The key difference is that enthalpy is the total energy (heat) content of a substance or system, while latent heat is just one component of that total, specifically the energy related to phase changes.
Based on the provided reference:
Enthalpy is the sum of the sensible and latent heat in a given air-vapor mix. It is sometimes referred to as the total heat of the air.
Understanding the Concepts
To grasp the difference, it's helpful to define each term within the context of thermal energy:
Latent Heat
Latent heat is the energy absorbed or released during a phase change (like melting, freezing, evaporation, or condensation) at a constant temperature. This heat energy changes the state of the substance rather than its temperature. Think about how water stays at 0°C while melting or 100°C while boiling – the energy added during these processes is latent heat.
Sensible Heat
Sensible heat is the energy that changes the temperature of a substance without changing its phase. It's the heat you can "sense" with a thermometer. Adding sensible heat to water increases its temperature (e.g., from 20°C to 30°C).
Enthalpy
As defined by the reference, Enthalpy (often denoted as H) is the total thermal energy content of a substance or system, particularly relevant in air-vapor mixtures. It includes both the energy related to temperature changes (sensible heat) and the energy related to phase changes (latent heat).
The relationship can be simply stated as:
Enthalpy = Sensible Heat + Latent Heat
This means latent heat is a part of enthalpy. Enthalpy represents the whole heat energy in the mix, while latent heat is just the portion associated with the moisture content changing phase (like water vapor condensing into liquid).
Units of Measurement
According to the reference:
- The units for sensible heat, latent heat, and enthalpy are the same: BTU/lb of dry air.
This standard unit allows for easy comparison and calculation when analyzing the energy content of air in applications like HVAC (Heating, Ventilation, and Air Conditioning). BTU stands for British Thermal Unit, a measure of heat energy, and it's normalized per pound of dry air in the mixture.
Summary Table
Here's a quick comparison:
| Feature | Latent Heat | Enthalpy |
|---|---|---|
| Definition | Heat related to phase change | Total heat content (Sensible + Latent Heat) |
| Effect | Changes phase, not temperature | Represents total energy, changes with temp AND phase |
| Relationship | A component of enthalpy | The sum including latent heat |
| Unit (per ref) | BTU/lb of dry air | BTU/lb of dry air |
Practical Context
In air conditioning, for example, dealing with moisture (latent heat) is crucial. Removing humidity from the air requires cooling the air below its dew point, causing water vapor to condense. This process involves removing latent heat. The total energy that the cooling coil needs to remove from the air to reach a desired temperature and humidity level is represented by the change in the air's enthalpy. Therefore, understanding both sensible and latent heat, and their sum (enthalpy), is vital for designing efficient systems.
Enthalpy provides a single value representing the total thermal energy of the air mixture, making it a convenient property for complex psychrometric calculations involving simultaneous temperature and moisture changes.
