7 replies to this topic

[vinod]

Sir......

Is the expansion of a compressible fluid through a control valve an isentropic process or isenthalpic process and why?

Regards

Vinod

[JoeWong]

Sir......

Is the expansion of a compressible fluid through a control valve an isentropic process or isenthalpic process and why?

Regards

Vinod

Vinod,

Isenthalpic...
Read more in

• Discussion on ISENTROPIC and ISENTHALPIC process via Relief Valve
• What is Isenthalpic Process

[vinod]

Okay thank you sir........

But in these cases there will be the loss due to friction(drop in enthalpy),so where is this drop in energy accounted.

Is there any difference in the flow from a nozzle and across an orifice?

Thanks

Vinod

[wnovaes]

Vinod,

In a throttling process (control valve, nozzle or orifice) overall process is isenthalpic. However, the isentropic process occurs during a short period of time.
Using as example the steam expansion through a control valve orifice. For the steam to squeeze through the narrow restriction it has to accelerate to a higher speed. It does so by borrowing energy from its enthalpy and converting it to kinetic energy. This incurs a heat drop. This part of the process is isentropic. Having passed through the narrow restriction, the steam expands into the lower pressure region in the valve outlet, and eventually decelerates as the volume of the valve body increases to connect to the downstream pipe. This fall in velocity requires a reduction in kinetic energy which is mostly re-converted back into heat and re-absorbed by the steam. The heat drop that caused the initial increase in kinetic energy is reclaimed (except for a small portion lost due to the effects of friction). Therefore, the final process is isenthalpic.
Even so, the isentropic process occurs during a short period of time, it must not be ignored. The isentropic process leads to a decreasing in the fluid temperature. For example, a PSV or pressure reduction valve in a gas system, depending of the fluid properties and pressure drop at the restriction, during the depressurizing process the valve body may reach a very low temperature. So, material specification for the valve internals and body should be compatible for low temperature operation.

Kinds Regards,
WSN.

[pleckner]

@Vinod,

To summarize what was said, a PSV is treated as a nozzle and thus is calculated as an Isentropic expansion, constant entropy. Orifice and valves (control valve being one of them) are treated as Isenthalpic expansions, constant entropy.

[djack77494]

@Vinod,

To summarize what was said, a PSV is treated as a nozzle and thus is calculated as an Isentropic expansion, constant entropy. Orifice and valves (control valve being one of them) are treated as Isenthalpic expansions, constant entropy.

I'm sure Phil intended for the last word to be "enthalpy".

[pleckner]

OOOOPS!

Thanks, it is "enthalpy".

[vinod]

Thanks everybody for the answers...........