How can I reduce the running costs of my compressed air system?
There are a number of things you can do to reduce the electrical costs of creating your compressed air. Some of these are easy to achieve with little or no incremental costs some are more complex and strategic which require planning and investment. All should be built into your business plans and company regular maintenance procedures. In this article we are looking at the more expensive or more complex things that can help you save electrical costs. Click here to see the first article in this series that looks at the quick, easy and low cost things you can do.
Now lets look at the low cost fixes
Check and fix air leaks in your pipework / attached equipment
This sounds a completely obvious fix but practically every compressed air system over a year old will have some degree of leaks. The leaks may be in the pipework or may be in the fittings connected to the pipework or may be within the machines that use the compressed air.
There are lots of ways of checking pipework for leaks. One of the best methods is using an ultrasonic tester which listens for the sound of an air leak. There are also systems that use leak detection fluid (a bit like putting a bike inner tube into a bucket of water) and looking for bubbles. However, the key is that even a very small leak, maybe a leak that is too small to hear with your ear will be wasting several cfm.
See our compressed air leak chart below
Leak noise level (dB) Sounds like leak size (cfm @100psi) approx cost (per hour)
10 normal breathing 0.5 £0.01
20 mosquito 0.7 £0.01
30 whisper 1.3 £0.03
40 domestic fridge 1.5 £0.03
50 quiet office 2.5 £0.05
A typical small compressed air system that is several years old will have between 10 and 30 separate leaks equating to a cost of leaks of between £2 and £7 per day.
Air Equipment can come and do a ultrasonic leak test on your compressed air system to identify and repair the significant leaks in your compressed air system. The cost of this can easily be saved in a few months by the energy saved.
Turn off your compressor when it is not in use
Again an obvious one, simply turn off your air compressor when you are not using any compressed air. As we know, almost all compressed air systems have some level of leak (see above). Leaving your air compressor turned on when you are not using any compressed air is basically just feeding the leaks in your system.
Reduce the air compressor working pressure
Keep the working pressure of the compressor to the minimum you can that still meet your process needs. Many people set the pressure to a higher level than is actually needed based on the ‘better safe than sorry’ principle. This uses more energy to compress the air to an unnecessarily high pressure. It also makes the effect of leaks worse, and also amplifies the effect of pressure losses through pipework and other auxiliary equipment.
Keep your compressor room well ventilated
This may sound a little odd, but keeping the temperature of the air entering your air compressor low improves energy efficiency. A 4’C rise in ambient temperature of your compressor house will result in 1% more energy usage.
Use the heat generated by your compressor for space or water heating
This is a slightly different approach to saving energy costs. You compressor creates a lot of heat as part of the compression process. Typically over 80% of the energy used in the compression process is given off as heat. For our 11KW compressor this means about 9KW of the input energy is given off as heat, this heat is usually released to the atmosphere and wasted. It is usually easy to duct this heat into a building to help keep the building warm. This reduces the amount of other heating required, saving energy costs.
Alternatively it is a relatively easy add a heat exchanger to the air compressor cooling system and then this can be used to pre-heat water for the central heating system, showers or warm process water etc. Again this is a relatively low cost approach that can save money used for heating elsewhere in the business.
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