How Air Compressor Loading and Unloading Works

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Air compressors are in an important role for a wide range of industrial applications. In this article we look at understanding their loading and unloading mechanisms to see the answer on how to best optimise performance and efficiency for air compressor service.

What does loading refer to in air compressor services?

Loading is a process where the air compressor is actively compressing air, a phase crucial for meeting the demand for compressed air in systems.

Unloading occurs in a similar way but when the demand for air is lower; the compressor stops compressing air but remains running, ready to resume loading as needed. This cycle is critical for energy efficiency and maintaining system pressure without overloading the compressor.

Our focused exploration will delve into the mechanics of these processes, their impact on compressor longevity, and how to balance them for optimal operation. Read on to learn more.

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Balancing Loading & Unloading for Optimal Air Compressor Service Efficiency

The key to maximising the efficiency of a quality air compressor service lies in effectively balancing its loading and unloading phases. This balance is not just about reducing energy consumption; it’s about optimising the entire system for reliability, longevity, and performance. A well-balanced compressor runs more efficiently, experiences less wear and tear, and provides consistent pressure, which is crucial for the quality of operations in various industrial applications.

Take a look at a comparison of the loading and unloading phases of an air compressor, highlighting the differences in purpose, operation, energy usage, impact on system pressure, effect on compressor longevity, and tips for optimisation.

FeatureLoading PhaseUnloading Phase
DefinitionThe compressor is actively compressing air.The compressor stops compressing air but remains running.
Purposeto meet the demand for compressed air in the reduce energy consumption when demand is low.
Operationengages when air pressure drops below a set point.engages when air pressure reaches the upper set point.
Energy Usagehigher, as the compressor is actively working.lower, as the compressor is in standby mode.
System Pressuremaintains the minimum required pressure.prevents overpressurisation and energy waste.
Impact on LongevityRegular wear is due to active operation.reduces wear and tear by avoiding constant operation.
Optimisation TipsRegular maintenance, correct sizing for demandAdjust set points for efficient operation and regular checks.

1. Inlet Valve

The inlet valve of your air compressor draws in outside air required for compressing gas. It filters the air through a fine mesh screen to prevent dirt or minerals like sand from entering the compressor chamber. Over time, however, these filters may become clogged, which prevents your inlet valve from opening as needed and leads to lost energy production or compressed air output from your compressor.

Most air compressors utilise a device known as a governor to monitor and regulate the flow of air entering their inlet valve. Depending on your compressor’s size and power, different governor types will likely be necessary; heavier machines with three-phase motors may use star-delta controllers, which help lower their starting current requirements.

These systems may also incorporate a timer that will signal when to stop unloading activity, known as dual-control or auto-dual systems.

Some air compressors utilise an alternative method for loading and unloading known as load/unload control or proportional capacity controls to manage loading and unloading, often at less expense than traditional start/stop control systems.

Under this control scheme, a compressor runs continuously at up to 25% of its capacity. An activating device uncovers part of the rotor and vents any remaining capacity into the atmosphere once it satisfies or reduces the demand for air.

This process is controlled by a pressure switch, which monitors air pressure within predetermined limits of maximum and minimum set values. Once pressure reaches its maximum pre-set value, an upward force exerted on a piston inside the pressure switch moves it upward and separates its contacts, shutting off the compressor motor. Once pressure decreases to the minimum setpoint value, the inlet valve opens, releasing excess air into the atmosphere as pressure resets back to the pre-set value, repeating the cycle.

2. Blow-Off Valve

The compressor draws air through its intake, generating internal pressure that must be vented off through a blow-off valve before unloading can occur. This helps balance the system and prevent the internal pressure from reaching dangerously high levels, avoiding further damage.

 You can adjust the size of the blow-off valve’s nozzle according to your requirements, which determines the amount of air vented off at any given time.  In general, however, the blow-off valve remains closed during loading and only opens during unloading in order to reduce wasted compressed air usage as well as lower overall compressor power consumption during unloading.

As soon as pressure falls below its cut-in setting or anticipated demand is lower than capacity, the air inlet valve is closed to enable the governor to activate the compressor unloading mechanism. The governor comprises a tower valve, a spring-loaded diaphragm, and a valve mechanism that allows air pressure from the reservoir to pass through them to open or close the respective inlet and exhaust valves, thus stopping further compression while venting excess air back out into the atmosphere.

Some systems use modulating controls to operate the compressor unloading mechanism. These controls utilise system pressure as a reference point and only permit air compressors to reach their maximum discharge pressure when under load. Although this reduces power consumption considerably at unload, they may consume up to 30% more power at zero load due to constant consumption from storage volume.

Modulating unloading controls have the potential to cause excessive short cycling of the compressor, leading to poor throttle response, coked spark plugs, or even backfiring engine performance. Turbo Performance recommends installing high-flow aftermarket blow-off valves, which can vent excess pressure generated by modulating controls without impacting air compressor performance at any point. Our selection range covers stock car applications all the way through big-boost racetrack applications.

3. Solenoid Valve

Solenoid valves are widely used control elements in pneumatic systems to regulate the flow of liquids and gases, such as liquids or gases that pass through them. With their ability to quickly shut off, release, dose, or mix fluids, they offer low control power usage as well as good medium compatibility due to the materials selected for their seals and bodies. Solenoid valves have quickly become an integral component in fluidics applications.

