Sizing Industrial Air Compressors: A Guide for Smart Buyers
Purchasing or expanding a compressed air system is a significant investment for any business. Whether you're setting up a brand-new facility or upgrading an existing one, getting the sizing right is crucial, since compressor size impacts productivity. Obviously, undersizing compressors will deliver insufficient air volume to keep pneumatic equipment running properly. This is a driving force that results in oversizing. A bit of oversizing is not usually an issue, but many systems are grossly oversized for the demand. While it might seem like a safe approach, oversizing can result in several negative consequences.
The Pitfalls of Oversizing
- Higher Capital Expense: Larger compressors cost more to purchase.
- Increased Energy Costs: Larger compressors have larger motors and consume more power. Plus oversized compressors are likely to run at partial load- which is less efficient. As the demand decreases, the specific power (kW/100cfm) increases, meaning it takes more energy to produce the same amount of compressed air.
- Higher Maintenance Costs: Larger compressors usually have more expensive parts.
- Increased Wear and Tear: Oversized compressors often cycle on and off more frequently. Increased cycling leads to higher maintenance and repair costs. Components like inlet valves, vent valves, motors, bearings, and couplings experience accelerated wear.
- Moisture Accumulation: If a compressor doesn't run long enough to reach the proper operating temperature, moisture can accumulate in the lubricant, potentially causing premature airend failures.
- Pressure Fluctuations: Oversizing can also lead to pressure swings, which can result in defective products and machinery downtime.
Sizing Fundamentals: Understanding CFM and PSIG
Before you can select a compressor, you must understand the two critical metrics that determine its size:
- Flow (CFM): Cubic Feet per Minute is the measure of the volume of air a compressor can deliver. Your required CFM is the total volume of air needed by all the tools, equipment, and processes that will be running at the same time. The CFM produced determines the size of the compressor, not the presrsure.
- Pressure (PSIG): Pounds per Square Inch Gauge is the measure of the force of the compressed air. Your system's required pressure is set by the tool with the highest minimum pressure requirement. Pro Tip: Running your system at a higher pressure than needed wastes energy. A general rule is that for every 2 PSI increase above 100 PSIG, energy consumption rises by about 1%.
- Air Quality (ISO 8573:2010): Air quality is a complicated subject. The standard helps facility personnel specify compressed air quality for solid particulates, humidity, and oil. Once the ISO Air Quality classes are determined, then the type and amount of filtration required, as well as the type of compressed air drying required (either refrigeration or desiccant dryers) can be selected to meet those class levels. The type of dryer selected will impact the total CFM demand of the system.
How to Determine Your Compressed Air Demand
An accurate assessment of your demand—including general production, any seasonal changes, and expected peak demands—is the foundation of smart sizing. Here are two methods to determine your needs.
Method 1: The Manual Estimate
This method is useful for a rough estimate or for planning a new system. It involves taking an inventory of every piece of equipment that uses compressed air. For each item, you'll need to find:
- Its required airflow in CFM.
- Its required pressure in PSIG.
- Its duty cycle (the percentage of time the tool is actually in use).
Once you have this information, you can estimate your total demand. This method can be a good starting point, but be careful—guessing duty cycles can easily lead to inaccurate results. Remember to consider if your duty cycles change with the seasons; for example, do certain processes run more in the summer than the winter?
Method 2: The Professional Air Demand Analysis (ADA)
Above, we mentioned the important metric called specific power ((kW/100cfm). This metric is commonly published data for specific compressor models, which helps buyers compare and select compressor models. But the specific performance of individual compressors is not a predictor of system performance, so this metric should be measured for the entire system.
For the most accurate measurement, a professional air system audit is the best approach. An auditor uses data loggers and flow meters to record your facility's precise air usage over a typical production cycle (often a full week). This process captures your true demand profile, including peaks, valleys, and variations between shifts. It's critical that this analysis is performed during a typical or peak production period to capture your true maximum demand, as a measurement taken during a slow season can lead to undersizing.
Beyond Today: Sizing for the Future and the Whole System
Smart sizing involves looking beyond just the compressor itself.
Planning for Future Growth
Don't just buy for today's needs. Consider your company's growth plans for the next 3-5 years. However, this doesn't mean you should grossly oversize your compressor now. Instead, consider:
- A Modular Approach: Installing two or more smaller compressors instead of one large one provides both redundancy and the flexibility to add more capacity as you grow. A master system controller can orchestrate these compressors to work together for peak efficiency.
- Variable Frequency Drive (VFD): If your load varies significantly, a VFD compressor automatically adjusts its motor speed to match demand. VFDs are typically most efficient when the load varies between 40-85% of the compressor's capacity.
The System as a Whole
A correctly sized compressor can still perform poorly if other components create bottlenecks. Ensure the rest of your system is sized correctly:
- Air Receivers (Storage Tanks): Proper storage helps meet intermittent peak demands and reduces wasteful compressor cycling.
- Piping: Undersized pipes cause pressure drops, forcing the compressor to work harder and starving tools of air.
- Dryers and Filters: Air treatment components must be sized to handle the compressor's full flow without causing a significant pressure drop
Conclusion
Sizing your compressed air system correctly by accurately determining your CFM and PSIG requirements is essential for optimizing performance, reducing energy consumption, and minimizing maintenance costs. By understanding the pitfalls of oversizing and implementing these best practices, you can make an informed decision that will both reduce operating costs and deliver more stable flow for better productivity.
Additional Resources