1. Terminology Related to Performance Parameters
Key performance parameters include:
Operating Pressure (commonly referred to as: Discharge Pressure): This refers to the maximum gas pressure at the air compressor's outlet. It is typically expressed as gauge pressure (P(G)), with common units being bar or MPa (1 bar = 0.1 MPa). In the air compressor industry, the term "kilograms" (or "kilos") when referring to pressure actually denotes *bar*; for example, "8 kilograms of pressure" equals 8 bar.
Note: Gauge pressure represents the difference between the pressure inside a vessel and the ambient atmospheric pressure; specifically, Gauge Pressure = Absolute Pressure – Atmospheric Pressure.
Pressure Unit Conversion: For instance, 1 MPa = 10 bar = 145 psi; 1 bar ≈ 0.98 kg/cm² (a unit commonly referred to as "kilogram pressure" in engineering practice).
Volumetric Flow Rate (commonly known as: Displacement): The volume of gas discharged by a compressor per unit of time, calculated based on the intake conditions. The standard units are m³/min or L/min. This metric reflects the air compressor's supply capacity and serves as a crucial indicator for assessing its gas-generating capability.
Unit Conversion: The standard unit for airflow volume in domestic markets is m³/min, whereas CFM is commonly used internationally.
1 CFM = 28.316847 Liters/minute (L/min)
1 CFM = 0.028316847 Cubic meters/minute (m³/min)
1 CFM ≈ 1.7 Cubic meters/hour (m³/h)
Specific Power: The power consumed per unit of volumetric flow rate; this is a key indicator for evaluating energy efficiency. It serves as the core parameter for assessing the energy efficiency of an air compressor, with units expressed in kW/(m³/min). The lower the specific power, the higher the energy efficiency.
Other important terms include:
Shaft Power: The power transmitted from the electric motor to the compressor shaft, measured in kW. 1 Horsepower (HP) ≈ 0.75 Kilowatts (kW).
Dew Point: The temperature to which compressed air must be cooled for water vapor to begin condensing and separating from the gas; this metric reflects the dryness of the gas. It is typically categorized into two types: Pressure Dew Point and Atmospheric Dew Point. The unit of measurement is °C.
Air-to-Power Ratio: The amount of electrical energy consumed to produce one cubic meter of compressed air, measured in kWh/m³.
2. Terms Related to the Compression Medium
Compression Medium: Refers to the working substance utilized within a compressor to undergo compression; typically, this consists of air or an inert gas.
Air is widely utilized as a compression medium due to its compressibility, transparency, ease of transport, safety, and inexhaustible supply. Inert gases (such as dry nitrogen or carbon dioxide) may also serve as compression media, as they do not react chemically with the surrounding environment.
Air Composition: The primary constituents of dry air are Nitrogen (N₂) at 78.08%, Oxygen (O₂) at 20.93%, and Carbon Dioxide (CO₂) at 0.03%. The density of dry air at 0°C and 760 mmHg is 1.293 kg/m³.
Compression Process: "Stages" and "Sections." In positive displacement compressors, each cycle of compression and cooling within a working chamber is termed a "stage"; in dynamic compressors, a single cooling process following multiple compression cycles by impellers is termed a "section."
Compression Efficiency: The ratio of actual compression work to theoretical compression work.
3. Terminology Related to Gas States
(1) Basic State Parameters
Discharge Pressure: The gas pressure at the compressor outlet, measured in bar or MPa.
Volumetric Flow Rate: The volume of gas drawn into the compressor per unit of time, measured in m³/min.
Compression Ratio: The ratio of the absolute discharge pressure to the absolute intake pressure of the compressor (e.g., discharge 20 bar / intake 5 bar = 4). In engineering practice-particularly in multi-stage compression-designs are often based on isothermal compression.
(2) Processes of Gas State Change
Isothermal Compression: The gas temperature remains constant during compression; this requires ideal heat transfer conditions (which are difficult to achieve in practice) and is used as a benchmark for economic efficiency.
Adiabatic Compression: There is no heat exchange between the gas and the external environment; entropy remains constant, and the process follows Poisson's Law (e.g., compression within an adiabatic cylinder or rapid expansion).
Polytropic Compression: The actual compression process falls between isothermal and adiabatic conditions; the gas temperature changes, and heat exchange occurs.
(3) Normal Conditions and Standard Conditions
Normal Conditions (Abbreviation: Normal): Pressure is 0.1 MPa (i.e., 1 standard atmosphere), temperature is 0°C, and humidity is 0%. Unit: Nm³/min.
Standard Conditions: The state of air at a temperature of 20°C, an absolute pressure of 0.1 MPa, and a relative humidity of 0%. Unit: m³/min.
Conversion: 1 Nm³/min = 0.932 m³/min.
The discharge capacity of air compressors-as well as the processing capacity of downstream equipment such as air dryers-is typically specified in terms of flow rate under standard conditions (unit: m³/min).
4. Terminology Related to Noise
Air compressor noise levels are typically expressed as A-weighted sound pressure levels, measured in decibels (A) and denoted as dB(A) or dBA. Reference: GB 3102.7, *Acoustics-Quantities and Units*. The measurement standards and specifications are based on the national standard GB/T 4980-2025, *Determination of Noise Emitted by Positive Displacement Compressors*. JB/T 6430, *Oil-injected Screw Air Compressors for General Use*, stipulates that, under specified operating conditions, the sound power level of a screw air compressor shall not exceed the values specified in the table below.
