Compressed Air Energy Audit

What you need before you start

• An inventory of every compressor: tag, rated kW, control type (fixed-speed, VSD, modulating), unloaded kW if known.
• Power (kW) logging on each compressor at 1–15 minute intervals for at least two weeks — ideally a full production cycle.
• The site electricity tariff (£/kWh) and grid carbon intensity (kgCO₂e/kWh) so findings can be monetised.
• Production schedule and shift hours so out-of-hours running can be detected reliably.

Step 1 — Establish the baseline

Plot total kW for the whole compressor house across the observation window. Identify the overnight / no-demand baseline (the floor the system never drops below). Anything drawn during zero-production hours is, by definition, not making useful air — it is leakage plus unloaded running.

Step 2 — Classify every interval

For each compressor and timestamp, label the state as off, unloaded, or loaded using power thresholds (e.g. unloaded ≈ 25–35% of rated kW for a fixed-speed screw). This lets you compute run-hours, unloaded fraction, start/stop counts, and overlap between machines.

Step 3 — Quantify leaks

Annualise the overnight baseline kW to 8,760 hours. Above a modest threshold (typically 5–10% of average loaded demand), the surplus is attributable to leaks. A tag-and-fix programme on a site drawing 12 kW of leakage saves on the order of ~52,500 kWh/yr — at £0.25/kWh, ~£13,000 and ~11 tCO₂e per year.

Step 4 — Check sequencing and unloaded running

When more than one compressor runs at the same time, a healthy system keeps one fully loaded as the base unit and trims with a VSD. If overlap intervals routinely show no fully-loaded unit, you have a sequencing problem — usually solved with a master controller. Excessive unloaded running on a fixed-speed unit is a strong indicator that a VSD retrofit or better sequencing will pay back fast.

Step 5 — Catch out-of-hours running and short cycling

Cross-reference compressor state with the shift schedule. Any run-time outside production is a candidate for a timer or isolation valve. Compressors starting and stopping more than ~6 times per hour are short-cycling — symptomatic of an undersized receiver or wrong pressure-band setup, and a cause of motor wear and transient energy losses.

Step 6 — Monetise and rank

Convert each kWh saving to £ at the site tariff and to tCO₂e at the grid carbon factor, then rank findings by annual £ saving. That ranked list — with rationale, threshold used, and an indicative payback — is the deliverable an engineering team can act on.

The ruleset, in one line each

• Leak burden — overnight baseline above threshold = leakage.
• Out-of-hours running — run-time outside production hours.
• Excessive unloaded — fixed-speed unit unloaded too much of its run-time.
• Poor sequencing — overlap intervals with no fully loaded base unit.
• Short cycling — starts/hour above threshold.