Psychrometry and Psychrometric Chart – Basic & Details

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Psychrometry and Psychrometric Chart – Basic & Details

Let’s learn psychrometry & psychrometric chart. The Psychrometry is the science of studying the properties of air-water mixtures. It is widely used to illustrate and analyze the characteristics of air-water mixtures in refrigeration, air conditioning processes, and many other processes. 

Psychrometric Chart is a graphical representation where all psychrometric parameters can be derived.

Psychrometry

Do you know where from the word Psychrometry comes? Psychrometrics is a word which derived from GREEK words, Psychro and Metrics.

  • Psychro means ‘to breathe’ or ‘to blow’ or ‘to make cold’
  • Metrics means to measure

Hence, psychrometry used to measure the properties of moist air, their control, the effect on materials, and human comfort. Moisture content in air varies significantly under different conditions like temperature, pressure, altitude, etc.

air water mixture
Air water mixture
  • Air can contain a large amount of moisture if it is hot.
  • When the air is cold, it’s capacity to hold the moisture is reduced.
  • When the temperature of hot air begins to reduce, the vapour also started to cool
  • If cooling continues, air reduces to hold the capacity of containing moisture content and it will condense into moisture droplets.

Psychrometric Chart

Let us take an example when we are inside a car, sometimes we use to see water droplets outside the windows. Where from this water comes? It comes from the surrounding air only. Moisture content present in the air comes in contact with cold window surface & becomes small water droplets. Now, psychrometry is entirely defining all the properties air-water mixtures.

What is Psychrometric Chart

Psychrometry is developed into a psychrometric chart which has a wide function to understand the properties of air-water mixture and calculate the various parameters related to it.

  • Any two parameters in the psychrometric chart can make the value of any other parameters.
  • Easy to find out the condensate temperature.
  • Easy to calculate the cooling or heating load requirements.
psychrometric chart graph
Psychrometric chart graph

All the psychrometric chart is represented in a Psychrometric which provides the states or condition of the air or water.

A typical psychrometric chart is attached for reference.

We all know, P-V diagram, this psychrometric chart is prepared by following three steps:

Psychrometry Considerations

There are few psychrometric considerations:

  • Dry air and water vapour present in atmospheric.
  • Both dry air and water vapour behave independently. Hence, we can easily consider dry air and water vapour separately as well as independently.
  • When dry air is referred to, it will mean the air part;
  • When water vapour or moisture is referred, we will simply consider water part only,
  • When referring to just air, it will mean the mixture of dry air and water vapour.
  • Complete dry air doesn’t exist, it always has some water vapour.
  • Oxygen and nitrogen are the main part and the proportion of gasses do not vary, and it acts as a single gas. Hence, dry air is considered a single gas
  • Dry air obeys the gas laws of Charles, Boyle etc
  • In the same way, water vapour is also obeyed gas laws.

In our rooms, the air is the mixture of dry air and water vapour.

Psychrometry Terms

All the psychrometric chart is represented in a Psychrometric which provides the states or condition of the air at any specific time. It displays the different properties of air:

  • Dry-bulb temperature
  • Wet-bulb temperature
  • Relative humidity (RH)
  • Humidity ratio
  • Specific volume
  • Dew point temperature
  • Enthalpy
  • Energy content
  • Moisture content
  • Specific volume and many more.

Psychrometric Chart Terms & Definitions

The Psychrometric terms are described with the explanation. If any of two psychrometric terms are known, one intersection can be plotted and from that point, other parameters can be determined using the chart.

Psychrometric Terms and definitions are defined below for a better understanding of the basic of air conditioning system:

Terms for Air

Atmospheric Air

Atmospheric air contains the followings:

  • Nitrogen
  • Oxygen
  • Carbon dioxide
  • Water vapour
  • Other gases and miscellaneous contaminants such as dust, pollutants and smoke.

The amount of pollutants or water vapour varies based on the location. These pollutants, moisture content decrease with the height from the earth surface.

atmospheric air components
Atmospheric air components

This is the air we breathe and use for air conditioning and ventilation system. Hence, it is necessary to filtrate this air before using it.

Dry Air

Normal air consists of water vapour. The quantity of water vapour varies depending on the conditions. If water vapour and all impurities can be able to remove from the normal air, then dry air will be formed.

  • The pure dry air does not exist in nature; it always contains some water vapor.
  • Dry air considered as perfect gases, as it exists in the atmosphere at low pressure.
  • Water vapour considered as perfect gases, as it exists in the atmosphere at low pressure.

The dry air is considered to have the composition as given in the following table:

There are some basic data of air, which is kept in mind:

composition of air
Composition of air

Moist Air

Moist means the presence of water. When moisture is present in the air, it is called as moist air. Moist air means a mixture of air and water.

