When is technical due diligence needed?
Compressed-air equipment is often treated as a line in a fixed-asset register. Its real value, and its real risk, only becomes clear when someone checks what it can still do. Due diligence is worth doing whenever a significant decision depends on the condition of the air system:
- buying or taking over a factory, or a production line that comes with its own compressor house;
- deciding whether to overhaul, refurbish or replace ageing compressors;
- planning a capacity expansion and asking whether existing machines can form part of it;
- agreeing a long-term service or AMC contract, where the starting condition affects scope and risk;
- buying a used or refurbished compressor;
- preparing a multi-year capital budget for utilities.
The aim is not to condemn old equipment or to endorse new equipment. A well-maintained twenty-year-old screw compressor can be a sound asset. A five-year-old machine that has run hot, wet and unserviced may not be.
Why age, hours and nameplate are not enough
Three shortcuts cause most poor decisions about existing compressors.
Age is not condition. Calendar age says nothing about how the machine was loaded, cooled, filtered and maintained. Two machines bought in the same year can be in completely different states.
Running hours are not condition either. Hours matter, but only alongside load factor, ambient temperature, dust, oil change history and overhaul records. Ten thousand hours in a clean, ventilated room are not the same as ten thousand hours beside a foundry or on a dusty construction site.
The nameplate is not current capacity. Free air delivery declines as airend clearances open up, inlet valves wear, intake filters restrict and coolers foul. Where capacity matters to the decision, it should be measured or at least estimated from logged load and pressure behaviour, not taken from the nameplate. Our FAD and reference conditions page explains why.
The due-diligence framework, step by step
- Define the decision. Acquisition, refurbishment, replacement, expansion, service contract or budget. The decision sets the depth of evidence needed.
- Define the boundary. List every compressor, dryer, filter, receiver, drain, controller, meter, main header and critical user included in the study.
- State the requirement. Required flow, minimum pressure at the point of use, air-quality class under ISO 8573-1, operating hours and how critical the air supply is to production.
- Add the future requirement. Committed growth, new products needing better air quality, or tighter uptime targets.
- Verify the asset register. Physically check makes, models, serial numbers and status against records. Missing or mismatched data is itself a finding.
- Review history. Service records, oil and separator changes, overhauls, recurring faults, trips and deferred work.
- Inspect and measure. Condition checks and, where needed, logging of pressure, power, temperature and dew point.
- Assess supportability. Can parts, controllers and service still be obtained for each machine?
- Grade risk and plan. Classify each asset and produce a lifecycle plan with priorities.
What we inspect on each asset
| Asset | Condition checks | Warning signs |
|---|---|---|
| Screw compressor airend | Noise, vibration, discharge temperature trend, oil condition, bearing history | Rising discharge temperature, metallic particles, bearing noise, falling output at the same speed |
| Reciprocating compressor | Valves, rings, crosshead or crank, intercooler, oil consumption | Hot valves, blow-by, knocking, high interstage pressure or temperature |
| Motor and drive | Insulation resistance, bearings, alignment, belts or coupling, VSD fault log | Overload trips, high current at rated pressure, drive faults |
| Coolers | Air and oil side fouling, fan condition, temperature approach | Discharge temperature rising in summer, frequent high-temperature trips |
| Dryers | Pressure dew point, refrigerant or desiccant condition, purge and switching | Water downstream, dew point above design, high pressure drop |
| Receivers | Inspection records, corrosion, safety valve, drain, gauges | Internal corrosion, no inspection history, seized safety valve |
| Controls | Controller model, sensor calibration, sequencing logic, settings record | Obsolete controller, no backup of settings, bypassed protections |
| Piping | Material, corrosion, leaks, layout, pressure drop | Rusted galvanised mains, many leaks, high drop to far users |
Where the airend condition is the deciding factor, deeper inspection may be needed. Our airend condition and overhaul decision page covers how that judgement is made.
Performance: can the system deliver what is needed?
Condition alone does not answer the question. The plant must also perform. We look at three things.
Capacity margin. Logged demand compared with the realistic output of the compressors, with the largest machine out of service. If the plant cannot meet peak demand with one compressor down, it has no standby, whatever the total installed capacity.
Pressure and quality. Pressure at critical users during peaks, and dew point and contamination where the process needs them. A plant that meets flow but delivers wet air to a pharma or food line is not fit for purpose.
Energy. Specific power in kW per m³/min at a stated pressure, unload hours, and how much pressure is generated above what users need. As a rule of thumb, each extra bar of generation pressure costs about 7% more power, so a high setpoint used to mask pressure losses is a lifecycle cost, not just an operating detail.
