When flowing fluid passes through throttling there is a drop in

1. Sep 22, 2008

when flowing fluid passes through throttling there is a drop in pressure and also there's a drop in temperature why?can anyone help??

2. Sep 22, 2008

FredGarvin

Re: throttling

Because there is a pressure drop, i.e. there is an expansion. Look at the gas laws for relationships between pressure and temperature.

http://en.wikipedia.org/wiki/Gas_laws

3. Sep 22, 2008

Q_Goest

Re: throttling

To help clarify, a pressure drop through a restriction does not do any work, and is normally estimated as an adiabatic process. As such, it is then an isenthalpic process (dH = 0).

If the gas is ideal (ie: PV = mRT) then the expansion (increase in V) will exactly counter the pressure drop (decrease in P) to give you a constant (ie: PV = constant for ideal gas). In the case of a real gas, this isn't always the case. Under typical circumstances, PV is slightly lower after the expansion, so the gas is very slightly cooler. However, there is often some range of values where PV will actually increase slightly after expansion, so the gas can actually increase in temperature. The best way to determine this temperature change is to determine the state downstream of your restriction from a gas properties database knowing the enthalpy upstream and the pressure downstream. These two (enthalpy and pressure) then define the state of the gas downstream and thus the temperature. The effects are generally very small and are due to the non-ideal nature of the gas.

4. Sep 22, 2008

stewartcs

Re: throttling

Also, for an Ideal Gas, the temperature remains constant.

CS

5. Sep 23, 2008

Re: throttling

well,u say there is expansion i dont know why there is expansion what causes this expansion??also i can get why pressure decreases in the constriction but according 2 bernoulli equation pressure will restore its value when fluid get out of constriction and back to flow in the same diameter before constriction but that doesnt happen??

6. Sep 23, 2008

stewartcs

Re: throttling

The pressure drop in a throttling device is caused by the increase in velocity. The velocity is increased due to the decrease in area of the device that the fluid is flowing into and then out of (e.g. throttling valve, porous plug, capillary tube). Take a look at the Bernoulli equation and the Continuity equation to see the relationship between pressure, area, and velocity. You'll see that energy is conserved.

Remember that a throttling process is defined as isenthalpic (h1 = h2).

Enthalpy is defined as:

h = u + Pv

Therefore, since pressure decreases, the specific volume must increase in order for h to remain constant (i.e. there is an expansion - assuming u is constant which is the case for an ideal gas). If u, the internal energy, is not constant, then you will see a temperature change like Q_Goest explained previously.

In order to expand (i.e. increase the specific volume), the fluid must do work. Since the process is adiabatic (and the KE and PE are negligible), the energy to do the work comes from the internal energy of the fluid. This typically results in a decrease in temperature (in real gases). However, depending on the inversion point, the temperature may actually increase. This is typically referred to as the Joule-Thompson effect, and the J-T coefficient determines if the temperature goes up or down. If the J-T coefficient is negative, the temperature will increase. If it is positive, the temperature will decrease. If it is zero, the temperature remains constant (again this is only for an ideal gas).

Hope this helps.

CS