Rotary screw compressor controls and sequencing

What does a screw compressor controller actually do?

Every industrial screw compressor has a controller that does two jobs. It protects the machine, and it decides how much air to produce.

Protection covers discharge temperature, motor overload, phase faults, oil and separator pressure differential, filter condition and service-hour counters. Air-output control reads the delivery or header pressure and acts on the inlet valve, the blowdown valve and, on VSD machines, the motor speed.

Once a plant has more than one compressor, a third job appears: deciding which machine runs and in what role. That is sequencing. It can be done with simple staggered pressure settings, with the built-in networking of one manufacturer’s controllers, or with a central controller that can handle mixed brands.

Load/unload, modulation and VSD control compared

The figure that matters is energy per unit of air delivered, measured as specific power in kW per m³/min at the working pressure. An unloaded fixed-speed compressor still draws a significant fraction of its full-load power while producing no air, depending on the design and how quickly the separator blows down. That is why unloaded hours on the service display are one of the first things we look at.

Start with the demand profile, not the controller

Before choosing controls, find out how the plant uses air. A logging exercise of at least a typical week, recording pressure and either flow or compressor power and load state, shows:

• Minimum, average and peak demand.
• How long peaks last and how often they occur.
• Differences between shifts, weekends and holidays.
• Night-time demand, which is often mainly leakage.
• Critical users that cannot tolerate low pressure.
• Expected growth.

With that data, the control choice is often obvious. Without it, plants buy a VSD that runs at full speed all day, or keep three fixed-speed machines idling unloaded. Our air demand profiling page explains how we measure it.

Pressure bands: the most common control mistake

Each compressor loads at a lower pressure and unloads at an upper pressure. The gap between them is the pressure band. In a multi-compressor room without a central controller, machines are often set with staggered bands, so the second loads only when the first cannot keep up.

The problem is that every stagger raises the top of the system pressure. Three or four machines in cascade can push the average header pressure well above what the plant needs. Each extra bar costs about 7% more power, and higher pressure also makes every leak and open blow-off use more air.

1. Find the lowest pressure the critical users actually need at the point of use.
2. Add the pressure drop through dryers, filters and pipework; measure it rather than guess.
3. Set the minimum header pressure from that total, with a small safety margin.
4. Keep all compressor bands as narrow as the receiver volume and controls allow.
5. Use a central controller where several machines would otherwise need a wide cascade.

Raising the set point to cover a pressure drop is rarely the cheapest fix. Our pressure profiling page covers how we find where pressure is lost.

Base load, trim and standby roles

In a well-run compressor room, each machine has a clear role.

• Base-load compressors run fully loaded for long periods, carrying the steady part of demand. Fixed-speed machines suit this role well.
• The trim compressor follows the changing part of demand. A VSD compressor sized so that the variable demand falls within its speed range is ideal.
• Standby compressors are ready to start if a running machine trips or demand rises beyond normal.

A common error is a trim machine larger than the base-load step it has to fill, or smaller than the variation it has to follow. If the trim compressor cannot cover the gap between base-load steps, the sequencer will keep starting and stopping a fixed-speed machine to fill it. Sizing the trim VSD so its range overlaps the largest base-load step avoids this “control gap”.

Our pages on fixed-speed and VSD compressors cover the machine side of these roles.

How a central sequencer works

A central controller reads one pressure signal, usually from the main header after the dryers and receivers, and commands each compressor to load, unload, start or stop. Instead of each machine reacting to its own discharge pressure, the whole room works to one narrow band.

1. The sequencer holds a target pressure band at the header.
2. When pressure falls, it loads the next machine in the chosen order or increases trim speed.
3. When pressure rises, it unloads or slows the trim first, then unloads base-load machines.
4. It stops machines that would otherwise idle unloaded for long periods.
5. It rotates the order on a timer or on hours to balance wear, if required.
6. It brings in standby machines when a running machine trips.

Mixed-brand rooms can be sequenced through a vendor-neutral central controller wired to each compressor’s remote load/unload and start/stop inputs, or through communication interfaces where the controllers support them. Each machine keeps its own safety protection; the sequencer only issues requests.

Receivers and control stability

Storage is part of the control system. A receiver smooths short peaks, reduces load/unload cycling and gives the controls time to respond. Too little storage makes even good controls look bad: the compressor cycles rapidly, the motor starts too often and pressure swings.