  • The moist air means a mixture of dry air and moisture or water vapour.
  • Composition of dry air is fixed and changes are negligible. Hence, it can be considered as constant.
  • The amount of water vapour present in the air depends upon the pressure and temperature of the mixture.
moist air components
Moist air components

Unsaturated air

Air is always consisting of some amount of water vapour. This water vapour depends on pressure and temperature.

Now we will take an enclosed space at some specific pressure and temperature and add moisture gradually. Initially, air consists in the enclosed space absorb the water vapour at the same pressure and temperature. This air is known as unsaturated air that means it can absorb or holds more water vapour.

Saturated air

But later, a stage will come when the air will not be able to absorb the moisture anymore. The extra moisture will be out in the form of dew or fog. The air which can hold the maximum water vapour or moisture at certain pressure and temperature is known as saturated air.

Saturated air and unsaturated air notes

  • The maximum quantity of water vapour air can hold depends on the temperature.
  • If the temperature of the air increases, air can hold more moisture,
  • As per Dalton’s law of partial pressure, P = Pa + Pw, where Pa = partial pressure of dry air and Pw = partial pressure of water vapour.
  • When the saturated air is cooled the water vapour starts to condense which is present in the air.
  • Unsaturated air means air contains less amount of water vapour than the saturated one.

Terms for Vapour

Let’s take some amount of water in an enclosed container and heat the water. Due to heating, the temperature of water gradually increases, and it will reach 100 deg. C.

At 100 deg. C, the addition of heat also is not able to change the temperature of the water. Here, instead of temperature changes, water is started to convert into vapour.

Unsaturated vapour

When the vapour is able to take the heat without temperature increase, it is known as unsaturated vapour.

Saturated vapour

Further increase of temperature, entire water will be converted to vapour. This vapour when all the water content will be converted to vapour is known as a saturated vapour.

Superheated vapour

If vapour is further heated, vapour will take the heat at 100 deg. C and there will not be any change in temperature. This steam which is having more heat content than the saturated vapour is called superheated vapour.

Terms for Air Temperature 

Dry Bulb Temperature

The Dry Bulb Temperature is simply the temperature of the ambient air that has no water on its surface. It is measured by normal thermometer & called “Dry Bulb Temperature” because the air temperature is indicated by a thermometer is not affected by the moisture of the air. When we talk about the temperature of the air, it is commonly referred to dry bulb temperature.

In the Psychrometric chart, Dry-bulb temperature is indicated on the X-axis, and all vertical lines in the chart represent constant temperature lines.

  • It is denoted as td, tdb , Tdb or DBT etc.
  • Dry-bulb temperature can be measured using a normal thermometer freely exposed to the air but shielded from radiation and moisture.
  • The temperature is usually given in degrees Celsius (°C) or degrees Fahrenheit (°F).
  • The SI unit is Kelvin (K). Zero Kelvin equals to -273o
dry bulb temperature psychrometry
Dry bulb temperature in Psychrometric chart

Measurement:  You take a thermometer and hold it in the room and note the temperature. It’s basically the ambient temperature that you see, is known as ‘Dry Bulb Temperature’.

Wet Bulb Temperature

The Wet Bulb Temperature is simply the temperature of the air which is associated with moisture. It is measured by normal thermometer & called “Wet Bulb Temperature” because the temperature is indicated by a thermometer is affected by the moisture of the air. When a normal thermometer is wrapped with a wet cloth, we get wet-bulb temperature.

  • It is denoted as Twb, tor twb
  • Wet-bulb temperature can be measured using a normal thermometer wrapped with wet cloth.
  • Wet-bulb temperatures are always lower than dry bulb temperatures and the only time that they will be the same is at saturation (i.e. 100% relative humidity).
  • The temperature is usually given in degrees Celsius (°C) or degrees Fahrenheit (°F).
  • The SI unit is Kelvin (K). Zero Kelvin equals to -273o

Measurement:  You take a thermometer and hold it in the room wrapped with a wet cloth and note the temperature. It’s basically the wet-bulb temperature that you see, is known as ‘Wet Bulb Temperature’.

wet bulb temperature graph
Wet bulb temperature in Psychrometric chart

Dew Point Temperature

If we cool moist air, we will see that moist air will start to condense at a certain temperature. This temperature is called as dew point temperature. Hence, we can say, Dew point temperature is the temperature at which moisture present in the air starts to condense.

When air is cooled, the relative humidity increases gradually until saturation is reached, and after the saturation level, condensation occurs. Condensation occurs on surfaces which are at or below the dew point temperature.

  • This is usually denoted by tdp or Tdp
  • Dew point is represented along the 100% relative humidity line on the psychrometric chart.
  • Dew point temperature is determined by moving from a state point horizontally to the left along constant humidity ratio up to curved saturation line.
  • At dew point, dry bulb temperature and wet bulb temperature are the same.
  • When the temperature of air reduces to the dew point, dew is formed.
  • The temperature is fixed at the dew point temperature.
  • The dew point temperature is directly related to the quantity of moisture in the air
  • It is the saturation temperature corresponding to the partial pressure of water vapor

Wet Bulb Depression

Web bulb depression means simply the difference between the dry-bulb temperature and wet bulb temperature at any point.

