- Thermodynamics Definition: Thermodynamics is defined as the study of energy, heat, and their interactions in physical systems.
- Types of Systems: In thermodynamics, a closed system does not exchange mass with its surroundings, while an open system allows mass and energy flow.
- Energy Forms: Energy exists in various forms like kinetic, potential, chemical, and electrical energy, each crucial for different applications.
- Thermodynamic Properties: Properties are categorized as extensive (dependent on mass) or intensive (independent of mass) and describe the system’s state.
- Equilibrium and Processes: A system in thermodynamic equilibrium has constant properties, and processes describe changes from one state to another.
Engineering thermodynamics studies energy, heat and work in plant and equipment. That heat-power branch of engineering is used to rate machines and processes.
Plant performance is judged by product output, feed or fuel use, production cost and environmental load. Engineers apply thermodynamics to equipment meant for safety and comfort.
Classical engineering thermodynamics grew through the 19th century, from Carnot, Clausius, Kelvin and Rankine. Scientists and engineers still use it to analyse plant.
Fundamentals of Thermodynamics
The word thermodynamics comes from Greek thermē (heat) and dynamis (power or force). Engineers study systems and how those systems exchange energy with their surroundings.
The definitions below are the working vocabulary of engineering thermodynamics, also called heat-power engineering.
System, Surrounding and Universe
A system is the region or quantity of matter under study. The usual aim is higher efficiency or lower loss. Examples include the refrigeration cycle in a cold store and the Rankine cycle in a power plant.
A closed system holds a definite mass inside a real or imagined surface. Composition inside that surface may stay fixed or may change with the process.
The volume of a system need not be constant. Air in a piston compressor is compressed; an inflated balloon can change size. Everything outside the chosen boundary is the surroundings. The system plus surroundings is the universe for that problem.
The surface that separates the system from the surroundings is the boundary. That boundary may be fixed or moving.
Heat and work cross that boundary. Those transfers are the interactions that heat-power engineering tracks.
Types of System in Thermodynamics
Thermodynamics uses two basic system types:
- Closed System or Control Mass: A closed system, or control mass, contains a definite quantity of matter. No mass crosses its boundary. An isolated system is a closed system that also exchanges no energy with the surroundings.
- Control Volume (Open System): A control volume is a region of space that mass and energy may enter and leave. The boundary of an open system is the control surface. That surface may be a real wall or an imaginary surface.
Pumps, steam turbines and air compressors are control volumes: water, steam or air crosses the boundary.
Microscopic Thermodynamics
The microscopic view is statistical thermodynamics. It starts from the particles that make up matter. It finds their average behaviour. From that average it recovers the macroscopic properties of the system.
Thermodynamics Property, States and Process
Thermodynamic Property
A thermodynamic property is a macroscopic feature of a system. Its value at a given instant does not depend on how the system reached that condition.
Extensive Property
Extensive properties depend on the amount of matter. The value for the whole system is the sum of the values for its parts. Volume, energy and mass are extensive. They scale with system size and can change with time.
Intensive Property
An intensive property does not depend on the amount of matter and is not additive. It does not scale with the size of the system. It may still vary from place to place inside the system. Pressure and temperature are intensive.
Thermodynamic State
The state of a system is the condition described by its properties. The same mass can exist in many distinct states. Properties are related, so a small subset of independent properties is enough to fix the state of a simple compressible system.
Thermodynamic Process
A process is a change from one state to another. If every macroscopic property at two times is the same, the system is in the same state at those times. A steady state means none of those properties changes with time.
System Equilibrium Cycle
A thermodynamic system cycle is a sequence of processes that starts and ends at the same state. After one cycle every property has its starting value. Regular cycles matter in plant work: condensate that circulates in a thermal power generating station follows a cycle.
Working Substance
Matter carries energy. It has mass, occupies volume and fills space. Bulk matter is made of molecules. It appears as solid, liquid or gas.
In a solid, molecules sit close together and cannot move freely. A large force is needed to change the shape.
In a liquid, molecules are not locked in place. A small force is enough to keep them together while the liquid still flows.
In a gas the molecules move freely and at high speed, almost independently of their neighbours. Gases are compressible because of the empty space between molecules. The phase of a substance (solid, liquid or gas) depends on its energy and on pressure and temperature.
Pure Substance
A pure substance has a uniform and definite chemical composition. It may exist as one phase (liquid water) or as more than one phase in equilibrium (water and steam). A uniform mixture of gases of fixed composition, such as dry air, is treated as a pure substance.
Pure-substance tables give the properties of the working fluid at stated pressure and temperature.
Example: Water as a pure substance is fixed by two independent intensive properties, usually pressure and temperature, when it is a single phase. Dry air as a gas is treated the same way. A non-homogeneous mixture needs more than two properties to fix the state.
Thermodynamic Equilibrium
In mechanics, equilibrium means the net force is zero. Thermodynamic equilibrium is stricter: mechanical, thermal, phase and chemical equilibrium must all hold between the system and its surroundings.
