What a compressed air audit should answer
An audit is not a collection of instrument screenshots or a generic savings figure. It is a structured investigation that should end in a decision. Before any instrument is installed, we agree the question the audit must answer, because each question needs different measurements.
| Your question | Main measurements | Typical outcome |
|---|---|---|
| Why does pressure drop at the end of the line? | Pressure profiling at several points, pressure drop across treatment | Location of restrictions and corrective work |
| Is our compressor too small, or too big? | Demand profiling, flow and load/unload logging | Sizing basis for new or rental capacity |
| How much are we losing to leaks? | Leak survey, pressure-decay or non-production flow | Tagged leak list and repair register |
| How efficient is our compressor station? | Real power and delivered flow, specific power | kW per m³/min baseline and control options |
| Why is there water or oil in the air? | Pressure dew point, air-quality sampling | Treatment fault or design gap identified |
| Did our changes work? | Repeat logging under comparable conditions | Verified before-and-after comparison |
Several questions can share one campaign, but the evidence needed for each stays explicit. An audit must also protect what production needs: lower electricity use is not a success if pressure, flow or air quality at critical users gets worse.
Audit services in this section
Each part of an audit has its own page explaining the method, the instruments and how to read the results:
- Air demand profiling: how much air the plant uses, when and how it varies.
- Power and flow logging: measuring real electrical input and delivered air together.
- Specific-power assessment: kW per m³/min for machines and the whole station.
- Pressure profiling: pressure behaviour across the system over time.
- Pressure-drop audit: locating losses across dryers, filters and pipework.
- Leak detection: ultrasonic surveys and leak quantification.
- Leak repair register and verification: tracking repairs until they are proven.
- Dew-point measurement: checking dryer performance at line conditions.
- Compressed-air quality measurement: particles, water and oil against ISO 8573-1 classes.
- ISO 11011 assessment framework: a structured basis for system assessment.
- Audit reporting and verification: how findings are classified, ranked and checked.
Drawing the system boundary
Every audit needs a clear boundary: where energy goes in and where useful air comes out. We list the compressors, dryers, filters, receivers, main pipework and important users, and we mark bypasses, temporary hoses and separately fed areas. The boundary decides which figures can be compared.
- For a station energy assessment, we state whether dryer power, cooling fans, pumps and other auxiliaries are included.
- For flow, we state whether the meter sits before or after the dryer and receiver, and whether desiccant purge air is counted.
- A manufacturer’s catalogue figure for one compressor is never compared directly with a whole-station result measured under different conditions.
Agreeing what production really needs
We ask the process owner to name critical users and their required pressure, flow and air quality, and to separate real requirements from habitual setpoints. If a line has always run at 7.5 bar (108.8 psig) but the equipment only needs 6 bar (87 psig), that difference is an opportunity, but it must be confirmed, not assumed. We also record what an interruption would cost and how long it can be tolerated, since this drives standby and storage decisions.
Shifts, product changes, batch cycles, cleaning, start-ups and shutdowns all shape demand. A plant may have low average demand but short peaks that set the minimum pressure or the receiver size. Seasonal heat matters too: in a Telangana summer, high ambient temperature raises dryer load and can cut compressor capacity.
How we run an audit
- Brief and scope. Agree the question, boundary, critical users and safety rules for installing instruments.
- Site walk-round. Inspect the compressor room, treatment, drains, receivers, pipework and main users; note obvious losses and misuse.
- Measurement plan. Define each signal: purpose, location, range, reference basis, instrument and logging interval.
- Install and commission instruments. Check readings are plausible, scaled correctly and on one time base.
- Log a representative period. Cover the shifts and operating states that matter for the decision, recording unusual events.
- Leak survey. Ultrasonic survey during production and, where possible, during quiet periods; tag each leak.
- Analyse. Combine power, flow, pressure and dew point; calculate specific power, leakage share and pressure losses.
- Report and rank actions. Separate measured findings from estimates, and rank actions by effect, cost and risk.
- Verify. After changes, repeat measurements under comparable conditions to confirm the result.
Choosing a representative measurement period
There is no universal number of days. The right duration depends on how variable the process is and what decision is being made. One shift may answer a narrow diagnostic question, while a sizing decision usually needs weekday, weekend and idle periods. We agree the coverage before installing instruments, and we label unusual events such as breakdowns, overtime, cleaning or temporary hoses. These are not automatically bad data; they may reveal a real constraint, but they are interpreted separately. If an important operating state was missed, we either measure again or limit the recommendation.
