Keith Barron, National Sales Manager at Atlas Copco Compressors, advises vehicle workshop managers to give priority to pressure and flow when specifying compressed air systems, in order to optimise performance and energy consumption.
It is now widely accepted that compressed air accounts for on average 12% of industrial electrical power consumption. In exceptional circumstances, it can even represent 30% of a company’s energy consumption. Furthermore energy expenditure is the dominant factor in a compressed air installation’s overall cost, often representing up to 80% of a compressor’s lifecycle operating budget.
Therefore, logic dictates that when specifying equipment for workshop applications, every care should be taken to ensure the system’s performance and output is closely matched to the application need. This precaution applies as much to the latest designs of rotary screw compressors installed in multi-bay commercial service operations and body repair shops, as it does to simple reciprocating units in a tyre fitting suppliers’ premises.
Sign of the times
In less energy-conscious times, air compressors were regarded as something of an ‘easy-fix’ when setting up a workshop operation. There was no real attempt to match equipment specification and performance with any degree of precision to actual air applications. It was also not an uncommon practice to allow equipment suppliers to over-specify air compressors’ power and output ratings without challenge or regard to outcome.
The legacy of that era still remains today in many areas of the vehicle workshop industry, whereby the common benchmark for equipment performance comparison is restricted solely to the kW power rating of the installed motor drive, as opposed to calculating the actual pressure and flow required to optimise productivity and energy-efficiency.
It pays to be sure
When it comes to replacing existing equipment, most companies would agree that to get the best return from their capital investment, it pays to carry out due diligence. One way to ensure a new compressed air system will be optimised, and provide the efficiency improvements that are promised, is by conducting an energy audit. This should be done both prior to replacement and at regular intervals after new equipment has been installed.
Applying simple and non-intrusive air audit data logging techniques to compressed air systems provides a true indication of an installation’s air use. Ultimately, it will flag up any wastage factors such as leakage and pressure drops. Most importantly, an audit establishes how well a compressed air system is actually performing, and will highlight opportunities to increase production uptime, reduce maintenance and improve operating costs.
Monitoring techniques for compressor systems have been applied to countless diverse applications throughout industry and as a result of implementing these energy-efficient measures, it is not uncommon for operating cost savings of as much as 40% to be realised.
It is the compressed air equipment in an installation that determines the requisite working pressure; not just the air compressor. In a large installation, that means all of its pipework, valves, dryers, receivers and filters. At the outset, the nominal air requirement for an application should be determined by calculating the air consumption for all the tools, machines and processes that will be connected to it. Also to be considered are additions for leakage, wear, and any future changes in the compressed air requirement, which, experience shows, should be based on a probability factor that is close to 60%.
A simple rule to bear in mind when determining the required size of an air compressor, based on its rated motor power, is that an increase in pressure creates a decrease in flow. For example, a basic 7 bar air compressor, driven by a 15kW motor, will deliver compressed air at a maximum volume of approximately 45 l/s. However, the free air delivery from a 10 bar version will deliver only 37 l/s from the same power unit. Therefore, an increase in working pressure by 1 bar increases the power requirement by approximately 6%.
For this reason, energy conscious manufacturers of stationary oil-injected air compressors adopt the mantra: pressure + flow = energy = cost, and offer potential customers a number of pressure/flow variants in each kW category.
It pays to apply a few essential rules when deciding on a new compressor:
- Assess present and future needs by first checking out the existing system with a thorough air audit. Take account of all variables from air end output right through to point of use
- Do not rely solely on stated motor power rating to match the compressor performance to the process air demand
- Start with the pressure and flow requirement and then find the best fit in terms of drive motor power
- Always keep in mind: pressure + flow = energy = cost
The enhanced performance of modern compressed air equipment, when correctly matched in terms of pressure and flow to application demands, may actually allow a lower kW rated machine to be specified at a correspondingly smaller purchase price. It may also allow reduced running costs in comparison to higher rated equipment.