It indicates the relative humidity of the air.

Dew point Depression

This is the difference between the Dry bulb temperature and dew point temperature of the air.

Terms for Humidity

It is the mass of water present in 1 kg of dry air and is generally expressed in terms of a gram per kg of dry air ( g/kg of dry air).

It is also called a humidity ratio or specific humidity.

Relative Humidity

RH is an expression of the moisture content of a given atmosphere as a percentage of the saturation humidity at the same temperature.

It is the ratio of an actual mass of water vapour in a given volume of moist air to the mass of water vapour in the same volume of saturated air at the same temperature and pressure.

This is briefly written as RH.

relative humidity graph
Relative humidity in Psychrometric chart

Absolute Humidity

The weight of water vapour in grains or pounds of moisture per pound of dry air or grams of water vapour per kg of air, i.e. g/kg. It is also known as humidity ratio or moisture content.

It is the mass of water vapour present in 1 m3 of dry air and is generally expressed in terms of a gram per cubic meter of dry air (g / m3 of dry air).

1 kg of water vapour is equal to 15430 grains.

Saturation Humidity

Air at a given temperature can support only a certain amount of moisture and no more. This is known as the saturation humidity.

Degree of Saturation

It is the ratio of an actual mass of water vapour in a unit mass of dry air (i.e., humidity ratio of moist air) to a mass of water vapour in the same mass of dry air when it is saturated (i.e., humidity ratio of saturated moist air)  at the same temperature.

degree saturation formula
Degree saturation formula

Degree of Saturation is expressed as;

μ = x / xs

where

μ = degree of saturation

x = humidity ratio of the air

xs = humidity ratio of the air at saturation at the same temperature and pressure

Terms for Energy

Enthalpy

Enthalpy is the measure of heat energy in the air due to sensible heat or latent heat. Sensible heat means the heat energy which changes the temperature of air and latent heat means the heat energy which cannot change the temperature.

This sensible heat energy and latent heat energy together known as enthalpy.

  • Unit of Enthalpy is Btu per pound of dry air (Btu/lb of dry air) or kilojoules per kilogram (kJ/kg).
  • Enthalpy is useful in air heating and air cooling applications.

Enthalpy Deviation

When we talk about enthalpy, it refers to the enthalpy of saturation. It should be corrected by the enthalpy deviation due to the air not being in the saturated state.

  • The unit of Enthalpy deviation is Btu per pound of dry air.
  • Enthalpy deviation is applied where extreme accuracy is required; however, on normal air conditioning, we don’t consider it.

Specific Air Volume

The cubic feet of the mixture per pound of dry air or cubic meter of the mixture per kg of dry air represented in m3/kg. It is the reciprocal of density

Vapour Pressure

Latent heat is the heat content due to the presence of water vapour in the atmosphere. It is the heat, which was required to evaporate the given amount of moisture.

Vapour pressure is a measure of the tendency of a material to change into the gaseous or vapour state, and it increases with temperature.

Terms for Heat

Sensible heat

Sensible heat is one kind of heat which is the reason to increase the temperature of air i.e dry-bulb temperature of the air.

Latent heat

Latent heat is the heat content due to the presence of water vapour in the atmosphere. It is the heat, which was required to evaporate the given amount of moisture.

Psychrometer

A psychrometer is an instrument is used to measure the state of the air.  It can easily measure,

  • barometric pressure
  • air dry-bulb temperature

Psychrometer consists –

  • Two thermometers with the bulb are covered by a wet wick.
  • Both the sensing bulbs are separated to avoid heat transfer between them

Two types of psychrometers are widely used:

Sling psychrometer – measuring state where air velocity is less.

Aspirated psychrometer – measuring the state of air for small fans or blower or ducts etc.

Other types of psychrometric instruments:

  1. DPT meter
  2. Hygrometer (Using human hair or horse’s hair)
  3. Dunmore Electric Hygrometer

Application of Psychrometry

There are wide applications of psychrometry in our life as well as industries, like:

  • Refrigeration industries.
  • HVAC industries.
  • All other industries related to refrigeration & air conditioning.
  • Meteorological application etc.

Example of Psychrometry

In extreme summer, we use the air conditioning system to reduce the temperature of the air. Psychrometry helps us to control the temperature of the air as well as moisture content and make us feel comfortable.

We do all the calculations in the air conditioning system taking psychrometry as the primary concern.

Conclusion

So, we have learned the basics of psychrometry, psychrometric chart, and all related terms. We will be very happy if we get any feedback or suggestion to improve the post.

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