Before and after comparison method for compressed air

Why most before-and-after claims fail

A plant replaces a compressor, fixes leaks or fits a variable-speed drive, and next month’s electricity bill is lower. It is tempting to call the difference a saving. Yet the bill may have fallen because a production line was down, because the monsoon cooled the compressor house, because a second shift was cut, or because the meter covers loads other than the compressors. Equally, a real improvement can be hidden by higher output in the “after” period.

A sound comparison separates the effect of the change from everything else that moved at the same time. The method below is the one we use when a customer asks us to show whether a piece of work actually helped. It applies equally to energy, pressure stability, dew point and running hours.

Start by writing the claim you want to test

Before any logger is installed, write the claim as a testable sentence, because each claim needs a different measurement plan.

• “Compressor-station electrical energy per tonne of output fell after leak repair” needs station kWh and production data in both periods.
• “Pressure at the paint shop stayed above 6 bar (87 psig) at peak demand after the header change” needs simultaneous pressure logging at the station and at the critical user.
• “Pressure dew point at the dryer outlet stayed at or below about +3 °C during the summer peak” needs a dew-point sensor at a defined sampling point and ambient temperature records.
• “Unloaded running hours fell after the sequencing change” needs controller hour counters or a power logger on each machine.

Draw the boundary before you measure

Sketch the air path and the electrical supply. Mark every compressor, dryer, filter, receiver, header and branch, the critical users and any parallel air source such as a rented portable. Then mark what the claim is about: one machine, the station, a branch or a process. A reading taken on a single compressor package cannot establish whole-station performance, and a station meter that also feeds cooling-tower pumps cannot isolate compressor energy.

Choosing a representative baseline

The baseline should represent normal operation, not the worst week. It should be long enough to include the usual shift pattern, weekends and typical production variation; for most plants that means at least one to two full working weeks of logged data. Record the dates, shifts worked, output, ambient conditions, control settings, maintenance state and any unusual events such as a breakdown or a rented standby machine.

Keep the raw baseline data unchanged. If adjustments are later agreed, they are applied to a copy, so anyone reviewing the result can see what was measured and what was calculated.

Matching the after period

The after period should match the baseline as closely as possible: same instruments in the same positions, similar length, similar shift pattern and similar season. Allow time for the change to settle. After a sequencing change, operators often adjust set points for a few days; after leak repair, new leaks start to appear within weeks. State when the after period started and why.

1. Confirm instruments are in the same positions and still within their verification status.
2. Record the date and description of the change exactly as made, including any set-point adjustments.
3. Allow a stabilisation period and exclude it from the comparison, stating its length.
4. Log for a period comparable to the baseline in length and operating pattern.
5. Note every event that could affect demand: new machines, line stoppages, holidays, power cuts.

Normalising for production, weather and pressure

Normalisation means adjusting for documented differences between the two periods, not for convenient ones. The usual factors are:

• Production. Compare energy per unit of output, or energy per delivered m³, where output differs between periods.
• Operating hours. A plant that ran fewer hours will use less air regardless of any improvement.
• Ambient temperature. Hyderabad summers of 40–45 °C raise inlet temperature, reduce dryer capacity and increase cooling load; comparing a May baseline with a December after period is not fair.
• Pressure. Generation pressure has a direct effect on power, typically about 7% more power for each extra bar on an oil-injected screw. If set pressure changed, say so and treat it as part of the intervention or adjust for it openly.

Every adjustment should be written down with its basis. A reviewer should be able to reproduce the adjusted figure from the raw data and the stated method.

Specific power: the most useful single comparison

For energy claims, the most robust single figure is specific power: electrical input divided by delivered free air, usually expressed in kW per m³/min (or kW per 100 cfm). It automatically accounts for changes in demand, provided both power and flow are measured at the same boundary. It still depends on pressure, so compare specific power at the same or corrected generation pressure.

A worked illustration: suppose a station averaged 110 kW while delivering 18 m³/min in the baseline (about 6.1 kW per m³/min) and 96 kW while delivering 17 m³/min after the change (about 5.65 kW per m³/min). The energy fell partly because demand fell, and partly because the station became more efficient. Specific power isolates the second effect. Whether the difference is real depends on the measurement uncertainty, which must be stated alongside it.

