What is scfm air compressor? Labeling methods and Technical Parameters

Four basic marking methods:

1. The use of motor horsepower for marking:

This in the early air pressure technology is not developed when it still works, but with the continuous development of technology, even if the use of the same engine, but also due to the pressure stage, compressor manufacturers and models of different sizes, the actual compressor discharged by the amount of air volume there is a world of difference.

Therefore, the catalog only indicates the engine power, this is the most irresponsible practice.

2. The use of intake volume (intake volume) to show:

This method of expression is generally in the inlet side of the orifice plate (orifice plate) to determine the current only used to specify the size of the centrifugal compressor.

The device uses ICFM, which is more accurate than the first two ways, but because the internal losses are not counted, so it is still higher than the actual volume of gas.

3. To show the use of piston displacement (piston displacement):

Because this is the design of the compressor information, only the cylinder size multiplied by the number of rotating, so this information is the easiest to get, but also used for many manufacturers to highlight.

There is no relationship between this theoretical value and the actual air output, depending on the technical capabilities of the manufacturer.

4. To show the use of free air delivery (Free Air Delivery):

This method is the use of orifice flow meter on the outlet side of the measurement, because of the accuracy and thus become the world’s major standards used to determine the actual compressor air volume, such as ISO, ASME, JIS, etc., however, in some Japanese manufacturers catalog, the use of F.A.D. to mark, but also added rated capacity, usually understandable that: the F.A.D. is not real, but only a design value.

It is usually understood that this F.A.D. is not real, but only a design value.

Unfortunately, there is a standard is one thing, there is no do is another thing, therefore, unless the original catalog in black and white on the basis of the standard, otherwise its credibility will have to be greatly reduced.

The actual air performance under different working conditions:

The actual air output refers to all losses are taken into account, in the compressor at the outlet (after the aftercooler) measured air output, usually with free air delivery (Free Air Delivery) to express.

Free Air Delivery is the volume of air compressed by a compressor and is expressed in terms of the free air conditions (temperature, pressure, humidity, etc.) at the inlet.

Therefore, even if the same test standard is used, the difference in the “free air” used can lead to a difference of more than 20% in the figures given.

1、Normal condition:

Expression method: Nm3/min (or specify the air intake conditions used for the test)

Air condition: 760 mmHg, 0 0C, 0% RH

Volume index: 1.00

2、Standard state:

Expression: SCFM (or specification of the air inlet condition used for the test)

Air condition: 1 bar, 20 0C, 0% RH

Volume index: 1.05 (approx.)

3、Actual condition:

Expression: ACFM, ICFM (or specification of the air intake conditions used for the test)

Air condition: 14.4 psi, 35 0C, 60% RH

Volume index: 1.20 (approx.)

For the same air flow rate, the figure can be increased by 20% if other air conditions are used.

Actual air performance, tested at different pressure conditions:

The value of the actual airflow rate (FAD) depends on the reference air condition and also on the pressure at which the test was carried out.

For example, a 55 kW Lutz blower has an FAD of approximately 40 m3/min at 0.5 bar, a 55 kW micro oil screw compressor has an FAD of approximately 9.1 m3/min at 8 bar and an FAD of approximately 6.8 m3/min at 13 bar.

When comparing the actual air output of different compressor brands with the same output, the pressure at the time of the test flow should therefore be taken into account.

There is no strict theoretical formula for conversion, but there is a recognized empirical formula for reference.

For screw compressors with oil injection, the rotor speed must be increased and the motor power increased by 6-7 to achieve the same actual air output with increasing pressure.

For a completely oil-free screw compressor, an increase in power consumption of around 10 % is required.

Suppose a 55 kW micro oil-screw compressor A has a measured air output of 9.54 m3/min at 7 bar, and another 55 kW micro oil-screw compressor B has a measured air output of 9.1 m3/min at 8 bar: Is A more efficient or is B more efficient?

According to the empirical formula above, to still get the same air output of 9.54 m3/min at 8 bar, A must increase the power input by more than 6%, i.e. 55 kW × 1.06 = 58.3 kW are the two specific energies:

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 compressor B is 1.16% higher.

The motor power of the air compressor

The efficiency of an air compressor depends on the actual air output of the compressor and the power absorbed by the motor.

The actual air output will be due to the test method and the method of expressing the different values in the big difference.

A similar situation exists when examining the motor power of a compressor.

At the same time, the efficiency of the compressor is also related to the motor operating factor, the motor efficiency.

1. Service factor (operating factor)

The motor power of an air compressor refers to the rated power of the motor, which, however, is not the same as the actual power consumption of the motor.

For European and Chinese domestic manufacturers, the actual power consumption of the motor is generally less than the nameplate rating, while the US manufacturers in the habit of using smaller motors, the service factor is larger, such as 1.25, and under full load, the motor power can exceed the nameplate 15%, such as labeling for the 100HP motor, the actual power can be more than 115HP.

“This violates the old rule that the actual power consumption of a motor must be less than the rated power on the motor nameplate”.

For the reasons mentioned above, the air flow rate for compressors of the same power rating is generally lower for European brands than for American brands.

Now some European brand manufacturers have adopted the practice of the United States.

2. The specific pressure of the shaft horsepower

People often called the energy (Specific energy) – unit of power consumption to measure the efficiency of the compressor, where the power consumption of a certain pressure of the shaft horsepower, which means that the compressor discharge pressure reaches a certain pressure point, the compressor spindle consumed by the power.

Due to different manufacturers, the selected pressure point is not the same, and therefore the labeling of the value of the shaft power is not the same.

The efficiency of the air compressor

Consider the efficiency of the air compressor to consider the test method, the state said, test pressure points, the actual power consumption, but also the efficiency of the motor, because the shaft horsepower is only the output part of the motor, the user pays for electricity is calculated according to the input power to consider the efficiency of the compressor, can not take into account the efficiency of the motor.

If you do not understand the standards used by each manufacturer when labeling compressors and selecting motors, it is almost useless to compare their efficiencies based solely on the information in their catalogs.

Even if all the details of each manufacturer were known, it would be very difficult to make comparisons as the relationship between the different standards would be difficult to determine.

Consequently, comparisons based on the information contained in catalogs can only be “indicative”.