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Plug those holes to improve energy costs

In the waste and recycling industry, compressed air is most commonly used at the heart of materials separation in the form of air knives.

As mixed waste moves along a conveyor belt, elements of it are removed by judicious blasts of compressed air.

First, paper and card are removed by such air jets. Later in the process, plastic bottles, which are usually made from PET, are scanned and identified by infrared camera. Precise air jets are then used to move the different plastics to separate areas.

The largest cost associated with compressed air is energy, yet the process is notoriously wasteful. Around just 45% of input energy is actually used to compress the air; the rest is lost through leaks, heat loss and other inefficiencies. It seems ironic that the recycling industry, with its focus on waste reduction, should be plagued by such inefficiency.

These losses are already being tackled, due in part to legislation. The recently introduced ISO 11011: 2013 standard sets out the requirements for conducting and reporting the results of a compressed air system assessment (audit) for the entire system. It sets requirements for analysing the data, reporting and documenting the findings, and identifying an estimate of energy savings.

A set of relatively simple alterations can cut the energy consumption of compressors in half. This is just as well, considering that compressed air can account for one-third of a site’s electricity bill.

Compressed air losses are not unique to the recycling sector. Some of the main factors that drag down efficiency are system leaks and heat loss. The good news is that these, and other problems, can be solved.

Leakage is a huge concern. According to the Carbon Trust, a 3mm hole in a compressed air line can cost a business up to £700 a year in wasted energy. But it can be prevented easily with regular pressure checks or by conducting a leak detection survey.

Air escaping through a leak hole creates a tiny sound that is inaudible to humans but detectable by ultrasound detection equipment, which converts it into an audible signal and indicates it optically.

Once leaks have been identified, a maintenance schedule must be put in place to plug them and ensure future leak detection surveys are carried out – or the benefits will only be temporary.

Many compressed air lines also run at excessive pressure, and this can be identified during an energy audit. Lowering system pressure could mean that fewer compressors are needed. As well as reducing energy and service costs, it will also cut down on maintenance.

Much of the energy used to power an air compressor is converted to heat. Traditionally, this was wasted but, now, up to 94% can be recovered and used to heat water or offices, or in industrial process applications where hot water is required.

Before investing in heat recovery, end users can calculate the potential energy and cost savings by assessing the heat or hot water demand in areas adjacent to the compressor installation.

Comparing this assessment with the average operating hours of the existing compressed air system will highlight the possible payback in terms of reductions in fuel, oil and gas costs. It makes sense to get help from a compressor specialist when making these calculations.

Operating two or more compressors via a central controller can also lead to savings, working in two ways. First, they constantly monitor pressure, keeping it within a narrow range by switching compressors on and off as needed. And second, they predict the best combination of compressors to use to meet demand.

This is particularly useful when using a combination of fixed- and variable-speed machines because it minimises off-load and part-load running of the compressors.

Multi pressure systems can make significant savings, with every 1 bar saved on pressure equating to a 7% saving on electrical running costs.

It is measures such as these that make an energy audit vital. Air compressors will always consume large amounts of energy, but running them more efficiently and recovering ‘waste’ heat will help to take the edge off utility bills.    

Mark Whitmore is general manager at BOGE Compressors

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