Equivalent to what?
When a compressor fails, is retired or must be standardised, the first question is usually “what is the equivalent model?”. The honest answer starts with another question: equivalent in which respects? Two machines with the same motor power can deliver noticeably different flow. Two with the same flow figure may quote it at different pressures. One may be oil-injected and the other oil-free, one air-cooled and the other water-cooled.
A proper equivalence check rebuilds what the existing machine actually does for the plant, then tests each candidate against that duty and against the site’s connections. Until that is done, a candidate is a possible substitute, not a verified equivalent.
Why are you replacing it?
The reason changes what “equivalent” must mean.
| Purpose | What matters most | What can be relaxed |
|---|---|---|
| Emergency replacement after failure | Flow, pressure, power supply, quick installation | Exact footprint and long-term efficiency, for a temporary period |
| Planned retirement | Lifecycle cost, efficiency, maintainability | Exact matching of old ratings if demand has changed |
| Standby machine | Ability to take the full critical load, reliable starting | Running efficiency |
| Energy improvement | Specific power, part-load control, heat recovery | Physical similarity |
| Fleet standardisation | Common spares, controls and service skills | Some differences in individual ratings |
For emergencies, hiring a machine while the permanent replacement is chosen properly is often the best route; see temporary air for compressor breakdown.
Record what you have now
Collect the full picture of the existing machine and how it is installed:
- Make, full model with suffix, package serial, airend or pump serial.
- Rated pressure and flow from the plate, and the actual working pressure set.
- Motor or engine power, voltage, frequency, starter type (direct-on-line, star-delta, soft starter, variable speed drive).
- Oil-injected or oil-free, air-cooled or water-cooled.
- Controller type and how it links to other compressors or a sequencer.
- Dryer, filters, receiver and drain arrangement downstream.
- Pipe connection size and position, cooling-air path, footprint and service access.
- Running hours, load hours and typical demand pattern.
Old machines are often oversized or undersized for today’s demand. If you have pressure or load data, or can run a short air audit, use it. Replacing like for like can carry an old mistake forward for another fifteen years.
Compare flow properly
Flow figures are where most false equivalence comes from. Compare free air delivery (FAD), the air actually delivered at the outlet referred to intake conditions, measured to a recognised method such as ISO 1217. Displacement or swept volume, sometimes quoted for piston machines, is always higher than FAD and should not be compared with it.
- Compare flow at the same working pressure. A machine’s FAD falls as its rated pressure rises.
- Check whether the figure is for the bare compressor or the complete package.
- Use the same units throughout: cfm or m³/min (1 m³/min is about 35.3 cfm).
- Check the ambient and intake conditions the figure assumes.
- For efficiency, compare specific power, kW per m³/min of FAD at the same pressure, for the complete package.
Pressure needs equal care. Running at a higher pressure than the plant needs costs about 7% more power per extra bar. A candidate with a higher maximum pressure is not “better” if the plant will run it there unnecessarily. More on this is in compressor performance data provenance.
Look at part-load behaviour
Few compressors run at full load all day. How a candidate behaves when demand falls matters as much as its full-load rating.
- Load/unload machines draw significant power while unloaded.
- Variable speed machines follow demand more closely but have a minimum speed below which they stop or unload.
- Piston machines may use start/stop or unloader control.
- Receiver size affects how often a load/unload machine cycles.
If the existing machine is one of several, the candidate must fit the control scheme: a variable speed trim machine with fixed-speed base machines, for example. Check that its controller can talk to the existing sequencer or that a new one is planned.
Check every connection
An equivalent that cannot be connected is not equivalent. Work through each interface:
| Interface | What to check |
|---|---|
| Electrical | Voltage, frequency, full-load and starting current, cable size, starter, protection settings |
| Mechanical | Footprint, weight, foundation or mounting, access for service |
| Air outlet | Connection size and position, check valve, isolation valve |
| Cooling | Air flow and duct pressure drop, or cooling water flow and quality |
| Air treatment | Dryer and filter capacity, inlet temperature to the dryer |
| Condensate | Drain type, oil-water separator capacity |
| Controls | Remote start/stop, sequencer link, alarms to plant systems |
Ventilation is a frequent problem in Indian plant rooms. A new machine that rejects more heat than the old one, in a room already near 45 °C in summer, will run hot and may trip. Electrical detail is covered in compressor electrical starter and protection interfaces.
Air quality must match too
If the existing machine is oil-free because the process needs Class 0 air under ISO 8573-1, an oil-injected machine with filters is a different solution, not an equivalent. It may be acceptable in some cases, but that is a deliberate engineering change that the process owner must approve. Likewise, a dryer sized for the old machine may not cope with a larger one, and pressure dew point will suffer. Check the full treatment train against the new flow.