Reading power and flow together
We measure real electrical power in kW, not current alone, because power factor and voltage change the relationship between current and energy. Flow must state its basis: free air delivery, actual volume at line conditions or a standard reference. When power and flow are logged on one time base, alongside compressor load state and speed, we can see how much energy goes into loaded running, unloaded running and modulation.
A station can use significant energy while delivering little useful air, for example when a large fixed-speed machine runs unloaded for long periods. Specific power, in kW per m³/min of delivered air, turns this into a figure that can be tracked over time and compared before and after changes. As a working rule, each extra bar of pressure needs about 7% more power, so pressure findings and power findings are always read together.
Pressure, dew point and air quality
Pressure logged at the compressor, after the dryer and filters, and at the furthest critical user shows where pressure is lost and whether the setpoint is higher than needed. Dew point shows whether the dryer is doing its job: a refrigerated dryer typically delivers a pressure dew point around +3 °C, while desiccant dryers are typically specified for −20 °C or −40 °C. Where products, instruments or coatings depend on clean air, sampling against the relevant ISO 8573-1 class confirms whether water, oil and particles are under control.
From findings to action
Audit findings usually fall into a few groups, and they are best tackled in order. Demand-side fixes come first because they reduce the air every later decision must supply.
- Reduce waste: repair leaks, stop open blowing and isolate idle areas.
- Reduce pressure losses: clean or resize filters, correct pipework and fittings.
- Lower the setpoint in steps once losses are fixed.
- Improve control: sequencing, base and trim operation, storage.
- Only then, re-size or replace compressors and treatment where data justifies it.
What an audit does not do
- It does not guarantee savings. It may show a system already working well.
- It does not replace nameplate data, but tests it against real operation.
- It does not justify a new compressor on its own; demand, losses and controls come first.
- It does not end with the report; actions need owners, dates and verification.
Types of compressed air audit compared
“Audit” covers several jobs of very different depth. Choose the type from the question you need answered, then check that the quotation describes the measurements, not just the visit.
| Audit type | What is measured | Typical site time | Typical output | Good for |
|---|---|---|---|---|
| Walk-through assessment | Spot readings of pressure and current, nameplate data, visual check of room, treatment, drains and pipework | Half a day to one day | Short list of obvious faults and a recommendation on whether deeper measurement is worth it | A first look; deciding what to measure next |
| Leak survey | Ultrasonic detection at joints, hoses, drains and tools; estimated leak flow per tag | One to three days for most single-site plants | Tagged leak register with location, size class and repair part | Plants with high idle or weekend demand |
| Full system assessment (ISO 11011 approach) | Logged real power, flow, pressure at several points, dew point, load/unload or speed, plus leak survey and end-use review | Installation and walk-round of two to three days, then logging over a representative period | Baseline in kW per m³/min and kWh per year, ranked measures with estimated savings and costs | Investment decisions, energy targets, sizing new or rental capacity |
| Air quality assessment | Particles, water and oil against ISO 8573-1 classes at agreed sampling points | One to two days | Measured class at each point against the required class | Food, pharmaceutical, paint, instrument air |
| Dew point survey | Pressure dew point logged after each dryer and at critical users | Spot checks in a day; logging over several days for trends | Dryer performance under real load and inlet temperature | Water in lines, monsoon problems, dryer sizing |
Site time depends on the number of compressors, the length of the distribution system and access. The logging period for a full assessment follows the guidance on representative periods above, not a fixed number of days.
What an ISO 11011 assessment examines
ISO 11011 sets out how to assess the energy efficiency of a whole compressed air system. It does not set pass or fail limits; it defines what the assessment must look at and how findings are reported. Its value is that supply, distribution and demand are examined together, so savings found on one side are not cancelled by problems on the other. The process steps are those described above; the content each area should cover is set out below.
| Area | What the assessment should establish |
|---|---|
| Supply: compressors | Rated and measured flow, input power, control mode, pressure band, loaded and unloaded hours for each machine |
| Supply: treatment | Dryer type and pressure drop, filter differential pressure, drain operation and the air lost by timed drains |
| Supply: station | Sequencing, receiver volume, ventilation and intake temperature |
| Distribution | Pressure drop from compressor outlet to the furthest critical user; pipe sizes, loops and dead legs; hose and fitting restrictions |
| Demand | Each significant end use: required pressure and flow, whether air is the right energy source, open blowing, idle equipment left pressurised, leakage, and artificial demand caused by running at higher pressure than needed |
| Reporting | Measured baseline, demand profile, ranked measures with saving, cost and risk, and the baseline that verification will be compared against |
A report that covers only the supply rows is a compressor-room study, not a system assessment. It may be useful, but it will miss most demand-side waste. The method in more detail is on the ISO 11011 framework page.
