How to read a compressor nameplate

What a nameplate is, and what it is not

A nameplate is the manufacturer’s statement about one assembly at the time it was built. It is the best starting point for identifying a machine, ordering parts and checking whether it suits a duty. It is not a performance test, a parts catalogue, a service certificate or proof that the machine is fit for a new application.

Photographs from Reddy Compressors’ own fleet and workshop.

A typical industrial compressor package carries several plates: one for the package, one for the airend, one for the motor or engine, one for each pressure vessel, and often one for the dryer and the electrical panel. Each describes only the item it is fixed to. A motor plate says nothing about the airend, and a receiver plate says nothing about the compressor’s rated pressure.

The package nameplate, field by field

Reading flow figures correctly

Flow on a nameplate is normally free air delivery, measured to a standard such as ISO 1217 and converted to the conditions at the compressor inlet. It is stated at one pressure. The same machine at a higher pressure delivers less, and at lower pressure may deliver a little more, depending on its design and control.

Flow may be shown in m³/min, l/s or cfm. Useful conversions are 1 m³/min ≈ 35.3 cfm and 1 l/s ≈ 2.12 cfm. A plate that shows 10 m³/min is therefore about 353 cfm. Do not confuse free air delivery with “displacement” or “piston displacement”, sometimes quoted for reciprocating compressors; displacement is the swept volume and is always higher than the air actually delivered.

The nameplate figure is the rated value when new. A worn airend, a blocked inlet filter, a slipping belt or a sticking inlet valve will reduce real flow. If you need to know today’s flow, measure it.

Reading pressure figures correctly

Pressure markings cause more mistakes than any other field. Keep these distinctions clear:

• Maximum working pressure of the package: the highest pressure the compressor may run at. Typical industrial values are 7.5, 8.5, 10 or 13 bar (about 110, 125, 145 or 190 psig).
• Set or unload pressure on the controller: what the machine is actually set to. It should sit below the maximum working pressure.
• Safety valve set pressure: the pressure at which the relief valve lifts, set above working pressure to protect the vessel and package.
• Vessel design pressure: shown on the receiver or separator tank plate. It relates to the vessel, not to the compressor’s rating.
• Gauge versus absolute: bar(g) or psig is above atmosphere; bar(a) includes atmospheric pressure. At sea level, 7 bar(g) is about 8 bar(a).

Remember that 1 bar = 14.5 psi. Raising set pressure costs about 7% more power for each extra bar on a typical screw compressor, so a plate rated at 10 bar does not mean the plant should run at 10 bar.

The motor and engine plates

The main motor plate gives rated power in kW, voltage, frequency, full-load current, speed, frame size, efficiency class where marked, insulation class, protection rating (IP) and sometimes a service factor. Use it to check the supply, the starter and cable sizing, and to order a replacement motor or bearings.

A motor’s rated kW is its output at the shaft. The electrical input of the whole package, including fan motors and losses, is higher, and that input is what counts when calculating specific power in kW per m³/min. On portable diesel compressors, the engine plate is separate and gives the engine model and serial, which are needed for engine parts and service.

The pressure vessel plate

Air receivers and oil separator tanks carry their own plate, usually showing the vessel maker, serial, year, design pressure, design temperature range, volume and test pressure. These values matter for safety inspection and for any decision to raise system pressure. The compressor must never be set to a pressure that the vessel or its safety valve is not rated for. Statutory inspection requirements for pressure vessels follow the applicable rules and the site’s inspection authority.

Dryer and auxiliary plates

Refrigerated dryer plates show the flow rating at stated reference conditions, the refrigerant type and charge, and the electrical supply. The rated flow assumes particular inlet temperature, ambient temperature and pressure; in hot Indian summers with high inlet temperature, the real capacity is often lower, so correction factors from the dryer maker should be applied. Desiccant dryers and filters also carry ratings that apply only at stated pressure and flow.

