Thermodynamics on the FE Exam
Thermodynamics is a major content area on the FE Mechanical and FE Chemical exams (7-14% each) and also appears on the FE Other Disciplines exam. This subject covers energy conversion, heat transfer, and the behavior of substances under different conditions. Building a solid understanding of thermodynamic principles is essential for a strong exam score.
The Laws of Thermodynamics
Zeroth Law
The Zeroth Law establishes the concept of thermal equilibrium: if two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other. This law is the foundation for temperature measurement.
First Law
The First Law is a statement of energy conservation. For a closed system, the change in internal energy equals the net heat added minus the net work done by the system. For an open system (steady-state, steady-flow), the energy balance includes enthalpy, kinetic energy, potential energy, heat transfer, and work.
Practice applying the First Law to both closed systems (pistons, rigid tanks) and open systems (turbines, compressors, nozzles, heat exchangers, throttling devices). Each device type has characteristic simplifications that make the problem easier to solve.
Second Law
The Second Law introduces entropy and limits the efficiency of energy conversion processes. Entropy always increases in an isolated system. The Carnot efficiency represents the maximum possible efficiency for a heat engine operating between two temperature reservoirs. Know how to calculate entropy changes for ideal gases, incompressible substances, and phase changes.
Thermodynamic Properties
Understanding how to determine thermodynamic properties (temperature, pressure, specific volume, internal energy, enthalpy, entropy) is crucial. Know how to use steam tables and refrigerant tables provided in the FE Reference Handbook. For two-phase mixtures, use quality (the mass fraction of vapor) to interpolate between saturated liquid and saturated vapor properties.
For ideal gases, use the ideal gas law (PV = nRT) and know the relationships for specific heats. For constant specific heats, internal energy change equals the specific heat at constant volume times the temperature change, and enthalpy change equals the specific heat at constant pressure times the temperature change.
Power Cycles
The FE exam tests your understanding of common power and refrigeration cycles:
- Carnot Cycle: The ideal reference cycle with maximum efficiency. Know how to calculate Carnot efficiency from hot and cold reservoir temperatures.
- Rankine Cycle: The ideal cycle for steam power plants. Includes pump, boiler, turbine, and condenser. Know how to calculate thermal efficiency, net work, and heat input.
- Otto Cycle: The ideal cycle for spark-ignition engines. Efficiency depends on compression ratio and specific heat ratio.
- Diesel Cycle: The ideal cycle for compression-ignition engines. Efficiency depends on compression ratio, cutoff ratio, and specific heat ratio.
- Brayton Cycle: The ideal cycle for gas turbines. Includes compressor, combustion chamber, and turbine. Efficiency depends on pressure ratio.
Refrigeration Cycles
The vapor-compression refrigeration cycle is the reverse of a power cycle. It uses a compressor, condenser, expansion valve, and evaporator to move heat from a cold space to a warm space. Know how to calculate the coefficient of performance (COP) for both refrigeration and heat pump applications.
Problem-Solving Strategy
For every thermodynamics problem, follow these steps: identify the system and its boundaries, determine whether the system is open or closed, list all assumptions (steady state, adiabatic, isentropic, ideal gas, etc.), write the appropriate energy balance, look up or calculate thermodynamic properties, and solve for the unknowns. Drawing a process on a T-s or P-v diagram can help you visualize the problem and catch errors.