How to select the right compressor.

Selecting the right compressor type is crucial for efficient and reliable operations. This article provides a depth guide to help you make the best decision based on your specific application requirements.

There are three instances when someone might want a new compressor, due to equipment upgrade, due to increased demand or they are getting it for the very first time. In the first two scenarios it is prudent to get your system audited by a compressed air specialist.?

Auditors provide a comprehensive and unbiased evaluation of the current compressed air system. Their expertise helps identify potential energy inefficiencies, areas of improvement, and opportunities to optimize the system.?

An auditor may not recommend a new compressor except when the existing compressors are old, inefficient, or cannot handle increased demand. They focus on identifying and implementing measures that can improve the existing compressed air system's performance without the need for additional equipment.?

When selecting a compressor for industrial applications, your choice will be influenced by the following factors:

1. Application

The first step in choosing the right compressor is to thoroughly understand the application requirements.

Identify the specific application. They are grouped into these three broad categories: Process, Power, and Pneumatic applications.

For process application (e.g., food and beverage, pharmaceutical, chemical), You will need to consider the required quality standards. (e.g., oil-free air for food processing).

When considering air quality, two types of compressors come into consideration: oil-free compressors and oil-lubricated compressors.?

• Oil free: Guaranteed Oil-free air

1. No contamination
2. Initial cost is higher, but payback exists.
3. Air quality conforms to Class 0 as per ISO 8573

• Lubricated compressors (with oil removal filters); Sometimes called technically Oil free air.

1. High chances of contamination
2. Lower initial costs but high on energy and maintenance
3. Air quality conforms at best to Class 1 as per ISO 8573 1 ed1 (1991)

Oil-Free Compressors: Provide guaranteed oil-free air with no contamination. Suitable for applications where air quality conformity to Class 0 as per ISO 8573 is required.

Lubricated Compressors: May have chances of contamination and conform at best to Class 1 as per ISO 8573 1 ed1 (1991). These are more cost-effective initially but higher on energy and maintenance costs.

2. Flow and working pressure.

Identify the pneumatic tools or equipment used in your industry and calculate the total air demand of all pneumatic tools, considering the duty cycle and peak usage periods.

Evaluate the flow rate and pressure demands of your pneumatic tools or equipment.

Let’s consider an example of a compressed Air System for an Auto Repair Workshop:

Step 1: Understand Application Requirements:

I. The auto repair workshop requires compressed air for various tasks, including pneumatic tools, tire inflation, and cleaning equipment.

II. The maximum air flow rate needed is 50 CFM during peak usage, and the minimum required is 20 CFM during regular operations.

Step 2: Calculate Total Air Demand:

Total air demand = Maximum air flow rate + Safety margin

Total air demand = 50 CFM + 30% safety margin

Total air demand = 50 CFM + 15 CFM = 65 CFM

Step 3: Consider Duty Cycle and Load Profile:

The auto repair workshop operates 8 hours a day, 6 days a week, and the compressor will run continuously during working hours (100% duty cycle).

Step 4: Assess Pressure Drop and Piping Losses:

The required working pressure for most tools and equipment is 90 psi (pounds per square inch).

After considering pressure drop and piping losses, the system requires an additional 10 psi, bringing the total pressure requirement to 100 psi.

• For medium flow and low - medium pressure applications between 4-10 bar, Screw compressors are most applicable.
• Centrifugal compressors are the most popular technology for high-capacity applications and continuous high flows above 4000m3/hour, medium to high pressure.
• Piston compressors are more applicable for low flow, high pressure (20 – 200 bar) applications because they’re capable of achieving high discharge pressure without risks of damage. Pistons should not be used for medium pressure (7 - 10barg) applications due to progressive loss of efficiency and reliability over the unit lifetime.

3. Demand Profile

The next step in selecting the appropriate compressor is to understand the application’s demand profile. Consider the following factors:

Choose a compressor type and capacity that aligns with your application's continuous or intermittent air demand and pressure requirements.

• Screw compressors are mostly where continuous operation is needed. This is because they have few moving parts, reducing the risks of mechanical failures. This makes them suitable for continuous-duty applications.
• For applications with intermittent or fluctuating demands, piston compressors offer versatility and the ability to handle varying flow rates and pressures. They can easily start and stop making them applicable where intermittent high-pressure gas is required.
• When dealing with continuous high-flow and high-pressure demands, centrifugal compressors excel at delivering large volumes efficiently. They are most efficient when operating at or near full capacity, which makes them ideal for applications with consistent high demand.

In some cases, a combination of compressor types might be used in a compressed air system to optimize efficiency and address different demand scenarios throughout the process.

Continuing with the compressed Air System for an Auto Repair Workshop example:

Step 6: Select the Compressor Type

Considering the continuous operation and varying flow requirements, a piston compressor is chosen for its ability to handle intermittent demands and moderate flow rates. We settled for a double stage piston compressor with a flow rate of 65CFM and 100psi.

Double-stage piston compressors are more efficient than single-stage compressors due to their two-stage compression process, intercooling, lower discharge temperatures, and ability to handle higher compression ratios.

Different compressor types offer flexibility for indoor, outdoor, or portable applications.

Conclusion:

By understanding the application requirements, considering flow, and working pressure, and evaluating air quality and installation location, you can make an informed decision about the right compressor type and size. Whether it's a screw compressor for continuous operation, a piston compressor for varying demands, or a centrifugal compressor for high-flow applications, selecting the optimal compressor will ensure efficient and reliable performance for your specific needs.

Lorna Olum

Application Engineer,

REN Engineering Solutions

T??+254 715 833 628

[email protected]

https://www.ren-engineering-solutions.com/