How is the displacement of screw air compressor defined?

The displacement of the screw air compressor is a key parameter to measure its core performance, and its definition, influencing factors and practical significance can be elaborated from the following perspectives:

definition of 1. displacement

the displacement of a screw air compressor (also known as volume flow or air supply) refers volume of compressed air discharged from air compressor per unit time, usually in cubic meters per minute (m & sup3;/min) or cubic meters per hour (m & sup3;/h). This parameter reflects the ability of the compressor to continuously provide compressed air under specific operating conditions.

Core characteristics of 2. displacement

1. standard Status and Actual Status
   • standard Status: Exhaust volume usually refers standard atmospheric pressure (101.325kPa) and specific temperature (e. g. 20 ℃ or 0 ℃) volume flow under. For example, the nominal displacement of an air compressor is 10m & sup3;/min, which means that 10 cubic meters of air can be discharged per minute under standard conditions.
   • Actual Status: Under actual working conditions, the temperature, pressure and humidity of compressed air will affect its volume. For example, in a high temperature and high humidity environment, the air volume expands, and the actual exhaust volume may be lower than the nominal value; when the exhaust pressure increases, the air mass per unit volume increases, but the volume flow rate may decrease due to the increase in the compression ratio.
2. Relationship with exhaust pressure
   • exhaust volume and exhaust pressure are inverse relationship. In the case of constant power, the higher the exhaust pressure, the more energy the air compressor needs to consume to compress the air, resulting in a decrease in the volume of air discharged per unit time. For example, the exhaust volume of the same air compressor at a pressure of 7bar may be lower than the exhaust volume at a pressure of 5bar.

3. Key Factors Affecting Exhaust Volume

1. air compressor design parameters
   • rotor diameter and length: The larger the diameter and length of the rotor, the larger the volume of compressed air in a single rotation and the higher the exhaust volume.
   • Speed: The higher the speed, the more times the rotor rotates per unit time, and the greater the displacement. However, high speed may lead to increased mechanical wear, the need to balance efficiency and life.
   • Screw profile design: Advanced screw profiles (such as asymmetric profiles) can optimize the compression process, reduce leakage, improve volumetric efficiency, thereby increasing exhaust capacity.
2. Operating condition
   • intake condition: Intake air temperature, humidity and pressure directly affect air density. For example, an increase in intake air temperature will cause a decrease in air density, a decrease in air mass for the same volume, and a decrease in exhaust gas volume (by mass).
   • Exhaust pressure: As mentioned earlier, an increase in exhaust pressure will reduce the exhaust volume.
   • Cooling effect: A good cooling system can maintain a low compression temperature, reduce air expansion, and increase the actual exhaust volume.
3. Maintenance status
   • air filter clogged: The increase in intake resistance will cause the intake air volume to decrease and indirectly reduce the exhaust volume.
   • Screw clearance wear: After long-term operation, the gap between the screw and the casing increases, the gas leakage increases, the volumetric efficiency decreases, and the exhaust volume decreases.
   • quality of lubricating oil: Lubricating oil viscosity is too high or too low will affect the sealing of the screw rotor, thereby affecting the displacement.

Practical application significance of 4. displacement

1. equipment selection basis
   • in industrial production, it is necessary to select an air compressor with appropriate exhaust volume according to the total air consumption of gas equipment. For example, a production line needs to consume 8m & sup3 per minute; For compressed air, it is necessary to select an air compressor with an exhaust volume ≥ 8m & sup3;/min, and reserve 10%-20% margin to meet peak demand.
2. energy efficiency evaluation index
   • the ratio of displacement to power (I. e. specific power, kW/(m & sup3;/min)) is an important indicator for evaluating the energy efficiency of air compressors. The lower the specific power, the lower the energy consumption per unit exhaust volume and the lower the operating cost. For example, the specific power of an energy-efficient air compressor is usually ≤ 7.2kW/(m & sup3;/min).
3. Fundamentals of System Optimization
   • in a system with multiple air compressors operating in parallel, the number and model of units should be reasonably configured according to the total displacement demand, so as to avoid the phenomenon of "big horse-drawn cart" or "small horse-drawn cart" and ensure the efficient operation of the system.

Measurement and correction of 5. displacement

1. measurement method
   • direct measurement method: Install a flowmeter at the outlet of the air compressor to directly measure the volume flow of compressed air.
   • indirect calculation method: By measuring the power, speed and pressure of the air compressor and other parameters, combined with the theoretical model to calculate the displacement.
2. Working condition correction
   • the actual displacement shall be corrected according to the standard state. For example, if the air compressor at 35 ℃, 80% relative humidity measured exhaust volume is 10m & sup3;/min, need to be converted to the standard state of the exhaust volume by formula, in order to compare with the nominal value.