Compressed-air controls and monitoring

What do compressed-air controls actually do?

Every compressor already has a controller. It reads discharge pressure and temperatures, decides when to load and unload, and trips the machine before damage occurs. That is enough for a plant with one compressor. Once two or more machines feed the same header, each controller works on its own pressure switch settings, and the machines start to fight each other.

The typical symptoms are familiar to any maintenance manager: two machines running part-loaded where one would do, pressure swinging between settings, a compressor that short-cycles every few minutes, and a header pressure set higher than any user needs “just to be safe”. Each of those costs energy. As a working rule, every extra bar of generation pressure needs about 7% more compressor power, and an oil-injected screw running unloaded still draws a significant fraction of its full-load power while producing no air.

Controls engineering addresses this in layers, from the individual machine up to the plant network.

The four layers of a compressor control system

Most stations we see in Indian plants have a mixed fleet: an older fixed-speed screw from one brand, a newer VSD from another, perhaps a reciprocating standby. The control design has to work with what is actually installed, not with an idealised single-brand catalogue system.

Guides in this controls and monitoring section

Each page below deals with one part of the control system in depth. Use them in roughly this order: start at the machine, then the station, then data and integration.

How do I know my compressor controls need attention?

• Header pressure regularly swings by more than about 1 bar (around 15 psi) between highest and lowest readings.
• Two or more compressors are often loaded and unloaded at the same time, or one machine short-cycles.
• Running hours are very uneven between identical machines, or a standby machine never runs and is not proven.
• Generation pressure is set well above what the most demanding user actually needs.
• Machines run unloaded for long periods at night, weekends or between shifts.
• The VSD compressor sits at full speed most of the day, or at minimum speed while a fixed-speed machine unloads.
• Operators ignore alarms because there are too many, or nobody receives alarms outside the compressor room.
• There is no record of power, flow or pressure, so no one can say what the air costs per m³.

Any one of these points is worth investigating. Several together usually mean the station is being run by a set of independent pressure switches rather than by a control strategy.

How we approach a controls review

1. Inventory. List every compressor, dryer and receiver with make, model, serial, drive type (fixed-speed or VSD), controller type and firmware where visible, and how each machine is currently started and loaded.
2. Check the machine layer. Verify pressure and temperature sensors, load/unload operation, inlet valve or speed response, blow-down timing and protection trips. Note which machines accept remote start/stop and load signals, and through what interface.
3. Measure the demand. Log pressure, and where possible power and flow, over representative production periods including nights and weekends. Our demand profiling work feeds directly into control design.
4. Agree the objective. Minimum acceptable pressure at the critical user, redundancy philosophy (N+1 or otherwise), and which machines are preferred as base load.
5. Design the strategy. Pressure band, base/trim allocation, sequence rules, storage, and what happens on a master control failure. Fallback to safe local pressure settings must always be defined.
6. Commission and verify. Implement in stages with site approval, test failure modes, then log again to confirm pressure stability and running pattern.

Common control problems and practical fixes

Safety and approval boundaries

Controls changes can stop production or defeat protection if handled carelessly. We work to a few firm rules. Machine protection trips, such as high discharge temperature (normal oil-injected screw discharge is about 75–95 °C) and motor overload, are never bypassed or raised beyond the manufacturer’s limits. Changes to sequencing, remote start or BMS/SCADA commands are commissioned with the site’s engineer present and tested for failure modes. Remote access and network connections need the site’s IT or OT approval. Not every older controller can be integrated, and we will say so plainly rather than promise what a machine cannot do.

Capacity control modes compared: what each one costs at part load

Every compressor has a way of matching output to demand, and that method sets how much power it draws when the plant does not need full flow. Most stations spend most of their hours between 40% and 80% of capacity, so the part-load behaviour matters more than the full-load rating on the nameplate.

Figures are package input power as a percentage of a fixed-speed machine’s full-load power, for typical oil-injected screws at 7 bar(g). A VSD machine draws about 3–5% more than a fixed-speed one at full flow because of drive losses, which is why a VSD running flat out all day is the wrong application. Load/unload figures assume adequate storage; with a small receiver the machine short-cycles and the blow-down losses push the figures up towards modulation levels.

Worked example: one 75 kW machine at 55% average load (illustrative)

Assumptions: package input 80 kW at full load, 6,000 running hours a year, average demand 55% of capacity, tariff ₹8.50/kWh (illustrative input, not a price). Power fractions at 55% interpolated from the table above.

• Modulation: 0.70 + 0.30 × 0.55 = 0.865. Energy = 80 × 0.865 × 6,000 = 415,200 kWh, about ₹35.3 lakh.
• Load/unload with good storage: about 0.75. Energy = 80 × 0.75 × 6,000 = 360,000 kWh, about ₹30.6 lakh.
• VSD: about 0.60. Energy = 80 × 0.60 × 6,000 = 288,000 kWh, about ₹24.5 lakh.

