Repair shops, tyre bays, service garages and small metalworking companies all rely on a source of compressed air to support their activities. Now major advances in design and technology make it possible for existing and potential compressed air users to take advantage of the guaranteed reliability, efficiency and economic benefits of the latest generation of workplace rotary screw compressors.
In this article, Chris Ferriday, Sales Manager for Industrial Air at Atlas Copco Compressors UK, explains how this type of innovative equipment provides high-end features and performance at realistic investment and running costs; which ticks all the boxes that matter most to small-to-medium sized enterprises (SMEs).
The compressed air marketplace offers a complex variety of options that range from the very simple equipment offered by hardware suppliers − and even high street discount stores − right up to premier high-spec units and systems designed and manufactured by leading compressor brands for major industrial production applications.
Within that broad sweep, some 75 per cent of applications are for compressors with unit power less than 30 kW (40hp). Until recently, this presented SMEs with the challenging task of matching a specific range of application requirements to the most appropriate type of smaller compressor with the right performance characteristics, and at an affordable price. In order to overcome this challenge, the latest generation of compact, robust and economical rotary screw compressors has been developed with the needs of small-business users firmly in mind.
Rotary screw compressors compared to piston and vane
Getting down to basics, but without too much technical detail, rotary screw compressors provide many distinct advantages over their reciprocating and vane counterparts in terms of size, air delivery, duty cycle and longevity. In addition to overall energy efficiency, rotary screw air compressors offer multiple benefits compared to other compressor technologies. These features include low noise output, light weight and small footprint for point-of-use plug-and-play installation. They also offer continuous operation at temperatures of up to 46 °C, extremely low oil-carryover into the delivered air, simplified maintenance procedures and zero loss of capacity over time.
Furthermore, smaller screw compressors can be tank mounted to enable additional air storage capacity, while full-feature variants with an integrated dryer and oil-water separator protect the compressed air network and end-user output.
To see why the rotary screw compressor has made such progress in comparison to piston and vane technologies, it may help to examine the pros and cons of their operating principles and performances relative to the average SME’s workplace requirements.
Comparing like for like
The piston or reciprocating air compressor has traditionally been regarded as the most economical and least expensive type of air compressor. In its simplest form, it is still the most common type found in the automotive service industry. In more demanding roles, technically advanced versions can incorporate multiple compression stages to make them particularly suitable for high-pressure applications, such as PET bottle blowing.
However, in small to mid-range piston compressors some major drawbacks need to be considered. For instance, they are primarily intended for intermittent use. The average piston compressor can only work for about 50 to 60 per cent of its duty cycle, or the amount of time a compressor can continuously operate without stopping to prevent overheating in a given period. These units require adequate cooling time between cycles, otherwise they can overheat and possibly fail. In contrast, rotary screw air compressors run cooler and are designed to operate continuously at 100 per cent duty cycle.
Component wear is also an issue. Just like a car engine, reciprocating compressors feature piston rings and other components that are in direct contact with each other and suffer wear over time. With that wear comes a decrease in performance, oil carryover and excessive heat generation. They tend to run hotter as efficiency decreases and operating time increases. A lubricated rotary screw compressor is designed so that the compressor oil seals the internal rotors, preventing parts from wearing out. Unlike a reciprocating compressor that loses performance with age, the rotary screw compressor can maintain the same level of performance throughout its long service life.
Generally speaking, rotary screw air compressors deliver more air per kW of power compared to reciprocating compressors of the same size. This enables the use of a smaller rotary machine in place of its piston equivalent with the same output, contributing to significant energy savings.
A further complication is that reciprocating compressors require air tanks to reduce air pulses created in the compression process. As a result, they need to employ receivers to smooth the airflow and handle large demands from workplace air users, tools and equipment. On the other hand, rotary screw compressors do not create air pulses, and – depending on capacity – are able to deliver large volumes of air when required without the need for a reservoir. Furthermore, they are built to operate for an extensive working life without requiring major repair.
It is a similar story with rotary vane compressors. The simple vane principle has the longest track record since its introduction almost 100 years ago. However, such design longevity and simplicity does not necessarily equate to energy efficiency. Neither does it allow for a full range of capabilities in comparison to those offered by rotary screw compressors in terms of turndown rates, speed range limitations and energy demand.
In much the same way as the piston compressor is affected by component wear, the vane operating principle involves continuous sliding in and out of the rotor vane slot within the stator casing. As a result, both the vane and slot are subject to excessive wear, eventually leading to failure of the vane and all that entails.
There are also significant disadvantages in terms of vane compressor’s maximum speed, which affects output. For instance, as vane compressors rely on centrifugal force to operate, their maximum speed is limited to 1000 to 1100 rpm, which is a third of that for a screw compressor air element.
Rotary screw – The real alternative
It is clear to see that the innovative developments embodied in advanced rotary screw compressor technology provide a clear advantage in performance and compatibility with SMEs needs. That is why Atlas Copco has introduced the all-new GL15-22kW low-investment compressor range, alongside its upgraded G7-15 and GX2-7 EP series, to offer robust performance, simple installation and maintenance, plus easy control and monitoring to air users in the 2-22kW (4.0 to 22.8 l/s) segment.
The all-new GL15-22 belt-driven, fixed-speed compressor family has been designed to provide quiet, efficient, powerful and reliable performance combined with minimum total cost of ownership. The 15, 18 and 22 kW compressors within the range increase free air delivery by a competitive 7.2 per cent margin, while also reducing the specific energy requirement by a significant 5.6 per cent compared to previous models.
These innovative, Industry 4.0 ready, machines have been developed from the outset to be easy to install, operate and maintain. They typify why the new generation of reliable, robust and compact rotary screw compressors is the real alternative and an affordable choice for small-business users.