Page 392 - Compression Machinery for Oil and Gas

370 SECTION II Types of Equipment

This harsh operating environment poses special challenges for expander
design and performance, reliability, and maintainability. Additional operational
considerations are increasingly demanding process parameters, extended main-
tenance cycles, and more stringent environmental regulations.

Radial Inflow Turboexpanders
In radial inflow turboexpanders, energy is transferred from the fluid to the
wheel in passing from a larger area at the wheel tip to a smaller area at the wheel
eye. Fig. 7.55 shows a schematic layout of a radial inflow turboexpander.
The fluid discharging from the wheel may have a considerable kinetic
energy (high velocity C 4 ). A diffuser is normally incorporated to recover the
kinetic energy, which would otherwise be wasted. In Fig. 7.55, the velocity tri-
angles are shown to clarify that the inlet relative velocity, W 3 , is radially inward,
and the absolute flow at rotor exit, C 4 , is axial. This configuration of velocity
triangles is popular for radial inflow turboexpanders.
In a turboexpander, there are three steps that convert energy:
1. The potential energy of the fluid is converted into velocity in the inlet guide
vanes. This conversion is approximately 95% efficient.
2. The potential energy remaining is converted to mechanical power in the
turboexpander wheel.
3. The exit velocity of the gas is relatively high and is decelerated in an exhaust
diffuser.

Thermodynamics of Gas Expanders
The load applied to the turboexpander serves as a sink for the power extracted
by the turboexpander.
Fig. 7.56 shows three reference expansion processes between the higher
pressure P 0 and the lower pressure P 5 . It shows the minimum discharge temper-
ature happens for an isentropic process, when energy is extracted from the fluid
stream. It also shows the discharge temperature for an isenthalpic expansion
process (e.g., across the JT valve) is higher than both the actual and isentropic
processes. The temperature difference between 5a and 5h (e.g., ΔT ¼ T 5h T 5a )
signifies the additional temperature drop, which can be gained if a turboexpan-
der is used in place of a JT valve.
The turboexpander efficiency can be defined in terms of static-to-total tem-
peratures when the temperature drop is the main purpose for utilizing a
turboexpander.

η exp ¼ T 0ðÞ T 5aðÞ
T t 0ðÞ T t 5sðÞ