Nonequilibrium thermodynamics: Properties of air

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 are 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.

?The LHS image below is a typical representation of air temperature fluctuation over 24 hrs in a day.?As it may be noticed, in every second air has a different temperature and so is its moisture-holding capacity, RHS image. This inhomogeneity of moisture in the air is the key that brings all other properties of air, like Relative humidity [RH], Humidity, Wet-bulb temperature [WBT], Dew-point temperature [DPT], Specific volume, etc., in a state of thermodynamic non-equilibrium.

Typical questions and answers

Q: In an air-conditioned room, when dews appear on window glass, what is the source of energy for phase transformation?

A:?When moist air on a low-temperature window glass cools,?its RH increases locally.?As the air cools its capability to hold moisture reduces and the RH of air increases. RH is defined as the ratio of moisture in the air at a temperature and the moisture that air could hold at that temperature. When air reaches saturation, air can’t hold water anymore. As air is cooled and the dew point approaches, the air gets compressed, and its density reduces. Dry air is denser than wet air. As the density of air reduces it expels out the moisture. The moisture appears as dews. Surrounding air provides energy and does the mechanical work on air to compress air. Air returns the energy to the surrounding by expelling out moisture from the air which condenses as water by releasing heat. Energy for compression of air at the dew point comes from the surroundings and goes back to the surrounding via exothermic condensation of water vapor. It is a typical isothermal process of heat transfer. ?t = 0, across the inside and outside glass surface. ?E=0, there is no change in the internal energy of air.

Q:?What assumption is involved in the method of measurement of WBT?

A: Principle of WBT measurement: The wet-bulb temperature (WBT) is the temperature read by a thermometer covered in water-soaked cloth (wet-bulb thermometer) over which air is passed.?Thermometer bulb wrapped in water-soaked cloth measures evaporative cooling which is only latent heat loss of water. There is an assumption that water and air reach thermal equilibrium at 100% RH.?What does it mean? It means, ?Q = 0, it further means the [mass of water soaked in cloth] x combined specific [heat of water + cloth] = [mass of surrounding air] x specific heat of air]. It assumes that ?Q =?H. WBT does not take into account, any work energy exchange between water-air.

Q: Why dew point temperature [DPT] is less than wet bulb temperature [WBT]?

A:?The dew point is the temperature where water vapor condenses into liquid water. When it is cooled, its RH increases, moisture in the air increases, and when the air becomes 100% saturated, the temperature at which air ejects water out is called the dew point. Unlike WBT which measures the temperature of the water and assumes water on the cloth and surrounding air are in thermal equilibrium, dew point measures the temperature of the air at which moisture vapor in the air condenses into water.?It could be thought of this way: Wet bulb temperature is a measure of water evaporation. Dew point temperature is a measure of water condensation.

Evaporation vs Condensation: Evaporation is an endothermic process. When water receives heat, the kinetic energy of molecules increases. Some molecules near the surface escape as a vapor to vapor space when they have enough kinetic energy. The heat carried by molecules to the vapor phase cools the water.?Phase change is an adiabatic process. Water uses its own internal energy to do work to break intermolecular bonds to make the water transit to vapor.

Condensation is just the opposite of evaporation. It’s an exothermic process. When air is cooled to saturation point, air can’t hold water anymore, little more cooling at this stage gets water ejected out water in the form of dew. As air cools, its kinetic energy reduces which is added back to its internal energy as potential energy . The process goes further, air’s volume reduces and density increases. This is an adiabatic cooling associated with, the latent heat of condensation being released.

Therefore, WBT : [1] WBT is the temperature of cold water that it can achieve [2] WBT measures latent heat when air is cooled to saturation [3] it does not take into account the work energy exchanged between air and water. DPT: [1] DPT measures the actual temperature of cold air at saturation point. It takes into account all forms of energy transacted at the dew point. WBT is more than DPT because the way they are measured is different.

Q: How heat transfer takes place from cold water [post evaporation took place] to ambient air at a higher temperature?

A: Water transfers energy in the form of work energy to air to enable it to do mechanical work to expand to receive enthalpy from water and raise its RH.

Q: Why the saturated line in the psychrometric chart is curved?

A: 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 [LHS image]. ?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.

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Q: How much sensible cooling and heating can one do at constant humidity?

A: Purpose of sensible cooling and heating:

Sensible Cooling of the Air: Sensible cooling of the air is one of the most common psychrometric processes in air conditioning systems. The basic function of air-conditioners is to cool the air absorbed from the room or the atmosphere, which is at higher temperatures. The sensible cooling of air is the process in which only the sensible heat of the air is removed so as to reduce its temperature, and there is no change in the moisture content (kg/kg of dry air) of the air. During the sensible cooling process, the dry bulb (DBT) and wet bulb (WBT) temperature of the air reduces, while the latent heat of the air and the dew point (DPT) temperature of the air remains constant. There is an overall reduction in the enthalpy of the air. The sensible heating process is the opposite of the sensible cooling process. In a sensible heating process, the temperature of the air is increased without changing its moisture content. During this process, the sensible heat, DBT, and WBT of the air increase while the latent of air, and the DPT of the air remain constant.

Credit: Google?