.
Question 6: What does enthalpy measure in an HVAC psychrometric context?
- The velocity of conditioned air through supply ductwork
- The pressure differential across the air filter at design airflow
- The total heat content of moist air, combining both sensible and latent heat (Correct answer)
- The static regain in a supply air duct system
Correct answer: The total heat content of moist air, combining both sensible and latent heat
Enthalpy represents the total heat content of moist air, expressed in BTU per pound of dry air, and includes both sensible (temperature) and latent (moisture) components.
Question 7: What indoor relative humidity range does ASHRAE recommend for occupant comfort in residential buildings?
- 10–20%
- 30–60% (Correct answer)
- 70–80%
- 85–95%
Correct answer: 30–60%
ASHRAE recommends maintaining indoor RH between 30% and 60% to ensure occupant comfort and prevent mold growth or dry-air discomfort.
313D Psychrometrics and Humidity Control 1
What does the term 'psychrometrics' refer to in HVAC?