One of my friends asked me this question, and I tried to explain it in a way anyone could understand.
In thermodynamics, enthalpy is a concept that combines the internal energy of a system and the energy associated with its potential to perform work.
Internal Energy (u):This is directly related to temperature. For example, the higher the temperature of something, the higher its internal energy.
Work Potential (pV):This is tied to pressure and volume. A system with higher pressure has a greater potential to perform work.
The formula for enthalpy is:
h=u+p⋅V
On the Mollier diagram (i-x chart):When we analyze air systems, it is often assumed that no work is done due to changes in pressure or volume. Therefore, the second term (p⋅V) is considered negligible, reducing the formula to:
h=u
Air Enthalpy:Air is a mixture of dry air and water vapor. Thus, the total enthalpy of air is the sum of the enthalpy of dry air and the enthalpy of water vapor:
h=hdry+hvapor
Why is water vapor special?Dry air and water vapor are gases with different physical and chemical properties. The behavior of water vapor is unique because it undergoes phase changes (condensation, evaporation) at temperatures we encounter daily. For example:
At a specific temperature, called the dew point, water vapor starts to condense into liquid water, while dry air remains unaffected.
At 0°C, water freezes, forming frost in the air, but for dry air, this is just another temperature without any phase change.
Heat changes on the h-x chart:
Moving vertically (constant X) on the chart represents a change in temperature only.
This change is called sensible heat because we can feel the temperature increase or decrease on our skin.
If we move only along a horizontal line on the chart, the air temperature does not change, but the moisture content (absolute moisture content, not to be confused with relative humidity) does.
Relative humidity is, as the name suggests, a relative measure, whereas absolute moisture content reflects the actual amount of water vapor present in the air.
When moving horizontally, we do not feel any temperature change, even though the internal energy (enthalpy) changes due to an increase or decrease in the amount of water vapor. This process involves latent heat – energy associated with phase changes, like evaporation or condensation, rather than temperature change.
Oblique processes on the h-x These represent a full range of possibilities, where we change the internal energy of air by altering both its temperature and moisture content. This means we can adjust both sensible heat and latent heat simultaneously.
The total heat is the sum of sensible heat and latent heat.
I hope this explanation helps clarify what enthalpy is! I did my best to make it understandable. 😊
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