FRL Unit – Function, Diagram, Construction, Working, Symbol, Advantages

FRL unit

Filter, Pressure Regulator, and Lubricator are combined in a unit. These three units together are called FRL units or Service units. Compressed air from compressor comes in FRL unit wherein, the air is filtered, controlled, and lubricated. Such prepared and controlled air is delivered to the pneumatic system.

Function of FRL Unit:-

1. Filter:

i) To prevent entrance of solid contaminants to the system.
ii)To condensate and remove the water vapour that is present in the air.
iii) To arrest submicron particles that may pose a problem in the system components.

2. Regulator:

To regulate the incoming pressure to the system so that the desired air pressure is capable of flowing at a steady condition.

3. Lubricator:

To provide lubrication for mating components of valves, cylinders forming a mist of oil and air.

FRL unit used in pneumatic system
FRL unit used in pneumatic system

Working of the FRC unit:

FRC unit means Filter Regulator Combine unit.
Most of the pneumatic system uses FRL unit. The main elements of the FRL unit are Filter, Regulator, and Lubricator.

1. Filter:

Air enters the inlet port of the air filter through angled louvers. The centrifugal action of the rotating air causes the larger pieces of dirt and water particles to be thrown against the inner wall of the filter bowl. These contaminants then flow down into the bottom of the filter bowl.
A baffle prevents turbulent air from splashing water on to the filter element. The air then passes through the filter element, where the fine dirt particles are filtered out. The compressed air then exits through the outer port to the regulator.

2. Relieving or Venting Type Pressure regulator:

Outlet pressure is sensed by a diaphragm preloaded with an adjustable pressure setting spring. The compressed air, which flows through a controlled cross-section at the valve seat, acts on the other side of the diaphragm. The diaphragm has a large surface area exposed to secondary (outlet) pressure and is quite sensitive to its fluctuations. The movement of the diaphragm regulates the pressure.

3. Lubricator:

As air enters the lubricator, its velocity is increased by a venture ring. The pressure at the venture ring will be lower than the atmospheric pressure and the pressure on the oil is atmospheric. Due to this pressure difference between the upper chamber and lower chamber, oil will be drawn up in a riser tube. Oil droplets mix with the incoming air and form a fine mist. The needle valve is used to adjust the pressure difference across the oil jet and hence the oil flow rate. The air –
the oil mixture is forced to swirl as it leaves the central cylinder so that large particles of oil goes back to bowl and only the mist goes to outlet.

FRL Symbols : 

FRL Unit symbol
FRL Unit symbol


What are Air Filters?

Functions: The function of air filters is to remove all foreign matter and allow dry, clean air to flow without restriction to the regulator and then on to the lubricator.

Filters are available in wide ranges starting from a fine mesh wire cloth (which only strains out heavier foreign particles) to elements made of synthetic materials (which are designed to remove very small particles).
Usually in-line filter elements can remove contaminants in the 5 to 50 pm range.

Factors Affecting Selection of Filters

While selecting the filters, the following factors at least should be taken into account:
1. Size of particles to be filtered from the system.
2. The capacity of the filter.
3. Accessibility and maintainability.
4. Life of the Filter.
5. Ability to drain the condensate.

air filter diagram
air filter diagram

Construction of Air Filter : 

The construction and operation of a typical cartridge-type filter system is illustrated in Fig. (a).

It consists of the filter cartridge, deflector, plastic bowl, baffle, water drain valve, etc.

Working of Filter :

The air to be filtered is allowed downward with a swirling motion that forces the moisture and the heavier particles to fall down. The deflector used in the filter mechanically separates the contaminants before they pass through the cartridge filter. The filter cartridge provides a random zig-zag passage for the airflow. This type of airflow arrests the solid particles in the cartridge passage.
The water vapor gets condensed inside the filter and is collected at the bottom of the filter bowl. Also, heavier foreign particles that are separated from the air are collected at the bottom of the bowl. Then the accumulated water and other solid particles at the bottom of the filter bowl are drained off with the use of an on-off drain valve located at the bottom of the filter bowl.


What are Air Pressure Regulators?


The function of the air pressure regulator is to regulate the pressure of the incoming compressed air so as to achieve the desired air pressure at a steady condition.

