1. Operating Principles
Screw compressors are rotary positive displacement machines that use two helical rotors (male and female) to trap and compress gas. As the rotors turn, gas is drawn into the inlet, trapped in the inter-lobe cavities, and progressively compressed as the cavities decrease in volume toward the discharge port.
Key Components
- Male rotor (drive): Typically 4 lobes, drives the female rotor through direct contact (oil-flooded) or timing gears (dry)
- Female rotor (driven): Typically 6 lobes/gates, creating a 4/6 lobe combination (most common profile)
- Casing: Houses both rotors with tight clearances, includes suction and discharge ports
- Slide valve: Adjustable internal valve for capacity control, changes the effective compression volume
- Bearings: Radial and thrust bearings support rotor loads
Compression Cycle
The screw compression cycle consists of three phases in each revolution:
- Suction (filling): Gas enters through the inlet port as the inter-lobe cavity opens and expands. Volume increases from zero to maximum displacement.
- Compression: As rotors continue to turn, the trapped gas cavity moves axially along the rotors. The cavity decreases in volume, compressing the gas. No valves are needed — compression is purely geometric.
- Discharge: The compressed gas cavity reaches the discharge port and gas is expelled. The built-in volume ratio determines the internal compression ratio.
Built-in volume ratio (Vi): Screw compressors have a fixed internal volume ratio determined by the discharge port geometry. If the actual system pressure ratio does not match the built-in ratio, over-compression or under-compression occurs, reducing efficiency. Modern machines offer adjustable Vi via slide valve position.
Rotor Profiles
The lobe profile determines sealing, efficiency, and manufacturing complexity. Common profiles include:
Symmetric Profile
Original SRM Design
Equal curvature on both rotors. Simpler to manufacture but lower efficiency due to blow-hole leakage.
Asymmetric Profile
Modern (e.g., Sigma, N-profile)
Optimized lobe shape reduces blow-hole area and improves sealing. 5-10% better efficiency than symmetric.
| Parameter | Typical Value | Notes |
|---|---|---|
| Male/Female lobe count | 4/6 (most common) | Also 3/5, 5/6, 5/7 combinations |
| L/D ratio | 1.0–1.65 | Length-to-diameter of male rotor |
| Male rotor tip speed | 20–80 m/s | Higher speed = more leakage but higher flow |
| Rotor speed | 1,800–6,000 RPM | Inversely proportional to rotor diameter |
| Clearance (oil-flooded) | 0.001–0.003 in | Oil film provides primary sealing |
| Clearance (dry) | 0.003–0.008 in | Larger to prevent rotor contact |
2. Oil-Flooded vs Dry Screw Compressors
The two main categories of screw compressors differ fundamentally in how sealing, cooling, and lubrication are achieved.
Oil-Flooded Screw Compressors
Oil is injected directly into the compression chamber, serving three critical functions:
- Sealing: Oil fills clearance gaps between rotors and between rotors and casing, dramatically reducing internal leakage (slip)
- Cooling: Oil absorbs compression heat, keeping discharge temperatures low (typically below 250°F even at high ratios)
- Lubrication: Oil lubricates the rotor contact surfaces, eliminating the need for timing gears
Oil-Flooded Advantages:
• Single-stage ratios up to 10-15:1 (vs 4-5:1 dry)
• Lower discharge temperature (better for downstream equipment)
• Higher volumetric efficiency (85-95%) due to oil sealing
• Simpler construction (no timing gears)
• Lower rotor speeds possible
• Tolerates moderate liquid carryover
Oil-Flooded Disadvantages:
• Requires oil separation system downstream
• Risk of oil contamination in process gas
• Oil degradation at high temperatures
• Potential foaming with some gas compositions
• Higher maintenance cost for oil system
Oil Separation System
Oil-flooded compressors require a multi-stage separation system to remove oil from the discharge gas:
- Primary separator: Gravity/impingement vessel removes bulk oil (99%+ by weight)
- Coalescing element: Fine coalescers remove oil aerosol to <3 ppm (typical specification)
- Activated carbon (optional): Final polishing to <0.01 ppm for critical applications
Oil Circulation Rate:
Typical: 2–5 gal/min per 100 ACFM inlet flow
Higher ratios for:
• Higher pressure ratio (more heat to absorb)
• Higher inlet temperature
• Heavier molecular weight gas
Oil Cooler Duty:
Q = m_oil × Cp_oil × ΔT_oil
Where:
m_oil = oil mass flow (lb/min) = GPM × 7.2 lb/gal
Cp_oil = 0.45 BTU/(lb·°F) typical
ΔT_oil = oil temperature rise across compressor
Oil injection temperature: Typically 140–180°F
Must be above gas dewpoint to prevent condensation
Dry (Oil-Free) Screw Compressors
Timing gears synchronize rotor rotation to maintain precise clearance without contact. No oil enters the compression chamber.