Moving from modulation to VSD saves 127,200 kWh, about ₹10.8 lakh a year in this example; simply converting modulation to load/unload and adding storage saves 55,200 kWh, about ₹4.7 lakh. Real results depend on the measured demand profile, which is why the demand logging described above comes before any control change.

Local controller versus master sequencer: who does what

A local (machine) controller looks after one compressor. A master controller or sequencer looks after the header and decides which machines run. The two are complementary: the master never replaces the local controller’s protection, and the local controller cannot see what its neighbours are doing.

Mixed-make fleets are usually connected through volt-free contacts (load/unload, remote start, run and fault feedback) or through a fieldbus where both ends support it. The important commissioning test is the fallback: switch off the master and confirm that every machine holds a safe header pressure on its local settings.

Designing the pressure band: cascade versus common band

Without a master, each machine is given its own band and the bands are stepped, or cascaded, so that one machine loads before the next. With four machines the result is typically:

• Compressor 1: load 7.0, unload 7.5 bar(g)
• Compressor 2: load 6.8, unload 7.3 bar(g)
• Compressor 3: load 6.6, unload 7.1 bar(g)
• Compressor 4: load 6.4, unload 6.9 bar(g)

The header can therefore wander over 1.1 bar (6.4 to 7.5 bar(g)), and at light load it sits near the top of that range. A sequencer reading one header transmitter can run all four in a single common band, for example 6.5–6.9 bar(g), and still protect the lowest acceptable header pressure.

Worked example: four 37 kW machines (illustrative)

Assumptions: average station input 150 kW, 7,000 hours a year, measured average header pressure 7.0 bar(g) with the cascade, 6.7 bar(g) with a common band; leaks and unregulated users are 20% of demand; tariff ₹8.50/kWh (illustrative input).

1. Compression power: 0.3 bar lower × about 7% per bar = about 2.1%. Saving = 150 × 0.021 × 7,000 = 22,050 kWh.
2. Unregulated flow falls in proportion to absolute pressure: (6.7 + 1.013) ÷ (7.0 + 1.013) = 0.963, a 3.7% reduction. Saving = 150 × 0.20 × 0.037 × 7,000 = about 7,860 kWh.
3. Total about 29,900 kWh a year, about ₹2.5 lakh at the assumed tariff.

Before lowering any band, confirm the pressure needed at the most demanding user and the pressure drop to it; lowering the header to cover a dirty filter or an undersized pipe only moves the problem.

Controller alarms and trips: what the categories mean

Codes and wording differ between makes and controller generations, but nearly every message falls into one of the categories below. A warning lets the machine run; a trip (shutdown) stops it and must be understood before reset.

Keep a log of every trip with time, ambient temperature and header pressure. Repeated temperature trips on summer afternoons point to cooling and ventilation; repeated electrical trips at shift change point to the supply.

Oil temperature control valve and portable compressor controllers

On an oil-injected screw, the oil temperature control valve (thermostatic or thermal valve) sends oil around the oil cooler until the oil is warm, then progressively through it. Its job is to keep the discharge temperature above the dew point of the air-oil mixture in the separator tank, so that water does not condense into the oil. At 35 °C and 70% RH intake, compressed to 8 bar(g), that dew point works out at about 72 °C (water vapour partial pressure 0.7 × 5.62 kPa × 9.01 ÷ 1.013 = about 35 kPa, which saturates at about 72 °C); it is why the valve holds the oil warm even when the cooler could cool it further. A valve stuck open shows as low running temperature, milky oil and short oil life in the monsoon; a valve stuck closed shows as high-temperature trips in summer. Element ratings differ by make and duty, so match the replacement to the original part. The valve and the oil circuit are covered in more depth on the compressed air filtration page, and replacement elements on the thermostatic valves page.

A portable diesel compressor controller does two jobs: it protects the engine and it regulates air delivery. Typical functions are listed below.

• Engine protection: low oil pressure, high coolant temperature, low coolant level, overspeed, charging failure, low fuel.
• Compressor protection: high discharge temperature, high separator differential, air filter restriction.
• Regulation: engine speed reduces together with inlet throttling as demand falls, so fuel use follows air use.
• Warm-up at idle before loading and a cool-down idle before shutdown, protecting the turbocharger.
• Dual pressure or flow mode selection on machines that offer it, for example drilling versus general site air.
• Hour meter, service reminders, fault history and, on newer units, telematics for location and fuel.

How Reddy Compressors helps

We are an independent multi-brand compressed-air specialist based 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. We are not an authorised dealer of any brand, which lets us look at a mixed fleet neutrally. We have tie-ups with all major and reputed brands.

