Enthalpy of air

The enthalpy of air, often denoted as ${ℎ}_{\text{air}}$, refers to the total energy content of a unit mass of air, including both its internal energy and the energy associated with its pressure and volume. Enthalpy is a state function commonly used in thermodynamics to describe the energy of a system, and it is particularly useful in engineering applications, including HVAC (heating, ventilation, and air conditioning) systems, combustion processes, and fluid flow analysis.

The enthalpy of air depends on several factors, including temperature, pressure, and humidity. It can be calculated using the following equation:

${ℎ}_{\text{air}}={𝑐}_{𝑝}\cdot 𝑇+𝑤\cdot {ℎ}_{\text{vap}}+{ℎ}_0$

where:

• ${ℎ}_{\text{air}}$ is the enthalpy of air,
• ${𝑐}_{𝑝}$ is the specific heat capacity of air at constant pressure,
• $𝑇$ is the temperature of the air,
• $𝑤$ is the specific humidity of the air (mass of water vapor per unit mass of dry air),
• ${ℎ}_{\text{vap}}$ is the enthalpy of vaporization of water at the given temperature and pressure (latent heat of vaporization),
• ${ℎ}_0$ is a reference enthalpy value, typically representing the enthalpy of dry air at a reference temperature and pressure.

The specific heat capacity of air (${𝑐}_{𝑝}$) and the enthalpy of vaporization of water (${ℎ}_{\text{vap}}$) vary with temperature and pressure, so accurate calculations of air enthalpy require consideration of these variations. Additionally, the enthalpy of dry air (${ℎ}_0$) is typically defined relative to a specific reference state, such as standard atmospheric conditions (e.g., 0°C, 1 atm).

In HVAC and other engineering applications, the enthalpy of air is used to analyze heating and cooling processes, determine energy requirements for air conditioning systems, and assess the performance of heat exchangers and air handlers. Enthalpy calculations are also essential for understanding psychrometric properties of air and designing systems for controlling air temperature and humidity.