First Law of Thermodynamics – Applications to Open Systems

In any systems, the rate of flow of working fluid is constant with respect to time, then the system is known as steady flow system. In this system, the mass of working fluid enters the system and leaves the system after doing the work. Therefore, this system is known as open system.

From first law of thermodynamics, the total energy entering the system is equal to the total energy leaving the system. This law is applicable to the steady flow systems.


It is defined as the capacity of the substance to do work. It can be classified as follows:

  1. Stored energy
  2. Transit energy.

Stored energy : It is the energy which is contained within the system boundaries.

Examples: Potential energy, kinetic energy, internal energy, etc.

Transit energy : The energy which crosses the boundary of a system is called transit energy.

Examples: Heat energy, work energy, electrical energy, etc.

Potential energy

It is the energy possessed by a system because of its height.

Potential energy, P.E = m x g x z


m be the mass of the system in kg
g be the acceleration due to gravity in m/s²
z be the height of the system above from datum in m

Kinetic energy

It is defined as the energy possessed by the system by virtue of its motion.

K E=\frac{m C^{2}}{2}


C be the velocity of the system in m/s

Flow energy

It is the energy associated with the flow of mass across the boundaries of a system. The displacing mass must do work on the mass being displaced. This work is known as flow work.

Flow energy = Force x distance moved

= p x A × χ

\begin{array}{c} {\left[\because \text { Pressure, } p=\frac{\text { Force }}{\text { Area }}=\frac{F}{A}\right]} \\ = p \times V  {[\therefore A \times x=\text { volume }, V]} \end{array}


V = volume of volume of the fluid flowing in m³/s

For unit mass, the flow energy is given by

Flow energy, F.E = p V

Total energy

Total energy is the sum of all energies.

Total energy = K.E + P.E + F.E + Internal energy + … etc.

For steady flow process, the following assumptions are to be made:

  1. The rate of mass flow through the control volume is constant.
  2. The rate of heat transfer is constant.
  3. The rate of work transfer is constant.
  4. The state of working fluid at any point remains constant.
  5. Only potential, kinetic, flow and internal energies are considered. There are no other forms of energy such as electrical, chemical, magnetic etc.

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Santhakumar Raja

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