• Site review of controllers, sensors, pressure settings and running pattern by our service engineers, across Telangana and Andhra Pradesh.
• Pressure and power logging as part of an air audit, used to set realistic pressure bands and sequence rules.
• Controller servicing, sensor replacement and set-point correction within manufacturer limits, often during scheduled service or AMC visits.
• Supply of new compressors, including VSD machines, and sequencing or master control where it suits the station, with supply across India.
• Rental compressors to cover a station while controls are modified or a machine is out for overhaul.

What to send us

• Make, model, serial and nameplate photo for each compressor and dryer.
• Photo of each controller display and its main pressure settings.
• Current pressure set points and the minimum pressure your critical users need.
• Rough running pattern: shifts, weekends, peak loads.
• Any existing sequencer, BMS or remote monitoring system.
• Main problems you see: pressure swings, trips, high power bills, alarms.

Frequently asked questions

Do I need a sequencer if I only have two compressors?

Often yes, if both run together for part of the day. With two machines on separate pressure switches, they commonly run part-loaded together or swap roles unpredictably. A simple sequencer or well-coordinated pressure bands can keep one machine as base, bring the second in only when needed and rotate duty. If one machine easily covers demand and the other is pure standby, careful pressure settings and a duty rotation routine may be enough.

Can compressors from different brands be controlled together?

Usually, but the method depends on each controller. Many modern controllers accept remote start/stop and load/unload signals through hard-wired I/O or a communication interface, which a vendor-neutral sequencer can use. Some older machines only allow simple contact inputs. We check each controller’s capability first, then decide whether a universal sequencer, a manufacturer’s own master or a staged pressure-band arrangement is the realistic choice.

How much energy can better compressor controls save?

It depends entirely on how the station runs today, so we measure rather than promise a figure. Savings come from lower generation pressure, at roughly 7% power per bar, from fewer machines running unloaded and from better use of a VSD. Logging pressure and power before and after the change shows the actual result, expressed as specific power in kW per m³/min of air delivered.

What pressure band should my compressor station run at?

Start from the minimum pressure your most demanding user genuinely needs at the point of use, then add the measured pressure drop through dryers, filters and piping. The control band sits just above that. Central sequencing typically allows a much tighter band than stacked individual pressure switches. Reducing piping and filter losses first often lets the whole band come down.

Is remote monitoring of compressors safe for my plant network?

It can be, if it is designed with your IT or OT team. Common good practice is read-only data by default, separate networks or gateways for compressor data, controlled user accounts and no permanent open remote control. Any remote command capability should be a deliberate, approved decision with a clear record of who can change what.

Can you service the controller on my existing compressor?

Yes. As part of service or AMC visits our engineers check sensors, settings, load/unload operation, protection trips and the controller’s fault history on most major brands. Where a controller or sensor has failed, we identify the correct part from the machine’s make, model, serial and nameplate, confirm fit and then quote.

What is the difference between a local compressor controller and a master compressor controller?

A local controller is fitted to each compressor. It reads that machine’s own discharge pressure and temperatures, loads and unloads it within its own band, and trips it for safety faults. A master controller reads the common header pressure and tells each compressor when to load, unload, start or stop, choosing the combination that holds pressure with the least energy. Safety protection always stays with the local controller. If the master fails, each machine should fall back to its own local set points.

What does compressed air sequencing do?

Sequencing decides which compressors run and in what order, so that only one machine trims while the others run fully loaded or switch off. It replaces stepped pressure-switch bands with one common band, which usually lowers the average header pressure and cuts unloaded running. Good sequencers also rotate the lead machine to even out running hours and start a standby machine automatically if one trips. Typical savings come from the lower pressure, about 7% per bar, and from removing part-loaded machines.

What does the oil temperature control valve on a screw compressor do?

The oil temperature control valve, also called the thermostatic or thermal valve, bypasses the oil cooler when the oil is cold and opens to the cooler as the oil warms. It keeps the discharge temperature above the dew point in the separator tank, which is about 72 °C for humid 35 °C intake air at 8 bar(g), so water does not collect in the oil. If it sticks open the oil turns milky; if it sticks closed the compressor trips on high temperature.

What does a portable compressor controller monitor?

A portable diesel compressor controller watches both the engine and the compressor. On the engine side it monitors oil pressure, coolant temperature, speed, battery charging and fuel level. On the compressor side it monitors discharge temperature, discharge pressure and filter restriction. It also regulates output by reducing engine speed and throttling the inlet as air demand falls, manages warm-up and cool-down idling, and records running hours, service intervals and fault history.

Related pages

• Compressed-air audits
• Compressed-air energy optimisation
• Base/trim and VSD energy savings
• Pressure profiling
• Compressor AMC
• VSD rotary screw compressors