A solenoid is composed of an inductive coil surrounding a ferromagnetic core or plunger. When activated, this creates a magnetic force that makes or breaks valve orifice seals; this results in linear motion within the valve body, which either opens or closes orifices depending on their design, assisted by an extra-core spring that enhances this magnetic force generated by the coil.

When not activated, the core is held in place by springs to maintain closed valves. Once the coil is activated, its magnetic field draws inward and pulls the core towards its centre, where magnetic force either makes or breaks seals, allowing or blocking fluid flow through seals. allowing or preventing fluid movement through seals allowing or preventing fluid movement through seals allowing or blocking fluid movement through channels or seals allowing movement through channels; also creating an outward magnetic field that attracts and pushes on core springs, which returns them back to their original position once the electrical current is switched off.

There is a range of solenoid valves to suit different purposes and designs. Two-way, three-way, and four-way solenoid valves each differ by having two outlets or actuator ports open or closed depending on the solenoid position; three-way and four-way solenoid valves have one inlet port with two outlet ports plus an exhaust port for use as actuator ports.

Piloted solenoid valves employ a separate circuit to assist in activating their plunger. Once energised, air from the valve inlet flows through its pilot orifice and opens it, stopping airflow from the upper inlet port while opening up the lower outlet port ports.

Pressure switches monitor and compare a tank’s air pressure over time with a preset minimum value. Once it reaches this threshold, an internal piston inside the pressure switch exerts force on the internal piston, which rises up and opens its contacts, thereby shutting off the compressor motor.

4. Pressure Switch

Pressure switches are devices designed to activate electrical contacts when air pressure reaches a predetermined point, unlike pressure sensors, which convert system pressure readings into electrical signals. A Bourdon tube, piston, or diaphragm moves conductive contacts on a pressure switch in order to open or close an electrical circuit as the desired amount of air pressure reaches its setpoint.

Pressure switches feature two settings known as cut-in and cut-out settings. These correspond with points where the inlet or discharge pressure exceeds the spring’s pretensioned force, and when this threshold is met, an NC contact opens while its counterpart closes automatically; then, when pressure decreases again, these contacts switch back into their regular positions.

As well as set-point settings, most switches feature a range adjustment screw that enables you to alter the activation pressure. Usually visible on the primary large spring, turning this screw clockwise increases both cut-in and cut-out pressure settings; turning it anticlockwise decreases them. The deadband of a switch actuates at different pressure ranges depending on its pressure sensing element, such as a Bourdon tube, piston, or diaphragm, with diaphragm switches having wider deadbands than their counterparts.

Load and unload control strategies may be available for your compressor and are often the optimal option in terms of energy savings and reliability. They ensure that only when compressing and stopping once compressed air needs are fulfilled does the motor run; they also eliminate wasted energy caused by operating full-sized compressors at a zero flow rate.

Modulation control can provide another alternative to your compressor, effectively reducing power consumption to just a fraction of what it would be under full-load conditions at zero flow, though it requires significant compressor control storage receiver capacity for proper functioning.

If you opt for this control method, a separate unloader valve may be required; fortunately, air compressor-specific pressure switches include built-in unloader valves. Unloader valves allow trapped air over the pistons to escape when they shut off, thus relieving excess load when restarting their compressors.

1. Recognising the Importance of Set Points

Set points are critical in managing the transition between loading and unloading. These are preset pressure levels at which the compressor switches modes. Ideally, the set points should be adjusted according to your system’s specific needs, considering factors like operational demand and the capacity of your compressed air system. Incorrect set points can lead to frequent loading and unloading, causing unnecessary stress on the compressor and increased energy usage.

2. Regular Maintenance for Consistent Performance

Regular maintenance is paramount to ensuring that both loading and unloading processes function smoothly. This includes routine checks of air filters, oil levels, and condensate drains. Well-maintained components prevent inefficiencies and failures, leading to a more stable operation of the compressor.

3. Understanding Demand Patterns

Analysing the demand patterns of your compressed air system allows for better adjustment of the loading and unloading phases. Systems with fluctuating demand might benefit from variable-speed drive (VSD) compressors, which can adjust their output to match the demand, thereby optimising the loading and unloading cycles.

4. The Role of Storage in System Efficiency

Air storage tanks play a vital role in balancing the loading and unloading cycles. By storing compressed air, they provide a buffer that reduces the frequency of the cycles, thereby decreasing energy consumption and wear on the compressor.

5. Monitoring and Adjustments

Continuous monitoring of the air compressor’s performance is essential. Modern compressors equipped with smart controls can provide valuable data on operation efficiency, allowing for timely adjustments to set points and other parameters.


Balancing the loading and unloading cycles of an air compressor is not something you can simply set up and then ignore. An air compressor service has to keep in mind regular monitoring and tweaks based on how the compressor is running, the air demands in your facility, and when maintenance occurs. If you keep an eye on finding the right equilibrium between loaded and unloaded operation, you can markedly boost the efficiency, longevity, and dependability of your compressor. This translates to cost savings and better productivity from the industrial processes relying on that compressed air. The key is actively managing that balance between loading and unloading the machine over time – it’s not something you can just put in place and walk away from. Careful attention to detail usually bears fruit.

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