How to photograph and record a nameplate

1. Stop and isolate the machine before opening canopy doors.
2. Photograph the whole package and its location first.
3. Photograph each plate square on, in good light, avoiding glare.
4. Do not scrape or polish the plate; take a second angled photo if stamping is faint.
5. Transcribe each field exactly, marking any unclear character.
6. Record running hours, controller set pressure and safety valve setting alongside the plate data.
7. Store the photos with the date and the asset tag.

Common misreadings to avoid

• Taking the receiver’s design pressure as the compressor’s maximum working pressure.
• Assuming the nameplate flow is delivered at the plant’s actual pressure.
• Using motor kW as total power input when comparing efficiency.
• Ignoring a 60 Hz rating on an imported or used machine.
• Reading the model as the serial, or the reverse.
• Treating the plate as proof that original components are still fitted.

When the nameplate is missing or unreadable

Plates are often lost during repainting, damaged by heat or corrosion, or removed when a used machine changes hands. Identity and ratings can usually be rebuilt from other evidence: the airend plate, the motor plate, the controller’s service menu, the safety valve setting, the separator and filter part numbers, and the frame layout. Each rebuilt value should be marked as probable until confirmed, for example by a part that fits or by a measured pressure and flow.

Never assume a pressure rating for a machine without a plate. Set it conservatively until the airend, motor and vessels have been identified, and fit a new identification label once the evidence is clear, recording how each value was established.

Abbreviations you will find on compressor and separator nameplates

Plates are small, so makers abbreviate heavily. Imported machines also mix European, American and Asian conventions. The table below lists the abbreviations seen most often on packages, motors and pressure vessels, with the meaning to record.

The oil separator vessel plate, field by field

On an oil-injected screw compressor, the separator tank holds hot air and oil at full discharge pressure. In most packages it is the largest pressure-containing part, so its plate deserves as much attention as the package plate. Separator plates are often hidden behind panels or painted over, which is a common reason the vessel is never inspected.

Two checks matter most. First, the safety valve on the separator must be set at or below the vessel’s design pressure. Second, the compressor’s unloading pressure must sit comfortably below the safety valve setting, typically by at least 0.5–1 bar. If a machine has been “uprated” by raising its set pressure, these are the first figures to check. Periodic thickness and hydraulic testing follows the state factories rules and the user’s own inspection scheme, where applicable. Corrosion from condensed water in the oil is the usual cause of wall loss in Indian conditions. For vessel inspection and replacement, see compressor pressure vessels: separator and receiver.

Worked example: turning a screw compressor plate into a usable record

The values below are illustrative, typical of an oil-injected screw compressor of this size. The aim is to show how each raw field becomes a derived value that a buyer, planner or service engineer can use without reading the plate again.

Package plate reads: FAD 6.2 m³/min at 7.5 bar(g); maximum working pressure 8.0 bar(g); motor 37 kW; 415 V, 3-phase, 50 Hz; FLA 64 A. Motor plate reads: 37 kW, 1,480 rpm, IE3, η 93.9%, cos φ 0.86, Ins. class F, IP55, S1. Separator plate reads: PS 14 bar, PT 20 bar, TS −10/+120 °C, V 40 litres.

1. Flow in cfm: 6.2 × 35.315 = 218.95, so about 219 cfm FAD at 7.5 bar(g).
2. Maximum pressure in psig: 8.0 × 14.5038 = 116.0 psig. The rating point is 7.5 × 14.5038 = 108.8 psig.
3. Maximum pressure in absolute terms: 8.0 + 1.013 = 9.013 bar(a), or about 9.0 bar(a) at sea level.
4. Motor power in hp: 37 × 1.341 = 49.6 hp. The usual trade description is “50 hp”.
5. Full-load current check: for a three-phase motor, I = P ÷ (√3 × V × η × cos φ). So I = 37,000 ÷ (1.732 × 415 × 0.939 × 0.86) = 37,000 ÷ 580.4 = 63.7 A. This agrees with the plate’s 64 A. A large mismatch suggests a rewound or replaced motor, or a plate from another machine.
6. Electrical input at full load (motor only): 37 ÷ 0.939 = 39.4 kW. The package input is higher once fan motors and drive losses are added.
7. Indicative specific power (motor shaft basis): 37 ÷ 6.2 = 5.97 kW per m³/min. This is a rough screening figure, not a measured package value.
8. Vessel cross-check: 14 bar PS is above the 8.0 bar(g) maximum. The safety valve should be set between the compressor’s unload pressure and 14 bar, typically about 9–10 bar on an 8 bar machine. The PS × V product is 14 × 40 = 560 bar·litre.
9. Test ratio: 20 ÷ 14 = 1.43 times design pressure, consistent with a normal hydraulic test.

