Compressed air is often called the fourth utility (after water, electricity and gas) due to its widespread use throughout industry. It is also the most expensive industrial utility, with about 10% of all electrical energy used by industry employed in compressing air. By reviewing the compressed air system as a whole, electricity consumption can be reduced and compression capacity increases avoided. This section discusses how this can be achieved by repairing air leaks, reducing intake air temperature, optimising system pressure, managing compressor operation and eliminating inappropriate uses of compressed air. It has often been found that a plant with multiple compressors can operate with one less compressor when upgrades have been made. Remember: to make effective improvements, a “systems approach” is crucial. Taking Action 1.Are there leaks in the system? > Info		Stop your air leaks and save 25–40 % of your energy costs
It is likely that a typical plant that has not been well maintained will have a leak rate of at least 20% of total compressed air production capacity. By detecting leaks and repairing them, air loss can be reduced to less than 10%. Whilst the air compressor may be efficient, leaks in the hoses, pipes, tools and joints can waste compressed air, and hence electricity and money. Fixing leaks may be as simple as tightening a connection or replacing faulty sections of fittings. As well as wasting energy, leaks cause a drop in system pressure, which can cause equipment to operate less efficiently. > Action
Methods for finding leaks: Shut off all equipment and start the compressor. Leaks can be found by listening for air escape sounds around the plant. If there are no leaks, the compressor should not run at all once it has reached the specified pressure. This test is best done on weekends or when the plant is not operational. You could also isolate one section of the pipework at a time and monitor the rate of loss of compressed air. This allows you to locate the areas of greatest loss. When your plant is idle and no compressed air-driven equipment is in use, monitor the energy use of your compressor. Compare the energy consumption of the compressor during idle periods with that during operating periods to identify the potential savings from cutting waste. Establish a maintenance program using compressed air leak tags. These tags are simply placed on leaks as they are identified by employees, making it easy for maintenance staff to locate the leaks. Another effective way to detect leaks is to use an ultrasonic leak detector, which can be used without shutting off equipment. These hand held detectors scan for leaks and through a headset, a leak can be heard, despite ambient noise. Although they cost about $800 to buy, they often have a fast payback. For example, three 2mm holes leaking 15 litres/second of compressed air increases annual energy consumption by around 10,000 kWh, costing $800 or more, so the cost of a meter can be repaid by the savings realised by fixing just a few leaks. To hire a detector costs around $300/month. Look under “Hire – electric and electronic test equipment” in the Yellow Pages for suppliers. Use the Compressed Air calculator on the website tofind out how much air leaks are costing you. 2.Is there too much pressure? > Info In many industrial plants, air is supplied at a higher pressure than is required. For example, some air compressors operate in excess of 700kPa at full load. However, some types of machinery and tools can operate efficiently at a lower air pressure of 500–600kPa. This means the system is running at an elevated pressure. The extra 100-200kPa includes a contingency factor and compensates for possible leaks and pressure drops across system. However, if your system is well maintained, you can lower this operating pressure. Pressure drops occur as the air travels from the compressor through dryers, separators, filters and piping to the point of use. This means that the pressure initially supplied by the compressor is reduced at the end of the distribution system. Unnecessary leaks and pressure drops create a situation whereby supply pressure must be increased to compensate for these losses. By ensuring that leaks and pressure drops are minimised, the pressure can be properly matched to the system demand without wasting energy and money. Every 100 kPa reduction in operating pressure saves 8% on energy costs.		Implement a regular maintenance program to identify and repair compressed air leaks. Place Compressed Air Leak Tags (see sample) where leaks are found and ensure the leaks are repaired promptly. Install compressor controls, such as timers. Consider replacing one large compressor at part load with several smaller ones operating at full load.

>Action

• Determine if you have a pressure drop.

Pipes should be correctly sized for velocities of 6–10m/s. The higher the velocity, the higher the pressure drop. Small pipe sizes often result in high air velocities which cause excessive pressure drop.

