Thermodynamics: First and Second Laws in Action
First Law: Open Systems
In open systems, there is both mass flow and energy transfer.
Mass Flow
The amount of mass flowing through a system per unit of area, volume, and time. If an open system has conservation of mass, and if the fluid is incompressible, it means that the density at the input is equal to the density at the output.
Work Flow
The work done by the fluid to move through the system. Due to the conservation of energy, the energy input must be equal to the energy output.
Enthalpy
A thermodynamic property that shows the energy changes that occur in a process between the input and output of an open system. It is an accurate property that depends only on the input and output of a system.
Equipment Operating at Constant Flow
- Boiler: A device that supplies heat to a fluid (usually water) in order to raise its temperature or vaporize it. It is a static device that does no work.
- Gas or Vapor Turbine: A device that receives a high-pressure and high-energy fluid, which expands to a low pressure, performing shaft work. We are interested in knowing the work of the shaft.
- Compressors and Pumps: Devices that aim to raise the pressure of the working fluid continuously. We are interested in determining the shaft work to be developed.
- Nozzle: This device accelerates a fluid by decreasing the area and reducing the pressure. The fluid flow is so fast that the device does no work, but it does lose heat.
- Valves: Regulatory elements where the fluid undergoes a pressure drop due to the size of the device and the speed with which it can pass through. There is no heat or work transfer because they have fixed or finite limits. It is an isenthalpic process where the energy input equals the energy output.
Thermodynamic Cycles
Duty Cycle
The objective is the transformation of heat into work in a physical place called a heat engine. Examples include steam power plants, diesel engine cycles, and gas turbine cycles.
Features of a Work Cycle
- Has a process in which at least the system or the working fluid receives heat at high temperatures.
- Has at least one process where the working fluid system rejects heat to a low point or sump temperature.
- Delivers work to the environment.
Primordial Power Plant Issues
The power plant takes into account two main aspects: Effectiveness and Efficiency. Effectiveness is related to the associated costs, while efficiency is related to the performance of the machine or process.
Performance of the plant: The ratio of the absolute value (+) of the net work and the heat absorbed.
Developed by Power Plant: The product of net work and the frequency with which the cycle repeats.
Calorific Value: The amount of energy absorbed by a motor, relative to the mass required or consumed to make it operate.
Refrigeration Cycle
Its purpose is to transform heat from a low-temperature source to a high-temperature source, using work, as in refrigerators, heat pumps, ventilation, and air conditioning.
Features
- Has at least one process or system where the working substance receives heat at a low temperature.
- Has at least one process where the system rejects heat to a high-temperature source.
- Requires extra work to be operational.
Note: Cooling cycles have no efficiency or performance in the traditional sense. Instead, they have an operating efficiency (COP), which is the ratio between the desired output (DI) and the net work (Wn).
Heat Pumps: The efficient operation is defined as the ratio between the desired output, corresponding to (Qh), and the net work (Wn).
Thermodynamics: Second Law
Based on experience, it describes the spontaneous sense of the realization processes and provides the necessary tools to measure the power quality and efficiency of thermodynamic transformations.
It recognizes the unidirectional nature of heat transfer and the transformation of work into heat.
Entropy
(Disorder) Used to optimize energy processes, considering the reversibility or irreversibility of the processes.
Irreversible Process
This is when the state of a process has the same features in one way or another, or there is no thermodynamic equilibrium.
Thermodynamic Equilibrium
For this to occur, the following should not happen in the process:
- Changes in mechanical product of the pressure variation.
- Chemical reactions (the process is at constant volume).
- Thermal equilibrium (the temperature is constant).
Irreversible Processes
Can be caused by:
- Internal irreversibilities of the system, such as internal friction and diffusion combustion.
- External irreversibilities of the system, which are mechanical and thermal.
In the Second Law
If the process is spontaneous, it is necessarily irreversible, which means that:
- Heat flows from high to low temperatures.
- Work is spontaneously converted into heat.
Clausius Statement
Pointing to the cycle of heat engines, it states that heat is not transferred spontaneously from a cold body to a higher temperature.
The only way is through a machine that is provided with external work. If there is no work, the process is reversible; if there is work, the process is irreversible.
Statement of Kelvin-Planck
No machine can convert all the heat supplied into work; it has to give up some heat to a cooler area that is naturally accessible.
Carnot Cycle
It bases and validates the second thermodynamic principle, establishing a relationship between heat and temperature. This cycle is based on four reversible processes (two adiabatic and two isothermal), allowing the determination of thermal performance as a function of the temperature of the bulbs delivering energy.
Entropy
The sum of all small states that are constantly changing, determining the heat disorder in a process. It is a state function that describes the irreversibility of processes. The Carnot cycle consists of four processes: two adiabatic, where the heat is zero and there is no change in entropy, and two isothermal processes, where heat is transferred and there is a change in entropy. In any process where there is or is no heat exchange, entropy is constant, defining the isentropic process.