Compressor sequencers for multi-machine stations

Why multiple compressors need a sequencer

Without central control, each compressor works on its own pressure settings. To stop them all loading together, installers “stack” the settings: machine one loads at 6.5 bar and unloads at 7.5 bar, machine two at 6.2 and 7.2 bar, machine three at 5.9 and 6.9 bar, and so on. The result is a wide overall pressure range, high average pressure, and a lead machine that may unload while a lag machine is still loaded. Every extra bar of average pressure needs about 7% more power, and unloaded machines burn energy without making air.

A sequencer replaces that stack with a single decision-maker. It reads one header pressure transmitter, compares demand against the capacity of the machines available, and switches machines in and out so that only the necessary number run, with at most one partly loaded.

How a sequencer works

1. One pressure reference. A transmitter in the main header, usually after the dryers and receiver, gives the sequencer the system pressure.
2. A single pressure band. The sequencer holds pressure between a lower and upper limit, typically much narrower than a stacked arrangement allows.
3. A running order. Machines are ranked lead, lag one, lag two and so on, either by a fixed priority or by rotation.
4. Load and unload decisions. When pressure falls and stays below the lower limit, the next machine is loaded or started; when it rises above the upper limit, the last machine in is unloaded, and stopped after its run-on time.
5. Time delays. Delays between steps prevent the sequencer from reacting to brief pressure dips and starting machines unnecessarily.
6. Rotation. At a set interval, or on running hours, the order changes so that wear is spread across identical machines.
7. Fallback. On loss of the sequencer or its signal, each machine reverts to its own local pressure settings.

Types of sequencing

For a larger station with different sized machines, dryers and storage to coordinate, see our page on master compressor control.

An illustrative example: three machines, before and after

Consider a hypothetical station with three similar fixed-speed screw compressors on a stacked cascade. The lead machine works between 6.8 and 7.8 bar, the second between 6.5 and 7.5 bar and the third between 6.2 and 7.2 bar. The plant’s critical user needs 6.0 bar at the point of use, and there is a measured 0.4 bar drop from the compressor room to that user.

On a cascade, the header can sit anywhere from 6.2–7.8 bar depending on which machines happen to be loaded, and during moderate demand the second and third machines may both run partly loaded, each spending time unloaded. Average generation pressure is high because the lead machine carries the top of the stack.

With a sequencer reading one header transmitter, the band could, for example, be set at about 6.5–7.0 bar: still above the 6.4 bar needed at the header to deliver 6.0 bar at the user. Only the machines needed run, the last one in carries the variation, and duty rotates weekly. The local cascade settings remain in place, set just outside the sequencer band, as a fallback. The actual band chosen on a real site would come from logged data, and the result would be confirmed by logging pressure and power again after commissioning, not assumed.

Setting the pressure band

The band is the heart of a sequencer design. Set it too narrow and machines start and stop too often; set it too wide and the energy benefit is lost.

• Start from the minimum pressure needed at the most demanding point of use, not at the compressor.
• Add the measured pressure drop from the header transmitter to that point, through filters, dryers and piping.
• Allow for how quickly pressure falls when a large user starts, which depends on storage volume.
• Check each compressor’s own maximum pressure so that the local fallback settings, placed just outside the sequencer band, stay within the machine’s rating and act as a backstop.
• Review the band after logging real behaviour, and tighten it in steps if pressure is stable.

Storage plays a big part. A properly sized and located receiver slows the rate of pressure change, giving the sequencer time to bring in the next machine without pressure dipping below the minimum. Our receiver sizing page explains the method.

Where should the VSD sit in the sequence?

A variable-speed compressor is normally the trim machine: fixed-speed machines carry the steady base load fully loaded, and the VSD absorbs fluctuations. For this to work, the VSD’s capacity range needs to cover the size of the step when a fixed-speed machine is added or removed. If it does not, there is a “control gap” where the VSD runs at maximum or minimum and a fixed-speed machine cycles. The VSD’s target pressure must also sit within the sequencer’s band, not compete against it. Our base/trim strategy page covers trim sizing in detail.

