Second Law Statements | Thermodynamics - Mechanical Engineering PDF Download

Second Law Statements

It need be mentioned that the second law is a product of experiential observations involving heat engines that powered the Industrial Revolution of the 19th century. A heat engine is a machine that produces work from heat through a cyclic process. An example is a steam power plant in which the working fluid (steam) periodically goes through a series of steps in a cyclic manner as follows:

• Step 1: Liquid water at ambient temperature is pumped into a boiler operating at high pressure
century. A heat engine is a machine that produces work from heat through a cyclic process. An example is a steam power plant in which the working fluid (steam) periodically goes through a series of steps in a cyclic manner as follows:

• Step 2: Heat released by burning a fossil fuel is transferred in the boiler to the water, converting it to steam at high-temperature and pressure

• Step 3: The energy contained in the steam is then transferred as shaft work to a turbine; during this process steam temperature and pressure are reduced.

• Step 4: Steam exiting the turbine is converted to water by cooling it and transferring the heat released to the surroundings. The water is then returned to step 1.

Like the steam power plant all heat engines absorb heat at a higher temperature body (source) and release a fraction of it to a low temperature body (sink), the difference between the two quantities constitutes the net work delivered during the cycle. The schematic of a heat engine (for example: steam / gas power plant, automotive engines, etc) is shown in fig. 4.1. As in the case of the steam cycle, a series of heat and work exchanges takes place, in each case a specific hot source and a cold sink are implicated. A schematic of such processes is suggested inside the yellow circle between the hot and cold sources.

Fig. 4.1 Schematic of Heat Engine

The opposite of a heat engine is called the heat pump (refrigerators being an example of such device) is shown in fig. 4.2. There are indeed a large number other types of practical heat engines and power cycles. Select examples include: Ericsson Cycle, Stirling cycle, Otto cycle (e.g. Gasoline/Petrol engine, high-speed diesel engine), Diesel cycle (e.g. low-speed diesel engine), etc. The Rankine cycle most closely reproduces the functioning of heat engines that use steam as the process fluid function (fig. 4.3); such heat engines are most commonly found in power generation plants. In such plants typically heat is derived from nuclear fission or the combustion of fossil fuels such as coal, natural gas, and oil.
Detailed thermodynamic analysis of the various heat engine cycles may be found in a number of textbooks (for example: J.W. Tester and M. Modell, Thermodynamics and its Applications, 3rd ed., Prentice Hall, 1999).

Fig. 4.2 Comparison of Heat Engine and Heat Pump

Fig. 4.3 Schematic of a Power Plant (Rankine) Cycle

As evident, the operation of practical heat engines requires two bodies at constant differential temperature levels. These bodies are termed heat reservoirs; they essentially are bodies with – theoretically speaking – infinite thermal mass (i.e., mC_P → ∞ ) which therefore do not undergo a change of temperature due to either release or absorption of heat. The above considerations may be converted to a set of statements that are equivalent descriptors of the second law 

Kelvin-Planck Statement: It is impossible to devise a cyclically operating device, the sole effect of which is to absorb energy in the form of heat from a single thermal reservoir and to deliver an equivalent amount of work. 

Clausius Statement: It is impossible to devise a cyclically operating device, the sole effect of which is to transfer energy in the form of heat from a low temperature body to a high temperature body.
Clausius Statement: It is impossible to devise a cyclically operating device, the sole effect of which is to transfer energy in the form of heat from a low temperature body to a high temperature body.