Throttling Process – Isenthalpic Process
A throttling process
is a thermodynamic process
, in which the enthalpy
of the gas or medium remains constant (h = const)
. In fact, the throttling process
is one of isenthalpic processes
. During the throttling process no work
is done by or on the system (dW = 0), and usually there is no heat tranfer
) from or into the system (dQ = 0). On the other the throttling process cannot be isentropic, it is a fundamentally irreversible process
. Characteristics of throttling process:
- No Work Transfer
- No Heat Transfer
- Irreversible Process
- Isenthalpic Process
A throttling of the flow causes significant reduction in pressure, becauses a throttling device causes a local pressure loss. A throttling can be achieved simply by introducing a restriction into a line through which a gas or liquid flows. This restriction is commonly done by means of a partially open valve or a porous plug. Such pressure losses are generally termed minor losses, although they often account for a major portion of the head loss. The minor losses are roughly proportional to the square of the flow rate and therefore they can be easy integrated into the Darcy-Weisbach equation through resistance coefficient K.
For example, consider a throttling of an ideal gas flowing through a valve that is partially open. From experience we can observe that: pin > pout, vin < vout, where p is the pressure and v is the specific volume. We can also observe that specific enthalpies remains the same, i.e. hin = hout.
The specific enthalpy is equal to the specific internal energy of the system plus the product of pressure and specific volume.
h = u + pv
Therefore, if pressure decreases then specific volume must increase if enthalpy is to remain constant (assuming u is constant). Because mass flow is constant, the change in specific volume is observed as an increase in gas velocity, and this is also verified by observations.
If there is a change in the internal energy, u, then there must be a temperature change. Normally the temperature of the fluid will drop. However in special cases, the temperature may remain the same or it may increase.