What is a Psychrometric Chart?
A Psychrometric chart represents various parameters relating to moist air in a graphical format. So, using this chart we can study the properties of air without using any complex mathematical equations.
These charts are available at various temperature and pressure. Psychrometric properties are also available as data tables, equations, and slide rulers. Going to any point on the Psychrometric chart will give you the full information about all the Psychrometric properties. Let's have a look at the Psychrometric Chart.
Now, let's make it easy my studying every single line and curve one-by-one. But for that we must know these 7 parameters;
Dry Bulb Temperature
Wet Bulb Temperature
Dew Point Temperature
Relative Humidity
Moisture
Enthalpy
Specific Volume
If you know any 2 parameters, then you can find the other five using this chart.
Let's discuss them individually along with their representation in the Psychrometric chart.
Dry-Bulb Temperature
It refers to the ambient air temperature.
It is called ‘dry bulb’ because the air temperature that is indicated by the thermometer is not affected by the moisture of the air.
Shown in Figure 1(a) as the horizontal axis of chart.
Figure 1. (a) Dry Bulb (b) Wet Bulb temperature representation
Wet-Bulb Temperature
It is measured by wrapping a wet cloth on the thermometer.
Moisture on the cloth evaporates when come in contact with air, lowering the temperature recorded by the thermometer.
Shown in Figure 1(b) as the diagonal lines on the chart.
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Dew Point Temperature
It is the temperature at which water vapors start to condense out of the air.
It depends on the moisture content in air. Higher the moisture content , higher the dew point temperature & vice-versa.
It is shown in Figure 2(a) as vertical axis (horizontal lines) on the chart.
Figure 2. (a) Dew point temperature (b) Relative Humidity (RH) representation
Relative Humidity (R.H.)
It is defined as the ratio of partial pressure of vapor to the vapor pressure of liquid at the gas temperature.
Shown in Figure 2(b) as exponential curves.
Moisture
It is the proportion of the mass of water vapor per unit mass of dry air.
It is the vertical axis of the chart; Figure 3.
Figure 3. Moisture content representation
Enthalpy
It is the total amount of heat energy of the moist air.
It includes the amount of heat required to raise the temperature of the mixture of dry air and the water vapors by 1 Degree Celsius (or 1 Kelvin) per kg of dry air.
It is indicated by the diagonal lines on the chart as shown in Figure 4(a).
Figure 4. (a) Enthalpy and (b) Specific Volume representation
Specific Volume
It is the inverse of density.
Volume of mixture of air + water vapor per unit mass of dry air at a particular temperature and pressure.
It is shown in Figure 4(b) as diagonal lines.
Applications of Psychrometric Chart
We use this chart in finding explanation for questions like why heated air can hold more moisture, designing the air conditioners or in troubleshooting and finding solutions to greenhouse.
Thankyou!
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