Air Compressor Unloader Valves
Figure 1: Air compressor unloader valve
An air compressor unloader valve is a device used by air compressors to release the trapped air inside the compression chamber and tank discharge line when the motor stops. This allows the motor to start up easier. In general there are two types of air compressor unloader valves, electrical and mechanical. Figure 1 shows an example of an electrical air compressor unloader valve.
Extracting the air by the unloader valve is essential for the compressor motor to start again without any effort. Otherwise, the generated load by the air in the compression chamber and tank discharging line creates a high initial torque that the motor may have difficulty overcoming.
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Table of Contents
- What does an air compressor unloader valve do?
- Types of air compressor unloader valves
- Mechanical vs electrical unloader valve comparison
- Selection criteria for electrical unloader valve
- Installing an electrical unloader valve
- Solving common problems
- FAQ
What does an air compressor unloader valve do?
The function of an air compressor unloader valve is to release the remaining air from the compression chamber and relative lines when the motor turns off. The operation of an air compressor is explained from the diagram in Figure 2 to understand how this valve works.
Figure 2: Air compressor components, check valve (A), pressure switch (B), pressure gauge (C), tank discharge line (D), safety valve (E), unloader valve (F).
Before the compressed air reaches the tank, it passes through a pressure switch (B) that evaluates the pressure between maximum and minimum established limits. While the compressor is working, the tank accumulates compressed air until the pressure reaches its pre-set maximum. Upon reaching its maximum pre-set value, the pressure exerts a force on an internal piston of the pressure switch, moving it up, separating its contacts, and shutting off the motor. The air contained in the compressor chamber and the tank discharge line (D) is immediately released through the unloader valve (F) into the atmosphere. This action makes the check valve (A) close and prevents the air contained in the tank from escaping. Once the tank pressure reaches its minimum, the pressure switch (B) contacts come back together, and the motor starts again.
Extracting the air by the unloader valve is essential for the compressor motor to start again without any effort. Otherwise, the generated load by the air in the compression chamber and tank discharging line creates a high initial torque that the motor may have difficulty overcoming.
Types of air compressor unloader valves
Although the air compressor unloader valve type varies depending on the compressor type and manufacturer, there are two main types, mechanical and electrical:
Mechanical unloader valves
This type of unloader valve works according to the pressure differential. It opens when the pressure reaches a predetermined pressure value. Figure 3 shows a mechanical air compressor unloader valve diagram.
Figure 3 step A shows air from the compressor enters the mechanical unloader valve through the inlet (1), travels through the unloader valve chamber (2) to the non-return valve (3). The force exerted by the air pushes open the non-return valve, allowing compressed air to go to the storage tank through the outlet (4) and also towards the diaphragm chamber (5) through the passage (6). As the tank fills with compressed air, the pressure increases until it reaches its maximum limit.
At step B, the diaphragm (7) lifts and allows air to pass into the upper chamber (8) of the piston (9). The air entering this chamber pushes the piston down, which allows the air trapped in the unloader valve chamber to be discharged into the atmosphere through the outlet (10). Due to the pressure drop, the non-return valve closes automatically.
Figure 3 step C shows that when the non-return valve closes, the air that goes towards the upper diaphragm of the valve decreases, the thrust force is low, and the diaphragm closes again. It causes the air trapped on the main piston to be released through the governor exhaust (11) at the top of the air compressor unloader valve (12).
Figure 3: Mechanical air compressor unloader valve diagram
Electrical unloader valves
A solenoid valve is an electrically operated device that controls the amount of air that passes through a line. It shuts off, releases, or doses the flow according to the system requirements. Heavier compressors (from 5 kW) often work with a star-delta system to reduce the motor starting current (compared to a direct start). This motor starts with the star connection, and then it switches from a star to delta circuit. During starting, the compressors air is unloaded, in the first few cycles, to a free outlet (rather than to the tank) to reduce the required torque of the motor. For this, a solenoid valve is used as an unloader valve.
