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Thermodynamics Laws

Laws of Thermodynamics


Thermodynamics is a branch of physics which deals with the energy and work of a system. It was born in the 19th century as scientists were first discovering how to build and operate steam engines. Thermodynamics deals only with the large scale response of a system which we can observe and measure in experiments. Small scale gas interactions are described by the kinetic theory of gases. The methods compliment each other; some principles are more easily understood in terms of thermodynamics and some principles are more easily explained by kinetic theory.

The four laws of thermodynamics define fundamental physical quantities (temperature, energy, and entropy) that characterize thermodynamic systems.

The Thermodynamics Laws

Energy exists in many forms, such as heat, light, chemical energy, and electrical energy. Energy is the ability to bring about change or to do work. Thermodynamics is the study of energy.

A working fluid is a substance which receives, transfers and transmits energy in a thermodynamic system. In most systems, the working substance is a fluid (liquid, vapour or gas). In a steam system, water is the working fluid.

Density (ρ), measured in kg/m3 or lbm/ft3, represents the mass of a substance per unit volume, or how tightly packed the molecules are. The more molecules packed in a given space, the more dense the material. The density of water in a given location of the boiler is critical to the steam generation process because relatively dense feed water will naturally push a less dense steam/water mixture through the boiler generating tubes.

Specific volume (vSP), measured in m3/kg or ft3/lbm, represents the space occupied per unit mass of a substance. It is the mathematical inverse of density. Most engineering equipment is designed for size and strength taking into consideration the specific volume of the intended working fluid.

Specific weight (γ), measured in kN/m3 or lbf/ft3, represents the weight of a substance per unit volume. This is the density of a substance acted upon by gravity. The pressure of a fluid at the bottom of a storage tank is a direct function of the height of the fluid in the tank and the specific weight of the feed water. This resultant pressure is an important shipboard consideration with respect to providing a minimum suction pressure for a pump below the tank to move the fluid through a system.

Thermodynamic Process

A thermodynamic process is any process which changes the state of the working fluid. These processes can be classified by the nature of the state change that takes place. Common types of thermodynamic processes include the following:

A reversible process is an ideal process where the working fluid returns to its original state by conducting the original process in the reverse direction. For a process to be reversible, it must be able to occur in precisely the reverse order. All energy that was transformed or distributed during the original process must be capable of being returned to its exact original form, amount and location. Reversible processes do not occur in real life.

An irreversible process is any process which is not reversible. All real life processes, such as the basic steam cycle, are irreversible.

An adiabatic process is a state change where there is no transfer of heat to or from the system during the process. Because heat transfer is relatively slow, any rapidly performed process can approach being adiabatic. Compression and expansion of working fluids are frequently achieved adiabatically with pumps and turbines.

An isentropic process or isoentropic process is one in which, the process takes place from initiation to completion without an increase or decrease in the entropy of the system, i.e., the entropy of the system remains constant. It can be proven that any reversible adiabatic process is an isentropic process. A simple more common definition of isentropic would be “No change in entropy”.

An isothermal process is a state change in which no temperature change occurs. Note that heat transfer can occur without causing a change in temperature of the working fluid. In the DFT, auxiliary exhaust heats incoming condensate, then condenses to liquid and falls to the bottom of the tank. Throughout this process, the temperature of the auxiliary exhaust remains constant at 246-249°F.

An isobaric process is a state change in which the pressure of the working fluid is constant throughout the change. An isobaric state change occurs in the boiler super heater, as the heat of the exiting steam is increased without increasing its associated pressure.

Thermodynamic Cycle

A thermodynamic cycle is a recurring series of thermodynamic processes through which an effect is produced by the transformation or redistribution of energy. In other words, it is a series of processes repeated over and over again in the same order. Thermodynamic cycles contain five basic elements: (1) a working fluid, (2) an engine, (3) a heat source, (4) a heat receiver, and (5) a pump. All thermodynamic cycles may be classified as being open cycles or closed cycles.

A closed cycle is one in which the working fluid is reused. Steam plants and refrigeration cycles are closed cycles. In a steam plant, the water undergoes a series of processes that change the state of the water. Eventually the water returns to its original state and is ready to begin the cycle again.

An open cycle is one in which the working fluid is not reused. Open cycles typically use the atmosphere as a working fluid. An internal combustion engine represents a typical open cycle. Air is drawn into the engine, combusted in the cylinders, and exhausted back to the atmosphere. Fresh air is drawn into the engine to begin the cycle again.

Zeroth law of thermodynamics

The Zeroth Law of Thermodynamics

Zeroth law of thermodynamics – If two thermodynamic systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other. If A and C are in thermal equilibrium with B, then A is in thermal equilibrium with C.

Law of Equilibrium

First law of thermodynamics

First law of thermodynamics (Conservation) – Energy can neither be created nor destroyed. It can only change forms. In any process, the total energy of the universe remains the same. For a thermodynamic cycle the net heat supplied to the system equals the net work done by the system.

Conservation of Energy

The change in internal energy of a system is equal to the heat added to the system minus the work done by the system. ΔU = Q – W

ΔU — the change in internal energy of a system, Q — the heat added to the system, W — the work done by the system.

First Law of Thermodynamics


Enthalpy is a measure of the total energy of a thermodynamic system.
Enthalpy (H, a measure of the total energy of a thermodynamic system) – The sum of the internal energy (which is the energy required to create a system) of the system plus the product of the pressure of the gas in the system and its volume. H = U + PV.


Entropy is the measure of disorder and randomness in a system.

Second law of thermodynamics

Second law of thermodynamics – The entropy of an isolated system not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium.

Second Law of Thermodynamics

The Second Law of Thermodynamics states that “in all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state.” This is also commonly referred to as entropy. In the process of energy transfer, some energy will dissipate as heat. Entropy is a measure of disorder.

Third law of thermodynamics

Third law of thermodynamics – As temperature approaches absolute zero, the entropy of a system approaches a constant minimum.


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