The adiabatic throttling process of a perfect gas is one of constant enthalpy

The throttling process (joule-expansion) is usually done using an pipe arranged with gate valve and the effect is found at different positions of valve this process is done under steady state and keeping the system in an adiabatic chamber during which due to the obstruction imparted by valve position the gas will lose some pressure energy in to frictional energy and hence the pressure decreases in an throttling experiment always.