Dry Screw Advantages:
• Oil-free gas discharge (no contamination risk)
• No oil separation system required
• Lower operating cost (no oil consumption)
• Suitable for oxygen, instrument air, food-grade gas
Dry Screw Disadvantages:
• Limited to 4-5:1 ratio per stage (thermal limits)
• Higher discharge temperatures (350-400°F)
• Lower volumetric efficiency (75-85%)
• Requires precision timing gears
• Tighter clearance tolerance
• More sensitive to liquid carryover
Comparison Table
| Parameter | Oil-Flooded | Dry |
|---|---|---|
| Max single-stage ratio | 10–15:1 | 4–5:1 |
| Max discharge temp | ~250°F | 350–400°F |
| Adiabatic efficiency | 70–82% | 65–78% |
| Volumetric efficiency | 85–95% | 75–85% |
| Oil in gas | Requires separation | Oil-free |
| Rotor contact | Direct (oil film) | No contact (timing gears) |
| Flow range | 100–12,000 ACFM | 500–30,000 ACFM |
| Typical speed | 1,800–4,000 RPM | 3,000–6,000 RPM |
Selection rule: Use oil-flooded when oil in gas is acceptable and maximum compression ratio per stage is desired. Use dry when oil-free gas is required (oxygen service, instrument air, food/pharma, or when downstream processes are oil-sensitive).
3. Compression Thermodynamics
Screw compressor thermodynamics span the range between isothermal and adiabatic compression. Oil-flooded machines approach near-isothermal behavior; dry machines follow near-adiabatic paths.
Adiabatic Compression
Adiabatic (Isentropic) Discharge Temperature:
T2 = T1 × r(k-1)/k
Where:
T1 = inlet temperature (°R)
T2 = discharge temperature (°R)
r = P2/P1 = pressure ratio (absolute pressures)
k = Cp/Cv = specific heat ratio
Example (dry screw):
T1 = 80°F = 539.67°R, r = 4.0, k = 1.3
T2 = 539.67 × 4.00.231 = 539.67 × 1.389 = 749.6°R = 289.9°F
Oil-Flooded Discharge Temperature
Oil-Cooled Discharge Temperature:
T2_actual = T1 + (T2_adiabatic - T1) × f_cool
Where:
f_cool = cooling factor (0.15–0.25 typical)
f_cool depends on oil/gas ratio and oil injection temperature
Example (oil-flooded screw):
T1 = 80°F = 539.67°R, r = 8.0, k = 1.3
T2_adiabatic = 539.67 × 8.00.231 = 539.67 × 1.678 = 905.4°R = 445.7°F
T2_actual = 539.67 + (905.4 - 539.67) × 0.20 = 539.67 + 73.1 = 612.8°R = 153.1°F
The oil-flooded machine achieves 8:1 ratio with only 153°F discharge
vs 446°F for dry/adiabatic compression.
Isothermal Compression (Ideal Limit)
Isothermal (constant temperature) compression represents
the thermodynamic ideal for oil-flooded screw compressors:
PV = constant (at constant temperature)
Isothermal work: W_iso = P1 × V1 × ln(P2/P1)
This is always less than adiabatic work:
W_adiabatic = P1 × V1 × (k/(k-1)) × [r(k-1)/k - 1]
Power savings ratio:
W_iso / W_ad = ln(r) / [(k/(k-1)) × (r(k-1)/k - 1)]
For r = 4, k = 1.3:
W_iso / W_ad = 1.386 / [4.33 × 0.389] = 1.386/1.685 = 0.822
Isothermal requires ~18% less power than adiabatic at 4:1 ratio.
Practical note: Oil-flooded screw compressors operate between isothermal and adiabatic. The actual compression path depends on oil injection rate, oil temperature, gas properties, and rotor speed. More oil injection moves the process closer to isothermal.
