Compressed-air demand assessment and load profiling

Why the compressor should follow the demand

Many compressors in Indian plants are sized by one of four shortcuts: adding up the air consumption figures of every tool, matching the motor rating of the old machine, reading a single pressure gauge, or simply buying “one size bigger to be safe”. Each figure contains some useful information, but none of them is the plant’s real demand.

Oversizing is expensive. A compressor much larger than the load spends long periods running unloaded, which still draws a large share of full-load power. Undersizing is worse, because production stops. Both mistakes come from the same root: nobody measured how the air is actually used. What a plant really needs is the right air system, and the right system starts with the demand.

A good assessment separates:

• Connected load (everything that could use air) from simultaneous load (what really runs together).
• Average demand from short peaks.
• Productive use from leakage and misuse.
• Present demand from planned future expansion.

Define what you are measuring

Decide the boundary before collecting a single number. A whole factory, one production block, one compressor room or one machine line can each have a different demand profile. Draw a simple sketch showing the compressors, receivers, dryers, filters, main headers, cross-connections and the major users, and mark where flow and pressure will be measured.

A measurement without a clear boundary can be numerically correct and still answer the wrong question. For example, flow measured at a compressor outlet includes desiccant dryer purge and drain losses; flow measured after the dryer does not.

Build a register of air users

Walk the plant and list every significant user. For each, record:

• What it does and where it is.
• The pressure it genuinely needs at the point of use.
• Its air consumption, and where that figure came from (manufacturer data, measurement or estimate).
• When it runs: continuously, per cycle, per shift, occasionally.
• How critical it is if air pressure falls.
• Whether it will change in the next few years.

Small users such as blow guns can be grouped, provided the basis is written down. The register also reveals the highest-pressure user, which usually decides the plant pressure, and the users that could be served from a smaller dedicated system.

Connected load is not simultaneous demand

The arithmetic sum of all consumers is an installed-load figure. It becomes a design figure only if everything really operates together, which almost never happens. Ten grinders on a shop floor rarely run at the same moment. A packaging line’s cylinders cycle in sequence, not all at once.

Simultaneity factors can be estimated from experience, but they should be checked against measurement. An estimated factor that cannot be checked is an assumption, and should be recorded as one. Measurement on a running plant always beats arithmetic on paper.

How to measure demand

There are three practical ways to measure demand on a working plant, each with its own accuracy and cost.

For sizing, a logging period of at least one full production week is usually needed, including a weekend, a holiday or a shutdown. That captures the full working pattern, the night shift and the non-production demand, which is very often mostly leakage. Our power and flow logging page explains the instruments and set-up.

Always record pressure alongside flow. The same flow at a different pressure is a different duty for the compressor. And record what production was doing at the time: shift, line running, product being made. A load profile without production context is hard to interpret later.

Reading a load profile

Once data is collected, look for these features:

• Base demand: the steady flow present for most of the working day. This is the load for the base compressor.
• Variation: the swing above base demand. This is what a trim or VSD machine must follow.
• Short peaks: events lasting seconds or minutes. These are often better met by storage than by extra compressor capacity.
• Non-production demand: flow during breaks, nights and holidays when nothing should be running. Most of this is leakage.
• Pressure dips: times when pressure falls below the minimum users need. Check whether they come from compressor capacity, response time, or pressure drop in the treatment and piping.

A demand duration curve, which sorts all logged readings from highest to lowest, is a useful way to see how many hours the plant spends at each level of demand. It shows clearly whether a large compressor will spend most of its life at part load.

Correct the numbers before sizing

Raw measured flow must be put on a consistent basis before choosing a compressor. Compressor capacity is normally stated as free air delivery (FAD) at reference inlet conditions, and a flow meter may report in different units or reference conditions. Our airflow, FAD and reference conditions page explains how to reconcile them.

Then adjust the measured demand for known changes:

1. Subtract leakage you intend to repair, but only after a credible repair plan, not as a hope.
2. Subtract artificial demand caused by running higher pressure than needed, and misuse such as open blowing.
3. Add dryer purge and drain losses if measurement was taken downstream of them.
4. Add confirmed future loads with their expected start dates.
5. Consider the Indian summer: high ambient temperatures and dust reduce what a compressor actually delivers, so allow margin based on the site, not a blanket percentage.
6. Decide standby needs: can the plant meet its peak with the largest compressor out of service?

