Compressors are most commonly used to compress and supply gas from one area to another. There are multiple types of air compressors designed according to the demand of different applications. A positive displacement (PD) compressor is one of the most commonly used types of compressor. This compressor uses a reciprocating or rotary component to compress the working fluid. The main objective of this article is to explain the positive displacement air compressor working, types, and applications.
What is a Positive Displacement Compressor?
A positive displacement compressor works by introducing a specified amount of air into a compression chamber where the air is compressed by mechanically reducing its volume.
It is known as a positive displacement compressor because it compresses the working medium by displacing the volume of the compression chamber.
This compressor has a forward and backward moving component such as a plunger, piston, diaphragm, or vanes inside the compression cylinder.
A positive displacement compressor can’t provide a continuous flow of fluid. Positive displacement compressors have low efficiency than centrifugal compressors.
The PD compressors have excellent quality, high reliability, affordable cost, and high versatility. Therefore, positive displacement compressors are most commonly used in LPG plants, refrigerators, chemical plants, fertilizer industries, shop cylinders, and many other applications.
Bike pumps and bicycle pumps are the most common examples of positive displacement compressors.
Positive Displacement Compressor Working Principle
A positive displacement compressor works on the displacement principle. The working of the PD compressor is very simple. It works in the following way.
First of all, the air is drawn from the suction valve to the compression chamber. The compressor uses a moving component such as a piston, plunger, or diaphragm to compress the air. This reciprocating component has TOO and FRO motion inside the compressor chamber.
The reciprocating component reduces the volume of the compression chamber and compresses the air. When the pressure of the air or gas reaches according to the requirements, the discharge valve opens, and compressed air is discharged. After this, the compressed air or gas is stored in a storage tank or delivered to the desired area.
These compressors have one or more compression chambers and multiple inlet ports. The positive displacement compressor has different types, but the working principle of all these types is the same.
The simplest example of a positive displacement air compressor is a bicycle pump, where air enters the cylinder and compresses by a moving piston; after that, it transfers to the bicycle tires.
Positive Displacement Compressor Types
The types of positive displacement air compressors are given below:
- Reciprocating compressor
- Rotary vane compressor
- Scroll compressor
- Screw compressor
- Diaphragm Compressor
1) Rotary Screw Compressor
The rotary screw compressor is a very famous and common positive displacement air compressor type. It uses rotary screws for air compression.
As the screws of the compressor start rotating, a vacuum starts producing inside the compressor chamber. Due to this cavity, the compressor sucks air into the cylinder. After the suction process, the inlet valve closes, and the compression process starts.
Two rotating screws compress the air that rotates regularly and runs through the chamber. With each revolution, the air pressure increases gradually until it reaches up to desired pressure.
These types of positive displacement air compressors collect pollutants from the air and lubricate the compressor parts themselves.
These compressors are lubricated by an air-oil mixture. This mixture lowers the temperature of the rotating parts. Oil in the air must be filtered properly before using it for final applications. However, the oil presence improves the operating efficiency of the equipment.
The maximum output of a rotary compressor is mostly 600 horsepower. It is compulsory to remove the oil from the air before it exits the rotary screw compressor.
This compressor requires a large combined decreasing filter, commonly referred to as an oil separator filter. This part should be replaced regularly. Failure to do so will result in excessive oil flow or high-pressure drops.
The previous screw compressors were inefficient compared to double-acting reciprocating compressors. However, the latest screw compressors tend to have outstanding efficiencies at full load.
A common screw compressor has motor control and all of the power, oil, and air cooler. It also has safety devices that are fully packaged and pre-installed. Most newer machines (except for the smallest) contain a microprocessor controller.
Read More: Screw Compressor Working and Types
2) Rotary Vane Compressor
This PD compressor works on the same principle as the rotary screw compressor works. But the difference is that these compressors use vanes as an alternative to screws for compressing the air in the chamber.
Like the oil injector, the vane compressor requires the same components for the separator and oil system parts. In general, smaller separator sizes result in more oil remaining in the vane compressor than a similar reciprocating piston compressor.
This type of compressor‘s full load efficiency is usually medium, and the part-load performance is heavily influenced by the control method. With the exclusion of the varying displacement, the control method of the vane compressor is essentially the same as that of the screw compressor.
Read More: Working of Centrifugal Compressor
3) Reciprocating Piston Compressor
A reciprocating compressor uses a piston or plunger that moves at a constant speed to introduce the air into the compression chamber and compresses it. Typically, one movement of the piston sucks the air in the chamber, and the other movement compresses it.
These types of positive displacement compressors are designed for double or single chambers. The reciprocating compressor has the ability to deliver up to 1000 horsepower. These machines are usually cheap to maintain and buy.
However, the performance of the reciprocating compressor gradually decreases over time. These compressors also have high levels of noise and low performance than centrifugal compressors.
If the oil content is high (around 50 ppm) and the exit temperature is also high, these compressors can cause the fluid to flow down. Due to the high manufacturing costs, the unique basis for dealing with vibrations, and the frequent and extensive maintenance, only a few manufacturers still produce double-acting reciprocating compressors.
Read Also: Different types of Reciprocating Compressors
4) Scroll Compressor
The scroll compressor has a pair of offset scroll blades. It has different solid scrolls. Typically, it has one fixed displacement and the other eccentric. This process sucks the gas in and compresses it.
It is one of the most silent types of positive displacement compressors. The spacing between the solid scrolls is minimal, which makes the volume very efficient.
Read Also: Working of Scroll Compressor
5) Diaphragm Compressor
A diaphragm compressor is the most famous type of PD compressor. This compressor uses a diaphragm to compress the air or gas inside the compression chamber. This diaphragm has a rotatory movement due to which it compresses the working fluid. The diaphragm compressor is mostly used to transfer toxic gases.
Read More: Diaphragm Compressor Working and Types
Advantages and Disadvantages of Positive Displacement Compressors
Advantages of Positive Displacement Compressor
- This compressor has a low cost of production.
- Components are easy to manufacture.
- These have a wide variety of pricing choices.
- It has high efficiency.
- This type of compressor has a high-pressure ratio.
Disadvantages of Positive Displacement Compressor
- Positive displacement compressors require high maintenance.
- It has high noise as compared to a dynamic compressor.
- Not appropriate for high flow rates.
- These compressors are not suitable for dirty gases.
- The positive displacement air compressor has a limited range of capacity.
- It is less reliable.
Applications of Positive Displacement Air Compressor
- Positive displacement compressors are used in refrigerators.
- These compressors are most commonly employed in chemical plants.
- They are used in hydraulic cylinders.
- They are used at auto shop cylinders.
FAQ Section
Why it is called positive displacement compressor?
It is known as a positive displacement compressor because it compresses the working fluid by displacing the cylinder volume. It uses a reciprocating component such as a piston or plunger for compression of the working fluid.
What are the examples of the Positive displacement compressor?
A positive displacement compressor has the following most common examples that are used in daily life:
- Bicycle Pump
- Reciprocating compressor
- Piston compressor
- Plunger compressor
- Diaphragm compressor
What is the Positive Displacement Compressor used for?
A positive displacement compressor uses to compress the compressible fluids (i.e., air and gas) and transfer them to the desired storage tank or pipeline system.
What are the types of the Positive Displacement Compressors?
Positive displacement compressors have the following types:
- Reciprocating compressor
- Rotary vane compressor
- Scroll compressor
- Screw compressor
- Diaphragm compressor