Question 1

What is the humidity ratio and how is it calculated?

Accepted Answer

The humidity ratio ω is the mass of water vapor per kilogram of dry air (kg/kg). It is calculated as ω = 0.622 × ps×φ / (P − ps×φ), where ps is the saturation pressure, φ is relative humidity (0–1), and P is total atmospheric pressure. At 25°C and 50% RH, ω ≈ 0.0099 kg/kg (9.9 g/kg).

Question 2

How is specific enthalpy of moist air computed?

Accepted Answer

Specific enthalpy h = cpa×T + ω×(hfg + cpv×T), where cpa = 1.006 kJ/(kg·K) is the specific heat of dry air, cpv = 1.805 kJ/(kg·K) is the specific heat of water vapor, and hfg = 2501 kJ/kg is the latent heat of vaporization at 0°C. At 25°C and 50% RH, h ≈ 50.5 kJ/kg.

Question 3

What does dew point temperature represent?

Accepted Answer

Dew point temperature Tdp is the temperature at which water vapor begins to condense when air is cooled at constant pressure. It is the temperature where the partial pressure of water vapor equals the saturation pressure, obtained by inverting the Antoine equation. Dew point depends only on absolute humidity, not on dry-bulb temperature. It is a key parameter for condensation and dehumidification calculations.

Question 4

How is the wet-bulb temperature different from dry-bulb?

Accepted Answer

The dry-bulb temperature is the ordinary air temperature measured by a thermometer. The wet-bulb temperature Twb is the temperature reached by adiabatic saturation — a thermometer covered with a wet wick cooling through evaporation. The difference (wet-bulb depression) reflects the drying potential of the air: larger difference means drier air. When air is fully saturated (φ=100%), Twb = Tdb. Wet-bulb temperature is used in cooling tower design and evaporative cooling performance.

Psychrometric Chart Calculator

Theory