Thermodynamic Diagrams, Cycles, and Properties

Vocabulary practice cards covering pressure-enthalpy (P-h) diagrams, Mollier (h-s) diagrams, vapor-compression refrigeration, steam power cycles, and standard thermodynamic symbols and SI units.

$$P\text{-}h\text{ Diagram}$$

A Pressure–Enthalpy diagram that plots pressure ($P$) on the vertical axis and specific enthalpy ($h$) on the horizontal axis, used especially in refrigeration, air-conditioning, and steam power systems.

$$P\text{-}h\text{ Diagram for Water}$$

A thermodynamic chart plotting pressure against enthalpy for water, illustrating the saturation dome, isobars, isotherms ranging from $10\,^\circ\text{C}$ to $600\,^\circ\text{C}$, isentropic curves, and vapor quality lines.

Subcooled (Compressed) Liquid Region

The thermodynamic region on the left side of a $P\text{-}h$ diagram where the fluid is entirely liquid at a temperature below its saturation temperature for a given pressure.

Two-Phase Mixture Region

The dome-shaped region in the center of a $P\text{-}h$ or Mollier diagram where liquid and vapor coexist in equilibrium, bounded by the saturated liquid and vapor lines.

Saturated Liquid Line

The curve forming the left boundary of the saturation dome on a thermodynamic diagram, along which the fluid exists entirely as liquid at its boiling point.

Saturated Vapor Line

The curve forming the right boundary of the saturation dome on a thermodynamic diagram, along which the fluid exists entirely as dry saturated vapor.

Superheated Vapor Region

The region to the right side of the saturation dome on a $P\text{-}h$ diagram where the fluid is entirely vapor at a temperature higher than its saturation temperature for a given pressure.

Isobars

Lines of constant pressure that appear as horizontal lines on a $P\text{-}h$ diagram and as sloping upward curves spreading outward from the saturated region on a Mollier diagram.

Isotherms

Lines of constant temperature that curve through single-phase regions and coincide horizontally with constant pressure lines inside the two-phase saturation dome.

Isentropic Lines

Lines of constant entropy that slope upward to the right on a $P\text{-}h$ diagram, useful for analyzing processes in compressors, turbines, and nozzles.

Quality ($x$)

The dryness fraction of a saturated liquid-vapor two-phase mixture, defining the ratio of vapor mass to total mixture mass inside the saturation dome.

R134a Pressure-Enthalpy ($P\text{-}h$) Diagram

A thermodynamic property diagram plotting pressure from $20\,\text{kPa}$ to $10\,\text{MPa}$ against enthalpy from $100\,\text{kJ}/\text{kg}$ to $500\,\text{kJ}/\text{kg}$ for refrigerant R134a, showing isotherms, quality lines ($x = 0.1$ to $0.9$), and the critical point (CP).

Compressor Process (Refrigeration Cycle)

The process from Point 1 to Point 2 on a refrigeration cycle where the compressor intakes suction vapor, increases refrigerant pressure along an upward line, and superheats it to the right of the saturation dome.

Condenser Process (Refrigeration Cycle)

The horizontal, constant-pressure process from Point 2 to Point 3 on a refrigeration cycle where the refrigerant rejects heat ($q_{\text{cond}}$), passes through the saturation dome, and exits slightly subcooled.

Throttle

A metering device (such as a capillary tube in household refrigerators) where high-pressure liquid drops drastically in pressure at constant enthalpy (a vertical line on a $P\text{-}h$ diagram), causing the temperature to plummet and partial vapor flashing.

Flash Evaporation (Point 4)

The rapid phase change during throttling where refrigerant instantly flashes from liquid to vapor, reaching approximately $35\%$ vapor quality upon entering the evaporator.

Evaporator Process (Refrigeration Cycle)

The constant-pressure process from Point 4 to Point 1 on a refrigeration cycle where the refrigerant absorbs heat energy ($q_{\text{evap}}$) from its surroundings, moving across to saturated vapor at the compressor inlet.