The resulting record line reads: “37 kW (49.6 hp) oil-injected screw, 6.2 m³/min (219 cfm) at 7.5 bar(g), max 8.0 bar(g) (116 psig), 415 V/3-ph/50 Hz, 64 A, separator PS 14 bar, 40 litres.” Add the airend and motor serial numbers and running hours, and the record is ready for spares selection through the compressor service kit finder or service kits.

What the nameplate implies about the operating envelope

A nameplate states a rating point and some limits. The operating envelope is the full set of conditions in which the machine can deliver that rating safely. Some limits are printed on the plate. Others are implied and stated in the maker’s manual. Record both before a machine is moved to a new site or new duty.

A full method for building this envelope from the plate, manual and site data is set out on the compressor operating envelope interpretation page.

Reading a nameplate when buying a used compressor

On a used machine, the nameplate is evidence to be cross-checked, not accepted. The useful question is whether every plate, the controller and the physical machine tell the same story.

1. Serial consistency. The package, airend, motor and separator serials should be plausible for the same build period. A newer airend or motor is not a problem in itself, but it should be explained by a service record.
2. Year against hours. Divide the running hours by the years in service. For example, a 2019 plate with 9,800 hours in 2026 gives 9,800 ÷ 7 = 1,400 hours a year, which is plausible for light single-shift use. A three-shift plant would log about 6,000–8,000 hours a year. A figure far below the site’s pattern suggests a replaced controller or hour meter.
3. Loaded and total hours. If the controller shows both, a loaded share far below about 50% suggests oversizing or long unloaded running. That affects wear on the motor and bearings, not just the airend.
4. Rating against motor. FAD per kW should be in line with the rest of the range. A 37 kW plate claiming 8 m³/min at 7.5 bar(g) deserves suspicion.
5. Plate integrity. Look for re-stamped, painted-over or newly riveted plates, and for frequency mismatches on imported units.

Where the serial cannot be resolved, the model and serial identity centre explains how to trace it, and the used compressor technical due diligence page covers the wider inspection.

How Reddy Compressors helps

Reddy Compressors, at Tarnaka in Secunderabad, has read and recorded nameplates on compressors of all major makes since 2002, including ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair and Doosan. We are an independent multi-brand specialist, not an authorised dealer, and brand names belong to their owners. We have tie-ups with all major and reputed brands.

Send us clear photos of your plates and we will help interpret them, identify the right spare parts, check whether a machine suits a new duty, or assess a used compressor before you buy. Our service engineers cover Telangana and Andhra Pradesh, and we supply compressors, rentals and spares across India. Where a measured flow or pressure check is needed, we can arrange it on site.

What to send us

• Photos of every plate on the package: compressor, airend, motor or engine, vessel and dryer.
• A photo of the whole machine.
• Running hours and the controller’s set pressure.
• What you want to decide: parts, service, new duty or purchase.

Frequently asked questions

What does FAD mean on a compressor nameplate?

FAD stands for free air delivery: the volume of air the compressor delivers, converted back to the conditions at its inlet, usually measured to ISO 1217. It is stated at one pressure, so the machine delivers a different amount at other pressures. It is the right figure for comparing compressors, unlike displacement, which is the swept volume and is always higher than real delivered air.

