How to calculate the air volume of air compressor

Air compressor air volume accounting needs comprehensive consideration rated parameters, actual working conditions, operation modes and loss factors here are the detailed steps and key points:

1. core parameters and units

1. rated gas volume:
   • marked by the manufacturer Free Air Delivery (FAD, Free Air Delivery), the unit is Nm & sup3;/min (standard cubic meters per minute) or m & sup3;/min (cubic meters per minute under actual conditions).
   • Standard Status: usually refers to 0°C(273.15K), 1 atmospheric pressure (101.325kPa), at this time 1Nm & sup3; The air volume is fixed.
   • Actual working condition: It is necessary to consider the influence of intake air temperature, pressure and humidity on the volume.
2. Key Terms:
   • Volume Flow (Displacement Volume): The geometric displacement of the compressor piston or rotor per revolution (such as 20L/revolution), the theoretical gas volume shall be calculated in combination with the speed.
   • Actual output gas volume: Affected by factors such as volumetric efficiency, leakage, and pressure ratio, it is usually lower than the theoretical value.

2. accounting steps

1. Determine the rated gas volume (FAD)

• refer to instructions: Directly get the FAD value marked by the manufacturer (e. g. 10Nm & sup3;/min).
• Unit conversion:

  • if it is marked as volume flow (e. g. 2m & sup3;/min @ 7bar), it needs to be converted to FAD through the ideal gas state equation:

FAD(Nm & sup3;/min)= volume flow (m & sup3;/min)×P standard ×T actual P actual ×T standard

Where: - $P_{ text {actual }}$: exhaust pressure (absolute pressure, such as 7bar = 700kPa + 101.325kPa = 801.325kPa) - $T_{ text {actual }$: intake temperature (such as 25°C = 298.15K) - $P_{ text {standard }}= 101.325text{kPa $,$T_{}}{ text {standard {K}

• example:
  compressor marked 2m & sup3;/min @ 7bar, inlet temperature 25°C:

FAD=2×101.325×298.15801.325×273.15 ≈14.7Nm³/min

2. Consider the correction of actual working conditions

• influence of intake air temperature: The increase in temperature will cause the air density to decrease and the actual output volume to decrease.

  • correction formula:

actual gas volume = FAD×T actual T standard

(It is only applicable when the pressure is constant, and comprehensive pressure and temperature correction is required for complex working conditions)

• example:
  FAD = 10Nm & sup3;/min, Inlet temperature 40°C(313.15K):

actual gas volume = 10 × 313.15273.15 & asymp;8.72Nm & sup3;/min

• pressure ratio effect: The increase of exhaust pressure will reduce the volumetric efficiency and the actual air volume will decrease.
  • Experience correction: For every 1bar increase in pressure, the gas volume will be reduced by about 3%-5% (refer to the compressor characteristic curve).

3. Operation mode and load rate

• continuous operation: Total gas volume = rated gas volume × operation time (e. g. 10Nm & sup3;/min× 8h = 80Nm & sup3;).

• Intermittent operation: Load rate to be calculated (e. g. 70% full load time, 30% no load time):

total gas volume = rated gas volume x load rate x operating time

example: 10Nm & sup3;/min compressor, load rate 70%, running for 8h:

total gas volume = 10 × 0.7 × 8=56Nm & sup3;

4. Loss factor

• pipeline leakage: Usually 5%-15% gas loss needs to be measured and corrected by pressure drop or flowmeter.
• Post-processing equipment: Dryers, filters, etc. may cause additional pressure drop and affect the end gas volume.
• compressor efficiency attenuation: After long-term operation, the volumetric efficiency may drop by 5%-10%.

3. simplified accounting methods

1. Theoretical volume flow method

• formula:

theoretical gas volume (m & sup3;/min)= displacement (L/revolution) × rotation speed (revolution/min)÷ 1000

example: Displacement 50L/revolution, rotating speed 1000 revolutions/min:

theoretical gas volume = 50 × 1000 ÷ 1000=50m & sup3;/min

• actual gas volume= theoretical gas volume x volumetric efficiency (usually 0.85-0.95).

2. Experience estimation table

compressor type	typical Volumetric Efficiency	gas volume range (Nm & sup3;/min)
piston compressor	0.85-0.90	0.1-100
screw compressor	0.90-0.95	1-500
centrifugal compressor	0.92-0.98	100-10000 +

4. practical application example

scene: A factory selects screw compressor, marked FAD = 20Nm & sup3;/min @ 7bar, intake air temperature 30°C, 16 hours of daily operation, load rate 80%, pipeline leakage rate 10%.
Accounting steps:

1. correct intake air temperature:

actual gas volume = 20 × 30+273.15273.15 & asynmp; 18.4Nm & sup3;/min

2. consider the load rate:

daily gas output = 18.4 × 0.8 × 16 & asynmp; 235.5Nm & sup3;

3. deduct leakage loss:

end usable gas volume = 235.5 ×(1 & minus;0.1)& asynmp; 212Nm & sup3;

5. considerations

1. unification of units: Ensure that the pressure (kPa/bar) and temperature (°C/K) units are consistent to avoid calculation errors.
2. Working condition matching: The rated gas volume shall be compared under similar pressure and temperature, and the data under different working conditions shall be converted.
3. dynamic monitoring: After long-term operation, it is recommended to measure the gas volume through the flowmeter to verify the theoretical accounting results.
4. Energy Efficiency Assessment: Combined with the specific power (kW/m & sup3;/min) to evaluate the compressor efficiency and optimize the selection.

Summary: Air compressor volume accounting shall be based on rated FAD based, combined actual working condition, operation mode and loss factor make corrections. Through formula conversion, empirical estimation or measured data, the available gas volume in different scenarios can be accurately calculated, which provides a basis for equipment selection and system design.