The compressed air leaving the compressor should be properly prepared before it goes into the circuit. The air should have the proper operating pressure for the circuit. The improper fluctuating pressure level in the piping system can adversely affect the operating characteristics of the system components such as valves, cylinders, etc.  Therefore, air pressure regulators are fitted to ensure the constant supply pressure irrespective of the pressure fluctuations in the compressor unit.

For example, the line from the compressor may carry a pressure of 10 bar, the air pressure regulator can reduce this pressure to 0 bar to any point between the full line pressure and zero pressure.

Thus the air pressure regulators act as pressure guards by preventing pressure surges or drops from entering the air circuits.

Types: The two types of air pressure regulators are :
1. Diaphragm-type regulator, and
2. Piston-type regulator.

air pressure regulator diagram
air pressure regulator diagram

Construction of Air Pressure Regulator : 

The construction and operation of a typical diaphragm-type air pressure regulator is illustrated in Fig. (a).

It consists of a diaphragm, valve, main and dampening springs, etc. Usually, the diaphragm is made of oil-resistant synthetic rubber with a nylon cloth reinforcements.

Working of air Pressure Regulator : 

  • The diaphragm allows the proper amount of movement for opening and closing at the valve seat. When the adjusting screw is in the fully retracted position, the valve is closed.
  • When the adjusting screw is turned to compress the adjusting and dampening springs, the valve is opened. Thus the air is allowed from the inlet port to the outlet port. The pressure of the outlet air depends upon the size of the valve opening that is maintained. This is determined by the compression of the adjustable spring. Higher the spring compression, more will be the amount of opening and hence more the pressure and vice versa.
  • The vent-holes are provided to let out the undesirable excessive outlet pressure, if any, into the atmosphere. The dampening spring is provided to act as a dampening device needed to stabilize the pressure.


What is an Air Lubricator?

Function : The function of an air lubricator is to add a controlled amount of oil with air to ensure proper lubrication of internal moving parts of pneumatic components.

The lubricator adds the lubrication oil in the form of a fine mist to reduce the friction and wear of the moving parts of pneumatic components such as valves, packings used in air cylinders, etc.
At the same time excessive lubrication is also undesirable. Excessive lubrication may result

(i) malfunction components,
(ii) increased environmental problems, and
(iii) seizing of components after prolonged downtime. 

What is the best type of Lubricant for a Pneumatic System?

1. Generally a good-quality, light-grade spindle oil is used in pneumatic systems.
2. Sometimes, a mixture of 50% kerosene and 50% SAE 30 oil is also used as a lubricant.

Air Lubricator diagram
Air Lubricator diagram

Construction and Operation Of Air Lubricator :

The construction and operation of a typical force-feed type air lubricator are illustrated in Fig. Its operation is similar to the principle of simple carburetor used in the petrol engines to obtain an air-fuel mixture.

As the air to be lubricated enters into the inlet pipe, the venturi ring located in the pipe increases its velocity of low. It causes a local reduction in the upper chamber. This pressure differential between upper and lower chambers causes suction of lubrication oil from the oil reservoir to the upper chamber. Now the oil in the form of mist is sprayed in the air stream and the air-oil mixture is obtained. This air-oil mixture is forced to swirl as it leaves the central cylinder causing more oil particles to be spread out of the air stream. The amount of oil dropping into the upper chamber can be controlled by a needle valve.

Advantages of FRL Units: 

  1. FRL Units ensure clean air delivery to pneumatic system parts hence system blockage may be avoided.
  2. Lubrication causes better life of parts. 
  3. Regulator devices avoid pressure drop or rise so that leakages and parts damages may be avoided.
  4. Equipment Lifetime may increase.
  5. Installation is easy.
  6. Easy to maintenance and servicing.

Disadvantages of FRL Units : 

  1. If FRL Unit is placed in the place of heavy vibration then it may damage the Unit.
  2. Piping materials without flexibility, such as steel tube piping, are prone to be affected by excess moment loads and vibrations from the piping side. Use flexible tubing in between to avoid such effects.

Sachin Thorat

Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.

One thought on “FRL Unit – Function, Diagram, Construction, Working, Symbol, Advantages

  1. Pingback: Difference between tube tyre and tubeless tyre | automobstars

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Recent Posts