Air demand duration curve tool

What a duration curve shows that a trend chart does not

A normal trend chart plots flow against time. It answers when demand changed: the morning start-up, the lunch dip, the night-shift low. It is good for spotting events, but poor at answering the sizing question, which is how often each level of demand actually occurs.

A duration curve takes the same data, throws away the order of events and sorts every reading from highest to lowest. Each point keeps the time it represents. The result is a falling line: at the left, the few hours of highest demand; at the right, the long hours of low demand. Reading across at any flow tells you how many hours the plant needed at least that much air.

This is the same idea power engineers use for electrical load-duration curves, applied to compressed air. It is one of the most useful pictures you can draw before buying, replacing or renting a compressor.

Which decisions it supports

The curve cannot tell you how fast demand rises, what sequence events followed, or how long pressure took to recover after a trip. Those answers stay in the original time-series data and the pressure trend diagnostic workbook.

What data you need

The input is a log of flow readings with timestamps. Optional fields make the result more useful.

Log for long enough to capture the real operating pattern. One full production week is a sensible minimum for most plants, including a weekend or shutdown day. Seasonal plants, such as those processing harvest crops in Andhra Pradesh, may need logging in both peak and off-peak months.

How the curve is built

1. Import the log and check each row: valid timestamp, no duplicates, no impossible negative flow.
2. Convert all flows to one unit and one reference basis.
3. Give each reading a time weight. For regular data this is the sampling interval; for irregular data it is the time to the next reading, capped at a maximum allowed gap.
4. Sort the readings by flow from highest to lowest.
5. Accumulate hours: for each flow value, cumulative hours = sum of weights of all readings at or above it.
6. Express as exceedance: exceedance % = cumulative hours ÷ total valid hours × 100.
7. Report the summary levels (E1, E10, E50, E90, maximum and minimum) and the data coverage.

The key rule is that the curve represents time, not the number of rows. A one-minute reading and a fifteen-minute reading must not count equally.

Missing data rules

Real logs have gaps: a meter that lost power, a controller that stopped communicating, a weekend when nobody restarted the logger. If a gap is silently filled with the last value, or counted as zero, the curve becomes misleading. The tool applies clear rules instead.

Never interpolate across a known start-up, shift change, compressor trip or major process change. Those are exactly the moments when demand behaves differently.

Coverage is reported as: coverage % = valid observed hours ÷ scheduled hours × 100. If you allow imputation, an effective coverage figure is shown alongside, never in place of, the raw figure. As a practical guide, treat a curve with low coverage as indicative only, and log again before a major purchase.

Worked example

A plant logs flow every minute for one week, 168 scheduled hours. Eight hours are lost when the logger battery fails, leaving 160 valid hours, a coverage of about 95%.

The installed plant is a single 20 m³/min (about 700 cfm) fixed-speed compressor. The curve shows it runs below 11 m³/min half the time, so it spends many hours unloaded. A more efficient arrangement might be a base machine sized near the E90 level with a variable-speed trim unit sized so that base plus trim covers the peak demand, and receiver storage kept for short transients. The receiver transient storage calculator checks short transients only, lasting seconds to minutes. The 16 hours a week above E10 and the 1.6 hours above E1 are far too long for storage: even 1 m³/min of excess for 1.6 hours is about 96 m³ of free air, which would need a receiver of roughly 97 m³ at a 1 bar usable pressure fall. Those hours need trim capacity, demand management or a rental machine. These numbers are illustrative; your own curve decides the answer.

Interpreting the curve without fooling yourself

• Check the flow basis. A curve in actual m³/min at line pressure cannot be compared with a compressor’s FAD rating.
• Look for a step at the right-hand end. A flat floor of demand when production is stopped is usually leakage. Measure it and see the leakage cost scenario calculator.
• Do not size to the maximum. A single one-minute peak may come from a blow-down or a tool left open. Ask whether it is a real requirement.
• Separate pressure bands if part of the plant needs higher pressure. One curve for the whole site may hide a small high-pressure user that sets the pressure for everyone.
• Remember future demand. Use the plant expansion scenario planner to add planned lines rather than adding a vague safety margin.
• Indian summers matter. Compressor FAD falls in high ambient heat; check that the base machine still covers E90 at 40–45 °C inlet temperature.

How Reddy Compressors helps

A duration curve is only as good as the data behind it. Reddy Compressors carries out demand logging and audits for plants across Telangana and Andhra Pradesh from our office at Tarnaka, Secunderabad, and supplies compressors, rentals and spares across India. We are an independent multi-brand specialist, selling new and used machines and servicing ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other brands, without being an authorised dealer of any.

We can:

• Log flow, pressure and power over a representative period.
• Build and interpret the duration curve with you, including the gap report.
• Propose base and trim arrangements, storage and control changes based on the curve.
• Supply new or used compressors, or rental machines from our own 50–2,000 cfm fleet to cover peaks and trials, through our rental service.

What to send us

• Your flow log as a spreadsheet or CSV, with timestamps and units.
• The source of the data and the meter location.
• Compressor make, model, nameplate photo and control type for each machine.
• Shift pattern, shutdown days and any known gaps in the log.
• Planned expansions or new air users.

Frequently asked questions

What is an air demand duration curve?

It is a chart of compressed-air flow readings sorted from highest to lowest, with the horizontal axis showing hours or percentage of time. Any point on the line tells you how long the plant needed at least that flow. It shows at a glance how much of the year is spent at base load, how much at medium demand and how rare the peaks really are, which is exactly what compressor sizing needs.

How do I size a compressor from demand data?

Build a duration curve from at least a week of logged flow, then read the demand exceeded most of the time, such as E90, for the base compressor, and the range up to E10 for a trim or variable-speed machine. Short peaks above that are often better handled by receiver storage than by a bigger compressor. Always check FAD at summer ambient temperature and allow for planned expansion.

What do E10, E50 and E90 mean?

They are exceedance levels. E10 is the flow exceeded for 10% of the valid time, E50 is the median flow exceeded half the time, and E90 is the flow exceeded 90% of the time. E90 indicates steady base demand, E50 typical running demand and E10 the upper working range. They are more reliable for sizing than the single maximum reading, which may be a brief, unusual event.

How long should I log compressed-air demand?

For most plants, one full production week including a weekend or shutdown day is a sensible minimum, because it captures every shift and the leakage floor. Seasonal operations need logging in both busy and quiet periods. Use one-minute or shorter intervals where possible. Longer logging improves confidence but only if the data is checked regularly for gaps and meter faults.

How should missing data be handled in a duration curve?

Missing periods should be reported, not hidden. Very short gaps may be filled by a method you choose, with the filled hours shown separately. Long gaps should stay missing and be excluded from the curve. Zero readings during a sensor fault must be treated as missing rather than as zero demand. The tool reports raw coverage so you can judge how far to trust the result.

Can a duration curve tell me how much rental capacity I need?

Yes, for temporary capacity. Draw a line at the capacity of your permanent compressors at site conditions. The hours above that line, and the flow at the top of the curve, show how much extra air is needed and for how long. That helps decide between storage, a rental compressor for peak periods, or a permanent addition. We can then propose a suitable machine from our rental fleet.

Related pages

• Air demand profiling
• Power and flow logging
• Base and trim control strategy
• Compressed air demand calculator
• Compressed air knowledge and calculation centre