download the exercise with solution: Exergy loss_ Throttling Process

An airflow of 5 g/s is throttled adiabatically to ambient pressure (p_{a}=1bar). The air enters the throttling at the pressure p_{1}. The flowing of exergy loss due to the throttling is 160 W. Determine the pressure p_{1} before the throttling.

The ambient temperature is T_{a}=298K. The changes of kinetic und potential energies can also be neglected. Air is assumed to be perfect gas.

The specific gas constant for air is R_{air}=0.287 J/g∙K and the specific heat capacity at constant pressure c_{p}=1 J/g∙K

Solution:

In order to calculate the exergy loss, we also need to determine the production of entropy during this process:

The general expression for the exergy loss is:

Considering the throttling process, we can reduce the conservation of energy equation to:

h_{1} = h_{2}

That means, enthalpy values at the inlet and outlet are the same.

h_{1} = h_{2} → c_{p}∙T_{1}=c_{p}∙T_{2} → T_{1} = T_{2}

Therefore the equation of exergy loss is reduced to: