At present, there are still big differences in the identification methods of air compressors on the market. In particular, the units used in the power difference between domestic and foreign products are generally different. The power unit of domestic air compressors is generally based on kW, while foreign products The products basically use Hp. In addition, the number marked on many domestic air compressors is not the specific power size, and in many cases it represents the manufacturer's product serial number. Today, I will take you to learn about the commonly used marking methods for air compressors.
One. Four basic marking methods:
1. Use motor horsepower to express:
This was feasible when the air compressor technology was not developed in the early days, but with the continuous development of technology, even if the same motor is used now, the pressure level, the air compressor manufacturer and the size of the air compressor will be different, and the actual There is a huge difference in the amount of air exhausted by an air compressor. Therefore, it is the most irresponsible practice to only indicate motor horsepower in the catalog.
2. Use Piston displacement to express:
Since this is the design data of the air compressor, you only need to multiply the size of the cylinder by the number of revolutions. Therefore, this data z* is easy to obtain and is also used by many manufacturers to mark it. There is no certain relationship between this theoretical value and the actual gas output, and it depends on the technical ability of the manufacturer.
3. Use Inlet volume to express:
This method of expression is usually measured by an Orifice meter on the side of the air inlet, and is currently only used to indicate the size of a centrifugal air compressor. The unit used is ICFM, which is more accurate than the previous two methods, but because the internal loss is not accounted for, it is still higher than the actual gas output.
4. Use free air delivery to express:
This method uses an aperture flow meter to measure on the outlet side. Because of its accuracy, it has become the world's main standard for measuring the actual air output of an air compressor, such as ISO, ASME, JIS, etc. However, in some Japanese manufacturers catalogs, Using FAD to mark, but adding Nominal capacity, it is usually understandable that this FAD is not real, but just a design value. It is a pity that it is one thing to have a standard, and it is another thing to do it or not. Therefore, unless the original catalog is written in black and white to indicate the standard on which it is based, its credibility will be greatly compromised.
two. The actual air output shown in the following table under different working conditions:
The actual air output refers to the air output measured at the outlet of the air compressor (after the aftercooler), taking all losses into account, and is usually expressed by the free air delivery (Free Air Delivery). The so-called free air volume refers to the volume of air compressed by the compressor expressed by the free air condition (temperature, pressure, humidity, etc.) of the air inlet. Therefore, even if the same test standard is used, the displayed number will differ by more than 20% due to the different "free air" used. The following are several commonly used free air conditions.
1. Normal Condition:
Representation method: Nm3/min (or indicate the gas inlet condition used in the test)
Refers to the air condition: 760 mmHg, 0 0C, 0% RH
Volume index: 1.00
2. Standard Condition:
Representation method: SCFM (or indicate the air intake condition used in the test)
Refers to the air condition: 1 bar, 20 0C, 0% RH
Volume index: 1.05 (approximately)
3. Actual Condition:
Representation method: ACFM, ICFM (or indicate the gas inlet condition used in the test)
Refers to the air condition: 14.4 psi, 35 0C, 60% RH
Volume index: 1.20 (approximately)
With the same air output, as long as you use different air conditions, you can increase the number by 20%.
three. Actual air volume tested under different pressures:
The actual air output (FAD) value is related to the reference air condition, and also related to the test pressure.
For example, the actual air output measured by a 55 kW Lutz blower at 0.5 barg is about 40 m3/min, and the actual air output measured by a 55 kW micro-oil screw air compressor at 8 barg is about 9.1 m3/min. min, and the actual air output measured at 13 barg is about 6.8 m3/min.
Therefore, when comparing the actual air output of different brands of air compressors with the same power, the pressure at the time of its test flow must be considered. There is no strict theoretical calculation formula for conversion, but there is a recognized empirical formula for reference. For oil-injected screw air compressors, if the same actual air output is to be maintained when the pressure increases, the speed of the rotor needs to be increased, and at the same time, 6-7% more motor power is consumed. For a completely oil-free screw air compressor, power consumption needs to be increased by about 10%.
For example, suppose that a 55 kW micro-oil screw compressor A measured the actual air volume of 9.54 m3/min at 7 barg, and another 55 kW micro-oil screw compressor B measured an actual air volume of 9.1 m3 at 8 bar. /min, is it efficient for A or B? According to the above empirical formula, if A is to maintain the air output of 9.54 m3/min at 8 barg, the power consumption needs to be increased by more than 6%, that is, the specific energy of 55 kW×1.06=58.3 kW is:
A: 58.3kW÷9.54m3/min=6.11kW/(m3/min)
B: 55kW÷9.1 m3/min=6.04kW/(m3/min) (6.11-6.04)÷6.04×100%=1.16%
This means that the efficiency of the B air compressor is 1.16% higher.
four. Motor power of air compressor
The efficiency of the air compressor is related to the actual air output of the air compressor and the power consumed by the motor. The actual air output will vary greatly in value due to different test methods and representation methods. There is a similar situation when looking at the motor power of an air compressor. At the same time, the efficiency of the air compressor is also related to the motor service factor and motor efficiency.
1. Shaft horsepower for specific pressure
People often use Specific energy-power consumption per unit of air output to measure the efficiency of air compressors. Power consumption here refers to the shaft horsepower of a specific pressure, which means that when the outlet pressure of the air compressor reaches a certain pressure point, the air compressor The power consumed by the main shaft of the machine. Because the pressure points selected by different manufacturers are different, the marked shaft horsepower values are also different.
2. Service Factor
The motor power of an air compressor refers to the nominal horsepower or rated power of the motor, but this is not equal to the actual power consumed by the motor. For European and Chinese domestic manufacturers, the actual power consumption of the motor is generally less than the rated power on the nameplate, while manufacturers in the United States have traditionally used smaller motors with a larger service factor such as 1.25, but at full load , The motor output can exceed 15% of the nameplate, such as a motor marked as 100HP, its actual output may exceed 115HP. This breaks the old rule that "the actual power consumption of the motor must be less than the rated power on the motor nameplate".
Therefore, generally speaking, for air compressors of the same horsepower, the air volume data of European brands is smaller than that of American brands, and the reasons are as above. Now some European brand manufacturers have started the American approach.
Fives. The efficiency of the air compressor
As mentioned above, when considering the efficiency of an air compressor, it is necessary to consider the test method, indicating the status, test pressure point, actual power consumption, etc., and also consider the motor efficiency, because the shaft horsepower is only the output part of the motor, and the user pays for the electricity according to the input power. Calculated, the efficiency of the air compressor cannot be considered without considering the efficiency of the motor.
In this case, if you don't understand the standards used by each manufacturer in labeling air compressors and selecting motors, it is almost meaningless to compare their efficiency based on the information in the catalog. In fact, even if you know all the details of each manufacturer, it is very difficult to compare, because the relationship between different standards is difficult to determine.
Therefore, comparisons based on the information provided on the catalog can only be "reference".