The Enthalpy of Moist Air

The heat content of the air, which is measured in kilojoules (kJ) per kilogramme (kg), is an important factor that air conditioning engineers need to take into consideration when designing their systems. This is because the air temperature is partially affected by the heat content of the air. The kinetic motion of the air molecules, which can be detected by a thermometer, and the latent heat (also known as “hidden heat”) contained within evaporated water are the two components that make up the heat content, also known as the enthalpy, of air. When attempting to determine the enthalpy of air, it is necessary to take into consideration both its temperature and the fact that it always contains at least a trace amount of water.

To determine the enthalpy of the air by itself, multiply the temperature of the air in degrees Celsius by 1.007, then deduct 0.026 from the resulting number. Take, for instance, air that is maintained at a temperature of 30 degrees Celsius.

Enthalpy of air equals 1.007 times 30 minus 0.026, which is 30.184 kJ per kilogramme.

Water Content of Air

Dry air is combined with water vapour to create what is known as moist or humid air. The humidity of the air is a dimensionless parameter that can also be referred to as absolute humidity or the humidity ratio. It is defined as follows:

Humidity=Mw/Ma

where Mw represents the mass of water vapour that is carried by Ma representing the mass of dry air. Hygrometers are the equipment that are used for the measurement of humidity.

The total pressure of humid air is the same as the sum of the partial pressures of its elements, which includes the pressure exerted by water vapour. The molar concentration of water in the gas phase is related to the partial pressure of water vapour, denoted by pw, which exists in the atmosphere.

Pw=YwPT

where yw represents the mole fraction of water vapour that is present in the air–vapour mixture and pT stands for the total pressure. At a certain temperature and pressure, the water vapour content of the air is said to be saturated if the relative humidity of the air reaches its utmost possible level under such circumstances. When more water vapour is introduced into air that is already saturated with water vapour, the consequence is the condensation of liquid water in the form of droplets or a mist. When circumstances are considered to be saturated, the partial pressure of water vapour in air is equivalent to the saturation vapour pressure psw of pure water at the given temperature.

The temperature at which a mixture of water vapour and air becomes saturated is referred to as the dew point or the saturation temperature. This temperature is reached when the mixture is cooled.

Dew Point vs. Humidity

The dew point is the temperature that air must be cooled to (at a certain pressure) in order to reach a relative humidity (RH) level of one hundred percent. At this time, the gaseous state of water cannot be contained by the air any longer. If the air were to be cooled much further, the water vapour in the atmosphere would have to condense back into liquid form and fall from the sky, typically in the form of fog or precipitation.

When there is a higher temperature than the dew point, there is a greater quantity of moisture in the air. This has a direct bearing on how “pleasant” the atmosphere will be outside. The relative humidity reading can be deceiving a lot of the time. For instance, if the temperature is thirty degrees and the dew point is thirty degrees, the relative humidity will be one hundred percent, however if the temperature is eighty degrees and the dew point is sixty degrees, the relative humidity would be fifty percent. On a day with temperatures of 80 degrees and relative humidity of 50 percent, it would feel considerably more “humid” than it would on a day with temperatures of 30 degrees and relative humidity of 100 percent. This is because the dew point is greater than normal.

Humidity of air

The amount of water vapour that is present in the air is referred to as the humidity. When there is a significant amount of water vapour in the atmosphere, the level of humidity will be elevated. When the humidity is high, the air around you feels wetter, and vice versa.

When discussing humidity in weather forecasts, the term “relative humidity” is typically used. The amount of water vapour that is actually present in the air is referred to as the relative humidity, and it is stated as a percentage of the maximum amount of water vapour that the air is capable of holding at the same temperature. Imagine that the temperature outside is brisk -10 degrees Celsius (14 degrees Fahrenheit). At such temperature, the air can only retain a maximum of 2.2 grams of water per cubic metre before it becomes saturated. Therefore, if there are 2.2 grams of water per cubic metre and the temperature outdoors is -10 degrees Celsius, we are experiencing an uncomfortably high level of relative humidity of 100 percent. If there were 1.1 grams of water in the air at a temperature of -10 degrees Celsius, then the relative humidity would be at fifty percent.

When there is a high level of humidity, the air is saturated with water vapour to the point where there is little place for anything else. If you perspire when the weather is humid, it may be difficult to cool off since your perspiration will not be able to dissipate into the air as it should.

Conclusion

The enthalpy of moist air is calculated by adding the enthalpy of dry air to the enthalpy of water vapour to arrive at a final value. The enthalpy of a thermodynamic system is the same as its total energy. There is no relationship between the pressure and the enthalpies of perfect gases. Within the framework of the International System of Units (SI), the joule serves as the unit of measurement. Dry air is combined with water vapour to create what is known as moist or humid air. The humidity of the air is a dimensionless parameter that can also be referred to as absolute humidity or the humidity ratio. It is defined as follows:

Humidity=Mw/Ma.