What base and trim mean
Plant air demand almost always has two parts. There is a floor that is nearly always present during production: continuous processes, instrument air, a baseline of leaks. Above it sits a variable part that rises and falls as machines, shifts and batch processes come and go.
A compressor is most efficient at, or close to, full load. Base machines should therefore cover the floor and stay fully loaded for long periods. One trim machine follows the variation. If several machines all follow demand together, each spends time part-loaded or unloaded, and the station’s specific power, in kW per m³/min of free air delivered, gets worse.
Start with the demand profile
Base/trim design cannot be done from nameplates alone. It needs a picture of how demand actually behaves.
- Log pressure and compressor power, and flow where a meter can be fitted, over at least one representative production cycle, including nights and weekends. Our demand profiling work does this.
- Plot demand over time and as a frequency distribution: how many hours the plant spends at each flow level.
- Identify the minimum sustained demand during production; that sets the base.
- Identify the range and speed of variation above the base; that sets the trim requirement.
- Note special events: shift starts, cleaning cycles, large intermittent users, night-time loads that may be mostly leakage.
Leakage matters here. If a third of the night-time load is leaks, fixing them changes the base. A leak survey before finalising a base/trim plan avoids designing around waste.
Trim sizing and the control gap
The most common base/trim mistake is a trim machine that cannot cover the gap between base combinations. Suppose the base machines each deliver about 20 m³/min. When demand rises past one base machine plus the trim’s maximum, a second base machine starts, and the trim must then drop back by roughly 20 m³/min. If the trim’s usable range is smaller than that step, it runs into its minimum, unloads or stops, and the station hunts between combinations.
| Trim choice | Strengths | Watch points |
|---|---|---|
| Fixed-speed load/unload trim | Simple, robust, low purchase cost | Unloaded running when trimming; needs adequate storage to limit cycling |
| Modulating fixed-speed trim | Steady pressure over a narrow range | Poor efficiency at low load because power falls much less than flow |
| VSD trim | Efficient over its turndown range, tight pressure | Usable range must exceed base machine step; avoid long running at minimum speed |
| Two trim machines (for large swings) | Covers wide variation | Needs master control to stop both trimming together |
A practical rule is that the VSD’s usable range, from minimum to maximum flow, should be at least as large as the largest fixed-speed base machine it works alongside, with some margin. Where it is not, options include a different trim size, a different mix of base machines, or more storage to smooth transitions. See our VSD integration page for drive and turndown detail.
Choosing the base machines
- Use the most efficient fixed-speed machines as base, based on measured specific power rather than age or brand loyalty.
- Prefer machines that can run fully loaded for long periods without temperature problems in your summer conditions.
- Avoid using an oversized base machine that must unload to match a smaller floor.
- Rotate identical base machines on a planned basis to even wear, while keeping the trim machine fixed.
- Keep a standby machine that can take over base or trim duty if one is out for service.
Pressure bands for base and trim
Base and trim machines must not compete. On a station without a sequencer, base machines are set to unload slightly above the trim machine’s target, so the trim always moves first. On a sequenced or master-controlled station, the controller holds the band and decides when to add or drop a base machine, while the trim follows pressure within that band. In both cases local settings remain as a fallback.
The lower the pressure the station can hold steadily, the less power it needs: about 7% for every bar. Good base/trim control, with storage and a properly located pressure transmitter, typically allows a tighter band than independent pressure switches, and that is often where much of the benefit comes from. Our base/trim and VSD energy page covers the energy side.
Storage and transitions
Receiver volume buys time. When a base machine starts or stops, pressure can move quickly on a station with little storage, pushing the trim to its limits and causing more switching. Adequate receiver capacity, located so that compressors and the pressure transmitter see the same system, smooths these transitions. A wet receiver before the dryer and a dry receiver after it serve different purposes, and both influence control behaviour. For sizing method, see storage and energy.
Signs your base/trim arrangement is wrong
| Symptom | Likely cause |
|---|---|
| VSD runs at maximum speed most of the day | Base too small; VSD effectively a base machine |
| VSD sits at minimum speed or stops often | Base too large, or demand has fallen |
| Fixed-speed machine cycles while VSD is at a limit | Control gap: trim range smaller than base step |
| Two machines unloaded at the same time | Overlapping pressure settings, no coordination |
| Header pressure set high to avoid complaints | Slow transitions, little storage, network pressure drop |
An illustrative station
Take a hypothetical plant whose logging shows production demand between about 28 and 50 m³/min, with a night load near 12 m³/min. It has two fixed-speed machines of about 20 m³/min each and is considering a VSD. A VSD with a usable range of roughly 6 to 26 m³/min could serve nights alone, trim above one base machine up to about 46 m³/min, and with a second base machine added, drop back without a gap. Whether such a machine suits the real plant would depend on measured data, room conditions and the machines’ actual delivery; the example only shows how the arithmetic is done.
