The only criterion to judge whether the air compressor can save energy is "specific power". In order to understand the specific power of a screw air compressor, first of all, we should have a complete understanding of the concept of input power. We know that the electricity charges paid by users are not the output power consumption of the whole machine, but the input power consumption of the whole machine, that is, the total input energy consumption of the machine. Next, I will give two examples to illustrate the input power calculation in two cases:
1. Calculating input power consumption based on motor nameplate parameters
(1) For any machine, the motor nameplate parameters should be specified as follows: (Take 132 kW single-stage air compressor as an example: flow rate = 24 m/min, working pressure = 7 Bar):
Rated motor power (rated output power or rated shaft power) P = 132 kw
Motor efficiency (for example, Huada Motor): _= 94.7%
Power factor: COS phi = 0.892
Service coefficient S.F = 1.15 (also adopted by manufacturers S.F = 1.2)
Based on the above parameters, we can know that the nominal rated input power of the motor of the machine (without considering service coefficient and full load): P1= (motor rated output power P_motor efficiency_) = 132 kw÷94.7%= 139.39 kw
Nominal rated input power of the machine (considering service coefficient and full load): P2= (rated output power of main motor P_main motor efficiency_) x (service coefficient S.F-0.05) = (132 kw_94.7%) x (1.15-0.05) = 153.33 kW (note: 5% margin should be taken into account when calculating service coefficient in theory, but not fully calculated)
The nominal specific power of the machine (at 7bar, considering the service factor and full load): ____________
PB1 = P2÷24m/min = 6.39kw/(m/min) (note: if it is an air-cooled machine, the input power of the fan should also be taken into account)
If the machine is air-cooled, the rated power of the fan motor is 4.5 kW and the efficiency is 85%, then the input power consumption of the fan motor is PF=4.5 kw÷85%=5.29 kw.
Then the nominal total input power of the machine (considering fan power consumption and service factor and full load): PZ = P2 + PF = 153.33 + 5.29 = 158.62 kw
The nominal specific power of the typhoon cooling machine (at 7bar, considering service coefficient and full load): PB2 = PZ÷24 m/min = 158.62÷24 = 6.61kw/(m/min)
2. Calculate input power according to measured current and voltage
Input power refers to the active power（KW）：P=（√3*U*I*COSφ）/1000 that is input by the power supply of the motor. U-is the line voltage (V), I-is the line current (A) input by the power supply of the motor, and COSφ_-is the power factor of the motor.
For example, you can measure the voltage and current of an air compressor when the exhaust pressure reaches 7 bar, and then calculate the corresponding input power consumption at 7 bar according to the above formula. According to the data of the main motor Y2-132M-4 in the table above, we know that the input power consumption of the motor can be calculated as follows: U = 380V, I = 237A, COS phi = 0.892, according to the rated current value and voltage value of the motor at full load, without considering the operating conditions (i.e. service factor). Then the total input power consumption of the main motor is: P3=(3*U*I*COS_)/1000= (1.731*380*237*0.892)/1000=139.14 kW after knowing the total input power, the specific power of the machine can be easily calculated to be 5.8 kw/(m/min).
