Low-pressure blowers: Selection matters

 

In the past, the means of providing a supply of low pressure air for purposes such as water treatment aeration, pneumatic conveying and other industry applications required relatively simple blower equipment. But with reliability, performance and energy efficiency higher on the agenda than ever before, specifiers should address a series of key considerations before committing to a purchase.

Positive displacement or dynamic / centrifugal?

There are two main methods of air compression: positive displacement, which creates pressure by reducing air volume, and dynamic or centrifugal compression, which uses speed to create pressure by turning kinetic energy into static energy. Positive displacement blowers usually handle lower flows than centrifugal blowers, but provide a bigger turndown, which maintains a steady level of efficiency. Centrifugal machines operate with bigger flows and can be optimised at specific points. The choice depends on the application. For processes that require wide variations of flow, positive displacement blowers are ideal; whereas those that require a big flow with little variation might find a centrifugal blower to be better suited. However, it’s not unusual to combine the two types. For example, a centrifugal machine might look after the base load, while the positive displacement machine manages variations of flow.

Is operating range important? 

In positive displacement, the market has been traditionally led by lobe blowers, which are commonly used for aeration in wastewater treatment plants or for pneumatic conveying within a factory. Low pressure applications invariably feature fluctuations in air demand for the process so you need to know that turndown on conventional lobe blowers can incur increased energy penalties. However, a blower incorporating more modern screw technology can operate from 100% capacity down to 25% with very little change in specific power requirement.

How much air can actually be delivered?

Make sure that you know the actual volume of air the blower package delivers, not just the air intake figure stated by the manufacturer, which is a common practice in the lobe blower market. The air output is a critical factor in sustaining essential oxygen levels within any aerobic process and, what’s more, it is a requirement of industry standard ISO1217 that performance data is stated as FAD (Free Air Delivery) – the actual volume of air at the blower outlet

Need to eliminate pressure drops in the system?

It is a proven fact that lobe blowers have to work harder, and consequently less efficiently, than screw technology blowers to deliver the same output. That is because the delivered air is compressed externally and depends on back pressure from the connected pipe work – or, in water treatment applications, water resistance in the aeration tank – in order to maintain the required air flow and pressure. Bear in mind that resistance to the internal air flow can lead to substantial pressure drops and increased energy consumption. In contrast, a rotary screw type blower demands up to 30% less energy because its proven internal compression principle is far more energy efficient.

Can power transmission losses be minimised?

Belt and pulley transmission from the motor shaft to the compression element has been the industry standard system for some time now but the downside is the need for constant maintenance and frequent replacement. A blower with close-coupled integrated gearbox transmission offers a more efficient alternative and can gain more than 3% in energy savings.

What about discharge temperature?

The discharge air temperature of a screw blower is, on average, some 5% cooler than that from a lobe blower. A lower operating temperature can benefit the aerobic process in wastewater aeration applications and allow for more options on the pipework layout.

Is low noise operation essential? 

Environmental impact is a vital consideration for many blower application sites.

It’s possible to enjoy quieter operation and reduce installation costs by choosing a blower with intelligent internal baffle design and canopy configuration. Sound levels can be down to 72 dB(A) so there is no need  to provide noise-insulated rooms and doors. 

How do the different types of blowers compare in terms of energy efficiency?

The evidence is provided from the test results of the independent TÜV organisation that measured the energy efficiency of an Atlas Copco ZS rotary screw blower against the performance of a conventional tri-lobe blower. In these like-for-like tests it was proven that the ZS unit was 23.8% more energy efficient at 0.5 bar(e)/7psig and 39.7% at 0.9 bar(e)/13psig.

Could pulsations in the air delivery be a problem?

It’s not generally understood that vibrations and compression element pulsations affect blower efficiency. So it is best if your unit can deliver high frequency, low pulsation air. This can be achieved when air is delivered via the flexible hoses that feature on screw technology blowers. The hoses help to provide a smooth discharge and vibration de-coupling between the compression element and the filter housing. They also overcome the possibility of serious cracking that inflexible connections can create on other types of blower.

Installation time

In meeting the needs of critical applications there are benefits to be derived from solutions such as Atlas Copco’s advanced screw technology incorporated in the plug-and-play ZS+ range units. Because they can be easily installed and quickly become active, the time and resource normally spent on set-up is minimised. Screw blowers that are supplied complete with Elektronikon control systems, deliver Class Zero 100% oil-free air and offer integrated VSD options for additional energy saving opportunities.

What is the cost?

Evaluating different technologies can be difficult because there is no industry standard for figures with which all suppliers comply. However, the lifecycle cost comparison can be divided into three main considerations: investment cost, power cost and maintenance cost. In terms of upfront investment, some suppliers apply extra charges to ancillary items. Atlas Copco has achieved success with its simpler plug-and-play concept. Power costs should be considered in terms of free air delivery and maintenance costs can depend heavily on the likely need for replacement parts and the availability of warranties.

Relative values

In addition to these considerations, there will be many other factors for the specifier to take into account such as bearing life, drive motor specification, pipework requirements, panel building and control systems. 

When it comes to basic mechanical performance, there is no doubt that the traditional lobe blower will continue to fulfil a role within industry but in terms of guaranteed continuity of performance, reliability and energy efficiency, it would seem it is the screw blower that ticks all the right boxes.