Multi-Stage Optimization for Balanced Power Distribution
| Overall Ratio | Stages | Ratio/Stage |
|---|---|---|
| Up to 4:1 | 1 | ≤4.0 |
| 4:1 to 16:1 | 2 | ≤4.0 |
| 16:1 to 64:1 | 3 | ≤4.0 |
| 64:1 to 256:1 | 4 | ≤4.0 |
Understand multi-stage compression principles
Compression ratio is the ratio of discharge pressure to suction pressure for a stage, calculated using absolute pressures (psia, not psig). For a multi-stage machine, the overall ratio R = Pfinal / Psuction equals the product of all stage ratios.
For minimum total power with perfect intercooling, the equal-ratio rule applies: rper_stage = R1/N where R is the overall ratio and N is the number of stages. This balances power across stages and minimizes discharge temperature spread (GPSA Section 13).
For natural gas (k ≈ 1.27), the practical single-stage limit is 3.5–4.0:1, set by discharge temperature. API 618 specifies 350°F absolute maximum, with 275–300°F preferred for valve and lubricant life. Lean low-MW gases run lower; rich gases tolerate slightly higher.
Rule of thumb at rmax=4.0: R<4 → 1 stage, 4–16 → 2 stages, 16–64 → 3 stages, 64–256 → 4 stages. The auto-optimize option computes N = ⌈log(R) / log(rmax)⌉ so each stage stays under the temperature-driven limit you specify.
Adiabatic compression follows T₂ = T₁ · r(k−1)/k. Higher T₁ (suction) yields higher T₂ proportionally. Intercoolers between stages drop the gas back to ~100–120°F before re-entering the next cylinder, resetting the temperature rise and limiting peak discharge T to the per-stage value rather than a cumulative one.