Worked example: from measured baseline to savings
This example is illustrative. It shows how measured data turn into a savings estimate and why measures must be applied in sequence, not added up separately.
Measured baseline (illustrative plant): average input power during production 102 kW; average delivered flow 15.0 m³/min (about 530 cfm); production 6,000 hours per year; setpoint 7.5 bar(g) (108.8 psig). Specific power = 102 ÷ 15.0 = 6.8 kW per m³/min. Annual energy = 102 × 6,000 = 612,000 kWh.
Findings: a non-production test with all machines idle shows 4.2 m³/min of flow, which is 28% of average flow, almost all leakage. Logged data from the trim compressor give a marginal power of 6.2 kW for each m³/min of demand removed. Pressure at the furthest critical user shows that the setpoint can safely come down by 0.5 bar. Open blowing with a plain pipe at one station uses 1.0 m³/min; an engineered nozzle would use 0.4 m³/min.
| Measure (in sequence) | Calculation | Power saved |
|---|---|---|
| 1. Repair leaks to halve leakage (2.1 m³/min removed) | 2.1 × 6.2 | 13.0 kW |
| 2. Replace open blow-off with nozzle (0.6 m³/min removed) | 0.6 × 6.2 | 3.7 kW |
| 3. Reduce setpoint by 0.5 bar (about 7% per bar, applied to the power that remains) | (102 − 13.0 − 3.7) × 0.035 = 85.3 × 0.035 | 3.0 kW |
| Total | 13.0 + 3.7 + 3.0 | 19.7 kW |
Annual saving = 19.7 × 6,000 = 118,200 kWh, which is 19.3% of the baseline. At an assumed tariff of ₹8.50 per kWh (illustrative), that is 118,200 × 8.50 = ₹10,04,700 a year.
Two points matter here. First, if the pressure reduction had been applied to the full 102 kW, it would have claimed 3.6 kW instead of 3.0 kW, double counting air that the first two measures had already removed. Second, specific power hardly changes: after the measures, 82.3 kW for 12.3 m³/min is 6.7 kW per m³/min. Leak and misuse savings show up in kWh and in demand, not in kW per m³/min, so a report must state both. Leak flow also falls slightly at lower pressure, so this estimate is on the conservative side.
Preparing the site for measurement
The documents an auditor needs are listed under “What to send us” below. The points here are about the site itself, so that logging starts on time and the data represent normal running.
- Controller hours: record running and loaded hours of every compressor at the start and end of logging; the difference cross-checks the logged load profile.
- Critical users: identify the machines that need a minimum pressure or a specific air quality, with the maker’s figures, so a pressure sensor can be placed at the furthest one.
- Electrical access: permits for the panels where power loggers will be clamped, with a person who can isolate safely if needed.
- Tapping points: safe points with isolating valves for pressure and dew point sensors, after each dryer and at the end of main headers.
- Non-production test: a person who can authorise a short test with production stopped and air still on, which shows leakage directly.
- Tariff structure: demand charge and any time-of-day rates, which change the value of each saving.
Avoid fixing known leaks or changing settings the week before measurement unless the change is recorded. Otherwise the baseline will not represent normal operation and verification will be distorted.
How to judge the quality of an audit report
A thick report is not necessarily a good one. Check it against these questions.
| Question | A good report | A weak report |
|---|---|---|
| Is the baseline measured? | Logged kW, flow and pressure with dates, instruments and logging intervals stated | Savings calculated from nameplate power and assumed load factors |
| Are instruments identified? | Type, range and calibration date for each logger and sensor | No mention of how readings were taken |
| Is demand quantified? | Profile by hour and day, with peak, average and non-production flow | A single average figure |
| Are savings sequenced? | Each measure calculated on the system left by the measures before it | Individual savings simply added together |
| Are leaks traceable? | Tag number, location, estimated flow and repair part for each leak | A total leak percentage only |
| Is critical-user performance protected? | Shows pressure, flow and air quality at critical users before and after | Recommends a lower setpoint without checking the furthest user |
| Is verification planned? | Defines how and when the baseline will be re-measured | Ends with a list of recommendations |
Also check whether the report recommends new equipment before cheaper demand-side measures. If it does, ask for the calculation showing why waste reduction was not enough.
