Air Compressor Sizing Guide for ASEAN Manufacturing Shops: FAD, Pressure and the Numbers Chinese Quotes Leave Out

Size an ASEAN shop's air compressor from **FAD (Free Air Delivery) measured to ISO 1217 at your working pressure** — never from the "air displacement" figure most Chinese datasheets print, which is theoretical and can leave you well short. Then set pressure at the lowest your worst tool tolerates, cover demand spikes with receiver storage instead of a bigger machine, and have the dryer re-selected for a 35–40 °C plant room rather than the 25 °C ambient its nameplate assumes.

Size From FAD, Not From "Displacement"

Free Air Delivery is air actually delivered at the outlet, corrected back to inlet conditions and measured to ISO 1217 — Annex C for fixed-speed packages, Annex E for variable-speed. "Air displacement" is the theoretical swept volume of the rotors or pistons, assuming 100% volumetric efficiency and ignoring internal leakage, heat and intake restriction. Trade sources put the gap at 20–40%; treat that band as an industry estimate rather than a controlled measurement, but the direction is certain — displacement always flatters the machine.

Here is what that looks like inside a real Chinese-supplier catalogue. We audited all **798 published products** in this site's database on 24 August 2026, including the 18 compressed-air listings:

• **0** cite ISO 1217 anywhere in their specifications
• **0** state FAD or "Free Air Delivery"
• **2** publish a figure explicitly labelled *"Air displacement"* — the [WITKOBOUT PM VSD screw](/en/products/witkobout-pm-vsd-screw) at 1.1–13.3 m³/min (7.5–75 kW) and the [two-stage screw](/en/products/witkobout-two-stage-screw) at 4.3–19.1 m³/min (22–90 kW)
• the [LONEWOLF piston range](/en/products/taizhou-lonewolf-machinery-lonewolf-belt-driven-air-compressor) tabulates its whole model line under a column header reading **DISPL — L/MIN**
• **0** state specific power, pressure dew point, an ISO 8573-1 class, or an ambient rating

Across the whole 798-product catalogue, "ISO 1217" appears **zero** times and "ISO 8573" appears **zero** times. This is not one careless supplier — it is the default unit of measure on the Chinese side of the trade, which means the m³/min figures in your comparison spreadsheet were, in most cases, never measured. They were calculated. Ask every bidder for the ISO 1217 datasheet with the annex named, then re-run the comparison on what comes back.

The Three Numbers That Decide the Bill Are Not in the Quote

A quote gives you motor kW, m³/min and MPa. Your electricity bill is set by three different numbers.

**Specific power** — kW consumed per m³/min delivered, at *your* pressure. The only honest efficiency metric, and the reason one 37 kW machine beats another 37 kW machine.

**Part-load and unloaded power.** A fixed-speed compressor running unloaded still draws roughly 30–70% of full-load power, per compressor-industry guidance. If your demand swings, that idle draw is the entire argument for variable speed. If demand is genuinely flat, a correctly sized fixed-speed unit is fine — VSD carries a reported 3–6% efficiency penalty at full load.

**Pressure.** The industry rule of thumb: each extra 1 bar of discharge pressure costs about 6–7% more shaft power. Most shops specify 8 bar because the datasheet says 8 bar, not because any tool needs it.

Compressed air is roughly **10% of a typical industrial facility's electricity** (US DOE survey), and above 30% in some plants — while only 10–20% of the energy put in reaches the point of use. The rest leaves as heat.

Worked example — arithmetic from stated assumptions, not a measured case study. A 37 kW compressor running 6,000 h/year at 70% average load draws 155,400 kWh. At 4.50 THB/kWh (inside the 4.10–5.50 blended band Thai factories currently pay) that is about **699,000 THB per year**. The US DOE puts leakage in a plant without a recent survey at 20–30% of output; at 25% that is roughly **175,000 THB a year escaping through holes**. Dropping the setpoint from 8 to 7 bar saves about **49,000 THB a year**. Neither is a purchasing decision, and both dwarf the price gap you are negotiating over.