Worked example: sizing from measurement

A plant asks for a replacement for its old 37 kW compressor and the purchase team proposes a 45 kW machine “for growth”. Before deciding, the plant logs one week of compressor power and header pressure.

The data shows a base demand that one modest machine could carry for most of the day shift, short peaks during line start-up, and a steady flow on Sunday when nothing was running. That Sunday flow is leakage. It means part of the “demand” the old machine was carrying was air escaping from the network.

The assessment leads to a different answer: repair the major leaks, add receiver capacity near the line that causes the start-up peak, and choose a compressor sized to the corrected base demand, with a VSD unit to follow variation if the swing is large. The confirmed future line is added on its real timetable rather than as a vague margin. The figures for each step come from the plant’s own log, not from a rule of thumb.

Common mistakes in demand assessment

• Measuring for a single day, missing the weekly pattern.
• Measuring flow without pressure, or pressure without flow.
• Treating leakage as genuine demand and buying capacity to supply it.
• Using the old compressor’s rating as a measure of demand.
• Adding a safety margin at every step, so the final size is far too large.
• Forgetting peaks caused by start-up, batch processes or cleaning cycles.
• Ignoring the effect of high ambient temperature on real compressor output.

How Reddy Compressors helps

Reddy Compressors, based at Tarnaka in Secunderabad and working with industrial compressed air since 2002, carries out demand assessments before recommending a compressor, rather than simply quoting the size requested. As an independent multi-brand specialist we can compare options from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and others, new or used, without being tied to one maker. We are not an authorised dealer of any brand. We have tie-ups with all major and reputed brands.

We can survey your users, log power, pressure and machine states, interpret the profile and advise on compressor size, storage, controls and leakage repair. If you need air while a decision is made or during a changeover, we can provide a rental compressor from our own fleet, with ranges from 50 to 2,000 cfm and 2 to 30 bar (about 30–435 psig). Our service teams cover the whole of Telangana and Andhra Pradesh, and we supply equipment and spares across India.

What to send us

• Nameplate photos of your existing compressors and dryers.
• A list of major air users with the pressure each needs.
• Your shift pattern and working days.
• Current pressure settings and any pressure complaints.
• Monthly electricity readings for the compressors, if separately metered.
• Planned expansion with approximate dates.

Frequently asked questions

How do I calculate how much compressed air my factory needs?

Start with a register of every air user, its pressure need and its consumption, then apply realistic simultaneity rather than adding everything together. The more reliable method is to measure: log header flow or compressor power and states for at least a week of normal production. Add confirmed future loads and dryer purge, subtract leaks you will genuinely repair, and correct the result to free air delivery.

Can I size a new compressor from my old compressor’s kW rating?

Not reliably. The old machine may have been oversized, may have spent much of its time unloaded, or may have been supplying significant leakage. Its kW rating tells you what was installed, not what the plant uses. Logging the old machine’s power and loaded hours for a week is a far better guide and often shows that a different size, or a VSD, is the better choice.

How long should compressed-air demand be logged?

For sizing decisions, at least one complete production week, including a weekend or holiday, is the usual minimum. This captures shift changes, start-up peaks, night running and non-production demand. Plants with monthly batch cycles or strong seasonal patterns may need longer logging, or a second logging period at a different time of year, to see the full range of demand.

How can I tell how much of my air demand is leakage?

Measure the flow, or the compressor’s loaded time, when production is completely stopped but the system is still pressurised, such as on a Sunday or during a break. Almost all the demand at that time is leakage, plus any users left running. A pump-up or draw-down test on a known receiver volume is a quick alternative that gives a useful snapshot.

What is the difference between cfm and FAD?

cfm is simply a unit of flow, cubic feet per minute, and can be quoted at different conditions. Free air delivery (FAD) is the compressor’s actual delivered output, expressed at its inlet conditions under a defined test standard. When comparing compressors or matching them to measured demand, make sure all figures are on the same basis, preferably FAD, otherwise the comparison can be misleading.

Should I add a safety margin when sizing a compressor?

Some margin is sensible, but it should be decided once, based on known factors such as site ambient conditions, confirmed future loads and standby requirements. Adding a separate margin at every step of the calculation is the commonest cause of oversizing. Storage and a VSD trim machine often handle peaks better than simply buying a larger compressor.

Related pages

• Compressor capacity selection and sizing
• Airflow, FAD and reference conditions
• Demand management and load profiling
• Air demand profiling
• Leak detection
• Working pressure and pressure budget