Positive displacement compressor

Definition

A positive displacement compressor is a type of compressor that increases the pressure of a gas by mechanically reducing its volume. This process involves trapping a specific amount of gas in a chamber and then forcing it out at a higher pressure, making it essential in various applications requiring consistent pressure and flow rates.

5 Must Know Facts For Your Next Test

1. Positive displacement compressors can be categorized into two main types: reciprocating and rotary types, each with its own unique design and application.
2. These compressors are widely used in refrigeration systems, air conditioning, and pneumatic tools due to their ability to deliver high-pressure outputs efficiently.
3. Positive displacement compressors operate based on the principle of trapping a specific volume of gas and then reducing that volume to increase pressure, which differentiates them from dynamic compressors.
4. The efficiency of positive displacement compressors can be affected by factors such as temperature, pressure ratios, and the properties of the gas being compressed.
5. Maintaining proper lubrication is crucial for positive displacement compressors, especially in reciprocating types, as it helps reduce friction and wear on moving parts.

Review Questions

• How do positive displacement compressors differ from dynamic compressors in terms of their operation?
  • Positive displacement compressors work by trapping a fixed volume of gas and mechanically reducing that volume to increase pressure. In contrast, dynamic compressors operate by imparting kinetic energy to the gas using rotating blades or impellers, which increases its velocity and converts that energy into pressure. This fundamental difference in operation means that positive displacement compressors are typically more effective for applications requiring high pressure at low flow rates.
• Discuss the advantages and disadvantages of using reciprocating versus rotary screw positive displacement compressors.
  • Reciprocating compressors are known for their high compression ratios and ability to handle varying loads effectively, making them ideal for applications requiring precise control. However, they tend to have higher maintenance needs due to more moving parts. Rotary screw compressors, on the other hand, provide continuous airflow with less pulsation and lower noise levels, making them suitable for industrial applications. They are generally more efficient for larger-scale operations but may not achieve as high compression ratios as reciprocating types.
• Evaluate how changes in operating conditions such as temperature and pressure ratios can impact the performance of positive displacement compressors.
  • Operating conditions like temperature and pressure ratios significantly affect the performance of positive displacement compressors. For instance, higher inlet temperatures can lead to reduced gas density, impacting the volumetric efficiency of the compressor. Similarly, increasing the pressure ratio can result in diminished performance if it exceeds the design limits of the compressor. Analyzing these factors helps in optimizing operational efficiency and ensuring that the compressor meets specific application requirements while preventing issues like overheating or mechanical failure.

Related terms

Reciprocating Compressor: A reciprocating compressor is a type of positive displacement compressor that uses a piston to compress the gas within a cylinder.

Rotary Screw Compressor: A rotary screw compressor is another type of positive displacement compressor that uses two interlocking screws to compress the gas, providing continuous flow.

Volumetric Efficiency: Volumetric efficiency refers to the ratio of the actual volume of gas delivered by a compressor to the theoretical volume it could deliver at the same operating conditions.