Indian operating conditions and base/trim
Base machines in Indian plants work hard. In April to June, compressor rooms in Telangana and Andhra Pradesh can reach 45 °C or more, and a fully loaded base machine with a partly blocked cooler will run close to its discharge temperature limit (normal oil-injected screw discharge is about 75–95 °C). If it trips, the trim machine and standby must cover its share at once. A sound arrangement therefore considers:
- Room ventilation and hot-air ducting, so base machines can stay loaded through the hottest hours.
- Regular cooler cleaning in dusty conditions, especially before summer.
- Power quality and voltage dips that can trip machines, and how the station restarts after a power failure or on DG supply.
- A standby machine that is genuinely ready, rotated into service at planned intervals.
- Seasonal review: demand and machine performance in summer can differ noticeably from winter, so the base/trim plan may need checking after the first hot season.
How Reddy Compressors helps
Reddy Compressors, Tarnaka, Secunderabad, has served industry since 2002 as an independent multi-brand specialist. We sell new and used compressors, rent from our own fleet, 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, so a trim or base recommendation is made on the station’s needs. We have tie-ups with all major and reputed brands.
- Pressure, power and flow logging to define base and trim from evidence.
- Assessment of existing machines’ condition and delivery before deciding which should be base.
- Supply of new VSD or fixed-speed machines sized to close a control gap, with supply across India.
- Setting pressure bands and coordinating with a sequencer or master controller, with commissioning on site across Telangana and Andhra Pradesh.
- Rental machines to trial a configuration or cover a station during changes.
What to send us
- Make, model, serial and rated flow and pressure of each compressor, and whether fixed-speed or VSD.
- Any pressure, power or flow logs, or monthly energy consumption for the compressor room.
- Shift pattern, night and weekend operation.
- Receiver sizes and positions.
- Current pressure settings and the minimum pressure your users need.
Frequently asked questions
Which compressor should be the trim machine?
Usually a VSD compressor, because it stays efficient across much of its speed range. Its usable flow range should be at least as large as the biggest fixed-speed base machine it works with, otherwise a control gap appears. Where there is no VSD, the fixed-speed machine with the best part-load behaviour and enough storage to limit cycling is the usual choice.
What is a control gap in a compressor system?
A control gap occurs when the trim machine’s capacity range is smaller than the step created when a base machine starts or stops. The trim then hits its maximum or minimum, and the station hunts between combinations, cycling a fixed-speed machine and wasting energy in unloaded running. It is fixed by resizing trim, changing the base combination or adding storage.
Can my VSD compressor run as the base machine?
It can, but it often wastes the VSD’s advantage. If the VSD runs at full speed for most hours, it is acting as base load, and a fixed-speed machine may carry that duty at similar or better efficiency while the VSD follows variation. The demand profile shows which arrangement suits your plant.
How do I set pressure for base and trim compressors without a sequencer?
Set the trim machine’s target, or load/unload band, at the lowest stable pressure your users need, and set base machines to unload slightly above it, so the trim always responds first. Keep the overall range as narrow as storage allows. A sequencer or master controller usually gives tighter, more reliable coordination than this manual approach.
Do I need flow measurement for base/trim design?
Flow measurement gives the clearest picture, but a good estimate is possible from logged power and load/unload states with known machine delivery. For major decisions, such as buying a new VSD trim machine, measured flow or a careful estimate from logged data is strongly recommended. Guessing from nameplates often leads to the wrong size.
Will fixing leaks change my base/trim arrangement?
Often yes. Leakage is steady demand, so it inflates the base load, especially at night and weekends. Once significant leaks are repaired, a smaller base may be enough, and the trim machine may then cover the night load alone. Repeating a short demand log after leak repairs confirms the new profile.
Related pages
Field guide
Base and trim: Let each machine do what it is best at
Most plants have a steady base demand and a variable part on top of it. The efficient arrangement is to run one or more fixed-speed compressors fully loaded on the base, and let a single trim machine, ideally a variable-speed compressor, follow the variable part.
