21 Pshychrometry

Updated on 01/04/26 by Mananjay MahatoShare on WhatsApp

Psychrometry is the study of the properties of moist air (a mixture of dry air and water vapor).

1. Key Definitions & Properties

  • Dry Air: Air containing no water vapor.

  • Moist Air: Mixture of dry air and water vapor ($0\%$ to $4\%$ by volume).

  • Dry Bulb Temperature (DBT): Temperature of air measured by an ordinary thermometer.

  • Wet Bulb Temperature (WBT): Temperature measured by a thermometer covered with a water-soaked wick. It indicates the “cooling effect” of evaporation.

  • Dew Point Temperature (DPT): The temperature at which water vapor starts condensing. (At saturation, $DBT = WBT = DPT$).

  • Relative Humidity (RH): Ratio of the actual mass of water vapor to the mass of water vapor required to saturate the same volume at the same temperature. Expressed in %.

  • Specific Humidity (Humidity Ratio): Mass of water vapor per kg of dry air ($g/kg$ or $kg/kg$).

  • Sensible Heat: Heat that changes temperature without changing the state.

  • Latent Heat: Heat that changes the state (moisture content) without changing the temperature.

2. The Psychrometric Chart: Lines & Directions

The chart is a graphical representation of air properties at a constant atmospheric pressure (usually $1.01325$ bar).

Line Type Orientation/Shape
DBT Lines Vertical and uniformly spaced.
Specific Humidity Lines Horizontal and uniformly spaced.
WBT Lines Inclined/Diagonal (Downward to the right).
Enthalpy Lines Parallel to WBT lines (almost).
RH Lines Curved lines (100% RH is the Saturation Curve).
Specific Volume Lines Steeply inclined lines.

3. Basic Psychrometric Processes

These processes are critical for exam numericals and conceptual questions:

  • Sensible Heating: Moving Right on the chart. DBT increases, Humidity Ratio remains constant.

  • Sensible Cooling: Moving Left on the chart. DBT decreases, Humidity Ratio remains constant.

  • Humidification: Moving Up. Moisture added, DBT constant.

  • Dehumidification: Moving Down. Moisture removed, DBT constant.

  • Heating and Humidification: Winter Air Conditioning.

  • Cooling and Dehumidification: Summer Air Conditioning (most common).

  • Adiabatic Cooling (Evaporative Cooling): Follows the constant WBT line. DBT decreases while Humidity increases (used in desert coolers).

4. Important Formulas for RAC Mechanics

For competitive exams, remember these relations:

  • Enthalpy of Moist Air ($h$):

    $$h = 1.005t + w(2500 + 1.88t) \text{ kJ/kg}$$

    (Where $t$ is DBT in $^\circ$C and $w$ is specific humidity)

  • Specific Humidity ($w$):

    $$w = 0.622 \times \frac{P_v}{P_b – P_v}$$

    (Where $P_v$ = Partial pressure of vapor, $P_b$ = Barometric pressure)

  • Sensible Heat Factor (SHF):

    $$SHF = \frac{\text{Sensible Heat}}{\text{Sensible Heat} + \text{Latent Heat}}$$
  • By-pass Factor (BPF): Represents the inefficiency of a cooling coil. For a cooling coil, $BPF = \frac{T_{out} – T_{surface}}{T_{in} – T_{surface}}$.

  • Apparatus Dew Point (ADP): The surface temperature of the cooling coil.

  • Human Comfort: Standard conditions are usually $24^\circ$C DBT and $50\% – 60\%$ RH.

  • Sling Psychrometer: Used to measure DBT and WBT simultaneously by rotating it in the air (minimum velocity required: $5$ m/s).

  • Hygrometer: An instrument specifically used to measure Humidity.

21 Pshychrometry