4. Power & Efficiency Calculations
Adiabatic Power
Adiabatic (Gas) Power:
W_ad = (P1 × V1 × k / (k-1)) × [r(k-1)/k - 1]
Where:
P1 = inlet pressure (lbf/ft²) = psia × 144
V1 = inlet volume flow (ft³/min) = ACFM
k = specific heat ratio
r = pressure ratio
Convert to HP: W_ad (HP) = W_ad (ft·lbf/min) / 33,000
Brake Horsepower:
BHP = W_ad / (η_ad × η_mech)
Where:
η_ad = adiabatic efficiency
η_mech = mechanical efficiency (~0.95)
Efficiency Definitions
Adiabatic Efficiency:
η_ad = Isentropic work / Actual shaft work
Oil-flooded screw: 70–82% (lower ratio = higher η)
Dry screw: 65–78%
Volumetric Efficiency:
η_vol = Actual delivered volume / Swept volume
Oil-flooded screw: 85–95% (oil sealing reduces slip)
Dry screw: 75–85% (larger clearances, more slip)
η_vol depends on:
• Pressure ratio (higher ratio = lower η_vol)
• Rotor clearances and profile
• Rotor speed (higher speed = relatively less leakage time)
• Gas molecular weight (heavier gas = less slip)
Mechanical Efficiency:
η_mech = Gas power / Shaft power = 0.93–0.97 typical
Losses: bearings, timing gears (dry), oil pump, shaft seals
Specific Power
Specific Power (energy intensity of compression):
SP = BHP / MMSCFD (HP per million standard ft³ per day)
SP = BHP / (ACFM/100) (HP per 100 actual ft³ per minute)
Lower specific power = more efficient compression.
Typical values (natural gas, SG 0.65):
• 3:1 ratio, oil-flooded: ~55–70 HP/MMSCFD
• 5:1 ratio, oil-flooded: ~90–120 HP/MMSCFD
• 3:1 ratio, dry: ~70–90 HP/MMSCFD
• 4:1 ratio, dry: ~100–130 HP/MMSCFD
Compare to reciprocating: ~45–60 HP/MMSCFD at 3:1 ratio
(reciprocating is more efficient but has higher capital cost)
Example: Full Power Calculation
Given:
2,000 ACFM natural gas (SG = 0.65, MW = 18.8, k = 1.28)
P1 = 50 psig = 64.7 psia
P2 = 200 psig = 214.7 psia
T1 = 80°F = 539.67°R
Oil-flooded screw compressor
Step 1: Pressure ratio
r = 214.7 / 64.7 = 3.318
Step 2: Adiabatic power
P1 = 64.7 × 144 = 9,317 lbf/ft²
W_ad = (9,317 × 2,000 × 1.28/0.28) × [3.3180.219 - 1]
W_ad = (9,317 × 2,000 × 4.571) × [1.288 - 1]
W_ad = 85,200,000 × 0.288 = 24,538,000 ft·lbf/min
W_ad = 24,538,000 / 33,000 = 744 HP
Step 3: BHP
η_ad = 0.78 (oil-flooded, 3.3:1 ratio, 2000 ACFM)
η_mech = 0.95
BHP = 744 / (0.78 × 0.95) = 744 / 0.741 = 1,004 HP
Step 4: Motor selection
Next NEMA size above 1,004 HP = 1,250 HP motor
Step 5: Discharge temperature
T2_ad = 539.67 × 3.3180.219 = 539.67 × 1.288 = 695°R = 235°F
T2_actual = 539.67 + (695 - 539.67) × 0.18 = 539.67 + 28.0 = 568°R = 108°F
Step 6: Standard flow
SCFM = 2,000 × (64.7/14.696) × (519.67/539.67) × (1/0.95)
SCFM = 2,000 × 4.403 × 0.963 × 1.053 = 8,910 SCFM
MMSCFD = 8,910 × 60 × 24 / 1,000,000 = 12.83 MMSCFD
5. Sizing & Selection Guidelines
API 619 Requirements
API 619 (Rotary-Type Positive Displacement Compressors for Petroleum, Chemical, and Gas Industry Services) establishes minimum requirements for screw compressor design, fabrication, and testing.
| API 619 Requirement | Specification |
|---|---|
| Rotor material | Forged steel or nodular iron (depending on gas service) |
| Bearing life (L10) | Minimum 25,000 hours (100,000 hours preferred) |
| Vibration limits | Per API 619 Table 1 (velocity-based at bearing housing) |
| Mechanical running test | 4-hour unloaded test at maximum continuous speed |
| Performance test | Optional; ASME PTC-9 for acceptance |
| Shaft sealing | Mechanical seal standard; dry gas seal optional for process gas |
| Oil separation (flooded) | Residual oil carry-over per purchaser specification (typically <3 ppm) |
Staging Criteria
Single-Stage Limits:
Oil-flooded: r ≤ 10–15:1 (temperature limited)
Max discharge temp: ~250°F
If r > 10, verify with manufacturer
Dry: r ≤ 4–5:1 (temperature limited)
Max discharge temp: 350–400°F
If r > 4, strongly consider two-stage
Two-Stage Compression:
Stage ratio: r_stage = r_total0.5 (equal split)
Intercooling: Cool interstage gas to near suction temperature
Power savings: 10–20% vs single-stage at same overall ratio
Example:
r_total = 16:1
r_stage = 160.5 = 4:1 per stage
With intercooling to 100°F between stages
Capacity Control Methods
Most Common
Slide Valve
Internal slide valve changes effective compression length. Continuous turndown to ~10% capacity. Simple and reliable. Standard on most packages.
