Friday, 22 July 2016

WHAT IS SHAFT WORK IN THERMODYNAMICS?

WHAT IS SHAFT WORK IN THERMODYNAMICS?

We have seen the basic concepts of work and heat transfer in the field of thermal engineering and simultaneously we have also seen the sign conventions used in thermodynamics for heat and work transfer.

Today we will focus here about the shaft work in thermodynamics. Shaft work in thermodynamics will also be discussed in relation with a control volume or steady flow process. Therefore we will first understand here the fundamental of control volume and then we will see the basic concept of shaft work in thermodynamics.

As we have already discussed the basic concepts of control volume in our previous post, but let us brief here the control volume for understanding of the shaft work in the field of thermal engineering.

So let us see here first, what is control volume?

Control volume system is basically a system where mass will enter and leave continuously the system such as air compressors, heat exchangers, turbines etc. Air compressor is one of the best examples of control volume systems. In an air compressor, atmospheric air will be entered in to the compressor i.e. control volume system at low pressure and will exit the system at high pressure.

Now let us again come to the shaft work fundamentals. 

Following figure indicates the basics of a typical control volume system. Let us assume mass is continuously entering from left side of control volume and leaving continuously the system at right end as shown in figure.

Control volume system and flow work
As we have seen above that mass is continuously entering from one side of control volume and leaving continuously the system by other side of the control volume system. Therefore few amount of work will be required here by the control volume or few amount of work will be given here by the control volume.

Let us consider here the example of air compressor. Atmospheric air will be entered in the compressor at lower pressure and will leave the system at high pressure.

Now as we have seen here that air compressor is a control volume system because mass i.e. air is continuously entering from suction port and discharging from exit port of air compressor. Control volume i.e. air compressor will require the work here for compressing the atmospheric air from lower pressure to higher pressure.

This work, which is required by the control volume here i.e. air compressor, will be provided by a rotating shaft as shown in above figure. Therefore work energy interaction with the control volume system through a rotating shaft will be termed as shaft work in thermodynamics.

So, we have classified various types of mechanical work energy transfer in thermodynamics. Shaft work has been discussed here and other forms of mechanical work energy transfer in thermodynamic could be easily secured by following links.
  1. Displacement work or PdV work
  2. Paddle wheel work
  3. Flow work in thermodynamics
We will come again with new topic i.e. "Enthalpy in thermodynamics" in our next post in area of thermodynamics.

Do you have suggestions? Please write in comment box.

Reference:

Engineering thermodynamics by P. K. Nag
Basic thermodynamics by Prof. S.K. Som
Image courtesy: Google

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