ASHRAE Psychrometrics and Moist Air Properties 1 — Questions and Answers
Question 1: What is the dry-bulb temperature of air?
- Temperature measured by a thermometer wetted with moisture
- Temperature measured by a standard thermometer shielded from radiation and unaffected by moisture evaporation (Correct answer)
- Temperature at which air becomes fully saturated with water vapor
- Temperature measured at the dew point condition
Correct answer: Temperature measured by a standard thermometer shielded from radiation and unaffected by moisture evaporation
Dry-bulb temperature is the temperature of air measured by a standard thermometer shielded from radiation and not influenced by moisture content or evaporation.
Question 2: What does wet-bulb temperature primarily indicate about an air sample?
- The actual sensible heat content of the air mass
- The dew point temperature of the air
- The cooling effect of evaporation, which correlates with the moisture content of the air (Correct answer)
- The temperature achieved only at 100% relative humidity
Correct answer: The cooling effect of evaporation, which correlates with the moisture content of the air
Wet-bulb temperature measures the cooling effect of water evaporating from a wetted thermometer wick, providing an indirect measure of the air's moisture content.
Question 3: On a standard psychrometric chart, what does the saturation curve (outer boundary) represent?
- The line of constant enthalpy values
- All states where relative humidity equals 100% (Correct answer)
- The boundary between sensible heat and latent heat zones
- The line of constant specific volume
Correct answer: All states where relative humidity equals 100%
The saturation curve is the locus of all air states at 100% relative humidity, representing fully saturated air at each dry-bulb temperature.
Question 4: How is humidity ratio (W) defined in psychrometrics?
- Percentage of water vapor in air by volume
- Ratio of water vapor mass to total moist air mass
- Ratio of water vapor mass to dry air mass (Correct answer)
- Relative saturation percentage of the air sample
Correct answer: Ratio of water vapor mass to dry air mass
Humidity ratio W is defined as the mass of water vapor per unit mass of dry air (lb_w/lb_da or kg_w/kg_da), and remains constant during sensible heating or cooling.
Question 5: What is the dew point temperature of an air sample?
- The temperature at which ice crystals form within the air mass
- The temperature to which air must be cooled at constant pressure for condensation to begin (Correct answer)
- The temperature difference between dry-bulb and wet-bulb readings
- The minimum temperature achievable through adiabatic evaporative cooling
Correct answer: The temperature to which air must be cooled at constant pressure for condensation to begin
Dew point is the temperature at which air, cooled at constant pressure and constant humidity ratio, reaches saturation and water vapor begins to condense.
Question 6: In psychrometrics, what does enthalpy (h) represent?
- Total sensible heat content of dry air only
- Total heat content per unit mass of dry air including both sensible heat and latent heat of water vapor (Correct answer)
- Partial pressure exerted by water vapor in the air mixture
- Temperature differential driving heat transfer across HVAC coils
Correct answer: Total heat content per unit mass of dry air including both sensible heat and latent heat of water vapor
Enthalpy in psychrometrics is the total heat content (sensible + latent) referenced per unit mass of dry air, encompassing both dry air energy and the energy of the associated water vapor.
Question 7: What standard atmospheric pressure does ASHRAE use as the baseline for psychrometric calculations?
- 14.0 psia (96.5 kPa)
- 14.696 psia (101.325 kPa) (Correct answer)
- 15.0 psia (103.4 kPa)
- 13.5 psia (93.1 kPa)
Correct answer: 14.696 psia (101.325 kPa)
ASHRAE uses the internationally recognized standard atmospheric pressure of 14.696 psia (101.325 kPa) as the baseline for all psychrometric tables, equations, and charts.
