Rotating Equipment

Capacity Control

Methods for regulating reciprocating compressor output to match fluctuating demand. Energy costs can be 65% of total operating costs, making efficient capacity control critical for economic operation.

Launch Calculator Control Methods

Best efficiency

Speed Control

Loss-free, power proportional to capacity

Most common

Valve Unloaders

Stepped control with ~proportional power

Avoid if possible

Bypass

100% loss - power constant at all loads

Contents

Use this guide when you need to:

  • Select capacity control methods
  • Calculate achievable turndown
  • Estimate power savings
  • Compare control system costs

1. Control Overview

Capacity control adjusts the quantity of gas delivered by the compressor to match fluctuating plant demands. In modern processing plants, energy costs amount to about 65% of overall operating costs, making efficient capacity control essential for economic operation.

Fundamental principle: The control system should be designed so that compressor power is almost proportional to the quantity of gas delivered. Methods that waste energy (like bypass) should be minimized or avoided.

Control Modes

Stepped

Discrete capacity levels

100%, 75%, 50%, 25%, 0% etc. Simple and reliable but limited flexibility.

Stepless

Continuous adjustment

Any capacity within range. More complex but better load matching.

Composite

Combined methods

Use stepped for coarse control, stepless for fine tuning.

Multi-Stage Considerations

For multi-stage compressors, all stages must be regulated simultaneously. If only one stage is unloaded, the compression ratios in individual stages change significantly, causing:

  • Excessive discharge temperatures in some stages
  • Increased piston rod loading
  • Potential valve damage
  • Reduced efficiency

2. Stepped Capacity Control

Stepped control regulates capacity in discrete steps. A three-step system delivers 100%, 50%, or 0% capacity. A five-step system delivers 100%, 75%, 50%, 25%, and 0%. Stepped control is preferred for its simplicity and reliability.

Stop-Start Control

The simplest method: run the compressor at full capacity or shut it down. A receiver tank absorbs demand fluctuations, with pressure switches controlling start/stop.

Stop-Start Control: Energy savings: 100% when stopped Suitable for: Small installations, intermittent demand Limitations: Frequent starts stress motor and valves Large receivers may be required Not suitable for continuous processes

Suction Valve Unloader Control

The most widely used method in process compressors. Mechanical "finger yokes" hold suction valves open, preventing compression in that cylinder end. The gas simply flows in and out without being compressed.

Unloader Capacity Steps (Double-Acting): 1 cylinder: 100%, 50%, 0% (3 steps) 2 cylinders: 100%, 75%, 50%, 25%, 0% (5 steps) 3 cylinders: 100%, 83%, 67%, 50%, 33%, 17%, 0% (7 steps) 4 cylinders: 100%, 87.5%, 75%, 62.5%, 50%, 37.5%, 25%, 12.5%, 0% (9 steps) Each cylinder end = 1/(2 x n_cylinders) of total capacity
Power savings: Power reduction is nearly proportional to capacity reduction. Slight losses (~2-5%) occur due to gas flowing through open valves, but these are usually negligible.

Unloading Sequence

For double-acting cylinders, the recommended unloading sequence is:

  1. Head-end (HE) first - maintains alternating load on gudgeon pin
  2. Crank-end (CE) second - if both ends unloaded, inertia forces maintain alternating load

This sequence ensures proper lubrication of the wrist pin bearings.

Fixed Clearance Pocket Control

Additional clearance volume connected to the cylinder via a valve. When opened, the expanding gas fills most of the cylinder before suction begins, reducing volumetric efficiency and capacity.

Clearance Pocket Effect: VE = 1 - C × [(P2/P1)^(1/n) - 1] Where: VE = Volumetric efficiency C = Clearance fraction (including pocket) P2/P1 = Compression ratio n = Polytropic exponent Typical pocket: 15-35% capacity reduction per pocket Power: Proportional to capacity (losses from valve throttling)

Pockets can be fixed volume or divided into sections with individual valves for multiple steps.

3. Stepless Capacity Control

Stepless control allows continuous regulation of capacity within a predetermined range. More complex than stepped control, but provides better load matching and smoother operation.

Speed Control

The simplest and most efficient method when the driver speed can be varied. Capacity is directly proportional to speed.

Speed Control: Q ∝ N (capacity proportional to speed) Power ∝ N (for constant pressure ratio) Drivers with variable speed: - Diesel/gas engines - Steam turbines - Gas turbines - Electric motors with VFD Typical range: 100% to 70% of rated speed Limitation: Driver torque characteristics limit turndown
Loss-free control: Speed control is 100% efficient - all energy goes into gas compression with no bypass or throttling losses.

