Key Properties of Atmospheric Air in Psychrometry

Every time paddy or any grain is dried, stored, or processed, the air surrounding it plays a decisive role. But not all air is the same – its temperature, moisture content, and energy level constantly shift, and those shifts directly affect how well grain dries, how long it stays safe in storage, and how efficiently processing equipment performs. Psychrometry – the science that studies the thermodynamic properties of moist air – gives us the tools to measure, understand, and control these variables. Before applying those tools, it’s essential to understand what atmospheric air is actually made of and what properties define its behavior.

What is atmospheric air, really? 🔗

Atmospheric air is not pure air. It is always a mixture of dry air and water vapor. Dry air itself is composed primarily of nitrogen (about 79% by volume) and oxygen (about 21% by volume), with trace amounts of carbon dioxide, argon, and other gases. For engineering purposes, dry air is treated as a single ideal gas with an apparent molecular weight of 28.9645 and a gas constant of 287.055 m³Pa/kgK.

The water vapor portion, although small in mass, has an outsized influence. Even a small amount of water vapor in atmospheric air greatly affects the drying process. Understanding the combined properties of this air-vapor mixture is what makes psychrometry indispensable in food and grain processing.

Key properties of atmospheric air 🔗

Nine properties describe the state of atmospheric air in psychrometry. Each one reveals a different aspect of the air’s condition and capacity to carry or release moisture.

1. Dry-bulb temperature (DBT) 🔗

Dry-bulb temperature is simply the actual temperature of the air, measured by an ordinary thermometer that is exposed to airflow but shielded from moisture and direct radiation. When people refer to “air temperature,” they are almost always talking about dry-bulb temperature. It is the primary reference point on any psychrometric chart and the starting input for most psychrometric calculations. In paddy drying, the DBT of the heated drying air determines how much moisture that air can potentially absorb from the grain.

2. Wet-bulb temperature (WBT) 🔗

Wet-bulb temperature is measured by a thermometer whose bulb is wrapped in a water-soaked wick over which air flows. As water evaporates from the wick, it cools the bulb. The wet-bulb temperature is the lowest temperature that air can reach through evaporative cooling alone, with all the latent heat supplied by the air parcel itself.

The drier the surrounding air, the faster the evaporation and the lower the wet-bulb reading. When dry-bulb and wet-bulb temperatures are far apart, the air is dry and has a low relative humidity – which means it has greater capacity to absorb moisture from grain. Wet-bulb temperature is therefore a direct indicator of drying potential.

3. Dew-point temperature (DPT) 🔗

Dew-point temperature is the temperature to which air must be cooled, at constant pressure and constant moisture content, before water vapor begins to condense into liquid. The dew point is always lower than the dry-bulb temperature – the two become equal only at 100% relative humidity, when the air is fully saturated.

In practice, for unsaturated air: dew-point temperature < wet-bulb temperature < dry-bulb temperature. For saturated air, all three temperatures are equal. Dew-point temperature is particularly important in storage design – if grain bin surfaces or cooled air ducts drop below the dew point, condensation forms, promoting mold and fungal growth.

4. Absolute humidity (humidity ratio) 🔗

Absolute humidity, also called humidity ratio or moisture content, is the mass of water vapor present per unit mass of dry air. It is typically expressed in grams of water per kilogram of dry air (g/kg). This quantity gives us the actual weight of water carried in the air, independent of temperature. Unlike relative humidity, absolute humidity does not change when air is simply heated or cooled – it changes only when moisture is physically added to or removed from the air. This makes it one of the most reliable measures for tracking moisture movement during drying operations.

5. Relative humidity (RH) 🔗

Relative humidity is the ratio of the actual amount of water vapor in the air to the maximum amount it can hold at that same temperature and pressure, expressed as a percentage. In other words, relative humidity tells us how much moisture the air contains relative to its capacity at that moment.

Crucially, relative humidity changes when temperature changes, even if the actual moisture content of the air remains exactly the same. Heating air at constant moisture content decreases its relative humidity; cooling it increases RH. This is why heated air is so effective at drying grain – warming the inlet air drops its RH, giving it the “appetite” to absorb more moisture from the product.

6. Percentage humidity 🔗

Percentage humidity (also called degree of saturation) is similar to relative humidity but is defined as the ratio of the actual humidity ratio of air to the humidity ratio of saturated air at the same dry-bulb temperature. While numerically close to relative humidity at low moisture levels, the two are not identical. Percentage humidity is more precise in certain engineering calculations involving drying air systems, particularly when working near saturation conditions.

7. Vapor pressure 🔗

Vapor pressure is the partial pressure exerted by the water vapor component within the air mixture. According to Gibbs-Dalton’s law, the total pressure of a moist air mixture equals the sum of the partial pressures of dry air and water vapor. The higher the vapor pressure of the water in the air, the more moisture the air already contains. Saturation vapor pressure is the maximum vapor pressure possible at a given temperature – it increases with temperature, which is why warm air can hold more moisture than cool air. Relative humidity can also be expressed as the ratio of actual vapor pressure to saturation vapor pressure at the same dry-bulb temperature.

