When it comes to compressor technology, there is a wealth of indicator figures to compare the energy efficiency of the various processes and machines – and, without purchasers realising it, this often results in apples being compared to oranges, so to speak.
Even an indicator as seemingly straightforward as “efficiency” is not necessarily as is it appears: is could be isentropic, isochoric, mechanical or overall efficiency, as well as several other types.
The situation is not much different when it comes to air flow: it could be the free air delivery, mass flow or volumetric flow rate as measured under inlet, discharge or standard conditions, etc.
It’s therefore often difficult for the lay person to make an objective comparison, as much uncertainty remains with regard to details. What exactly was measured? Where was it measured, and how? Were all losses and energy-consuming equipment taken into account?
These are all questions that must be asked. After all, it’s precisely these aspects that are decisive when it comes to evaluating the energy efficiency of the complete system once installed.
Standard ISO 1217:2009 has been used for decades in the compressed air sector for evaluation of positive displacement compressors. The standard allows for objective comparison of the scope of the quotation and scope of delivery in terms of output and volumetric flow rate. Yet although this and other standards reflect the current state of technology, their application is not legally required. For instance, there has been hardly any application of ISO 1217:2009 to the low pressure range (which includes reciprocating and screw blowers) although doing so would be highly beneficial in this area as well. Annex B of the standard describes measurement of the airend (or compressor block);
Annex C deals with electrically driven compressors as a complete unit (i.e. a complete machine, ready to be put in service, with a IIA declaration of conformity, as provided for in the Machinery Directive); and Annex E covers variable speed machines, such as those with frequency converters.
The standard first describes the physical values and their reference points. For example, the inlet and discharge parameters, such as pressure and temperature, must refer to the real inlet/discharge of the complete unit or its transfer points and must not refer to measurements taken somewhere within the machine itself. Precise definition of the measurement method used to calculate the effective free air delivery additionally ensures that all influences capable of reducing the actually usable volume or mass flow of air are taken into account, such as pre-warming of inlet air within the sound enclosure.
Permitted deviations of the physical values between measurement conditions and contract conditions are also quantified, as is the allowable tolerance of calculated performance data and their conversion to contract conditions.
In essence, ISO 1217:2009 states that when measurements have been performed in compliance with the standard, the actual free air delivery (usable volumetric flow rate) and the specific power, depending on the size, must not differ from the parameters indicated in the quotation by more than 4 or 5 percent. Specific power plays a key role here, as it expresses the power consumption in relation to the volumetric flow rate produced – i.e. kW per m³/h.
A familiar analogy is the specific fuel consumption indicated for cars, expressed in litres per 100 kilometres driven.
To therefore ensure that you are comparing ‘apples with apples’ certain criterion should be requested and present in each quote.