Signs your sequencer is not working well

• Pressure still swings widely, or the band has been widened over time “because of complaints”.
• Machines start and stop many times an hour.
• One machine has far more running hours than its identical partner.
• The sequencer has been switched off or set to manual and machines run on local settings.
• A machine shows “remote” but does not respond, so the sequencer brings in the next one.
• The header transmitter reads differently from local gauges.

Commissioning and fail-safe behaviour

A sequencer must make the station more reliable, not less. During commissioning we test what happens when the sequencer loses power, when its pressure transmitter fails, when a machine trips while it is lead, and when communication to one machine is lost. In every case the station should keep supplying air: either by the sequencer skipping the faulty machine, or by machines falling back to local pressure settings set slightly below and above the sequencer band. These settings are recorded and left with the site.

Critical sites, such as pharmaceutical plants and continuous process industries, also need an agreed standby philosophy. A sequencer can rotate a standby machine into duty regularly so that it is proven, rather than discovering a fault only during an emergency.

How Reddy Compressors helps

Reddy Compressors is an independent multi-brand compressed-air specialist at Tarnaka, Secunderabad, serving industry since 2002. We sell, rent and service ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar, Gardner Denver and other machines, without being an authorised dealer of any brand. That matters on mixed fleets, where a sequencer must work with several controllers.

• Check each machine’s controller for remote start/stop and load capability and the interface it needs.
• Log header pressure and running pattern to set a realistic band, through our pressure profiling work.
• Advise on a suitable sequencer, whether a manufacturer’s unit or a vendor-neutral one, and on storage and transmitter location.
• Commission sequencing with your engineer, test fallback modes and record settings.
• Service the compressors under the sequencer through regular visits or AMC, across Telangana and Andhra Pradesh.

What to send us

• List of compressors with make, model, serial, rated flow and pressure, and drive type.
• Photos of each controller and its current load/unload settings.
• Receiver sizes and positions, and a simple sketch of the compressor room.
• Minimum pressure needed by your critical users.
• Running pattern by shift and any existing sequencer details.

Frequently asked questions

What is the difference between a sequencer and a master controller?

A sequencer mainly decides which compressors run, in what order, to hold a pressure band, and rotates duty. A master controller usually goes further: it chooses the best combination of differently sized machines for the measured demand, coordinates a VSD trim, and may include dryers, storage and energy data. On small stations a sequencer is often enough; larger mixed fleets benefit from master control.

How narrow can the pressure band be with a sequencer?

Narrower than a stacked cascade, but the right figure depends on storage, machine response, demand swings and pressure drop. Many stations can run a band well under 1 bar once sequenced. We set the band from logged data, test it under real production, then tighten it gradually. The goal is the lowest stable pressure that still meets the most demanding user.

Will a sequencer work with compressors of different brands?

In most cases yes. Vendor-neutral sequencers commonly control machines through volt-free contacts for remote start/stop and load/unload, which most controllers provide. Some brands also allow communication links. We check each controller first; a very old machine may need a small modification or can be left as a manually started standby.

Why does my compressor sequencer start machines too often?

Common reasons are a pressure band that is too tight for the storage available, step delays set too short, a header transmitter placed where it sees sharp local dips, or a trim machine too small to cover the gap between fixed-speed machines. Logging pressure alongside machine states usually shows which one it is, and the fix is often in settings rather than hardware.

What happens to air supply if the sequencer fails?

A correctly commissioned system keeps running. Each compressor’s local pressure settings are left as a backstop, set just outside the sequencer band, so machines revert to local control on loss of signal. This should be tested during commissioning and after any change, and the fallback settings written down for your maintenance team.

Is a sequencer worth it for a small plant?

If two or more machines regularly run together, or pressure is set high to cover poor coordination, sequencing usually brings a clear benefit through lower pressure and less unloaded running. If one machine meets demand and the other is standby, simple duty rotation may be enough. Logging a week of pressure and running data answers the question with evidence.

Related pages

• Compressed-air controls and monitoring
• Master compressor control
• Base/trim control strategy
• Multi-compressor optimisation
• Reducing unloaded and idle running
• Air receiver sizing