Air compressor

01

Set the lowest pressure that does the job

The US Department of Energy's compressed-air sourcebook gives a rule of thumb for systems around 100 psig: every 2 psi of extra discharge pressure raises compressor energy by about 1% at full output. Higher pressure also increases leaks and other unregulated use; for a system in that range with 30–50% unregulated usage, DOE puts the total penalty at about 1.6–2% per 2 psi. [1]

DOE notes that "flow restrictions of any type require higher operating pressures than necessary", so pressure drop through filters, dryers and piping raises the discharge pressure needed. Find the equipment that needs the highest pressure and check whether it can be served locally instead of raising the whole system. [1]

As a calculation example only, lowering a 100 psig system by 6 psi at the DOE's 1.6–2% per 2 psi saves about 4.8–6% of compressor energy. On a compressor drawing 75 kW for 6,000 hours a year, 5% is about 22,500 kWh. Use your own load and hours.

Before asking for quotations, log flow and pressure over a normal production week. DOE describes oversized air compressors as "extremely inefficient" at part load and suggests multiple, smaller compressors with sequencing controls for times when demand is less than peak. [1]

02

Choose the compressor type

DOE describes the main industrial types. The ranges below are DOE's general descriptions, not limits of any model offered.

Suppliers also offer variants for specific needs: belt or direct drive, permanent-magnet variable speed, low- and high-pressure, oil-free scroll and water-lubricated, and diesel portable units. [2]

03

Choose the part-load control for your demand profile

DOE warns that an unloaded rotary screw compressor still consumes power "while delivering no useful work", so, in DOE's words, "some load/unload control schemes can be inefficient", especially with too little storage, which also causes short cycling. [1]

Variable-speed drives match motor speed to demand with nearly proportional power and can hold pressure within 1–2 psi. DOE adds that near full output a VSD unit uses more power than a fixed-speed one because of drive losses, efficiency falls at low speed, and best efficiency comes when it runs between 30 and 80 percent speed for more than half of its operating hours. [1]

For an installation with two or more compressors, ask for a sequencing scheme in which one unit trims the variable load and the others run fully loaded or stop, and size the receiver for the control method chosen.

04

Measure leaks before buying more capacity

DOE gives a simple test with all air-using equipment off: time the compressor's loaded (T) and unloaded (t) periods and calculate leakage as T × 100 ÷ (T + t) percent of capacity. It considers less than 5–10% normal for a well-maintained system and says poorly maintained systems can lose 20–30% of capacity and power. [1]

If leaks are high, a larger compressor only feeds them. Fix leaks and pressure problems first, then size any new machine to the reduced demand.

05

Specify dryers and treatment by the dew point you need

DOE says refrigerant-type dryers "provide a pressure dew point of 35 to 39°F" (2–4 °C), and that pressure-swing twin-tower desiccant dryers use 10–18% of their rating as purge air. A refrigerant dryer cannot reach a dew point below about 35 °F. [1]

Drier than necessary costs energy. State the pressure dew point and oil and particle limits your processes need, and treat only the branch that needs higher quality where practical.

Medical and dental compressed air are regulated separately in each market; suppliers offering medical air systems and dental compressors should state the standard their system meets. [3] [4]

06

Plan heat recovery

DOE states that 80–93% of the electrical energy used by an industrial air compressor becomes heat, and that a properly designed heat-recovery unit can recover 50–90% of it for space heating, process or water heating, or boiler make-up water preheating. [1]

Ask whether heat-recovery options are available for the offered model and what water or air temperature they deliver at your duty.

07

Air end, separator, coolers, receiver and dryer in one package

Which rows apply depends on the type chosen above: the separator only on oil-injected screws, piston rings on reciprocating units, cooling-water flow only on water-cooled packages. [1] [5]

08

Pressure, delivery, input power and dew point to log before quoting

Discharge pressure leads the list because of DOE's rule of thumb on energy per extra 2 psi; the part-load and dew-point rows give DOE figures to hold offers against. [1]

09

Send the demand profile, not just a size

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Demand: [measured flow profile over a week; peak and minimum; future additions]
Pressure: [highest pressure any user needs; pressure at the compressor; pressure drops in treatment and piping]
Type: [oil-injected or oil-free screw, reciprocating or centrifugal; reason if oil-free]
Control: [load/unload, variable speed or both; number of units; sequencing; receiver volume]
Air quality: [pressure dew point; oil and particle limits; branches needing higher quality]
Cooling: [air- or water-cooled; design ambient; cooling water data]
Heat recovery: [use for recovered heat; required water or air temperature]
Electrical: [voltage; frequency; starting method; motor efficiency class]
Site: [room size; ventilation; noise limit; altitude]
Documents: [performance data at your pressure; specific power; maintenance schedule; pressure-vessel certificates]

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