Time: 2018-11-26

The only criterion for determining whether an air compressor is energy-saving is “specific power”. To understand the specific power of a screw air compressor, you must first have a complete understanding of the concept of its input power. We know that the electricity bill that the user has to pay is not the output power of the whole machine, but the input power consumption of the whole machine, that is, the total input energy consumption of the machine. Below I will give an example of the input power calculation in two cases:

1.Calculate input power consumption based on motor nameplate parameters

1) For any machine, the parameters to be marked on the motor nameplate are as follows (for example, a conventional single-stage air compressor 132kw model: flow = 24m3/min, working pressure = 7bar):

Motor rated power (rated output power or rated shaft power):

P = 132kw

Motor efficiency (example of a domestic motor):

= 94.7%

Power factor：COSφ=0.892

S.F (Service factor) =1.15 (There are also manufacturers with a service factor of 1.2)

Based on the above parameters, we can know:

The machine's nominal rated input power of the machine (regardless of service factor and full load):

P1 = (Motor rated output power P ÷ Motor efficiency η)

= 132kw÷94.7%

= 139.39kw

The nominal rated input power of the machine (when considering the service factor and full load):

P2 = (P÷主马达效率η) x (S.F-0.05)

=（132kw÷94.7%）x (1.15–0.05)

= 153.33kw

(Note: In theory, when calculating the service factor, you need to consider leaving a margin of 5%, which cannot be calculated.)

The nominal specific power of the machine (at 7 bar, considering the service factor and full load):

PB1 = P2 ÷ 24 m³/min

= 6.39kw/( m³/min)

Note: If it is an air-cooled machine, you also need to consider the input power of the fan. If the machine is an air-cooled machine with a fan motor rated at 4.5kw and an efficiency of 85%, the fan motor's input power consumption is:

PF = 4.5kw ÷ 85%

= 5.29kw

The nominal total input power of the machine (considering fan power consumption and considering the service factor and full load):

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

The nominal specific power of the chilled machine (at 7 bar, considering the service factor and full load):

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

2. Calculate the input power based on the measured current and voltage values

Input power refers to the active power (kW) input by the power supply to the motor:

P= （√3*U*I*COSφ）/1000

U — Line voltage for motor power input（V）

I — Line current of motor power input （A）

COSφ— Motor power factor

For example: You can measure the voltage and current of an air compressor when the exhaust pressure reaches 7bar, and calculate the corresponding input power consumption at 7bar according to the above formula. According to the above main motor data, we know that without considering the working condition (ie, the service factor), we can calculate the input power consumption value of the motor according to the rated current value and voltage value when the motor is fully loaded:

U = 380V

I = 237A

COSφ = 0.892

Then the total input power consumption of the main motor is:

P3 = （√3*U*I*COSφ）/1000

= (1.731 * 380 * 237 * 0.892)/1000

= 139.14kw

When we know the total input power consumption value, we can easily calculate the specific power of this machine is 5.8kw / ( m3 / min).

After understanding the calculation method of the specific power of these two machines, I believe that everyone has a certain understanding of the power.

However, the first method only considers the situation under full load conditions. In most cases, the user site is partially loaded and loaded with no-load switching, and the nominal parameters are not calculated. Generally speaking, the input specific power / 60 = electricity per cubic air (kwh), the user immediately knows how much electricity to pay for the compressed air they consume.