There are three types of solenoid valves: direct-acting, semi-direct acting, and indirect (pilot-operated) acting. The direct-acting valve function depends only on a electromagnetic field created in the solenoid coil to close or open the valve, and the indirect valve depends on the system pressure differential. The semi-direct acting valve combines the functions of a direct and indirect valve. In addition, the circuit function of a solenoid valve determines the ports (2, 3, 4-way) and the position of the valve when it is de-energized (open or closed). Read our technical article about solenoid valves for more information.
The 2/2-way pilot-operated valve is the most commonly used type of solenoid valve for an electrical unloader valve for air compressors. One of the reasons is that this valve type is suitable for larger flows of heavy air compressors compared to direct-acting valves. It is also the valve type with the lowest energy consumption. This valve type is also known as a servo-assisted solenoid valve. Its function depends on the behavior of the solenoid coil and the pressure differential of the system (at least 0.5 bar is required for operation). Generally, the valve has two connections, one for the inlet and the other for air discharge as seen in Figure 4.
Figure 4: Diagram of an air compressor unloader valve, electrical type (indirect valve)
Air compressor unloader valve parts
- Coil (A): A cylindrical and hollow coil made from enameled copper wire. Through the use of induction this coil stores energy through in a magnetic field.
- Armature (B): The metallic cylinder upon which the coil is wound
- Spring and plunger (C): When there is no magnetic field in the coil, the spring keeps the plunger in a specific position, meaning normally open or normally closed. The spring yields to force applied to the plunger by a magnetic field.
- Membrane (D): Closing membrane that stops or allows air to be discharged to the atmosphere.
- Inlet port (E): The air enters the solenoid valve through this port.
- Outlet port (F): This port releases the air into the atmosphere.
In an indirect solenoid valve, the air enters and goes from the inlet connection (E) into the area above the membrane through a control orifice. From there, the air goes to the pilot orifice of the solenoid valve which is initially closed, as in Figure 5 (left side).
Figure 5: Diagram of a normally closed indirect valve: de-energized (left) and energized (right)
When the tank reaches its maximum pressure, the solenoid coil is energized, and the plunger of the solenoid valve moves upwards, opening the pilot orifice. With this line open, the pressure of the air housed in the area above the membrane begins to decrease until its value is lower than the pressure exerted by the air over the membrane. Once this membrane moves up, the inlet air passes to the outlet as seen in figure 5 right. When the tank pressure reaches its minimum pressure limit, the electrical current to the solenoid coil stops and the pilot orifice closes immediately. The air pressure in the membrane upper-chamber is re-established, and the membrane returns down to its initial position, preventing air passage from the inlet to the outlet.
A normally closed indirect valve is the most common valve used; it is energized (open) during the star circuit and closed during the delta circuit connection. However, there are air compressors with the star-delta circuit that use a normally open valve too. In this case, the tank pressure reaches its maximum limit, the pressure switch separates its contacts, the motor stops, and the solenoid valve opens (de-energized) to discharge the air into the atmosphere. Once the tank pressure reaches the minimum pressure limit, the pressure switch contacts connect, the motor starts, and the solenoid valve closes (energized) after the connection changes from star to delta, as in Figure 6.
Figure 6: Diagram of a normally open indirect valve: de-energized (left) and energized (right).
Mechanical vs solenoid unloader valves
| Mechanical | Electrical | |
| Adjustment | Can be manually adjusted | Adjusted at the pressure switch or timer |
| Timer | Cannot be operated with a timer | Can be operated with a timer |
| Position | Can be installed in any position | Should be installed upright or with a maximum deflection of 90° |
| Obstructions | Not sensitive to dirt | More sensitive to dirt then a mechanical valve |
| Application | A mechanical unloader valve is commonly used in smaller compressors (less than 5 kW) | In heavier compressors (three-phase motor) the unloader valve used is typically an indirect solenoid valve. |
Selection criteria for electrical unloader valve
In addition to the valve being suitable to work with air, consider the following when selecting a solenoid valve to be used as an air compressor unloader valve:
- Capacity (Kv value): The Kv value determines the airflow through the solenoid valve. Knowing the required Kv value ensures you choose the correct valve with the required capacity. Calculate Kv or Cv values with our valve sizing calculator.