Rules that make base and trim work
- Only one trim machine at a time. Two machines trimming together both run part-loaded.
- Size the trim range to cover the swing. If the trim machine’s range is smaller than the step between base machines, there will be a “control gap” where a fixed machine unloads and the trim machine cannot fill the hole.
- Keep the VSD in its efficient range. Below its minimum speed a VSD starts to load and unload like a fixed-speed machine.
- Give the system enough storage so that pressure does not collapse while a base machine starts.
A quick check
Look at the hours meters. If a fixed-speed machine shows a large share of unloaded hours, it is acting as a trim machine and wasting energy. Look at the VSD’s speed trend: if it spends most of its time at full speed, it is too small for the swing; if most of its time at minimum, it is too big or the base is too small.
What to send for a quotation
- Make, model and serial number of every compressor and dryer in the station.
- Running hours and a typical week of operation (shifts, weekends).
- Current pressure settings and controller type on each machine.
- Any alarm or fault history you have.
Send it through find my part or the spare parts enquiry, or call +91 99590 60718 or +91 92468 88812.
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Part-load fleet dispatch (single demand point)
For one plant demand, list every feasible combination of up to three compressors, put the others on base load and one on trim, and rank the states by total package kW.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Plant demand to be met (m³/min free air): 10
- Machine A: type: Fixed speed, load/unload
- Machine A: full-load flow, FAD at duty pressure (m³/min): 6
- Machine A: package power at full flow (kW): 40
- Machine A, fixed speed only: unloaded power (kW): 12
- Machine A, VSD only: minimum stable flow (m³/min): 0
- Machine A, VSD only: power at minimum flow (kW): 0
- Machine B: type: Fixed speed, load/unload
- Machine B: full-load flow, FAD at duty pressure (m³/min): 6
- Machine B: package power at full flow (kW): 40
- Machine B, fixed speed only: unloaded power (kW): 12
- Machine B, VSD only: minimum stable flow (m³/min): 0
- Machine B, VSD only: power at minimum flow (kW): 0
- Machine C: type: Variable speed (VSD)
- Machine C: full-load flow, FAD at duty pressure (m³/min): 7.5
- Machine C: package power at full flow (kW): 48
- Machine C, fixed speed only: unloaded power (kW): 0
- Machine C, VSD only: minimum stable flow (m³/min): 2
- Machine C, VSD only: power at minimum flow (kW): 15
- Running hours per year at this demand: 6000
Result
- 1. A + C: 67 kW, 6.7 kW per m³/min; trim C at 4 m³/min (VSD), others fully loaded
- 2. B + C: 67 kW, 6.7 kW per m³/min; trim C at 4 m³/min (VSD), others fully loaded
- 3. A + B: 70.7 kW, 7.07 kW per m³/min; trim A at 4 m³/min (67% loaded), others fully loaded
- Lowest-power state: A + C trim C at 4 m³/min (VSD), others fully loaded
- Specific power of that state: 6.7 kW per m³/min
- Annual energy in that state: 4,02,000 kWh per year
- N+1 check (lose machine C): Passes other available machines can give 12 m³/min against 10 m³/min demand
- Lowest-power state that also passes N+1: Same as above
- Saving against the next higher-power state (A + B): 3.67 kW = 22,000 kWh per year
Method. Single-demand-point mode. Every on/off combination of the available machines is tried, with each online machine in turn as the trim machine. Base machines run fully loaded at their FAD and full-flow kW; the trim machine takes the remainder. A state is feasible only if the remainder is above 0, no more than the trim machine’s full flow, and (for a VSD trim) not below its minimum stable flow. Fixed-speed trim on load/unload: kW = x × loaded kW + (1 − x) × unloaded kW, with x = trim flow ÷ full flow (ignores extra blowdown losses with small storage). VSD trim: kW interpolated linearly between the minimum-flow and full-flow points. Specific power = total kW ÷ demand. N+1 check: with the largest online machine lost, all other available machines together must still cover the demand (assumes standby machines can start in time). Annual kWh = kW × hours at this demand. The lowest-power state that also passes N+1 is shown when the cheapest state does not. Use package kW and FAD at the same duty pressure for every machine. Not included: the demand sweep, annual load profile and scenario modes described on the page; run the tool once per demand band and weight each result by the hours in that band (annual kWh = Σ kW × hours).
What to take from it. Run the base machines fully loaded and give the trim role to the machine with the flattest part-load curve; two fixed-speed machines both partly unloaded is usually the most expensive way to meet the same flow.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.