Specific enthalpy is a thermodynamic property representing the total energy contained within a unit mass of a substance. This concept is fundamental for analyzing open systems, such as pipelines or engines, where mass continuously moves in and out. It is a convenience property defined to simplify the energy bookkeeping for flowing fluids in engineering. Specific enthalpy measures the energy available for transfer or conversion as a substance flows through a process. It allows engineers to track the total energy content of a fluid without constantly recalculating the work required to move that fluid.
The Physical Meaning of Specific Enthalpy
Specific enthalpy (\(h\)) is a composite measure built from two distinct forms of energy that a flowing substance possesses. The first component is the specific internal energy (\(u\)), which is the energy stored within the substance at the molecular level. This includes the kinetic energy of the molecules and the potential energy stored in chemical bonds. The second component is the flow work, represented by the product of pressure and specific volume (\(Pv\)). Flow work is the energy required to push a unit mass of the fluid into or out of a control volume, such as a section of pipe or a turbine casing.
Combining internal energy and flow work into the single property of specific enthalpy dramatically simplifies thermodynamic equations for flowing systems. Without this flow work term, the energy analysis of any open system would require continuous calculation of the work done by the surrounding fluid to maintain movement. This combination captures the total energy carried by the moving mass across the system boundary.
How Specific Enthalpy is Calculated
Specific enthalpy (\(h\)) is mathematically defined as the total enthalpy (\(H\)) of a system divided by its total mass (\(m\)), expressed as \(h = H/m\). Since total enthalpy is the sum of internal energy (\(U\)) and the product of pressure (\(P\)) and volume (\(V\)), the specific equation is \(h = u + Pv\). The standard SI unit is the joule per kilogram (\(\text{J/kg}\)), or often the kilojoule per kilogram (\(\text{kJ/kg}\)).
Enthalpy is a state function, meaning its value depends only on the current state of the substance, such as temperature and pressure. For practical engineering, the absolute value of specific enthalpy is rarely used; instead, the change in specific enthalpy (\(\Delta h\)) is the focus. This change represents the amount of heat energy added to or removed from the substance as it passes through a device.
Engineers often calculate specific enthalpy using tabulated data, such as “steam tables,” which list pre-calculated values for various substances. These tables establish a reference state, commonly setting the specific enthalpy of saturated liquid water at \(0.01^\circ\text{C}\) to zero. This provides a consistent baseline for measuring changes. Using \(\Delta h\) across a process allows for the direct calculation of heat transfer or work output in real-world equipment.
Specific Enthalpy Versus Specific Internal Energy
The primary difference between specific enthalpy (\(h\)) and specific internal energy (\(u\)) is the inclusion of the flow work term (\(Pv\)). Specific internal energy (\(u\)) represents only the energy stored within the substance’s molecules. Specific enthalpy (\(h\)) includes this internal energy plus the energy required to physically displace the surroundings and maintain flow.
This distinction dictates their use in thermodynamic analysis. Specific internal energy is appropriate for closed systems, where a fixed amount of mass is contained within a rigid boundary, such as a gas sealed in a calorimeter. In these static, non-flowing systems, the \(Pv\) term is negligible.
Specific enthalpy is mandatory for open systems, which involve mass flow across boundaries, such as pumps or heat exchangers. In these flowing systems, the \(Pv\) term is significant because work is continuously done to push the fluid into and out of the control volume. For liquids and solids, where specific volume (\(v\)) is very small, \(h\) and \(u\) are nearly identical. However, for gases and vapors, the difference is substantial and must be considered.
Where Specific Enthalpy is Applied in Practice
Specific enthalpy is widely used across industrial and scientific disciplines, primarily in the design and analysis of energy systems. In power generation, it is fundamental for calculating energy conversion efficiency in steam turbines and boilers. The design of the Rankine and Brayton cycles, which govern power plants and jet engines, relies entirely on tracking specific enthalpy changes.
Specific enthalpy is also the backbone of refrigeration and air conditioning systems. Engineers use it to calculate the precise heat removal required for cooling, as specific enthalpy values determine the capacity of refrigerants to absorb and release heat during compression and expansion.
In chemical engineering, specific enthalpy calculations are used in thermochemistry to determine the heat required or released during chemical reactions, which is essential for designing industrial reactors. Property tables facilitate the practical application of specific enthalpy by tabulating values for common working fluids like water, steam, and refrigerants. By measuring the specific enthalpy at the inlet and outlet of a machine, the net energy transferred as heat or work can be accurately determined.