Supportability and obsolescence
An asset can be in good condition and still be a risk if it cannot be supported. We check:
- whether genuine or reliable aftermarket parts are still available for the airend, valves, controller and drive;
- whether the controller and VSD are still supported, and whether settings and software are backed up;
- whether the machine is a one-off model in a fleet of other makes, which makes spares harder to stock;
- whether local service skills exist for that type, such as centrifugal or water-injected machines;
- receiver and pressure-vessel inspection records required under applicable rules.
For mixed fleets, standardisation over time can reduce spares and training burden, as explained on our fleet standardisation page.
Grading risk and building the lifecycle plan
Each asset is graded so that decisions can be compared on the same basis.
| Grade | Meaning | Typical action |
|---|---|---|
| Retain | Sound condition and performance, supportable | Continue preventive maintenance, monitor trends |
| Restore | Defects that service or minor repair can correct | Targeted service: coolers, valves, drains, controls, leaks |
| Overhaul | Core wear affecting output, temperature or reliability | Planned airend or major overhaul with rental cover during the outage |
| Replace | Uneconomic to repair, unsupported or unfit for future duty | Specify a replacement based on measured demand, plan cutover |
| Retire | Not needed after demand and leakage are reduced | Decommission, keep as standby only if it adds genuine resilience |
The lifecycle plan should then group actions by urgency: safety and immediate risks first, then reliability, then energy and future capacity. Our lifecycle cost and TCO page covers how energy, maintenance and capital costs are weighed over time.
Buying a used compressor or a plant with one
For a single used machine, a compact version of the same checks applies: verify the serial number and running hours, review service history, run it loaded at its rated pressure, check discharge temperature and oil condition, listen for bearing noise, inspect coolers and the control panel, and confirm that parts are available. For a factory acquisition, ask early for the compressor list, service history and electricity data, because a compressor house that needs heavy investment changes the value of the deal.
How Reddy Compressors helps
Since 2002, Reddy Compressors has sold, rented, serviced and supplied spares for compressors of many makes from our base at Tarnaka, Secunderabad. Because we are an independent multi-brand specialist and not an authorised dealer of any manufacturer, our assessments are neutral between ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar and other brands. We have tie-ups with all major and reputed brands.
- Condition and performance assessments of existing compressor houses across Telangana and Andhra Pradesh, with project-based support elsewhere in India.
- Pre-purchase checks on used compressors, and refurbished machines of our own with honest condition notes.
- Overhauls, service and AMC to restore assets graded for repair, through our team of over 20 service engineers and technicians.
- Rental compressors from our own fleet to cover overhauls, cutovers and gaps in capacity.
- New compressors where replacement is the right answer, specified from measured demand rather than nameplate totals.
What to send us
- The decision you need to make and its timeline.
- Compressor, dryer and receiver list with make, model, serial number and nameplate photos.
- Running hours, service records and any overhaul history.
- Required pressure, air quality and operating shifts.
- Known problems: trips, low pressure, moisture, high power.
- Planned expansion or change in production.
Frequently asked questions
How do I know if an old air compressor is worth repairing?
Compare the cost and risk of repair with replacement on the same basis. Check measured output and discharge temperature, airend and motor condition, parts availability and the energy it uses at your pressure. If core wear can be corrected by an overhaul, parts are available and the machine still suits your demand, repair is often sensible. If it is unsupported, oversized for real demand or inefficient at part load, replacement may cost less over its life.
What should I check before buying a used screw compressor?
Verify the serial number, running hours and service history. Run the machine loaded at rated pressure and watch the discharge temperature, which on an oil-injected screw is typically about 75–95 °C. Check the oil for darkening or particles, listen for bearing noise, inspect coolers and the control panel, check for oil carry-over, and confirm that airend, controller and filter parts are still available for that model.
Does a high number of running hours mean a compressor needs replacing?
Not by itself. Running hours must be read alongside load factor, operating environment, oil and filter change history and previous overhauls. A machine with high hours that has been serviced on time in a clean, cool room can be in better condition than a younger one run hot and dusty. Measured temperature, vibration, oil condition and output tell you far more than the hour meter alone.
Why is my compressor delivering less air than the nameplate?
Nameplate FAD is measured under defined test conditions on a new machine. In service, output falls with airend wear, worn inlet or unloader valves, a restricted intake filter, slipping belts, high intake temperature and fouled coolers. Running at a pressure different from the rating also changes output. Measuring delivered flow, or analysing load time against demand, shows the real capacity before decisions are made.
What does a compressed air lifecycle plan include?
A lifecycle plan lists each asset with its condition grade, the actions needed and when, and the expected effect on reliability, energy and capacity. It covers maintenance and overhaul timing, replacements, controls upgrades, piping and treatment changes, spares strategy and rental cover for outages. It should be reviewed as demand changes and as new condition data comes in, rather than written once and filed.
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.