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Compressed Air Basics

Compressed air is a versatile and widely used source of energy in various industrial applications. However, it's essential to understand certain fundamental concepts related to PSI (pressure), stored air, moisture, heat, and airflow to ensure the efficient and reliable operation of air compressor systems.

PSI & Stored Air

Understanding pressure (PSI) and stored air volume is crucial for determining the energy stored in the compressed air system.

PSI (Pounds per Square Inch)

PSI measures the pressure resulting from the force exerted on a specific area. In the context of compressed air, it indicates the pressure at which the air is stored or delivered.

PSIG (PSI at the Gauge)

PSIG refers to the pressure measured by a gauge, indicating the pressure above atmospheric pressure.

Atmospheric Pressure

At sea level, the atmospheric pressure is approximately 14.7 psi.

Thinking of PSI and Volume as Stored Energy

In a compressed air system, the pressure and volume of air work together to store energy. Understanding this relationship is essential for sizing air compressors and determining the appropriate storage capacity.

Units of Measurement for Air and Tanks

Tools typically use air measured in cubic feet (CFM), while tanks are measured in gallons. It's important to know the conversion factor between these units (1 cubic foot = 0.133681 gallons, and 1 gallon = 7.48 cubic feet).

Key Factor: Atmospheric Pressure

The relationship between PSI, volume, and stored air is influenced by atmospheric pressure. To calculate the stored air in a tank, multiply the atmospheric pressure (14.7 psi) by the volume of the tank in gallons.


Moisture is a common challenge in compressed air systems due to the presence of water vapor in the air. Understanding how relative humidity, temperature, and dew point affect moisture levels is crucial for managing this issue effectively. To combat these issues, proper moisture management techniques like air dryers and moisture separators should be incorporated into the compressed air system.

Relative Humidity

Relative humidity measures how "full" the air is with water vapor compared to the maximum amount it can hold at a given temperature. Warmer air can hold more water vapor than colder air. For every 20ºF reduction in temperature, the amount of water vapor that air can hold is cut in half.

Dew Point

The dew point is the temperature at which the air becomes fully saturated with water vapor, reaching 100% relative humidity. When the dew point is reached, excess moisture condenses into liquid water, leading to potential issues in the compressed air system.

Effects of Moisture

As compressed air cools during storage or transportation, it releases moisture, which can lead to the following problems:

Condensate Formation

Condensation of moisture can occur within the compressed air system, leading to damage to equipment and reduced performance.

Diminished Tool Life and Performance

Moisture in pneumatic tools can cause corrosion, wear, and reduced efficiency, negatively impacting tool life and performance.

Freezing in Winter

In cold environments, moisture can freeze, causing blockages in the system and affecting the air supply.


Understanding the relationship between pressure and temperature is vital for maintaining safe and efficient compressed air systems. To manage heat generation during compression, proper ventilation, intercooling, and efficient heat exchange mechanisms are implemented in air compressor systems.

Pressure-Temperature Relationship

As air is compressed, its temperature increases due to the energy imparted during compression. Conversely, when air expands, it cools down.

Temperature Rise During Compression

The temperature rise during compression can be significant and should be monitored and managed to prevent potential issues, such as:

Equipment Damage

Excessively high temperatures can damage compressor components and downstream equipment.

Increased Moisture Holding Capacity

Higher temperatures can lead to increased moisture-carrying capacity of the air, which may result in more condensate formation during cooling.

Air Flow

Understanding air flow rates, commonly measured in CFM (Cubic Feet per Minute), is essential for designing and operating efficient compressed air systems.

CFM (Cubic Feet per Minute)

CFM measures the volume of air flowing through a system in one minute. It indicates the system's capacity to deliver air to end-use applications.

ACFM (Actual Cubic Feet per Minute)

ACFM provides a more specific way to define CFM, considering the actual conditions (pressure and temperature) at which the air flows.

SCFM (Standard Cubic Feet per Minute)

SCFM, on the other hand, refers to the CFM value standardized to specific conditions (e.g., 68°F and 14.7 psi).

CFM Displacement

CFM displacement is calculated by multiplying the stroke volume of the compressor by the pump's RPM. This value helps in understanding the compressor's theoretical air delivery potential.

Grasping these fundamental concepts of PSI, stored air, moisture, heat, and airflow is crucial for maintaining efficient, reliable, and safe compressed air systems. By implementing proper moisture management, heat control, and sizing the system correctly, businesses can optimize their compressed air usage and enhance overall productivity. Regular maintenance and proper monitoring are also essential for keeping the system in optimal condition and preventing potential issues that may arise with the compression and storage of air.

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