How do I convert m³/min to cfm on a nameplate?

Multiply by about 35.3. A nameplate showing 5 m³/min is roughly 177 cfm, and 17 m³/min is about 600 cfm. For litres per second, multiply by about 2.12 to get cfm. Always note the pressure at which the flow is stated, because converting the units does not change the fact that flow depends on the operating pressure.

Can I run my compressor above the pressure on the nameplate?

No. The maximum working pressure on the package plate is a design limit for the airend, motor, vessels and valves. Running above it risks overload, high temperature, safety valve lifting and damage. If the plant needs higher pressure, first check whether distribution losses can be reduced, and if not, choose a compressor and vessels rated for the higher pressure.

Why is my compressor delivering less air than the nameplate says?

The nameplate shows rated flow when new, at a stated pressure and reference conditions. Real delivery falls with a worn airend, blocked inlet filter, sticking inlet valve, slipping belts, high inlet temperature or operation at a higher pressure. Leaks downstream can also make it seem that the compressor is short of air. A flow measurement and inspection show which cause applies.

Is the motor kW on the plate the compressor’s power consumption?

No. The motor kW is its rated output at the shaft. The package draws more electrical power because of motor losses, fan motors, the dryer if integrated, and controls. For efficiency comparisons, use the total package input power divided by free air delivery, giving specific power in kW per m³/min, at the same pressure for every machine compared.

What information is on an air receiver nameplate?

An air receiver plate usually shows the maker, serial number, year of manufacture, design pressure, design temperature range, volume and hydraulic test pressure. These relate to the vessel only. The compressor and the safety valve must be set so that the vessel’s design pressure is never exceeded. Keep the plate legible, as inspectors and service engineers need it.

What is on a compressor separator nameplate?

The separator vessel plate on an oil-injected screw compressor normally shows the vessel maker, serial number, year of manufacture, design (maximum allowable) pressure PS, hydraulic test pressure PT, design temperature range TS, internal volume in litres and, on many vessels, the construction code and an inspection stamp. The serial is the vessel’s own, not the compressor’s. Use the plate to confirm that the safety valve is set at or below PS, that the compressor’s maximum pressure is comfortably lower, and to schedule thickness checks and hydraulic tests over the vessel’s life.

Where is the name plate on an air compressor?

The main package plate is usually riveted to the canopy or frame near the control panel, or inside the front door on enclosed screw compressors. Reciprocating units often carry it on the crankcase or the receiver. The motor plate is on the motor body, the airend plate is on the airend casting, and the separator and receiver each carry their own plate on the shell or a welded bracket. Some separator plates are only visible with a side panel removed. Photograph all of them, because each plate covers only the part it is fixed to.

What is the compressor operating envelope on a nameplate?

The operating envelope is the range of conditions in which the compressor can run safely and deliver its rating: maximum and minimum pressure, maximum and minimum ambient temperature, supply voltage and frequency tolerance, altitude and vessel temperature limits. The nameplate gives some of these directly, such as maximum pressure, voltage, frequency and sometimes maximum ambient. Others come from the motor standard and the maker’s manual. Running outside the envelope, for example at 48 °C in an unventilated room or on 360 V supply, causes trips, overheating and shortened life.

How do I check the full-load current on an air compressor nameplate?

Use the three-phase formula I = P ÷ (√3 × V × η × cos φ), with P being the motor’s rated output in watts, V the line voltage, η the efficiency and cos φ the power factor from the motor plate. For a 37 kW, 415 V motor with 93.9% efficiency and 0.86 power factor: 37,000 ÷ (1.732 × 415 × 0.939 × 0.86) = about 63.7 A. If the plate’s full-load current is far from your calculation, the motor may have been changed or rewound, or the plate may belong to another machine.

Related pages

• Compressor model and serial identity
• Reading compressor performance data sheets
• cfm, psi and bar conversion
• Air receiver lifecycle integrity
• Safety valve identification and service
• Compressor spare parts