Selecting the right compressed air system for your business is a significant capital investment decision, and the size you choose will significantly impact operational efficiency, energy costs, and equipment reliability. A unit that is too small cannot keep up with operations, while a unit that is too large results in significant inefficiencies. This article will provide you with a technical walkthrough on how to properly size an air compressor for your business.
The Importance of Correct Sizing
Incorrect compressor sizing leads to operational and financial problems. Both undersizing and oversizing create distinct issues that reduce productivity and increase costs.
Problems Caused by Undersizing
An undersized air compressor acts as a bottleneck in your system; it cannot produce the required volume of air, leading to several problems:
- Pressure Drops: Operating your tools without the pressure needed to reach their specified performance levels, can result in inconsistent work and low-quality output.
- Production Halts: Operations may need to pause while the system reestablishes the correct air pressure, which will cause downtime and decrease your productivity.
- Continuous Running and Overheating: Undersized compressors must operate continuously to meet the excessive air demand, exceeding their designed duty cycle, leading to overheatin,g which increases component wear and shortens their operational life.
Problems Caused by Oversizing
An oversized compressor presents a different set of issues. Oversized units meet air demand too quickly and enter an inefficient pattern of short operating cycles.
- Energy Inefficiency: A compressor is most efficient when running at full load. Short-cycling forces it into an ‘unloaded’ state where it consumes significant amounts of energy without producing useful air, wasting costly resources.
- Accelerated Mechanical Wear: Constant starting and stopping puts stress on the motor, the couplings, and airend bearings, which is likely to result in higher maintenance requirements and breakdowns.
- Moisture and Internal Corrosion: When run times are too short, your compressor will not reach its optimal operating temperature. This temperature is required to vaporise any condensed moisture in the system. If it is not reached, water will be present in the lubricant, encouraging rust and internal corrosion.
Correct sizing has a direct impact on lifecycle costs. Energy can account for as much as 80% of a compressor’s total cost, making sizing a significant factor when it comes to your ROI.
Decoding the Metrics That Matter
To size a compressor correctly, it is necessary to understand the technical metrics that define its performance.
- Flow (FAD vs. CFM): Flow refers to the volume of air a machine produces in a given time, and is the most critical sizing factor for determining machine capacity. It is often expressed in Cubic Feet per Minute (CFM). The most accurate measure, however, is Free Air Delivered (FAD), which is the actual volume of usable air delivered at the discharge point after internal losses have been accounted for. Compressors should be compared based on their FAD and not their CFM.
- Pressure (PSI/bar): Pressure is the force exerted by your compressed air. The single tool or process with the highest PSI requirement is used to determine the system pressure your facility requires.
- Duty Cycle: The duty cycle describes whether a compressor is designed for binary (on/off) operation or continuous duty. Piston compressors are manufactured for on/off duty (60-75% duty cycle), whereas rotary screw compressors are designed for a 100% duty cycle, and are the industry standard for continuous duty applications.
- The Horsepower (HP) Myth: Sizing by HP is a big mistake. Horsepower is a measurement of motor input power, but that does not determine the amount of air the compressor can produce. An efficient modern air compressor can generate a much higher FAD at a lower HP than previous models, so compressors should be sized by FAD and PSI only.
A Step-by-Step Guide to Calculating Your Air Demand
Determining your facility’s air demand is a systematic 5-step process.
Step 1: Inventory Your Air Operated Equipment and Processes
List every piece of equipment, each tool, and every process that uses compressed air in your facility.
Step 2: Determine Individual Consumption & PSI Requirements
Consult the manufacturer’s manual for each item to determine its specific air consumption (CFM or FAD) and the PSI each requires.
| Tool / Equipment | Average CFM Requirement (at 90 PSI) |
| 1/2″ Impact Wrench | 4-5 CFM |
| Blow Gun (1/4″) | 3-4 CFM |
| Production Spray Gun | 8-12 CFM |
| Sander / Polisher | 10-20 CFM |
| Tyre Inflator | 2-3 CFM |
Note: These are average values. Always use the specific requirements stated for your tools.
