8 Steps to read Psychometric Chart , Psychometric Conversion Chart & related important terms.

Psychometrics (Hygrometry)-

·         It is the study of the properties of mixtures of air and water vapour.

·         It’s a determination of physical and thermodynamic properties of gas-vapor mixtures.

·         Atmospheric air is a mixture of many gases plus water vapour and a number of pollutants. Composition of standard air are listed below-

Constituent	Molecular Weight	Molecular fraction
Oxygen	32.000	0.2095
Nitrogen	28.016	0.7809
Argon	39.944	0.0093
Carbon dioxide	44.010	0.0003

·         

      The amount of water vapour and pollutants vary from place to place. The concentration of water vapour and pollutants decrease with altitude, and above an altitude of about 10 km, atmospheric air consists of only dry air.

·         Moist air is mixture of various gases that constitute air and water vapour.

As we know the mixture of water vapor and air is the most common system of interest because of its several application such as, heating, ventilating, and air-conditioning and meteorology.

In air conditioning systems is a mixture of dry air and water vapour. While the composition of dry air is constant, the amount of water vapour present in the air may vary from zero to a maximum depending upon the temperature and pressure of the mixture (dry air + water vapour).

What is Saturated air?

·         At a given temperature and pressure the dry air can only hold a certain maximum amount of moisture. When the moisture content is maximum, then the air is known as saturated air, which is established by a neutral equilibrium between the moist air and the liquid or solid phases of water.

Properties of psychometric air-

Dry-bulb temperature (DBT)-

·         DBT is the temperature of the moist air as measured by a standard thermometer or other temperature measuring instruments.

·         It is indicated by thermometer exposed to the air in a place sheltered from direct solar radiation.

Wet-bulb temperature (WBT)-

·         The wet-bulb temperature is the lowest temperature that can be reached under current ambient conditions by the evaporation of water only.

·         Wet-bulb temperature is largely determined by both actual air temperature (dry-bulb temperature) and the amount of moisture in the air (humidity).

·          At 100% relative humidity, the wet-bulb temperature equals the dry-bulb temperature.

Saturated vapour pressure (p_sat)

It is the saturated partial pressure of water vapour at the dry bulb temperature. This is readily available in thermodynamic tables and charts. ASHRAE suggests the following regression equation for saturated vapour pressure of water, which is valid for 0 to 100^oC.

In (P_sat)= C₁ / T + C₂ + C₃ T + C₄ T² + C₅ T³ + C₆ In (T)

Where p_sat = saturated vapor pressure of water in kilo Pascal

T = temperature in K

The regression coefficients c₁ to c₆ are given by:

c₁ = -5.80022006E+03, c₂ = -5.516256E+00, c₃ = -4.8640239E-02 , c₄ = 4.1764768E-05, c₅ = -1.4452093E-08, c₆ = 6.5459673E+00.

Humidity-

Humidity is the amount of water vapor present in the air. Water vapor is the gaseous state of water and is invisible.

Relative Humidity (Φ) or (RH)-

It is defined as the ratio of the mole fraction of water vapour in moist air to mole fraction of water vapour in saturated air at the same temperature and pressure.

Dew Point temperature

It is the temperature when vapour changes into liquid (Condensation) i.e dew.

Enthalpy-

Enthalpy is the measure of the total thermal energy in air.

Energy content is expressed as energy per unit weight of air (Btu/lb_air, J/kg_air).

Enthalpy in the psychrometric chart can read from where the appropriate wet-bulb line crosses the diagonal scale above the saturation curve.

Air with the same amount of energy may either be drier hotter air (higher sensible heat) or cooler moister air (higher latent heat).

Psychrometric ratio

 

Ratio of heat transfer coefficient to the product of mass transfer coefficient and humid heat at a wetted surface.

r= h_c / k_y c_s

where:

r{\displaystyle r} = Psychrometric ratio, dimensionless

{\displaystyle h_{c}}h_c = convective heat transfer coefficient, W m⁻² K⁻¹

{\displaystyle k_{y}}k_y = convective mass transfer coefficient, kg m⁻² s⁻¹

{\displaystyle c_{s}}c_s = humid heat, J kg⁻¹ K⁻¹

Psychometric conversion chart

·         Standard psychrometric charts are bounded by the dry-bulb temperature line (abscissa) and the vapour pressure or humidity ratio (ordinate).

·         The Left Hand Side of the psychrometric chart is bounded by the saturation line.

·         Psychrometric charts are readily available for a standard barometric pressure of 101.325 kPa at sea level and for normal temperatures (0-50^oC). ASHRAE has also developed psychrometric charts for other temperatures and barometric pressures (for low temperatures -40 to 10^oC, high temperatures 10 to 120^oC and very high temperatures 100 to 120^oC).

Eight Step of Reading of Psychometric Chart

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