Defining a Positive Displacement Compressor
Click here for a detailed overview of positive displacement compressors, including benefits, drawbacks, and examples.
Compressors are primarily used for compressing and delivering gas or fluid from one area to another. Several types of compressors exist, each designed according to the specific demand of different applications. One of the most common types of compressors is the positive displacement compressor. The latter uses a rotary or reciprocating component to compress a working fluid.
Here’s what you need to know about the positive displacement compressor.
What is a Positive Displacement Compressor?
A positive displacement compressor is a machine that works by introducing a specific amount of fluid into a compression cylinder. The fluid’s volume is reduced by mechanical compression. The equipment has its name since it only compresses the working medium by efficiently displacing the volume of the compression cylinder.
They cannot deliver a continuous flow of fluid. This is because they have forward and backward components, such as diaphragms, pistons, plungers, or vanes inside the compression cylinder. As a result, they also have low efficiency compared to centrifugal compressors.
Positive displacement compressors have high reliability, excellent quality, high versatility, and affordable cost. For these reasons, they are commonly employed in fertilizer industries, LPG plants, and chemical plants, as well as many other applications.
Bicycle and bike pumps are some of these most common examples.
The Working Principle of a Positive Displacement Compressor
Positive displacement compressors operate on the displacement principle. Here’s how it works:
- Air or gas is sucked in from the suction valve and straight to the compression cylinder.
- The compressor uses a moving component such as a plunger, diaphragm, or piston to compress the gas or air. This reciprocating component has a ‘to and fro’ motion within the compression chamber.
- This reciprocating component minimizes the compression chamber’s volume and compresses the air.
- When the gas or air pressure reaches the specified requirement, the discharge valve opens immediately, and the compressed gas is discharged.
- Afterward, the compressed air or gas is stored in a specified storage tank or delivered to a designated area.
These compressors usually have multiple inlet ports and one or more compression chambers. Although different types of positive displacement compressors exist, they share similar working principles based on specified applications.
The bicycle pump is the simplest and most perfect example. First, air enters the cylinder and compresses due to a moving piston. Then, the air transfers to the bicycle tires, where they take up space until the tires reach their specified measurements.
Types of Positive Displacement Compressor
As mentioned earlier, different types of positive displacement compressors exist. Here they are in no particular order:
A reciprocating compressor uses a piston that moves at constant speeds to draw in the gas/air into the chamber and compresses it. One movement of the piston sucks in the gas/air into the chamber while the other movement efficiently compresses it.
This type of positive displacement compressor has two unique designs: single and double chambers which can deliver as much as 1,000 horsepower, are affordable, and are cheap to maintain.
Rotary Screw Compressor
The rotary screw compressor is one of the most common positive displacement compressors. It makes use of rotary screws to compress gas/air.
When the screws of the compressor begin to rotate, a vacuum is produced within the chamber. As a result of this development, the compressor sucks gas or air into the cylinder. The inlet valve closes after suctioning, and the compression procedure kicks off.
Two rotating screws compress the gas/air rotating regularly and travels through the chamber. As a result, the gas pressure increases with every revolution until the designated pressure is reached.
This compressor lubricates using an air-oil mixture that lowers the temperature of the active components. The maximum output of this type of compressor is about 600 horsepower.
The scroll compressor has a pair of offset scroll blades with different solid scrolls. It has one eccentric displacement and the other fixed, which allows it to suck in the gas/air and compress it.
The scroll compressor is one of the most silent machines, with the spacing between the solid scrolls minimized, making the volume quite efficient.
Rotary Vane Compressor
This positive displacement compressor shares a similar working principle as the rotary screw compressor. However, the significant difference between them is that this compressor uses vanes – instead of screws – for compressing the gas or air in the chamber.
The full load efficiency of this positive displacement compressor is usually medium, and the control method greatly influences the part-load performance.
The diaphragm compressor is the most popular compressor type today. It uses a diaphragm to deliver and compress the fluid within the compression chamber efficiently. The diaphragm compresses the fluid due to the rotatory movement it undergoes. Diaphragm compressors are generally used for transferring toxic gases.
Benefits of Positive Displacement Compressor
Positive displacement compressors have several advantages, as specified below:
- High efficiency
- Low cost of production
- High-pressure ratio
- The components of this compressor type are easy to fabricate or manufacture
- Several pricing choices
But as with everything in life, positive displacement compressors have a few drawbacks or disadvantages you need to know.
Drawbacks of Positive Displacement Compressors
- Require incredibly high maintenance
- Not the ideal choice to use for dirty gases/air
- Not appropriate for high-flow rates
- Limited ranges of capacity
- Very noisy compared to dynamic compressors
- The engine has many more component failures than other compressor types
- Only somewhat reliable
Despite these drawbacks of the positive displacement compressor, it remains one of the most commonly used machines in several manufacturing industries.
Compressors compress and deliver gas/air from one designated area to another, and different compressors exist and are designed according to the specified demand of different applications. The positive displacement compressor is one of the most commonly used compressor types. It uses a rotary or reciprocating component for compressing working fluid.
Peripheral Valve Internals
Complete Valve Repair Kits
P.E.T. Compressor Parts