Step 3: Apply a Utilisation Factor
It is unlikely that all of your tools will be used simultaneously, so determine your peak demand by selecting the family of tools most likely to be used simultaneously during your busiest period, and adding their consumption requirements together to reach a total.
Step 4: Add a Buffer
Add a buffer of 25-30% to the number calculated in Step 3 to account for small air leaks in your pipework, to allow for future growth, and to prevent your compressor from running continuously at 100% of its capacity.
Step 5: Allow for Pressure Drops
Air treatment systems (filters, dryers) create pressure drops, and this loss must be added to your peak PSI requirement. If a tool needs 90 PSI, for example, and the treatment system causes a 10 PSI drop, the compressor must be set to deliver at least 100 PSI to account for both.
Selecting the Right Compressor Technology
With your required FAD and PSI calculated, you can now choose the most appropriate technology for your facility. As a Premier Distributor for Atlas Copco, J Ll Leach provides expertise in matching businesses with their ideal air compression solutions.
For intermittent demand, a piston compressor, such as the Atlas Copco LF series, is the most suitable, while a rotary screw compressor is required for continuous industrial use. The control technology for your air compressor is also a key consideration:
- Fixed Speed Compressors: These are efficient when operating at or near 100% capacity. They are best for facilities with stable and predictable air demand.
- Variable Speed Drive (VSD) Compressors: For applications with variable air demand, the most efficient compressor is a VSD compressor. The Atlas Copco GA VSD⁺ is an example of a VSD compressor; the motor speed is continuously matched to actual air demand, resulting in energy savings of up to 60%. A Variable-Speed-Drive (VSD) Air Compressor is the answer when you have variable air demand.
Oil-free compressors, such as the Atlas Copco ZR/ZT series, deliver ISO 8573-1 Class 0 clean air for food & beverage, and pharmaceutical applications. For large or scalable applications, an AIRCUBE containerised station provides a complete plug-and-play compressed air solution.
Sizing the Entire System, Not Just the Compressor
Even correctly sized compressors can be made inefficient by other undersized system components. That is why a professional Air Compressor Installation always considers the entire system:
- Air Receiver (Tank): The tank buffers demand peaks and reduces wear on the compressor from cycling. It also helps to remove moisture from the air.
- Air Treatment: Air dryers and filters are essential for protecting your equipment. Effective condensate management (the process of safely removing collected water and oil from your system) is also crucial for your system’s health and compliance with environmental industry standards.
- Heat Recovery Systems: A significant amount of the energy used in air compression is converted to heat. Atlas Copco heat recovery systems can capture up to 90% of this waste heat so that it can be repurposed for other uses, such as heating water or spaces. This converts pure waste into a valuable resource and reduces your facility’s energy bills.
UK Compliance Requirements
Operating a compressed air system in the UK requires adherence to strict legal standards. Key regulations include:
- Pressure Systems Safety Regulations (PSSR) 2000: Requires most systems to have a Written Scheme of Examination (WSE).
- Pressure Equipment Regulations 2016: Ensures equipment is designed and manufactured to be safe.
- Air Quality & Efficiency Standards: ISO 8573-1 defines air purity, BS EN 12021 sets standards for breathing air, and ISO 11011 provides guidelines for energy-efficient systems.
- F-Gas Regulations: These regulations apply to systems using refrigerant dryers and govern the handling of fluorinated greenhouse gases.
The Role of a Professional Air Audit
While this guide provides a useful technical framework, the most accurate sizing method is to conduct a professional air audit. Using data loggers and ultrasonic flow meters, a comprehensive energy audit from J Ll Leach provides precise usage data. It measures your peak, average, and minimum air demand, quantifies the cost of any concealed air leaks, and provides accurate up-to-date data for planning your system upgrades.
Sizing an air compressor is a strategic technical decision. By focusing on FAD and PSI, understanding the duty cycle, and taking a holistic view of the entire system, you can invest in an asset that delivers efficiency and reliability.
Ready to size your system correctly?
Take the next step towards an efficient and reliable compressed air system.
- Download our free Sizing Checklist to guide your internal calculations.
- Book a no-obligation consultation with our engineering team to discuss your specific needs.
- Schedule a professional Air Audit for a data-driven plan to optimise your facility.
Contact J Ll Leach today to get started.