Expander Models in Simulators

Either of two methods can be used to take into account efficiency when calculating power requirements for compressors, whether they are centrifugal, reciprocating, or screw. One method is the polytropic method, based on the expression, PV = constant during compression, where V is the gas volume and n is the polytropic coefficient, which lies between 1 and the specific heat ratio. 

Since the advent of simulation programs that routinely calculate entropy, the second method, called the isentropic method, has become preferred, because it has a sound theoretical basis. The theoretical horsepower delivered to the gas is computed for a reversible, adiabatic (isentropic) compression firom inlet, 1, to outlet, 2. The entropy balance in terms of the molar entropy, s, is 

Since Tj, Pi, and P2 are known, Eq. (15.7) is solved iteratively for T2,iseiiiropic- With T2 known, the exit enthalpy can be computed. Then the first law of thermodynamics for an adiabatic compression of molar gas flow, F, assuming no change in potential or kinetic energy of the gas and written in terms of molar enthalpy, h, can be applied to calculate the theoretical or isentropic powen 

The excess power required, because of inefficiency of the compressor, is the difference between the brake power, Wbrake. and the isentropic power, Wi niropic- These two powers define an isentropic efficiency, with the assumption that the excess power increases the enthalpy to 

The actual temperature of the discharged compressed gas, T2, is then computed iteratively from the actual enthalpy, h2- The actual temperature T2 can be significantly higher than the 

---