# What is the specific power?

Specifi power: an indicator of efficiency of air compressor, calculated with electric motor power [kW] and free air delivery [m3/min] of the air compressor under the specific conditions of discharge pressure at 0.7MPa(gauge pressure) and suction air temperature of 20 degrees Celsius, 0% relative humidity.

**How to calculate the specific power?**

Assuming a Screw Air Compressor(one-stage 132kw), FAD flow is 24m3/min, working pressure is 7bar

Rated power P=132kw;

Efficiency η = 94.7%; Power Factor COSφ=0.892

Service factor=1.15

Nominal rated input power(in full load and do not take service factor into account) is:

P1 = （P ÷ η）= 132kw ÷ 94.7%= 139.39kw

Nominal rated input power(in full load and take service factor into account) is:

P2 = （P ÷ η）x (service factor-0.05)=（132kw ÷ 94.7%） x (1.15 – 0.05)= 153.33kw

Note: Theoretically, a 5% margin should be considered when calculating the service factor, which cannot be calculated in full)

Nominal specific power(at 7bar in full load and take service factor into account ) is:

PB1 = P2 ÷ 24 m3/min= 6.39kw/( m3/min)

If by air colling, should consider the input power of the fan. Assuming rate power of fan is 4.5kw, efficiency is 85%.

Power consumption of fan is PF = 4.5kw ÷ 85%= 5.29kw

Nominal total power is:

PZ = P2 + PF = 153.33 + 5.29 = 158.62 kw

A air colling screw air compressor nominal specific power is:

PB2 = PZ ÷ 24 m3/min = 158.62 ÷ 24 = 6.61kw/( m3/min)

**The relationsships between working pressure, volume air and power of air compressor**

**What is the working pressure of air compressor**

Working pressure of air compressed or we called discharge pressure is the max pressure on the output side of air compressor when generate the air

Unit transfer

Unit | bar | Mpa | psi | kg |

bar | 1 | 0.1 | 14 | 1.0 |

Mpa | 10 | 1 | 145 | 10 |

psi | 0.06 | 0.006 | 1 | 0.07 |

kg | 0.9 | 0.09 | 14 | 1 |

**What is the volume flow of air compressor?**

“Actual volume flow rate of a compressor is the actual volume of gas, compressed and delivered at the standard discharge point, referred to conditions of total temperature, total pressure and composition prevailing at the standard inlet point.” – IS01217

Unit is m3/min on the name plate of air compressor

**What is the power of air compressor?**

The power of the air compressor refers to the nameplate power of the matched drive motor or diesel engine. The unit of power is: KW or HP. The conversion method is: 1KW ≈ 1.333HP. Unit is KW or HP; 1KW=1.333HP

**The relationship between working pressure, volume flow and power?**

Under a certain of power, the volume flow and pressure also change accordingly with he speed changes.

For example, a 22KW air compressor, working pressure at 7bar, calculating the speed according to the technical curve of the compressor host, capacityis 3.8m3/min. When pressure at 8bar, speed must reduce(otherwise the drive motor will be overloaded), then capacity is 3.6m3/min. The speed are reduced and the volume of discharge air is reduced accordingly.