Psychrometry

Psychrometry is the study of the thermodynamic properties of moist air (a mixture of dry air and water vapor) and the processes used to condition it. Key properties include dry-bulb temperature (DBT), wet-bulb temperature (WBT), dew-point temperature (DPT), relative humidity (RH), humidity ratio (W), specific volume (v), and enthalpy (h). These properties are graphically represented on a psychrometric chart at a constant atmospheric pressure, allowing engineers to visualize and calculate air-conditioning processes. Sensible heating and cooling change the DBT while keeping the humidity ratio constant (horizontal lines on the chart). Humidification and dehumidification change the moisture content. Cooling and dehumidification occurs when air is cooled below its dew-point temperature, causing moisture to condense. The Sensible Heat Factor (SHF) defines the ratio of sensible to total heat load, determining the slope of the process line. When two air streams mix adiabatically, the resulting state point lies on the straight line connecting the two initial states, dividing the line inversely proportional to their mass flow rates. Understanding these principles is essential for calculating HVAC loads, designing air handling units, and ensuring indoor thermal comfort and air quality.

Governing FormulaHumidity Ratio: W = 0.622 · P_v / (P_atm - P_v) Relative Humidity: φ = P_v / P_sat(T) Enthalpy (SI, T in °C): h = 1.006 · T + W · (2501 + 1.86 · T) Sensible Heat Load: Q_s = m_dot_a · c_p · ΔT Latent Heat Load: Q_l = m_dot_a · h_fg · ΔW ≈ m_dot_a · 2501 · ΔW Sensible Heat Factor: SHF = Q_s / (Q_s + Q_l) where: P_v = partial pressure of water vapor, P_sat = saturation pressure at DBT m_dot_a = mass flow rate of dry air, c_p = specific heat of moist air (~1.02 kJ/kg·°C)

Knowledge Check

10 Questions

1.What does the humidity ratio (or specific humidity) represent in psychrometrics?

2.If the partial pressure of water vapor in an air-water vapor mixture is 1.5 kPa and the total atmospheric pressure is 101.325 kPa, what is the humidity ratio?

3.On a standard psychrometric chart, what psychrometric process is represented by a horizontal line moving to the right?

4.What is the physical significance of the wet-bulb temperature?

5.An air handling unit supplies 1.5 kg/s of dry air. The air is heated from 15°C to 30°C. Assuming the specific heat of the moist air is 1.02 kJ/(kg·°C), what is the sensible heat load added to the air?

6.How does the relative humidity of moist air change when it undergoes sensible heating?

7.What is the Sensible Heat Factor (SHF) for an air conditioning process?

8.When two air streams are mixed adiabatically, how is the resulting state point located on the psychrometric chart?

9.What thermodynamic condition must be met for an air stream to undergo cooling and dehumidification?

10.What does the dew-point temperature represent?