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Science of Engineering Thermodynamics Part 2

The objective of the Engineering Thermodynamics (Part-2) is to familiarize the readers with the following concepts:
  1. Understanding about Working Substance
  2. What is Pure Substance?
  3. Meaning and importance Thermodynamic Equilibrium
  4. Actual and Quasi-equilibrium Process
  5. Importance of Dimensions
  6. Energy and its Forms
  7. Heat
  8. Work

Working Substance

Theory of matter is helpful in understanding the concept of energy. Matter is known for its mass, volume & space and irrespective of its structure and nature it has certain characteristics like consistency and reliability. Matter is made from large number of particles called molecules. One can find matters of Solid, liquid or gas everywhere. In Solid matter molecules are close to each other and strongly bounded and cannot able to move freely. Thus large force required to change its shape.

Molecules in liquid matter are not firmly held and thus very small force is sufficient to keep the molecules together. In gaseous state the molecules moves randomly and freely as if it is in unbound state then it moves very fast irrespective of its adjacent molecules. Compressibility is associated with gases, are having plenty of empty spaces between the connecting molecules. Energy is the reason for matter to exist in different phases.

Pure Substance

Material of solo chemical structure or homogeneity in variant chemical structure is known as pure substances. Material can exist in single phase like liquid or can also exist in more than one phase in equilibrium with each other. A uniform mixture of gases having similar chemical composition is also termed as pure substance. The importance of pure substance is in the determination of properties of the working substance at different conditions of pressure and temperature. Example For pure substance like water can be described fully by two sovereign intensive properties termed as pressure and temperature. Another pure substance is air in the gaseous state. But for non homogenous substance more than two properties are required to describe the state.

Thermodynamic Equilibrium

In mechanics equilibrium is said to have reached, when we equalises the opposing forces. But the meaning of thermodynamic equilibrium is different and far reaching as it involves balancing act for many other influences (between and system and surrounding) apart from balancing opposing forces). In order to attain the complete equilibrium with in a system, one need to fulfil the condition for mechanical, thermal, phase and chemical equilibrium.

In this section we are limiting our discussion to thermodynamic equilibrium. Emphasis on having equilibrium states and its change from one equilibrium to another is best described by Classical Thermodynamics If the state is fixed then the system is said to be in equilibrium. Intensive properties like pressure and temperature to be accurately measured in order to assign the state. A system is said to be in thermodynamic equilibrium if its intensive properties not changes on account of very little disturbance. Under this situation the system is in complete stability with the restraints offered by the surroundings.

Actual and Quasi-equilibrium Process

During an actual process, system can be considered as non-equilibrium due to the various non equilibrium effects present in the system and thus shows change in pressure and temperature. In Quasi-equilibrium (quasi-static) process the deviation from the thermodynamic equilibrium state is extremely small. Thus all states through which the system passes is considered to be in equilibrium state during a Quasi-equilibrium process. Quasi-equilibrium is used for deriving relation-ships among various extensive properties like entropy, internal energy, specific heats and enthalpy etc.

Significance of Dimensions and Units

Physical quantity is known by its dimensions and if the magnitude is given to this dimension then it is termed as unit. Prerequisite of engineering calculations is to have the same unit of physical quantities.

Some vital dimensions like mass(m), length(L), time(t), and temperature(T) are termed as primary-dimensions. Engineers and scientists around the globe are performing their calculations mainly on two types of units called English system and Metric system (SI). SI system of unit representation is more logical and commonly used by professionals.

Primary DimensionsMetric (SI ) UnitEnglish Unit
LengthMeters (m)Foot(ft)
MassKilogram(kg)Pound(lb)
TimeSeconds (s)Seconds (s)
TemperatureKelvin (K)Fahrenheit
Electric Currentamperes (A)amperes (A)

Significance of Secondary-Dimensions and Units

Secondary dimensions or derived dimensions are expressed in terms of primary dimensions like Velocity(V), Energy(E), and Volume(V), Force, Power, Heat etc. Force is considered as secondary dimension in SI units, since its unit is derived from Newton’s second law that is Force = (Mass) × (Acceleration) 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 W = (Mass) × (local gravitational acceleration) Specific Weight(y) is defined as the gravity force acting on 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 same. When gravitational acceleration changes then 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 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 Pascal1 N/ m2
1 K - Pascal103 - N/ m2
1 bar105 - N / m2
1 M - Pascal106 - N/ m2
1 atm 101.325 kpa=1.01325 bars
1 Bar 100 kpa = 0.1 M - Pascal
1 kgf/cm29.807 N/cm2 = 0.9807 Bar = 0.9679 atm

Absolute pressure is related with the actual pressure at a given point and the measurement is done with respect to absolute vacuum or absolute zero pressure. The actual pressure at a given point is refered as absolute pressure and is measured relative to absolute vacuum (absolute zero pressure). Gauge Pressure = Absolute Pressure – Atmospheric Pressure Pressures below atmospheris pressure are called Vacuum pressure and measured by vacuum gauge that indicates the difference between ‘atmospheric pressure and the ‘absolute pressure. Vacuum Pressure = Atmospheric Pressure – Absolute Pressure

closed system The above figure best describe the relationship between absolute pressure, atmospheric pressure, gauge pressure and vacuum pressures

Energy and forms of Energy

Energy is defined as the capability to do work. Energy input always produces some effect on the matter of the system. Two-major types of energy are: Stored Energy and Energy in Transit
  1. In material stored energy can exit in several forms like Internal Energy(IE), Kinetic energy(KE), Potential Energy(PE), Chemical Energy(CE), Electrical Energy(EE), Nuclear Energy(NE). Details of this energy will be discussed in next section.
  2. Energy in Transit

Heat

Heat is defined as the energy transferred without transfer of mass across the boundary of a system due to 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

closed system 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 system as a result of motion of 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 area, F is force, x is Incremental distance-travelled, W is work, V is volume Lower case letters in equation 1 & 2 above are for work and volume based on per-unit mass. Extensive properties with lower case characters are called specific-properties. Equation – (1) is in English unit, while British thermal unit (BTU) is used in Equation - 2 and the two sets of units are linked by a conversion factor called Mechanical-Equivalent of heat and its value is 778 ft-lbf / Btu.


By Pratosh Saxena, B.E(Mech), PG in Mgmt, B.O.E, Energy Auditor, PGDOM
with over 23 yrs of experience in Thermal Power Plants

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