What is the difference between nozzle (or a ventury) and flow control valve?

[Mohankpvk]

In both the cases, the flow cross section area of the pipe is reduced.But in case of nozzles the velocity increases,pressure reduces and the flow rate is maintained.But in case of flow control valve, flow rate changes.(Assuming the fluid to be incompressible) Why is there a difference between the two cases?

 

[JBA]

For any identical flow area and coefficient both the nozzle/orifice and the control valve act exactly in the same manner, reducing the size of the flow area in either case equally reduces the mass flow through the device.

 

[Mohankpvk]

For any identical flow area and coefficient both the nozzle/orifice and the control valve act exactly in the same manner, reducing the size of the flow area in either case equally reduces the mass flow through the device.

Could you please explain how the flow area differs for the case of flow rate reduction from the case where velocity increases(flow rate maintained the same)?
Please suggest some books or other sources from which I can understand this.

 

[Dullard]

I may misunderstand your question, but:

A nozzle 'attempts' to preserve total energy by reducing cross-section gradually, with a minimum of friction/turbulence. Much of the potential energy due to pressure is converted to kinetic energy (velocity). Control valves reduce cross-section, but make no attempt to be gentle.

 

[Mohankpvk]

I may misunderstand your question, but:

A nozzle 'attempts' to preserve total energy by reducing cross-section gradually, with a minimum of friction/turbulence. Much of the potential energy due to pressure is converted to kinetic energy (velocity). Control valves reduce cross-section, but make no attempt to be gentle.

Nice answer.So, in case of flow control valves, sudden reduction in cross sectional area is a reason for reduction in flow rate.