Psychrometric Chart: What is Psychrometry, Diagram, Application

By Mohit Uniyal|Updated : August 1st, 2022

Psychrometry was first proposed and developed by German engineer Richard Mollierin the 19th century. The psychrometry chart represents the thermodynamic properties of moist air to analyze air conditioning processes. The degree of freedom of the Psychrometric chart is three (barometric pressure, DBT and WBT). The degree of freedom of the Psychrometric chart means the number of independent parameters required to define the position of a point in the Psychrometric chart.

A psychrometric chart is used to design winter and summer air conditioning by analyzing different air conditioning processes like humidification. Dehumidification, heating and cooling processes. This article discusses the thermodynamic properties of moist air and the representation of these properties in psychrometric charts.

Table of Content

What is Psychrometry?

Psychrometry describes the thermodynamic properties of moist air. Moist air is a mixture of water vapour and dry air. The psychrometric chart represents dry bulb temperature Tdb, wet bulb temperature Twb, dew point temperature Tdp, specific humidity or humidity ratio(ω), relative humidity(Φ), degree of saturation(μ), specific volume(ν) and specific enthalpy(h). Here, DBT and WBT have measured the properties of moist air, and the remaining properties are derived from basic thermodynamic relations and fundamental definitions of psychrometry.

Measurement of DBT and WBT in Psychrometry

A sling Psychrometer is used to measure dry bulb temperature and wet bulb temperature. Dry bulb temperature (DBT) is the temperature of moist air measured by a sling thermometer and which is freely exposed to the air. DBT does not give information about moisture details in the moist air.

Wet bulb temperature (WBT) is the temperature of moist air measured by a sling thermometer and which is covered by a wet cloth. WBT gives the moisture details of moist air. DBT and WBT have measured the properties of moist air. Measurement of DBT and WBT by using a sling psychrometer is shown in the fig. below.

Psychrometry Properties

There are various properties that can help in determining climatic conditions. A few essential Psychrometry properties are:

  • DBT
  • WBT
  • Dew Point Temperature (DPT)
  • Specific Humidity or Humidity Ratio in Ps
  • Relative Humidity
  • Degree of Saturation
  • Specific Volume
  • Specific Enthalpy

DBT and WBT in Psychrometry

Dry bulb temperature (DBT) is represented as vertical lines on the Psychrometric chart, as shown in Fig. below. Wet bulb temperature (WBT) is represented as inclined lines on the Psychrometric chart, as shown in Fig. below. The curve on the Psychrometric chart is called the saturation curve. DBT and WBT will be equal at any point on the saturation curve.

Dew Point Temperature (DPT)

Dew point temperature is the temperature at which the moist air starts condensing. The temperature corresponding to the partial pressure of water vapour(Pv) is defined as moist air's Dew Point Temperature (DPT). Horizontal lines drawn on the Psychrometric chart represent the DPT lines, as shown in fig. below. The DBT, WBT and DPT values are the same for moist air on the saturation curve. DBT, WBT and DPT lines are represented on the Psychrometric chart as shown in Fig. below.

Specific Humidity or Humidity Ratio

Specific humidity(ω) is one of the important thermodynamic properties of moist air. Moist air consists of water vapour and dry air. Specific humidity or humidity ratio is the mass of water vapour in one kg of dry air. Specific humidity is denoted with ω and expressed in terms of kg/kg of dry air. The horizontal lines drawn on the Psychrometric chart represent the specific humidity lines, as shown in the figure below.

Specific humidity or Humidity ratio (ω in kg/kg of dry air)

ω = Mass of water vapour/Mass of dry air

ω = mv/ma = (0.622 Pv)/(Pb-Pv)

Relative Humidity in Psychrometry

Relative humidity in Psychrometry is defined as the ratio of the mass of water vapour in moist air and the mass of water vapour in saturated air at the same temperature and pressure. The relative humidity is denoted with ϕ and represented in terms of percentage %. Relative humidity at any point on the saturation curve is 100%. Relative humidity curves on Psychrometry chat are shown in fig. below.

Relative humidity(ϕ)

ϕ = Mass of water vapour in moist air/Mass of water vapour in saturated air

ϕ = mv/mvs = Pv/Pvs=Pv/Ps

Degree of Saturation

The degree of saturation in Psychrometry is defined as the ratio of specific humidity of moist air and specific humidity of saturated air at the same temperature. The degree of saturation is represented as μ. The degree of saturation is a measure of moisture absorbing capacity of air. It gives the relationship between normal specific humidity(ω) and maximum possible specific humidity (ωs).

Degree of Saturation(μ)

μ = ω/ωs

Here,

  • ω = (0.622 Pv)/(Pb-Pv)
  • ωs = (0.622 Pvs)/(Pb-Pvs) = (0.622 Ps)/(Pb-Ps);

μ = ω/ωs = [(0.622 Pv)/(Pb-Pv)] / [(0.622 Ps)/(Pb-Ps)]

= [Pv/ Ps][(Pb-Ps)/(Pb-Pv)] = ϕ[(Pb-Ps)/(Pb-Pv)]

Specific Volume and Specific Enthalpy

Specific volume in Psychrometry is defined as the volume occupied by one kg of dry air, and it is represented with v and expressed in terms of m3/kg. As shown in fig. below, specific volume lines are represented as inclined lines on the Psychrometric chart. Specific enthalpy of moist air is defined as the algebraic sum of specific enthalpy of dry air and specific enthalpy of water vapour. Specific enthalpy lines are represented as inclined lines on the Psychrometric chart, as shown in fig below.

Specific enthalpy of Moist air (h)

h = 1.005Tdb+ω(2500+1.88 Tdb) kJ/kg

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FAQs on Psychrometry

  • Psychrometry is the study of the conditions of the gas-vapour mixture. The properties of moist air is measured and tested in this. Various methods such as DBT, WBT, DPT, etc are used to measure the properties of moist air

  • The degree of freedom of the Psychrometric chart is three (i.e., barometric pressure, DBT and WBT). To define the position of any point in the Psychrometric chart is three because the Psychrometric chart is drawn to represent the thermodynamic properties of moist air. Moist air contains water vapour and dry air. According to Gibbs phase rule

    P+F=C+2

    For Moist Air

    Number of phases P=1

    Number of components in moist air C=2

    P+F=C+2

    1+F=2+2

    F=3

  • Bypass factor (BPF) is the ratio of amount of air bypassing the coil and total amount of air passed through the coil during sensible heating or sensible cooling processes in Psychrometry.

    BPF = Amount of Air Bypass the Coil/Total Air Passed through the coil

  • In Psychrometry, the process of addition of moisture at constant dry bulb temperature is called the humidification process. Specific humidity and relative humidity of moist air increases in the humidification process. The process of removal of moisture at constant dry bulb temperature is called dehumidification process. Specific humidity and relative humidity of moist air decreases in the dehumidification process.

  • The process of introducing finely atomized water into air is called adiabatic humidification. Specific humidity and relative humidity of moist air increases in adiabatic humidification process.

  • Psychometry properties are the aspects that are used to measure the conditions of moist air. There are various properties such as:

    • DBT
    • WBT
    • Dew Point Temperature (DPT)
    • Specific Humidity or Humidity Ratio in Ps
    • Relative Humidity
    • Degree of Saturation
    • Specific Volume
    • Specific Enthalpy

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