Oil-free air with a reduction in TCO

Where clean compressed air is concerned, all compressor manufacturers and end-users seek compliance with ISO 8573.1, an international standard that specifies levels of air quality for use across industry. Air quality is defined by the content of particles, water and oil (liquid, vapour and aerosol), with each level described as a ‘Class’ - Class 1 being the tightest standard definition specifying an oil concentration not exceeding 0.01mg/m3, entrained particulates of less than 0.1µm in size and humidity no greater than -70°C dew point. Setting aside particulates and water, for the purposes of this article we will be concentrating on oil and how novel and innovative technology can be deployed to eliminate oil entirely from compressed air streams.

In addition to the Classes defined in ISO 8573.1, there is what is called a ‘user defined Class 0’, which is described as ‘specified by the equipment user or supplier and more stringent than Class 1’. While a Class 1 compressor is often referred to as an 'oil-free' solution, it cannot be considered 'truly' oil-free if used to deliver compressed air to the most sensitive of downstream applications. A Class 0 oil-free compressor, on the other hand, can guarantee 100% oil-free air.

Compressed air that is free of oil in its liquid, vapour and aerosol forms is a vital process resource for a number of industries, notably those operating in the electronics, pharmaceuticals, automotive finishing and food & beverage sectors. The compressed air system of choice for these industries has inevitably been one that is designated as being 'oil-free', but oil-free compressors tend to be more expensive to operate and maintain, making total cost of ownership one of the key factors to consider when planning fresh investment in retrofits or additional plant.

Oil is traditionally injected into the compression cavities of a machine, to improve the sealing properties of each air path, reduce pressure drop and facilitate heat dissipation during the compression cycle. While this is an efficient method of generating compressed air, there is inevitably the risk of oil vapour and aerosols suspended in the air stream finding their way to the point of use. If the downstream systems are completely sealed, then this risk is acceptable; in reality, even the best maintained pneumatic system is likely to have a degree of leakage into the production environment, leading to unacceptable levels of contamination.

While some modern rotary screw and piston compressors are designed to address this problem, as the internal moving parts of these systems need to be lubricated, there is a risk that lubricants will ultimately enter the air stream and therefore these machines cannot be classified as being totally oil-free.

On the other hand, if the air drawn into the compressor contains oil, this will be concentrated during the compression process and subsequently transmitted to downstream processes. To eliminate this risk, a number of compressor manufacturers - BOGE among them - fit catalytic converters at the compressor outlet, to ‘open’ the long chain hydrocarbon molecules and leave a clean supply of air containing carbon dioxide and water; the latter may require a separate dryer to be installed for optimum air quality.

While rotary screw and piston compressors offer an efficient and reliable source of compressed air, in their ‘oil-free’ versions they can be expensive to purchase, operate and maintain, with relatively large numbers of moving parts and high levels of energy consumption. They also tend to be noisy and rather cumbersome units because they have to accommodate all the necessary lubrication, cooling, oil reservoir and control devices.

High-speed turbo technology

In an effort to find a solution to these problems, BOGE has been investigating the potential of new high-speed turbo technology, which eliminates the need for oil altogether. The result of this engineering design project is a new range of machines from the company – prefixed ‘HST’ – that is not only oil-free in every particular, but which also delivers to the customer a total cost of ownership that is at least 30% lower than that of a traditional ‘oil-free’ machine. BOGE’s HST 55, 110 and 220kW ISO 8573.1 Class 0 compressors are 50% smaller and lighter than traditional oil-free machines; they are also quieter - typically 63dBA for the 55kW unit, as opposed to 80dBA for a traditional screw compressor.

At the heart of the compressor are two compact permanent magnet (PM), high-speed motors each having a central rotor suspended on self-stabilising air foil bearings. Two precision balanced titanium impellors of different sizes and blade arrangements are mounted at each end of each rotor. At rotational speeds, typically in excess of 100,000rpm, air is drawn in via an intake funnel and a specially designed spiral casing to be boosted in three stages to reach operating pressures quickly and efficiently, with minimal energy losses thanks to the frictionless air bearing configuration.

Indeed, the air bearing is a novel feature of BOGE’s HST design. It is virtually maintenance free, unlike magnetic bearings, which are widely used in other types of turbo compressor. Magnetic bearings also require a power back-up device to be fitted to protect them in the event of a mains power failure.

Other operating efficiencies are derived from the impellor, diffuser and spiral casing design, which effectively reduces air pressure on the intake side. This has the effect of sucking fresh air through the motor, eliminating the need for a separate fan cooling motor and control unit. The inherently highly efficient PM motors are also inverter controlled enabling supply to be matched precisely and instantaneously to the demand.

This simple design has far fewer moving components, which minimises the risk of wear. For example, each drive motor has only one moving part, and unlike a traditional oil-free compressor there is no oil system, no conventional bearings and no gear units, while the number of seals is reduced from 17 to just three. Moreover, the low levels of internal friction, the type of motor and motor control devices used, and the elimination of supplementary parts such as fan motors, ensure significant improvements in overall energy efficiency. For example, energy use in the idle state is less than 1.9% of nominal power, while CO2 emissions are about 20% lower than traditional compressors.

Precision high speed turbo technology, linked with the use of advanced electronic control systems, has created a new generation of 100% oil-free, Class 0 compressors that offer companies operating in sensitive industries such as the food and beverage sector the potential to improve the quality, productivity and reliability of their production processes, while simultaneously reducing operating costs.