How Reddy Compressors helps
Our audit team works from Tarnaka, Secunderabad, across Telangana and Andhra Pradesh, with project support elsewhere in India where feasible. As an independent multi-brand specialist, we audit systems built around ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other compressors, and our recommendations follow the evidence rather than a product range.
- Demand, pressure, power and flow logging, with leak surveys.
- Dew-point and air-quality checks on treatment equipment.
- A clear report separating measured facts from estimates.
- Follow-up work: leak repair support, service, control changes, treatment upgrades.
- New, used or rental compressors sized from measured demand where change is justified.
What to send us
- A simple sketch of the compressor room and main pipework.
- List of compressors, dryers and filters with make, model and rating.
- Operating pressure, shift pattern and production schedule.
- Electricity bills or compressor energy data, if available.
- The problem or decision the audit should support.
Frequently asked questions
What is a compressed air audit?
A compressed air audit is a measured investigation of how a plant produces, treats, distributes and uses compressed air. Instruments log power, flow, pressure and often dew point over a representative period, and a leak survey locates losses. The result is a report that shows real demand, efficiency and losses, with ranked actions based on evidence rather than assumptions.
How long does a compressed air audit take?
It depends on how variable the plant is and what decision the audit supports. A narrow diagnostic check may need only a shift or two of logging, while sizing or energy decisions usually need weekdays, weekends and idle periods. We agree the coverage before installing instruments and state clearly in the report which operating states were observed.
Will an air audit guarantee energy savings?
No. An honest audit may find large opportunities, small ones, or a system that is already performing well. Savings can only be confirmed by measuring again after changes are made, under comparable conditions. We do not promise a percentage before measuring, and our reports separate measured results from estimates.
Can nameplate data replace audit measurements?
No. Nameplate and catalogue data describe what a compressor can do under defined test conditions. They do not show your actual demand, operating hours, control behaviour, leakage or pressure losses. We use nameplate data as a labelled input and check it against measured power and flow on site.
What happens after the audit report?
Each action is assigned an owner and a date, approved changes are made, and their effect is checked by repeat measurement under comparable conditions. Typical follow-up includes leak repairs, pressure reduction, control and sequencing changes, treatment fixes and, where justified, new or rental capacity. Unresolved findings stay visible until they are closed.
Can our maintenance team help with the audit?
Yes, and their input is valuable. Maintenance staff know the fault history, access routes, unusual events and how the plant really runs. We record their observations alongside the measured data, clearly labelled, and use them to explain what the instruments show. Their help also makes instrument installation and leak tagging quicker and safer.
What is an ISO 11011 compressed air audit?
It is an assessment carried out following ISO 11011, the standard for assessing the energy efficiency of compressed air systems. It covers the supply side (compressors, treatment, controls), the distribution network and the demand side (end uses, leaks, misuse and artificial demand) together. Power, flow, pressure and dew point are measured to set a baseline, and the report ranks improvement measures by saving, cost and risk. That baseline is what results are verified against after changes are made.
What is the difference between an air leak detection audit and a full compressed air audit?
A leak detection audit finds and tags leaks, usually with an ultrasonic detector, and estimates the flow lost at each one. It produces a repair register. A full compressed air audit also logs power, flow and pressure over a representative period, reviews controls, treatment, pipework and end uses, and sets an energy baseline. A leak survey is often one part of a full audit, but on its own it cannot tell you whether your compressors are correctly sized or controlled.
What instruments are used in a compressed air audit?
A full audit typically uses clamp-on or panel power loggers that record real power in kW, not current alone; flow meters, either inserted into the header or clamped on as ultrasonic meters; pressure loggers at the compressor outlet, after treatment and at the furthest critical user; a dew point sensor after each dryer; and an ultrasonic leak detector. All loggers should run on synchronised clocks, and the report should state each instrument’s range and calibration date.
How are savings from a leak audit calculated?
The leak flow removed is multiplied by the marginal power the compressors save for each m³/min of reduced demand, then by operating hours and tariff. For example (illustrative), removing 2.1 m³/min at 6.2 kW per m³/min saves 13.0 kW; over 6,000 hours that is 78,000 kWh. Using full-load specific power instead of marginal power overstates savings on load/unload machines, because they still draw power when unloaded.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 7 November 2026. How our engineering content is reviewed.