Preliminary Estimate Using Industry-Standard Methodology
| Parameter | Formula |
|---|---|
| Compression Ratio | CR = Pdischarge / Psuction |
| Number of Stages | INT(ln(CR) / ln(3.5) + 0.90) |
| Estimated BHP | 21 × CR1/n × n × Q × 1.154 |
| Interstage Pressure | Ps × CRstage/n |
Understand reciprocating compressor sizing methodology and cylinder selection
It estimates the number of stages, brake horsepower, interstage pressures, and per-stage discharge temperatures using the GPSA quick-look method (BHP/MMscfd ≈ 21 × R_stage × N × 1.154 for natural-gas service).
Suction and discharge pressures (psia), flow rate (MMscfd), suction temperature, gas specific heat ratio (k), and specific gravity. Optional inputs include the interstage cooler outlet temperature and the maximum allowed compression ratio per stage.
Stages are sized to keep each-stage compression ratio at or below the user-set maximum (default 3.5). The calculator uses n = INT(ln(Roverall)/ln(Rmax) + 0.9) — the smallest integer stage count that keeps rstage ≤ Rmax with a small overshoot margin. Stage ratios are then equal: rstage = Roverall1/n.
For k≈1.27 and 100°F suction, a 3.5:1 stage ratio gives an isentropic discharge temperature near 300°F — the API 618 preferred limit (350°F absolute max). Higher ratios accelerate valve and packing wear, oil coking, and rod-load problems.
The GPSA/Ariel quick-look formula (21 × Rstage × N × Q × 1.154) is typically within ±10–15% of detailed OEM cylinder selection software. Use it for budgetary scoping and feasibility; finalize with manufacturer software (Ariel A6, Dresser-Rand) for procurement and rod-load checks.