A Sizing Method That Survives Contact With the Floor

1. **List every air consumer** with its FAD demand and realistic duty cycle — not its peak. 2. **Apply a simultaneity factor.** Summing peaks is how shops end up two frame sizes too large. 3. **Do not pad heavily.** An oversized fixed-speed unit idles for hours a day, burning electricity to do nothing. 4. **Cover bursts with storage, not kW.** A larger [air receiver](/en/products/witkobout-air-tank) absorbs short spikes far more cheaply than the next frame size up. Sandblasting, large cylinders and pulse-jet filter cleaning are storage problems, not compressor problems. 5. **Size for the system you will have after a leak survey**, then commission with one. 6. **Add headroom only if you can name it** — a specific machine on a specific line, not a vague 30%.

Air Quality: ISO 8573-1 Is the Only Spec That Travels

Say **[2:4:2]** or **[1:4:1]**, not "oil-free". ISO 8573-1 classes particles, water and oil separately, so a class triplet is unambiguous across languages and legal systems. Class 1 for oil is commonly quoted at ≤0.01 mg/m³ total oil — but published guidance on the exact food-contact triplet **conflicts between sources**, and SQF, BRC and IFS each reference the standard differently. Confirm the triplet with your own scheme auditor, not your compressor vendor.

Then match the treatment to the class:

• **Refrigerated dryer** → roughly +3 °C pressure dew point; adequate for general shop air.
• **Desiccant dryer** ([heated-purge type](/en/products/shandong-xinchen-gas-equipment-lugas-externally-heated-purge-desiccant-air-dryer)) → −40 °C class dew points, for outdoor pipework, instrument air, and anything that cannot tolerate condensation.
• **Staged filtration** ([grade-rated filters](/en/products/shandong-xinchen-gas-equipment-lugas-filter)) → particulate and coalescing stages, in the order the manufacturer specifies.

An [oil-free compressor](/en/products/witkobout-oil-free-piston) removes lubricant risk at the source but does not by itself deliver a class — ambient air arrives with its own oil vapour and particulate.

The Tropical Derate Nobody Quotes

ISO 7183 rates compressed-air dryers at 35 °C inlet air, saturated, 7 barg, in a **25 °C ambient**. A compressor room in Rayong, Bekasi or Johor is not 25 °C. Above the reference condition both compressor and dryer lose capacity and the dew point drifts upward — which is when water starts arriving at pneumatic valves and paint lines.

Our own catalogue carries the tell: the refrigerated dryer listing is titled ["High Inlet Temperature"](/en/products/shandong-xinchen-gas-equipment-lugas-high-inlet-temperature-air-cooling-refrigerated-air-dryer) — an admission that the standard model is not for that duty. Yet **0 of the 18** listings state an ambient rating. We could not verify a published derate factor for 45 °C ambient from any authoritative source, so do not accept one from a salesperson either. Demand three things in writing:

• The manufacturer's **capacity correction table** for inlet and ambient temperature, with your selection re-run at your real plant-room temperature.
• **Compressor-room ventilation design**, including heat rejection in kW.
• **Condensate handling** — tropical humidity produces far more condensate than temperate design assumptions, and oil-contaminated condensate is a discharge-compliance issue in Thailand, Vietnam and Malaysia alike.

Conclusion

The compressor decision is not "how many kW". It is **which measurement standard, at which pressure, in which ambient, at which air-quality class** — and all four belong in the purchase order, alongside the certificate evidence in our [certifications guide](/en/news/quality-certifications-chinese-equipment) and [supplier audit checklist](/en/news/verify-chinese-equipment-supplier-audit). Browse our [air compressor range](/en/products?category=gc-air-compressor) and [WITKOBOUT screw compressors](/en/brands/witkobout), see how [China sourcing](/en/china-sourcing) works, or [talk to our team](/en/contact) about a specification review before you release the PO.