Uncertainty and rival explanations

Every measured difference has an uncertainty, made up of instrument accuracy, installation effects, sampling interval and the natural variation in demand. If a claimed improvement is smaller than the combined uncertainty, the honest conclusion is “no measurable change”, not “a small saving”. Where the result matters commercially, it is worth investing in tighter instruments or a longer logging period before the decision rather than after it.

Then list rival explanations and test each one: a production drop, a closed branch, a new leak, a rented machine taken off line, a dryer bypassed, a controller reset. A comparison that cannot rule out obvious rivals should be reported with that limitation in plain words.

Reporting the result honestly

A good comparison report states the claim, the boundary, the baseline and after periods, the instruments, every adjustment and its basis, the result with its uncertainty, and the rival explanations considered. It also says plainly what the result does not show, for example that a station-level saving says nothing about air quality at the users.

How Reddy Compressors helps

We carry out before-and-after measurement as part of air audits and improvement work for plants across Telangana and Andhra Pradesh, following the ISO 11011 assessment method. We log pressure at the station and at critical users, delivered flow, power on each compressor and dew point where air quality is part of the claim. We then report the boundary, instruments, conditions, adjustments and remaining uncertainty, so your management can rely on the result.

As an independent multi-brand specialist we work on ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other makes, and we are not tied to selling any one brand’s replacement machine. If the measured result shows that a change did not help, we say so and look for the reason. Our office is at Tarnaka, Secunderabad; call +91 99590 60718 or +91 92468 88812.

What to send us

• A description of the change you made or plan to make, and the date
• The claim you want to test, in one sentence
• A sketch or list of compressors, dryers, receivers and main branches
• Any existing data: energy bills, controller hour counters, pressure logs, production records
• Shift pattern and any known changes in production between periods

Frequently asked questions

How do I prove a compressed-air energy saving?

Measure electrical power and delivered flow at the same boundary for a representative baseline period, make one defined change, then measure again for a comparable period with the same instruments. Compare specific power in kW per m³/min rather than raw kWh, adjust openly for documented differences in production, hours and pressure, and state the uncertainty. Bills alone rarely prove a saving.

How long should a baseline measurement period be?

Long enough to capture the plant’s normal pattern of shifts, weekends and production swings. For most factories that means at least one to two full working weeks of continuous logging. Seasonal plants or batch processes may need longer, or separate baselines for each operating mode. A single day, or the worst week of the year, gives a misleading baseline.

Why is my electricity bill lower but my compressor no more efficient?

Energy use falls whenever demand falls, for example through lower production, fewer shifts, a stopped line or cooler weather. None of these means the compressor station has become more efficient. Specific power, which divides input power by delivered air, shows whether efficiency itself changed. If specific power is unchanged, the saving came from lower demand, not the intervention.

Can I compare a summer baseline with a winter result?

Only with care. High ambient temperature raises inlet and cooling-air temperature, reduces dryer capacity and changes compressor loading. In Telangana and Andhra Pradesh the difference between May and December conditions is large. Where seasons differ, record ambient temperature in both periods, compare like-for-like operating hours where possible and state the seasonal limitation clearly in the result.

What if the improvement is smaller than the measurement uncertainty?

Then the honest result is that no measurable change was detected within the accuracy of the method. It does not prove the change failed, but it does not prove success either. Options include extending the logging period, using more accurate instruments at a tighter boundary, or accepting the result and recording it as inconclusive rather than claiming a saving.

Does pressure reduction count as part of the saving?

It can, if it was a deliberate part of the intervention and the lower pressure still meets every critical user. Typically about 7% more power is needed for each extra bar on an oil-injected screw, so pressure changes have a large effect. Report the pressure change explicitly and do not attribute its effect to a different part of the work.

Related pages

• Compressed air project evidence centre
• Instrument calibration and measurement evidence
• Specific-power assessment
• Power and flow logging
• Measurement uncertainty worksheet
• Measurement, verification and energy performance