Most Important Power Plant Fundamentals – Part -2
Before discussing boilers and power plants in detail, it’s important to understand some basic mechanical engineering principles. These fundamental concepts are essential for boiler operation engineers in their daily tasks.
HEAT
Heat refers to the energy in a system and is transferred from objects at higher temperatures to those at lower temperatures through radiation, conduction, or convection. An object’s heat represents the total kinetic energy of its molecules, while temperature measures the average energy of these molecules. Q denotes heat, with the SI unit being the joule (J).
Heat is measured by the energy needed to raise the temperature of a known mass of water by a known amount. The central units for measuring heat include:
- Calorie: The energy needed to raise the temperature of one gram of water by one degree Celsius. A kilocalorie (kcal) equals 1000 calories and is used for larger quantities (e.g., raising the temperature of one kilogram of water by one degree Celsius).
- 1 kcal = 1000 cal
- Centigrade Heat Unit (CHU): The energy needed to raise the temperature of one pound of water by one degree Celsius.
- 1 CHU = 453.6 cal
- British Thermal Unit (BTU): The energy needed to raise the temperature of one pound of water by one degree Fahrenheit.
In SI units, heat is measured in joules (J) or kilojoules (kJ), with 1 kcal = 4.1868 kJ.
Specific Heat
Specific heat (C) is the energy required to raise the temperature of a unit mass of a substance by one degree Celsius. The particular water heat is 1 kcal/kg°C since raising one kilogram of water by one degree Celsius requires one kilocalorie.
WORK
Work occurs when a force acts on an object, causing it to move. It is calculated as the product of the force applied and the displacement in the direction of the force:
- W = F × X, where F is the force in newtons and X is the displacement in meters.
In the MKS system, work is measured in kilograms (kg), and in the SI system, it is measured in newtons (Nm) or joules (J).
POWER
Power is the rate at which work is done, defined as work done per unit of time:
- Power (P) = Work done / Time taken, or P = dW/dT.
The SI unit of power is the watt (W), where:
- 1 W = 1 J/s,
- 1 kilowatt (kW) = 1000 W.
ENERGY
Energy is the capacity to work and exists in various forms, such as heat, light, chemical, electrical, and atomic energy.
ENTHALPY
Enthalpy (H) is a thermodynamic property representing the total heat content of a system, defined as the sum of the internal energy (E) and the product of pressure (P) and volume (V):
- H = E + PV.
LAWS OF THERMODYNAMICS
Zeroth Law of Thermodynamics States that if system A is in thermal equilibrium with system B, and system B is in thermal equilibrium with system C, system A is also in thermal equilibrium with system C. This establishes the concept of temperature.
First Law of Thermodynamics (Law of Conservation of Energy): Energy cannot be created or destroyed but only transformed from one form to another. For example, heat energy can be converted into mechanical work and vice versa:
- W = JH, where J is the joule equivalent of heat.
Total heat supplied is the sum of work done and the change in internal energy:
- H = E + W.
Second Law of Thermodynamics:
- Kelvin-Planck Statement: It is only possible to convert some of the heat supplied to an engine into equivalent work. Some heat will always be lost, making thermal efficiency less than 1 (or less than 100%).
- Clausius Statement: Heat cannot flow from a colder object to a hotter one without external energy. Heat flows naturally from high to low temperatures; an external source is needed for reverse flow.
This law introduces entropy (S) as a state variable, where the change in entropy (dS) is the heat transfer (dH) divided by the temperature (T):
- dS = dH / T.