2022-09-13 14:09:38

Air compressor identification method

At present, there are still large 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 kW. The product basically uses Hp mainly. In addition, the numbers marked on many domestic air compressors are not the specific power size, and in many cases represent the product serial number of the manufacturer. Today, the editor will take you to understand the commonly used marking methods of air compressors.


one. Four basic flag methods:


1. Use motor horsepower to express:


This was feasible in the early days when the air compressor technology was not developed, but with the continuous development of technology, even if the same motor is used now, the actual pressure will be affected by the pressure, the manufacturer of the air compressor and the size of the model. The amount of air that the air compressor discharges makes a huge difference. Therefore, it is the most irresponsible approach to only indicate the motor horsepower on the catalog.


2. Use Piston displacement to express:


Since this is the design data of the air compressor, it is only necessary to multiply the cylinder size by the number of revolutions. Therefore, this data is the easiest to obtain and is also used by many manufacturers to indicate. There is no certain relationship between this theoretical value and the actual gas output, which depends on the technical ability of the manufacturer.


3. Use the Inlet volume to express:


This representation method is usually measured by an Orifice meter on the air inlet side, 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 first two methods, but it is still higher than the actual gas output because the internal loss is not counted.


4. Use Free air delivery to express:


This method is measured on the outlet side with an aperture flowmeter. Because of its accuracy, it has become the main standard in the world to measure the actual air output of the air compressor, such as ISO, ASME, JIS, etc. However, in some Japanese manufacturers catalogs, Using F.A.D. to mark, but adding Nominal capacity, usually understandable statement is: this F.A.D. is not real, but just a design value. Unfortunately, it is one thing to have a standard, and another to have it, so unless the original catalogue describes the standard based on black and white, its credibility will be greatly reduced.


two. The actual air volume shown under different working conditions:


The actual air output refers to the air output measured at the outlet of the air compressor (after the aftercooler) considering all losses, and is usually expressed as Free Air Delivery. The so-called free air output refers to the volume of air compressed by the compressor, which is expressed by the free air condition (temperature, pressure, humidity, etc.) at the air inlet. Therefore, even if the same test standard is used, the number expressed 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 inlet gas condition used in the test)


Indicated air conditions: 760 mmHg, 0 0C, 0% RH


Volume Index: 1.00


2. Standard Condition:


Representation method: SCFM (or indicate the gas inlet condition used in the test)


Indicated air conditions: 1 bar, 20 0C, 0% RH


Volume Index: 1.05 (approx.)


3. Actual Condition:


Representation method: ACFM, ICFM (or indicate the gas inlet condition used in the test)


Air conditions referred to: 14.4 psi, 35 0C, 60% RH


Volume Index: 1.20 (approx)


The same air output can be increased by 20% as long as different air conditions are used.


three. The actual air volume tested under different pressures:


The actual air flow (FAD) value is related to the reference air condition and the pressure at which the test is performed.


For example, a 55 kW Lutz blower at 0.5 barg measures an actual air output of about 40 m3/min, and a 55 kW micro-oil screw air compressor measures about 9.1 m3/min at 8 barg. min, while the actual airflow measured at 13 barg is approximately 6.8 m3/min.


Therefore, when comparing the actual air output of different brands of air compressors with the same power, the pressure during the test flow should 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 still to be maintained when the pressure increases, the rotor speed needs to be increased, and at the same time, 6~7% more motor power is consumed. For a completely oil-free screw air compressor, the power consumption needs to be increased by about 10%.


As an example, suppose a 55 kW micro-oil screw machine A measures an actual air output of 9.54 m3/min at 7 barg, and another 55 kW micro-oil screw machine B measures an actual air output of 9.1 m3 at 8 bar /min, is A more efficient or B more efficient? According to the above empirical formula, if A still maintains the same air output of 9.54 m3/min at 8 barg, it needs to increase the power consumption by more than 6%, that is, 55 kW×1.06=58.3 kW The specific energy of the two are:


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. Air compressor motor power


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 volume will vary greatly in value due to different test methods and representation methods. The same is true when examining the motor power of the air compressor. At the same time, the efficiency of the air compressor is also related to the motor service factor, motor efficiency, etc.


1. Shaft horsepower at specified pressure


People often use specific energy - the power consumption per unit of air output to measure the efficiency of the air compressor. The 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 pressure The power consumed by the main shaft of the machine. Due to the different pressure points selected by different manufacturers, the indicated 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 it does not equal the power actually 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 American manufacturers are accustomed to use smaller motors with a larger service factor such as 1.25, and at full load , The motor output can exceed 15% of the nameplate. For example, a motor marked as 100HP may have an actual output of more than 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, in general, for the same horsepower air compressor, the air volume data of European brands is smaller than that of American brands, for the above reasons. Now some manufacturers of European brands have begun to do the same in the United States.


five. air compressor efficiency


As mentioned above, when considering the efficiency of the air compressor, it is necessary to consider its test method, indicating the state, 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 electricity according to the input power. Calculated, the efficiency of the air compressor cannot be ignored without considering the efficiency of the motor.


In this way, it is almost meaningless to compare the efficiency of the data on its catalog without knowing the standards used by each manufacturer to label air compressors and select motors. In fact, even knowing all the details of the various manufacturers, it is very difficult to compare, because the relationship between the different standards is difficult to determine.


Therefore, comparisons made on the basis of the information provided on the catalogue are only "references".