1. Find out the volume of air produced by your compressor (this is written on the compressor).
2. Determine the cross sectional area of the pipe = (3.14) x radius2.
3. Divide the volume by the area and then divide by 3600 (if not already in volume of air per second) to get velocity, i.e. velocity = volume of air per second/ cross sectional area.

For example, if you have a compressor producing air at 126L/sec (or 0.126 m3/sec) and a pipe leading directly from the compressor which has a 50mm radius:
Area of pipe = 3.14 x (0.05)2 m2 = 0.00785. The velocity is (0.126m3/0.00785m2) = 16m/s which is too high and is causing drastic pressure drops. Therefore, take the steps listed below to reduce your pressure drop.

• Reduce pressure drops by fixing leaks, selecting components with the lowest possible pressure drop, reducing the complexity of the distribution network (e.g. by minimising bends in the system) and reducing the distance the air travels through the distribution system. Ideally, the pressure drop should be no more than 10%. For example, if the air coming directly from the compressor is at 800kPa, and by the end of the pipe it is 690kPa, the pressure drop is over 10% which means you are wasting energy and money.

• Reducing system pressure means immediate savings but requires careful matching of tools and air compressor capacity.
• If more pressure at point of use is needed, try to reduce pressure drops and leaks before increasing compressor pressure.
• Remove unused compressed air lines and separate unused equipment with a valve.
• Consider dividing the distribution network into high and low pressure sections to minimise unnecessary use of high pressure. This is applicable when you:
  • have large quantities of air required at both low and high pressure and need to use more than one compressor, or
  • have one specific machine that operates at a higher pressure or is a long way from the compressors.

3.Is the temperature too high?

• Energy savings can be made if the temperature of the intake air is reduced. For every 3oC reduction in inlet temperature, energy use is reduced by 1%. This can often be achieved by ducting fresh air from outside (from a cool, shaded area), generally with a payback period of 2-5 years.

4.What controls does the system have?

> Info

Since compressor systems are typically sized to meet a system’s maximum demand, a control system is almost always needed to reduce the output of the compressor during times of lower demand. Large energy savings can be made through accurate matching of system supply and system demand. A control system can shut off unneeded compressors and turn on additional compressors when needed. Many compressors have inbuilt controls to shut the compressor off when not required. In a multi– compressor situation, controls can chose the most efficient compressor to use for any given demand.

>Action

• Determine the type of control system needed

There are many types of controls. The one best suited to your situation is determined by the number and type of compressors being used and the demand profile. It is important to seek advice from a compressed air system control supplier.

• Install a Variable Speed Drive (VSD)

A VSD allows the compressor to operate under a range of loads by accurately matching the motor speed to the actual load. This can help limit peak electricity demand by running equipment for longer at a lower output. This is important since peak demand charges can comprise 30% – 50% of total electricity costs.
Note: Some compressors run less efficiently at part load, so seek expert advice. Also, you cannot retrofit a VSD to a compressor, you must purchase a compressor already fitted with a VSD.

• Timer control

This is applicable to most compressed air systems and switches compressors off after production periods.

5.What other options are there?

• Compressed air is often used when other energy sources are more economical.
• One example is the use of a blower or fan for cooling, drying and cleaning processes. This results in both energy and dollar savings because the electric motors that run these systems are much cheaper to operate than air compressor motors. Modern electric hand tools (both plug-in and batterypowered) have increasingly compact and powerful motors, so they might sometimes replace compressed air powered equipment.
• Consider utilising the waste heat from your compressors to pre-heat boiler feed water, process hot water, or for other useful tasks. Up to 90% of the electricity used by the compressor is converted to waste heat.
• If you use compressed air to move particles or liquid, consider installing a specially designed nozzle which can be added onto the end of existing tubing. Such a nozzle draws in surrounding air to increase the volume of air and can reduce the amount of compressed air needed for a task by 70%.

Calculate your savings

The Compressed Air Calculator calculates leak costs, pressure savings and temperature savings.