Vapor-Compression Refrigeration Cycle ($P\text{-}h$)

A closed refrigeration cycle plotted on a $P\text{-}h$ diagram tracing compression (1–2), condensation (2–3), isenthalpic throttling (3–4), and evaporation (4–1).

Simple Steam Power Plant Flowsheet

A closed loop thermodynamic power system consisting of a boiler ($Q_B$), a turbine ($W_T$), a condenser ($Q_C$), and a feedwater pump ($W_P$).

Steam Turbine Process

The expansion process from Point 1 to Point 2 where high-pressure, high-temperature steam expands through turbine blades to produce shaft power ($w_{\text{turbine}} = h_1 - h_2$), discharging low-pressure steam to the condenser.

Steam Condenser Process

The process from Point 2 to Point 3 in a steam cycle where heat is extracted from turbine exhaust steam by cooling water, condensing it into the subcooled liquid region.

Feedwater Pump

A mechanical pump that draws condensed water from the condenser outlet at Point 3 and elevates its pressure to the boiler system pressure at Point 4.

Thermal Efficiency ($\eta_{\text{th}}$) of a Steam Plant

The ratio of turbine work output to boiler heat input, defined mathematically as $\eta_{\text{th}} = \frac{w_{\text{turbine}}}{q_{\text{boiler}}} = \frac{h_1 - h_2}{h_1 - h_4}$.

Steam Power Cycle on a $P\text{-}h\text{ Diagram}$

A thermodynamic cycle plot illustrating steam plant processes: constant-pressure boiler heating ($q_{\text{boiler}} = h_1 - h_4$), turbine expansion ($w_{\text{turbine}} = h_1 - h_2$), condensation, and feedwater pumping.

Mollier Diagram

A thermodynamic diagram, typically plotting enthalpy versus entropy ($h\text{-}s$) or pressure versus enthalpy ($P\text{-}h$), created by German engineer Richard Mollier to simplify fluid energy process calculations.

Richard Mollier

A German engineer who developed thermodynamic enthalpy diagrams to simplify the analysis of energy processes involving fluids, especially water and steam.

Mollier Diagram for Steam

An enthalpy–entropy ($h\text{-}s$) chart displaying lines of constant pressure, constant temperature up to $800\,^\circ\text{C}$, and dryness fraction curves for steam and water.

Specific Enthalpy ($h$)

A thermodynamic property representing total heat content per unit mass, with common SI units of $\text{J}/\text{kg}$ or $\text{kJ}/\text{kg}$.

Specific Entropy ($s$)

A thermodynamic measure of microscopic disorder per unit mass, with common SI units of $\text{J}/(\text{kg}\cdot\text{K})$ or $\text{kJ}/(\text{kg}\cdot\text{K})$.

Constant-Pressure Specific Heat ($C_p$)

The amount of heat required to raise the temperature of a unit mass of a substance by one Kelvin at constant pressure, with units of $\text{J}/(\text{kg}\cdot\text{K})$ or $\text{kJ}/(\text{kg}\cdot\text{K})$.

Constant-Volume Specific Heat ($C_v$)

The amount of heat required to raise the temperature of a unit mass of a substance by one Kelvin at constant volume, with units of $\text{J}/(\text{kg}\cdot\text{K})$ or $\text{kJ}/(\text{kg}\cdot\text{K})$.

Heat Capacity Ratio ($k$)

A dimensionless ratio of the constant-pressure specific heat to the constant-volume specific heat ($C_p / C_v$).

Compressibility Factor ($Z$)

A dimensionless thermodynamic property representing the deviation of a real gas from ideal gas behavior.

Specific Volume ($v$)

The volume occupied per unit mass of a substance, symbolized by lowercase $v$ with common SI units of $\text{m}^3/\text{kg}$.

Density ($\rho$)

The mass per unit volume of a substance, represented by the Greek symbol $\rho$ with common SI units of $\text{kg}/\text{m}^3$.

Efficiency ($\eta$)

A dimensionless thermodynamic measure of performance, symbolized by the Greek letter $\eta$.