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:


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