Air compressors are essential tools in an auto body workshop, providing the compressed air necessary to operate various pneumatic equipment such as spray guns, impact wrenches, and sanding machines. A thorough understanding of air compressor types and their operation ensures that technicians select the appropriate compressor for specific tasks, optimizing performance and safety. In Kenya’s automotive service industry, where workshops range from small garages to large service centers, knowledge of compressor classification helps in maintaining efficient, cost-effective operations.

6.1 Classification of Air Compressors

Classification of air compressors involves grouping them based on different operational principles and design features. This classification aids auto body technicians in identifying the best type of compressor for their workshop needs, depending on factors like pressure requirements, air volume, and energy efficiency.

6.1.1 According to Working

Air compressors can be classified according to their working principle, which determines how air is compressed and delivered. Understanding this classification helps technicians match compressors to the demands of specific pneumatic tools used in auto bodywork, such as paint sprayers and air ratchets.

Positive Displacement Compressors

Positive displacement compressors operate by trapping a fixed volume of air and reducing its volume to increase pressure. This category is highly common in auto body shops because it provides a steady and reliable air supply.

• Reciprocating (Piston) Compressors: Use pistons moving in cylinders to compress air. They are widely used in small to medium workshops due to their ability to generate high pressure and ease of maintenance.
• Rotary Screw Compressors: Employ two meshing helical rotors to compress air continuously. These are preferred in larger workshops with high air demand because they provide smooth, uninterrupted airflow.
• Rotary Vane Compressors: Contain a rotor with sliding vanes that compress air within a chamber. They are suitable for moderate pressure needs and are quieter, which benefits workshops in residential areas.
• Scroll Compressors: Utilize two interleaved spiral scrolls to compress air. Though less common in auto body shops, they offer compact design and low noise operation.
• Diaphragm Compressors: Use a flexible diaphragm to compress air, mainly for specialized applications requiring oil-free air.

Dynamic Compressors

Dynamic compressors increase air velocity and convert it into pressure through diffusers or volutes. These compressors are generally used in industrial applications but are less common in auto body workshops due to their complexity and large size.

• Centrifugal Compressors: Use a rotating impeller to impart velocity to air, which is then converted to pressure. They are suitable for continuous high-volume air supply but require significant space and maintenance.
• Axial Compressors: Compress air by accelerating it axially through multiple stages of rotating blades. These are typically found in jet engines and are not practical for auto body workshops.

Practical Considerations in Kenyan Workshops

Reciprocating and rotary screw compressors dominate Kenyan auto body workshops due to their adaptability and cost-effectiveness. For example, a SACCO-owned vehicle repair workshop in Nakuru uses a rotary screw compressor to handle multiple spray booths simultaneously, ensuring consistent paint application quality.

6.1.2 According to Action

Classifying air compressors according to their action involves the method by which air is compressed within the device. This classification focuses on the mechanical movement and flow characteristics of the compressor.

Single-Acting Compressors

In single-acting compressors, compression occurs on one side of the piston during its movement. These compressors are simpler and usually less expensive, making them suitable for small workshops with low to moderate air demands.

• They have fewer moving parts, which reduces maintenance complexity.
• The air delivery is intermittent, which might require a storage tank to smooth out the supply.
• They typically have lower power requirements, making them energy efficient for occasional use.
• Noise levels tend to be higher due to the piston’s single compression stroke.
• Single-acting compressors are common in rural garages where cost and simplicity are prioritized.

Double-Acting Compressors

Double-acting compressors compress air on both sides of the piston during each stroke, effectively doubling the output compared to single-acting units of the same size.

• They provide a more continuous and smoother air flow.
• These compressors are larger and more complex, requiring skilled maintenance.
• Energy consumption is higher but justified by increased output.
• Double-acting compressors are suitable for busy workshops, such as county government vehicle maintenance depots, where continuous air supply is critical.
• Their higher initial cost is offset by improved efficiency and productivity.

Other Action Types

• Rotary Action: Rotary compressors, such as screw and vane types, use continuous rotary motion to compress air, providing steady airflow ideal for professional auto body shops.
• Reciprocating Action: Involves back-and-forth piston movement, typical of piston compressors.
• Oscillating Action: Less common, involves parts moving in oscillation to compress air.

6.1.3 According to Number of Stages

The number of compression stages affects the pressure output and efficiency of air compressors. Understanding this classification helps technicians select compressors that meet specific pressure requirements for various auto body applications.

Single-Stage Compressors

Single-stage compressors compress air once per cycle, suitable for low to medium pressure applications.

• They are simpler in design and less costly to purchase and maintain.
• Typically deliver pressure up to about 8 bar (116 psi), adequate for many pneumatic tools used in auto body workshops.
• The cooling system is less complex, reducing operational costs.
• Single-stage compressors are common in small garages and retail auto shops.
• Their air delivery is less efficient for high-demand tasks like continuous spray painting.

Two-Stage Compressors

Two-stage compressors compress air in two successive stages, increasing pressure more efficiently.

• Air is first compressed to an intermediate pressure, cooled, then compressed again to the final pressure.
• This process improves efficiency and reduces the temperature of compressed air, protecting tools and increasing compressor lifespan.
• Two-stage compressors can generate pressures above 10 bar (145 psi), necessary for heavy-duty pneumatic equipment.
• They are preferred in professional workshops with higher air volume and pressure requirements, such as large county government fleet service centers.
• Maintenance involves more components but results in longer service intervals.

Multi-Stage Compressors

Multi-stage compressors have three or more compression stages, designed for very high pressure and industrial-scale applications.

• They provide very high pressure and volume, often exceeding 20 bar (290 psi).
• Cooling between stages is critical to prevent overheating and maintain efficiency.
• These compressors are rarely used in typical auto body workshops due to their size and cost.
• They find use in specialized sectors like manufacturing and power plants.
• Multi-stage compressors require advanced maintenance skills and infrastructure.

Chapter Summary

This chapter began by exploring the classification of air compressors based on their working principles, types of action, and the number of compression stages involved. It then focused on the single stage reciprocating air compressor, detailing the work done during compression when no clearance volume is present. The discussion progressed to the thermodynamic processes involved in compression, examining the work done during isothermal, polytropic, and isentropic compression, highlighting how each process affects compressor performance. The power required to drive a single-stage reciprocating air compressor was analyzed through practical calculations. Further, the chapter addressed the impact of clearance volume on the work done by a reciprocating air compressor and introduced methods to determine multistage compression effects. The power requirements for operating a two-stage reciprocating air compressor were then calculated, considering the complexities introduced by multiple stages. Finally, the chapter concluded with the determination of the minimum work necessary for two-stage compression, emphasizing the efficiency gains achievable through optimal compressor design.