During throttling process:

Explanation:

If steam is throttled, its enthalpy remains constant and a pressure drop takes place.

Throttling (Isenthalpic) process:

• A throttling process occurs when a fluid flowing through a passage suddenly encounters a restriction in the passage.
• The restriction could be due to the presence of an almost completely closed valve or due to sudden and large reduction in flow area etc.
• The result of this restriction is a sudden drop in the pressure of the fluid as it is forced to flow through the restriction. This is a highly irreversible process and is used to reduce the pressure and temperature of the refrigerant in a refrigeration system.
• This is a highly irreversible process. Hence entropy in the throttling process increases
• Since generally, throttling occurs in a small area, it may be considered as an adiabatic process (as the area available for heat transfer is negligibly small) (δQ = 0) also since no external work is done (W = 0).

∴ h1 = h2