Compressors

This lesson covers compressor selection and operation in process plants. Learn the type map (reciprocating, screw, centrifugal, axial), staging and intercooling, discharge-temperature limits, capacity control, surge and anti-surge control, drivers, and API 617/618 specifications.

Compressors are among the most expensive and critical machines in a chemical plant — recycle compressors, refrigeration compressors, air compressors and gas-export compressors. Choosing the right type and control strategy determines efficiency and reliability.

Thermodynamic work calculations are in the Fluid Mechanics module; here we focus on selection and operation.

An analogy

Compressor selection is like choosing a vehicle: a van (reciprocating) for small loads at high pressure, a bus (screw) for medium duty, a train (centrifugal/axial) for massive continuous flow.

Types and ranges

Reciprocating: small–medium flow, very high pressure (hydrogen make-up). Screw: medium flow, moderate pressure (plant air, refrigeration). Centrifugal: large flows, multiple stages (process gas, air separation). Axial: very large flows, lower ratios (gas turbines, big air plants).

Staging and temperature limits

Discharge temperature is limited (≈ 150–200 °C) for lubricant, seals, polymerisation or safety. Split high ratios into stages with intercoolers — saving power and keeping temperatures safe; knockout drums remove condensed liquid.

Capacity control

Variable speed (steam turbines, VFD motors), inlet guide vanes, suction throttling, recycle (spillback), clearance pockets and valve unloaders (reciprocating), slide valves (screw).

Surge and protection

Centrifugal compressors surge at low flow — violent flow reversal that can destroy the machine. Anti-surge controllers keep operation to the right of a surge control line using fast recycle valves. Protection also includes vibration, axial displacement, bearing temperatures, seal gas systems (dry gas seals) and API 617/618/670 standards.

Formulas

• Isentropic discharge temperature: T₂s = T₁ r^((γ−1)/γ); actual T₂ = T₁ + (T₂s − T₁)/η
• Stage ratio: r_stage = r^(1/N)
• Mass flow from normal volume: ṁ = Q_N × ρ_N (air ρ_N = 1.293 kg/Nm³)
• Power: P = ṁ · w / η_mech

Worked example: Staging an air compressor

Given: 5000 Nm³/h air, 1 → 8 bar; T₁ = 20 °C, η = 0.8

Find: Mass flow, power single vs two-stage, single-stage discharge temperature.

• ṁ = 5000 × 1.293/3600 = 1.80 kg/s
• Single stage 298.7 kJ/kg → 536 kW; two-stage intercooled 254.6 kJ/kg → 457 kW
• Tâ‚‚ single = 20 + 293 × 0.811/0.8 = 317 °C (too hot)

Answer: Two stages: 457 kW and safe temperatures (vs 536 kW, 317 °C)

Worked example: Stage ratios

Given: Overall ratio 50, max 4 per stage

Find: Stages.

• 50^(1/3) = 3.68 ≤ 4 → 3 stages

Answer: 3 stages

Common mistakes

• Wrong: Letting liquid reach the compressor. — Right: Install knockout drums and heat-traced suction lines.
• Wrong: Relying on slow control for surge. — Right: Use dedicated fast anti-surge controllers and valves.

Applications

• Refinery hydrogen: Reciprocating make-up and recycle compressors.
• Ammonia & methanol: Large centrifugal synthesis-gas compressors on steam turbines.
• Refrigeration & air: Screw and centrifugal compressors for utilities.

Key takeaways

• Type by flow and pressure: recip, screw, centrifugal, axial.
• Stage to respect discharge-temperature limits.
• Capacity control and anti-surge protection are essential.
• Specify to API 617/618/670.

Going further

• Dry gas seals: Non-contacting seals minimise leakage on centrifugal compressors.
• Performance monitoring: Polytropic efficiency and head tracking detect fouling.
• API 670: Machinery protection systems for vibration and position.