Operating a screw air compressor efficiently requires a clear understanding of its internal mechanisms. Among the most critical of these is the load and unload system, which regulates the machine’s output in response to air demand. This process not only ensures operational efficiency but also helps prevent excessive wear, mechanical strain, and unnecessary energy consumption.
When functioning correctly, the load/unload cycle balances air supply with system requirements, maintaining consistent pressure levels without overburdening the equipment. However, when faults occur, they can lead to downtime, increased operational costs, and even safety risks. If you’re using a compressor in Singapore, understanding how this mechanism works, recognising common failure modes, and applying targeted troubleshooting measures are essential for keeping it in peak condition.
How Does a Screw Compressor Load and Unload?
A screw air compressor uses a load/unload control method to adapt to fluctuating air demand. In the loading phase, the compressor operates at full capacity, actively compressing air to meet the system’s requirements. To understand how the technology works, the unloading phase begins when the compressor stops compressing air but continues to run in standby mode, ready to resume loading when demand increases. This prevents unnecessary stop-start cycles, reduces power surges, and prolongs component life.
The load/unload process is crucial to overall system efficiency, but it is not immune to malfunction. Without proper maintenance, problems in this cycle can arise, leading to wasted energy, excessive pressure build-up, or inadequate air supply.
Why Do Load/Unload Problems Occur in Screw Air Compressors?
Being able to correctly load and unload is central to a screw compressor’s performance. Units that cannot load naturally cannot produce any compressed air, resulting in operational disruption. Conversely, if the compressor cannot unload, it may continue compressing air beyond the required level, triggering the protection features against overpressure or forcibly opening the safety valve. The following sections outline the most common causes for these issues.
Loading Failure
If a compressor fails to load, it will run on idle without generating compressed air, which is a situation that wastes electricity and reduces productivity. The first step in diagnosing this problem is to inspect the time relay.
- No load signal displayed: This often indicates a faulty time relay, which prevents the compressor from initiating the loading cycle.
- Load signal displayed but no air output: This may point to a malfunctioning loading solenoid valve or intake valve. Alternatively, air leaks in the system could be preventing sufficient pressure from reaching the intake valve.
Unloading Failure
When unloading fails, the compressor continues to pressurise the system, potentially exceeding the set safety threshold. Once the limit is reached, the safety valve opens or the machine triggers overpressure protection. In this situation, it is important to check:
- Unload solenoid valve: A defective valve may remain open or closed when it should not, disrupting the unloading sequence.
- PLC main controller: Faults in the controller can prevent the correct unloading signal from being sent.
Possible Reasons for Pressure Build-up Failure
If a screw compressor cannot create enough pressure, the underlying cause could be electrical or mechanical in nature. One of the most frequent electrical issues is a defective loading solenoid valve. If this valve breaks down, it may remain closed despite receiving the correct signal from the PLC, preventing air from entering the compression stage.
As for the mechanical causes, several faults can hinder pressure build-up:
1. Severely clogged air filter – Restricts airflow into the compressor, limiting the amount of air available for compression.
2. Stuck intake valve – Prevents air from entering the compression chamber.
3. Airline leaks – Cause pressure loss between components, reducing efficiency.
4. Minimum pressure valve leak – Allows air to escape before adequate pressure is built.
To diagnose these faults, technicians can check the intake valve and remove the air filter, start the compressor, and see if the intake valve opens in the loading phase. If it does not, the solenoid valve should be examined. For compressors with a cylinder near the intake valve, lifting the cylinder manually can help determine whether the valve mechanism is operational.
What Happens When the Unloading Issue Persists?
If unloading issues remain unresolved, the compressor will continue to generate pressure unnecessarily. This can lead to frequent activation of the safety valve, excessive wear on internal components, and inefficient energy usage. Several underlying factors may be responsible:
1. Unload Solenoid Valve Malfunction
When this valve fails, it may remain in the wrong position even if the control panel shows that the compressor is in unloading mode. As a result, compressed air continues flowing, and the intake valve is kept from closing.
2. Faulty Intake Valve
A damaged or sticking intake valve may not close fully during unloading, allowing the compressor to keep producing air until the safety valve opens.
3. Pressure Switch Defects
Should the pressure switch become defective, it will not trigger unloading when the set pressure is reached. For instance, if the unloading point is 0.8 megapascal, the compressor may keep running in load mode beyond this threshold.
4. Blockage In The Oil Separator Filter
When the compressor’s air output exceeds the system’s discharge rate, pressure within the oil separation tank rises. A blocked filter can accelerate this build-up, causing the safety valve to release pressure.
5. Low Air Consumption Compared to Output
If the demand for air is significantly lower than the compressor’s capacity, it may remain in loading mode for prolonged periods, even if unloading should occur.
Preventing Load/Unload Failures
Many load/unload problems can be prevented through routine maintenance and periodic inspection. Key preventive measures include:
- Regular filter replacement to prevent blockages and maintain airflow.
- Checking solenoid valves for wear or contamination that could hinder operation.
- Inspecting the intake valve for mechanical damage or sticking components.
- Testing the pressure switch to ensure accurate signalling for loading and unloading.
- Monitoring air demand to avoid running a compressor at unnecessarily high capacity for extended periods.
By implementing these measures, operators can minimise downtime, reduce repair costs, and maintain optimal performance.
Conclusion
The load and unload mechanism in screw air compressors is integral to their efficiency, reliability, and safety. When faults arise, they can lead to wasted energy, overpressure incidents, and reduced productivity. Understanding the symptoms, identifying the root causes, and applying systematic troubleshooting steps allows operators to address problems swiftly and effectively. Through regular maintenance and proper operational practices, the risk of load/unload issues can be greatly reduced, ensuring that the compressor continues to meet air demand reliably while preserving its service life.
