Air Compressor Capacity Control Methods Compared

Compressor capacity control ultimately solves one problem: air demand is changing, compressor output capacity needs to keep up.

Fixed Speed Era Approaches

Fixed speed motor speed is fixed. 60Hz power supply gives that synchronous speed, can't directly adjust output capacity. To regulate, gotta find other ways.

Control

Capacity Regulation Systems

Most direct is start-stop. Pressure below lower limit, start. Above upper limit, stop. Under 20 hp small piston machines commonly do this. One pressure switch handles it. Problem is startup inrush current is big, six or seven times rated current. Motor windings can't handle frequent abuse. Manufacturers limit to no more than 6 starts per hour. Applications with frequent demand fluctuation can't keep that limit at all.

Screw machines with power going up, start-stop control doesn't work anymore. 100 hp motor startup current over 500A. Dozens of times a day, motor and contactors can't take it. Load-unload control came along: motor keeps running, inlet valve switches states. Pressure high, close valve and unload. Pressure low, open valve and load. Start count drops from dozen per hour to three to five per day. Much less stress on electrical system.

Unloaded state isn't zero power consumption. Rotors spinning idle, oil pump running, cooling fan turning, adds up to 20% to 30% of full load power. A 50 hp machine unloading burns 10-15 kW. Doesn't look like much, accumulates to real money. If air load long-term is only 50%, half the time unloading, average power consumption over 60%, but output only 50%. Specific energy consumption over 20% higher.

Inlet throttling tried to solve this problem. Inlet valve opening continuously adjustable, not just full open or full closed, can be half open. Output follows opening, continuously adjustable in 50% to 100% range. Sounds good. Actual results are mediocre. Throttling's essence is creating resistance at inlet. Inlet pressure gets pulled low, compression ratio forced higher, efficiency drops accordingly. Throttle to 70% load, specific power worsens 10% to 15%. Throttle to 50% load, deterioration expands to over 25%. Narrow adjustment range, poor efficiency. After VFDs got cheap, nobody goes this route anymore.

Throttling

Inlet Valve Control

Load/Unload

Fixed Speed Operation

What VFD Does

VFD changed this situation. Power frequency adjustable, motor speed adjustable, output directly controlled through speed.

Key to electricity savings is in screw machine power characteristics. Shaft power to speed relationship is close to cubic. 80% speed, power just over 50%. 50% speed, power only 12% to 15%.

What does this mean?

At 50% load condition, load-unload control average power consumption is over 60%. VFD speed control power consumption only around 15%. Same output, electricity cost differs four times. 100 hp machine running 6,000 hours per year at average 60% load, difference of 30,000-40,000 kWh per year.

Electricity savings is one aspect.

Pressure stability is also different. Load-unload control adjusts through valve switching, pressure fluctuates between upper and lower limits. VFD speed control adjusts continuously through speed. PID responds fast. Pressure fluctuation can be controlled within 3 psi. Paint lines, precision machining sensitive to air pressure, this difference sometimes determines whether the process can be done.

Startup characteristics changed too. Fixed speed motor direct start has big inrush current. Star-delta start is better but still has impact. VFD soft start keeps current always controlled near rated value. Speed ramps up slowly from zero. Friendly to motor windings, friendly to grid too. Old factories with tight electrical capacity, this characteristic sometimes matters more than electricity savings.

VFD systems are expensive. VFD itself $2,000-3,000, high power more expensive. Matching motor needs VFD-specific model, higher insulation class required, bearing configuration needs upgrade. Complete machine cost 30% to 40% higher than fixed speed.

Complex electrical system means more failure points. VFD parameters, ordinary electricians can't understand. Problems need professional people. VFD hates heat, dust, moisture. Installed in compressor room, heat dissipation and dust protection need careful consideration.

Adjustment range has lower limit. Speed too low, oil film can't form, bearings will have problems. Generally marked 30%, some manufacturers can do 25%. Below limit, can only shut down.

How to Coordinate Multiple Machines

Single machine capability is limited. Even biggest VFD machine adjustment range is only 30% to 100%. Compressor stations with over 100 hp installed usually need multiple units. Coordination issues come up.

Simplest approach is pressure sequence control. Set several pressure thresholds. Pressure drops to first level, machine 1 starts. Drops to second level, machine 2 follows. Logic is simple, few pressure switches can do it. Problem is machines each manage themselves, often ends up with one at full load and another frequently loading-unloading. Station efficiency won't come up.

Controller

Central Management

Station

Multi-Unit Setup

Central controller does something different. It knows header pressure, knows each machine's status and capability, it calculates an overall optimal plan.

Fixed speed machine efficiency curve is steep. Full load efficiency highest, deviating from full load drops noticeably. VFD machine efficiency curve is flat, partial load also acceptable. Common configuration is fixed speed machines handle stable base load, VFD follows fluctuations. Three 75 hp fixed speed plus one 60 hp VFD, total installed 285 hp. Air demand 80 hp, run one fixed speed plus VFD at low speed. Demand 200 hp, two fixed speed full load plus VFD making up difference. Demand 270 hp, all running high load. Controller switches automatically, no one needs to watch.

Coordinated control can also balance runtime across machines. Controller records cumulative operating hours, periodically rotates startup priority. Wear distributed across equipment, won't have one wear out early while others are still new.

New compressor stations with three machines or more basically all have coordinated control. Retrofitting old stations also increasing. Machines that each managed themselves get controller installed for unified management. Not much investment, obvious effect.

Selection Considerations

Wrote all this, when it comes to actual selection still gotta look at conditions.

Power is first dividing line. Under 20 hp, start-stop control is simple and sufficient. No need to complicate. Over 20 hp, fixed speed screw machines come standard with load-unload. This system is mature and reliable.

Air usage characteristics is second dividing line. Load stable above 80% without much variation, fixed speed machine full load efficiency isn't worse than VFD. Equipment investment saved is more real than electricity difference. Load swings big, frequently changing between 30% and 100%, VFD advantage becomes obvious.

Single unit or multiple is third dividing line. Installed over 100 hp, multiple machines running, coordinated control is mandatory. Without coordination, each fighting alone, station efficiency can't come up no matter what. Fixed speed and VFD mixed is more economical approach. Balances equipment investment and adjustment flexibility.

Some special requirements need separate consideration. Applications requiring high pressure stability, VFD machine advantage isn't in saving electricity but in pressure control precision. Old factories with tight electrical capacity, VFD's soft start characteristic sometimes matters more than electricity savings.

No control method is optimal for all situations. Start-stop is simple but limited by power. Load-unload is reliable but has idling losses. VFD is efficient but equipment is expensive. Coordinated control is powerful but needs extra investment. Figure out your air usage conditions and constraints, then selection has basis.