To achieve a highly efficient compressed air system that reduces the cost of ownership and improves sustainability, a review of the compressors and distribution system is required, argues Allen Cockfield, of the compressed air treatment specialist, Artic Driers International.
Is your compressor set up correctly? Is it fully loading and unloading at the correct pressures? If more than one compressor is in operation, are they working in unison or competing for control of the air system? These are typical questions that need to be answered to ensure that the operational performance and cost-effectiveness of a compressed air supply system are being achieved.
From an efficiency perspective, the output volume (m³/min) per kW of absorbed power is crucial. “An underperforming compressor is not an asset; it’s a power drain. The efficiency of the compressed air system needs to he regularly checked, especially after a major rebuild,” notes Allen Cockfield, Chief Executive Officer at Artic Driers International.
Compressors and dryers can also be affected by heat build-up in the compressor room. By reducing the ambient air temperature by 1.0°C, the efficiency of the compressor can be increased by 1.0%. Lower temperatures make dew points more achievable and filtration more effective for the air dryers.
Lower inline filtration temperatures result in significantly reduced oil carryover into the compressed air. For every 10 °C increase in inlet air temperature, the oil carryover increases by a decimal point. All filters are rated at a 20°C inlet air temperature; therefore, at this temperature, 0.01 mg/m³ of the oil remains in the compressed air line. At 30 °C, the oil remaining in the compressed air will be 0.10 mg/m3. “Getting the room air temperature down is, therefore, crucial to air quality in a plant, and to long-term compressor and dryer performance,” Cockfield points out.
“Is waste heat recovery from your compressor a possibility?” he asks. A vast percentage of the power a compressor uses is ejected as heat. Hot oil at 90°C from a compressor can be used to feed a heat exchanger. This can be used to heat a changing room, shower, or part of a production process. There are many possibilities,” he suggests.
Filtration and drainage
On the issue of compressed air quality, he warns that ignoring the filtration and drainage of condensates is perilous. “If the refrigeration dryer is not ejecting condensates to waste, it will be injecting thousands of litres of condensate into the distribution system every week. And if desiccant dryers are used, the lack of drainage may well lead to bed contamination and the need to replace the absorption desiccant packs – and these are not inexpensive!” he warns.
He advises that dew point monitors be installed and alarms set to ensure that compressor users stay on top of condensate issues. These can connect to audible alarms (horns) or flashing LED light towers, and they can send alarm signals to plant SCADA systems. The investment in these monitoring systems will pay for itself very quickly.
Regular mobile or fixed air quality audits to monitor dew point, and particle and oil carryover performance are a wise habit. A fixed-base monitoring station also makes sense, depending on the end-user requirements. There are numerous types of condensate drains available on the market. These include timed solenoids, float level drains, timed ball valve drains, large-capacity ball float valves, and capacitance level-controlled drains.
“These will all require some routine maintenance to ensure adequate drainage: a condensate drain that is malfunctioning is a condensate generator! Liquids can destroy adsorption materials, and they affect a refrigeration air dryer’s ability to reach its target dew point.
Air system pressure losses
Determining the pressure losses in a compressed air system is best achieved by logging the compressed air flow and pressure using a data logger. The following pressure losses must be determined: the compressor’s cut-in and cut-out pressure; the delivery manifold pressure losses; the pre-filtration loss; the air dryer loss; the after filter pressure loss; and the air distribution system pressure losses.
Simultaneous data logging is preferred, as it provides a more comprehensive overview of the data. If using a compressed air and power auditor, choose with care. Auditors play a critical role in conducting in-depth analyses. The power required to increase the pressure of a compressed air system by 1.0 bar is not a linear relationship; it requires a 5% to 8% increase in energy consumption relative to the full-load power of the compressor.
Compressed air leaks
Leaks cost millions of rands every year. It is vital to minimise leaks. With ultrasonic leak detection, this is simple. “Ultrasonic leak detection not only finds the leaks, but photographs them, quantifies the actual air lost and the cost to the company. It even writes a report for you!” Cockfield advises.
Table 1: The typical costs of air leaks at an assumed electricity tariff of R1.80 per kWh.
It is estimated that 25-30% of a compressed air system's energy is wasted into the atmosphere every year. Pressure losses cost money: for every bar of pressure loss, 8.0% of the input power (kW) is wasted. “A compressed air system leak audit should be undertaken at least once a year, to tag the leaks and fix them! We often see last year’s audit leak tag on a leak that was not rectified,” he says.
In conclusion, he asserts that a properly installed and well-maintained compressed air system is a sustainable asset that significantly enhances a production line's effectiveness, resulting in reduced downtime and substantially lower maintenance costs. If neglected, it can cost a small fortune and could dramatically affect the business's bottom-line profit, making it unsustainable.
