Studying thermodynamics I face this following text from Cengel's book.

> 
> Throttling valves are usually small devices, and the flow through them may be assumed to be adiabatic (q = 0) since there is neither
>  sufficient time nor large enough area for any effective heat transfer
>  to take place. Also, there is no work done (w=0), and the change in
>  potential energy, if any, is very small (pe=0). Even though the exit
>  velocity is often considerably higher than the inlet velocity, in
>  many cases, the increase in kinetic energy is insignificant (ke=0). 
>  Then the conservation of energy equation for this single-stream
>  steady-flow device reduces to
>  \*\*h2>h1 (kJ/kg)\*
> 
> 
> 

Previously, I thought that enthalpy meant internal energy + work energy (PV). So If the book says throttling valves have no work involved, how is energy equation about enthalpy (u + PV)?