Most Efficient
Variable Speed Drive
VFD adjusts rotor speed for capacity control. Best energy efficiency at part load. 50-100% speed range. Higher capital cost.
Simplest
Bypass / Recycle
Full-speed operation with discharge recycled to suction. Least efficient but simplest. Used for on/off service or minimum flow protection.
Rotor Sizing
Male Rotor Diameter Estimation:
D_male is determined by required displacement volume:
V_disp = ACFM / (N × η_vol × z_lobes)
Where:
V_disp = displacement per revolution (ft³/rev)
N = rotor speed (revolutions per minute)
z_lobes = number of male rotor lobes (typically 4)
Rotor diameter is then derived from displacement geometry.
Preliminary estimate:
D_male (inches) ≈ C × ACFM0.4
Where C = 1.8 (oil-flooded), 2.0 (dry)
L/D ratio: 1.0–1.65 (typical 1.2–1.5)
Rotor length = D_male × L/D
Speed range by size:
D = 4″: 4,000–6,000 RPM
D = 8″: 2,500–4,000 RPM
D = 12″: 1,800–3,000 RPM
D = 16″+: 1,800–2,500 RPM
Flow Conversion
ACFM to SCFM Conversion:
SCFM = ACFM × (P1/P_std) × (T_std/T1) × (Z_std/Z1)
Where:
P_std = 14.696 psia
T_std = 519.67°R (60°F)
Z_std = 1.0 (ideal at standard conditions)
SCFM to MMSCFD:
MMSCFD = SCFM × 60 × 24 / 1,000,000
MMSCFD to ACFM (reverse):
SCFM = MMSCFD × 1,000,000 / (60 × 24)
ACFM = SCFM × (P_std/P1) × (T1/T_std) × (Z1/Z_std)
Always size on ACFM: Screw compressors are volumetric machines — they move actual cubic feet per minute regardless of gas density. Convert all flow rates to ACFM at inlet conditions before selecting frame size.
6. Midstream Applications
Screw compressors occupy an important niche in midstream operations where flow rates are moderate (500–12,000 ACFM) and operational simplicity is valued.
Gas Gathering
- Wellhead compression: Boost well pressure from 15–50 psig to gathering line pressure (100–300 psig). Oil-flooded preferred for tolerance to liquids.
- Field boosting: Increase pressure for pipeline transport. Ratios of 3–8:1 typical.
- Low-pressure gathering: Collect gas from multiple wells at sub-atmospheric to low pressure. Screw handles wide suction pressure range.
Vapor Recovery
- Tank vapor recovery: Capture flash gas and tank vapors for sale or flare reduction. Low inlet pressure (0.5–5 psig), variable flow.
- Casing gas: Recover casing-head gas that would otherwise be vented or flared.
Process Applications
- Fuel gas compression: Boost gas for engine or turbine fuel. Moderate ratio, continuous duty.
- Refrigerant compression: Process refrigeration (propane, ammonia) for NGL recovery. Oil-flooded excels at refrigerant service.
- Landfill gas: Compress biogas/landfill gas containing CO2, moisture, and contaminants. Oil-flooded tolerates impurities.
Screw vs Reciprocating: Decision Factors
| Factor | Screw Compressor | Reciprocating |
|---|---|---|
| Flow pattern | Continuous, pulsation-free | Pulsating (requires dampening) |
| Efficiency | 70–82% adiabatic | 82–88% adiabatic |
| Maintenance | Fewer parts, simpler | Valves, rings, rod packing |
| Vibration | Low (no reciprocating forces) | High (requires foundation analysis) |
| Dirty gas tolerance | Good (oil-flooded) | Poor (damages valves) |
| Capital cost | Lower for same HP | Higher but better efficiency |
| Availability | 95–98% | 92–96% |
| Footprint | Smaller, lighter | Larger, heavier |
When to choose screw over reciprocating: (1) Continuous, steady flow service, (2) Dirty or wet gas with liquid carryover, (3) Minimal operator attention available, (4) Lower capital cost priority, (5) Short delivery time needed, (6) Pulsation-sensitive downstream equipment, (7) Flow range 500–12,000 ACFM.
Package Considerations
- Suction scrubber: Always install upstream to remove free liquids and protect rotors from slugs
- Discharge cooler: Required for dry compressors; recommended for oil-flooded to reduce downstream temperatures
- Oil separator (flooded): Primary vessel with coalescing elements; size for residence time and velocity
- Control panel: Monitor discharge temperature, bearing temperature, vibration, oil pressure, and differential pressure
- Safety devices: High discharge temperature shutdown, high vibration shutdown, low oil pressure shutdown, relief valve
Ready to size a screw compressor?
→ Launch Screw Compressor Sizing Calculator