• A wet receiver after the compressor helps the machine run longer load cycles.
• A dry receiver after the dryer supports the header and protects against dew-point spikes during peaks.
• Point-of-use receivers can serve one large intermittent user without raising the whole plant pressure.

See air receiver tank sizing for the method.

Symptoms of poor control and what they usually mean

Monitoring and data

Modern controllers record running hours, loaded hours, starts, alarms and service counters. A remote-monitoring system or plant SCADA link can add trends of pressure, power and dew point. The value of that data comes from someone reviewing it: rising unloaded hours, more frequent starts or a creeping discharge temperature are early warnings.

• Record loaded and total hours for each machine at every service.
• Log header pressure and, where possible, flow or power.
• Review alarms and trips monthly, not only when a machine fails.
• Keep the controller’s sensor calibration and wiring in good condition; a drifting pressure transducer can make a whole room misbehave.

Our controls and monitoring hub covers sensors, alarms and network options in more detail.

How Reddy Compressors helps

Reddy Compressors, at Tarnaka, Secunderabad, has served industry since 2002. As an independent multi-brand specialist, not an authorised dealer of any manufacturer, we work on mixed compressor rooms with machines from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other brands. We have tie-ups with all major and reputed brands.

• Review of load/unload hours, set points and pressure bands across every machine.
• Demand and pressure logging to decide base-load, trim and standby roles.
• Advice on vendor-neutral sequencing and on whether a VSD trim machine is justified.
• Controller, sensor and wiring diagnosis, and set-point correction.
• Supply of new and used/refurbished fixed-speed and VSD compressors, and rentals from our own fleet to cover peaks or trials.
• Service and AMC from more than 20 service engineers and technicians covering all of Telangana and Andhra Pradesh, with supply of equipment and spares across India.

Call +91 99590 60718 or +91 92468 88812, or email reddycompressors@gmail.com.

What to send us

• List of compressors with make, model, rating and nameplate photos.
• Current load and unload set points for each machine.
• Total and loaded hours from each controller display.
• Header pressure and the minimum pressure your critical users need.
• Receiver sizes and positions, and dryer types.
• A sketch or photo of the compressor room and header.

Frequently asked questions

What is the difference between load/unload and VSD control on a screw compressor?

Load/unload control runs the motor at fixed speed and opens or closes the inlet valve, so the machine is either fully producing air or idling unloaded. VSD control changes the motor and airend speed to follow demand within a speed range. VSD suits variable demand and trim duty, while load/unload suits machines that stay loaded most of the time.

Why does my screw compressor run unloaded for so long?

Long unloaded running usually means the compressor is larger than the demand, too many machines are running at once, or the pressure bands make machines wait unloaded. Leaks at night can also keep a machine running lightly. Check total versus loaded hours on the controller, log demand for a week, and adjust machine roles or run-on timers before considering new equipment.

Do I need a central controller for two or three compressors?

Not always. Two machines with sensible staggered bands and enough storage can work well. A central controller becomes worthwhile when cascading bands push header pressure too high, machines fight each other, unloaded hours are high, or you want automatic rotation and standby start. Mixed-brand rooms can use vendor-neutral sequencers that work through each compressor’s remote inputs.

What pressure should I set my compressors to?

Set the header to the lowest pressure your critical users need at the point of use, plus the measured pressure drop through treatment and pipework, plus a small margin. Every extra bar costs about 7% more power and increases leakage. If the pressure drop is large, fixing filters, dryers or pipework is usually cheaper than raising the set point.

Should the VSD compressor be base load or trim?

In most mixed rooms the VSD compressor should be the trim machine, following the changing part of demand while fixed-speed machines carry the steady base load fully loaded. A VSD running at full speed all day gains little. Size the VSD so its speed range covers the largest step between base-load machines, avoiding a control gap.

Why does header pressure keep swinging with several compressors running?

Swinging pressure usually means machines are fighting on overlapping bands, the pressure sensor sits in a poor position, or storage is too small for the demand peaks. Large intermittent users, such as blow-off or test benches, can also cause swings. Trend the pressure, compare all set points and check receiver volume before replacing any controller.

Related pages

• Rotary screw compressors hub
• Multi-compressor sequencing and optimisation
• Variable-speed drive (VSD) screw compressors
• Fixed-speed rotary screw compressors
• Compressed-air audits and measurement
• Compressed-air energy efficiency