Compressed Air - The ultimate fight against energy waste

In industrial facilities compressed air is needed for uncountable applications. Moreover, as years go by, air demand changes and compressed air systems grow with increasing demand. Far to often new equipment is acquired with the aim of keeping production running without risking stand-stills, without having a closer look whether the system could be optimized. 

Keep in mind however that compressed air systems tend to be a major lever for energy savings as ultimately all energy put into compressing air becomes heat. 

There are plenty of considerations to make, when it comes to conceiving a good system. Let’s start with the air itself.

Air Quality

The air quality class is set according to ISO 8573-1. This standardised system defines parameters from the least to most contaminated sources of compressed air. In the context of compressed air specifications, air compressors are graded according to the purity class after compression. The choice of air quality defines a plant’ equipment. 
 

Type of compressors

Depending on the required pressure many different compressor types can be used. In heavy industry procutions the following types are the most common:
Screw compressors with oil injection, oil-free screw compressors and turbo-compressors. All of them have their strong and week spots.

Turbo compressors have the best isentropic efficiency, or in other words the specific energy needed to compress a cubic meter of air (kJ/Nm3) is approx. 10% lower compared to screw compressors. However, turbo compressors are not good when a wide range of flow with relatively constant pressure is required, since they usually use IGVs (inlet guide vanes) to regulate flow. They are considered to be a god choice for base-loads, and are also oil-free by design.

Screw compressors are less efficient than turbos, but can be controlled easily via VSD (variable speed drive), becoming an optimal choice for large flow regulations. Oil-free screw compressors are slightly less efficient since the gap between the screws (male and female shafts) needs to be slightly bigger than for oil-injected compressors. If highest air quality is required and good control is required, they are still the number one choice.
Oil-injecting has two functions. Firstly it allows smaller gaps between the make and female shaft yielding better efficiency, and second the oil absorbes most of the heat, which resides in the compressed air. This cools down the air, and stores much of the heat in oil, which can be easily be passed on to a secondary heat source (e.g. water), with relatively small heat exchanger. 

Heat

Make sure to recover heat from the compressors, whenever possible. We are talking about big quantities of heat and therefore money! Find adeguate heat sinks in your plant!  About 75% of electric energy put into air compression can be recovered via uesful heat, depending on the temperature level of the heat sinks.

Control system

A very important aspect for energy savings is the use of an efficient control system. Make sure that all compressors are controlled by a smart common control unit. Get familiar with common compressor control logic (load-unload-stop or blow-off) and “by-pass” their shortcomings.

General considerations 

Optimizing a compressed air system also involves the following levers:
  1. Leaks: Identify and fix leaks in the system to prevent wastage.
  2. Pressure Settings: Adjust operating pressures to the minimum required for each application. Establish a second low pressure system if useful.
  3. Load Matching: Match the system’s capacity to actual demand to avoid overworking compressors.
  4. Efficient Equipment: Use energy-efficient compressors and ancillary equipment.
  5. Maintenance: Regularly service and maintain equipment to ensure optimal performance.
  6. Air Treatment: Implement proper filtration, drying, and cooling to improve air quality and efficiency.
  7. Distribution System: Design and maintain an efficient piping network to minimize pressure drops.
  8. Storage: Optimize the use of storage tanks to buffer demand fluctuations.
  9. Control Systems: Use advanced controls to regulate compressor operation based on demand.
  10. Seasonal demand: Change control settings depending on demand, since it may not be constant 24/7.
 

Remember that every industrial setup is unique, so it’s important to tailor these steps to your specific equipment, operational requirements, and goals.