Psychrometrics: A non-equilibrium thermodynamics

?Questions and answers

Psychrometrics

Psychrometrics is an air and water vapour science that studies the properties of moist air. It is critical to have a thorough understanding of psychrometrics. It is important not only for heating, cooling, and humidification processes and the comfort of building occupants, but also for building insulation, roofing properties, and the stability, deformation, and fire resistance of building materials. As a result, a thorough understanding of the fundamental concepts and principles of psychrometrics is essential.

This post includes a psychrometric chart that depicts moist air properties such as dry-bulb temperature, wet-bulb temperature, relative humidity, humidity ratio, and enthalpy. Three of these characteristics are sufficient to identify moist air. The post goes on to explain the key terms and their interrelationships.

What is air?

Atmosphere thermodynamics

What is it?

In atmospheric thermodynamics, the air is a mixture of dry air and any amount of water vapor. When moist air does not include a liquid, the air is a pure gas and behaves like dry air. The atmosphere is an example of a non-equilibrium system. Almost all systems found in nature are not in thermodynamic equilibrium. Air is no exception. It is always in a state of non-equilibrium. Temperature, vapor pressure, and the volume of air is continuously changing in ambient air and shifting internal energy, enthalpy, and entropy. Non-equilibrium thermodynamics is a work in progress, not an established structure. A non-thermodynamics system is changing or can be triggered to change over time, and are continuously and discontinuously subject to flux of matter and energy to and from other systems.?Since the composition of vapor is generally inhomogeneous in a moist atmosphere, vapor has a diffusive process. Energy transport is also associated with the diffusion of vapor. Diffusion is the net movement of anything (for example, atoms, ions, molecules) from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in concentration.

More details

Air is a mixture of various gases. The air in the Earth's atmosphere is composed of approximately 78% nitrogen and 21% oxygen [diatomic gases]. Air also contains trace amounts of many other gases, including carbon dioxide, neon, and hydrogen. On a hot, muggy summer day, you've most likely heard the term "humid." But what exactly does that mean? The amount of water that the air can hold before it rains is referred to as relative humidity. Humidity is usually measured in percentages, so the highest level of relative humidity is 100 percent just before it rains. Although air appears to be light, there is a lot of it pushing down on the Earth's surface. This is referred to as air pressure. At sea level, you have high air pressure because the entire atmosphere is pressing down on you. The reason for this is that the atmosphere has mass and is being pulled downward by the Earth's gravitational forces. Because there is less atmosphere pushing down on you on top of a mountain, the air pressure is low.

Because the molecules in the air are so far apart, there are almost no intermolecular forces. There is no surface to air. Air is a solitary point. There is no surface tension in the air.

What is water?

Water is a triatomic molecule composed of one oxygen atom bonded to two hydrogen atoms. It is a highly polar molecule because of the significant difference in the electron affinity of hydrogen and oxygen atoms. Water, due to its polar structure, forms intermolecular bonds with many other substances, changing their physical properties by modifying the physical geometry of molecules.

Is there a special affinity for moisture in the air?

Yes, the air is attracted to moisture. The reason for this is the unequal energy distribution in air and water. The molecules in the air are far apart, and there are few intermolecular forces. In comparison to water, air has very little potential energy. At 20 degrees Celsius, the surface tension of water-air is approximately 73 dynes/cm. Nature does not permit this disparity in energy. Nature causes water molecules to enter open voids in the air and generate intermolecular forces to counteract surface tension at the air-water interface. As a result, air has a natural attraction to water vapour.

How moisture changes the properties of air?

Air properties such as density, specific heat capacity, thermal conductivity, and viscosity are assumed to be temperature dependent. This assumption applies only to dry air. The fluid properties of water vapour and dry air differ. As a result, as relative humidity changes, so do density, specific heat capacity, thermal conductivity, and viscosity.

Specific terms in psychrometry and their interactions

Psychrometric chart

This is a typical psychrometric chart. We will use this chart to explain various terms used in psychrometry. A psychrometric chart is a graphical representation of the psychrometric processes of air at constant pressure. Psychrometric processes include physical and thermodynamic properties such as dry bulb temperature, wet bulb temperature, humidity, dew point, enthalpy, air density, etc.

Fundamentals of a psychrometric chart

Why the saturated line in the psychrometric chart is curved?

The relative humidity lines in the psychrometric chart are curved lines that move upwards to the right. The line representing saturated air where the relative humidity is 100% is no exception. A psychrometric chart is like the P-H diagram [below].?As you go to the right on a psychrometric chart away from the 100% saturation line along with an increase in temperature you see a reduction in the RH of air. The RH lines you see in a psychrometric chart are the vapor pressure line.?At constant vapor pressure, the RH reduces as a curved line with temperature. The reason is the slope of the RH lines continuously changes as you go from left to right on a psychrometric chart. It becomes shaper at a higher humidity ratio.