8. Specific volume 🔗

Specific volume is the volume occupied by one kilogram of dry air, including its associated water vapor, at given conditions of temperature and pressure. It is the inverse of density. Warmer air has a greater specific volume and is therefore lighter than cool air – which explains thermal buoyancy in drying chambers. On the psychrometric chart, specific volume lines run nearly vertically, with values typically ranging from about 0.80 to 0.95 m³/kg of dry air at normal processing temperatures. Engineers use specific volume to calculate actual air volume flow rates and to size fans, ducts, and dryer chambers correctly.

9. Enthalpy 🔗

Enthalpy is the total heat energy content of the moist air, combining both sensible heat (the heat associated with temperature) and latent heat (the heat locked in water vapor). The enthalpy of moist air accounts for the heat in dry air plus the sensible and latent heat of water vapor. In psychrometric calculations, the reference point is taken as 0°C and one atmosphere, where the latent heat of water vapor is 2501.4 kJ/kg. The total enthalpy of moist air is expressed in kJ per kilogram of dry air.

Enthalpy is the property engineers rely on most when calculating the energy required for heating drying air. Specific enthalpy is the total energy content per unit mass, and it includes both sensible and latent heat components. In grain dryers, knowing the enthalpy difference between inlet and outlet air allows precise calculation of heater capacity and fuel consumption.

How these properties relate to each other 🔗

These nine properties are not independent – they are mathematically linked, and knowing any two of them is enough to determine all the others. Given any two psychrometric properties, the other values can always be determined – either through calculation or by reading a psychrometric chart.

For example, if you know the dry-bulb temperature and relative humidity of incoming air, you can immediately determine its wet-bulb temperature, dew point, absolute humidity, specific volume, and enthalpy. This interconnectedness is what makes the psychrometric chart such a powerful tool for designing and managing drying and storage systems.

Why these properties matter in paddy processing and storage 🔗

The main applications of psychrometry in the food industry include food drying, postharvest operations for fruits and vegetables, and grain storage with aeration. Each of these applications relies directly on the properties described above.

In paddy drying, heated air with a low relative humidity and high enthalpy is passed over wet grain. The air’s capacity to absorb moisture – determined by its dry-bulb temperature, absolute humidity, and vapor pressure – drives moisture from the grain surface into the airstream. Monitoring the wet-bulb temperature of exit air tells the operator how much moisture the air has picked up and when drying efficiency is declining.

In grain storage, the dew-point temperature of ambient air must stay well below the grain temperature to prevent condensation on grain surfaces or bin walls. Aeration systems use psychrometric data to decide when to switch fans on or off, pushing air through the grain only when that air will cool or dry the grain rather than humidify it further. Maintaining the right storage atmosphere preserves grain quality indicators like whiteness of rice and low free fatty acid content in oilseeds.

For system design, specific volume and enthalpy determine fan sizing, heater capacity, and duct dimensions. Getting these calculations wrong wastes energy, causes uneven drying, or leads to storage losses.

The psychrometric chart: bringing all properties together 🔗

A psychrometric chart is a graphical representation of the physical and thermal properties of atmospheric air, drawn at a fixed barometric pressure – typically standard atmospheric pressure of 101.325 kPa. It plots dry-bulb temperature on the horizontal axis, humidity ratio on the vertical axis, and overlays curves for relative humidity, lines for wet-bulb temperature, enthalpy, and specific volume – all on a single diagram.

The power of the chart lies in its efficiency: locate two known properties, find their intersection, and read off all remaining properties instantly. The outcome of a drying process can often be followed more easily by monitoring the condition of the air with a psychrometric chart than by directly measuring moisture content of the material being dried. For food engineers and grain processors alike, fluency with this chart translates directly into better process control and less product loss.

What do you think? Given that relative humidity changes with temperature even when the actual moisture content stays the same, how should a grain storage manager decide the right time to run aeration fans – based on RH alone, or by using a combination of properties like dew-point and dry-bulb temperature? And with nine interconnected properties all influencing each other, which two do you think are the most practical pair to measure on the field to characterize drying air effectively?

References

1. https://link.springer.com/chapter/10.1007/978-1-4419-7662-8_9
2. https://ebooks.inflibnet.ac.in/ftp02/chapter/introduction-to-psychrometry/
3. https://egyankosh.ac.in/bitstream/123456789/11334/3/Unit-4.pdf
4. https://www.engineeringtoolbox.com/dry-wet-bulb-dew-point-air-d_682.html
5. https://en.wikipedia.org/wiki/Wet-bulb_temperature
6. https://mepacademy.com/understanding-dry-bulb-wet-bulb-and-wet-bulb-depression/
7. https://www.haidinggroup.com/industry-news/understand-correctly-the-drybulb-temperature-dewpoint-temperature-and-wetbulb-temperature-article-406/
8. https://edis.ifas.ufl.edu/publication/AE406
9. https://energy-models.com/training/21-properties-moist-air-and-psychrometric-chart
10. https://macinstruments.com/resources/moisture-handbook/
11. https://www.adicot.com/psychrometric-chart-calculator
12. https://extension.psu.edu/psychrometric-chart-use
13. https://www.sciencedirect.com/article/abs/pii/B9780128184738000098
14. https://www.researchgate.net/publication/286643928_Significance_and_application_of_psychrometric_chart_in_food_processing_A_review