- Maximum operating pressure: The solenoid valve must be capable of handling the maximum working pressure of the compressor.
- Electrical control: Determine the valve voltage is available at the installation site. Check if the system is powered during the air discharging period or, it uses an integrated timer in the valve for the opening time to be adjusted. In addition, check if the system requires a normally open or normally closed valve.
- Connection type and size: Ensure the pipe threads match the solenoid inlet thread type and size. If the required solenoid valve connection size is not available, a fitting can be used.
- Solenoid valve material: For the valve body, a good choice for a compressed air system is the use of brass as housing material and an FKM (Viton) seal since they have good heat and chemical resistance.
- IP classification: Solenoid valves used in air compressor systems should have an IP65 rating to ensure an effective seal against external agents.
Installing an electrical unloader valve
When replacing or installing an electrical air compressor unloader solenoid valve, follow the steps below:
- Turn off the compressor, wait until the system cools and confirm that the circuit is not under pressure.
- To remove a damaged solenoid valve, remove the coil and then the valve body. When disconnecting the damaged valve, be sure to use a suitable tool to hold the system piping.
- Carry out an internal inspection of the lines that connect to the solenoid valve to verify that dirt particles are not inside them.
- To start the solenoid valve installation, make sure it is positioned correctly according to the airflow direction. Solenoid valves generally indicate the direction of airflow with an arrow.
- If the new valve has plugs for the inlet and outlet connections, it is advisable to remove them just before installation to prevent external agents from entering the valve.
- Install the valve body first. Use a suitable tool for tightening the solenoid valve and another one to hold the system piping. Never tighten the valve by leaning on the coil, as this could cause internal damage.
- Position the valve body with the coil upward or with a maximum deflection of 90 degrees to minimize the risk of external agents collecting in the solenoid valve plunger.
- Install the coil. Place it on the valve body, with its gasket and nut over it, and tighten. A guideline for torque is 5 Nm, but the solenoid valve’s manual should have a tightening specification.
- Proceed with the electrical connection of the coil. From the three pins, the central one connects to the earth. The two pins remaining are coil terminals used as needed (for phase or neutral supply). Never use the system piping as an earth connection. Connect the coil to the power only when completing its installation; otherwise, the coil might burn out.
- Test the solenoid valve to check the performance of the system.
Solving common problems
A malfunction of an air compressor unloader valve can cause leaks if it does not close properly, and even the compressor motor could not start if the valve does not open correctly. The causes and solutions for these common problems are the below:
- Dirt in the valve: Sometimes external agents (dust, Teflon, etc.) contaminate the inside of the solenoid valves and cause a device malfunctioning. To confirm this is the cause of the problem, disassemble the valve, inspect the internal condition, perform a suitable cleaning (if required), reinstall the solenoid valve, and test it to check performance. Checking of the upstream lines of the air compressor unloader valve is recommended too. The dirt could come from another point of the system and, the internal cleaning of the valve could be a momentary solution, but the problem would happen again.
- Internal components damaged: During the internal inspection of a solenoid valve, any piece (membrane, seals, or o-rings) found damaged must be replaced.
- Electrical issues: Confirm that the voltage and frequency are correct. Check the condition of the coil and measure its resistance, in case it tends to zero, this coil is burnt and needs a replacement. Read our technical article about how to replace a solenoid valve coil for more information.
- Valve position: Ensure the valve is installed in the correct orientation with the airflow, by looking at the arrow indication on the valve housing.
FAQ
What does the unloader valve do on an air compressor?
This device is used by air compressors to release to the atmosphere the trapped air inside the compression chamber when the tank pressure reaches its maximum pre-set value.
Does an air compressor need an unloader valve?
Yes, an air compressor needs an unloader valve. Extracting the air by the unloader valve is essential for the compressor motor to start again without any effort.
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