It’s the same with compressed air as with many other things in life: a small cause can have a large effect – both in a positive and negative sense. Upon closer inspection things are often different from how they at first appear. In unfavourable conditions compressed air can be expensive, but in the right circumstances it is very economical. In this first chapter we will explain the terms used in compressed air engineering and the things you should watch for in connection with them.
Free air delivery
The air delivery of a compressor (known also as the free air delivery or FAD) is the expanded volume of air it forces into the air main (network) over a given period of time. The correct method of measuring this volume is given in the following standards: DIN 1945, Part 1, Annex F and ISO 1217, Annex C. The measurement process is performed as follows as shown in the following figure:
The temperature, atmospheric pressure and humidity must first be measured at the air inlet of the compressor package. Then, the maximum working pressure, temperature and volume of compressed air discharged from the compressor are measured. Finally, the volume V2 measured at the compressor outlet is referred back to the inlet conditions using the shown equation:
The result is the free air delivery (FAD) of the compressor package. This figure is not to be confused with the airend delivery (block delivery).
Note: DIN 1945 and ISO 1217 alone only define the airend delivery.
Motor shaft power
The motor shaft power is the power that the motor delivers mechanically to its output shaft. The optimal value for motor shaft power is the point at which optimum electrical energy efficiency is achieved and the cos ϕ power factor is reached without the motor overloading. This figure lies within the range of the rated motor power. The rated power is shown on the motor’s nameplate.
Note: If the motor shaft power deviates too far from the rated motor power, the compressor will run ineffi ciently and/or will be subject to increased wear.
Electrical power consumption
The electric power consumption is the power that the drive motor draws from the mains power supply with a defined mechanical load on its shaft (motor shaft power). The power consumption exceeds the motor shaft power by the value of the motor losses – both electrical and mechanical – from bearings, fan, etc. The ideal electric power consumption P can be calculated using the formula:
Un, ln, and cos ϕn are quoted on the motor nameplate.
The specific power of a compressor is the relationship between the electric power consumed and the compressed air delivered at a given working pressure. The electrical power consumption is the sum of the power consumed by all consumers in a compressor, for example, drive motor, fan, oil pump, auxiliary heating, etc. If the specific power is needed for an economic appraisal, it should refer to the compressor package as a whole and the maximum working pressure. The overall electrical power consumption at maximum pressure is then divided by the FAD at maximum pressure: Pspec= Electrical Power Consumption/Delivery
IE – The new formula for energy-saving drives
Efforts in the USA to reduce the energy requirements of three-phase asynchronous motors resulted in the Energy Policy Act (EPACT) becoming law in 1997. A short while later, an efficiency classification system was also introduced in Europe. The international IEC standard for electric motors has been in place since 2010. Classifications and legal requirements subsequently resulted in significantly improved energy efficiency for premium class electrical motors. High efficiency motors provide significant advantages:
a/ Lower operating temperatures
The internal efficiency loss caused by heat generation and friction can be as high as 20 percent in small motors and 4-5 percent in motors upward of 160 kW. IE3/IE4 motors operate with significantly less heating and, as a result, with much lower losses: A conventional motor with F class insulation operates at about 80 K, giving it a temperature reserve of 20 K, whereas an IE motor, working under the same operating conditions, will run at only about 65 K, increasing its reserve to 40 K.
b/ Longer life
Lower working temperatures mean less thermal stress on the motor, the motor bearings and terminals. Motor service life is significantly extended as a result.
c/ Six percent more compressed air for less power consumption
Less heat loss leads to increased efficiency. Thus, with precise matching of the compressors to the enhanced efficiency motors, KAESER is able to achieve up to a six percent increase in air delivery and a five percent improvement in specific power. This means improved performance, shorter compressor running time and less power consumed per cubic metre of compressed air delivered.