This section stays with thermodynamic equilibrium. Classical thermodynamics describes equilibrium states and the change from one equilibrium state to another.
A fixed state means the system is in equilibrium for the properties in use. Pressure and temperature must be uniform enough to measure. A system is in thermodynamic equilibrium if a small disturbance does not change those intensive properties.
In that case the system is at rest with the constraints applied by the surroundings.
Actual and Quasi-equilibrium Process
A real process usually passes through non-equilibrium states. Pressure and temperature can vary inside the system while the change is happening.
A quasi-equilibrium (quasi-static) process stays so close to equilibrium that every intermediate state can be treated as an equilibrium state.
Quasi-equilibrium paths are used to relate extensive properties such as entropy, internal energy, specific heats and enthalpy.
Dimensions and Units
A physical quantity has a dimension. A unit is a chosen size of that dimension. Engineering calculations require consistent units.
Primary dimensions include mass (m), length (L), time (t) and temperature (T). Two unit systems are in wide use: US customary (English) units and the SI metric system. The SI system of unit is the usual choice in engineering work. SI thermodynamic temperature is the kelvin. The table lists Fahrenheit for the English column; the thermodynamic English scale is Rankine.
| Primary Dimensions | Metric (SI) Unit | English Unit |
| Length | Meters (m) | Foot(ft) |
| Mass | Kilogram (kg) | Pound (lb) |
| Time | Seconds (s) | Seconds (s) |
| Temperature | Kelvin (K) | Fahrenheit |
| Electric Current | amperes (A) | amperes (A) |
Secondary Dimensions and Units
Secondary or derived dimensions are written from primary dimensions. Examples are velocity, energy, volume, force, power and heat.
Force is considered as a secondary dimension in SI units, since its unit is derived from Newton’s second law that is
Force is defined as the force required to accelerate a mass of 1 kg at a rate of 1 m/s2.
Weight and mass are not to be considered as same. Weight is gravitational-force act on a body and its magnitude is determined from Newton’s second law
Specific Weight(y) is defined as the gravity force acting on a unit volume of a substance and is determined by y = (density ) × g N/ m3
Thus regardless of location in the universe, the mass of the body remains the same. When gravitational acceleration changes then the weight of the body also changes. At top of the mountain, the body weighs less, as its g decreases with altitude.
Specific Volume(ϑ) and Density(ρ) both are intensive property and can differ from point to point. Reciprocal of density is a specific volume.
Pressure: Pressure is defined as a normal force exerted by a fluid per unit area in case of liquid or gas. In solid pressure is equivalent to normal stress.
SI Unit of pressure and stress is Pascal (N/ m2). Other units of pressure are given below:
| 1 Pascal | 1 N/ m2 |
| 1 K – Pascal | 103 – N/ m2 |
| 1 bar | 105 – N / m2 |
| 1 M – Pascal | 106 – N/ m2 |
| 1 atm | 101.325 kpa=1.01325 bars |
| 1 Bar | 100 kpa = 0.1 M – Pascal |
| 1 kgf/cm2 | 9.807 N/cm2 = 0.9807 Bar = 0.9679 atm |
Absolute pressure is the pressure at a point measured from absolute vacuum (zero absolute pressure).
Gauge pressure is that absolute pressure minus the local atmospheric pressure.
Vacuum pressure is atmospheric pressure minus absolute pressure when the absolute pressure is below atmosphere. A vacuum gauge reads that difference.
The figure relates absolute, atmospheric, gauge and vacuum pressure.
Energy and forms of Energy
In engineering teaching, energy is the capacity to produce a change, often stated as the capacity to do work. An energy transfer changes the state of the matter in the system.
Energy is grouped as stored energy and energy in transit.
- Stored energy in matter includes internal energy (U), kinetic energy (KE), potential energy (PE), chemical energy, electrical energy and nuclear energy. Those forms are taken up later.
- Energy in Transit
Heat
Heat is defined as the energy transferred without the transfer of mass across the boundary of a system due to the temperature difference between the system and the surroundings. The energy in transition alone is called heat. The amount of heat transferred during a process is dependent on the path followed and not on the end conditions only.
Work
When force F is used to move one or more particle through a distance x then it is called Work. In a given below piston cylinder arrangement in order to decrease volume V of a system, work is required to be done. Therefore in an extremely small volume change in the system as a result of the motion of the piston is related to the differential in work through the force-distance product by the formula:
dW = Fdx = pAdx = pdV [ ft-ldf | Nm] …………….. (1)
dw = pdv [Btu / lbm | KJ/kg ] …….. (2)
Where, p is system-pressure, A is the area, F is the force, x is Incremental distance-travelled, W is work, V is volume
Lower-case letters in (1) and (2) are work and volume per unit mass. Those are specific properties.
Equation (1) is in US customary units. Equation (2) uses the British thermal unit. US customary teaching links them with the mechanical equivalent of heat, about 778 ft-lbf/Btu (the exact conversion is 778.169 ft-lbf/Btu).
