How can I reduce the running costs of a compressed air system? (part 2)
This article is part two of the ‘how do i reduce the running costs of my compressed air system?’ and looks at the higher cost or strategic routes to save money on your compressed air system. To go to part 1 click here
Strategic / higher cost fixes
These ways of saving electrical costs are quite expensive to implement, usually requiring replacement of capital plant, however, they are things that need to be considered when looking at replacing worn out equipment or when purchasing new equipment for a new factory extension or new production line etc.
Replace pipework with leak free smooth bore systems
The modern aluminium smooth bore pipework systems are much more efficient than the earlier galvanised steel systems. There are numerous reasons for this but the key ones are
- Smooth interior bore resulting in lower pressure drops
- Corrosion resistance that prevent leaks occurring once the pipework has been installed for a year or two.
- ‘O’ Ring seals that allow for some pipework movement without leaking
- Multiple pipe sizes enabling the most appropriate size to be selected during installation to minimise pressure drops for planned flow rates
In addition to this the modern pipework systems are light weight for easy installation and are easy to modify once installed so you can keep your pipework aligned with your requirements as your business grows and changes
Click here to find out more about the high efficiency pipework we recommend
Pipework tends to evolve rather than be designed!
We frequently find that a company’s compressed air pipework hasn’t been changed for many years. Frequently, additional compressors and additional applications have been added but the original pipework remains in place. Often this is like trying to squeeze a lot more air down a small pipe, resulting in high pressure drops and shortage of air at the application.
There are two basic distribution systems for compressed air pipework. The single main and the ring main. The single main is a ‘branch’ where air is supplied down a single pipe to multiple applications. You often find the applications towards the front of the single main have plenty of air whilst those at the far end are starved of air.
The other system the ring main is a better approach. In this case air can flow in either direction around a ring. This tends to lessen the chance of equipment at the far end of the ring being starved of air.
As compressed air systems evolve and grow it is always necessary to consider the system as a whole. If you add more applications or add more compressors you need to review your pipework system to make sure it can efficiently cope with the additional load. This may require larger pipe, or modification of the pipework system, such as adding a ring main.
Replace you current fixed speed compressor with a variable speed unit
Traditionally air compressors are fixed speed so they run at a fixed and consistent speed, which produces a fixed amount of compressed air per minute. This is great if you are using this exact amount of compressed air since the air compressor is constantly running at its most efficient point.
However, as we all know, this is never the case in the real world. The air compressor tends to run to fill up the receiver tank and to supply enough air for the current usage, it then stops running as the required compressed air is drawn off the receiver tank. A few minutes to a few tens of minutes later the air has been drawn off the receiver to the point that the compressor has to start up again to pump the system back up to working pressure and so it goes on through the day.
Since large electric motors aren’t designed to start up frequently, due to the stresses that a start-up puts through the motor, air compressors are designed to ‘run on’ in an ‘unloaded’ condition for a few minutes after the system air pressure has been reached. When unloaded, a compressor is still turning but by shutting a few valves within the compressor no compressed air is being made. If more air is required when a compressor is running in an unloaded state it is simply a case of opening the valves again to start making compressed air again.
The downside of running in an unloaded condition is that the air compressor is still using between 25% and 50% of the energy it is using when creating compressed air. This energy is totally wasted just keeping the compressor turning over.
If a fixed speed air compressor is running at 50% load and has a run on of 5 minutes. It will run for say 10 minutes at full power, then 5 minutes unloaded. It will then stop. 5 minutes later it will restart and run for a further 10 minutes followed by a further 5 minutes unloaded and so on. As you will appreciate the energy usage over the whole hour will be approximately 25% higher due to the unloaded running. With a 30KW air compressor this will equate to an additional 7KW for nothing every hour!
Variable speed compressors are an innovation that enables the compressor to rotate more slowly, producing less air, absorbing less energy. In this way the air compressor slows its rotation speed until it is producing exactly that amount of compressed air that is being used by the downstream equipment. It doesn’t need to run unloaded saving significant amounts of energy. On average a variable speed compressor will save 30% of the energy used by a fixed speed compressor, this is particularly true when the compressor isn’t running permanently at full load.
So why isn’t every compressor sold a variable speed compressor? The main reason that fixed speed compressors are still sold is that they are considerably cheaper than variable speed units. Also, at smaller sizes (less than 10KW), the energy saved over the life of the compressor isn’t enough to warrant the additional spend required to get a variable speed unit.
For larger compressed air systems with multiple air compressors and backup air compressors we recommend having a mix of fixed and variable speed air compressors, with a control system to manage the start up of the various compressors to give maximum efficiency of the whole system.
The British Compressed Air Society (BCAS) the UK trade body for Compressed Air Manufacturers and Distributors have produced a white paper on saving energy costs on compressed air systems. Click here to download the BCAS efficiency white paper