P-H diagram of water

Dry bulb temperature [DBT]

What is DBT?

DBT points are given on the x-axis. The vertical lines emerging from each DBT point on the x-axis are DBT lines. It is the ambient temperature of the air. It means that if you hold a thermometer in the ambient air, it is the temperature of the ambient air. Normally, this is how books explain what is DBT. ?Actually, DBT has a far greater meaning. The temperature that you call DBT is the temperature of air + moisture in it. Since air and moisture have different specific heats, DBT is strongly influenced by the relative humidity of air [RH] which modifies the specific heat of the mixture. The larger the moisture in the water, the larger is the specific heat of the mixture, and the lesser the temperature. [ specific heat of moisture = 4.186 j/g/k specific heat of dry air = 1.005 j/g/].

Effect of Pressure on DBT

Boyle's law states that at a constant temperature if pressure is Increased on a gas, volume decreases and vice-versa. When pressure is increased at a constant temperature the volume reduces, and the vapor molecules respond to the reduction in volume by condensation of water vapor.

What is relative humidity [RH]?

In the psychrometric chart, steady RH is represented by the bent lines running from the base left to the upper right of the graph. RH is the amount of water vapor present in air expressed as a percentage of the amount needed for saturation at the same temperature. RH is a ratio of the partial pressure of moisture in air / Vapor pressure of air at a given temperature. 60% Relative humidity means, air contains 60% of water vapor compared to how much it can hold at that temperature, if Relative Humidity is 100%, air no more can hold water vapor.

Effect of temperature on RH at constant pressure

Temperature and RH are inversely proportional. As moisture in the air increases the specific heat of the mixture increases, and the temperature reduces. What does it mean? The larger the specific heat capacity of a substance, the more energy it takes to raise the temperature by a given amount. If you just remember the relationship between the specific heat of moist air and dry air you can answer many questions of psychrometry.

Effect of pressure on RH at a constant temperature

if you have some moist air at a constant temperature and you compress it, effectively you reduce volume, which means you increase the density of air. Since moisture is 18 g/ mole and air is 29 g /mole, you increase the dry air: moisture ratio. RH decreases. The opposite applies when you reduce the pressure.

Wet bulb temperature [WBT]

Wet-bulb temperature (WBT) combines air temperature with humidity – in essence, it is a measure of heat-stress conditions in humans.

It is one of the most confusing subjects in the psychrometry. Please see the psychrometric chart.

?The WBT line emerges from the intersection of DBT and RH and goes along the diagonal line. The point where this diagonal line intersects the saturation line is WBT. It is an indicator of moisture in the air at 100% saturation. WBT is the temperature of air if air gives off all its absorbed enthalpy [moisture] adiabatically and returns to its initial RH.

More details

Wet bulb temperature is a property of moist air

Just imagine a wet bulb-dry bulb hygrometer. It has one dry bulb thermometer and one wet bulb thermometer arm with a moist water-soaked cloth around the bulb of the thermometer.

The difference between dry and wet bulb temperatures represents the difference in the enthalpy of air at these two bulbs. ?A dry bulb represents the enthalpy or heat of the given air at a given RH. The wet bulb represents the enthalpy of the given air at 100% saturation [wet bulb is surrounded by maximum water vapor possible].

The difference is the maximum loss of enthalpy that can occur in moist air at a given RH at constant pressure if it gives off all absorbed enthalpy. WBT is the temperature of air if air gives off all its absorbed enthalpy [moisture] adiabatically and returns to its initial RH

Effect of temperature on WBT

RH and WBT are related at a given temperature and constant pressure. As RH increases at constant pressure so is the increase in WBT. There will be less evaporation.

Effect of pressure on WBT

Fundamentally it means that for a given RH an increase in ambient pressure means less evaporation rate and consequently an increase in WBT. In a similar manner, a reduction of ambient pressure means an increased evaporation rate of moisture and therefore a decrease in the WBT.

Dew-point temperature [DPT]:

The dew point is defined to be the temperature at which the equilibrium vapor pressure equals the actual partial pressure of the water in the atmosphere.

?DPT lines originate from humidity ratio points on the humidity ratio axis and go parallel to the x-axis. The point where this line intersects the saturation line is DPT. The dew point is the temperature to which air must be cooled to become saturated with water vapor. When further cooled, the water vapor in the air will condense to form liquid water. The dew point is affected by humidity. When there is more moisture in the air, the dew point is higher.

Effect of pressure on the dew point

Increasing the barometric pressure increases the dew point. This means that, if the pressure increases, the mass of water vapor per volume unit of air must be reduced in order to maintain the same dew point.

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