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Sunday, 4 September 2016


We were discussing various basic concepts of thermodynamics such as “Thermodynamic point and path function” in our recent post. We have also discussed the “Flow work or flow energy in thermodynamics and also “Mass balance and energy balance for a steady flow process” in the field of thermal engineering.

Today we will see here the steady flow energy equation for throttling devices with the help of this post. Finally we will also see the applications of throttling devices. After this post, we will see the steady flow energy equation for heat exchanger in our next post.

Let us first see here the basic concepts of throttling devices

When a fluid (gas or liquid) flows through a restricted passage, such as partially opened valve, significant pressure reduction of the fluid will be secured over there. Such type of flow of fluid where significant pressure reduction of fluid is secured by introducing a restriction in to its passage will be termed as throttled flow.

Devices used for providing the restriction in the flow of fluid and simultaneously causing significant pressure drops will be termed as throttling devices. There are many live examples of throttling devices such as adjustable valves, porous plugs and capillary tubes etc.
Throttling devices
Let us see here the assumptions and law of throttling devices in order to analyse the energy equation for throttling process.
  1. Throttling devices are generally very small in size and hence there will not be much area for transfer of heat energy. It must be noted here also that there will not be much time for transfer of heat energy because pressure drop will take place rapidly. Therefore we can assume that transfer of heat energy will be zero or Q = 0
  2. There will not be any mechanism in throttling devices that will allow transferring of work energy between system and surrounding i.e. there will not any work interaction between throttling devices and surrounding. Therefore for a throttled process, we will have W =0
  3. Throttling devices are generally very small in size and hence change in gravitational potential energy will be assumed as zero. Therefore we will have ∆Z =0 and hence ∆PE =0
  4. When fluid flows through a throttling device, change in kinetic energy will be assumed as zero as we can neglect the small changes in kinetic energy of the fluid. Therefore, we will have ∆KE =0

Let us recall the steady flow energy equation

Let us implement here the assumptions made above in order to secure the energy equation for throttling devices in quite simple terms.
H1 = H2
Where, H1 and H2 are the enthalpy of the fluid at inlet and outlet respectively.


So what we have noted here?
  1. A throttling process is a process where, there will not be any change in enthalpy between inlet state and outlet state of the fluid and therefore throttling devices are also termed as isenthalpic devices.
  2. There will not any work energy transfer during throttling process and hence we can say that they will provide pressure drop without providing any work energy.
  3. Transfer of heat energy during throttling process is ignored as explained above
  4. Change in potential energy is also ignored during the throttling process as explained above
  5. Change in fluid velocity and hence change in kinetic energy is also ignored during throttling process as explained above.
We will see the “Steady flow energy equation for turbine and compressor” in our upcoming posts.
Do you have suggestions? Please write in comment box


Engineering thermodynamics by P.K. Nag
Engineering thermodynamics by R. K. Rajput
Image courtesy: Google

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