Score candidates and record differences
Lay out the existing machine and each candidate side by side and list every difference. For each, decide whether it is acceptable, needs a change elsewhere (a bigger cable, a new duct), or rules the candidate out. A simple matrix is enough:
- List the requirements: flow at pressure, air quality, power supply, footprint, cooling, controls, maintenance support.
- Mark which are essential and which are preferences.
- Enter each candidate’s value for each requirement, from verified data.
- Reject any candidate that fails an essential requirement.
- Compare the remaining candidates on lifecycle cost: energy, service and installation.
- Record the decision, the differences accepted and who approved them.
That record is valuable later, when someone asks why a particular machine was chosen.
Maintenance and spares support
An equivalent machine must also be one you can keep running. Before choosing, check how its service needs compare with the old machine and with the rest of your fleet.
- Are filters, separators, oil and wear parts readily available in India, and from more than one source?
- Does it use the same oil type as your other machines, or will you need to manage two oils on site?
- Can your own technicians or your service provider carry out routine work, or does the controller need special software?
- How are major services such as airend bearing replacement handled for that make?
- Will standardising on one family reduce the number of different spares you hold?
A slightly less efficient machine with good local parts support can be the better choice for a remote site, where a week waiting for a separator element costs more than the energy difference. Our compressor AMC page explains how we keep mixed fleets serviced.
Commissioning the replacement
When the chosen machine arrives, prove it before the old one is removed for good, where space and time allow.
- Check the electrical supply, protection settings and direction of rotation.
- Run it loaded at the plant’s working pressure until temperatures settle.
- Confirm load and unload points, and the link to any sequencer.
- Check the dryer inlet temperature and pressure dew point under the new flow.
- Record the readings as the new baseline for maintenance.
How Reddy Compressors helps
Reddy Compressors sells new and used or refurbished compressors, rents them and services them, working across ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar and many other brands. Because we are an independent multi-brand specialist, not an authorised dealer of any one brand, we can compare candidates from several makers on the same basis. We have tie-ups with all major and reputed brands.
We record your existing machine and installation, check actual demand where possible, and propose replacements with the differences stated plainly. If you need air while the decision is made, we can rent a machine from our own fleet of 50–2,000 cfm, 2–30 bar diesel and electric units. Our office is in Tarnaka, Secunderabad, our service teams cover the whole of Telangana and Andhra Pradesh, and we supply equipment and spares across India.
What to send us
- Nameplate photos of the existing compressor, motor and dryer.
- Working pressure and any known flow demand or load hours.
- Reason for replacement and your timeline.
- Power supply details and starter type.
- Photos of the installation: room, ducting, piping and connections.
- Air quality needs of the process, if any.
Frequently asked questions
How do I find an equivalent replacement for my old air compressor?
Start with what the existing machine actually does: its working pressure, real flow demand, air quality and how it is powered, cooled and controlled. Then compare candidates on free air delivery at that same pressure, not on motor kW or a headline figure. Check every connection, from cable size to ventilation, and list all differences before choosing.
Can I replace a compressor with one of the same kW from another brand?
Possibly, but kW alone does not prove equivalence. Machines of the same motor power can deliver noticeably different flow at the same pressure, and their package efficiency differs. Compare free air delivery at your working pressure and specific power in kW per m³/min for the complete package. Also check air quality, cooling, controls and footprint before deciding.
What is the difference between FAD and displacement?
Free air delivery is the air actually delivered at the compressor outlet, referred back to intake conditions, usually measured to ISO 1217. Displacement is the theoretical swept volume of the compression element. Displacement is always higher than FAD because of leakage and clearance losses, so comparing a displacement figure with an FAD figure overstates the smaller machine.
Can an oil-injected compressor replace an oil-free one?
Only if the process owner approves it as a deliberate change. An oil-injected compressor with good filtration can give very clean air, but it is not the same as an oil-free compressor where the process requires ISO 8573-1 Class 0 air. Pharma, food and electronics plants often specify oil-free. Treat the swap as an engineering change, not a like-for-like replacement.
Should I rent a compressor while choosing a replacement?
Often, yes. Buying in a hurry after a breakdown tends to repeat old sizing mistakes. A rental machine keeps production running while you collect demand data, compare candidates and plan the installation properly. Our rental fleet covers 50–2,000 cfm at 2–30 bar, diesel and electric, so a suitable temporary match is usually possible.
Related pages
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.