## Throttling

A fluid can be throttled by several means. Examples are: a partly open valve, an orifice or any other sudden
reduction in the cross-section of the flow. The enthalpy remains almost constant and pressure reduces in
this process. Throttling is an irreversible process due to eddying of the fluid.

Consider a perfectly thermally insulated pipe which fluid flows steadily through an orifice. Applying
the first law of thermodynamics to the steady flow system
defined by the control volume between sections 1-1 and 2-2, gives:

**dQ/dt+dW/dt=m [h + C/2 + g Z]**

**dQ / dt = 0** because the system is thermally insulated. **dW / dt** is also zero. If velocities at sections 1-1
and 2-2 are small or approximately equal and the height difference between these two sections,
**Z**, is negligible, then we can write:

**h=h2-h1=0**

where **h1** and **h2** represent the enthalpy of the working fluid at sections 1-1 and 2-2 respectively.

Copyright © 1998 Taftan Data. All Rights Reserved