Reverse Flow Control (Hoerbiger System)

An ingenious stepless method that delays suction valve closing during the compression stroke. Gas flows back through the suction valve until dynamic pressure from the reverse flow forces the valve closed.

Reverse Flow Control: Operating principle: 1. Spring-loaded fingers hold suction valve open 2. Piston begins compression stroke 3. Gas flows back through valve 4. Dynamic pressure builds on valve disc 5. When pressure exceeds spring force, valve closes 6. Remaining gas is compressed normally Range: 100% to 40% (down to 15% in special cases) Power: Nearly proportional to capacity Losses: Flow losses through valve in both directions

Variable Clearance Pocket Control

A piston on a threaded spindle allows continuous adjustment of clearance volume. Completely loss-free since gas is compressed and re-expanded without throttling.

Variable Clearance: Adjustment: Manual or pneumatic/hydraulic actuator Range: Depends on pocket volume Power: 100% proportional to capacity (loss-free) Advantage: No restriction between cylinder and pocket Energy in clearance gas is recovered on re-expansion

Bypass Control

Connects discharge to suction via a throttle valve. Simple and reliable, but provides no power savings - the compressor works at full power regardless of net delivery.

Bypass Control: Power savings: ZERO (100% loss system) Use only when: Other methods not available As supplement to other methods For very fine capacity trim Note: Temperature may change during throttling Check with Mollier diagram for real gas effects May need cooling after bypass valve
Avoid bypass when possible: Bypass control wastes 100% of the capacity reduction as heat. A compressor at 50% capacity via bypass uses 100% power. The same capacity via unloaders uses ~50% power.

4. Composite Control

Each control method has limitations. Best results come from combining methods to leverage advantages of each. With today's energy costs, the payback on sophisticated control systems is often less than one year.

Common Combinations

Combination Application Benefit
Unloaders + Fixed Clearance More capacity steps Finer control without complexity
Unloaders + Variable Clearance Stepped + stepless Wide range with good efficiency
Unloaders + Speed Control VFD applications Loss-free fine tuning
Unloaders + Bypass Minimum cost Steps + continuous trim
Unloaders + Clearance + Bypass Full flexibility Many steps + fine adjustment
Unloaders + Reverse Flow High turndown Stepless down to 15%

Example: 4-Cylinder Double-Acting Compressor

Configuration:

  • 4 cylinders, double-acting = 8 ends
  • Suction valve unloaders on all ends
  • One fixed clearance pocket per cylinder (25% reduction)

Available steps with unloaders only:

100%, 87.5%, 75%, 62.5%, 50%, 37.5%, 25%, 12.5%, 0%

Additional steps with clearance pockets:

Each unloader step can be reduced by ~6% increments

Total: 20+ discrete capacity steps

5. Selection Guidelines

Choosing capacity control methods requires balancing initial cost, operating efficiency, reliability, and control flexibility.

Selection Matrix

Factor Unloaders Clearance Speed Bypass
Capital cost Low Medium High (VFD) Low
Operating cost Low Low Lowest Highest
Reliability High High Medium Highest
Control type Stepped Stepped/Stepless Stepless Stepless
Typical range Steps by end 15-35%/pocket 70-100% 0-100%
Power savings ~Proportional ~Proportional Proportional None

When to Use Each Method

Suction Valve Unloaders

  • Standard on most process compressors
  • Multiple cylinders provide adequate steps
  • Low cost, high reliability
  • Power savings nearly proportional to capacity

Clearance Pockets

  • When more steps needed than unloaders provide
  • Combined with unloaders for finer control
  • Variable type for stepless control
  • Higher cost than unloaders but loss-free

Speed Control

  • When driver already has variable speed (engine, turbine)
  • New installations where VFD cost is justified
  • Applications requiring fine continuous control
  • Best efficiency at partial loads

Bypass Control

  • Only when other methods not feasible
  • As supplement for fine trim between steps
  • Temporary or emergency use
  • Accept high operating cost for low capital cost

Economic Justification

Annual Savings Calculation: Savings = (Power_bypass - Power_controlled) × Hours × $/kWh Example: 500 hp compressor at 50% capacity - Bypass: 500 hp (100% power) - Unloaders: 250 hp (50% power) - Savings: 250 hp × 0.746 kW/hp = 186.5 kW - At 8000 hrs/yr and $0.08/kWh: 186.5 × 8000 × 0.08 = $119,360/year savings ROI on control system often < 1 year
Energy costs dominate: With energy at 65% of operating costs, investing in efficient capacity control pays back quickly. Evaluate total cost of ownership, not just initial cost.

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